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Nitrogen limitation, toxin synthesis potential, and toxicity of cyanobacterial populations in Lake Okeechobee and the St. Lucie River Estuary, Florida, during the 2016 state of emergency event
Abstract and Figures
Lake Okeechobee, FL, USA, has been subjected to intensifying cyanobacterial blooms that can spread to the adjacent St. Lucie River and Estuary via natural and anthropogenically-induced flooding events. In July 2016, a large, toxic cyanobacterial bloom occurred in Lake Okeechobee and throughout the St. Lucie River and Estuary, leading Florida to declare a state of emergency. This study reports on measurements and nutrient amendment experiments performed in this freshwater-estuarine ecosystem (salinity 0–25 PSU) during and after the bloom. In July, all sites along the bloom exhibited dissolved inorganic nitrogen-to-phosphorus ratios < 6, while Microcystis dominated (> 95%) phytoplankton inventories from the lake to the central part of the estuary. Chlorophyll a and microcystin concentrations peaked (100 and 34 μg L⁻¹, respectively) within Lake Okeechobee and decreased eastwards. Metagenomic analyses indicated that genes associated with the production of microcystin (mcyE) and the algal neurotoxin saxitoxin (sxtA) originated from Microcystis and multiple diazotrophic genera, respectively. There were highly significant correlations between levels of total nitrogen, microcystin, and microcystin synthesis gene abundance across all surveyed sites (p < 0.001), suggesting high levels of nitrogen supported the production of microcystin during this event. Consistent with this, experiments performed with low salinity water from the St. Lucie River during the event indicated that algal biomass was nitrogen-limited. In the fall, densities of Microcystis and concentrations of microcystin were significantly lower, green algae co-dominated with cyanobacteria, and multiple algal groups displayed nitrogen-limitation. These results indicate that monitoring and regulatory strategies in Lake Okeechobee and the St. Lucie River and Estuary should consider managing loads of nitrogen to control future algal and microcystin-producing cyanobacterial blooms.
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Supplementary resources (18)
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... Additionally, much of the P in the lake is legacy phosphorus bound to sediment which, when resuspended, can further increase P concentrations (i.e., internal loading; Moore et al., 1998;Fisher et al., 2005). Because of this increased P loading, primary productivity within Lake Okeechobee has been considered N-limited (Havens, 1995;Havens et al., 2003;Kramer et al., 2018), and periods of increased N loading into the lake have increased cyanoHABs Lapointe et al., 2017;Kramer et al., 2018). ...
... Additionally, much of the P in the lake is legacy phosphorus bound to sediment which, when resuspended, can further increase P concentrations (i.e., internal loading; Moore et al., 1998;Fisher et al., 2005). Because of this increased P loading, primary productivity within Lake Okeechobee has been considered N-limited (Havens, 1995;Havens et al., 2003;Kramer et al., 2018), and periods of increased N loading into the lake have increased cyanoHABs Lapointe et al., 2017;Kramer et al., 2018). ...
... Lake Okeechobee has gained notoriety for its Microcystis-dominated cyanoHABs in the past decade. Until now, characterizations of the cyanobacterial community structure in Lake Okeechobee have been carried out via microscopy (e.g., Havens et al., 1994;Cichra et al., 1995;Beaver et al., 2013), apart from Kramer et al. (2018) which focused on full metagenomic sequencing of the cyanoHAB that occurred in 2016. Thus, few data exist on molecular characterizations of the bacterial/cyanobacterial community structure within Lake Okeechobee and this study is the first of its kind. ...
