Unveiling the Environmental Drivers of Pelagia noctiluca Outbreaks: A Decadal Study Along the Mediterranean Coastline of Morocco, Algeria and Tunisia

Abstract

Pelagia noctiluca blooms are a significant ecological event in the Mediterranean, with profound implications for marine ecosystems and coastal economies. This study aims to investigate the interannual and seasonal variability of Pelagia noctiluca bloom patterns along the Moroccan, Algerian, and Tunisian Mediterranean coasts, focusing on the influence of environmental factors such as sea surface temperature, nutrient availability, and oceanographic conditions on bloom intensity and distribution. The analysis reveals significant seasonal and interannual fluctuations in bloom size across the three regions, with the most substantial blooms occurring from June to August during the warmer months. In 2014 and 2018, peak bloom sizes of up to 775 jellyfish per unit area were recorded in Morocco and Algeria, while Tunisia also experienced notable blooms, particularly in 2015 and 2017. However, from 2020 to 2023, a marked decline in bloom intensity was observed across all three regions, with bloom sizes dropping, particularly in Tunisia and Morocco. Correlation analysis of environmental variables showed weak to moderate relationships between bloom size and key factors. Sea surface temperature showed a positive correlation (r = 0.13), suggesting that warmer waters contribute to increased bloom intensity. Nitrate and current direction also exhibited weak positive correlations (r = 0.12 and r = 0.27), indicating that nutrient availability and ocean circulation patterns could enhance bloom formation. The correlations with carbon (r = 0.08) and phosphate (r = 0.04) further suggest that organic matter and nutrient availability play a minor role in supporting bloom growth. On the other hand, negative correlations with factors such as oxygen (r = −0.04), solar radiation (r = −0.04), and wave height (r = −0.05) suggest that these factors may slightly inhibit bloom intensity, possibly by influencing nutrient dynamics and dispersing jellyfish populations. Notably, pH level (r = −0.29) and current speed (r = −0.23) exhibited stronger negative correlations, indicating a more pronounced inhibitory effect. In conclusion, this research highlights the complex and multifactorial nature of Pelagia noctiluca bloom dynamics, where temperature, nutrient availability, and oceanographic conditions interact to influence bloom size and distribution across the Moroccan, Algerian, and Tunisian coasts. While these environmental factors contribute to bloom variability, other ecological and anthropogenic factors likely play a significant role. Further research is necessary to better understand the synergistic effects of climate change, nutrient loading, and biological interactions on jellyfish bloom dynamics in the Mediterranean, with implications for effective management strategies.

1. Introduction

Pelagia noctiluca

Outbreaks have become an intriguing concern due to their frequent and intense in the Mediterranean, especially along the coasts of Morocco, Algeria, and Tunisia. These jellyfish blooms can cause significant disruptions to marine ecosystems and local economies, affecting biodiversity, fisheries, tourism, and coastal communities [1,2,3,4,5,6]. Pelagia noctiluca blooms frequency and intensity highlight the importance of a deep understanding of the oceanic parameters that drive such events, which are crucial for effective mitigation strategies [7]. The Mediterranean’s semi-enclosed nature and unique oceanographic and climatic characteristics make it particularly vulnerable to jellyfish blooms. Research has proven that some sea parameters like sea surface temperature (SST), sea surface salinity (SSS), nutrient levels, and ocean currents cause jellyfish outbreaks [8,9,10,11,12]. Additionally, human behavioral and economic activities such as coastal development, pollution, and overfishing have altered the natural conditions, potentially exacerbating the frequency and intensity of jellyfish blooms [13,14,15,16,17,18]. Previous studies have linked P. noctiluca blooms to a high sea surface temperature and changes in sea salinity, which results in favorable ecosystem for jellyfish proliferation [19,20,21]. However, the mechanisms resulting in the appearance of these blooms remain unclear, especially over extended periods that could reveal trends and fluctuations in jellyfish populations.

Our research focuses on the south Mediterranean coasts, including Morocco, Algeria, and Tunisia, a study area with distinct oceanographic parameters and specific seasonal climatic patterns that make it an ideal case for studying P. noctiluca blooms. Despite the region’s localization, there has been limited research on the drivers of jellyfish blooms in these countries, highlighting a gap in our understanding. The study aims to explore the ecological factors behind P. noctiluca blooms along the Moroccan, Algerian, and Tunisian coastlines from 2011 to 2023. The study investigates eighteen key environmental drivers, including SST, SSS, nutrient concentrations, dissolved oxygen, turbidity, wave height, Current speed-direction, and atmospheric variables such as precipitation and wind speed. Data will be sourced from satellite remote sensing and in situ measurements [22,23,24]. A significant part of the analysis will focus on how these factors interact to influence the timing and intensity of the blooms, with particular attention to potential lag effects where environmental changes like warming may take time to impact the blooms [1]. We will consider analyzing climate variability and examine how long-term climate trends may shape jellyfish populations [25,26]. For jellyfish bloom size, we collected data from the Global Biodiversity Information Facility Denmark (GBIF) and Jellywatch EU, and we will integrate previous studies to provide a comprehensive view of jellyfish activity and variability along the coast. Our statistical analysis, such as correlation and regression analyses, will be applied to identify bloom events’ most significant environmental drivers [11,27,28,29,30]. This study aims to clarify the causes that lead to these blooms and offer practical insights for coastal management in the Mediterranean. By identifying the key parameters causing these outbreaks will contribute to developing early warning systems and predictive models, particularly in our research areas, where tourism and fisheries are highly vulnerable to the impacts of jellyfish blooms.

