Eutrophication in Australian Rivers, Reservoirs and Estuaries – A Southern Hemisphere Perspective on the Science and its Implications

Australian science has made rapid advances in the last decade in understanding eutrophication processes in inland waters and estuaries. The freshwater research on which these advances are based was triggered by well-publicised blooms of cyanobacteria during the 1980s and early 1990s, particularly a 1000 km long bloom on the Darling River. In estuaries the study which greatly enhanced our understanding but simultaneously served to stimulate further research into estuarine eutrophication, the Port Phillip Bay Study, was initially designed to address perceived problems of toxicants in the Bay but provided profound insights into drivers for, and ecosystem responses to, eutrophication. Subsequent estuarine research has largely been stimulated by management questions arising from Australia’s increasing coastal development for residential purposes. The research has shown that some of the beliefs extant at the time of the blooms were incorrect. For example, it is now clear that stratification and light penetration, not nutrient availability, are the triggers for blooms in the impounded rivers of southeastern Australia, although nutrient exhaustion limits the biomass of blooms. Again, nitrogen seems to play as important a role as phosphorus does in controlling the biomass of these freshwater blooms. The research has also shown that aspects of eutrophication, such as nutrient transport, are dominated by different processes in different parts of Australia. Many of the biophysical processes involved in eutrophication have now been quantified sufficiently for models to be developed of such processes as sediment-nutrient release, stratification, turbidity and algal growth in both freshwater and estuarine systems. In some cases the models are reliable enough for the knowledge gained in particular waterbodies to be applied elsewhere. Thus, there is now a firm scientific foundation for managers to rely upon when managing algal blooms. Whilst these findings have already been presented to managers and communities throughout Australia, there is still a considerable way to go before they are absorbed into their modus operandi.     

Composition of inorganic and organic nutrient sources influences phytoplankton community structure in the New River Estuary, North Carolina

The New River Estuary, NC, is a nutrient-sensitive, eutrophic water body that is prone to harmful algal blooms. High annual loading from the watershed of varying nutrient forms, including inorganic phosphorus and inorganic and organic nitrogen, may be linked to the persistence of algal blooms in the estuary. In order to evaluate phytoplankton response to nutrient inputs, a series of in situ nutrient addition experiments were carried out during June 2010 to July 2011 on water from an estuarine site known to support algal blooms. Estuarine water was enriched with nutrients consisting of individual and combined sources of dissolved inorganic nitrogen, orthophosphate, urea, and a natural dissolved organic nitrogen (DON) addition derived from upstream New River water. The combined inorganic N and P addition most frequently stimulated phytoplankton biomass production as total chlorophyll a. The responses of diagnostic (of major algal groups) photopigments were also evaluated. Significant increases in peridinin (dinoflagellates), chlorophyll b (chlorophytes), and myxoxanthophyll (cyanobacteria) were most frequently promoted by additions containing riverine DON. Significant increases in zeaxanthin (cyanobacteria) were more frequently promoted by inorganic nitrogen additions, while increases in fucoxanthin (diatoms) and alloxanthin (cryptophytes) were not promoted consistently by any one nutrient treatment. Evaluating the impact of varying nutrient forms on phytoplankton community dynamics is necessary in order to develop strategies to avoid long-term changes in community structure and larger-scale changes in ecosystem condition.     

Spatial and temporal variations of hydrological conditions,nutrients and chlorophyll <Emphasis Type="Italic">a</Emphasis> in a Mediterranean coastal lagoon (Mar Menor,Spain)