Lake Okeechobee is a large eutrophic, shallow, subtropical lake in south Florida, United States. Due to decades of nutrient loading and phosphorus rich sediments, the lake is eutrophic and frequently experiences cyanobacterial harmful algal blooms (cyanoHABs). In the past, surveys of the phytoplankton community structure in the lake have been conducted by morphological studies, whereas molecular based studies have been seldom employed. With increased frequency of cyanoHABs in Lake Okeechobee (e.g., 2016 and 2018 Microcystis -dominated blooms), it is imperative to determine the diversity of cyanobacterial taxa that exist within the lake and the limnological parameters that drive bloom-forming genera. A spatiotemporal study of the lake was conducted over the course of 1 year to characterize the (cyano)bacterial community structure, using 16S rRNA metabarcoding, with coincident collection of limnological parameters (e.g., nutrients, water temperature, major ions), and cyanotoxins. The objectives of this study were to elucidate spatiotemporal trends of community structure, identify drivers of community structure, and examine cyanobacteria-bacterial relationships within the lake. Results indicated that cyanobacterial communities within the lake were significantly different between the wet and dry season, but not between periods of nitrogen limitation and co-nutrient limitation. Throughout the year, the lake was primarily dominated by the picocyanobacterium Cyanobium . The bloom-forming genera Cuspidothrix , Dolichospermum , Microcystis , and Raphidiopsis were highly abundant throughout the lake and had disparate nutrient requirements and niches within the lake. Anatoxin-a, microcystins, and nodularins were detected throughout the lake across both seasons. There were no correlated (cyano)bacteria shared between the common bloom-forming cyanobacteria Dolichospermum , Microcystis , and Raphidiopsis . This study is the first of its kind to use molecular based methods to assess the cyanobacterial community structure within the lake. These data greatly improve our understanding of the cyanobacterial community structure within the lake and the physiochemical parameters which may drive the bloom-forming taxa within Lake Okeechobee.
... In recent years, cyanobacterial harmful algal blooms (cyanoHABs) have become a growing problem for Southwest Florida (SWFL). Lake Okeechobee, the largest lake in the Southern United States, and its connecting waterways (the Caloosahatchee River and Saint Lucie River) frequently experience Microcystis blooms [3,4]. Recent cyanoHABs in Lake Okeechobee and both waterways led to the declaration of a state of emergency in 2016 and 2018 [4][5][6][7]. ...
... Lake Okeechobee, the largest lake in the Southern United States, and its connecting waterways (the Caloosahatchee River and Saint Lucie River) frequently experience Microcystis blooms [3,4]. Recent cyanoHABs in Lake Okeechobee and both waterways led to the declaration of a state of emergency in 2016 and 2018 [4][5][6][7]. Nutrient reduction is currently considered the best strategy to reduce bloom events; however, these strategies can take many years to show positive results [7]. The characteristically shallow water depth of these subtropical freshwater systems and reduced stratification periods also leave them more vulnerable to nutrient loading, making nutrient reduction strategies more difficult [8,9]. ...
Harmful cyanobacterial blooms of the toxin-producing Microcystis have become a growing problem for Southwest Florida freshwater bodies. Recently, a 2016 bloom in Lake Okeechobee and a 2018 bloom in the Caloosahatchee River both led to the declaration of a state of emergency for the state of Florida. Fast-acting suppression methods are needed to protect residents and wildlife. Hydrogen peroxide and L-lysine have shown promising results in selectively inhibiting the growth of Microcystis aeruginosa and are more ecologically friendly due to fast degradation in water or the biological enhancement of nontarget organisms, respectively. We further explored the use of hydrogen peroxide, L-lysine, and combined treatments of both chemicals, which have never been tested before, for the rapid suppression of Microcystis. We assessed the susceptibility of seven M. aeruginosa strains and six other phytoplankton (Cyanobium spp., Synechococcus sp., Dolichospermum planctonica, Mychonastes homosphaera, and Chromochloris zofingiensis) commonly found in Florida, and revealed that susceptibility was diverse. All three treatments were effective at inhibiting the growth of M. aeruginosa, mixed treatments (16.7 mg/L hydrogen peroxide: 8 mg/L L-lysine) were most effective with a median growth inhibition ratio of 94.2% on the last day of the experiment, while hydrogen peroxide (16.7 mg/L) (83.8%) and L-lysine (8 mg/L) (78.5%) were less so. We found axenic M. aeruginosa to be significantly more sensitive to hydrogen peroxide when compared with nonaxenic strains (p < 0.01, n = 18). L-lysine was found to be significantly more toxic to M. aeruginosa than other examined cyanobacteria and chlorophyte strains at the end of the experiment (p < 0.001, n = 33), demonstrating its specificity to this cyanobacterium, while hydrogen peroxide and mixed treatments had varying effects on the other tested phytoplankton.
... It is well known that microcystin concentrations can be variable and can change within monthly periods; for example, in Lake Manatee in Florida microcystin concentrations were variable and reached up to 0.47 µg/L over the summer during algal blooms [66]. In Lake Okeechobee, an intense increase in CyanHAB was detected where Microcystis was identified as the dominant phytoplankton species in 2016, when a state of emergency was declared due to the toxicity of the water [67]. In Mexico, microcystins have been reported in concentrations between 4.9 and 78.0 µg/L in water that is used as a drinking source, as recreational areas, or for irrigation [68], in concentrations of >1 µg/L in drinking water sources [69], and in concentrations ranging from 0.2 and 2.4 µg/L in lake water [70]. ...
