Phytoplankton and suitability of derived metrics for assessing the ecological status in a limno-reservoir, a Water Framework Directive nondefined type of Mediterranean waterbody

Limno-reservoirs are small waterbodies constructed in the riverine zone of large Mediterranean reservoirs to preserve a constant water level, with the goal of mitigating the undesirable effects of reservoir operation, including those related to water level fluctuations. The Pareja limno-reservoir was among the first limno-reservoirs, and its purposes are both environmental and recreational. Phytoplankton is one of the compulsory biological indicators used to estimate ecological status in lakes and reservoirs in accordance with the European Union Water Framework Directive (WFD). Chlorophyll a (Chl-a) and phytoplankton biovolume in the Pareja limno-reservoir were studied between spring 2008 and winter 2011. Phytoplankton composition was analyzed employing the functional group approach. The suitability of several phytoplankton metrics to assess the ecological status of the limno-reservoir was verified, including Chl-a, total biovolume, cyanobacteria contribution and absolute biovolume, IGA (index de grups algals), MedPTI (Mediterranean Phytoplankton Trophic Index), Assemblage (Q) Index, and PTI (Phytoplankton Trophic Index). Phytoplankton composition was dominated by centric diatoms from functional group B, although they were replaced by other groups in some periods as a response to environmental changes. Checking the response of the phytoplankton metrics to a trophic gradient in addition to Chl-a, we determined that total biovolume, Q Index, and especially PTI were the most appropriate metrics to assess the ecological status of the Pareja limno-reservoir. This approach has relevant management implications for Mediterranean limno-reservoirs. Ecological status assessment may not yet be mandatory in countries such as Spain, but considering environmental and recreational goals, performing such assessments is advisable.