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Article

Winter Bloom of Marine Cyanobacterium, Trichodesmium erythraeum and Its Relation to Environmental Factors

by
Nowrin Akter Shaika
1,
Eman Alhomaidi
2,
Md. Milon Sarker
3,
Abdullah An Nur
1,
Md. Ashfaq Sadat
1,
Sadiqul Awal
4,
Golam Mostafa
5,
Shanur Jahedul Hasan
5,
Yahia Mahmud
5 and
Saleha Khan
1,*
1
Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh
2
Department of Biology, College of Science, Princess Nourah bint Abdulrahman University, Riyadh 11671, Saudi Arabia
3
Department of Fisheries and Marine Science, Noakhali Science and Technology University, Noakhali 3814, Bangladesh
4
Department of Arts, Education & Agri-Tech, Melbourne Polytechnic, Epping, VIC 3076, Australia
5
Bangladesh Fisheries Research Institute, Mymensingh 2202, Bangladesh
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(2), 1311; https://doi.org/10.3390/su15021311
Submission received: 9 October 2022 / Revised: 16 December 2022 / Accepted: 25 December 2022 / Published: 10 January 2023
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Abstract

:
A winter bloom event of Trichodesmium erythraeum was monitored for the first time in the southeastern coastal parts of Bangladesh along the Bay of Bengal. This study presents the brownish to light pinkish bloom that appeared in mid-winter and disappeared abruptly during spring. Heavy blooms of T. erythraeum revealed the highest concentration of 91.47 ± 52.94 × 103 colonies/L in the Bakkhali River Estuary, and 66.93 ± 12.95 × 103 colonies/L in the Maheshkhali Channel of the Bay of Bengal. Three distinct morphological shapes, namely puffs, tufts and asymmetrical colonies, were depicted as major types. Several environmental factors, such as water temperature, salinity, pH, dissolved oxygen, NO3–N and PO4–P, were analyzed to determine their relationship with the occurrence, abundance and bloom formation of T. erythraeum. The abundance of the species showed a positive correlation with salinity and pH while exhibiting a negative correlation with temperature and DO. A cluster analysis revealed a clear indication of T. erythraeum bloom during winter. Thus, the prevalence of the highest density of the bloom in the present study area strongly suggests increased monitoring and research efforts in order to effectively manage or impede harmful algal blooms.

