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Diversity of primary producer is often surprisingly high, despite few limiting factors such as nutrients and light to facilitate species coexistence. In theory, the presence of herbivores could increase the diversity of primary producers, resolving this "paradox of the plankton." Little experimental evidence supports this natural enemies hypothesis, but previous tests suffer from several deficiencies. Previous experiments often did not allow for multigeneration effects; utilized low diversity assemblages of herbivores; and limited opportunities for new primary producer and herbivore species to colonize and undergo species sorting that favors some species over others. Using pond plankton, we designed a mesocosm experiment that overcame these problems by allowing more time for interactions over multiple generations, openness to allow new colonists, and manipulated higher diversity of primary producers and grazers than have previous studies. With this design, the presence of zooplankton grazers doubled phytoplankton richness. The additional phytoplankton species in grazed mesocosms were larger, and therefore likely more grazer resistant. Furthermore, phytoplankton richness in grazed mesocosms was similar to that observed in natural ponds whereas it was much lower in mesocosms without grazers. However, stoichiometric imbalance caused by variation in nitrogen : phosphorus ratios and light supply did not alter phytoplankton richness. Therefore, grazers enhanced primary producer richness more strongly than ratios of nutrient supply (even though both grazing and ratios of resource supply altered composition of primary producer assemblages). Taken together, these experimental and field data show that grazing from a diverse assemblage of herbivores greatly elevated richness of phytoplankton producers in pond ecosystems.

To further clarify empirical relationships between maximum phytoplankton biomass per unit total phosphorus (TP) and the maximum:mean phytoplankton biomass ratio, predictive models were created from a meta-analysis of datasets from lakes worldwide. Peak concentrations of planktonic chlorophyll a were on average 2.6 times greater than the mean, with most maximum:mean chlorophyll a ratios falling within less than an order-of-magnitude band (from 1.2 to 6 times the mean value). Peak concentrations of planktonic algal biovolume in 22 Organisation for Economic Co-operation and Development (OECD) lakes were on average 3.3 times greater than the mean, with maximum:mean biovolume ratios ranging from 1.2 to 6.3. By contrast, the temporal dynamics of planktonic (suspended) algal biomass in Minnesota streams and rivers were far more constrained; peak concentrations of planktonic chlorophyll a were only on average 1.8 times greater than the mean, with maximum:mean chlorophyll a ratios ranging from 1.3 to 3.8. These data suggest regional differences in maximum chlorophyll a values may exist. Parallel analyses of periphyton biomass in streams and rivers showed far greater variability in the maximum:mean chlorophyll ratio for benthic algae. Peak concentrations of attached chlorophyll a were on average 4.3 times greater than the mean, with the observed maximum:mean chlorophyll a ratios exhibiting exceptionally high variability (ranging from 1.0 to 50 times the mean value), likely reflecting differences in the temporal dynamics of planktonic and benthic algal growth in freshwater ecosystems.

The use of persistent organic pollutants (POPs), such as herbicides, pesticides, pharmaceutical and personal care products (PCPPs), and polycyclic aromatic hydrocarbons (PAHs), has more than doubled since 1950. POPs find their way into aquatic ecosystems through agricultural and industrial runoff, wastewater treatment effluent discharge, and atmospheric deposition. Cyanobacterial harmful algal blooms (CyanoHABs), which can produce toxins potent enough to cause human death, have been increasing in intensity, frequency, and spatial scale throughout the same time period as accelerated POP usage. Here, we provide a meta-analysis and suggest that POP stressors may be significantly aggravating nutrient-driven CyanoHABs by suppressing the growth of competing phytoplankton, and/or by indirectly or directly stimulating cyanobacterial growth.