The Mar Menor is a sheltered and hypersaline lagoon, with salinity ranges between 38 and 51 psu. The lagoon is threatened by several pressures and in the last decades detrimental impact on the natural community structure and dynamics have increased. In the watershed, agricultural practices are rapidly evolving from extensive dry crop farming to intensively irrigated crops, with increasing loads of nutrient and pollutants to the lagoon. Hydrological conditions, nutrients and chlorophyll a concentrations were analysed in 1997 and 2002–2003 in a grid of 20 stations in the lagoon. Different time scales, from daily to inter-annual, were considered. In the considered periods, the dissolved inorganic nitrogen (DIN) increased whilst phosphate decreased significantly. These contrasting patterns depended upon the increased agricultural loading for DIN and were due to the implementation of the wastewater works for phosphates. In 1997 and 2002, the highest nitrate concentrations were usually found on the west coast of the lagoon, close to the mouths of the main watercourses. In parallel, the lowest concentrations were detected at the inner coastline along “La Manga” sandy bar and “El Estacio” channel. Based on weekly data, correlations between chlorophyll a concentrations and environmental variables disagreed with traditional eutrophication models. Relationships between chlorophyll a and nutrients were negative, suggesting that in the short term phytoplankton controlled nutrient concentrations. Moreover, nitrate and phosphorous seemed to alternate as limiting factors. The relationships between chlorophyll a became positive when considering time lags and analysed at longer time scales (monthly or seasonal means), thus suggesting a very rapid response of primary producers to nutrient enrichment. A significant correlation between chlorophyll a concentration and fish larvae density was also found at all time scales analysed, suggesting a top-down control of the trophic web.     

Temporal dynamics and spatial variation of algae in relation to hydrology and sediment characteristics in the Okefenokee Swamp,Georgia

In a 20-month study, phytoplankton and periphyton chl a, and dry mass of macroscopic algal aggregates in four marshes and a lake within the Okefenokee Swamp (Georgia, USA) were comparable to other wetlands and lake littoral areas. Chlorophyll levels in two marshes were inversely related to water level and phytoplankton at three marshes developed unimodal maxima following macrophyte dieback. Standing stocks in a vernally inundated marsh were greater than a nearby marsh which was permanently inundated; chlorophyll levels displayed longer blooms in the inundated marsh during periods of low rainfall or after drought. Field dynamics, sediment sorption characteristics and algal bioassays suggest that evaporative drawdown stimulates algae by release of nutrients from exposed peat, while high water levels reduce nutrient release from sediments and disperse phytoplankton through flushing. Equilibrium phosphate concentrations of sediments and algal levels were higher at an abandoned rookery than a nearby non-rookery area, indicating nutrient enrichment from residual guano deposits.     

POSITIVE FEEDBACK AND THE DEVELOPMENT AND PERSISTENCE OF ECOSYSTEM DISRUPTIVE ALGAL BLOOMS1

Harmful algal blooms (HABs) have occurred with increasing frequency in recent years with eutrophication and other anthropogenic alterations of coastal ecosystems. Many of these blooms severely alter or degrade ecosystem function, and are referred to here as ecosystem disruptive algal blooms (EDABs). These blooms are often caused by toxic or unpalatable species that decrease grazing rates by planktonic and benthic herbivores, and thereby disrupt the transfer of nutrients and energy to higher trophic levels, and decrease nutrient recycling. Many factors, such as nutrient availability and herbivore grazing have been proposed to separately influence EDAB dynamics, but interactions among these factors have rarely been considered. Here we discuss positive feedback interactions among nutrient availability, herbivore grazing, and nutrient regeneration, which have the potential to substantially influence the dynamics of EDAB events. The positive feedbacks result from a reduction of grazing rates on EDAB species caused by toxicity or unpalatability of these algae, which promotes the proliferation of the EDAB species. The decreased rates also lower grazer‐mediated recycling of nutrients and thereby decrease nutrient availability. Since many EDAB species are well‐adapted to nutrient‐stressed environments and many exhibit increased toxin production and toxicity under nutrient limitation, positive feedbacks are established which can greatly increase the rate of bloom development and the adverse effects on the ecosystem. An understanding of how these feedbacks interact with other regulating factors, such as benthic/pelagic nutrient coupling, physical forcing, and life cycles of EDAB species provides a substantial future challenge.     