The eutrophication of freshwater ecosystems allows the proliferation of cyanobacteria that can produce secondary metabolites such as microcystins. The main aim of this study was to explore the occurrence and concentration of microcystin and the mcyA gene in water bodies located in agricultural, urban, and recreational areas in the karst aquifer of the Yucatan peninsula of Mexico (YPM) and to analyze the water quality variables and chlorophyll-a (Chl-a) associated with their presence. Water samples were collected from 14 sites, and microcystin concentrations were quantified using antibody-based ELISA test. Total DNA was isolated from filters and used for PCR amplification of a fragment of the mcyA gene. Amplicons were cloned and sequenced to identify toxin-producing cyanobacteria present in water. Results showed that water bodies had different trophic status based on Carlson’s trophic state index. Dissolved inorganic nitrogen (DIN: NH4+ + NO3− + NO2−) and P-PO43− concentrations were within a range of 0.077–18.305 mg DIN/L and 0.025–2.5 mg P-PO43−/L, respectively, per sampled site. All sampled sites presented microcystin concentrations within a range of ≥0.14 µg/L to ≥5.0 µg/L, from which 21.4% (3/14) exceeded the limit established in water quality standards for water consumption (1 µg/L). The mcyA gene fragment was detected in 28.5% (4/14) of the sites. A total of 23 sequences were obtained from which 87% (20/23) shared >95% nucleotide identity (nt) with the genus Microcystis and 13% (3/23) shared >87% nt identity with uncultured cyanobacteria. No correlation with the presence of the mcyA gene and microcystins was found; however, a positive correlation was detected between microcystin concentrations with pH and Chl-a.
... Heatwaves are considered omens of future climate change, 85,86 particularly in relation to the incidence of harmful cyanobacterial blooms in nutrient-rich lakes. 26,27,87,88 Previous studies have already noted that differences in growth response of cyanobacterial species to warming. 33,89 For example, Microcystis generally displays a high rate of growth accelerated (Q 10 ), which is the acceleration of the growth rate over a 10°C step. ...
Climate change and eutrophication are two environmental threats that can alter the structure of freshwater ecosystems and their service functions, but we know little about how ecosystem structure and function will evolve in future scenarios of climate warming. Therefore, we created different experimental climate scenarios, including present-day conditions, a 3.0 °C increase in mean temperature, and a "heatwaves" scenario (i.e., an increase in temperature variability) to assess the effects of climate change on phytoplankton communities under simultaneous stress from eutrophication and herbicides. We show that the effects of climate warming, particularly heatwaves, are associated with elevated cyanobacterial abundances and toxin production, driven by a change from mainly nontoxic to toxic Microcystis spp. The reason for higher cyanobacterial toxin concentrations is likely an increase in abundances because under the dual pressures of climate warming and eutrophication individual Microcystis toxin-producing ability decreased. Eutrophication and higher temperatures significantly increased the biomass of Microcystis, leading to an increase in the cyanobacterial toxin concentrations. In contrast, warming alone did not produce higher cyanobacterial abundances or cyanobacterial toxin concentrations likely due to the depletion of the available nutrient pool. Similarly, the herbicide glyphosate alone did not affect abundances of any phytoplankton taxa. In the case of nutrient enrichment, cyanobacterial toxin concentrations were much higher than under warming alone due to a strong boost in biomass of potential cyanobacterial toxin producers. From a broader perspective our study shows that in a future warmer climate, nutrient loading has to be reduced if toxic cyanobacterial dominance is to be controlled.
... There are many kinds of HACBs, which are caused by diverse organisms, including toxic phytoplankton and cyanobacteria (blue-green algae). Harmful algal blooms consisting of a variety of algae have been observed in Biscayne Bay, the Indian River Lagoon, the St. Lucie Estuary, Lake Okeechobee, the Florida Keys, and the Caloosahatchee Estuary [175][176][177][178][179]. For example, Phlips et al. [180] investigated the scales of spatiotemporal variability in harmful algal species dispersal in the Indian River Lagoon and observed five potential toxin-producing algal species at bloom level, namely the diatom Pseudo-nitzschia calliantha and the dinoflagellates Pyrodinium bahamense var. ...