Although aquatic ecosystems are a major carbon reservoir, how their carbon dynamics will respond to increasing concentrations of atmospheric CO2 is not well understood. The availability of essential nutrients has the potential to modify carbon fluxes under elevated CO2 by altering carbon processing and storage in the biota. Here, we describe a semi-continuous culture experiment with natural phytoplankton and bacteria assemblages designed to investigate (1) how carbon dynamics in aquatic ecosystems respond to continuously elevated atmospheric CO2, and (2) whether carbon fluxes resulting from elevated CO2 are modified by changes in inorganic nitrogen and phosphorus availability. Our results showed that elevated CO2 led to significant increases in photosynthetic carbon uptake, despite a decrease in the algal chlorophyll a concentrations and no significant change in total algal biovolume. This enhancement of inorganic carbon uptake was accompanied by a significant increase in dissolved organic carbon (DOC) production. Concurrent increases in the C/N and C/P ratios of dissolved organic matter also suggested that DOC stability increased. Nutrient availability, especially nitrogen availability, had strong effects on inorganic carbon uptake and biomass carbon pools. In contrast, CO2-enhanced DOC production was not significantly affected by varying concentrations of inorganic nitrogen and phosphorus. Our study underscores the importance of DOC as a potential carbon sink in aquatic ecosystems. The observed responses to changes in CO2 and nutrient availability could have important implications for long-term carbon cycling in aquatic ecosystems, and highlight the potential buffering capacity of aquatic ecosystems to future environmental change.

One of the most visible consequences of eutrophication in freshwater lakes is an increase in the biomass of cyanobacteria. While growth of cyanobacteria has been shown to be coupled to water column concentrations of total phosphorus (TP) in natural lakes, the ecological behavior of cyanobacteria in turbid impoundments is less well understood. Patterns of cyanobacterial biovolume in 7 turbid Kansas (USA) reservoirs exhibited overlap with published data from natural lakes in North America. Non-algal turbidity (NAT) influenced the relationship between cyanobacteria and TP in these systems. Relative to yields predicted from natural lakes, the biomass of cyanobacteria per unit TP was typically higher in the reservoirs, except in those systems that had exceptionally high levels of NAT (>∼2.0 2/m) where observed concentrations were mostly lower than predicted. A more consistent pattern was observed with respect to the effects of NAT on yields of chlorophyll-a per unit TP in the reservoirs: across almost...

The Three Gorges Dam (TGD) has greatly altered ecological and environmental conditions within the reservoir region, but it is not known how these changes affect phytoplankton structure and dynamics. Here, a bimonthly monitoring program was implemented from 2007 to 2009 to study the impact of damming on phytoplankton assemblages in the backwater area of the Pengxi River (PBA). By application of the phytoplankton functional group (C strategists, competitive species; S strategists, stress-tolerant species; R strategists, rapid propagation species), seasonal changes in phytoplankton relative to environmental variations were evaluated using ordination analysis. Seasonal patterns of phytoplankton dynamics were detected during this study, with CS/S strategists causing algal blooms from mid-spring to early summer, CS/CR strategists often observed during flood season, and CS strategists dominant during mid-autumn. CR/R groups dominated during winter and caused algal blooms in February. Our results indicated that phytoplankton assemblages were directly related to reservoir operation effects. Generally, the TGD had a low water level during flood season, resulting in a relatively short hydraulic retention time and intensive variability, which supported the cooccurrence of CS and CR species. During the winter drought season, water storage in the TGD increased the water level and the hydraulic retention time in the PBA, enabling R/CR strategists to overcome the sedimentation effect and to out-compete S/CS species in winter. As expected, these diversity patterns were significantly correlated with the hydraulic retention time and nutrient limitation pattern in the PBA. This study provides strategic insight for evaluating the impacts of reservoir operations on phytoplankton adaptation.