The influence of a coastal upwelling event on chlorophyll <Emphasis Type="Italic">a</Emphasis> and nutrient dynamics in the surface layer of the Gulf of Finland,Baltic Sea

Temporal variation and distribution of chlorophyll a and nutrients concentration was evaluated on the basis of field observations in August 2006 in the Gulf of Finland. Strong easterly winds in August 2006 generated an upwelling event along the Estonian coast of the Gulf of Finland. It caused a drop of the water-surface temperature and nutrient enrichment of the upper layer. At first, the chlorophyll a declined in the area affected by the upwelled water due to the strong advective transport of the chlorophyll a rich waters towards the northern coast and due to the intensive water mixing and low seed population in the upwelling waters. After stabilization of the upwelling, nutrients from the upper mixed layer were consumed fast: there were no nitrites + nitrates left one week later, and phosphate concentration was under the detection limit 2 weeks later. The smaller phytoplankton size fraction showed faster response to the upwelled nutrients compared with the bigger size fraction, showing the increase in chlorophyll a content already during the stabilization of the upwelling. The increase in chlorophyll a concentration in >20-μm size fraction at stations influenced by upwelling was observed only after the relaxation of the upwelling and formation of stratification.     

Assessment of the eutrophication status of the Great Barrier Reef lagoon (Australia)

Current scientific consensus is that inshore regions of the central and southern Great Barrier Reef, Australia, are at risk of impacts from increased nutrient (as well as sediment and pesticide) loads delivered to Reef waters. Increases in the discharge of water quality contaminants to the Reef are largely a consequence of the expansion of agricultural practices in northern Queensland catchments following European settlement in the 1850s. In particular, the presence of elevated chlorophyll a and nutrient concentrations in many parts of the inshore Great Barrier Reef together with intense and extensive phytoplankton blooms following the discharge of nutrient-rich river flood waters suggest that the central and southern inshore area of the Great Barrier Reef is likely to be significantly impacted by elevated nutrient loads. The biological consequences of this are not fully quantified, but are likely to include changes in reef condition including hard and soft coral biodiversity, macroalgal abundance, hard coral cover and coral recruitment, as well as change in seagrass distribution and tissue nutrient status. Contemporary government policy is centered around promotion and funding of better catchment management practices to minimize the loss of catchment nutrients (both applied and natural) and the maintenance of a Reef wide water quality and ecosystem monitoring program. The monitoring program is designed to assess trends in uptake of management practice improvements and their associated impacts on water quality and ecosystem status over the next 10 years. A draft set of quantitative criteria to assess the eutrophication status of Great Barrier Reef waters is outlined for further discussion and refinement.     

Increased sea ice melt as a driver of enhanced Arctic phytoplankton blooming

Phytoplankton primary production in the Arctic Ocean has been increasing over the last two decades. In 2019, a record spring bloom occurred in Fram Strait, characterized by a peak in chlorophyll that was reached weeks earlier than in other years and was larger than any previously recorded May bloom. Here, we consider the conditions that led to this event and examine drivers of spring phytoplankton blooms in Fram Strait using in situ, remote sensing, and data assimilation methods. From samples collected during the May 2019 bloom, we observe a direct relationship between sea ice meltwater in the upper water column and chlorophyll a pigment concentrations. We place the 2019 spring dynamics in context of the past 20 years, a period marked by rapid change in climatic conditions. Our findings suggest that increased advection of sea ice into the region and warmer surface temperatures led to a rise in meltwater input and stronger near-surface stratification. Over this time period, we identify large-scale spatial correlations in Fram Strait between increased chlorophyll a concentrations and increased freshwater flux from sea ice melt.     