Graphical Abstract

1. Introduction

Climate change is altering aquatic ecosystem functioning and health. Marine and coastal waters have experienced progressive warming, acidification, and areas of low oxygen water [1] threaten to undermine the functioning of natural ecosystems, especially when combined with the myriad of additional anthropogenic stressors that may intensify in this century [2]. At the same time, there is a scientific consensus that global trends in the occurrence, toxicity and risk posed by harmful algal blooms (HABs) plague many coastal and inland waters throughout the world [2,3,4]. There are several different bloom-forming alga species found in coastal waters globally, including cyanobacteria, dinoflagellates and diatoms. The toxin-producing species listed in the IOC-UNESCO Taxonomic Reference List of Harmful Microalgae [5] include 105 dinoflagellates, 37 marine cyanobacteria, 31 diatoms, eight haptophytes, six raphidophytes, three dictyochophytes and two pelagophytes. Thus, it is evident that marine dinoflagellates and cyanobacteria are the key harmful algal bloom species constituting the first and probably the second most appearing harmful algal class, respectively [6,7].
Planktonic marine cyanobacteria of the genus Trichodesmium [8] have fascinated scientists for well over a century [9]. These colonial cyanobacteria have been reported throughout the tropical and subtropical ocean waters of the Atlantic, Pacific, and Indian oceans, as well as the Caribbean and South China seas [10,11,12]. Modern interest in Trichodesmium dates to the early 1960s with the recognition that the biological productivity of large expanses of the ocean is often limited by the availability of nitrogen [9,13] and the observation that Trichodesmium is a diazotrophic [14], nitrogen-fixing cyanobacterium. In the Indian Ocean, Trichodesmium blooms have been recorded since 1942 [15]. In the Southeastern Arabian Sea, an unusual occurrence of Trichodesmium blooms was reported by Sathish et al. [16] during the winter monsoon (January 2020). The pre-Southwest monsoon is the predominant favourable season of Trichodesmium bloom in the Northern Indian Ocean [17,18,19]. The monsoon period promotes Trichodesmium biomass highly in these regions.
Generally, the climate of the Bay of Bengal (BoB) is dominated by the monsoons. During the Northern summer, the rain-bearing Southwest monsoon prevails from June to September, while the Northern winter monsoon blows over the Bay from November to April. During winter (north-east monsoon), atmospheric and subsurface turbulent heat fluxes contribute about 50% of the net sea surface cooling in BoB. The Northeast winds of winter lead to the upwelling of water columns and supply of nutrients that persists in a longer winter bloom of phytoplankton [20]. The biological productivity of the Bay is, however, controlled by regulating vertical nutrient flux to the euphotic column through density and temperature-initiated stratification and weaker winds over the region [21]. In the surface layer, winter-cooled freshwater remains stratified and thermal inversion regularly takes place in the northern BoB [22] that could provoke the bloom events of Trichodesmium [23]. Moreover, the climate change-induced river discharge during a Southwest monsoon will increase the inundation depth and sea level in the coastal region of Bangladesh. The progression of climate change pressure on key variables, such as alteration in temperature, salinity, nutrient inputs, rainfall, stratification, coastal upwelling and precipitation will eventually lead to HAB trends and responses [24].
It is worthy of mention here that the coastal landmass of Bangladesh is very fertile due to the regular flush of nutrient-rich silts and the supply of organic matter from mangrove litter fall. As a result, BoB has become one of the richest areas in terms of biological diversity and nutrient-rich habitat [25]. Nutrient influx through riverine discharge, wind-induced mixing of the surface ocean, coastal upwelling, ocean currents, eddy pumping and aeolian processes are the incomparable characteristic features of BoB [26,27]. Materially, many countries in South Asia and Southeast Asia are dependent on BoB including littoral and landlocked countries for maritime usage [28,29]. However, the marine and estuarine ecosystems of BoB and particularly in Bangladesh waters are threatened by different types of pollutants dumped directly into the bay or washed down through a large number of rivers and tributaries [30,31]. Upwelling, the formation of mud banks, nutrient discharges from estuaries and run-off from the land promote algal blooms in the coastal waters of the region. Of late, climate change and eutrophication might lead to harmful algal blooms and pose significant impacts on the biological and economic aspects of the ‘Blue Economy’ [32]. Negative impacts of the diverse group of harmful, noxious and toxic algae that comprise HABs reported from the bay increasingly threaten the economic viability of marine resources, recreational activities, the diversity of the ecosystem, and particularly aquaculture, as well as fisheries and public health. Being one of the most significant areas for fish stock improvement and development of commercial mariculture, BoB remains an understudied area in the world [25]. To date, reports on HABs from Bangladesh waters have received minimal attention compared to other regions.
T. erythraeum blooms release nitrogen, carbon and other nutrients into the environment, which contributes to nutrient loading [33,34]. Trichodesmium blooms with an alarming incidence of harmful effects on marine biota, especially on coastal fish and shellfish fauna, are frequently reported [35,36,37]. Further reports on cyanotoxin production from Trichodesmium blooms with allergenic symptoms and neurotoxin accumulation are available [38,39,40,41,42,43,44,45,46]. Detoni et al. [37] quantified neurotoxins in Trichodesmium blooms in the open waters of the South Atlantic and also investigated fish and shrimp larvae and plankton community mortality as a consequence of the harmful effects of the blooms. Hence, several studies have been carried out on T. erythraeum blooms mostly during warm weather with high water temperatures (>24 °C) worldwide [8,44,47,48,49].
As before, scarce records of the proliferation of T. erythraeum were found at low water temperatures. In the case of BoB, the bloom occurrence of this genus had not been documented thoroughly in winter. The formation of T. erythraeum blooms is often linked with several environmental factors that trigger the species to dominate upon other phytoplankton. In general, the HAB impacts have grown rapidly in recent decades in the noted BoB, particularly in the coastal waters of Bangladesh responding to population overgrowth, pollution expansion, climatic shifts and coastal development. This could lead to fisheries resource disasters and a comprehensive shock for fishery communities, ecosystem diversities, recreational activities and especially for public health. Based on a new and systematic data set, therefore, this study reports a comprehensive winter bloom of T. erythraeum in the coastal waters of Bangladesh, along with its distribution patterns in relation to environmental factors. This is the first study on Trichodesmium in the Southeastern coastal parts of Bangladesh along BoB.

