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Article

Spatial and Seasonal Variations in Iron and the Response of Chlorophyll-a in Zhanjiang Bay, China

by
Zi-Liang Chen
,
Li-Lan Shi
,
De-Meng Peng
,
Chun-Liang Chen
*,
Ji-Biao Zhang
and
Peng Zhang
College of Chemistry and Environmental Science, Guangdong Ocean University, Zhanjiang 524088, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(16), 2338; https://doi.org/10.3390/w16162338
Submission received: 17 July 2024 / Revised: 15 August 2024 / Accepted: 16 August 2024 / Published: 20 August 2024
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Abstract

:
Iron (Fe) is a crucial trace element in marine ecosystems, playing a vital role in regulating marine primary productivity and driving marine biogeochemical cycling processes. However, understanding seasonal iron variations and the response of chlorophyll-a (Chl-a) to coastal waters remains limited. The aim of this study was to find out about the spatial and seasonal variations in iron concentrations and their impact on chlorophyll-a levels in Zhanjiang Bay. We conducted seasonal monitoring of surface seawater for iron in 2019, alongside assessments of terrestrial iron inputs during three precipitation seasons. The monitoring results showed that the iron content in Zhanjiang Bay ranged from 0.83 to 339.2 μg·L−1 with an average of 54.34 ± 75.91 μg·L−1. The annual average iron content in the central bay is higher than that in the bay mouth and inner bay. The iron content in autumn is much higher than that in other seasons, which may be due to the influence of river dredging. Correlation analysis revealed that temperature and pH are the main factors affecting the iron content in Zhanjiang Bay and the spatial distribution of iron is influenced by rainfall, river inputs, and human activities, particularly channel dredging. Iron content and chlorophyll-a were negatively correlated between different seasons, which was more significant with the increase in iron content. This may mean that the increase in iron concentration may inhibit the synthesis of chlorophyll-a, thus affecting primary productivity. We need to carry out more research experiments to verify this hypothesis. This study reveals the spatial and temporal changes in iron in urban coastal waters and its relationship with environmental factors, which is of great significance for understanding the marine biogeochemical cycle of iron in coastal eutrophic waters and specifying effective environmental management strategies.

Graphical Abstract

1. Introduction

Iron (Fe) is the fourth most abundant element in the Earth’s crust and a crucial trace element in marine ecosystems [1,2]. It plays a vital role in regulating marine primary productivity and driving marine biogeochemical cycling processes. Although nitrogen (N), phosphorus (P), and silicon (Si) are commonly recognized as essential nutrients, the existence of iron is also critical for the growth of marine phytoplankton and the increase in marine biomass, particularly in iron-limited regions [3,4]. Some studies show that iron plays a significant role in the growth, development, and energy conversion of plants [5,6]. As a coenzyme factor, iron is involved in the photosynthesis, respiration, and nitrogen fixation processes of phytoplankton [6,7] and is an essential trace element for the growth of phytoplankton in the ocean [8]. Iron is an essential element for chlorophyll synthesis in photosynthesis, so iron restriction can lead to a decrease in the photosynthetic efficiency of phytoplankton, which in turn affects their growth and productivity. The abundance of species with high iron demand decreases under iron-restriction conditions, whereas the abundance of species with low iron demand increases. For instance, diatoms exhibit a higher affinity for iron; therefore, their abundance declines in iron-restricted marine environments. The iron utilization capacity and biogeochemical behavior vary among species; for instance, diatoms sequester iron within their cell walls, whereas nitrogen-fixing cyanobacteria release iron into the surrounding environment. Alterations in community structure can impact the process of iron cycling. Low iron solubility in oxygenated marine environments [9,10] and its strong particle reactivity lead to its removal from the water column through particle scavenging, making it a scarce nutrient component for marine communities [11].
The distribution of iron in the ocean is heterogeneous, with higher concentrations typically found in coastal regions and lower concentrations in the open ocean [12]. This distribution pattern is closely related to suspended particulate matter brought by river and land runoff and the absorbed iron on their surfaces. Terrestrial particle inputs can rapidly settle down to the seabed, potentially releasing iron into the dissolved phase during sedimentation, which can then be supplied to organisms in the upper layers of the water column [13]. Iron in seawater predominantly exists in the trivalent form (Fe3+), which readily forms insoluble ferric hydroxide and quickly precipitates to the seabed [3]. This makes dissolved iron, which is bioavailable, a limited resource, especially in oceanic environments far away from terrestrial influences [14]. Consequently, microorganisms and phytoplankton must employ special strategies, such as producing iron chelators or specialized iron acquisition systems [15,16,17,18], to effectively obtain and utilize this key nutrient element [19]. As for the sources of iron, in coastal areas, they mainly originate from river inputs, the resuspension of sediments, and anthropogenic activities. Atmospheric deposition is also the primary source of iron in the open ocean [1,6,8]. In some specific sea areas, such as those heavily influenced by coastal human activities, increased dissolved iron concentrations can be observed due to land-based inputs and the resuspension of sediments [20,21]. In these environments, the cycling of iron interacts in complex ways with ecological processes and water quality conditions. For instance, iron can interact with organic matter, phosphorus, sulfur, and other heavy metals, influencing the chemical properties of water bodies, ecosystem function, and the survival of marine organisms [3,7,21,22].
The estuary, serving as the transitional zone between river and ocean, plays a pivotal role in facilitating their connection. This region predominantly captures particulate matter transported by rivers and also witnesses the alteration of dissolved material prior to its entry into the ocean. In most estuarine areas, there is generally a decline in the concentration of dissolved iron accompanied by its removal from the water body as salinity increases [23,24]. However, certain estuarine areas exhibit an opposite trend where dissolved iron increases upon entering the ocean [25]. Iron may become a limiting factor for primary productivity in estuaries. Although Sanggou Bay is rich in nitrogen, phosphorus, and silicon in autumn and the water temperature is suitable, the level of chlorophyll-a and primary productivity is relatively low. This phenomenon may be caused by the limitation of iron [26]. Human activities such as agriculture, industry, and the discharge of domestic wastewater have led to the enrichment of nitrogen and phosphorus in certain sea areas beyond the needs of phytoplankton growth. These activities not only alter the nutrient composition of local water bodies but also potentially change the biogeochemical cycling of trace elements, including iron [6,7].
Zhanjiang Bay, located on the western coast of Guangdong Province, is a typical bay seriously affected by human activities, faced with multiple sources and types of pollution, e.g., industrial and agricultural contamination, as well as the direct discharge of port activities and domestic sewage. The main environmental problems in this bay were water eutrophication and heavy metal pollution [27,28,29]. The majority of the waters within Zhanjiang Bay have been classified as exceeding Class IV water quality standards, with dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) identified as the primary pollutants [30,31,32]. The imbalance in the nitrogen-to-phosphorus ratio is notably severe. At present, there are few studies on iron in eutrophic bay water, and the response of chlorophyll-a is not clear, which needs further study. Through investigating iron content in Zhanjiang Bay waters and terrestrial iron inputs, this study aimed to understand the spatiotemporal distribution and transformation characteristics of iron in the bay to explore the potential impact of land-based iron inputs on the iron content of the bay waters and to elucidate the impact of iron input on the regional phytoplankton community and nutrient cycling. Ultimately, this knowledge is essential for formulating more precise pollution control measures and improving the ecological environment of Zhanjiang Bay.

