Vertical dynamics of planktonic communities at Sofala Bank, Mozambique

Abstract

Coastal ecosystems are largely influenced by the interaction of several factors operating at various temporal and spatial scales, specifically those responsible for primary and secondary production processes that modulate marine resources. Hydrological processes (e.g. tides and coastal currents), nutrients availability, phytoplankton groups, zooplankton and larval fish (specifically the clupeid Herklotsichths quadrimaculatus) abundance and distribution were investigated at the Sofala Bank (Mozambique), with special emphasis on their horizontal distribution and vertical dynamics. Horizontal distribution has shown onshore‐offshore gradients in all analysed parameters, as well as inshore waters intrusion probably related to Zambezi River delta runoff. Tidal currents were responsible for major hydrological vertical variations and for horizontal and vertical advection of phytoplankton biomass in the surface and deepest layers, respectively. Nutrient concentrations were typical from oligotrophic regions, and nutrient ratios were strongly influenced by depleted nitrite + nitrate concentrations, suggesting low estuarine discharges typical from the dry season. Both phytoplankton pigments and zooplankton were found mainly near bottom (40 m deep), despite the latter displayed vertical migrations triggered by light variations. Phytoplankton community was dominated by microflagellates, specifically prymnesiophyceans, and vertical distribution changes were similar for the whole community. Cyanobacteria was the only phytoplankton group that displayed a different vertical distribution pattern, mainly concentrated at mid water column depths (10 20 m). Herklotsichthys quadrimaculatus larvae displayed typical diel vertical migrations and were mainly distributed in the upper water column (0 20 m) during the night and almost absent from all sampled strata during the day. Larger larvae dominated the neuston layer during the night while during daylight periods remained close to the bottom. This investigation enhances the importance of physico‐chemical phenomena determining the planktonic communities vertical dynamics at a tropical coastal ecosystem of the Western Indian Ocean, where planktonic dynamics are still poorly described and understood

Resumo alargado disponível em português