Diversity of the phytoplankton assemblages of a polymictic hypertrophic lake

Diversity of phytoplankton assemblages from the shallow hypertrophic lake, the Albufera of Valencia (Spain), was studied for the period 1980 - 1988. The lake serves as a reservoir for the surrounding rice cultivation, with two seasonal periods of water retention to keep ricefields flooded. The seasonal hydrological cycle seems to be the primary factor driving phytoplankton community changes, compounding the disturbances due to water mixing. The lake is dominated throughout by filamentous cyanophytes. A Shannon-Weaver function was used to measure phytoplankton diversity. Phytoplankton steady-states, defined as the periods when more than 80 % of total phytoplankton biomass was shared by only 1-3 species, were assumed for comparative purposes. Phytoplankton diversity and the assemblages showed a high degree of interannual regularity during the study period. Phytoplankton was mainly limited by light and water flushing from November to April, and more by nitrogen and phosphorus during summer. Low diversities were related to stable community states, while higher diversities corresponded to non-steady state periods, the former lasting longer. Four annual phytoplankton associations were found, both steady and non-steady states being characterized by two different algal communities. Diversity reached a steady-state, lasting from November to june. From january to March, water flushing high nutrient levels and oligophotic conditions selected in favour of large unicellular phytoplankton species. The onset of spring quiescence resulted in the lowest values of diversity and species richness, due to the overwhelming dominance of vernal crops of Planktothrix agardhii, whose selfshading populations would have excluded many less well-adapted species. By june, nutrient decrease and increasing temperatures and irradiance, moved phytoplankton towards an intermediate assemblage dominated by smaller and faster growing species. Thereafter, long-summer water stabilization and nutrient depletion interacted to enhance diversity and the onset of a non-steady state, characterized by an immature summer assemblage of large colonies, potentially motile (e.g. Microcystis) and with high nutrient storage capacity. During the autumnal period when the ricefields are drained for harvest, diversities reached maximal levels, including species from the surrounding habitats (ricefields and drainage ditches). Lake reisolation by November-December led to another phytoplankton steady-state, when the phytoplankton reverted toward a composition similar to that of June. Storms increased diversity and the presence of small-sized species, contributing to a more rapid species replacement or arresting the establishment of an steady-state, depending upon their intensity frequency and occurence. Stagnation always selected in favour of cyanophytes, while flushing rates enhanced the presence of other algal groups, mainly diatoms. The annual flushing cycle of the lake is regarded as a potential tool for the future lake restoration. Our results confirm the view of regarding the phytoplankton of polymictic Oscillatoria-dominated lakes, as sub-climatic communities mainly limited by light, and agree with some postulates based on the IDH-diversity theory