Combining a regional climate model with a phytoplankton community model to predict future changes in phytoplankton in lakes

1. Linking a regional climate model (RCM) configured for contemporary atmospheric greenhouse gas concentrations, with a phytoplankton community model (PROTECH) produced realistic simulations of 20 years of recent phytoplankton data from Bassenthwaite Lake, in the North‐West of England. 2. Meteorological drivers were derived from the RCM to represent a future climate scenario involving a 1% per annum compound increase in atmospheric CO₂ concentrations until 2100. Using these drivers, PROTECH was run for another 20 year period representing the last two decades of the 21st century. 3. Comparison of these present and future simulations revealed likely impacts on the current seasonal phytoplankton development. Under future climate conditions, the simulated spring bloom showed an increase in cyanobacteria dominance caused by greater success of Planktothrix. Also, the summer cyanobacteria bloom declined earlier because of nutrient limitation caused by the increased spring growth. Overall productivity in the lake did not change. 4. Analysis showed that these predicted changes were driven by changes in water temperature, which were in turn triggered by the higher air temperatures predicted by the RCM.     

Strong spatial differentiation of N and P deficiency,primary productivity and community composition between Nyanza Gulf and Lake Victoria (Kenya,East Africa) and the implications for nutrient management

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Phosphorus and nitrogen limitation of phytoplankton growth in eutrophic Lake Inba, Japan

Lake Inba is one of the most eutrophic lakes in Japan. In this study, field sampling and nutrient enrichment bioassays were conducted to determine the seasonal patterns of nutrient limitation for phytoplankton growth in this lake. Phytoplankton biomass increased significantly with the additions of phosphorus (P) on almost all sampling dates, indicating P limitation of phytoplankton growth from spring to autumn. However, nitrogen (N) limitation was also observed during summer (i.e., 19 August). On 10 August, a typhoon struck Lake Inba. After this event, dissolved inorganic nitrogen (DIN) and phosphorus concentrations increased, probably because of increased river discharge. At the same time, phytoplankton growth in the control treatment became relatively high, with the addition of neither P nor N stimulating the growth. However, 10 days after the typhoon, the phytoplankton growth rate in the control treatment decreased, with only the addition of N having a significant positive effect on phytoplankton growth. N limitation during summer is caused by the low concentrations of DIN, as well as changes in the N:P ratio due to allochthonous nutrient loads. These results indicate that a reduction of both P and N input is necessary to control phytoplankton blooms in Lake Inba.     

Modelling the effects on phytoplankton communities of changing mixed depth and background extinction coefficient on three contrasting lakes in the English Lake District

1. The process‐based phytoplankton community model, PROTECH, was used to model the response of algal biomass to a range of mixed layer depths and extinction coefficients for three contrasting lakes: Blelham Tarn (eutrophic), Bassenthwaite Lake (mesotrophic) and Ullswater (oligotrophic). 2. As expected, in most cases biomass and diversity decreased with decreasing light availability caused by increasing the mixed depth and background extinction coefficient. The communities were generally dominated by phytoplankton tolerant of low light. Further, more novel, factors were identified, however. 3. In Blelham Tarn in the second half of the year, biomass and diversity did not generally decline with deeper mixing and the community was dominated by nitrogen‐fixing phytoplankton because that nutrient was limiting to growth. 4. In Bassenthwaite Lake, changing mixed depth influenced the retention time so that, as the mixed depth declined, the flushing rate in the mixed layer increased to the point that only fast‐growing phytoplankton could dominate. 5. In the oligotrophic Ullswater, changing the mixed depth had a greater effect through nutrient supply rather than light availability. This effect was observed when the mixed layer was relatively shallow (<5.5 m) and the driver for this was that the inflowing nutrients were added to a smaller volume of water, thus increasing nutrient concentrations and algal growth. 6. Therefore, whilst changes in mixed depth generally affect the phytoplankton via commonly recognized factors (light availability, sedimentation rate), it also affected phytoplankton growth and community composition through other important factors such as retention time and nutrient supply.     

Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Seasonal variation in phytoplankton composition and physical-chemical features of the shallow Lake Doïrani,Macedonia, Greece

Phytoplankton species composition, seasonal dynamics and spatial distribution in the shallow Lake Doïrani were studied during the growth season of 1996 along with key physical and chemical variables of the water. Weak thermal stratification developed in the lake during the warm period of 1996. The low N:P ratio suggests that nitrogen was the potential limiting nutrient of phytoplankton in the lake. In the phytoplankton of the lake, Chlorophyceae were the most species-rich group followed by Cyanophyceae. The monthly fluctuations of the total phytoplankton biomass presented high levels of summer algal biomass resembling that of other eutrophic lakes. Dinophyceae was the group most represented in the phytoplankton followed by Cyanophyceae. Diatomophyceae dominated in spring and autumn. Nanoplankton comprised around 90% of the total biomass in early spring and less than 10% in summer. The seasonal dynamics of phytoplankton generally followed the typical pattern outlined for other eutrophic lakes. R-species (small diatoms), dominant in the early phase of succession, were replaced by S-species (Microcystis, Anabaena, Ceratium) in summer. With cooling of the water in September, the biomass of diatoms (R-species) increased. The summer algal maxima consisted of a combination of H and M species associations (sensu Reynolds). Phytoplankton development in 1996 was subject to the combined effect of the thermal regime, the small depth of mixing and the increased sediment-water interactions in the lake, which caused changes in the underwater light conditions and nutrient concentrations.     

Ecology under lake ice

Winter conditions are rapidly changing in temperate ecosystems, particularly for those that experience periods of snow and ice cover. Relatively little is known of winter ecology in these systems, due to a historical research focus on summer ‘growing seasons’. We executed the first global quantitative synthesis on under‐ice lake ecology, including 36 abiotic and biotic variables from 42 research groups and 101 lakes, examining seasonal differences and connections as well as how seasonal differences vary with geophysical factors. Plankton were more abundant under ice than expected; mean winter values were 43.2% of summer values for chlorophyll a, 15.8% of summer phytoplankton biovolume and 25.3% of summer zooplankton density. Dissolved nitrogen concentrations were typically higher during winter, and these differences were exaggerated in smaller lakes. Lake size also influenced winter‐summer patterns for dissolved organic carbon (DOC), with higher winter DOC in smaller lakes. At coarse levels of taxonomic aggregation, phytoplankton and zooplankton community composition showed few systematic differences between seasons, although literature suggests that seasonal differences are frequently lake‐specific, species‐specific, or occur at the level of functional group. Within the subset of lakes that had longer time series, winter influenced the subsequent summer for some nutrient variables and zooplankton biomass.     

A two-part model linking multidimensional environmental gradients and seasonal succession of phytoplankton assemblages

Phytoplankton dynamics in Lake Müggelsee, a eutrophic and polymictic lake in Berlin, and in the inflowing lowland River Spree have been comprehensively investigated during the last two decades. Zooplankton dynamics, nutrient supply, light climate, duration of ice cover and of summer stratification have also been regularly measured to help to explain phytoplankton development. The first period (1978–1990) was characterised by high nutrient loads and dominance of cyanobacteria from spring to autumn. Since then, loads of phosphorus and nitrogen have been lowered by 40–50%. Oscillatoria-like cyanobacteria (Limnothrix redekei, Planktothrix agardhii) were favoured under hypertrophic conditions in both the polymictic lake and the river, but they have disappeared nearly completely after nutrient reduction. Development of these species depended on meteorological conditions and nutrient supply in spring rather than on seasonal averages of nutrient concentrations. Diatoms have became dominant and chlorophytes have increased their share of the biomass since the nutrient load was reduced. Species com- position changed even within the algal groups. Retention time of water and duration of thermal stratification of the water column modified phytoplankton structure. Mobile algae like Microcystisor Ceratium occurred in the lake during stratification periods. Otherwise, species composition in the shallow, polymictic lake was very similar to that in the inflowing lowland river. Species with high starting biomass, fed by high riverine import, resting stages or perennation were selected in this flushed system.     

Response of a shallow Mediterranean lake to nutrient diversion: does it follow similar patterns as in northern shallow lakes?