2. Materials and Methods

2.1. Study Area

BoB, in the Northeastern part of the Indian Ocean, with an average depth of more than 2600 m, occupying an area of about 2,172,000 km2, is bordered in the West by Sri Lanka and India, in the North by Bangladesh, and in the East by Myanmar and Thailand. The Andaman and Nicobar Islands separate BoB from the Andaman Sea, its Eastern arm [25]. The study area (Figure 1) is located in the Bakkhali River Estuary (Lat. 21°28′36.3″N; Long. 91°58′25.5″E) and the Maheshkhali Channel (Lat. 21°28′17.9″N, Long. 91°56′06.8″E) which are situated at the Southeastern coast of BoB, Bangladesh.
Three representative sampling stations were established from each site and illustrated as S1, S2, S3 in the Bakkhali River Estuary (BRE), and S4, S5, S6 in the Maheshkhali Channel (MC). The adjacent land is intensively used for agriculture, settlements, forests, shrimp ponds (locally known as ghers), water bodies and fisheries, salt production, industrial and infrastructure developments, tourism and the management of environmentally important and special areas [50]. Consequently, the Bakkhali River carries a large volume of agricultural, industrial and domestic waste which flows into the Maheshkhali Channel leading to increased nutrient concentrations and algal growth [51]. The Channel itself is remarkable as an extensive fishing ground as well as a popular spot for recreational activities.

2.2. Sample Collection, Enumeration and Identification

Three cruises were conducted at both sites during January (mid-winter, MW), February (late-winter, LW) and March (spring, SP) of 2021. Samples were collected using a conical plankton net (mesh size, 25 µm) during high tide in the daytime. For qualitative plankton study, the plankton net was towed just under the water surface at a speed of approximately 1 m/s for one minute. The collected samples were passed in a bottle from the net and were preserved with a 5% buffered formaldehyde solution in seawater. For the quantitative study, a known volume of 50 L of sub-surface water sample was collected in a plastic bucket and passed through the plankton net (mesh size, 25 µm), and the concentrate was collected from the bucket and preserved in 5% buffered formalin in seawater. The quantitative enumeration of phytoplankton was carried out by counting colonies in a 1 mL Sedgewick-Rafter counting chamber with an Optima Biological microscope (G-206) following the sedimentation method [52]. A subsample (1 mL) was drawn from the concentrated sample and dispensed into the counting chamber, which was scanned thoroughly, ensuring that all colonies were counted. As the extensive bloom of T. erythraeum colonies was observed in samples, they were counted following the dilution technique.
T. erythraeum was identified following the morphological description given by Janson et al. [53] and Aziz and Mohid [54]. There were three major colony types of the species composed of aggregates of several to several hundred trichomes. There are enormous differences in the nature of trichomes of peripheral and interior colonies [54]. Spherical colonies (“puffs”) of T. erythraeum were composed of trichomes radiating in all directions, whereas fusiform colonies (“tufts”) were composed of straight trichomes oriented in parallel [53]. Other mixed forms of asymmetrical colonies were identified with different shapes and trichome aggregates.

2.3. Environmental Factors

Environmental factors were determined from sub-surface water samples. During sampling, water temperature, salinity, dissolved oxygen (DO) and pH were measured using a HI-9829 multi-parameter meter. For the nutrient analysis, the collected surface water samples were filtered on board through glass microfiber filter paper (Whatman GF/C) with the help of a Nalgene hand operated vacuum pump. The filtered water sample was used to determine the concentration of NO3-N. The concentration of NO3-N was determined by using a direct reading spectrophotometer (DR/2010, HACH, Loveland, CO, USA) with high range NitraVer 5 powder pillows (Nitrate Reagent HACH 14034-46 USA) for 25 mL filtered water samples. Phosphate-phosphorus, PO4-P of the filtered water sample was determined by a direct reading spectrophotometer (DR/2010, HACH, USA). PhosVer 3 powder pillows for the 25 mL water sample was used to determine the PO4-P.

2.4. Statistical Analysis

The correlation between Trichodesmium abundance and environmental factors was conducted following Pearson’s correlation method. The canonical correspondence analysis (CCA) was performed to find out the controlling environmental factors of T. erythraeum abundance. A cluster analysis was undertaken to elucidate the similarity of T. erythraeum abundance based on Bray-Curtis. All of the univariate and multivariate analyses were performed using PAST Version 3 (The Paleontological Association, London. UK).