2. Materials and Methods

2.1. Study Area

The Mediterranean coastline of Morocco, Algeria, and Tunisia (Figure 1, latitudinal range: 28° N–37° N; longitudinal range: 10° W–12° E) is a crucial area for studying the expansion of the jellyfish species Pelagia noctiluca; this species causes frequent bloom events in the region [1,5]. This zone encompasses various oceanographic environments, from the Alboran Sea to the Algerian Basin and the Gulf of Gabès. Each area has distinct hydrographic conditions influenced by mixing Atlantic and Mediterranean waters, seasonal upwelling, and mesoscale eddies [6,9,31]. The gradients in sea surface temperature (SST), which range from 16 °C to 28 °C on a seasonal scale, along with other abiotic factors like wind-driven currents and localized nutrient enrichment, particularly near river mouths like the Moulouya and Medjerda rivers, create a highly variable environment for P. noctiluca. These conditions help define the triggers for jellyfish blooms [32,33,34]. Along Morocco and Algeria, the Atlantic inflow through the Strait of Gibraltar impacts the western part of this region and controls the variability in surface salinity and mixing. Meanwhile, the eastern Tunisian coast experiences strong summer stratification and oligotrophic conditions, with occasional nutrient enrichment [16,35]. These environmental gradients, along with human pressures such as overfishing of jellyfish predators and coastal urbanization, provide an ideal setting to distinguish between natural and human-driven factors affecting jellyfish populations [21,36]. The Gulf of Gabès, with its shallow and often productive waters (less than 50 m deep), offers a “natural laboratory” for studying these dynamics. This area is susceptible to climate-related SST anomalies, making it a hotspot for P. noctiluca aggregations [12,37,38]; this region’s combination of temperature- and nutrient-variable coastal upwelling systems creates an excellent model for studying the complex relationships between hydroclimate variability, trophic cascades, and jellyfish outbreaks which is an issue in Mediterranean marine ecology [28,39,40,41].

2.2. Data Collection

From 2011 to 2023, a thorough data-collecting effort has been used to enable all-encompassing research of Pelagia noctiluca outbreaks in the study area. Carefully chosen from eminent scientific databases and minimal literature, that give the study a strong and reliable basis. Seeking to explain the dynamics behind these phenomena, this work compiles several high-resolution datasets to offer a whole awareness of the complex environmental and biological elements affecting jellyfish blooms. We daily collected Pelagia noctiluca outbreak data from the Global Biodiversity Information Facility Denmark (GBIF), Jellywatch Spain, and published papers, which recorded sightings of jellyfish and bloom events across the Moroccan, Algerian, and Tunisian coastlines [6,9,10,11,12,14,20,21,37,42].

Sea surface temperature and salinity provided by the Copernicus Marine Environment Monitoring Service France and the National Oceanic and Atmospheric Administration USA produced a high-quality dataset allowing statistical analysis of the physical elements influencing jellyfish distribution. Moreover, we gathered daily data from 2011 to 2023 utilizing NOAA’s AMSR2 US aboard the GCOM-W1 US satellite for oceanic observations. Furthermore, we employed CMEMS’ Mercator Ocean Biogeochemical Global Ocean Analysis and Forecast System France, which operates at a 1/4-degree resolution and delivers 10-day ocean predictions updated weekly. This dataset comprises biogeochemical metrics such as chlorophyll, nitrate, phosphate, and pCO2, encompassing 50 vertical levels from 0 to 5700 m [43]. CMEMS France and NOAA US looked further at columns of dissolved oxygen, a vital factor determining the condition of marine ecosystems and their resistance to P. noctiluca blooms. Obtained data on water clarity, wave height, and ocean currents provides important new perspectives on the physical environment that can affect jellyfish activity and distribution [44]. CMEMS France and NOAA US also provide atmospheric data, including precipitation and wind speed; these are augmented by data from past published research [38,42,45]. By means of a multidisciplinary approach, the integration of several datasets presents a unique chance to improve the whole knowledge of jellyfish bloom dynamics in this sensitive marine ecosystem and acquire insights into the environmental factors influencing P. noctiluca outbreaks.

2.3. Data Processing and Analysis

Consistent, exact, and repeatable methods across the numerous datasets were guaranteed using Python 3.12.3. tools included in the Anaconda environment [4]. The next phases were conducted in order to do a thorough investigation of the biological and environmental elements affecting Pelagia noctiluca blooms:

➢

Data Standardization:

Environmental and bloom data were normalized and interpolated to a common grid reflecting space across the complete study area. All variables were calculated as monthly averages to temporally coincide with the jellyfish bloom data. Standardization was performed with the following equation:

Standardized Value = (X_i − μ)/σ

where

• X_i is the value of the variable at each data point,
• μ is the mean of the variable,
• σ is the standard deviation of the variable.

The standardization procedure process ensured the dataset was homogeneous, which was a requirement for analyses that tested associations between environmental conditions and bloom events later.

➢

Correlation Analysis:

Using Pearson correlation coefficients to investigate the relationships between environmental factors and Pelagia noctiluca bloom incidence [24]; r is the Pearson correlation coefficient:

$$\mathrm{r}=\mathsf{\Sigma } \left[\right({\mathrm{X}}_{\mathrm{i}}-\overline{\mathrm{X}}) ({\mathrm{Y}}_{\mathrm{i}}-\overline{\mathrm{Y}}\left)\right]/\surd [\mathsf{\Sigma } {({\mathrm{X}}_{\mathrm{i}}-\overline{\mathrm{X}})}^2 \mathsf{\Sigma } {({\mathrm{Y}}_{\mathrm{i}}-\overline{\mathrm{Y}})}^2]$$

where

• X_i and Y_i are the values of the two variables being compared,
• $\overline{\mathrm{X}}$ and $\overline{\mathrm{Y}}$ are the means of the variables X and Y, respectively,
• n is the number of data points.

This approach helped to measure the degree and direction of interactions between bloom episodes and environmental factors [46,47]. Heatmaps then helped to clearly show these relationships, therefore clarifying the main environmental causes of jellyfish outbreaks. The heatmaps also visually represented the variables most strongly associated with P. noctiluca bloom occurrences.

➢

Temporal Pattern Analysis:

Temporal pattern analysis was undertaken to analyze the temporal distribution of Pelagia noctiluca blooms during the study period [48,49]. The monthly bloom size B_(t) was articulated as a function of significant environmental factors E_j(t) within the framework of a linear regression model.

B_(t) = β₀ + Σ _{βj Ej (t)} + ε_t

where

• B_(t) is the bloom size at time t,
• β₀ is the intercept,
• β_j are the coefficients for each environmental variable E_j(t),
• ε_t is the error term.

This modeling technique allows the identification of the main environmental factors of bloom dynamics across several temporal scales [15,50]. It also elucidates the temporal dynamics of P. noctiluca bloom episodes during the 10-year study period.