1. In view of the paucity of data on the response of warm shallow lakes to reductions in nutrient loading, this paper presents a long‐term limnological data set to document changes in the food‐web of a shallow Mediterranean lake (Lake Albufera, Valencia, Spain) that has experienced reductions in phosphorus (P) (77%) and nitrogen (N) (24%) loading following sewage diversion. 2. Nine years after sewage diversion, P concentration in the lake was reduced by 30% but remained high (TP = 0.34 mg L^?1), although the mean water retention time in the lake was only 0.1 years. Nitrate concentrations did not significantly change, probably because the lake continued to receive untreated effluents from ricefields. 3. Chlorophyll a concentration was reduced by half (annual mean of 180 μg L^?1). Cyanobacteria abundance remained high but its composition changed towards smaller species, both filamentous and chroococcal forms. 4. Cladocera abundance increased and reached peaks twice a year (December to March and July to September). After nutrient reduction, short‐term clear‐water phases (up to 5 weeks) occurred during February to March in several years, concomitant with annual flushing of the lake and lower fish densities. The abundance of Cladocera in winter contrasted with the spring peaks observed in northern restored shallow lakes. The zooplankton to phytoplankton biomass ratio remained lower than in northern temperate shallow lakes, probably because of fish predation on zooplankton. 5. Improvement of the water quality of Lake Albufera remained insufficient to counteract littoral reed regression or improve underwater light allowing submerged plants re‐colonise the lake. 6. Sewage diversion from Lake Albufera impacted the food web through the plankton, but higher trophic levels, such as fish and waterfowl, were affected to a lesser degree. Although the fish species present in the lake are mainly omnivorous, long‐term data on commercial fish captures indicated that fish communities changed in response to nutrient level and trophic structure as has been observed in restored shallow lakes at northern latitudes. 7. Phosphorus concentrations produced similar phytoplankton biomass in Lake Albufera as in more northern shallow lakes with abundant planktivorous fish and small zooplankton. However, in Lake Albufera, high average concentrations were maintained throughout the year. Overall, results suggest that nutrient control may be a greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. This synthesis examines 35 long‐term (5–35 years, mean: 16 years) lake re‐oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in‐lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10–15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in‐lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100–150 μg L^?1. This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Lake Horowhenua: a computer model of its limnology and restoration prospects

Lake Horowhenua, a small (2.9 km²) shallow (< 2 m deep), coastal dune lake on the west coast of the North Island of New Zealand, receives the runoff from intensive agriculture within its catchment and, until 1987, the treated sewage effluent from the town of Levin. Consequently the lake is highly enriched but with an annual cycle of algal P-limitation in winter and N-limitation in summer. There have been several schemes proposed to accelerate the improvement of the lake's water quality for recreational use. A computer hydraulic and nutrient model of Lake Horowhenua was developed using rainfall, evaporation and nutrient data to describe the nutrient budget. To match the lake nutrient concentrations, terms for in-lake processes of sedimentation, seasonal sediment nutrient release, phytoplankton production, and denitrification were required. The computer model results indicated that denitrification was the major natural restoration process accounting for a net loss of more than 50% of the N from the lake each year. Application of the model also allowed lake managers to evaluate the potential effects of a number of proposed restoration schemes.     

Origin and succession of phytoplankton in a river-lake system (Spree,Germany)

The River Spree (Germany) flows through an impoundment and several shallow lakes in its middle and lower course. In this river-lake system, the seasonal and longitudinal dynamics of dominant phytoplankton populations were studied in relation to retention time of water, mixing conditions and nutrient supply from 1988–92. Some phytoplankton species populated the same river section for weeks or months each year at their season. Such stable populations have to origin from river zones functioning like mixed reactors. In the Spree system, centric diatoms originated from an impoundment and filamentous cyanobacteria from a flushed lake with longer retention time of water. Downstream, biomass and composition of phytoplankton altered nearly simultaneously along the system.The fate of planktonic organisms washed from mixed reactors into the flow depended on the conditions at the zones of origin. During spring, populations dominating phytoplankton communities of the well-mixed lakes grew further under river conditions. However the biomass of summer species, adapted to intermittent stratification, was halved along the river course. These seasonal differences were probably caused by lower maximum growth rates of summer species and enhanced losses (photorespiration, sedimentation or grazing of benthic filter feeders, but not of zooplankton) of algal populations under river conditions in summer.Phytoplankton assimilation, settlement of diatoms, or denitrification caused declining (probably growth limiting) concentrations of dissolved inorganic phosphorus (spring), silicon (early summer) or nitrogen (summer) along the river course, respectively. The minimum content of DRP was often followed by a clear-water phase. Reduced DSi supply selected against diatoms and additional DIN shortage favoured N₂-fixing cyanobacteria in the last lake of the system.R-strategists (sensu Reynolds) were selected in both the flushed, shallow lakes and the lowland river. In general, the biomass of cyanobacteria increased within the lakes and declined along the river course. Some diatom populations grew in the river, but were grazed or settled down in the lakes. Beside this general picture, different populations from the same phylogenetic group did not necessarily perform in similar ways.     