3. Results

3.1. Blooms of Trichodesmium erythraeum

At the end of December 2020, the surface of BRE and MC, BoB, Bangladesh were found to have turned from their normal colour of bluish-green to pale brownish. The brownish discolouration of the water surface was easy to sample because of excellent calm weather with a bright sunny day. From early January, the colour of the bay started to change from brownish to light pinkish along with the sawdust-like thick scums on the surface water (Figure 2). Analysing the samples collected in MW (January 2021), extensive blooms of Trichodesmium erythraeum were observed in all of the studied stations. In BRE, the highest concentrations of T. erythraeum were found at S1 (91.47 ± 52.94 × 103 colonies/L) in LW and S3 (88.40 ± 41.17 × 103 colonies/L) in MW. The highest concentrations of T. erythraeum in MC were observed at S5 (66.93 ± 12.96 × 103 colonies/L) in MW with the second-highest during LW at S6 (64.93 ± 12.50 × 103 colonies/L). In general, the winter bloom-forming condition of T. erythraeum sharply decreased in SP with very low concentrations of colonies at each station (Figure 3).

3.2. Morphological Variations among T. erythraeum Colonies

A microscopic study of the phytoplankton samples collected from all six stations revealed that the brownish-thick scums observed at the water surface were caused by heavy blooms of T. erythraeum. The three distinct morphological shapes, namely “puffs”, “tufts” and “asymmetrical” colonies that differed in MW, LW and SP were identified (Figure 4). The maximum composition of puffs (35% at S4 and S6) and asymmetrical colonies (51% at S4) occurred in MW (Figure 5). Tuft colonies showed their composition of > 50% at all the stations during spring and <50% composition (except at S3 and S6, LW) during winter. The maximum composition of tufts (75%, SP) was found at S6. In most of the stations, puff colonies appeared in a lower percent composition than the two other morphologies. No asymmetrical colonies were recorded at S6 in SP. On average, puff and asymmetrical colonies showed their highest percent composition during both MW and LW and tuft colonies during SP.

3.3. Relationship between T. erythraeum Colonies and Environmental Factors

A strong relationship was observed between T. erythraeum colonies and environmental factors in the present study. Puff, tuft and asymmetrical colonies showed a very strong negative correlation (p < 0.01) with temperature (r = −0.870, −0.809, −0.854 for puff, tuft and asymmetrical colonies, respectively) and a positive correlation with salinity (r = 0.822, 0.779, 0.816 for puff, tuft and asymmetrical colonies, respectively). Colonies were found to be significantly (p < 0.01) positively related to pH and negatively to DO. Tuft colonies exhibited a correlation (p < 0.05) with PO4-P that was significantly different from puff and asymmetrical colonies. The water temperature and salinity values, however, differed in the bloom (MW, LW) and non-bloom (SP) periods of T. erythraeum (Figure 6). pH maintained a value of 7.83 ± 0.25 to 8.04 ± 0.03 with lower colonial abundance in SP (Figure 7). Dissolved oxygen concentration in the bloom period varied from 6.70 ± 0.1 to 8.40 ± 0.1 mg/L, while in the non-bloom period, it remained above 8.40 ± 0.16 mg/L except S1 (8.22 ± 0.03 mg/L). Although NO3-N concentration did not differ significantly between winter and spring, a station-wise variation in PO4–P values was noticed differently in MW, LW and SP (Figure 8). Three peaks of PO4–P values were measured during the bloom-forming winter months of January (MW, 0.38 ± 0.01 mg/L, 0.34 ± 0.01 mg/L and 0.30 ± 0.01 mg/L) and one peak in February (LW, 0.29 ± 0.04 mg/L).
A canonical correspondence analysis (CCA) was also done between environmental factors and T. erythraeum colonies (Figure 9). Eigenvalues of axis 1 (0.012) revealed 85.29% correlation and axis 2 (0.002) revealed 14.71% correlation between them. Puffs were positively and tufts were negatively correlated with both axes. Temperature and salinity exhibited the most profound impact, while DO, NO3–N and pH exhibited a comparatively lower impact on the abundance of T. erythraeum colonies in conformity with CCA.

3.4. T. erythraeum Assemblage

No significant variation in T. erythraeum abundance was found through cluster analysis among different seasons and stations (Figure 10). At the similarity of 70%, two clusters were found; one cluster contained MW and LW, while SP remained isolated (Figure 10a). At a similarity of 85%, three major clusters were formed among stations (Figure 10b). The large cluster contained three stations (S2, S4, S5), one cluster contained two stations (S1, S6), while another cluster (S3) remained isolated.