Emerging changes in water availability in the U.S. state of Florida have been recognized as a combined result of human perturbations, natural variability, and climate change. Florida is particularly susceptible to the impacts of the sea level rise due to its extensive coastline, low elevation, and lack of topographic relief to promote drainage. Owing to the porous nature of the state’s aquifer systems, saltwater intrusion into coastal areas is an evolving threat. Additionally, anthropogenic intervention has increased the contribution of nutrients and sediments to many lakes, reservoirs, and rivers, subsequently causing eutrophication and sedimentation problems. The state is facing the challenges of ocean acidification head-on since, in many regions, groundwater aquifers are connected to coastal waters, where water circulates from land to sea through the underlying porous limestone. Additionally, as Earth’s atmosphere warms up, extreme weather events are expected to change the environmental fate of contaminants in the aquatic environment, and this, in turn, may impact the type and distribution of contaminants in source waters. This review paper highlights five major emerging themes that are of significance for sustainable long-term management of Florida’s water resources: (i) influences of changing climate on groundwater aquifers; (ii) implications of climate change on eutrophication; (iii) impacts of changing climate on the Everglades; (iv) climate-change influence on runoff and sediment loads; and (v) influence of ocean acidification on coastal water. The findings of this review indicate that, in the future, the changing global climate will likely alter numerous environmental conditions in Florida, and the resulting changes may impact the natural properties of the state’s fresh and coastal waters. The findings are expected to mobilize knowledge in support of the changing climate to assist Floridians to adapt to its effects.
... Seasonal blooms of Pyrodinium bahamense are also common in the northern IRL system and have been associated with elevated concentrations of saxitoxins (Laureano-Rosario et al., 2021). In the southern IRL and St. Lucie Estuary, the toxic cyanobacterium Microcystis aeruginosa can be transported into the system with water discharged from Lake Okeechobee through the St. Lucie C-44 Canal (Kramer et al., 2018) Diatoms in the genus Pseudo-nitzschia have been observed at many locations in the lagoon, although few studies have investigated their taxonomy and ecology within the IRL (Lopez et al., 2021;Phlips et al., 2012;E.J. 2011a. This diatom has the potential to produce the toxin domoic acid (DA) and may represent an emerging threat to the IRL system. ...
The Indian River Lagoon (IRL) spans approximately one-third of the east coast of Florida and, in recent years, has faced frequent harmful algal blooms (HABs). Blooms of the potentially toxic diatom, Pseudo-nitzschia, occur throughout the lagoon and were reported primarily from the northern IRL. The goal of this study was to identify species of Pseudo-nitzschia and characterize their bloom dynamics in the southern IRL system where monitoring has been less frequent. Surface water samples collected from five locations between October 2018 and May 2020 had Pseudo-nitzschia spp. present in 87% of samples at cell concentrations up to 1.9×103 cells mL-1. Concurrent environmental data showed Pseudo-nitzschia spp. were associated with relatively high salinity waters and cool temperatures. Six species of Pseudo-nitzschia were isolated, cultured, and characterized through 18S Sanger sequencing and scanning electron microscopy. All isolates demonstrated toxicity and domoic acid (DA) was present in 47% of surface water samples. We report the first known occurrence of P. micropora and P. fraudulenta in the IRL, and the first known DA production from P. micropora.
Blue-green algae, or cyanobacteria, may be prevalent in our rivers and tap water. These minuscule bacteria can grow swiftly and form blooms in warm, nutrient-rich water. Toxins produced by cyanobacteria can pollute rivers and streams and harm the liver and nervous system in humans. This review highlights the properties of 25 toxin types produced by 12 different cyanobacteria genera. The review also covered strategies for reducing and controlling cyanobacteria issues. These include using physical or chemical treatments, cutting back on fertilizer input, algal lawn scrubbers, and antagonistic microorganisms for biocontrol. Micro-, nano-and ultrafiltration techniques could be used for the removal of internal and extracellular cyanotoxins, in addition to powdered or granular activated carbon, ozonation, sedimentation, ultraviolet radiation, potassium permanganate, free chlorine, and pre-treatment oxidation techniques. The efficiency of treatment techniques for removing intracellular and extracellular cyanotoxins is also demonstrated. These approaches aim to lessen the risks of cyanobacterial blooms and associated toxins. Effective management of cyanobacteria in water systems depends on early detection and quick action. Cyanobacteria cells and their toxins can be detected using microscopy, molecular methods, chromatography, and spectroscopy. Understanding the causes of blooms and the many ways for their detection and elimination will help the management of this crucial environmental issue. Key Contribution: This article focuses on cyanobacteria, looking into their origins, how they spread, and any issues connected to this type of toxicity diffusion in water. Many strategies are also discussed to reduce these risks and secure drinking water.