2. Materials and Methods

2.1. Overview of the Study Area and Monitoring Station Arrangement

Zhanjiang Bay is located in the northeastern part of Leizhou Peninsula in western Guangdong, enclosed by many islands such as Nansan Island, Techeng Island, Dongtoushan Island, Donghai Island, and Naozhou Island [29]. The bay has shallow water and small waves, with a long deep-water channel, making it the largest natural harbor in South China. The tidal pattern in Zhanjiang Bay is irregular semi-diurnal, with an average tidal range of 2.17 m over the years. The maximum flow rate during flood tide is 1.54 m/s, and during ebb tide it is 1.96 m/s. There are more than 200 rivers and sewage outlets along the coast of Zhanjiang Bay. The main rivers are Suixi River, Nanliu River, Lvtang River, Wenbao River, and so on. Among them, the annual average water flux of Suixi River in the top reaches is the largest, with a runoff of 7.9 × 109 m3/yr, accounting for over 90% of the total river inflow in the bay [33,34,35,36]. The most critical water exchange channel between Zhanjiang Bay and the outer seawater is located between the Nansan and Donghai islands, and it is directly connected to the South China Sea [35].
To comprehensively understand the levels and spatiotemporal distribution of iron content in Zhanjiang Bay, based on the natural environmental characteristics of Zhanjiang Bay, tidal changes, and the characteristics of iron input from coastal land and in conjunction with the “Specifications on Spot Location of Monitoring Sites Related to Coastal Area Environment” (HJ 730-2014) in China [37], we established 23 bay water monitoring stations from the bay mouth to the inner bay. The specific locations of each station are shown in Figure 1. For the convenience of discussion, stations S1 to S3 were designated as the bay mouth area, stations S4 to S11 were designated as the central bay area, and stations S12 to S23 were designated as the inner bay area. According to the spatial distribution characteristics of the main land-based estuary and sewage outlet in Zhanjiang Bay, the survey stations of the land-based river and sewage outlet were designed. These stations cover the estuary of the river and the main municipal sewage outlet entering Zhanjiang Bay (Table 1).

2.2. Sample Collection and Analysis

In 2019, we carried out simultaneous land and sea investigation and monitoring on four voyages (January, April, July, and November). For the convenience of discussion, based on the climatic and precipitation characteristics of the Zhanjiang area, the survey was categorized as follows: April represents the normal season, January and November represent the dry season, and July represents the wet season in the investigation of terrestrial inputs.
The steps of water sample collection and analysis are shown in Figure 2. The surface water samples were collected using a portable water sampler, and the flow rate of water at the terrestrial input points was monitored using a rotameter. The water depth at different points was measured using a combination of lead hammer, rope, and tape measure, and the flux was calculated by considering the changes in the width and depth of the water body. Conventional environmental factors such as water temperature, salinity, and pH were measured on site using a multi-parameter water quality meter (AQUAREAD AP7000, Aquaread, Broadstairs, UK). Referring to the “Code of practice for marine monitoring technology” (HY/T 147.1-2013) in China [38], the collected water samples were transported to the laboratory on the same day and filtered using a 0.45 μm cellulose acetate filter membrane. The filtered water samples for nutrient analysis were frozen for later use. The water sample used for the determination of iron content was acidified to a pH less than 2 with guaranteed reagent (GR) concentrated nitric acid, and the acidified water sample was stored in a transparent glass bottle. The ultra-pure water was filtered with a 0.45 μm cellulose acetate filter membrane and the pH was adjusted with nitric acid to less than 2; this was used as the analytical blank solution. An inductively coupled plasma mass spectrometer (Agilent ICP-MS 7500CX, Agilent, Santa Clara, CA, USA) was used for analysis and determination. Dissolved oxygen in the water was measured using the iodometric method, chlorophyll-a was extracted with acetone and then measured using spectrophotometry, and the contents of dissolved inorganic nitrogen, phosphorus, and silicon were determined according to the standard methods described in the “The specification for marine monitoring” (GB 17378.4-2007) [39] in China.