4. Discussion

A thick bloom and the accumulation of brownish scum of T. erythraeum colonies in the coastal waters of BoB, Bangladesh were observed during the cruise track in the present study. The bloom onsets indicate a greater significance, as it is the first report from Bangladesh with higher colony densities of the species. The highest T. erythraeum density was 91.47 × 103 ± 52.94 × 103 colonies/L and 66.93 × 103 ± 12.95 × 103 colonies/L at bloom sites of BRE and MC, respectively, during LW and MW. More closely related colonial density in Trichodesmium bloom was previously reported from San Pedro Bay, Leyte, the Philippines in April 2013 [48]. Morphological variation was also found in T. erythraeum species which are differed by puffs, tufts and asymmetrical colonies. Supporting evidence can be drawn from the findings of Gradoville et al. [14], who reported spherical “puffs”, fusiform “tufts” and “mixed” morphologies of Trichodesmium colonies in the North Pacific Subtropical Gyre.
Carpenter [55] articulated that temperature has long been acknowledged as a key factor that regulates Trichodesmium abundance. There are uncertainties over the temperature range that influence Trichodesmium blooms, and the present results support this uncertainty. In this study, an inverse relationship was found between water temperature and T. erythraeum abundance. While studying the influence of environmental settings on the prevalence of Trichodesmium, Hegde et al. [56] also found a non-correlation between temperature and the abundance of the species. Although Trichodesmium scarcely blooms at <25 °C [9], it was found to dominate between 23.04 to 23.96 °C at all the studied stations in BRE and MC. Sahu et al. [57] also reported the dominance of T. erythareum at <24 °C in the coastal waters of the northwestern BoB during winter (January 2015). In an earlier study in the Southwestern South Atlantic Ocean, the lower surface seawater temperature (ranging between 24 °C and 25.2 °C) was observed during the toxic Trichodesmium bloom [37]. The bloom was more predominant when the temperature was relatively low, whereas a decline in the bloom coincided with a temperature rise. Thus, it can be assumed that temperature may be a significant factor in the onset of T. erythraeum blooms in the coastal waters of Bangladesh.
The present observed salinity resembles a highly significant (p < 0.01) positive correlation with the T. erythraeum bloom. Jiang et al. [8] and Zhang et al. [47] also determined a similar positive correlation between them in the East China Sea which was influenced by river plumes. The winter maximum in Trichodesmium abundance in this observation was associated with the ranges of higher salinity (33–35‰) values which were similarly attributed to the experimental studies by Bell et al. [58]. Salinity seems to play a crucial role in the Trichodesmium distribution in large estuaries and adjacent shelves [8]. The coastal waters of BoB, Bangladesh experiences huge freshwater influxes through rivers and adjacent areas during monsoon months that reduces the surface water salinity, but in winter when normally no rain happens, the freshwater influx is at its minimum, allowing salinity to increase, and that along with other factors might have favoured Trichodesmium proliferation.
Again, in BRE and MC, the colony density of T. erythraeum was highest when pH ranged between 8.1 to 8.3, agreeing with the findings of Kumar et al. [59], who reported similar ranges of pH values during the occurrence of the bloom of the same species in the Andaman Sea. Trichodesmium exhibited a significant response to changes in pH [60]. Dao et al. [61] reported that the cyanobacterial biomass seemed to be positively correlated (p < 0.01) with pH, which supports the present findings of T. erythraeum. pH within the water column often increases during cyanobacterial blooms, as the organisms remove carbon dioxide and bicarbonate from the water through photosynthesis. During blooms, higher DO values due to the photosynthetic release of oxygen by the highly-dense algal biomass have been reported elsewhere [9]. With a range of DO values from 6.77 to 8.40 mg/L during winter, a significant negative correlation (p < 0.01) was observed between T. erythraeum and DO in this study. Okogwu and Ugwumba [62] found that cyanobacteria are negatively correlated (p < 0.01) with dissolved oxygen content, which may be attributed to the high degradation of cyanobacteria subsequent to blooms leading to dissolved oxygen depletion. Therefore, the decomposition of Trichodesmium in the water column causes a high biological oxygen demand (BOD).