A preocupação com o crescente aumento de cianobactérias nos reservatórios de abastecimento populacional ocorre devido ao potencial de produção de toxinas no interior de suas células, que ao serem liberadas nas águas bruta ou tratada, ocasionam inúmeros malefícios a saúde humana, ao meio ambiente e a economia. Nesse contexto, o presente estudo objetivou avaliar as correlações entre a ocorrência da cianotoxina denominada Saxitoxina (SXT) e a sua relação com importantes variáveis ambientais no rio Piranhas. As amostragens de água bruta foram realizadas no período de junho a agosto de 2022, e, além da SXT, os seguintes parâmetros foram analisados: cor aparente, turbidez, condutividade elétrica, pH, nitrogênio amoniacal, alumínio, fosforo total, ferro, alumínio e Chl-a. Os parâmetros com correlações significativas foram investigados quanto às suas variações ao longo do tempo elaborando-se gráficos de dispersão descritivos. Os resultados apontam que a SXT foi significativamente correlacionada com cinco parâmetros de qualidade da água, sendo quatro positivamente (cor, turbidez, nitrogênio amoniacal e clorofila-a) e um negativamente (Al), em que as mais fortes aconteceram entre a STX com o nitrogênio amoniacal (0,84) e a clorofila-a (0,77). De forma geral, a densidade da comunidade fitoplanctônica se mostrou bem diversificada, com destaque para a divisão Cyanophyceae, que foi superior em relação as demais divisões, com destaque para o gênero Oscillatoria sp. Os níveis de STX identificados estavam dentro dos limites recomendados pela legislação brasileira, contudo estes valores ainda estabelecem potencial de risco à saúde pública.
Florida's environments are suitable reservoirs for many disease-causing agents. Pathogens and toxins in Florida waterways have the potential to infect mosquito vectors, animals, and human hosts. Through a scoping review of the scientific literature published between 1999 and 2022, we examined the presence of water-related pathogens, toxins, and toxin-producers in the Florida environment and the potential risk factors for human exposure. Nineteen databases were searched using keywords relating to the waterborne, water-based toxins, and water-related vector-borne diseases which are reportable to the Florida Department of Health. Of the 10,439 results, 84 titles were included in the final qualitative analysis. The resulting titles included environmental samples of water, mosquitoes, algae, sand, soil/sediment, air, food, biofilm, and other media. Many of the waterborne, water-related vector-borne, and water-based toxins and toxin-producers of public health and veterinary importance from our search were found to be present in Florida environments. Interactions with Florida waterways can expose humans and animals to disease and toxins due to nearby human and/or animal activity, proximal animal or human waste, failing or inadequate water and/or sanitation, weather patterns, environmental events, and seasonality, contaminated food items, preference of agent for environmental media, high-risk populations, urban development and population movement, and unregulated and unsafe environmental activities. A One Health approach will be imperative to maintaining healthy waterways and shared environments throughout the state to protect the health of humans, animals, and our ecosystems.
Our understanding of drivers of cyanobacterial harmful algal blooms (cHABs) is evolving, but it is apparent that not all lakes are created equal. Nitrogen (N) is an important component of all cHABs and is crucial for cyanotoxin production. It is generally assumed that external nitrogen inputs are the primary N source for cHABs. However, in northern lakes, nitrogen inputs are typically low, and suggests that internal nitrogen cycling, through heterotrophic organic matter decomposition or nitrogen fixation, may play a significant role in cHAB development and sustainment. Using Lake of the Woods as a testbed, we quantified nutrients, cyanotoxins, nitrogen fixation, and the microbial community in the southern extent of the lake. During our temporal study, inorganic nitrogen species (NO 3 ⁻ +NO 2 ⁻ and NH 4 ⁺ ) were either at very low concentrations or below detection, while phosphorus was in excess. These conditions resulted in nitrogen-deficient growth and thereby favored nitrogen fixing cyanobacterial species. In response, nitrogen fixation rates increased exponentially throughout the summer and coincided with the Aphanizomenon sp. bloom. Despite nitrogen limitation, microcystin, anatoxin, saxitoxin, and cylindrospermopsin were all detected, with microcystin being the most abundant cyanotoxin detected. Microcystin concentrations were highest when free nitrogen was available and coincided with an increase in Microcystis. Together, our work suggests that internal nitrogen dynamics are responsible for the dominance of nitrogen fixing cyanobacteria and that additions of nitrogen may increase the likelihood of other cyanobacterial species, currently at low abundance, to increase growth and cyanotoxin production.