2.3. Quality Control

The sampling process and analytical procedures were conducted in strict accordance with the requirements of the “The specification for marine monitoring” (GB 17378.4-2007) in China [39]. Sampling tools were rigorously cleaned and rinsed with water samples twice before use. All instruments were calibrated meticulously, and detailed records were kept of all sampling locations, times, and environmental conditions in time. All reagents used in the experiment were of high purity, and the water used was ultra-pure, with 2% nitric acid serving as the blank sample. To ensure the accuracy and reliability of the test results, the laboratory used standard seawater samples (provided by the National Center of Ocean Standards and Metrology) for quality control, with the quality control error range within 96–103%.

2.4. Research Methods

2.4.1. Calculation Method for Land-Based Iron Input Flux

The monthly input flux of land-based iron was estimated using the following formula:
Q i , j = ∑ j = 1 n C i , j × F i , j × 10 − 6
Qi,j (kg/month) represents the cumulative iron flux from land source i in month j, Ci,j (μg·L−1) represents the iron content in the water sample from land source i in month j, and Fi,j (m3/month) represents the cumulative flow rate at land source i in month j. The annual flux, represented by the symbol ∑ Q , is calculated by accumulating the monthly fluxes from all months. For comparison purposes, the study categorizes March to April and October as the normal season, January to February and November to December as the dry season, and May to September as the wet season. The total fluxes for the normal, wet, and dry seasons are calculated by multiplying the fluxes of April, July, and November by the number of months in each season, respectively.

2.4.2. Correlation Analysis

In this study, the correlation analysis method was used to explore the relationship between iron and other environmental factors. We used Origin 2018 software to draw the correlation heat map of coastal surface water monitoring stations in different seasons and throughout the year to analyze the interaction between iron and environmental parameters such as temperature, salinity, DO, pH, chlorophyll-a, and nutrients. When p ≤ 0.05, the observed correlation is statistically significant.

2.4.3. Data Statistics and Analysis Methods

The distribution of sampling stations and the regional variations in iron, dissolved oxygen, and chlorophyll-a content were mapped using the Ocean Data View (ODV) software (5.7.0). The box plots for seasonal variations in iron content in Zhanjiang Bay and the bar graphs were created using Origin 2018. The data were analyzed by Excel software (https://www.microsoft.com), and the data were expressed as arithmetic (mean ± SD).

3. Results and Analysis

3.1. Spatiotemporal Variations in Iron Content in the Coastal Seawater of Zhanjiang Bay

Based on the monitoring results from the four-season surveys in 2019, there were significant differences in the content levels of iron element among different stations in Zhanjiang Bay (Figure 3). The iron content in Zhanjiang Bay waters varied from 0.83 to 339.2 μg·L−1 in 2019, with an average of 54.34 ± 75.91 μg·L−1. There were notable differences in iron content among the seasons (Figure 3). In winter (January), the iron content ranged from 22.80 to 28.69 μg·L−1, with an average of 25.94 ± 1.28 μg·L−1. In spring (April), the iron content ranged from 31.45 to 85.94 μg·L−1, with an average of 60.97 ± 19.33 μg·L−1. In summer (June), the iron content ranged from 21.77 to 26.84 μg·L−1, with an average of 24.44 ± 1.40 μg·L−1. In autumn (November), the iron content ranged from 0.83 to 339.17 μg·L−1, with an average of 110.93 ± 138.57 μg·L−1.
The annual average iron content in the bay mouth region (S1–S3) of Zhanjiang Bay was 72.06 ± 106.12 μg·L−1, the middle bay region (S4–S11) was 78.43 ± 99.89 μg·L−1, and the inner bay region (S12–S23) was 33.86 ± 26.33 μg·L−1.The annual monitoring results in different water areas of Zhanjiang Bay showed that the iron content in the bay mouth and middle bay regions was relatively close. The concentration of iron in the inner bay exhibited a significantly lower iron content compared to other regions, indicating a pronounced disparity in the regional distribution of iron.
The seasonal variation in iron content in the waters of Zhanjiang Bay was distinct, with the lowest values in summer and the highest in autumn (Figure 4). In spring, the average iron content in the bay mouth region was relatively low, while the average iron content in the middle bay and inner regions was more similar. In summer, the average iron content in the waters of Zhanjiang Bay decreased compared to spring and the iron content in various regions was relatively close. In autumn, the iron content was generally higher between Donghai Island and Nansan Island, reaching a maximum of 339.2 μg·L−1, and gradually decreased towards the water areas of Xiashan District and Potou District. In winter, the iron content in Zhanjiang Bay was relatively low overall and the distribution was more uniform across the regions. Overall, the iron content in the nearshore waters of Zhanjiang Bay gradually decreased from spring to summer, significantly increased in autumn, and then decreased in winter.