Trichodesmium growth is further linked with some nutrients. The colony abundances of the species were positively related to phosphate-phosphorus and had no significant relationship with nitrate-nitrogen. This finding is similar to those observed in Protoperidinium divergens blooms in the Southeastern coastal waters of BoB [63]. Being a nitrogen-fixing cyanobacterium, Trichodesmium holds competitive precedence to dominate in nitrogen-poor waters [64], where other studies have displayed phosphorus availability as a prerequisite for growth [65]. The highest value of PO4-P was found at 0.38 ± 0.01 mg/L during the peak bloom period in MW. The lower soluble phosphorus concentration from late winter was observed due to most of the soluble phosphorus having already been taken up and incorporated into the Trichodesmium biomass, leaving little remaining in the water column. Phosphate can be assumed to be sourced from a large number of industrial effluents, tributaries and bivalve and shrimp farm wastes alongside the coastal waters of BoB.
Coastal environments of Bangladesh are routinely exposed to runoffs from different anthropogenic sources. These abrupt runoffs contain many organic and inorganic nutrients that can shift the phytoplankton community structure to form large blooms of harmful algae. The major distinctive feature of BoB, however, is the tremendous fresh-water influx that it receives from rivers that trigger the seasonality of the phytoplankton regime [66]. In addition to river-sourced nutrients, coastal areas of the Bay are driven by inputs of nutrients from vertical mixing due to coastal currents. It is well known that seasonally reversing monsoon winds significantly affect BoB circulation. The flows of the Ganges and Brahmaputra in Bangladesh are highly seasonal, and heavily influenced by the monsoon rainfall. Both BRE and MC are located in the transporting zone of freshwater to the marine environment, considered the bloom expansion and discharge site. These sampling sites in this study were determined to be highly productive because of excessive nutrients washed in from agricultural lands, industrial wastes, rural and urban sewage and the nearby shrimp and bivalve farms [67], which sometimes induces the growth of many harmful algal blooms [63]. An earlier report claimed that phytoplankton biomass increased especially in the Northern part of BoB due to strong light conditions and nutrient inputs from estuarine mechanisms and river runoff in January-February (the northeast monsoon) [66].
The intriguing feature of Trichodesmium blooms is the abruptness with which blooms appear and disappear [68]. This study shows that T. erythraeum is dominant during winter compared to spring. According to cluster analysis in the winter season, both MW and LW were contained in one cluster, which stated a clear indication of T. erythraeum bloom during the winter season in the study area. However, a fluctuation in water temperature alone is not effective at allowing the preponderance of diazotrophic cyanobacteria in the sea [69]. Phytoplankton abundance patterned different trends at different stations due to variations in environmental factors [70]. This study result supports the hypothesis that a set of environmental factors, all typical for the coastal waters of Bangladesh, demonstrate a favourable condition for the bloom of T. erythraeum. T. eryuthraeum has been a widely studied species throughout tropical and subtropical oceans, but it is not described adequately along the Bangladesh part of the BoB. Therefore, the preponderance of Trichodesmium in low water temperatures drives further work for a better grasp of the monsoon impacts on the ecological behaviour and response of this cyanobacterium. Additional comprehensive field studies covering more sampling stations and frequency will provide a greater understanding of the different harmful algal populations in the coastal waters of the BoB, Bangladesh.