Statement of Significance
This study is the first assessment of nitrogen fixation rates and water column 16S rRNA gene amplicon sequencing in Lake of the Woods during a harmful algal bloom season. The aim of this study is to better understand nitrogen dynamics and the microbial ecology of cyanobacterial harmful algal blooms on Lake of the Woods. Result from this study reveal that internal nitrogen cycling via nitrogen fixation may alleviate nitrogen deficiencies, and structure and control the cyanobacterial community and cyanotoxin production. Molecular analysis reveals that cyanotoxins in Lake of the Woods are produced by less abundant cyanobacteria that are limited by nitrogen. This study has significant management implication as agencies continue to mitigate toxic blooms on Lake of the Woods, the largest shoreline lake in the United States. Our work is an important initial assessment and jumping off point for further research on Lake of the Woods when assessing how nitrogen plays a role in bloom formation and toxicity. Submitting to L&O, we believe would allow for the greatest outreach and access to an audience that will continue to build upon our findings. Additionally, submitting with L&O our work will reach beyond the scientific audience, but also reach other parties participating in the mitigation of harmful algal blooms.
Nutrient enrichment is a significant global-scale driver of change in coastal waters, contributing to an array of problems in coastal ecosystems. The St. Lucie Estuary (SLE) in southeast Florida has received national attention as a result of its poor water quality (elevated nutrient concentrations and fecal bacteria counts), recurring toxic Microcystis aeruginosa blooms, and its proximity to the northern boundary of tropical coral species in the United States. The SLE has an artificially large watershed comprised of a network of drainage canals, one of which (C-44) is used to lower the water level in Lake Okeechobee. Public attention has primarily been directed at nutrient inputs originating from the lake, but recent concern over the importance of local watershed impacts prompted a one-year watershed study designed to investigate the interactions between on-site sewage treatment and disposal systems (OSTDS or septic systems), groundwaters, and surface waters in the SLE and nearshore reefs. Results provided multiple lines of evidence of OSTDS contamination of the SLE and its watershed: 1) dissolved nutrients in groundwaters and surface waters were most concentrated adjacent to two older (pre-1978) residential communities and the primary canals, and 2) sucralose was present in groundwater at residential sites (up to 32.0 mg/L) and adjacent surface waters (up to 5.5 mg/L), and 3) d 15 N values in surface water (+7.5 o / oo), macroalgae (+4.4 o / oo) and phytoplankton (+5.0 o / oo) were within the published range (>+3 o / oo) for sewage N and similar to values in OSTDS-contaminated groundwaters. Measured d 15 N values in M. aeruginosa became increasingly enriched during transport from the C-44 canal (5.8 o / oo) into the mid-estuary (8.0 o / oo), indicating uptake and growth on sewage N sources within the urbanized estuary. Consequently, there is a need to reduce N and P loading, as well as fecal loading, from the SLE watershed via septic-to-sewer conversion projects and to minimize the frequency and intensity of the releases from Lake Okeechobee to the SLE via additional water storage north of the lake. These enhancements would improve water quality in both the SLE and Lake Okeechobee, reduce the occurrence of toxic harmful algal blooms in the linked systems, and improve overall ecosystem health in the SLE and downstream reefs.
The present investigation is aimed to understand how a bloom-forming cyanobacterium Microcystis aeruginosa adapts to changing climatic conditions. The cyanobacterium was exposed to stresses of UV-B (2 Wm⁻²) radiation and temperature (45 °C) for desired time intervals. Results showed that both the stresses affect growth and photosynthetic efficiency of M. aeruginosa. More than 50% loss of survival and content of photosynthetic pigments was noted after 4 h treatment of both the above stresses. Such changes were mainly due to the generation of reactive oxygen species which cause damage to proteins, DNA, lipids, and modulation of the membrane stability. An increase in the proline accumulation was noted in the cells which probably negates the harmful effects. In addition, activity of antioxidative enzymes namely, catalase, superoxide dismutase, ascorbate peroxidase, and peroxidase was induced by 1.5–3.0-fold on 3 h of UV-B and temperature treatment indicating their possible role in protection. Interestingly, induction of photoprotective compound, mycosporine-like amino acids (MAAs) were also found under UV-B stress which might be an additional strategy of defense mechanism for the survival of the cyanobacterium. Analysis of photoprotective compound revealed shinorine as the main MAA synthesized by the cyanobacterium.
