Thursday 23 April 2020

Looking for the causes of recurring Cyanobacterial blooms iin Epe Lagoon, Nigeria.

Phytoplankton form the base of aquatic food webs and are important biological component of marine ecosystems and biogeochemical cycles. Their excessive growth can cause significant threat to biodiversity and ecosystem functioning as in the case of toxic Algal blooms.Toxic Algal blooms occur naturally because of increased water temperature and inorganic nutrients (phosphate, ammonium and nitrate). Global reports suggest Algal blooms have increased because of Human activities resulting from elevated nutrient loadings and water flow modifications. Toxic Algal blooms can result in food web alterations and the dominance of introduced species.

In a paper published in the journal Applied Water Science on 16 April 2020, Sandra Chinwendu Akagha and Dike Ikegwu Nwankwo of the Department of Marine Sciences at the University of Lagos, and Kedong Yin of the School of Marine Science at Sun Yat-Sen University discuss the possible causes and consequences of reoccurring Cyanobacterial blooms and dynamics of nutrient and phytoplankton in Epe Lagoon, Nigeria.

Toxic Cyanobacterial blooms can potentially serve as signs or ecological indicators of changes in aquatic ecosystem integrity. It is important to identify and understand what triggers bloom events to enable prediction, surveillance and management strategies for bloom occurrences. The relationships among hydrologic discharge (flushing and residence time), vertical/horizontal salinity and thermal gradients, rainfall and drought, wind and tidal mixing determine the frequency, severity, spatial and temporal extent of bloom events in coastal ecosystems.

Oxygen depletion in aquatic ecosystem can result from Algal blooms. Harmful Algal blooms reduce the aesthetic value of coastal and aquatic environment which causes economic loss from ecotourism, fisheries resources and Human health threats. Human health threats occur from the consumption of Shellfish or Fish contaminated with Cyanobacterial toxins. Shellfish such as Clams, Mussels and Oysters rapidly accumulate Algal toxins in their tissues because they filter large volume of water. Cases of toxic Algal bloom-related neurotoxic Shellfish poisoning, paralytic Shellfish poisoning, amnesic Shellfish poisoning, diarrhetic Shellfish poisoning, and saxitoxin Fish poisoning have been reported.

Blooms can produce toxins (e.g., microcystin) which are released by Cyanobacteria, Diatoms and Dinoflagellates. Extensive study of population dynamics of Cyanobacteria in relation to environmental factors (light, temperature, nutrient, pH, mixing or zooplankton grazing) have been documented. Cyanobacteria can outcompete other Algal groups because of their resistance to zooplankton grazing, buoyancy and nitrogen fixing ability by heterocystous forms. In addition, Cyanobacteria can thrive in carbon dioxide-deficient and low-pH conditions in aquatic ecosystems. The frequency of eutrophic events has increased over the last several decades in many coastal ecosystems where blooms of phytoplankton are particularly affected by riverine discharge. For instance, bloom incidences have been reported in Chesapeake Bay, the northern Adriatic Sea, the Baltic Sea, the Neuse River Estuary, the Pearl River Estuary, the Kopački Rit floodplain, South African inland waters, the Wadden Sea, Eurasian Arctic and Hypoarctic large River Estuaries, and Lake Taihu, China.

In recent years, the south-west coastal Nigeria is one of the fastest developing regions in West Africa. Epe lagoon boarders the eastern section of the largest metropolitan city (Lagos) in West Africa. Lagos has over 21 million inhabitants with associated rapid economic development and coastal degradation by human activities. Epe Lagoon is connected to the Lagos Lagoon and, hence, receives large amounts of anthropogenic nutrients from increased agriculture, fish farming, poorly treated sewage effluent and domestic/industrial waste discharge. In Nigeria, the incidences of Cyanobacteria blooms have been documented over the past three decades. Blooms of Anabaena flos-aquae, Anabaena spiroides, Microcystis aeruginosa, Microcystis flos-aquae and Microcystis wesenbergii have been reported in the Lagos Lagoon, Microcystis aeruginosa in the Iju Ogun River, Microcystis sp. in Kuramo Lagoon, Anabaena flos-aquae in Owo river, Microcystis aeruginosa in Awba Reservoir, Ibadan, Microcystis aeruginosa and Microcystis wesenbergii in Oyan Dam, Ogun State, as well as a proliferation of Oscillatoria sp. in the Lagos Lagoon. These bloom series were reported to cause colouration, anoxia, odour and bad taste of the water in the affected aquatic ecosystems.