5. Conclusions

Although planktonic blooms have been observed in other studies, the present study was the first large-scale description with a number of important features of T. erythraeum blooms in the coastal waters of the BoB, Bangladesh. A thick bloom and accumulation of brownish scum of T. erythraeum colonies in the surface water were observed throughout the cruise track. This study records a significant winter bloom of T. erythraeum with a comparative analysis of the environmental conditions in spring. The fluctuation in water temperature alone was not effective at allowing the preponderance of diazotrophic cyanobacteria in the sea; other abiotic factors were also favourable for blooms of the species along the coastal waters of Bangladesh. However, a dynamic stretch of T. erythraeum blooms to greater latitudes could have emergent regional and global implications. Unquestionably, the aforementioned observation needs more detailed monitoring in the future to determine the drivers of bloom formation and propagation. Such types of blooms require the continuous monitoring of coastal and open waters to understand the triggering mechanisms behind bloom events, as the Bay is greatly suffering from coastal pollution or anthropogenic nutrification. Hence, the progression of climate change and eutrophication invigorate HAB trends and responses that in turn affect the coastal waters for their respective economic growth.

Author Contributions

Conceptualization: S.K.; Sampling and data analysis: N.A.S., M.M.S., A.A.N., M.A.S. and G.M.; Original draft writing: N.A.S.; Supervision, funding acquisition and project administration: S.K.; Editing and review: S.K., S.A., E.A., S.J.H. and Y.M. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by Bangladesh Fisheries Research Institute (BFRI), Bangladesh, and also funded by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2023R317), Riyadh, Saudi Arabia.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data generated in the present study are available on request from the corresponding author.

Acknowledgments

We are grateful to the staff of the Marine Fisheries and Technology Station of Bangladesh Fisheries Research Institute at Cox’s Bazar for their help during sample collection.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Map showing the six sampling stations in the Bakkhali River Estuary and Maheshkhali Channel on the Southeastern coast of the Bay of Bengal, Bangladesh.
Figure 1. Map showing the six sampling stations in the Bakkhali River Estuary and Maheshkhali Channel on the Southeastern coast of the Bay of Bengal, Bangladesh.
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Figure 2. Surface patches of T. erythraeum blooms observed in the southeastern coastal part of Bangladesh (photo taken on 17 January 2021).
Figure 2. Surface patches of T. erythraeum blooms observed in the southeastern coastal part of Bangladesh (photo taken on 17 January 2021).
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Figure 3. The abundance (mean ± SD; n = 3) of T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
Figure 3. The abundance (mean ± SD; n = 3) of T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
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Figure 4. Microscopic view (10×) of T. erythraeum colonies; (a) puffs, (b) tufts, and (c) asymmetrical.
Figure 4. Microscopic view (10×) of T. erythraeum colonies; (a) puffs, (b) tufts, and (c) asymmetrical.
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Figure 5. Percent composition of different forms of colonies during the blooms of T. erythraeum in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) at different stations.
Figure 5. Percent composition of different forms of colonies during the blooms of T. erythraeum in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) at different stations.
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Figure 6. Influence of temperature and salinity on the abundance of T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
Figure 6. Influence of temperature and salinity on the abundance of T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
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Figure 7. Influence of pH and dissolved oxygen on the abundance of T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
Figure 7. Influence of pH and dissolved oxygen on the abundance of T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
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Figure 8. Influence of nitrate-nitrogen and phosphate-phosphorus on the abundance of a T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
Figure 8. Influence of nitrate-nitrogen and phosphate-phosphorus on the abundance of a T. erythraeum colony in winter and spring (MW = Mid-Winter, LW = Late-Winter, SP = Spring) along the coast of the Bay of Bengal, Bangladesh.
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Figure 9. Canonical correspondence analysis (CCA) between environmental factors and abundance of T. erythraeum colonies along the coast of the Bay of Bengal, Bangladesh.
Figure 9. Canonical correspondence analysis (CCA) between environmental factors and abundance of T. erythraeum colonies along the coast of the Bay of Bengal, Bangladesh.
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Figure 10. Cluster analysis based on Bray-Curtis similarity matrix of (a) three seasons and (b) six sampling stations in the Bay of Bengal, Bangladesh.
Figure 10. Cluster analysis based on Bray-Curtis similarity matrix of (a) three seasons and (b) six sampling stations in the Bay of Bengal, Bangladesh.
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Shaika, N.A.; Alhomaidi, E.; Sarker, M.M.; An Nur, A.; Sadat, M.A.; Awal, S.; Mostafa, G.; Hasan, S.J.; Mahmud, Y.; Khan, S. Winter Bloom of Marine Cyanobacterium, Trichodesmium erythraeum and Its Relation to Environmental Factors. Sustainability 2023, 15, 1311. https://doi.org/10.3390/su15021311

AMA Style

Shaika NA, Alhomaidi E, Sarker MM, An Nur A, Sadat MA, Awal S, Mostafa G, Hasan SJ, Mahmud Y, Khan S. Winter Bloom of Marine Cyanobacterium, Trichodesmium erythraeum and Its Relation to Environmental Factors. Sustainability. 2023; 15(2):1311. https://doi.org/10.3390/su15021311

Chicago/Turabian Style

Shaika, Nowrin Akter, Eman Alhomaidi, Md. Milon Sarker, Abdullah An Nur, Md. Ashfaq Sadat, Sadiqul Awal, Golam Mostafa, Shanur Jahedul Hasan, Yahia Mahmud, and Saleha Khan. 2023. "Winter Bloom of Marine Cyanobacterium, Trichodesmium erythraeum and Its Relation to Environmental Factors" Sustainability 15, no. 2: 1311. https://doi.org/10.3390/su15021311

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