沿岸海域富营养化与赤潮发生的关系

综述了赤潮的发生与沿岸海域富营养化的关系。近几十年来,人类活动使得天然水体的富营养化进程大大加速。营养负荷的增加与高生物量水华的增多相联系。控制营养输入后,浮游植物生物量或有害藻类水华事件也相应减少。营养的组成与浮游植物的种类组成及水华的形成有密切联系。有机营养对有害藻类水华的促进作用受到关注。营养输入时机影响浮游植物种间竞争的结果,因而对浮游植物的群落演替具有深远影响。由于浮游植物存在生理差异,因而对营养加富的反应因种而异。营养在调控某些有毒藻类的毒素产量方面也发挥着重要作用。此外,营养输入与藻类水华之间存在复杂的间接联系。当然,营养状况并非浮游植物群落演替的唯一决定因素。研究结果提示,控制营养输入、减缓水域富营养化是减少有害藻类水华发生的有效途径,而深入研究典型有害藻类的营养生理对策则为防治并最终消除有害藻类水华提供了理论基础。     

The construction of a diatom-based chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment

1. Canonical correspondence analysis of a diatom and water chemistry dataset from fifty-nine maritime Antarctic lakes situated on Signy and Livingston Islands showed that nutrients and functions of nutrients (NH₄⁺, chlorophyll a) accounted for a significant fraction of the variance in the diatom data. 2. Weighted averaging regression was used to construct a diatom-based transfer function for inferring chlorophyll a concentrations from sediment core diatom assemblages. 3. The transfer function was applied to ²¹⁰Pb-dated sediment cores from three lakes (Moss, Sombre and Heywood) receiving different levels of nutrient input from fur seal populations, i.e. low, medium and high, respectively. 4. Moss Lake showed relatively stable reconstructed chlorophyll a values, and no evidence of recent eutrophication, agreeing with measured chlorophyll a concentrations at the site. 5. Changes in diatom assemblages and results of chlorophyll a reconstructions at Sombre Lake suggested that nutrient enrichment had occurred, which could be clearly linked to fluctuations in the measured water chemistry over the last 10–14 years. 6. Despite recorded increases in recent nutrient inputs there was no apparent diatom response at Heywood Lake.     

Differences in food webs and trophic states of Brazilian tropical humid and semi-arid shallow lakes: implications of climate change

Global warming may intensify eutrophication of shallow lakes by affecting nutrient loading, evaporation rates, and water level and thus produce major changes in food webs. We investigated to what degree food webs in tropical humid lakes differed from those in more eutrophic semi-arid lakes of the same latitude. Our results indicate that the catchment area-to-lake area ratio, nutrients, chlorophyll a, suspended solids, abundances of phytoplankton, zooplankton, and omnivorous fish as well as total fish catch per unit effort were all higher in the semi-arid lakes, whereas inlet water-to-evaporation ratio (proxy for water balance), water transparency, percentage macrophytes cover, and the piscivores:omnivores ratio were higher in the humid lakes. Our results suggest that reduced inlet water-to-evaporation ratio will increase lake eutrophication, which, in turn, as in temperate regions, will alter trophic structure of the freshwater community.

     

Phytoplankton Production at Two Stations in Lindåspollene,a Norwegian Land-locked Fjord,and Limiting Nutrients Studied by Two Kinds of Bio-assays

At two stations in a Norwegian land-locked fjord, one more and one less influenced by freshwater run-off, studies were made of the primary production, chlorophyll a concentrations, and the quantity and composition of phytoplankton. Samples were taken for analyses of phosphate, nitrate, and silicate, and temperature and salinity were measured. Two kinds of bio-assays were performed: combinations of phosphate, nitrate, and silicate were added to I, natural water incubated in situ, and II, natural water incubated in the laboratory. Lower salinities and higher concentrations of all nutrients were found at the station most influenced by fresh-water. Although the bio-assays indicated that nitrogen was the main limiting nutrient, the biological parameters showed no consistent differences between the two stations. The similarity is tentatively explained by the growth of nanoplankton algae, partly depending on nutrients other than those analysed.     