In 2019 a  new Cyanobacterial species, Lagosinema tenuis, was documented in the Lagos Lagoon which further highlights the ecological relevance of Cyanobacterial dominance in tropical African Lagoon systems. 

Akagha et al. set out to elucidate: (1) environmental variables and phytoplankton dynamics, (2) nutrient dynamics and eutrophication and (3) annual Cyanobacterial blooms and associated environmental factors in Epe Lagoon through an 18-month investigation of three ecological sites.

Epe Lagoon is a tropical water body with a surface area of 243 km² located at the eastern part of the Lagos Lagoon complex. Epe lagoon is fresh, lotic (has a high water throughput), non-tidal and sandwiched between Lagos and Lekki lagoons. It is connected to the Atlantic Ocean via the Lagos Lagoon year round. Epe Lagoon is influenced by freshwater input from creek and river inflow. Riparian (river bank-dwelling) vegetation such as Bahiagrass, Paspalum orbiculare, Ivory Coast Raffia Palm, Raphia hookeri, Oil Palm, Elaeis guineensis, Golden Leather Fern, Acrostichum aureum, Coconut Palm, Cocos nucifera and the Mangroves, Rhizophora racemosa, and Avicennia nitida, are found along the fringing wetland. Notable fauna found in this area are Amphipods, Oligochaetes, Polychaetes, Isopods, Barnacles, Oysters, Nematodes, Fiddler Crabs and migratory Birds that feed on exposed biota. The bordering wetland has experienced rapid population growth, agricultural and urban development over the past decade. As a result, anthropogenic activities including domestic waste deposition, fishing, indiscriminate sand mining and inland water transportation are increasing in Epe Lagoon. Human influenced sites along the Epe Lagoon include the hydrothermal plant at Egbin and agricultural sites on the bordering wetland at Ikosi. Three sites on the Epe Lagoon (Imope, Ikosi and Egbin) were investigated by Akagha et al. chosen for their ecological uniqueness and bloom incidences.

Map of study area indicating the three sampling sites: Egbin, Ikosi and Imope. Akagha et al. (2020).

Samples of water and phytoplankton were collected monthly for 18 months between November 2012 and April 2014, in the morning. Water samples were collected in a labelled 1 litre plastic bottle with screw cap and transported in a cooler with ice to the laboratory for chemical analysis. Three 250 ml amber bottles were used to collect water samples for analysis of dissolved oxygen, biological oxygen demand and chlorophyll a. Samples for dissolved oxygen were fixed with Winkler reagent. Phytoplankton samples were collected with a 35 μm plankton net tied unto a motorized boat and towed at low speed (4 knots) for 5 min. The plankton net was hauled in and the samples were emptied into a 500 ml labelled plastic container with screw cap and fixed with 4% unbuffered formalin.

Rainfall ranged between 0 and 413.6 mm during the study period. In January 2013 (Bloom Episode I), there was no rainfall, but this increased to 80.1–132.7 mm in January and February 2014 (Bloom Episode II). Water temperature oscillated between 23 and 34°C with the minimum recorded in August 2013. Total suspended solids and total dissolved solids were between (4–3166 mg per litre) and (30–5290 mg per litre), respectively. pH was 6.1–7.7, indicating acidic conditions, while salinity was in the range of 0–5‰. Dissolved oxygen, biochemical oxygen demand and chemical oxygen demand were in the range of 2–15 mg per litre, 0.5–13 mg per litre and 15–160 mg per litre, respectively There were no significant differences in the environmental variables across all the sampling sites except for total suspended solids which was significantly different across the sampling sites.

Chlorophyll a ranged from 1 μg per litre (below the limit of detection) at Imope in February 2013 to 201 μg per liter at Ikosi in February 2014. Chlorophyll a values were higher (at least 55 μg per litr) in the dry months (November 2012–January 2013) at the three sites during the first annual cycle, and higher values (at least 153 μg per litr) were recorded at Ikosi during the dry month (January 2014–February 2014) of the second annual cycle.

