Spring and Summer Proliferation of Floating Macroalgae in a Mediterranean Coastal Lagoon (Tancada Lagoon, Ebro Delta, NE Spain)

During the last 10 years, a drastic change in the structure of the community of primary producers has been observed in Tancada Lagoon (Ebro Delta, NE Spain). This consisted of a decrease in the abundance of submerged rooted macrophyte cover and a spring and summer increase in floating macroalgae. Two spatial patterns have been observed. In the west part of the lagoon, Chaetomorpha linum Kützing, dominated during winter and decreased progressively in spring whenCladophora sp. reached its maximum development. In the east part of the lagoon, higher macroalgal diversity was observed, together with lower cover in winter and early spring. Cladophora sp., Gracilaria verrucosa Papenfuss andChondria tenuissima Agardh, increased cover and biomass in summer. Maximum photosynthetic production was observed in spring for G. verrucosa (10· 9 mg O2 g-1 DW h-1) and C. tenuissima (19·0 mg O2 g-1 DW h-1) in contrast with Cladophora sp. (15·9 mg O2 g-1 DW h1) and Chaetomorpha linum (7·2 mg O2 g-1 DW h-1) which reached maximum production in summer. Increased conductivity from reduced freshwater inflow, and higher water temperatures during periods of lagoon isolation, mainly in summer, were the main physical factors associated with an increase in floating macroalgal biomass across the lagoon. Reduced nitrogen availability and temperature-related changes in carbon availability during summer were related to a decrease in abundance of C. linum and increases in G. verrucosa and Cladophora sp.