A cross-ecosystem comparison of data obtained from 92 coastal zone ecosystems worldwide revealed a strong positive response of marine phytoplankton biomass to nutrient enrichment that is highly consistent with the general patterns reported previously in the limnological literature for freshwater lakes and reservoirs. Average concentrations of chlorophyll a in estuarine and coastal marine systems were strongly dependent on the mean concentrations of total nitrogen and total phosphorus in the water column. Moreover, as is true of freshwater ecosystems, the identity of the primary growth-limiting nutrient for marine phytoplankton appeared to be generally predictable from watercolumn total nitrogen : total phosphorus (TN : TP) ratios. This similarity in physiological response to nutrients likely derives from the shared evolutionary histories of marine and freshwater phytoplankton.

A current principle of ecological stoichiometry states that the nitrogen to phosphorus ratio (N:P) of primary producers should closely match that from environmental nutrient supplies. This hypothesis was tested using data from ponds in Michigan, USA, a freshwater mesocosm experiment, a synthesis of studies from diverse systems (cultures, lakes, streams, and marine and terrestrial environments), and simple dynamic models of producer growth and nutrient content. Unlike prior laboratory studies, the N:P stoichiometry of phytoplankton in Michigan ponds clustered around and below the Redfield ratio (7.2:1 by mass), despite wide variation in N:P supply ratios (2:1-63:1 by mass) and the presence of grazers. In a mesocosm experiment, the N:P stoichiometry of phytoplankton cells again deviated from a nearly 1:1 relationship with N:P supply. Phytoplankton seston exhibited lower N:P content than expected at high N:P supply ratios, and often higher N:P content than anticipated at low N:P supply ratios, regardless of herbivore presence. Similar devi- ations consistently occur in the N:P stoichiometry of algae and plants in the other diverse systems. The models predicted that both high loss rates (sinking, grazing) and physiological limits to nutrient storage capacity could attenuate producer stoichiometry. In the future, research should evaluate how limits to elemental plasticity of producers can influence the role of stoichiometry in structuring communities and ecosystem processes.

Species-area relationships have been observed for virtually all major groups of macroorganisms that have been studied to date but have not been explored for microscopic phytoplankton algae, which are the dominant producers in many freshwater and marine ecosystems. Our analyses of data from 142 different natural ponds, lakes, and oceans and 239 experimental ecosystems reveal a strong species-area relationship with an exponent that is invariant across ecosystems that span >15 orders of magnitude in spatial extent. A striking result is that the species-area relationship derived from small-scale experimental studies correctly scales up to natural aquatic ecosystems. These results significantly broaden our knowledge of the effects of island size on biodiversity and also confirm the relevance of experimentally derived data to the analysis and understanding of larger-scale ecological patterns. In addition, they confirm that patterns in microbial diversity are strongly consistent with those that have been repeatedly reported in the literature for macroorganisms.

Summary 1. Fine-scale physical and chemical gradients and deep photosynthetic microbial populations were assessed to provide an initial characterisation of a small, thermally stratified reservoir (Cross Reservoir, Kansas, U.S.A.) and its deep chlorophyll maxima (DCM). Factors were identified that may affect vertical positioning of subepilimnetic photosynthetic sulphur bacteria (PSB) in lakes. 2. Results indicate that Cross Reservoir is a mesotrophic, dimictic lake with large subepilimnetic chlorophyll maxima containing dense layers of PSB. Characteristics of the deep PSB community of Cross Reservoir strongly correlate with both light and nutrient gradients. 3. The deep bacterial community mostly contained single-celled and aggregating green sulphur bacteria, specifically free-living Chlorobium limicola and the conspicuous motile ectosymbiotic consortium known as ‘Chlorochromatium aggregatum’. The bacteria were within the anaerobic hypolimnion, beneath a metalimnetic plate of Cryptomonas spp. and within very low sulphide and light conditions [mean of 67 μgS L−1 and 0.05% photosynthetically active radiation (PAR)]. Pigment concentrations and fluorescence trends indicate that the bacteria made up a larger proportion of the DCM biomass than did phytoplankton in 1996. 4. Cross Reservoir shares characteristics with natural lakes world-wide that also include a deep PSB community containing dense layers of ‘C. aggregatum’. Correlation analyses indicate that PSB community positioning and density are related to light, sulphide supply, redox potentials and pH. A 2-factor principal components analysis (PCA) and other data trends supported these interpretations and indicated that PSB are sensitive to the thermal stability of the water column, are nitrogen limited and regulated more by sulphide or sulphide to light ratios than local levels of light. The sensitivity of these deep photosynthetic bacteria to environmental gradients, and their significance to some aquatic systems, demonstrate their potential as indicators of environmental disturbance.

