Selective withdrawal short-term effects of phytoplankton community composition in a Medieterranean reservoir

A combination of different factors as light-nutrient availability, mixing regimes and biological interactions between species regulate phytoplankton composition and abundance in reservoirs. A correspondence between withdrawals events and changes in phytoplankton community composition was pointed out by few studies based on the analysis of field observations. In this work we want to examine the specific reaction of phytoplankton groups, aggregated depending on their particular response to environmental conditions, to withdrawals level variation. We start from the concept that changes in the environmental conditions generated by withdrawals operations may favor the development of certain species in detriment of others. Our analysis was conducted adopting a conceptual model in which the reservoir is conceived as a stack of horizontal homogeneous layers having the same volume, and the only forcing is associated to outflows. Short term experiments (15 days) were conducted taking as reference the bathymetry of a Mediterranean medium-size reservoir (El Gergal- Seville). The response of several phytoplankton groups, with different sensitivities to the environmental conditions (e.g. tolerance to light availability), was evaluated simulating several scenarios, extracting water at surface, intermediate or bottom level. The changes experienced by the phytoplankton groups are dependent on the bathymetry of the lake under study, the withdrawal rate and the relative position between the level of extraction and the depth of the phytoplankton group development. Experiments results show that the modification of the outflow level may induce a change of the dominant phytoplankton group. Our work, understanding how withdrawals affect phytoplankton behavior, is oriented to the developing of alternative strategies for reservoir water quality management.

Anna Rigosi and Francisco Rueda

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