Implications of rapid changes in chlorophyll-a of plankton, epipelon, and epiphyton in a Pampean shallow lake: an interpretation in terms of a conceptual model

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 m² 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 m²). 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 m²) 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 m² in February). The epipelon biomass varied between 50 and 252 mg chlorophyll-a per m², 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.