Annual changes in the algal density and concentrations of chlorophyll a, total phosphorus, and organic matter were analyzed in water and sediments at four sites characterized by the presence or absence of submerged and emergent macrophytes, during turbid-and clear-water conditions to determine the contribution of the algal components of the plankton and the epipelon and to identify the most typical species in each community. Three states were recognized: one turbid and two clear, with different submerged macrophyte cover. The peaks of phytoplankton and epipelon occurred in the turbid phase, whereas the highest proportion of true epipelic algae in sediments was reached in the second clear phase. The Oscillatoriaceae dominated during the turbid phase in the water and throughout the entire year within the sediments. IntroductionIn shallow lakes where alternative equilibrium states can occur, the importance of the relationships between epipelic and pelagic algae has recently been highlighted (VADEBON-COEUR et al., 2002). In such lakes the structure and biomass of epipelon varies in relation to spatial heterogeneity created by the presence or absence of emergent and submerged macrophytes. According to CYR (1998), the biomass of benthic algae varies among sites and according to depth in small oligotrophic and mesotrophic lakes.The benthic assemblages are made up of autochthonous species plus others incorporated from the plankton and periphyton. Thus, changes in environmental conditions can promote an increase in the density of those algae that can develop in both the sediment and the water column, giving them a competitive advantage over the algae that can live in only one of those lake habitats. The seasonal distribution of algal productivity observed by HANSSON (1996) in North American lakes, and by BARKO et al. (1977) andFLÖDER et al. (2006) in small ponds, suggests an exchange of algal organisms between the planktonic and epipelic populations.The phosphorus in shallow lakes tends to accumulate in the sediments and in the macrophytes (SØNDERGAARD et al., 2003;ROONEY and KALFF, 2003). Thus, the nonplanktonic algae 154 M. A. CASCO et al.

Biomass assessments of algae in wetlands usually include only the phytoplankton community without considering the contribution of other algal associations to total algal biomass. This omission prevents an accurate evaluation of the phytoplankton community as an integral part of the total ecosystem. In the present work, the biomass contributions (expressed as chlorophyll-a content per m 2 of lake) of phytoplankton, epiphyton on both submerged and emergent macrophytes, and epipelon were measured in Lacombe Lake, Argentina, for the purpose of (1) establishing the relative importance of the phytoplankton and (2) evaluating the entire contribution of algal biomass within the context of the Goldsborough & Robinson conceptual model. Our sampling was carried out monthly for a year in sites representative of different conditions with respect to water depth and type of macrophytes. Physicochemical analyses of water were performed following standard methods. Plankton was collected in a five-level profile at deeper stations and in subsurface samples at the shallow one. Samples of sediment obtained with corers were collected for epipelon sampling and segments of plants were cut at different levels, so as to obtain the epiphytes by scraping. Pigment was extracted with aqueous acetone and calculations were made by means of the Lorenzen equation. According to the Goldsborough & Robinson model, a Lake State developed here during the winter (phytoplankton maxima: 150 mg chlorophyll-a per m 2 ). Then, through the subsequent growth of the submerged macrophytes, an Open State was observed, characterized by a maximum epiphyton biomass (at 3,502 mg chlorophyll-a per m 2 ) along with lower levels of phytoplankton biomass. The epiphytic algae on the emergent macrophytes were always present but attained only relatively low biomass values (maximum: 120 mg of chlorophyll-a per m 2 in February). The epipelon biomass varied between 50 and 252 mg chlorophyll-a per m 2 , registering a considerable contribution of settled algae from the water column (phytoplankton). This study contributes to our knowledge of wetland dynamics through its assessment of the rapid changes in the relative contributions of both planktonic and attached algae to the total algal biomass within the context of specific environmental factors.

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