A total of 116 phytoplankton species belonging to 54 genera were recorded during the study, and there were six phytoplankton groups including Diatoms, Bacillariophyceae (40.7%), Green Algae. Chlorophyceae (30.1%), Golden Algae, Chrysophyceae (0.89%), Cyanobacteria, Cyanophyceae (20.4%), Dinoflagellates, Dinophyceae (0.89%) and Euglenids, Euglenophyceae (7.08%). The Centric Diatom, Aulacoseira granulata, was the dominant species except in January 2013, 2014 and February 2014 (dry season I and II) when there was an incidence of Cyanobacterial bloom. At Imope, the lowest Diatom abundance (48%) was recorded in April 2013, while Cyanobacterial abundance was highest (22–47.0%) in May, June and July 2013. At Ikosi, the lowest Diatom abundance (37.0%) was recorded in May 2013, while Cyanobacterial abundance peaked (at least 98.0%) in January 2013, January 2014 and February 2014. Diatoms were lowest (5.0%) in April 2013, while Cyanobacteria were highest (at least 36.0%) in April and May 2013 at Egbin. The prevalent bloom species were Anabaena circinalis, Anabaena flos-aquae, Anabaena limnetica and Anabaena spiroides.

The Centric Diatom, Aulacoseira granulata, was the dominant phytoplankton species in the Epe Lagoon throughout most of Akagha et al.'s study. Diatoms of North America.

Twenty-three Cyanobacterial taxa were recorded. Species richness was observed in the Order Nostocales with 10 taxa, Chroococcales with 5 taxa, Oscillatoriales with 4 taxa, Spirulinales with 2 taxa and Synechococaccales with 2 taxa, respectively. Two Cyanobacterial bloom episodes were observed at Ikosi, Epe Lagoon, during the two annual cycles of the study when rainfall was low. The first bloom began in January and ended in February 2013, while the second bloom started in January 2014 and ended in March 2014. There was notable increase in phytoplankton density (over 40 550 cells per millilitre) and the dominance of Cyanobacteria (to over 98% of all cells). During the first bloom episode, Anabaena circinalis (30 000 cells per millilitre, 73.0% of all cells) and Anabaena limnetica (10 000 cells per millilitre, 24% of all cells) were dominant, but the bloom collapsed in February 2013. Conversely, during the second bloom episode, Anabaena flos-aquae (40 000 cells per millilitre, 99.0% of all cells) was dominant. Anabaena flos-aquae (15 050 cells per millilitre, 27% of all cells), Anabaena circinalis (30 000 cells per millilitre, 53% of all cells) and Anabaena spiroides (10 000 cells per millilitre, 18% of all cells) were recorded in February 2014 at Ikosi. The bloom collapsed in March 2014. At Ikosi, Cyanobacterial cells were at least 98.0% of the total phytoplankton density which was higher (2000-40,000 cells per millilitre) than the Alert Level 1 (over 2000 cells per millilitre) for raw waters during the two bloom episodes. The Cyanobacteria bloom did not reach Alert Level 2 (over 100 000 cells per millilitre) at the sampling sites during the study period.

The Cyanobacterium Anabaena circinalis reached 73% of all the recovered phytoplankton cells in the Epe Lagoon in January 2013, and  53% of all cells at the Ikosi testing station in February 2014. Phytoplankton Identification.

Seasonality and distribution of terrestrial and aquatic organisms in the tropics are determined by rainfall patterns. Rainfall is a driving force in aquatic ecosystems of south-west, Nigeria because it influences flow rate, mixing, dilution and nutrient recycling. The observed lowest temperature (27 °C) in the wet season (Late Wet I) might be attributed to the influx of cooler flood waters from wetlands, adjoining creeks and rivers. These assertions confirm temperature regimes in coastal lagoons of south-west, Nigeria. The low total suspended solids and total dissolved solids in the wet months agree with previous studies of coastal waters of south-west Nigeria. Flood water intrusion causes dilution, high flushing rate and low retention time during the rainy season, which is different from the situations in the dry season and the onset of the rainfall. pH indicated an acidic condition in certain occasion (July 2013) which could be attributed to the seepage of humic and fulvic acid exudates from surrounding wetlands. The slightly acidic to neutral nature of Epe lagoon probably accounted for the dominance of Aulacoseira granulata at Epe during the off-bloom season. Proliferations of Aulacoseira granulata have previously been associated with low pH in studies of Central African lakes. The observed pH remained within the acceptable limits of 6.3–8.5 for inland waters. At Ikosi, a pH range between 6.7 and 7.4 was found to correspond to the period of high prevalence of Anabaena flos-aquae, Anabaena circinalis, Anabaena limnetica, Anabaena sphaerica and Anabaena spiroides.