The Lake Okeechobee and the Okeechobee Waterway (Lake Okeechobee, the St. Lucie Canal and River, and the Caloosahatchee River) experienced an extensive harmful algal bloom within Lake Okeechobee, the St. Lucie Canal and River and the Caloosahatchee River in 2016. In addition to the very visible bloom of the cyanobacterium Microcystis aeruginosa, several other cyanobacteria were present. These other species were less conspicuous; however, they have the potential to produce a variety of cyanotoxins, including anatoxins, cylindrospermopsins, and saxitoxins, in addition to the microcystins commonly associated with Microcystis. Some of these species were found before, during, and 2 weeks after the large Microcystis bloom and could provide a better understanding of bloom dynamics and succession. This report provides photographic documentation and taxonomic assessment of the cyanobacteria present from Lake Okeechobee and the Caloosahatchee River and St. Lucie Canal, with samples collected June 1st from the Caloosahatchee River and Lake Okeechobee and in July from the St. Lucie Canal. The majority of the images were of live organisms, allowing their natural complement of pigmentation to be captured. The report provides a digital image-based taxonomic record of the Lake Okeechobee and the Okeechobee Waterway microscopic flora. It is anticipated that these images will facilitate current and future studies on this system, such as understanding the timing of cyanobacteria blooms and their potential toxin production
Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing phosphorus (P) inputs was the prescribed solution for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the " P-only " paradigm is overgeneralized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both nutrients are important for HAB control. The changing paradigm from P-only to consideration of dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic N and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N and P inputs will help control HABs in some lakes and also reduce N export to downstream N-sensitive ecosystems. Managers should consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum.
Preventing harmful algal blooms (HABs) is needed to protect lakes and downstream ecosystems. Traditionally, reducing phosphorus (P) inputs was the prescribed solution for lakes, based on the assumption that P universally limits HAB formation. Reduction of P inputs has decreased HABs in many lakes, but was not successful in others. Thus, the ?P-only? paradigm is over-generalized. Whole-lake experiments indicate that HABs are often stimulated more by combined P and nitrogen (N) enrichment rather than N or P alone, indicating that the dynamics of both nutrients are important for HAB control. The changing paradigm from P-only to dual nutrient control is supported by studies indicating that (1) biological N fixation cannot always meet lake ecosystem N needs, and (2) that anthropogenic N and P loading has increased dramatically in recent decades. Sediment P accumulation supports long-term internal loading, while N may escape via denitrification, leading to perpetual N deficits. Hence, controlling both N and P inputs will help control HABs in some lakes and also reduce N export to downstream N-sensitive ecosystems. Managers should consider whether balanced control of N and P will most effectively reduce HABs along the freshwater-marine continuum.
Shallow lakes, particularly those in low-lying areas of the subtropics, are highly vulnerable to changes in climate associated with global warming. Many of these lakes are in tropical cyclone strike zones and they experience high inter-seasonal and inter-annual variation in rainfall and runoff. Both of those factors strongly modulate sediment-water column interactions, which play a critical role in shallow lake nutrient cycling, water column irradiance characteristics and cyanobacterial harmful algal bloom (CyanoHAB) dynamics. We illustrate this with three examples, using long-term (15-25 years) datasets on water quality and plankton from three shallow lakes: Lakes Okeechobee and George (Florida, USA) and Lake Taihu (China). Okeechobee and Taihu have been impacted repeatedly by tropical cyclones that have resulted in large amounts of runoff and sediment resuspension, and resultant increases in dissolved nutrients in the water column. In both cases, when turbidity declined, major blooms of the toxic CyanoHAB Microcystis aeruginosa occurred over large areas of the lakes. In Lake George, periods of high rainfall resulted in high dissolved color, reduced irradiance, and increased water turnover rates which suppress blooms, whereas in dry periods with lower water color and water turnover rates there were dense cyanobacteria blooms. We identify a suite of factors which, from our experience, will determine how a particular shallow lake will respond to a future with global warming, flashier rainfall, prolonged droughts and stronger tropical cyclones.