3. Results and Discussion

3.1. Pelagia noctiluca Outbreaks in the Southern Mediterranean Coastline

• Temporal patterns of Pelagia noctiluca blooms in the Moroccan Mediterranean coastline

The data clearly shows seasonal and annual fluctuations in Pelagia noctiluca bloom magnitudes along the Moroccan Mediterranean coast from 2011 to 2023 (Figure 2), with notable maxima in particular years and months.

Chronological Patterns: Summer (June to August): Shows a persistent tendency for larger bloom sizes, particularly in August. Often reaching levels starting from 10 to 775. August bloom sizes indicate maximum bloom intensity. For instance, bloom sizes of 775 and 357.1 were documented in 2018 and 2015, indicating substantial blooms in those years. Autumn (September to November): The sizes of blooms begin to diminish after the summer zenith. Nonetheless, September continues to see substantial blooms, reaching up to 100 in certain years. During October and November, bloom sizes diminish, with numerous measurements noted at 10 to 61, signifying reduced or nearly absent blooms in these months. Winter (December to February): Bloom sizes are consistently low, generally approximately 10, signifying negligible blooming activity during these cold months. We must highlight that we recorded only three isolated cases of high bloom size during the winter season, which were in 2015, 2018, and 2019.

Interannual Fluctuation: 2018 is significant for experiencing some of the greatest bloom sizes, especially in August, when bloom sizes reached 775. This year is anomalous in terms of bloom magnitude relative to previous years in the dataset. In 2015, comparable extensive blooms (with values of 357.1) were recorded in August, underscoring the possibility of occasional, significant bloom occurrences in specific years. In August 2017, there was a significant prevalence of blooms, with values reaching 100, constant over several months during this specific year, indicating that this timeframe may be especially conducive to the proliferation of Pelagia noctiluca.

Yearly Consistency: Specific years such as 2011, 2012, and 2017 demonstrate comparatively large bloom volumes during the summer months, sustaining moderate intensity. On the other hand, the years 2020, 2021, and 2022 have significantly lower bloom counts; most months show values of 10, indicating fewer bloom events over that period.

The overarching tendency suggests that Pelagia noctiluca blooms are more frequent and stronger in the summer months, although their magnitude and occurrence can fluctuate significantly from year to year. The years 2015 and 2018 are noted for their extraordinary bloom intensity, while the years 2020 and 2021 have considerably lower bloom sizes.

Potential Influences: The substantial blooms noted in particular years (e.g., 2015, 2018) may be affected by distinct environmental factors, like water temperature, nutritional availability, or oceanographic conditions that promote jellyfish proliferation. In contrast, the lower bloom sizes in years such as 2020 may suggest environmental variables that were less conducive for jellyfish proliferation; these could be due to inadequate prey abundance or water temperatures [8,15]. The high bloom sizes in summer months are consistent with findings from previous studies, which suggest that jellyfish populations, including Pelagia noctiluca, are more likely to bloom in response to increased food availability and warmer temperatures during these months (36, 54). On the other hand, the consistently low bloom sizes have pointed out environmental or ecological stressors during these years. Such reduced bloom sizes could be linked to shifts in oceanographic conditions, such as changes in water circulation, as well as anthropogenic factors like overfishing or pollution, which may disrupt the availability of food sources for jellyfish [34]. The seasonal patterns observed, with peak bloom sizes in the warmer months, reflect broader ecological trends in jellyfish biology. Jellyfish, including Pelagia noctiluca, typically thrive in the summer due to the increase in plankton availability and warmer water temperatures, which provide optimal growth conditions for these species [51]. In contrast, the decrease in Pelagia noctiluca bloom sizes observed during colder months of the years, show less favorable conditions for jellyfish proliferation, which aligns with established patterns of seasonal jellyfish abundance [49,52].

The data indicates that Pelagia noctiluca has a significant seasonal pattern at the Moroccan Mediterranean coastline, with the most substantial outbreaks transpiring during the summer months, particularly in August. Certain years have significant blooms, such as 2015 and 2018, but others show minimal bloom activity. The variations show the complex and varied traits of jellyfish blooms molded by seasonal and environmental factors. More investigation could clarify the exact factors triggering these notable bloom episodes and the basic processes responsible for the observed variation [2,17,21].

• Temporal patterns of Pelagia noctiluca blooms in the Algerian Mediterranean coastline

The data reveals pronounced interannual and seasonal variability in Pelagia noctiluca bloom magnitudes, with striking peaks in specific years and months along the Algerian coast (Figure 3), with notable maxima in particular years and months.

Chronological Patterns: Summer (June to August): Shows a persistent tendency for bloom sizes reaching 100, particularly in August. However, June, July, and August bloom sizes did indicate frequent bloom intensity as these bloom sizes of 10 to 100 were documented in separate years, like the one recorded in 2013, 2020, 2022, and 2023, indicating substantial blooms in those years. Autumn (September to November): The sizes of blooms begin to diminish after the summer zenith. Nonetheless, September continues to see very few blooms, reaching up to 10 in two years only. During October and November, bloom sizes diminished, signifying reductions and absences during October. Winter (December to February): Bloom sizes are consistently low, generally approximately 10, signifying negligible blooming activity during these cold months. We must highlight that during the year 2016, the jellyfish blooms were observed in almost all the months, including February until June, September, and December, but with a very low intensity of 10.

Interannual Fluctuation: 2014 is significant for experiencing some of the greatest bloom sizes, recorded in March, with bloom sizes reaching 670. This year is anomalous in terms of bloom magnitude relative to previous years in the dataset. In 2013, comparable extensive blooms (with values going from 10, 55 to 100) were recorded between April, July, and August, underscoring the possibility of occasional, significant bloom occurrences in this specific year. In July 2020, June 2022, and August 2013, 2017, there was a significant prevalence of blooms, with values reaching 100, but not constant over several months during these specific years, indicating that this timeframe may be especially conducive to the proliferation of Pelagia noctiluca under specific factors.

Yearly Consistency: The greatest bloom size was recorded by 2014 during March; the other specific years, such as 2013, 2020, 2022, and 2023, demonstrate comparatively large bloom volumes during the summer months, sustaining moderate intensity. On the other hand, the years 2012, 2015, 2016, and 2017 have significantly lower bloom counts; most months show values of 10, indicating fewer bloom events over that period.

