Abstract
Reservoirs are primary water resources for many uses in the Mediterreanean region and need dedicated studies for understanding the complexity of their dynamics particularly vulnerable to local and global stressors. This study focused on phytoplankton variations in relation to seasonal environmental changes on a multiannual time scale (2006–2015) at a Mediterranean eutrophic reservoir (Bidighinzu Lake, Italy) belonging to the Italian, European and International Long Term Ecological Research networks. Phytoplankton cell density, volume and biomass and chlorophyllaconcentrations were analysed together with meteo-climatic, hydrological, physical and chemical variables to detect trends and correlations. The period under study was also compared with previous years to assess the presence of significant differences in the environmental and planktonic compartments. Multiannual changes were more pronounced in summer than in the other seasons during the decade under study. The most conspicuous environmental changes were a significant decrease in summer nutrient concentrations in the reservoir and a simultaneous anthropic pressure reduction in the watershed. In addition, the mixing zone and euphotic zone ratio also increased. Multiannual changes in summer phytoplankton composition consisted of an increased density of smaller Bacillariophyceae and Cyanophyceae, which replaced larger species of the same phytoplankton classes. This resulted in opposite trends of total phytoplankton cell density (increasing) and mean phytoplankton cell volume (decreasing) over the study years. The nutrient decrement was statistically the strongest environmental driver of the phytoplankton changes observed in the reservoir. However, the mixing zone and the euphotic zone ratio and water temperature also significantly affected the multiannual phytoplankton variations. Therefore, we conclude that the success of small cell-sized phytoplankton in Bidighinzu Lake was most probably due to the synergic interactions of more environmental forces related to changing anthropic pressures and climate variability. Our results highlight the importance of long-term monitoring of reservoirs in the Mediterranean basin, especially in semi-arid regions where the need and scarcity of high quality water will be further exacerbated due to the global climate change.
Highlights
IntroductionPhytoplankton growth depends on water temperature and on light and nutrient availability (Winder and Sommer 2012)
In all aquatic ecosystems, phytoplankton growth depends on water temperature and on light and nutrient availability (Winder and Sommer 2012)
A significant multiannual decreasing trend in nutrient concentrations was detected in summer in the Bidighinzu Lake simultaneously with reduced anthropic pressure in its watershed
Summary
Phytoplankton growth depends on water temperature and on light and nutrient availability (Winder and Sommer 2012). Being the basis of pelagic trophic web as the primary producers, phytoplankton is a fundamental element for understanding the dynamics of the ecosystem and for predicting and managing the ecosystem responses to local and global changes (Guinder et al 2010). Different selective pressures, such as light availability, nutrient limitation, fluctuating nutrient supply or grazers, can influence the size structure in natural phytoplankton communities (Litchman et al 2010). Nutrients and carbon throughout the food web can be expected when small phytoplankton dominates (Legendre and Rassoulzadegan 1996, Falkowski et al 1998), with consequences on the ecosystem functioning (Kamenir and Morabito 2009)
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