3.2. Spatiotemporal Variations in Terrestrial Input Iron in Zhanjiang Bay

The survey of Zhanjiang Bay revealed that the annual iron content at the terrestrial input sites ranged from below the detection limit to 80.71 μg·L−1, with an average of 11.81 ± 19.33 μg·L−1. There was a clear variation in the annual average flux of terrestrial input iron at the 11 input sites (Figure 5). The annual total flux of terrestrial input iron was 6.40 × 105 kg, with the input flux in the wet season (July) being 6.01 × 105 kg, accounting for 93% of the annual flux; the input flux in the normal season (April) was 9.58 × 103 kg, accounting for 1.5% of the annual flux; and the input flux in the dry season (November) was 2.60 × 104 kg, accounting for 5.5% of the annual flux. Compared to the normal and dry seasons, the terrestrial input iron flux significantly increased during the wet season. Among these, the Suixi River estuary (P9) had the highest annual input flux of iron into the sea (reaching 6.23 × 105 kg) and it was also the highest in all three seasons, with the iron flux accounting for over 90% of the total flux in each period (93.63% in the normal season, 97.69% in the wet season, and 91.06% in the dry season).
In terms of spatial distribution, there were also significant differences in iron content at different terrestrial input sites. During the dry season, the Donghai Island aquaculture sewage outlet (P2) had the highest terrestrial input iron content of 80.71 μg·L−1 for the entire year, which was significantly higher than other terrestrial input sites. In the November survey, the iron content at the Nanliu River Estuary (P4) and the Dengta Park floodgate estuary (P10) could not be detected, indicating the lowest iron content for the entire year. During the normal season, the Lutang River Estuary (P5) had an input iron content of 55.25 μg·L−1, which was significantly higher than other terrestrial input sites at the same time. During the wet season, the Suixi River estuary (P9) had an input iron content of 53.63 μg·L−1, which was the highest value at the same time.
In 2019, the average content of terrestrial input iron in Zhanjiang Bay was the highest in the dry season, followed by the wet season, and the lowest in the normal season. During the dry season (Figure 6A), the fluctuation range of terrestrial input iron was between below the detection limit and 80.71 μg·L−1, corresponding to the Dengta Park floodgate estuary (P10) and the Donghai Island breeding sewage outlet (P2), respectively, with an average terrestrial input iron content of 12.06 ± 25.20 μg·L−1 for this season. During the wet season (Figure 6B), the fluctuation range of terrestrial input iron content in Zhanjiang Bay was between 2.48 μg·L−1 and 53.63 μg·L−1, with the lowest value at the Dengta Park floodgate estuary (P10) and the highest value at the Suixi River estuary (P9), with an average terrestrial input iron content of 11.74 ± 16.72 μg·L−1 for this season. Furthermore, during the normal season (Figure 6C), the fluctuation range of terrestrial input iron content in Zhanjiang Bay was between 2.32 μg·L−1 and 55.25 μg·L−1, with the lowest value at the Dengta Park floodgate estuary (P10) and the highest value at the Lutang River Estuary (P5), with an average terrestrial input iron content of 11.62 ± 16.12 μg·L−1 for this season.

3.3. Variations in Environmental Factors in the Coastal Seawater of Zhanjiang Bay

The variations in various environmental factors in the waters of Zhanjiang Bay in 2019 are shown in Table 2. Throughout the year, the water temperature in the bay waters ranged from 17.4 to 33.6 °C, with an average of 24.53 ± 5.08 °C. The salinity of the bay waters varied between 18.2 and 30.57, with an average salinity of 25.63 ± 2.82. The pH of the bay waters ranged from a minimum of 6.84 to a maximum of 8.46, with an average pH of 7.94 ± 0.29. The DO of the bay waters varied between 5.42 and 9.90, with an average DO of 7.64 ± 1.15 (Figure 7). According to the statistical report of the Zhanjiang Meteorological Bureau, the total annual rainfall in Zhanjiang City in 2019 was 1496.26 mm, with an average monthly rainfall of 124.69 ± 105.25 mm. The total rainfall during the dry season was 42.65 mm, during the normal it was 291.59 mm, and during the wet season it reached 1162.02 mm.
In 2019, the chlorophyll-a (Chl-a) content in the waters of Zhanjiang Bay varied between 1.11 μg/L and 47.24 μg/L, with an average of 9.34 ± 11.43 μg/L (Table 2). The average Chl-a content was highest during the wet season (June) and lowest during the dry season (November); the annual average content in the bay mouth and middle regions was similar, while the content in the inner bay region was relatively higher (Figure 8). The annual average content of dissolved silicate (DSi) in the waters of Zhanjiang Bay varied between 0.21 mg/L and 3.39 mg/L, with an average of 1.23 ± 0.78 mg/L; the highest average DSi content was observed during the normal season (April), while the lowest average content was in the dry season (January); the annual average content in the three regions was relatively close. The annual average total dissolved inorganic nitrogen (TDN) content in the waters of Zhanjiang Bay varied between 1.16 mg/L and 5.46 mg/L, with an average of 2.20 ± 0.73 mg/L; the highest average TDN content was observed during the dry season (November), while the lowest average content was in the wet season (June); the annual average content was highest in the middle bay region, followed by the bay mouth region, and lowest in the inner bay region. The total dissolved inorganic phosphorus (TDP) content in the waters of Zhanjiang Bay varied between 0.04 mg/L and 0.20 mg/L, with an average of 0.10 ± 0.03 mg/L; the highest average TDP content was observed during the normal season (April), while the lowest average content was in the dry season (January); the annual average content was relatively low in the bay mouth region, while the middle bay and inner regions had higher contents, with the difference among them being minimal.

3.4. The Relationship between Iron and Nutrients, Chlorophyll-a, and Other Environmental Factors in Zhanjiang Bay

The relationships among nutrients, chlorophyll-a, and other environmental factors in Zhanjiang Bay are shown (Figure 9). The results showed that iron and DSi showed a significant negative correlation in spring (p ≤ 0.05). The results showed that iron and temperature (p ≤ 0.01), TDP (p ≤ 0.05), DSi (p ≤ 0.01), and Chl-a (p ≤ 0.05) showed a significant negative correlation in summer but significantly positive correlation with salinity and urea (p ≤ 0.01). Moreover, a significant negative correlation was found between iron and temperature (p ≤ 0.01), pH (p ≤ 0.05), TDP (p ≤ 0.01), and Chl-a (p ≤ 0.05) in autumn. In addition, there was no significant correlation between iron and other indicators in winter. For the whole year, iron and salinity showed a significant positive correlation (p ≤ 0.05) but there was no significant correlation with other indicators (Figure 10).