Dissolved oxygen is dependent on water temperature and decreases as water temperature increases. Photosynthesis, respiration and other hydrological dynamics such as river influx, flushing rate and mixing influence dissolved oxygen levels in aquatic ecosystems. In Akagha et al.'s study, dissolved oxygen and biochemical oxygen demand were relatively low especially during the bloom episodes in the dry season. The relatively higher dissolved oxygen and biochemical oxygen demand values that were observed during the rainy season might be due to mixing of surface water with atmospheric oxygen by current, wave action and river inflow. Previous studies have observed high dissolved oxygen levels attributed to current and wave action in a study of the Bonny River in Niger Delta.

It has been suggested that in lotic fresh waters biochemical oxygen demand between 6.0 mg per litre and 8.0 mg per litre indicate moderate pollution while values greater than 8.0 mg per litre indicate severe pollution. Biochemical oxygen demand values at Epe Lagoon suggest that Epe Lagoon is moderately to highly polluted during the wet season, owing to influx of land-originated pollutants. The acceptable limit of biochemical oxygen demand set by the World Health Organisation for international water quality standards is 15.9–37.5 mg per litre with warning limit of 18.9–34.9 mg per litre. An increase in the amount of organic materials in aquatic ecosystems results in high levels of chemical oxygen demand. Chemical oxygen demand values were high at Ikosi in January 2013 and February 2014, Imope in March 2014 and Egbin in April 2014. The observed higher values of chemical oxygen demand at these sites during this period may be due to the collapse of the bloom and horizontal mixing of the surface water. There was no significant difference in water temperature, pH, salinity and total disolved solids across all the sampling sites, whereas significant difference was observed for total suspended solids which was high at the onset of the rainy season.

Nutrient concentrations and variations in stoichiometric nutrient ratio (silicon/nitrogen, nitrogen/phosphorus and silicon/phosphorus) are influenced by rainfall, human activities, internal nutrient cycling and regeneration (e.g. denitrification, nitrogen fixation). The rate of nitrogen and phosphorus cycling through sediment influences nitrogen or phosphorus limitation in aquatic ecosystems. In freshwater and coastal marine ecosystems, nitrogen is removed in sediments through denitrification. Phosphorus is readily released from sediments through mineralisation. In tropical waters of west Africa, nitrogen concentrations are mostly introduced through anthropogenic sources. The reduction of nitrogen and/or phosphorus inputs into aquatic systems can improve the quality of the water. Nutrient reduction effort can be challenging especially when it is introduced into the aquatic ecosystem through non-point sources such as agricultural run-off. The improvement of water quality may be slowed by internal loading of nutrients from sediments after external loading of nutrients is reduced. Nutrient stoichiometry elaborates the role of resource availability in aquatic ecosystems as well as resource competition in phytoplankton. The nitrogen/silicon/phosphorus ratio of marine Diatoms is about 16/16/1, and deviations from this ratio may result in nutrient limiting for phytoplankton. Silicate was limiting at the study sites, possibly resulting from the absence of tidal sea water influence in Epe Lagoon, introduction of organic nitrogen and phosphates by flood water and agricultural run-off. Nutrient-laden influx from agricultural practices could result in increased levels of nitrate and phosphate, as well as relatively low or declining silicate concentrations. Previous studies have attributed higher silicate values in the dry season to the cessation of flood water discharge and the influx of tidal sea water into the brackish Lagos Lagoon. In this study, silicate values were higher than the threshold value at Imope in February 2013 and at Ikosi in March 2013. These values coincided with the lower Cyanobacterial biomass at Imope in February 2013, the collapse of a Cyanobacterial bloom and the proliferation of Diatoms (84.38% of all cells) at Ikosi in March 2013 after the bloom regime.

A fishing boat on Epe Lagoon. Nigerian Tribune.