Potential Influences: The overarching tendency suggests that Pelagia noctiluca blooms are more frequent and stronger in the Spring-Summer months, although their magnitude and occurrence can fluctuate significantly from year to year. The years 2014, 1013, 2020, 2022, and 2023 are noted for their huge bloom intensity, while the years 2012, 2015, 2016, and 2017 have considerably lower bloom sizes. Overall, the results from Figure 3 reveal that Pelagia noctiluca blooms tend to peak during the warmer months, particularly in years like 2014, 2013, 2020, 2022, and 2023. However, the decline in bloom activity suggests that factors beyond just temperature and food availability, such as environmental and ecological shifts, could influence the frequency and intensity of jellyfish blooms along the Algerian coast [53,54].

• Temporal patterns of Pelagia noctiluca blooms in the Tunisian Mediterranean coastline

The data reveals distinct seasonal trends in Pelagia noctiluca bloom sizes along the Tunisian coast (Figure 4), with the largest blooms typically occurring in the warmer months, from late spring to early autumn.

Chronological Patterns: Over the years, there is a clear temporal pattern whereby major blooms mostly occur in the warmer months, especially from late spring to early autumn. Regularly displaying significant bloom sizes, the months of May, June, July, and September often reach the maximum value of 1000, therefore signifying peak bloom times. For instance, May and June saw some really large blossoms in 2013, and June and July in 2015. These months line up with the monthly cycle in which increased temperatures and improved nutrient availability help jellyfish blooms to occur. On the other hand, the milder months of January, February, and March usually show far fewer blooms, usually reaching values around 100, suggesting a drop-in jellyfish activity during these times.

Interannual Fluctuation: Bloom sizes exhibit substantial interannual variability, with notable month-to-month discrepancies in certain years. In 2014, bloom sizes fluctuated significantly throughout the year across several months (e.g., January to April, June to August, and October to December), with certain months experiencing substantial blooms (775) and others exhibiting low sizes (100). This variation indicates that elements affecting bloom size, including oceanic conditions, food availability, and environmental stresses, may differ annually. While 2013, 2015, and 2016 show great blooms all year long, other years, such as 2017, 2018, 2019, and 2022, show more rare and small blooms, reflecting changing environmental conditions during these years.

Yearly Consistency: Notwithstanding the variations, specific months reliably demonstrate either substantial or minimal blooming years. Recurring instances of extensive blooms (200−1000) throughout several years, including 2013 (May, June, September, and December), 2014 (January, March, June, July, November, and December), 2015 (February-April and June-December), and 2016 (June, July, and September). This consistency suggests that these months could offer a more fit habitable ecosystem for Pelagia noctiluca to bloom. Conversely, January and February often show reduced bloom sizes, which highlights how less suited these cold months are for the evolution of the species.

Potential Influences: Many environmental elements could help to explain the noted variations in bloom size. Temperature is a major factor since jellyfish blooms are usually connected to warmer seas, usually found in late spring to summer [22,32]. Bloom development may also be much influenced by nutrient availability, which can be affected by seasonal upwelling or variations in water currents. The variation between years could be attributed to changes in oceanic conditions, such as shifts in water temperature, salinity, or the availability of plankton, which serves as food for jellyfish [12]. Furthermore, influencing the timing and intensity of blooms would be geographical localization (next to Italy and Greece, where Jellyfish blooms occur several times) and climate change, therefore changing the seasonal and interannual patterns seen in this dataset. In general, whereas Pelagia noctiluca blooms show obvious seasonality—that is, maxima in warmer months—interannual oscillations point to the major influence of temperature and nutrient availability on bloom sizes. Despite these fluctuations, certain months show consistency in bloom patterns, pointing to potential seasonal and environmental cues that guide jellyfish proliferation along the Tunisian coast [13,37].

3.2. Correlation Between Environmental Variables and Bloom Intensity

• Correlation between environmental variables and bloom intensity along the Moroccan Mediterranean coast

The heatmap analysis shows the correlation between bloom size and many environmental conditions over the Moroccan coast (Figure 5), therefore clarifying the link between Pelagia noctiluca bloom size and important environmental factors. The correlation between bloom size and sea surface temperature is positive (r = 0.13), suggesting that as the sea surface temperature increases, bloom size tends to increase, although the relationship is weak. Other notable Positive correlations include bloom size, which shows positive, though weak, correlations with several variables such as Nitrate and Currents Direction (r = 0.12); Carbon (r = 0.08); Current speed (r = 0.06); sea surface height (r = 0.05); phosphate, and wind direction (r = 0.04); turbidity, Chlorophyll, and sea surface salinity (r = 0.01). This suggests that while the relationships are weak, these environmental factors may still play a minor role in influencing Pelagia noctiluca bloom size on the Moroccan Mediterranean coastline. The favorable link between nitrate and the current direction suggests that patterns of nutrient availability and oceanic circulation could help to create blooms, maybe by improving food supply and distribution circumstances. Similarly, the correlation with carbon suggests that higher organic matter content in the water could support bloom formation, possibly by fueling microbial activity and the lower trophic levels in the ecosystem. The weak positive correlations with current speed and sea surface height imply that water movement and vertical displacement of water masses may have slight effects on bloom distribution, possibly by redistributing nutrients or aggregating jellyfish populations. The minimal correlations with phosphate and wind direction suggest that these factors exert little influence on bloom size in this dataset, though local wind-driven upwelling or mixing events could play a role in nutrient availability over different spatial and temporal scales. The near-zero correlations observed with turbidity, chlorophyll, and sea surface salinity indicate that these factors have negligible effects on bloom formation in this region, at least based on the current dataset. This is somewhat surprising, particularly for chlorophyll, which is often linked to primary productivity and food availability for jellyfish. This could imply, therefore, that other elements—such as predation, competition, or life cycle dynamics—may be more important in influencing bloom intensity than these physical and biological ones taken by themselves. Though these interactions are minor overall, they draw attention to possible environmental factors that, in concert with other ecological and oceanographic processes, can help to generate variability in bloom development.