4. Discussion

4.1. Comparison of Fe Concentration in Zhanjiang Bay with Other Estuaries and Bays around the World

The results indicated that the surface iron concentration in the Zhanjiang Bay area exhibited distinct spatial and temporal distribution patterns throughout the survey period. In each voyage, the iron concentration in the surface water of Zhanjiang Bay in autumn was significantly higher than that in the other three seasons (Figure 3). It is speculated that this phenomenon may be related to the channel dredging project. After considering the potential impact of channel dredging activities, our investigation revealed that Zhanjiang Bay was engaged in dredging work for the “Zhanjiang Port 300,000-ton Channel Reconstruction and Expansion Project” at the time [40,41]. Dredging, as a sediment management measure, may disturb sediments and increase material exchange at the sediment–water interface, including the release of nutrients such as iron [42]. In summer, with the surge of precipitation, rainwater directly injects more iron into the bay in the form of precipitation and the runoff of the river increases, carrying more iron and transporting it to the bay. Not all dissolved iron input from rivers can exist stably in the bay. In fact, in the process of mixing salt and fresh water, a considerable part of iron will precipitate from the solution and be removed from the water body [23,24,42,43]. On the other hand, the content of chlorophyll-a in summer was significantly higher than that in the other three seasons (Figure 8), which proved that phytoplankton grew vigorously and the absorption of iron by phytoplankton primary production increased [44,45,46], which may be another reason for the low iron content. The average concentration and maximum concentration of iron in Zhanjiang Bay were higher than those in St. Lawrence River, Scheldt estuary, Yangtze estuary, Periyar rivers, Chalakudy River, Sanggou Bay, and Jiaozhou Bay (Table 3) [26,47,48,49,50,51]. The concentration range and maximum value of iron in Zhanjiang Bay were similar to those in Massachusetts’ North River and Orinoco River [52,53] but much lower than those in Beaulieu River and Broadkill River [54,55]. One of the reasons for the high average iron concentration in Zhanjiang Bay may be that, as a semi-enclosed bay, the hydrodynamic exchange conditions are poor due to the influence of reclamation projects [56], so the iron enrichment in coastal waters is more serious than other sea areas.

4.2. Correlation Analysis of Iron in Zhanjiang Bay Waters with Other Environmental Factors

As a trace nutrient metal, iron plays a crucial role in marine biogeochemical processes [57]. Although the total iron content in seawater is not high, it is an essential nutrient for marine organisms, especially in supporting the growth of phytoplankton [6,7,11,58,59]. The content levels and distribution of iron in the water body are influenced by water temperature, salinity, pH, and dissolved oxygen (DO) [60,61]. In this study, iron exhibited a moderate negative correlation with water temperature, while it displayed a weak negative correlation with pH, with correlation coefficients of −0.41 and −0.20, respectively, indicating a trend for iron content to decrease as water temperature increases and pH value improves. Iron exhibited a significant positive correlation with salinity (p ≤ 0.05), with a correlation coefficient of 0.47 (Figure 10), indicating that the iron content increases as salinity levels rise. The moderate negative correlation between iron content and temperature in the waters of Zhanjiang Bay could be due to the fact that temperature increases may promote microbial activity [59,62], thereby increasing the biological utilization of iron and causing the decrease in dissolved iron content [63]. On the other hand, the increase in temperature can also accelerate the chemical oxidation of iron, leading to the conversion of dissolved iron to particulate iron [64,65]. The weak negative correlation between iron content and pH in the waters of Zhanjiang Bay could be attributed to the fact that, as the pH value of the water body increases, the solubility of iron decreases, leading to more iron transforming from dissolved to particulate form, thereby reducing the dissolved iron content in the water body [66].
Typically, as salinity increases, the ionic strength of the water increases, leading to a decrease in the solubility of iron, and thus more iron exists in particulate form. This results in a negative correlation between dissolved iron and salinity [67]. Therefore, an increase in rainfall brings more iron-rich particles from the land to the ocean, increasing the iron content in the water body. However, by combining summer field observations from Zhanjiang Bay with rainfall data from the Zhanjiang Meteorological Bureau, we noted that despite the high rainfall-induced decrease in seawater salinity, the iron content remained relatively low, showing a positive correlation with salinity (Figure 9b). The reason for this result may be that the increased rainfall leads to enhanced surface runoff, carrying more organic matter and nutrients into the bay. This creates suitable conditions for the absorption and utilization of phytoplankton, promoting the increase in their biomass. This process may intensify the competitive absorption of iron among phytoplankton [68], to some extent reducing the iron content in the waters of Zhanjiang Bay.
The moderate correlation between iron content in Zhanjiang Bay waters and temperature, coupled with a weak correlation with pH, suggests that the behavior and cycling of iron in seawater are influenced by a complex interplay of environmental factors rather than being solely determined by a few conventional ones [69,70,71] but may be more controlled by local conditions such as external inputs. The sources of iron in Zhanjiang Bay mainly include river input, surface runoff, atmospheric deposition, and anthropogenic activities in the coastal area [56,72]. Among these, rivers carry a large amount of terrestrial material into the bay, including iron from soil erosion and urban sewage. Atmospheric deposition is caused by airborne particles from industrial, transportation, and other activities and are deposited into the water through dust fall processes. Human activities, especially port activities and aquaculture, may directly or indirectly increase the iron content in the waters of Zhanjiang Bay [29,73,74].

