Temporal and diel cycling of nutrients in a barrier–lagoon complex: Implications for phytoplankton abundance and composition

The cycling of nutrients (silicate, nitrate, ammonium and phosphate) in a quasi-pristine barrier–lagoon complex (Louro Lagoon, NW Iberian Peninsula) was studied both in a diel and temporal timescale covering different seasonal conditions. Hydrographical and meteorological conditions exert a major control on the nutrient cycling in the lagoon. From late autumn to early spring the lagoon is characterized by oxic conditions due to significant freshwater inputs and sporadic communication with the sea. In summer, elevated primary production coupled with water stagnancy leads to bottom waters hypoxia and marked diel variation of pH, oxygen and redox potentials. Diel variation of nutrients was only evident when mixing of surface and bottom waters during the night was observed.Temporal changes in nutrients concentrations and their relative abundance were found to induce a clear shift in the phytoplankton composition. In winter, during silicate and nitrate abundance and low phosphate, diatoms predominate. In summer, under hypoxic conditions, sediments take control on the supply of nutrients leading to increased phosphate availability relative to nitrate/ammonium or silicate. Diatoms are then replaced during the summer blooms – linked to low atomic N:P ratios (typically <2) – by flagellates and green algae with minor contributions of cyanophytes and dinoflagellates.

► The Louro lagoon shifts from winter oxic conditions to stagnant hypoxic waters in summer.
► Marked diel variations of pH, oxygen and redox potentials during summer.
► Summer hypoxia leads to increased phosphate availability.
► Seasonal changes in nutrients concentrations and their relative abundance induces a shift in phytoplankton composition.
► Flagellate and green algae blooms are associated with low N:P ratios.