On the other hand, other notable negative correlations, including Oxygen, Solar radiation, Precipitation (r = −0.04); pH level, Wave height (r = −0.05); Silicate (r = −0.03); Wind speed (r = −0.02); indicating that these environmental factors may have a slight inhibitory effect on Pelagia noctiluca bloom size, although the relationships are weak. The negative correlation with oxygen, solar radiation, and precipitation suggests that higher levels of these variables might contribute to conditions that are less favorable for bloom development. Lower oxygen levels could be linked to eutrophic conditions that sometimes favor jellyfish proliferation, while reduced solar radiation and precipitation could affect nutrient dynamics, stratification, or primary production in ways that indirectly influence bloom intensity. Similarly, the weak negative correlation with pH levels and wave height suggests that slight increases in acidity and rougher sea conditions might marginally reduce bloom size. Lower pH levels can affect marine organisms’ physiology, including gelatinous plankton, while increased wave height might contribute to higher dispersal and reduced bloom aggregation, preventing the formation of large jellyfish swarms [35,49]. The correlation with silicate, although weak, may indicate that silica-driven phytoplankton communities (such as diatoms) do not necessarily contribute to conditions that favor jellyfish blooms. Furthermore, the extremely slight negative correlation with wind speed implies that stronger winds could have a small dispersive effect, thereby maybe breaking apart bloom formations and lowering localized aggregation. Though these negative connections are modest, they suggest possible environmental variables that might somewhat inhibit bloom development. However, given the low correlation values, other biological or ecological factors likely have a stronger influence on bloom size dynamics than these physical and chemical parameters alone [55,56].

• Correlation between environmental variables and bloom intensity along the Algerian Mediterranean coast

The association between bloom size and different environmental conditions throughout the Algerian coast is shown by the heatmap analysis (Figure 6), therefore clarifying the relationship between Pelagia noctiluca bloom size and important environmental parameters. The correlation between bloom size and Current direction (r = 0.27), Carbon (r = 0.20), and Phosphate (r = 0.13) suggests that as these three factors increase, the bloom size tends to increase, although the relationship is weak. Other notable Positive correlations include bloom size, which shows positive, though weak, correlations with several variables such as Oxygen (r = 0.09); Wind speed (r = 0.08); sea surface height, and Solar radiation (r = 0.05); Sea surface temperature and Sea surface salinity (r = 0.03). This suggests that while these environmental factors exhibit weak to moderate positive correlations with Pelagia noctiluca bloom size, they may still play a role in influencing bloom formation and intensity. The positive correlation with the current direction is the strongest among these variables, indicating that water movement patterns may significantly influence bloom distribution and size. Alterations in the current direction can relocate jellyfish populations to advantageous regions with plentiful food resources or ideal environmental conditions, hence facilitating bloom proliferation. The association with carbon indicates that elevated organic carbon levels in the water may be connected to larger bloom sizes. This may suggest that elevated concentrations of dissolved organic matter and productivity in the water column create more favorable conditions for jellyfish survival and growth. Organic carbon is essential in marine food webs, sustaining microbial and planktonic communities that eventually influence jellyfish feeding behavior. The correlation with phosphate, although weaker, suggests that phosphate availability may have some influence on bloom intensity. Phosphate is an essential nutrient for primary productivity, and its presence in the water may support phytoplankton growth, which serves as the base of the food web. Increased phytoplankton biomass resulting from elevated phosphate concentrations would indirectly benefit jellyfish populations by better food availability. Other important positive correlations show oxygen and wind speed, implying that higher oxygen levels and improved water circulation somewhat increase bloom development. Higher oxygen levels support a more productive ecosystem, indirectly benefiting jellyfish populations, while increased wind speed enhances nutrient mixing and facilitates bloom expansion. Weak positive correlations were also observed for sea surface height, solar radiation, sea surface temperature, and sea surface salinity. These connections suggest that minor changes in oceanic and atmospheric factors affect bloom development, hence affecting possibly water column stability and nutrient distribution. Still, given the low correlation values, their influence seems to be small. These facts imply that more than other environmental factors, oceanographic characteristics like current direction and biogeochemical conditions such carbon and phosphate availability may greatly affect bloom intensity. Nevertheless, even although these interactions suggest probable consequences, more study is needed to clarify the basic processes controlling Pelagia noctiluca bloom dynamics since additional biological and physical factors are probably involved in bloom formation in this field. On the other hand, other notable negative correlations, including pH level (r = −0.29); Current speed (r = −0.23); Precipitation (r = −0.18); Wave Height (r = −0.12); Chlorophyll (r = −0.11); Wind speed (r = −0.08); Nitrate and Turbidity (r = −0.06); Wind Direction and Silicate (r = −0.04) indicating that these environmental factors may have a slight inhibitory effect on Pelagia noctiluca bloom size, although the relationships are weak. The negative correlation with pH level suggests that more acidic conditions may be associated with reduced bloom intensity. This may be associated with physiological stress in jellyfish or have an indirect impact on planktonic food supplies resulting from alterations in carbonate chemistry. Current speed exhibits a modest negative connection, suggesting that intensified currents may hinder bloom formation by dispersing jellyfish populations, thereby obstructing dense aggregations in certain locales. The inverse association with precipitation suggests that more rainfall can cause changes in salinity and nutrient dilution, thereby maybe creating less favorable conditions for bloom growth. Moreover, wave height exhibits a weak negative link, suggesting that increased wave energy may disrupt jellyfish aggregations, hence diminishing bloom size by the dispersal of individuals across a broader area. The negative correlation with chlorophyll is somewhat unexpected, as chlorophyll is a proxy for primary productivity, which supports jellyfish populations. This weak inverse relationship may suggest that bloom formation is not directly driven by phytoplankton abundance alone or that other factors, such as competition or predation, play a stronger role in bloom regulation. Likewise, wind speed demonstrates a weak negative link, indicating that increased wind intensity may facilitate water mixing, dispersing bloom-forming organisms and constraining bloom magnitude. Reduced associations with nitrate and turbidity, together with wind direction and silicate, suggest that these variables probably exert negligible inhibitory effects on bloom production [41,51]. Although a necessary mineral, nitrate shows a minor negative correlation, implying that other minerals like carbon and phosphate might have a major influence on bloom dynamics. Similarly, lower turbidity might not directly improve bloom development, suggesting that water clarity has not much effect on jellyfish. Overall, while these negative correlations indicate that certain environmental factors may slightly suppress Pelagia noctiluca bloom formation, the relationships remain weak. This suggests that although physical and chemical oceanographic conditions can influence bloom dynamics, other ecological factors, including life cycle stages, predation, and competition, may have a more major impact on bloom intensity [57,58,59,60,61]. Further research integrating biological interactions and hydrodynamic modeling would help clarify the extent to which these environmental variables influence jellyfish bloom patterns on the Algerian coastline [62,63,64,65,66,67,68,69].