Relating Phytoplankton Dynamics to Environmental Forcing in the Lower St. Johns River Estuary: A Multivariate Assessment

The St. Johns River, a 300‐mile riverine and estuarine system located along the north‐eastern coastline of Florida, has undergone extensive eutrophication through point and nonpoint source nutrient inputs. Moreover, recent reports of sudden fish kills and a high incidence of fish with lesions suggest the potential for harmful algal blooms. As part of a NOAA‐funded project involving the characterization of water quality parameters in relation to water inflows and nutrient inputs and the development of species‐specific markers/probes for instrumental‐based monitoring efforts, we used primer‐based multivariate analyses to examine the relationship between abiotic variables and both total and phylogenetic‐group chlorophyll a concentrations (derived from photopigments and ChemTax matrix factorization of diagnostic carotenoids) during 2001. Seven sampling sites (identified through principle components ordination of physical/chemical parameters as ranging from oligo‐ to mesohaline) were sampled intensively over 2‐week periods on a seasonal basis. Chlorophyll a concentrations typically ranged from 5 to 35 µg/L with the greatest concentrations occurring at the oligohaline sites. Phytoplankton assemblages were dominated by diatoms, cryptophytes, and cyanobacteria, and together typically comprised up to 90% of the total chlorophyll a. Temporal variability in phytoplankton assemblages followed seasonal trends impacted by meteorological and hydrological forcing. Spatial variability in phylogenetic‐group abundance (illustrated through multidimensional scalar ordination of sample dissimilarity) was dramatic and associated with differences in abiotic variables along the estuarine gradient.     

A multilevel trait-based approach to the ecological performance of Microcystis aeruginosa complex from headwaters to the ocean

The Microcystis aeruginosa complex (MAC) clusters cosmopolitan and conspicuous harmful bloom-forming cyanobacteria able to produce cyanotoxins. It is hypothesized that low temperatures and brackish salinities are the main barriers to MAC proliferation. Here, patterns at multiple levels of organization irrespective of taxonomic identity (i.e. a trait-based approach) were analyzed. MAC responses from the intracellular (e.g. respiratory activity) to the ecosystem level (e.g. blooms) were evaluated in wide environmental gradients. Experimental results on buoyancy and respiratory activity in response to increased salinity (0–35) and a literature review of maximum growth rates under different temperatures and salinities were combined with field sampling from headwaters (800 km upstream) to the marine end of the Rio de la Plata estuary (Uruguay-South America). Salinity and temperature were the major variables affecting MAC responses. Experimentally, freshwater MAC cells remained active for 24 h in brackish waters (salinity = 15) while colonies increased their flotation velocity. At the population level, maximum growth rate decreased with salinity and presented a unimodal exponential response with temperature, showing an optimum at 27.5 °C and a rapid decrease thereafter. At the community and ecosystem levels, MAC occurred from fresh to marine waters (salinity 30) with a sustained relative increase of large mucilaginous colonies biovolume with respect to individual cells. Similarly, total biomass and, specific and morphological richness decreased with salinity while blooms were only detected in freshwater both at high (33 °C) and low (11 °C) temperatures. In brackish waters, large mucilaginous colonies presented advantages under osmotic restrictive conditions. These traits values have also been associated with higher toxicity potential. This suggest salinity or low temperatures would not represent effective barriers for the survival and transport of potentially toxic MAC under likely near future scenarios of increasing human impacts (i.e. eutrophication, dam construction and climate change).     