Humans now strongly influence almost every major aquatic ecosystem, and their activities have dramatically altered the fluxes of growth-limiting nutrients from the landscape to receiving waters. Unfortunately, these nutrient inputs have had profound negative effects upon the quality of surface waters worldwide. This review examines how eutrophication influences the biomass and species composition of algae in both freshwater and costal marine systems. An overview of recent advances in algae-related eutrophication research is presented. In freshwater systems, a summary is presented for lakes and reservoirs; streams and rivers; and wetlands. A brief summary is also presented for estuarine and coastal marine ecosystems. Eutrophication causes predictable increases in the biomass of algae in lakes and reservoirs; streams and rivers; wetlands; and coastal marine ecosystems. As in lakes, the response of suspended algae in large rivers to changes in nutrient loading may be hysteretic in some cases. The inhibitory effects of high concentrations of inorganic suspended solids on algal growth, which can be very evident in many reservoirs receiving high inputs of suspended soils, also potentially may occur in turbid rivers. Consistent and predictable eutrophication-caused increases in cyanobacterial dominance of phytoplankton have been reported worldwide for natural lakes, and similar trends are reported here both for phytoplankton in turbid reservoirs, and for suspended algae in a large river. A remarkable unity is evident in the global response of algal biomass to nitrogen and phosphorus availability in lakes and reservoirs; wetlands; streams and rivers; and coastal marine waters. The species composition of algal communities inhabiting the water column appears to respond similarly to nutrient loading, whether in lakes, reservoirs, or rivers. As is true of freshwater ecosystems, the recent literature suggests that coastal marine ecosystems will respond positively to nutrient loading control efforts. Our understanding of freshwater eutrophication and its effects on algal-related water quality is strong and is advancing rapidly. However, our understanding of the effects of eutrophication on estuarine and coastal marine ecosystems is much more limited, and this gap represents an important future research need. Although coastal systems can be hydrologically complex, the biomass of marine phytoplankton nonetheless appears to respond sensitively and predictably to changes in the external supplies of nitrogen and phosphorus. These responses suggest that efforts to manage nutrient inputs to the seas will result in significant improvements in coastal zone water quality. Additional new efforts should be made to develop models that quantitatively link ecosystem-level responses to nutrient loading in both freshwater and marine systems.

Ecological Stoichiometry theory predicts that the production, elemental structure and cellular content of biomolecules should depend on the relative availability of resources and the elemental composition of their producer organism. We review the extent to which carbon- and nitrogen-rich phytoplankton toxins are regulated by nutrient limitation and cellular stoichiometry. Consistent with theory, we show that nitrogen limitation causes a reduction in the cellular quota of nitrogen-rich toxins, while phosphorus limitation causes an increase in the most nitrogen-rich paralytic shellfish poisoning toxin. In addition, we show that the cellular content of nitrogen-rich toxins increases with increasing cellular N : P ratios. Also consistent with theory, limitation by either nitrogen or phosphorus promotes the C-rich toxin cell quota or toxicity of phytoplankton cells. These observed relationships may assist in predicting and managing toxin-producing phytoplankton blooms. Such a stoichiometric regulation of toxins is likely not restricted to phytoplankton, and may well apply to carbon- and nitrogen-rich secondary metabolites produced by bacteria, fungi and plants.