4.3. Impact of Terrestrial Iron Inputs on the Coastal Seawater of Zhanjiang Bay

To understand the impact of terrestrial iron inputs on the changes in iron content in the waters of Zhanjiang Bay, we selected the nearest sea stations from each terrestrial input site and compared their iron content results to assess the contribution of terrestrial inputs to the changes in iron content in the waters of Zhanjiang Bay (Table 4). In the seven comparison groups, the iron content of Hongxing Reservoir Estuary (P3) was significantly higher than that of its adjacent offshore station (S10), up to 31 times higher; similarly, the iron content in the estuary of Nanliu River Estuary (P4) was also significantly higher than that in the adjacent offshore station (S12), about 25 times higher. These significant differences were likely due to autumn channel dredging activities, which resulted in the transport of particulate iron into the upper water column and its diffusion into the surrounding sea areas [75]. The iron content of Potou Primary School sewage outlet (P11), Jinsha Bay sewage outlet (P7), and Dengta Park flood gate (P10) showed a medium level of difference compared with their adjacent sea stations S17, S20, and S19, which were about 5 times, 4 times, and 10 times, respectively. These moderate differences may be attributed to the resuspension of iron from sediments into the water column due to physical disturbances such as shipping activities and fishing operations [76]. The differences in iron content between Donghai Island breeding sewage outlet (P2) and S11 station and between Lutang River Estuary (P5) and S15 station were small, at 1.23 μg/L and 8.77 μg/L, respectively. These smaller differences indicate that the terrestrial input iron at these stations had a smaller impact on the iron content of the adjacent water bodies.
The comparison results showed that, in the seven comparison groups, the average iron content at the sea stations was higher than that at the terrestrial stations in five groups, indicating that the iron content from terrestrial inputs into the water bodies was generally lower than the iron content in the sea areas. This suggests that terrestrial inputs had a relatively small impact on the iron content in the sea areas. The higher iron content in the sea areas of Zhanjiang Bay may have had other input sources. This is likely because Zhanjiang Bay is a busy water area with frequent ship traffic and channel dredging activities, which disturb the sediment at the bottom of the sea [77], increasing the desorption and solubility of particulate iron and promoting the increase in iron content in the seawater to some extent.

4.4. Impact of Iron on Primary Productivity in the Bay Waters

In the marine ecosystem of Zhanjiang Bay, a region known for its rich biodiversity, productivity is influenced by various factors. Generally, there is a significant positive correlation between chlorophyll-a content and primary productivity [78,79]. Chlorophyll-a has been widely accepted as an effective biological indicator for assessing primary productivity in the sea area [80,81]. Furthermore, considering the key role of iron as a micronutrient in photosynthesis, especially in some sea areas, the availability of iron may limit the growth and photosynthesis efficiency of phytoplankton. Therefore, exploring the relationship between iron and chlorophyll-a content can provide us with an in-depth understanding of the impact of iron on primary productivity. The surface water iron concentration in Zhanjiang Bay was significantly higher, averaging 54.34 ± 75.91 μg/L, indicating relatively elevated levels of iron in this area (Table 3). The results of correlation analysis showed that iron and chlorophyll-a showed a weak negative correlation in spring and winter, and the correlation coefficients were −0.28 and −0.094, respectively. In summer and autumn, there was a significant negative correlation (p ≤ 0.05), and the correlation coefficients were −0.51 and −0.43, respectively (Figure 9). Correlation analysis showed that there was a weak negative correlation between annual average iron and chlorophyll-a (correlation coefficient −0.20) (Figure 10). The rainfall in summer caused the river to carry iron into the bay, and the iron content in the bay increased significantly due to the influence of the channel dredging project in autumn. It may be that this change leads to the correlation between chlorophyll-a and iron content being a weak negative correlation to a strong negative correlation. This may be due to the iron concentration in Zhanjiang Bay far exceeding the trace amounts required for chlorophyll-a synthesis, with excess iron potentially inhibiting chlorophyll-a synthesis and thereby limiting the primary productivity of Zhanjiang Bay. Iron plays a crucial role in the photosynthesis of phytoplankton, as it is a component of various enzymes and proteins involved in key steps of the photosynthetic electron transport chain [82]. Excessive iron levels can lead to oxidative stress, inhibiting chlorophyll-a synthesis and photosynthetic efficiency [83,84,85]. Therefore, although iron is an essential trace element for phytoplankton growth, its excessive presence may have a negative impact on primary productivity. For those species already adapted to iron-limited conditions, a sudden increase in iron concentration can cause them to lose their competitive advantage, potentially becoming a limiting factor for the microphytoplankton community [86]. We need to conduct more research experiments to verify this hypothesis. This further explains the observed weak negative correlation between iron and chlorophyll-a. Therefore, changes in iron content not only affect the biogeochemical cycling of marine nutrients but may also regulate ecosystem productivity by directly acting on photosynthetic organisms. A weak positive correlation was found between iron and TDN (correlation coefficient 0.19). This may reflect the role of iron in the nitrogen cycle. TDN is an important nitrogen source, and its bioavailability may be influenced by iron content.
In marine ecosystems, productivity depends not only on the availability of trace nutrients but also on the regulation of macro-nutrients such as nitrogen, phosphorus, and silicate [87]. The bioavailability of iron plays a crucial role in promoting the uptake and utilization of nitrate, phosphate, and silicate by algae under specific conditions, thereby supporting primary productivity [37]. Even though no significant correlations were found in this study, based on the comprehensive characteristics of the iron biogeochemical cycle, we cannot ignore the central role of iron in the ecological geochemistry of trace elements or its potential impact on the role of these macro-nutrients in the ecosystem.