Shallow lakes respond to nutrient loading reductions. Major findings in a recent multi-lake comparison of data from lakes with long time series revealed: that a new state of equilibrium was typically reached for phosphorus (P) after 10–15 years and for nitrogen (N) after
<5–10 years; that the in-lake Total N:Total P and inorganic N:P ratios increased; that the phytoplankton and fish biomass often decreased; that the percentage of piscivores often increased as did the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankto biomass, and cladoceran size. This indicates that
enhanced resource and predator control often interact during recovery from eutrophication. So far, focus has been directed at reducing external loading of P. However, one experimental study and cross-system analyses of data from many lakes in north temperate lakes indicate that nitrogen may play a more significant role for
abundance and species richness of submerged plants than usually anticipated when total phosphorus is moderate high. According to the alternative states hypothesis we should expect ecological resistance to nutrient loading reduction and P hysteresis. We present results suggesting that the two alternative states are less stable than
originally anticipated. How global warming affects the water clarity of shallow lakes is debatable. We suggest that water clarity often will
decrease due to either enhanced growth of phytoplankton or, if submerged macrophytes are stimulated, by reduced capacity of these plants to maintain clear-water conditions. The latter is supported by a cross-system comparison of lakes in Florida and Denmark. The proportion of small fish might increase and we might see higher
aggregation of fish within the vegetation (leading to loss of zooplankton refuges), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. Moreover, lakes may have
prolonged growth seasons with a higher risk of long-lasting algal blooms and at places dense floating plant communities. The effects of global warming need to be taken into consideration by lake managers when setting future targets for critical loading, as these may well have to be adjusted in the future. Finally, we highlight some
of the future challenges we see in lake restoration research.
Cyanobacteria are most often the dominant group of organisms in the phytoplankton of eutrophied freshwater lakes. In the first section the distribution of cyanobacterial populations in the Netherlands was described in relation to depth, chlorophyll concentration and average light climate. Long term dominance in shallow lakes was mainly caused by Oscillatoria like organisms. Microcystis was found to be dominant in deeper lakes. Although the dominance of Oscillatoria was closely related with high chlorophyll concentrations, the relation between the Oscillatoria dominance and the average underwater light climate was not clear. A good explanation could not be given. In the second section the dominance of Oscillatoria was followed during the restoration of three lakes. Here it was found that in two shallow lakes of different depth Oscillatoria disappeared from the epilimnion at a Zeu/Zm value of 0.4. This is in good agreement with competition experiments done in the laboratory. The stratified population of Oscillatoria rubescens disappeared from the metalimnion at a Zeu/Zm ratio of 1.2. This fitted also with the physiological information.
Light-driven vertical migration is critical in dense aggregation of Microcystis aeruginosa during algal blooms under reduced turbulence conditions in natural water bodies. This study examined the vertical migration characteristics of Microcystis aeruginosa in calm water based on a colony migration model with consideration of cell density change, and demonstrated the effects of mucilage fraction, colony size, irradiance intensity, and water turbidity. The results suggest that colonies with larger radii and under higher irradiance usually had a larger daily averaged retention time at the water surface (DRT). In addition to colony size, mucilage was found to be important in changing the vertical migration behavior of Microcystis colonies, in which increasing mucilage volume fraction can increase the migrating velocity of the colony as well as the length of time it remains at the water surface. Increase of light extinction also favors the aggregation of colonies at the surface. An approximate critical value of 2400 µmol photons m² s⁻¹ for maximum irradiance was found for persistent algal bloom development under the given simulation conditions. Extremely small colonies exposed to irradiance below the critical value were not likely to migrate to the water surface to form algal blooms. According to the DRT values, three regions with different ranges of irradiance and colony size were proposed as the critical conditions for algal bloom development. Rough comparisons to field observations suggested that these results were reasonable and meaningful, and have the potential to be applied in real cases following further validation by more detailed investigations.
A bloom of the cyanobacteria, Microcystis aeruginosa occurred in the St. Lucie Estuary during the summer of 2016, stimulated by the release of waters from Lake Okeechobee. This cyanobacterium produces the microcystins, a suite of heptapeptide hepatotoxins. The toxin composition of the bloom was analyzed and was compared to an archived bloom sample from 2005. Microcystin-LR was the most abundant toxin with lesser amounts of microcystin variants. Nodularin, cylindrospermopsin and anatoxin-a were not detected.