Biomass of phytoplankton assemblages in Lake Geneva 1974-2010

Sampling has been continuously undertaken in the middle of the lake at the deepest point, once or twice a month. The studied environmental factors used in this study were selected based on previous analyses that emphasized their role on phytoplankton abundance: phosphorus concentration , water temperature , abundance of Daphnia sp. , and solar radiation (https://intranet.inra.fr/climatik_v2). Water temperature was measured with a thermometer from 1974 to 1998; a multiprobe was used after 1998. Dissolved phosphorus concentrations (P-PO43-) was measured at a series of discrete depths between the surface and the bottom of the lake, using colorimetric analyses and spectrophotometric assay according to a standardized protocol (AFNOR NF EN 1189). Water samples for estimating phytoplankton species and biomass were collected in the upper 10 m from 1974 to 2001; after 2001 in the top 18 m, using an integrating bell sampler. Phytoplankton identifications and cell counts were performed in sedimentation chambers under an inverted microscope. More than 400 cells were counted per samples. In this study, phytoplankton quantity was expressed in terms of biomass. Species biovolumes were derived from cell numbers and mean cell volumes using geometrical models. Finally, species biomass was computed considering a ratio of 1 µg per unit of volume. Daphnia sp. were sampled by a vertical tow from 50 m to the surface using a 200 µm plankton net. The zooplankton samples were preserved in a 5% buffered formaldehyde solution. Daphnia sp. were identified and individuals were enumerated in a 0.1ml sedimented zooplankton sub-sample. Abundances were given as the number of individuals per square meter. Phytoplankton functional groups were previously identified in Anneville et al. (2018).