5. Conclusions

The results of four surveys in Zhanjiang Bay in 2019 and three investigations of terrestrial inputs show that the average iron content in the waters of Zhanjiang Bay is relatively high throughout the year. The middle bay has higher iron content than the bay mouth and inner bay, and the iron content in autumn is much higher than that in other seasons due to the impact of channel dredging. The terrestrial input iron content in the water bodies is generally lower than that in seawater, fully reflecting that the iron content in the waters of Zhanjiang Bay is controlled by natural seasonal changes and human activities.
The correlation analysis in Zhanjiang Bay reveals that iron concentrations are influenced by a complex array of environmental factors, with temperature and pH showing negative correlations, while salinity exhibits a positive correlation. There is a complex interaction between iron concentration and chlorophyll-a level. Iron content and chlorophyll-a were negatively correlated between different seasons, which was more significant with the increase in iron content. This may mean that the increase in iron concentration may inhibit chlorophyll-a synthesis and thus affect primary productivity. We need to conduct more research experiments to verify this hypothesis.
In conclusion, our study shows that the iron concentration in Zhanjiang Bay is affected by natural seasonal changes and human activities, especially dredging. The correlation analysis results show that iron and chlorophyll-a are negatively correlated, which may have a potential impact on primary productivity, but more research is needed to verify this speculation.

Author Contributions

Z.-L.C.: formal analysis, software, validation, writing—original draft, writing—review and editing. L.-L.S.: original draft preparation. D.-M.P.: visualization. C.-L.C.: visualization, supervision. J.-B.Z.: conceptualization, funding acquisition, methodology, project administration, resources, supervision, writing—review and editing. P.Z.: funding acquisition, investigation, methodology, project administration, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

Thanks for the financial support provided by the Research and Development Projects in Key Areas of Guangdong Province (2020B1111020004); the Guangdong Basic and Applied Basic Research Foundation (2023A1515012769); and Guangdong Basic and Applied Basic Research Foundation (2020A1515110483).

Data Availability Statement

Data will be made available on request.

Acknowledgments

The authors are grateful for the anonymous reviewers’ careful review and constructive suggestions to improve the manuscript. Thanks for the financial support provided by the Research and Development Projects in Key Areas of Guangdong Province (2020B1111020004); the Guangdong Basic and Applied Basic Research Foundation (2023A1515012769); and Guangdong Basic and Applied Basic Research Foundation (2020A1515110483).