The study is a first attempt to generate a series of Total Phosphorus–Chlorophyll a (TP–Chl a) predictiveOC models relating the quantitative responses of four algal size fractions to phosphorus gradients. The study was carried out in 27 glacial lakes from two regions in southern Quebec, the Laurentians and the Eastern Townships, and covered a relatively modest range of trophic conditions (TP, 3–34 µg P L-1; Chl a, 0.3–7.6 µg L−1). Algal biomass was estimated using measurements of Chl a, and the total Chl a was divided into four operational size fractions: picophytoplankton 20 mm (inedible fraction). We tested the hypothesis that the slopes of the TP–Chl a regression models developed for algal size fractions would increase consistently from the smallest to the largest algal size fraction, as suggested by the first half of the sigmoidal TP–Chl a models. Although there was no consistent trend in the magnitudes of the slopes of TP–Chl a relationships for picophytoplankton (slope = 1.14), nanophytoplankton (0.93), and microphytoplankton (1.22), Chl a concentrations in the largest size fraction increased more rapidly with phosphorus enrichment than in either of the smaller fractions. When included as an additional variable, lake water alkalinity improved the prediction of Chl a and presented differential effects on size fractions. The effect of TP enrichment on microphytoplankton is more pronounced in well-buffered lakes, whereas TP enrichment has a stronger effect on nanophytoplankton in low-alkalinity lakes. The effects of alkalinity may be the result of either a pH influence on phytoplankton carbon uptake or a stronger top-down grazing effect on small algae in well-buffered lakes.

A cross-ecosystem comparison of data obtained from 20 French Mediterranean lagoons with contrasting eutrophication status provided the basis for investigating the variables that best predict chlorophyll a (Chl a) concentrations and nutrient limitation of phytoplankton biomass along a strong nutrient enrichment gradient. Summer concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) comprised only a small fraction of total nitrogen (TN) and total phosphorus (TP). On the basis of inorganic nutrient concentrations, the most oligotrophic lagoons appeared to be phosphorus-limited, with a tendency towards the development of nitrogen limitation as eutrophication increased, as evidenced by decreasing DIN:DIP ratios. A weak but significantly positive relationship was found between dissolved silicate (DSi) and Chl a, reflecting DSi accumulation in the water column along the trophic state gradient and implying a progressive shift away from potential Si limitation of phytoplankton growth. Observed concentrations of Chl a were far better explained by TN and TP than by DIN and DIP concentrations, suggesting that a total nutrient based approach is likely to be the most appropriate for managing eutrophication in Mediterranean lagoons and other coastal waters. These results give credence to the idea that marine and freshwater environments respond in a similar fashion to nutrient enrichment., Une comparaison inter-sites des concentrations de nutriments et de chlorophylle a (Chl a) issues de 20 lagunes de Mediterranee francaise de degres divers d'eutrophisation a ete menee afin de rechercher la meilleure variable expliquant le niveau de Chl a et d'explorer la limitation de la production primaire par les nutriments au long d'un fort gradient d'enrichissement en azote et phosphore. En ete, l'azote et le phosphore inorganiques dissous (DIN et PID respectivement) constituaient une faible fraction des concentrations d'azote et de phosphore totaux (TN et TP respectivement). Sur la base des concentrations en nutriments inorganiques, la plupart des lagunes oligotrophes semblaient limitees par le phosphore avec une tendance vers des rapports DIN:DIP decroissants, et donc une limitation par l'azote, lorsque l'eutrophisation augmentait. Une correlation positive mais faible a aussi ete obtenue entre les concentrations de silicates (DSi) et Chl a, indiquant une accumulation de DSi dans la colonne d'eau et donc une diminution de la limitation de la croissance du phytoplancton par la silice au fur et a mesure que l'eutrophisation augmentait. Le fait que Chl a etait beaucoup mieux expliquee par TN et TP que par DIN et DIP suggere meme que l'approche limnologique basee sur l'ensemble des nutriments est vraisemblablement la meilleure pour gerer l'eutrophisation dans les lagunes mediterraneennes et dans d'autres ecosystemes marins cotiers. Ces constats renforcent l'idee que les ecosystemes marins et d'eau douce repondent de facon similaire a l'enrichissement en nutriments.