Thermal stratification, nutrient dynamics, and phytoplankton productivity during the onset of spring phytoplankton growth in Lake Baikal, Russia

Lake Baikal, Russian Siberia, was sampled in July 1990 during the period of spring mixing and initiation of thermal stratification. Vertical profiles of temperature, dissolved nutrients (nitrate and soluble reactive phosphorus), phytoplankton biomass, and primary productivity were determined in an eleven-station transect encompassing the entire 636 km length of the lake. Pronounced horizontal variability in hydrodynamic conditions was observed, with the southern region of the lake being strongly thermally stratified while the middle and north basins were largely isothermal through July. The extent of depletion of surface water nutrients, and the magnitude of phytoplankton biomass and productivity, were found to be strongly correlated with the degree of thermal stratification. Horizontal differences likely reflected the contribution of two important factors: variation in the timing of ice-out in different parts of the lake (driving large-scale patterns of thermal stratification and other limnological properties) and localized effects of river inflows that may contribute to the preliminary stabilization of the water column in the face of intense turbulent spring mixing (driving meso-scale patterns). Examination of the relationships between surface water inorganic N and P depletion suggested that during the spring and early summer, phytoplankton growth in unstratified portions of the lake was largely unconstrained by nutrient supplies. As summer progressed, the importance of co-limitation by both N and P became more apparent. Uptake and regeneration rates, measured directly using the stable isotope ¹⁵N, revealed that phytoplankton in stratified portions of the lake relied primarily on NH₄ as their N source. Rates of NH₄ regeneration were in approximate equilibrium with uptake; both processes were dominated by organisms <2 µm. This pattern is similar to that observed for oligotrophic marine systems. Our study underscores the importance of hydrodynamic conditions in influencing patterns of biological productivity and nutrient dynamics that occur in Lake Baikal during its brief growing season.     

Phytoplankton periodicity of the Waitaki Lakes,New Zealand

The Waitaki River system in the South Island of New Zealand includes three large glacially-formed headwater lakes, Tekapo, Pukaki and Ohau, which drain into the manmade Lake Benmore. Phytoplankton periodicity was followed from December 1975 to January 1980 as part of a study investigating possible changes in these lakes as a consequence of hydroelectric development. The phytoplankton was highly dominated by diatoms, e.g., Diatoma elongatum, Cyclotella stelligera, Asterionella formosa, and Synedra acus, but in lakes Ohau and Benmore populations of green algae occasionally developed. In all four lakes seasonal phytoplankton periodicity was observed with maximum biomass in spring and summer. In Lake Tekapo, the first lake in the chain, maximum biomass did not exceed 300 mg m^–3, but in the very turbid Lake Pukaki the maximum summer biomass ranged between 300 and 800 mg m^–3. In Lake Ohau, the least turbid lake, maximum biomass was around 1 000 mg m^–3. In the newly created Lake Benmore periodicity was less evident and summer maxima reached over 1 500 mg m^–3. The phytoplankton periodicity in these lakes is greatly influenced by seasonal patterns of turbidity from inflowing glacial silt.     