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Distribution map of terrestrial (P1–P11) and coastal surface water monitoring stations (S1–S23) in Zhanjiang Bay.
Figure 1. Distribution map of terrestrial (P1–P11) and coastal surface water monitoring stations (S1–S23) in Zhanjiang Bay.
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Figure 2. Procedure for determination of dissolved iron concentration in seawater by inductively coupled plasma mass spectrometer.
Figure 2. Procedure for determination of dissolved iron concentration in seawater by inductively coupled plasma mass spectrometer.
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Figure 3. Iron content distribution in surface waters of Zhanjiang Bay: a comparative analysis of spring (a), summer (b), autumn (c), and winter (d).
Figure 3. Iron content distribution in surface waters of Zhanjiang Bay: a comparative analysis of spring (a), summer (b), autumn (c), and winter (d).
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Figure 4. The seasonal variations in iron in the Zhanjiang Bay coastal water.
Figure 4. The seasonal variations in iron in the Zhanjiang Bay coastal water.
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Figure 5. Annual iron flux and average iron concentration variations from the estuaries and sewage outlets of Zhanjiang Bay (the pie chart represents the seasonal proportion of annual iron flux).
Figure 5. Annual iron flux and average iron concentration variations from the estuaries and sewage outlets of Zhanjiang Bay (the pie chart represents the seasonal proportion of annual iron flux).
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Figure 6. (A) Dry season iron concentration (CFe) and iron discharge flux in Zhanjiang Bay estuary and sewage outlet. (B) Wet season iron concentration (CFe) and iron discharge flux in Zhanjiang Bay estuary and sewage outlet. (C) Normal season iron concentration (CFe) and iron discharge flux in Zhanjiang Bay estuary and sewage outlet.
Figure 6. (A) Dry season iron concentration (CFe) and iron discharge flux in Zhanjiang Bay estuary and sewage outlet. (B) Wet season iron concentration (CFe) and iron discharge flux in Zhanjiang Bay estuary and sewage outlet. (C) Normal season iron concentration (CFe) and iron discharge flux in Zhanjiang Bay estuary and sewage outlet.
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Figure 7. DO content distribution in surface waters of Zhanjiang Bay: a comparative analysis of spring (a), summer (b), autumn (c), and winter (d).
Figure 7. DO content distribution in surface waters of Zhanjiang Bay: a comparative analysis of spring (a), summer (b), autumn (c), and winter (d).
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Figure 8. Chl-a content distribution in surface waters of Zhanjiang Bay: a comparative analysis of spring (a), summer (b), autumn (c), and winter (d).
Figure 8. Chl-a content distribution in surface waters of Zhanjiang Bay: a comparative analysis of spring (a), summer (b), autumn (c), and winter (d).
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Figure 9. Correlation relationships among iron and nutrients, chlorophyll-a, and other environmental factors during the spring (a), summer (b), autumn (c), winter (d).
Figure 9. Correlation relationships among iron and nutrients, chlorophyll-a, and other environmental factors during the spring (a), summer (b), autumn (c), winter (d).
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Figure 10. The relationship between annual average iron and nutrients, chlorophyll-a, and other environmental factors in Zhanjiang Bay. (Correlation coefficient r ≥ 0.8 is strong correlation; 0.5 ≤ r < 0.8 is moderate correlation; r < 0.5 is weak correlation.)
Figure 10. The relationship between annual average iron and nutrients, chlorophyll-a, and other environmental factors in Zhanjiang Bay. (Correlation coefficient r ≥ 0.8 is strong correlation; 0.5 ≤ r < 0.8 is moderate correlation; r < 0.5 is weak correlation.)
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Table 1. Investigation of estuaries and sewage outlets.
Table 1. Investigation of estuaries and sewage outlets.
StationTerrestrial Estuaries and Sewage OutletsEast Longitude/(°)North Latitude/(°)
P1Donghai Island breeding sewage outlet110.347821.0739
P2Donghai Island breeding sewage outlet2110.401721.0864
P3Hongxing Reservoir River Estuary110.417521.0603
P4Nanliu River Estuary110.382521.1519
P5Lutang River Estuary110.414721.2128
P6Wenbao River Estuary110.397221.2531
P7Jinsha Bay sewage outlet110.391921.2703
P8Binghu Park flood control gate110.391421.2792
P9Suixi River Estuary110.388021.3928
P10Dengta Park floodgate estuary110.433121.2536
P11Potou Primary School sewage outlet110.448121.2397
Table 2. The status of conventional environmental elements and nutrient content of nitrogen, phosphorus, and silicon in Zhanjiang Bay in 2019.
Table 2. The status of conventional environmental elements and nutrient content of nitrogen, phosphorus, and silicon in Zhanjiang Bay in 2019.
Environmental ElementUnitMinimum ValueMaximum ValueMean Value
temperature°C17.4033.6024.53 ± 5.08
salinity 18.2030.5725.63 ± 2.82
DOmg/L5.429.597.64 ± 1.15
pH 6.848.467.94 ± 0.29
Chl-aμg/L1.1147.249.34 ± 11.43
TDNmg/L1.165.462.20 ± 0.73
TDPmg/L0.040.200.10 ± 0.03
DSimg/L0.213.391.23 ± 0.78
Table 3. Comparison of Fe concentration in Zhanjiang Bay with other bays and estuaries.
Table 3. Comparison of Fe concentration in Zhanjiang Bay with other bays and estuaries.
Study AreaSurvey TimeAverage Concentration of CFe (μg/L)Range of CFe Concentration (μg/L)Reference
St. Lawrence River1995–19964.02 ± 2.01 [44]
Scheldt estuary200229.95 [45]
Yangtze estuary1997–20024.020.56–4.52[46]
Periyar rivers2003.74.30 [47]
Chalakudy river2003.73.80 [47]
Massachusetts’ North River2006–2007 77.62–389.8[49]
Orinoco River2004–2006138.037.00–312.0[50]
Beaulieu River2012–2013558.055.85–1172[51]
Broadkill River2015–2016 22.34–1049[52]
Sanggou Bay20140.188 ± 0.1160.079–0.520[26]
Jiaozhou Bay20111.29 ± 0.401 [48]
Zhanjiang Bay201954.34 ± 75.910.830–339.2This study
Table 4. Contrast of 2019 annual average iron concentrations in terrestrial and adjacent offshore monitoring stations in Zhanjiang Bay.
Table 4. Contrast of 2019 annual average iron concentrations in terrestrial and adjacent offshore monitoring stations in Zhanjiang Bay.
Terrestrial StationThe Average Iron Content of Terrestrial Stations (μg/L)Adjacent
Bay Station		Average Iron Content of Bay Stations
(μg/L)		Iron Content Difference (μg/L)
Donghai Island breeding sewage outlet (P2)33.71S1134.941.23
Hongxing Reservoir River Estuary (P3)3.62S10113.61109.99
Nanliu River Estuary (P4)2.63S1266.1463.51
Lutang River Estuary (P5)32.78S1524.018.77
Potou Primary School sewage outlet (P11)3.70S1719.7116.01
Jinsha Bay sewage outlet (P7)5.80S2021.4715.67
Dengta Park flood gate (P10)1.60S1917.0015.40
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Chen, Z.-L.; Shi, L.-L.; Peng, D.-M.; Chen, C.-L.; Zhang, J.-B.; Zhang, P. Spatial and Seasonal Variations in Iron and the Response of Chlorophyll-a in Zhanjiang Bay, China. Water 2024, 16, 2338. https://doi.org/10.3390/w16162338

AMA Style

Chen Z-L, Shi L-L, Peng D-M, Chen C-L, Zhang J-B, Zhang P. Spatial and Seasonal Variations in Iron and the Response of Chlorophyll-a in Zhanjiang Bay, China. Water. 2024; 16(16):2338. https://doi.org/10.3390/w16162338

Chicago/Turabian Style

Chen, Zi-Liang, Li-Lan Shi, De-Meng Peng, Chun-Liang Chen, Ji-Biao Zhang, and Peng Zhang. 2024. "Spatial and Seasonal Variations in Iron and the Response of Chlorophyll-a in Zhanjiang Bay, China" Water 16, no. 16: 2338. https://doi.org/10.3390/w16162338

APA Style

Chen, Z.-L., Shi, L.-L., Peng, D.-M., Chen, C.-L., Zhang, J.-B., & Zhang, P. (2024). Spatial and Seasonal Variations in Iron and the Response of Chlorophyll-a in Zhanjiang Bay, China. Water, 16(16), 2338. https://doi.org/10.3390/w16162338

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