Phytoplankton changes in a shallow Mediterranean lake (Albufera of Valencia, Spain) after sewage diversion

Abstract

Phytoplankton from Lake Albufera, a shallow, polimictic, freshwater lake, was studied before and after sewage diversion. The lake is used as a reservoir for rice cultivation in the surrounding lake area. Due to antropogenic eutrophication in the 1960s, the lake turned from a mesotrophic to a hypertrophic, turbid state. In 1991, a restoration plan of the lake started which has reduced 30% of sewage effluents. Total phosphorus in the lake was reduced about 31%, from 0.49 to 0.34 mg l−1, but nitrate did not vary. Chlorophyll-a mean annual values diminished to half, although hypertrophic levels remained (mean value from 318 μg l−1 to 180 μg l−1). The algal community structure indicates a response to P reduction in the direction of increasing equal contribution of algal groups. Cyanobacteria are still the dominant group, but reduced their biomass by about 15%, due to filamentous cyanobacteria depletion (from 78% to 48% of total phytoplankton biovolume), in favour of chroococcal cyanobacteria (changed from 7 to 23% in the 1990s). Other algal groups, especially diatoms and chlorophytes, also increased their contribution in the phytoplankton after nutrient diversion, while euglenophytes, indicative of organic matter pollution in the lake, decreased. There were pronounced changes in phytoplankton composition and a general trend toward presence of smaller algal species. The dominant species in biovolume during the 1980s, Planktothrix agardhii, almost disappeared in the lake in the 1990s, and was replaced seasonally by the slender Pseudanabaena galeata. Disappearance of Planktothrixagardhii, allowed large Cladocera to control algae early in the year (dominant small diatoms and chlorophytes), during several clear water phases occurring in recent years that lasted up to five weeks. Even though some improvement of the lake water quality was observed, complementary restoration measures are suggested, such as a reduction of phosphorus below 0.05 mg l−1, control of pesticides in the catchment area, and management of benthivorous-planktivorous fish species, in order to re-establish phytoplankton composition and submerged plants as at the beginning of the 20th century.