A long-term (28-year) data set was used to investigate historical changes in concentrations of phosphorus (P), nitrogen (N), N:P ratios, and Secchi disk transparency in a shallow subtropical lake (Lake Okeechobee, Florida, USA). The aim was to evaluate changes in the risk of N2-fixing cyanobacterial blooms, which have infrequently occurred in the lake's pelagic zone. Predictions regarding bloom risk were based on previously published N:P ratio models. Temporal trends in the biomass of cyanobacteria were evaluated using phytoplankton data collected in 1974, 1989-1992, and 1997-2000. Concentrations of pelagic total P increased from near 50 microg l-1 in the mid-1970s to over 100 microg l-1 in the late 1990s. Coincidentally, the total N:P (mass) ratio decreased from 30:1 to below 15:1, and soluble N:P ratio decreased from 15:1 to near 6:1, in the lake water. Published empirical models predict that current conditions favor cyanobacteria. The observations confirm this prediction: cyanobacteria presently account for 50-80% of total phytoplankton biovolume. The historical decrease in TN:TP ratio in the lake can be attributed to a decreased TN:TP ratio in the inflow water and to a decline in the lake's assimilation of P, relative to N. Coincident with these declines in total and soluble N:P ratios, Secchi disk transparency declined from 0.6 m to near 0.3 m, possibly due to increased mineral turbidity in the lake water. Empirical models predict that under the turbid, low irradiance conditions that prevail in this lake, non-heterocystous cyanobacteria should dominate the phytoplankton. Our observations confirmed this prediction: non-N2-fixing taxa (primarily Oscillatoria and Lyngbya spp.) typically dominated the cyanobacteria community during the last decade. The only exception was a year with very low water levels, when heterocystous N2-fixing Anabaena became dominant. In the near-shore regions of this shallow lake, low N:P ratios potentially favor blooms of N2-fixing cyanobacteria, but their occurrence in the pelagic zone is restricted by low irradiance and lack of stable stratification.

The factors determining the relative biomass of blue-green algae during the growing season were studied using data from 22 lakes worldwide. Multiple linear regression analyses suggest that total nitrogen (TN), total phosphorus (TP), and light (as estimated from Secchi disc transparency and the depth of the mixed layer) interact to determine the relative biomass of planktonic blue-green algae. At a fixed TN: TP ratio, blue-green relative biomass increases as light availability decreases. At a fixed light level, blue-green relative biomass also increases as the TN: TP ratio decreases. Both effects are consistent with current knowledge of algal physiology, and with a recently proposed theoretical framework for algal community structure.

The validity of a recent nutrient-based modal of algal photosynthesis was tested using phosphorus-limited semi-continuous cultures of the green alga, Scenedesmus quadricauda. The data suggest that light-saturate photosynthesis (P opt, mg C · mg chla−1· h−1) is a hyperbolic function of cellular phosphorus quota (Qp), whether Qp is expressed per unit cellular chlorophyll a, per unit dry weight, or per cell number. Furthermore, the light availability during growth appears to have little effect on these hyperbolic relationships over the range of light intensities studied here (65–110 μE · m−2· s−1). These data, and data presented from other published studies, suggest that the cell quota model is a valid empirical description of the photosynthetic behavior of nutrient-limited cells.

An analysis of growing season data from 17 lakes throughout the world suggests that the relative proportion of blue-green algae (Cyanophyta) in the epilimnetic phytoplankton is dependent on the epilimnetic ratio of total nitrogen to total phosphorus. Blue-green algae tended to be rare when this ratio exceeded 29 to 1 by weight, suggesting that modification of this ratio by control of nutrient additions may provide a means by which lake water quality can be managed.