Suspended algae, or phytoplankton, are the prime source of organic matter supporting food webs in freshwater ecosystems. Phytoplankton productivity is reliant on adequate nutrient supplies; however, increasing rates of nutrient supply, much of it manmade, fuels accelerating primary production or eutrophication. An obvious and problematic symptom of eutrophication is rapid growth and accumulations of phytoplankton, leading to discoloration of affected waters. These events are termed blooms. Blooms are a prime agent of water quality deterioration, including foul odors and tastes, deoxygenation of bottom waters (hypoxia and anoxia), toxicity, fish kills, and food web alterations. Toxins produced by blooms can adversely affect animal (including human) health in waters used for recreational and drinking purposes. Numerous freshwater genera within the diverse phyla comprising the phytoplankton are capable of forming blooms; however, the blue-green algae (or cyanobacteria) are the most notorious bloom formers. This is especially true for harmful toxic, surface-dwelling, scum-forming genera (e.g., Anabaena, Aphanizomenon, Nodularia, Microcystis) and some subsurface bloom-formers (Cylindrospermopsis, Oscillatoria) that are adept at exploiting nutrient-enriched conditions. They thrive in highly productive waters by being able to rapidly migrate between radiance-rich surface waters and nutrient-rich bottom waters. Furthermore, many harmful species are tolerant of extreme environmental conditions, including very high light levels, high temperatures, various degrees of desiccation, and periodic nutrient deprivation. Some of the most noxious cyanobacterial bloom genera (e.g., Anabaena, Aphanizomenon, Cylindrospermopsis, Nodularia) are capable of fixing atmospheric nitrogen (N2), enabling them to periodically dominate under nitrogen-limited conditions. Cyanobacteria produce a range of organic compounds, including those that are toxic to higher-ranked consumers, from zooplankton to further up the food chain. Both N2- and non-N2-fixing genera participate in mutualistic and symbiotic associations with microorganisms, higher plants, and animals. These associations appear to be of great benefit to their survival and periodic dominance. In this review, we address the ecological impacts and environmental controls of harmful blooms, with an emphasis on the ecology, physiology, and management of cyanobacterial bloom taxa. Combinations of physical, chemical, and biotic features of natural waters function in a synergistic fashion to determine the sensitivity of water bodies. In waters susceptible to blooms, human activities in water- and airsheds have been linked to the extent and magnitudes of blooms. Control and management of cyanobacterial and other phytoplankton blooms invariably includes nutrient input constraints, most often focused on nitrogen (N) and/or phosphorus (P). The types and amount of nutrient input constraints depend on hydrologic, climatic, geographic, and geologic factors, which interact with anthropogenic and natural n...

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Experiments were conducted to test the following proposed mechanisms whereby blooms of colonial blue-green algae (Microcystis aeruginosa) may more negatively affect large-bodied cladocerans than other zooplankton taxa: that colonial morphology most strongly interferes with the feeding of larger cladocerans, and that larger cladocerans more readily ingest toxic or nonnutritious colonial blue-greens. Experiments were conducted with both colonial and unicellular M. aeruginosa, in order to distinguish the effects of coloniality from chemical inhibitory factors associated with bluegreen algae. Experiments with colonial M. aeruginosa provided some support for both mechanisms, particularly with the cladoceran Diaphanosoma brachyurum. However, some small-bodied species associated with blue-green algal blooms did not conform to the proposed mechanisms, showing relatively strong reductions in clearance rates in the presence of colonial M. aeruginosa (Bosmina longirostris, Ceriodaphnia quadrangula) or relatively high consumption of colonial M. aeruginosa (Brachionus cafyciflorus, B. Iongirostris). Our results indicate that, contrary to earlier proposals, neither body size, taxonomic position, nor association with blue-green algal blooms are good predictors of the response to colonial M. aeruginosa.It has often been noted that blooms of colonial or filamentous blue-green algae are associated with changes in zooplankton composition, including losses of large cladocerans and increases in importance of smaller cladocerans, rotifers, and copepods (Gliwicz 1977;Edmondson and Litt 1982; Richman and Dodson 19 8 3; Orcutt and Pace 1984). Among the explanations for such changes in zooplankton composition is that the colonial morphology of the blue-green algae most strongly inhibits large-bodied cladocerans. Two major, but not necessarily mutually exclusive, mechanisms have been proposed for the greater susceptibility of large cladocerans to colonial or filamentous blue-green algae (Gliwicz 1977;Webster and Peters 1978). First, it has been suggested that colonial or filamentous algae clog the filtering appendages of larger cladocerans, reducing their feeding rates on co-occurring nutritious food sources or increasing their respiration rates or both. Several studies have supported this mechanism, showing that feeding or respiration rates of larger cladocerans are most strongly affected by the presence of filamentous blue-greens (Webster and Peters 1978;Richman and Dodson 1983; Porter and McDonough 1984). However, this mechanism has not been examined with more loosely organized colonial blue-greens, such as Microcystis aeruginosa. Moreover, these studies do not clearly demonstrate that the effects on feeding and respiration are due solely to the filamentous morphology; toxic or inhibitory chemicals produced by the blue-green algae could also contribute to the effect. Lampert ( 198 1, 1982) has shown that unicellular M. aeruginosa can strongly inhibit cladoceran feeding rates, presumably due to a chemical effect. Webster and...

This study compares and contrasts the dynamics of phytoplankton, zooplankton, and nutrients in two of the largest shallow lakes in the USA (Lake Apopka, Florida) and Europe (Lago Trasimeno, Umbria, Italy) and considers particularly the biomass ratio of zooplankton to phytoplankton (BZ:BP) in relation to nutrient levels and in the context of data from other subtropical and temperate lakes. Lake Apopka is hypereutrophic with higher concentrations of total phosphorus (TP), nitrogen (TN), and nearly an order of magnitude higher BP than Lago Trasimeno. However, combined data from the two lakes can be fit to a single log-log regression model that explains 72% of the variability in BP based on TP. In contrast, BZ has a significant positive log-log relationship with TP only for Lago Trasimeno, and is much lower than expected based on the TP concentrations observed in Lake Apopka. Lake Apopka has a fish assemblage that includes high densities of gizzard shad (Dorosoma cepedianum) and threadfin shad (D. petenense), similar to other eutrophic Florida lakes that also have extreme low BZ. The ratio BZ:BP is below 0.01 in Lake Apopka, 10-fold lower than in Trasimeno and among the lowest values reported in the literature. Although stress of high water temperature and a greater proportion of inedible cyanobacteria may be contributing factors, the collective results support an emerging view that fish predation limits the biomass of crustacean zooplankton in subtropical lakes.