Potassium-induced inhibition of nitrogen and phosphorus metabolism as a strategy of controlling <Emphasis Type="Italic">Microcystis</Emphasis> blooms

To explore potassium toxicity in Microcystis sp., growth, chlorophyll a, carotenoid and phycocyanin content, uptake of nitrate, phosphate and ammonium and activities of the assimilatory enzymes nitrate reductase, alkaline phosphatase and glutamine synthetase (GS) were studied. Nitrate, phosphate, ammonium uptakes and chlorophyll a and phycocyanin contents decreased with increase in the concentration of potassium, but carotenoid content registered an increase at increasing potassium concentration. Alkaline phosphatase and GS activities followed the trend of inhibition of their respective nutrients, whereas nitrate and nitrate reductase showed negative correlation (p < 0.01). Potassium was found to inhibit the activities of all the assimilatory enzymes in a non-competitive manner. Inhibitions of these parameters support the view that potassium has the potential to regulate Microcystis blooms in an eco-friendly manner.     

The Status and Characteristics of Eutrophication in the Yangtze River (Changjiang) Estuary and the Adjacent East China Sea, China

Eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the Yangtze River Estuary and in the adjacent East China Sea. In 2003 and 2004, four cruises were undertaken in three zones in the estuary and in the adjacent sea to investigate nitrate (NO₃–N), ammonium (NH₄–N), nitrite (NO₂–N), soluble reactive phosphorus (SRP), dissolved reactive silica (DRSi), dissolved oxygen (DO), phytoplankton chlorophyll a (Chl a) and suspended particulate matter (SPM). The highest concentrations of DIN (NO₃–N+NH₄–N+NO₂–N), SRP and DRSi were 131.6, 1.2 and 155.6 μM, respectively. The maximum Chl a concentration was 19.5 mg m⁻³ in spring. An analysis of historical and recent data revealed that in the last 40 years, nitrate and SRP concentrations increased from 11 to 97 μM and from 0.4 to 0.95 μM, respectively. From 1963 to 2004, N:P ratios also increased from 30–40 up to 150. In parallel with the N and P enrichment, a significant increase of Chl a was detected, Chl a maximum being 20 mg m⁻³, nearly four times higher than in the 1980s. In 2004, the mean DO concentration in bottom waters was 4.35 mg l⁻¹, much lower than in the 1980s. In comparison with other estuaries, the Yangtze River Estuary was characterized by high DIN and DRSi concentrations, with low SRP concentrations. Despite the higher nutrient concentrations, Chl a concentrations were lower in the inner estuary (Zones 1 and 2) than in the adjacent sea (Zone 3). Based on nutrient availability, SPM and hydrodynamics, we assumed that in Zones 1 and 2 phytoplankton growth was suppressed by high turbidity, large tidal amplitude and short residence time. Furthermore, in Zone 3 water stratification was also an important factor that resulted in a greater phytoplankton biomass and lower DO concentrations. Due to hydrodynamics and turbidity, the open sea was unexpectedly more sensitive to nutrient enrichment and related eutrophication processes.     

Molecular approaches to diagnosing nutritional physiology in harmful algae: Implications for studying the effects of eutrophication

Every year harmful algal blooms (HABs) cause serious impacts to local economies, coastal ecosystems, and human health on a global scale. It is well known that nutrient availability can influence important aspects of harmful algae biology and ecology, such as growth, toxin production, and life cycle stage, as well as bloom initiation, persistence and decline. Increases in the rate of supply of organic matter to ecosystems (eutrophication) carries many possible ramifications to coastal systems, including the potential for nutrient enrichment and the potential for stimulation of harmful algal blooms. Traditional studies on algal nutrition typically use either cultured isolates or community level assays, to examine nutrient uptake, nutrient preference, elemental composition, and other metrics of a species’ response to nutrients. In the last decade, technological advances have led to a great increase in the number of sequences available for critical harmful species. This, in turn, has led to new insights with regards to algal nutrition, and these advances highlight the promise of molecular technologies, and genomic approaches, to improving our understanding of algal nutrient acquisition and nutritional physiological ecology, in both cultures and field populations. With these developments increased monitoring of nutritional physiology in field populations of harmful algae will allow us to better discriminate how eutrophication impacts these groups.     

