Macrophyte influence on the structure and productivity of photosynthetic picoplankton in wetlands

We used multiple approaches to analyze photosynthetic picoplankton (PPP) structure and production in a wetland in the Lower Parana Basin (Argentina). A seasonal field survey was combined with an experimental study to analyze PPP variability under different light conditions. Flow cytometry analyses showed differences in PPP structure among the aquatic environments. Three main picoplankton populations were distinguished: phycocyanin-rich picocyanobacteria (Pcy), picoeukaryote (Peuk) and anaerobic anoxygenic photosynthetic bacteria (AnAnPB). The experimental study revealed important changes in PPP structure in relation to the light conditions imposed by floating plants: enclosures exposed to light showed a higher proportion of Peuk and Pcy over AnAnPB; in mesocosms covered by plants, just as in the highly vegetated relict lakes (ROLs), AnAnPB were dominant. Total picophytoplankton abundances varied from 1.7 x 10(4) to 4.6 x 10(5) cells mL(-1) in the shallow lakes, and were lower (0.69 x 10(4) to 2.5 x 10(5) cells mL(-1)) in the ROLs. Annual variations in temperature and hydrological conditions influenced the PPP abundances, observing maximum values during the warm dry phase. The photosynthetic rates per unit area of PPP (P-APPP) and algae > 3 mu m (P-A > 3 mu m) were measured in the aquatic environments in winter and summer: P-A PPP (1.5 to 100 mg C m(-2)h(-1)) was lower than P-A > 3 mu m and was directly affected by light, which was limiting under the dense floating plant cover.