• Correlation between environmental variables and bloom intensity along the Tunisian Mediterranean coast

The heatmap analysis shows the association between bloom size and different environmental conditions throughout the Tunisian coast (Figure 7), therefore clarifying the relationship between Pelagia noctiluca bloom size and important environmental parameters. The correlation between bloom size and Oxygen (r = 0.15), pH level (r = 0.10), and sea surface temperature (r = 0.08) suggests that as these three factors increase, the bloom size tends to increase, although the relationship is weak. Other notable positive correlations include bloom size, which shows positive, though weak, correlations with several variables such as Wive height (r = 0.06); Silicate and Precipitation (r = 0.05); Turbidity (r = 0.03); Current Direction (r = 0.01). This suggests that while these environmental factors exhibit weak to moderate positive correlations with Pelagia noctiluca bloom size, they may still play a role in influencing bloom formation and intensity. This suggests that while these environmental factors exhibit weak to moderate positive correlations with Pelagia noctiluca bloom size, they may still play a role in influencing bloom formation and intensity. The correlation with oxygen suggests that higher oxygen levels contribute to bloom growth by supporting a more productive ecosystem where jellyfish can thrive. The positive link with pH implies that somewhat more alkaline circumstances could be related to bigger bloom sizes, hence maybe changing plankton dynamics and food availability for jellyfish. The weak positive correlation with sea surface temperature suggests that warmer waters create favorable conditions for bloom formation, possibly by accelerating jellyfish metabolism, reproduction, or food supply. Furthermore, wave height shows a little positive correlation that would suggest that moderate wave activity might help mix nutrients and plankton in the water column, therefore fostering bloom development. The weak bloom correlation with silicate and precipitation suggests that higher silicate levels may play a minor role in bloom formation, likely through its influence on phytoplankton communities, while precipitation might contribute by altering salinity or nutrient dynamics in coastal waters. Likewise, turbidity—which denotes water clarity—shows a very small positive link, implying that somewhat more turbid conditions may not greatly prevent bloom development, presumably because of jellyfish’s adaptation to different water transparencies. At last, the almost zero association with the current direction suggests that in this dataset, variability in water circulation patterns would not have a major influence on bloom size. Ocean currents can nevertheless, however, affect the accumulation and distribution of blooms over more extensive geographical areas. Though these interactions are minor overall, they draw attention to possible environmental factors influencing Pelagia noctiluca bloom dynamics. Bloom’s development and intensity are probably determined more by the interaction of physical and chemical elements as well as biological and ecological ones. Further studies incorporating long-term monitoring and hydrodynamic modeling could clarify these interactions and improve bloom prediction in the region. On the other hand, other notable negative correlations, including Carbon (r = −0.13); Wind speed (r = −0.06); Phosphate, sea surface height and wind direction (r = −0.05); Solar radiation (r = −0.04); Chlorophyll (r = −0.03); Nitrate (r = −0.02); Current speed (r = −0.01) indicating that these environmental factors may have a slight inhibitory effect on Pelagia noctiluca bloom size, although the relationships are weak. The negative link with carbon implies that larger degrees of dissolved organic carbon may not support bloom development, maybe due to changes in microbial activity or changed food chain interactions that do not directly benefit jellyfish populations. Similarly, wind speed exhibits a weak negative connection, therefore higher winds could aid to increase water mixing and dispersion, thereby limiting bloom aggregation in certain locations. The negative correlations between phosphate, sea surface height, and wind direction indicate a modest reducing effect on bloom size from these factors. Lower phosphate availability may slightly reduce phytoplankton productivity, indirectly affecting food availability for jellyfish, while changes in sea surface height and wind direction may influence water column stability and transport dynamics, impacting bloom distribution. Furthermore, displaying a weak negative correlation is solar radiation, implying that more sunshine does not really encourage bloom development. Indirect impacts like more water column stratification could be responsible for this; this would limit nutrient mixing and lower favorable circumstances for jellyfish development. The negative correlation with chlorophyll is somewhat unexpected, as chlorophyll is an indicator of phytoplankton abundance. This may suggest that bloom formation is not directly dependent on phytoplankton biomass alone and that other ecological factors, such as predation, competition, or life cycle stages, play a more significant role. The very weak negative correlations with nitrate and current speed suggest that these factors have minimal impact on bloom size in this dataset. While nitrate is an essential nutrient for phytoplankton growth, its limited influence on bloom formation implies that other nutrients, such as phosphate and silicate, maybe more important drivers of jellyfish population dynamics. Similarly, current speed appears to have an almost negligible effect, indicating that water movement intensity alone is unlikely to significantly regulate bloom size. Overall, these findings indicate that while certain environmental factors may have slight inhibitory effects on bloom formation, their individual influence is weak, suggesting that a combination of multiple ecological, biological, and oceanographic factors likely governs Pelagia noctiluca bloom dynamics in this region. Future research integrating long-term datasets, species interactions, and hydrodynamic modeling may provide a more comprehensive understanding of the mechanisms controlling jellyfish bloom intensity [9].

3.3. Temporal Trends in Bloom Intensity Relative to Environmental Variables

• Temporal trends in bloom intensity relative to environmental variables along the Moroccan Mediterranean coast

The data shown in Figure 8 illustrates the relationship between Pelagia noctiluca bloom intensity and key environmental variables, precisely sea surface temperature, nitrate, and phosphate levels along the Moroccan Mediterranean coastline.

The first plot (left) shows the trend between bloom size and sea surface temperature, where bloom size appears to increase slightly with rising sea surface temperature. The data points range from approximately 16 °C to 30 °C, with most bloom sizes clustering below 200 individuals per unit area, although a few outliers exceed 800 individuals. The fitted regression line suggests a weak but positive trend, indicating that higher sea surface temperature may be associated with increased bloom intensity. This is consistent with the ecological realization that higher water temperatures could increase jellyfish survival and reproduction, hence generating possibly more significant blooms.

