Increase of rotifer diversity after sewage diversion in the hypertrophic lagoon, Albufera of Valencia, Spain

The Albufera of Valencia is a large oligohaline hypertrophic lagoon, regula ted by sluice gates according to the needs of the surrounding rice field cu ltivation. It is in a turbid state with permanent cyanobacterial blooms. A slight improvement was detected after diversion in the 1990s of part of the sewage flowing into it. After sewage diversion, we found that: (1) Chlorop hyll concentration and rotifer densities decreased; (2) Rotifer proportions declined, due mainly to a relative increase in cladocerans; (3) Rotifer di versity increased. The two dominants of the 1980s, Polyarthra spp. in the c older period and Brachionus angularis in the warmer one, reverted after sew age diversion to a more diverse assemblage reminiscent of the 1970s, with a higher number of dominant species. In the summer of 1998, both Brachionus calyciflorus and its predator Asplanchna brightwelli, dominant in 1973, bec ame abundant again. In 1998, an increase in the number of dominant species was also observed during water renewal periods, some of these species were new or seldomly found before in the lagoon ( Proalides tentaculatus-digitus , Trichocerca pusilla at the end of rice culture, Brachionus variabilis at the end of winter flooding). Another change that indicates an improvement o f water conditions is a more distinct and longer clear water phase, which o ccurs in the water renewal period at the end of winter and involves a Daphn ia magna peak. The increased importance of this phase, promoted the flouris hment of Brachionus variabilis, a facultative Daphnia epibiont never found before in the lake.