Limnetica 31

This study was aimed at describing the phytoplankton dynamics and structure of two eutrophic to hypereutrophic Mediterranean shallow lakes (Santa Olalla and Dulce). The two systems are close together and can function as one water body at times of heavy rainfall, but once separated, they evolve differently. The Shannon diversity index was low for both shallow lakes (average values of 1.11 in Santa Olalla and 1.79 in Dulce). The average phytoplankton Chl a concentrations and primary production rates were very high, although slightly higher in Santa Olalla (365.2 mg m^–3 and 1.29 g C m^–3 h^–1, respectively) than in Dulce (230 mg m^–3 and 0.88 g C m^–3 h^–1). Phytoplankton variation in the lakes was related to shifts in the physical and chemical features of lake water as well as hydrological conditions, a finding that is corroborated by the canonical correspondence analysis results, which showed a different pattern of evolution in each system. Eight functional groups were found in Santa Olalla (D, H₁, J, K, M, S₁, W₂ and Y), although the D, W₂ and Y groups were only predominant during the first four months of the study. For the rest of the period, the system was particularly dominated by the H₁, K and S₁ groups. Dulce exhibited a more complex distribution of phytoplankton functional groups over time. Ten functional groups were observed in this system (D, H₁, J, K, M, P, S₁, S₂, W₂ and Y). Some characteristics of these systems, such as rapid water volume fluctuations, low light penetration and low concentration of inorganic nutrients, are stressful conditions for phytoplankton, which may account for the low phytoplankton diversity and the equilibrium phases recorded for many months in both wetlands.