The second middle plot examines the relationship between bloom size and nitrate concentrations, therefore indicating the availability of nutrients. Nitrate concentrations range from approximately 0.001 to 0.05 mmol/L, with most bloom sizes staying below 200 individuals. The regression line suggests a very weak increasing trend, meaning that higher nitrate levels might contribute to increased bloom size, but the effect appears minimal.

The third plot (right) assesses the trend between bloom size and phosphate levels, ranging from approximately 0.002 to 0.01 mmol/L. Similarly to the nitrate trend, bloom size remains mostly below 200 individuals, with a few high-density outliers exceeding 900 individuals. The regression line suggests a weak positive trend, indicating that phosphate levels may have a minimal influence on bloom intensity.

Overall, these trends indicate that bloom intensity does not exhibit a strong relationship with these environmental factors alone in this special research area of the Moroccan Mediterranean coastline [51]. At the same time, sea surface temperature shows a slightly stronger positive trend, and nutrient levels (nitrate and phosphate) exhibit only marginal effects. This suggests that while warmer waters may create more favorable conditions for Pelagia noctiluca blooms, other ecological and oceanographic factors—such as food availability, ocean currents, and interspecies interactions—may play a more significant role in determining bloom intensity. Consequently, future research must include supplementary variables, including zooplankton abundance and hydrodynamic circumstances, to enhance the comprehension of these bloom processes [53,70,71].

• Temporal trends in bloom intensity relative to environmental variables along the Algerian Mediterranean coast

The first plot in Figure 9 illustrates the relationship between Pelagia noctiluca bloom intensity and key environmental variables, precisely sea surface temperature, nitrate, and phosphate levels along the Algerian Mediterranean coastline.

The first plot (left) shows the trend between bloom size and sea surface temperature, where bloom size appears to increase slightly with rising sea surface temperature. The data points range from approximately 15 °C to 30 °C, with most bloom sizes clustering below 200 individuals per unit area, although a few outliers exceed 900 individuals. The fitted regression line suggests a weak but positive trend, indicating that higher sea surface temperature may be associated with increased bloom intensity. This is consistent with the ecological realization that higher water temperatures could increase jellyfish survival and reproduction, hence generating possibly more significant blooms.

The second middle plot examines the relationship between bloom size and nitrate concentrations, therefore indicating the availability of nutrients.

Nitrate concentrations range from approximately 0.001 to 0.05 mmol/L, with most bloom sizes staying below 200 individuals. The regression line suggests a very weak decreasing trend, meaning that lower nitrate levels might contribute to increased bloom size, but the effect appears minimal.

The third plot (right) assesses the trend between bloom size and phosphate levels, ranging from approximately 0.001 to 0.01 mmol/L. Similarly to the sea surface temperature and nitrate trend, the bloom size remains mostly below 200 jellyfish per observation, with a few high-density outliers exceeding 1000 jellyfish per observation. The regression line suggests a weak positive trend, indicating that phosphate levels may have a minimal influence on bloom intensity.

Overall, these trends indicate that bloom intensity does not exhibit a strong relationship with these environmental factors alone in this special research area of the Algerian coastline. At the same time, phosphate shows a slightly stronger positive trend, and sea surface temperature with the nitrate exhibits marginal effects [22]. This suggests that phosphorus may have a significantly stronger influence on bloom formation, perhaps supporting phytoplankton development and indirectly benefiting Pelagia noctiluca [51]. Meanwhile, the marginal influences of sea surface temperature and nitrate imply that these factors have minimal direct impact on bloom intensity, suggesting that other environmental or biological causes may play a more major role. Overall, while nutrient availability contributes to bloom dynamics, bloom formation is likely governed by a combination of numerous interacting factors rather than a single environmental variable [72,73,74,75].

• Temporal trends in bloom intensity relative to environmental variables along the Tunisian Mediterranean coast

The analysis of trends in Figure 10 shows the intensity of Pelagia noctiluca blooms related to environmental factors along the Tunisian coast reveals enjoyable temperature and nutrient availability patterns. The first plot (left) shows the trend between bloom size and sea surface temperature, where bloom size appears to increase slightly with rising sea surface temperature. The data points range from approximately 15 °C to 30 °C, with most bloom sizes clustering below 300 jellyfish per observation, although a few outliers exceed 900 jellyfish per observation. The estimated regression line shows a faint but positive trend, meaning that a little rise in bloom size could be connected with greater sea surface temperature. This is consistent with the knowledge that more favorable conditions for jellyfish development and reproduction may arise from warmer seas, which could result, in certain cases, more major blooms. The second middle plot examines the relationship between bloom size and nitrate concentrations, therefore indicating the availability of nutrients. Nitrate concentrations range from approximately 0.001 to 0.05 mmol/L, with most bloom sizes staying above 400 jellyfish per observation. The regression line suggests a nearly flat trend, meaning that nitrate levels have minimal influence on bloom size. This suggests that nitrate availability is not a primary driver of Pelagia noctiluca bloom formation in this dataset. The third plot (right) assesses the trend between bloom size and phosphate levels, where bloom sizes also remain fairly stable, ranging below 200 jellyfish per observation across phosphate concentrations from approximately 0.001 to 0.01 mmol/L, with a few high-density outliers reaching 1000 jellyfish per observation. The fitted regression line suggests a slight negative trend, indicating that higher phosphate levels may be associated with a slight decrease in bloom size. This could potentially be linked to conditions that are less conducive to bloom formation, possibly due to shifts in planktonic community composition or competition with other species at the Tunisian coast.

Overall, these trends indicate that bloom intensity does not exhibit a strong relationship with these environmental factors alone in this special research area. At the same time, sea surface temperature shows a slightly stronger positive trend, while Nitrate and Phosphate show a slightly negative trend that exhibits marginal effects. This suggests that sea surface temperature may significantly influence bloom formation, perhaps supporting phytoplankton development and indirectly benefiting Pelagia noctiluca. Meanwhile, the marginal influences of phosphate and nitrate imply that these factors have minimal direct impact on bloom intensity, suggesting that other environmental or biological causes may play a more major role [12,38].