Phytoplankton periodicity in a subtropical lake (Lake Kinneret,Israel)

Lake kinneret is a subtropical monomictic lake characterized by a Pyrrhophyta-Chlorophyta assemblage, supplemented by Cyanophyta in some years. Concerning their abundance and seasonal occurrence, the phytoplanktonic algae belong to two groups: algae appearing in quantity at a definite annual period and algae present throughout the year. Four stages of algal succession occur in the lake. There is a marked periodicity in the phytoplankton composition with a high standing stock in winter-spring, due to the dinoflagellate water-bloom, and a low one during the summer months, related to the high stability of summer stratification. The annual succession at the species level has been an almost constant event in the lake for many years.The increase in nutrient concentrations in 1973 and 1974 increased the diversity and abundance of algae (except Peridinium) but did not lead to significant changes in algal succession. Conversely, the decrease of the zooplankton grazing pressure in 1975 and 1976 facilitated the development of algal maxima during summer-fall. They were caused by nanoplanktonic forms, and they developed without additional enrichment of nutrients. The algal abundance and diversity decreased. The years 1981 and 1982 were characterized by both an increase in phosphorus and a decrease in zooplankton. These conditions favored the concomitant abundance of many species and an increase of non-Pyrrhophyta biomass.     

Long-term nutrient trends and harmful cyanobacterial bloom potential in hypertrophic Lake Taihu,China

Rapid economic development in China’s Lake Taihu basin during the past four decades has accelerated nitrogen (N) and phosphorus (P) loadings to the lake. This has caused a shift from mesotrophic to hypertrophic conditions, symptomized by harmful cyanobacterial blooms (CyanoHABs). The relationships between phytoplankton biomass as chlorophyll a (Chla) and nutrients as total nitrogen (TN) and total phosphorus (TP) were analyzed using historical data from 1992 to 2012 to link the response of CyanoHAB potential to long-term nutrient changes. Over the twenty year study period, annual mean Chla showed significantly positive correlations with both annual mean TN and TP (P < 0.001), reflecting a strong phytoplankton biomass response to changes in nutrient inputs to the lake. However, phytoplankton biomass responded slowly to annual changes in TN after 2002. There was not a well-defined or significant relationship between spring TN and summertime Chla. The loss of a significant fraction of spring N loading due to denitrification likely weakened this relationship. Bioavailability of both N and P during the summer plays a key role in sustaining cyanobacterial blooms. The frequency of occurrence of bloom level Chla (>20 μg L^?1) was compared to TN and TP to determine nutrient-bloom thresholds. A decline in bloom risk is expected if TN remains below 1.0 mg L^?1 and TP below 0.08 mg L^?1.     

The importance of nutrient source in determining the influence of retention time on phytoplankton: an explorative modelling study of a naturally well-flushed lake

Two models were used to examine the relationship between hydraulic retention time, nutrient source and total chlorophyll in a shallow lake (Bassenthwaite Lake, UK). The first model was a derivation of the Vollenweider model and the second was the phytoplankton community model, PROTECH. The adapted Vollenweider model produced two different responses to changing the retention time that were phosphorus source dependent. If the phosphorus was totally from a point source, then annual mean chlorophyll steadily declined with increasing flushing rate. However, when a diffuse source was used, the chlorophyll changed little and even increased with short retention times (retention time <40 days). The PROTECH model produced some similar responses but they were more season dependent. Winter mean chlorophyll always declined with decreasing retention time, regardless of nutrient source, but the summer mean curves were source dependent and similar to those produced by the adapted Vollenweider model. Further simulations with PROTECH using a standardized flow regime provided strong evidence as to the mechanisms behind these responses. Analysis showed that the decline in chlorophyll with decreasing retention time was the prevalent response of the PROTECH simulations due to flushing loss of both nutrients and algae. Furthermore, the curve formed an asymptote at long retention times because other factors (e.g. light) limited growth; retention times >100 days had little effect on chlorophyll. However, with a diffuse phosphorus source and short retention times, an increase in biomass was observed when the nutrient was limiting for growth. Handling editor: Luigi Naselli-Flores     

Effects of different N- and P-loading on primary and secondary production in an experimental marine ecosystem