An increase in nitrate levels during the wet season could be attributed to the introduction of floodwaters that contained organic materials from adjoining wetland and nutrient rich agricultural run-off due to poor farm practices. Of all the five seasonal partitions, a low nitrogen to phosphorus ratio was recorded during the Dry Season I (November 2012–February 2013), Early Wet Season I (March 2013–June 2013) and Early Wet Season II (March 2014–April 2014) during this study. The observation during the dry season I might be a consequence of faster utilisation of nitrate by phytoplankton in aquatic environments, while a low nitrogen to phosphorus ratio during Early Wet Season I and Early Wet Season II might be a sign of the effect of high flushing rate and low retention time during this period. The nitrogen to phosphorus ratio was high at the three sites during the Late Wet Season I (July 2013–October 2013) and Dry Season II (November 2013–February 2014). 

Rainfall values were high during Late Wet Season I which caused intense nutrient-laden flood water intrusion into the lagoon. However, the absence of rainfall in August 2013 possibly slowed flushing and increased residence time of the lagoon during this period. Flood water inflow influenced eutrophication in coastal waters because nitrate and phosphate from agricultural run-offs (fertilisers) are released. The resulting effect is the change in nitrogen/phosphorus, silicon/phosphorus and silicon/nitrogen leading to potential silicate limitation because of the increased nitrate and phosphate concentrations. The Dry Season II period was associated with reduced volume of freshwater inflow from adjoining river, less perturbation stress related to mixing, reduced flushing and higher residence time in the lagoon. This situation probably explained the high nitrogen/phosphorus ratio values and Cyanobacterial bloom during this period. Relationships between riverine nutrient input, land based flood water and eutrophication in coastal waters have previously been reported in the Chesapeake Bay, the northern Adriatic Sea, some areas of the Baltic Sea, the Mississippi River, the Neuse River Estuary, the Pearl River Estuary. and the Kopački Rit floodplain.

Sources of sulphate in aquatic environments can be either natural or anthropogenic. Industrialization, burning of fossil fuel and agricultural practices contribute to sulphate availability in coastal ecosystems. Sulphate enters aquatic ecosystems through leachates from soil, precipitation, petroleum spill and ammonium sulphate fertilisers. Phytoplankton utilise sulphate for physiological and metabolic processes, although little is known about its role in phytoplankton blooms. Sulphate concentrations regulate the flux of phosphorus from sediment to an extent. At low sulphate levels, phosphorus is usually adsorbed to sediment while high sulphate levels support the release of phosphorus to the water column. Sulphate concentrations in Akagha et al.'s study were generally higher in the dry than the wet season because of the reduced flood water influx and longer retention time during the dry season. The presence of petroleum products in this lagoon could be the source of observed sulphate.

Chlorophyll a followed a seasonal trend and was closely related to phytoplankton abundance. Similarly, a previous study reported that low chlorophyll a value implied limited phytoplankton growth in a turbid Mexican lagoon. The rise in chlorophyll a during the dry season may be related to increased insolation, photosynthetic depth, retention time as well as less perturbation stress from flood waters. Flushing of planktonic microalgae by flood could lead to the low chlorophyll a values in the rainy season. A single major rainfall peak in the dry season (January–February) and a minor peak in the late rainy season (August–November) were documented at Ikpoba reservoir, Edo state, Nigeria.

The dynamics and seasonality of chlorophyll a pointed to the interplay between phytoplankton growth and loss rate which are caused by multiple mechanisms in aquatic ecosystems. Phytoplankton population dynamics are dependent on changes in the proportion of dissolved silicon, nitrogen and phosphorus. It has been hypothesised that decreasing silicon to nitrogen ratio may increase eutrophication by reducing the potential for Diatom growth in favour of harmful phytoplankton species. Long-term decline in the silicon to phosphorus ratios have been responsible for significant blooms of non-siliceous Algae in coastal waters worldwide.

The phytoplankton successional pattern at Ikosi showed a clear transition from dDiatom to Cyanobacterial dominance in the dry season during the two annual cycles. The coincidence of the Cyanobacterial bloom in the dry season could be attributed to higher insolation, water temperature, photosynthetic depth and increased stability of water. Anabaena catenula, Anabaena circinalis, Anabaena limnetica and Anabaena spiroides which are nitrogen-fixing Cyanobacteria dominated the phytoplankton community during the bloom episode in the Dry Season I and II at Ikosi. The bloom episode during the dry season I coincided with a low nitrogen to phosphorua ratio, possibly due to improved water stability and phosphorus being more available as a result of biogeological process and mineralisation. The prevalence of nitrogen fixing Cyanobacteria is a response to nitrogen limitation in aquatic ecosystems. In Akagha et al.'s study, higher nitrogen concentrations during the bloom episode in the Dry Season II may suggests an input from the surrounding wetland. In the tropics, rainfall controls physical, chemical and biological dynamics in aquatic ecosystem. Consequently, this might be the reason for the low nitrogen level during Bloom Episode I (January 2013) when there was no rainfall. However, rainfall values (80.1 mm) accounted for the nitrate availability during Bloom Episode II (January and February 2014) which lasted longer because of the associated nutrient-laden river influx.