3.4. Interannual Variability and Environmental Influences on Pelagia noctiluca Bloom Patterns in the Mediterranean

Pelagia noctiluca interannual variability of the bloom patterns across the Mediterranean is influenced by a complex interplay of environmental factors that vary both spatially and temporally. Figure 11 presents a series of annual maps depicting the distribution and intensity of Pelagia noctiluca blooms from 2010 to 2023, visually representing the fluctuations in bloom patterns. We documented an evident distinct variability in bloom intensity and spatial distribution, with some years experiencing concentrated blooms in specific regions and others exhibiting more dispersed, lower-intensity occurrences. From 2010 to 2014, notable bloom peaks were visible in the western Mediterranean, particularly in 2014, when the highest bloom intensities were recorded in specific regions along the Algerian coastline. This exceptional bloom may reflect a convergence of optimal environmental conditions, including favorable sea surface temperatures and nutrient availability, both of which are known to drive jellyfish proliferation [8,51]. The distinct bloom patterns observed in 2014 stand in contrast to the years immediately following, particularly 2015 to 2023, which saw a general decline in bloom intensity across the region. The 2017 and 2019 peaks, although less pronounced, highlight the continued role of temperature and food availability, with summer months showing more intense blooms.

The distribution and lower frequency of Pelagia noctiluca outbreaks following 2017 could be ascribed to oceanic factors such as changed currents or fluctuations in plankton production. Larger-scale climate events, like the North Atlantic Oscillation, which has been proven to affect marine ecosystems and species distribution, are frequently connected to these shifts [54]. Notably, the maps indicate a marked reduction in bloom activity in the eastern Mediterranean from 2020 onward, possibly related to shifts in local environmental conditions, including decreased nutrient concentrations and increased anthropogenic pressures such as overfishing and pollution [61]. The environmental factors influencing Pelagia noctiluca blooms in the Mediterranean include a combination of sea surface temperature, nutrient levels (particularly chlorophyll, nitrate, and phosphate), and hydrodynamic conditions.

In years with higher bloom intensity, such as 2014 and 2017, favorable conditions for plankton growth—chiefly driven by higher nutrient concentrations and optimal SST—are evident. These conditions promote plankton productivity, which serves as the primary food source for Pelagia noctiluca, facilitating large-scale blooms [15]. In contrast, reduced nutrient availability and fluctuating sea surface temperature during the 2020–2023 period likely led to the observed decline in bloom sizes, with many months showing negligible bloom activity, particularly in the eastern Mediterranean. Our results line up with studies showing that nutritional dynamics significantly influence jellyfish bloom patterns [51]. Furthermore, the spatial distribution of blooms in the southern Mediterranean region, as shown in Figure 11, points to different regional trends possibly influenced by local oceanographic conditions. The increase in nutrition availability is clearer in some years, promoting more jellyfish blooms. Conversely, during years with unfavorable wind patterns or increased wave height, blooms may be dispersed or reduced in size due to the disruption of aggregation conditions [66,76]. Salinity influences bloom intensity, so freshwater influx from rivers or upwelling may enhance bloom development by altering local water column stability [38,77]. Some regions display greater blooms connected with low salinity [10,27]; bloom patterns of Pelagia noctiluca in the Mediterranean from 2010 to 2023 demonstrate considerable annual variability influenced by a complex array of environmental factors. The pronounced reliance of these blooms on temperature and nutrient availability underscores the impact of both natural climate fluctuations and anthropogenic activities on bloom dynamics. Future research must continue to investigate the synergistic effects of climate change, nutrient loading, and other ecological stressors to better understand the potential shifts in jellyfish populations in the face of a changing marine environment [15].

4. Conclusions

This paper’s findings examine Pelagia noctiluca bloom patterns along the Moroccan, Algerian, and Tunisian Mediterranean coastlines and reveal notable seasonal and interannual variability with different regional patterns impacted by different environmental conditions.

For the Moroccan coast, the most substantial blooms occur during the warmer months, particularly from June to August, aligning with typical jellyfish seasonal patterns driven by warmer temperatures and increased plankton availability. Notably, the year 2014 showed extraordinary bloom intensities, with bloom sizes reaching up to 775 jellyfish. Bloom intensity dropped, though, in 2020–2023; many months in the eastern parts especially reported lesser blooms. Reflecting the complex dynamics influencing bloom formation, these losses could be caused by elements including lower nutrient availability or changes in oceanographic circumstances.

Similar trends were noted on the Algerian coast, where 2014 saw the highest bloom intensities, especially in the western areas. Bloom intensity dropped; nevertheless, following 2014, 2017, and 2019 showed modest peaks. The years following 2017 showed ever-decreasing bloom sizes, presumably related to changing nutrient levels and marine circumstances, including variations in currents and upwelling. Large blooms in particular years point to the importance of oceanographic elements, including nutrient dynamics and circulation patterns, in influencing bloom intensity in Algeria.

The Tunisian coast shows a somewhat different trend, with 2015 and 2017 experiencing significant bloom sizes, similar to Morocco and Algeria, but with a noticeable reduction in bloom activity from 2020 onward. Changes in local environmental conditions—including lower nutrient concentrations and higher anthropogenic pressures like overfishing and pollution—may be the cause of Tunisia’s declining bloom sizes. Fascinatingly, compared to other areas, Tunisia’s bloom patterns seem to be more directly impacted by local oceanographic changes as well as salinity and food accessibility.

The study emphasizes the intricate interaction of environmental elements affecting bloom patterns over the Mediterranean overall. Although Morocco and Algeria saw notable highs in several years—most notably in 2014—both countries have seen a downturn recently. Conversely, Tunisia observed a clear drop in bloom intensity starting in 2020, which could be connected to local as well as more general environmental changes.

In conclusion, while the seasonality of blooms—with peaks in the warmer months—is consistent across the three regions, each coast exhibits distinct interannual patterns [78,79,80]. While Tunisia saw a clear decline in bloom activity in recent years, the Moroccan and Algerian coasts exhibit more notable annual variations in bloom size. These results underline the need for region-specific study; it became obvious that we need innovative research objectives, including studying the adaptability of this jellyfish along the Mediterranean Sea, as we noticed significant differences between the key factors that lead to Pelagia noctiluca blooms along the Southern Mediterranean coastlines compared to the Northern Mediterranean; and not excluding comprehensive research about the relevance of climate change, nutritional availability, and oceanographic changes in forming jellyfish numbers. Future studies focusing on long-term monitoring, oceanographic models, and biological interactions will be essential for understanding and predicting jellyfish bloom dynamics in these regions.