The effects of nutrient loading on phytoplankton, zooplankton and macrozoobenthos in experimental ecosystems was studied in a 7-month experiment. The mesocosms were designed to mimic the major physical characteristics (irradiance, temperature, mixing) of the Dutch coastal zone in the river Rhine plume. Three different nutrient loading scenarios were used, representing present and future conditions. The level of the spring phytoplankton bloom was determined by phosphorus loading, whereas during summer the nitrogen loading determined phytoplankton biomass. The differences in nutrient loading did not result in shifts in phytoplankton species composition. With exception of the early phase of the spring bloom, diatoms dominated phytoplankton biomass in all nutrient treatments. This was ascribed to microzooplankton grazing on smaller algal species. Microzooplankton biomass showed a positive correlation with primary production, and also significant differences between nutrient treatments. Copepod development was limited, probably due to competition with microzooplankton and predation by benthic fauna. Macrobenthos biomass correlated with primary production, and was lower in the lowest nutrient treatment.     

Effects of freshwater input on nutrient loading, phytoplankton biomass, and cyanotoxin production in an oligohaline estuarine lake

Pulsed river water events can increase nutrient levels potentially translating into enhanced primary production, phytoplankton community shifts, and bloom formation. The Bonnet Carré Spillway is a managed river diversion which can be used to redirect a significant amount of Mississippi River water into Lake Pontchartrain, reducing the risks of flood in the downstream communities during runoff seasons. We investigated nutrient enrichment and consequent changes in phytoplankton biomass, including toxic species in Lake Pontchartrain during and after a 1-month Bonne Carré Spillway opening in 2008. Water samples were collected along a 30 km transect. A freshwater plume was found to have formed by the strong river input that had limited mixing with the lake during the opening. The plume and lake water gradually mixed together after the Spillway was closed, indicated by the reduction of the horizontal salinity gradient. The river pulse increased the lake nitrate and dissolved reactive phosphorus concentrations to more than five times the lake background in the plume stations. Nutrient concentrations decreased rapidly after the Spillway closure as the plume dissipated. Diatoms and chlorophytes dominated the system during the opening. After the Spillway closure, there was a shift over time from diatom dominance to toxic cyanobacteria dominance that corresponded to more stable, warmer, and nutrient-limited water conditions. Associated toxins were present and varied over time and space. Further research on the phytoplankton assemblages on the lake is needed in subsequent, non-Spillway opening years to evaluate the impact of river water pulses on the development of these toxic cyanobacterial blooms.     

A five-year study of autotrophic winter picoplankton in Lake Balaton,Hungary

Picoeukaryotes dominate the phytoplankton of Lake Balaton—the largest shallow lake in Central Europe—in the winter period. We examined the annual dynamics of picoplankton abundance and composition in the lake in order to establish if the picoeukaryotes merely survive the harsher winter conditions or they are able to grow in the ice-covered lake when the entire phytoplankton is limited by low light and temperature. Lake Balaton has an annual temperature range of 1–29°C, and it is usually frozen between December and February for 30–60 days. In the spring-autumn period phycocyanin and phycoerythrin rich Cyanobacteria are the dominant picoplankters, and picoeukaryotes are negligible. Our five-year study shows the presence of three types of picophytoplankton assemblages in Lake Balaton: (1) Phycoerythrin-rich Cyanobacteria—the dominant summer picoplankters in the mesotrophic lake area; (2) Phycocyanin-rich Cyanobacteria—the most abundant summer picoplankters in the eutrophic lake area and; (3) Picoeukaryotes—the dominant winter picoplankters in the whole lake. The observed winter abundance of picoeukaryotes was high (up to 3 × 10⁵ cells ml⁻¹), their highest biomass (520 μg l⁻¹) exceeded the maximum summer biomass of picocyanobacteria (500 μg l⁻¹). Our results indicate that the winter predominance of picoeukaryotes is a regular phenomenon in Lake Balaton, irrespective of the absence or presence of the ice cover. Picoeukaryotes are able to grow at as low as 1–2°C water temperature, while the total phytoplankton biomass show the lowest annual values in the winter period. In agreement with earlier findings, the contribution of picocyanobacteria to the total phytoplankton biomass in Lake Balaton is inversely related to the total phytoplankton biomass, whereas no such relationship was observable in the case of picoeukaryotes.