The Cyanobacterium Anabaena flos-aquae comprised 27% of phytoplankton cells sampled in the Epe Lagoon in February 2014. US Environmental Protection Agency/Wikimedia Commons.

Despite the increased stability of the water column during Bloom Episode I when there was no rainfall, the bloom duration was shorter than Bloom Episode II. This suggests that in addition to optimal environmental conditions (light, temperature, stability etc.), riverine and land-based nutrient influx might be an important factor that influenced the bloom duration in the tropical aquatic environments.

A visible greenish colouration was seen on the surface water at Ikosi on three sampling occasions. The two Cyanobacterial bloom regimes occurred in the dry season and were related to environmental conditions. The duration of the bloom was determined by the rainfall pattern which controlled nutrient influx, flushing rate and residence time. Higher insolation and improved stability of the lagoon during the dry months probably favoured the proliferation of nitrogen fixing Cyanobacteria. Domestic waste discharge, agricultural run-off and poor sewage systems are sources of organic nutrient enrichment in coastal waters of Nigeria. Coastal lagoons are residual sink for large nutrient loading associated with anthropogenic and agricultural activities. These introductions cause an imbalance in the aquatic ecosystem which alter food chain that lead to loss of biodiversity and productivity. Nutrient loading has led to intensified eutrophication causing reoccurring Algal bloom episodes including harmful species, accrual of organic matter and oxygen depletion (hypoxia and anoxia). In addition to eutrophication, climatic and hydrogeological factors such as rainfall pattern, flood, temperature rise, flow rate, river and storm water discharge influence anoxia and hypoxia conditions in aquatic ecosystems. The complex interactions of these environmental factors determine the magnitude of the temporal and spatial dynamics of Algal blooms and oxygen depletion. The complexity of these systems affects the physical, chemical and biological processes controlling the production and accumulation of organic matter, oxygen dynamics and nutrient cycling. Disolved oxygen depletion (anoxia and hypoxia) is one of the major consequences of eutrophication in coastal waters. The variation between relatively high rates of oxygen consumption and low rates of oxygen introduction results in a decrease in disolved oxygen levels. Very low or hypoxic conditions (disolved oxygen below 2.6 mg per litre) were observed at the three sites in January 2013 and at Ikosi (disolved oxygen below 5 mg per litre) in January 2014 due to the bloom episode and subsequent oxygen utilisation by microbial degradation of organic matter. 

Anabaena catenula, Anabaena circinalis, Anabaena limnetica and  Anabaena spiroides were the bloom species observed in the lagoon. Some of these species have been reported to be potentially toxic. In reference to the alert framework for Cyanobacteria in coastal waters, Alert Level 1 (over 2000 cells per millilitre) was exceeded at Epe Lagoon (Ikosi). Cyanobacterial biomass was high (40 000 cells per millilitre), almost reaching Alert Level 2 (over 100,000 cells per millilitre). These observations call for concern and demand continuous environmental monitoring and management of this coastal lagoon.

The factors regulating the severity of Algal blooms in aquatic ecosystems are complex. In this study, rainfall, anthropogenic activities along bordering wetlands and hydrodynamics are forcing factors which influence phytoplankton The hypoxic conditions observed during the two bloom episodes reflect the vulnerability of the biological component of this lagoon. Rainfall controlled nutrient dynamics which triggered Cyanobacterial bloom development and influenced the bloom duration. Nitrate, phosphate, sulphate and other environmental factors were related to the seasonality whereas silicate was relatively low. Blooms were dominated by potentially toxic species: Anabaena circinalis, Anabaena flos-aquae, Anabaena limnetica and Anabaena spiroides, and were related to nitrate dynamics. This study provides an important scientific information to the development of tools to assess, manage and mitigate risk of noxious bloom occurrences in tropical lagoons.

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