LAUSR

Abstract Maputo Bay is highly affected by large tidal changes and riverine freshwater input with a phytoplankton biomass peak during March each year. Microscopy analysis was used to describe how the phytoplankton community composition was affected by tidal changes, during four in situ incubation experiments. Using stable isotope tracers, new and total primary production, based on nitrate (15NO3−)- and carbon (13C–bicarbonate)-assimilation were estimated. The highest biovolume of phytoplankton (> 2 μm) and also the highest C– and NO3−-assimilation rates (nM h−1) were found at spring-high tide. The C:N (mol:mol) ratio of particulate organic matter (POM) varied between 6.0 and 8.2. The proportion of diatoms in the phytoplankton community was higher at spring-high tide as compared to neap-low tide, whereas dinoflagellates were found in a reverse pattern. New production ranged between 6.3% and 10.4% of total primary production and was thus within the range previously reported for tropical regions. The largest proportion of NO3−-based new production relative to total production was estimated during calm conditions and spring-high tide. Concordantly, a large fraction of the microplanktonic community covered their N-demand by other sources of N than NO3−.