Contrasting responses of intertidal microphytobenthos and phytoplankton biomass and taxonomic composition to the nutrient loads in the Jiulong River Estuary

Microphytobenthos (MPB) and phytoplankton are important primary producers in the estuarial ecosystem, and their functions are critical to the ecosystem's biodiversity and environmental safety. The aim of this study was to compare the response of MPB and phytoplankton to the nutrient loads in a eutrophic estuary, which has seldom been studied. We used high‐performance liquid chromatography (HPLC) and CHEMTAX software to examine the biomass and taxonomic composition of both MPB and phytoplankton at Da‐yu Island (DYI) and Ji‐yu Island (JYI) in the Jiulong River Estuary from July 2010 to March 2012. The results showed that MPB chlorophyll a was low in the summer and high in the winter at both DYI and JYI, indicating a unimodal pattern. However, the phytoplankton chlorophyll a showed a mirrored pattern. Diatoms were the dominant class in both benthic and pelagic environments. Although redundancy analysis indicated that the effects of different environmental factors could not be easily separated, it is likely that phosphate and temperature were the most important factors regulating the seasonal patterns of MPB and phytoplankton diatoms, respectively. MPB and phytoplankton cyanobacteria was co‐limited by salinity and temperature. The high N/P ratio and low phosphate favored chlorophytes and cyanobacteria. Our study demonstrates the use of HPLC and CHEMTAX in an integrated survey of the spatial and temporal distribution patterns of MPB and phytoplankton in an estuarial ecosystem. The contrasting responses of MPB and phytoplankton to nutrient loads indicate the critical role of MPB in subtropical estuarial ecosystem function. The relationship between nutrients and MPB may indicate a significant contribution to carbon and nutrient cycling.     

The Environment of Lake Victoria (East Africa): Current Status and Historical Changes

Beginning in the mid‐1980s Lake Victoria experienced severe eutrophication and it was suggested that deteriorating water quality might lead to a collapse of its fisheries. A series of lake‐wide surveys carried out 1999–2001 and 2005–2009 revealed that the temperature of the lake had risen by > 1 °C since 1927, with more rapid warming of the deeper waters reducing the thermal gradient in the water column and thus weakening stratification and the extent and severity of deoxygenation. The chlorophyll a concentrations in open water decreased since the 1980s, while Secchi disc visibility increased, indicating a reduced severity of algal blooms. Chlorophyll a was higher and Secchi disc visibility lower in inshore waters but there has been no deterioration in these areas since the 1980s. The conductivity remained unchanged, although it was about 50% greater in the semi‐enclosed Nyanza Gulf than in the open lake. The water quality of the lake has therefore improved considerably despite the fact than concentrations of plant nutrients have not decreased and the reasons why this may be the case are discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Predicting cyanobacterial dynamics in the face of global change: the importance of scale and environmental context

There is growing concern that harmful cyanobacterial blooms are increasing in frequency and occurrence around the world. Although nutrient enrichment is commonly identified as a key predictor of cyanobacterial abundance and dominance in freshwaters, several studies have shown that variables related to climate change can also play an important role. Based on our analysis of the literature, we hypothesized that temperature or water‐column stability will be the primary drivers of cyanobacterial abundance in stratified lakes whereas nutrients will be the stronger predictors in frequently mixing water bodies. To test this hypothesis, as well as quantify the drivers of cyanobacteria over different scales and identify interactions between nutrients and climate‐related variables, we applied linear and nonlinear mixed‐effect modeling techniques to seasonal time‐series data from multiple lakes. We first compared time series of cyanobacterial dominance to a published lake survey and found that the models were similar. Using time‐series data of cyanobacterial biomass, we identified important interactions among nutrients and climate‐related variables; dimictic basin experienced a heightened susceptibility to cyanobacterial blooms under stratified eutrophic conditions, whereas polymictic basins were less sensitive to changes in temperature or stratification. Overall, our results show that due to predictable interactions among nutrients and temperature, polymictic and dimictic lakes are expected to respond differently to future climate warming and eutrophication.