Modelling phytoplankton biomass evolution in a micro-tidal estuary: the case of Fangar Bay.

Abstract

The primary production in coastal bays and estuaries is influenced by multiple physical variables, such as wind, tides, freshwater inputs or light availability. At short temporal scales, these factors can influence other variables such as phytoplankton biomass, as well as the amount of nutrients present in the water body. Observations made in Fangar Bay, a small, shallow, microtidal bay in the NW Mediterranean Sea, have shown that there are spatial and temporal variations in phytoplankton biomass depending on different wind events. This bay is characterised by the presence of sea breezes and episodes of very strong NW winds, which homogenize the water column for variable periods of time. The Regional Ocean Modelling System (ROMS) coupled to a nitrogen-based nutrient, phytoplankton, zooplankton and detritus (NPZD) model has been applied to understand this spatio-temporal variability of the phytoplankton biomass in Fangar Bay under a range of conditions. Four sampling points have been chosen to visualize the differences in concentration within the bay. When breezes blow (6 m·s-1), stratification predominates, featuring higher concentrations at the surface than at the bottom.  In contrast, during strong wind episodes (> 10 m·s-1), the water column is well mixed, which homogenises nutrient concentrations throughout the column and increases the phytoplankton biomass in the lower layers. The dispersion of freshwater plumes from irrigation channels conditions the spatial distribution of phytoplankton biomass within the bay. The performed numerical analyses demonstrate the influence of freshwater plume evolution on the distribution of phytoplankton biomass and other water properties, in agreement with remote sensing observations. These results provide a quantitative basis for a more sustainable management of such constrained coastal domains. Keywords: phytoplankton biomass, ROMS-NPZD model, wind, biological parameters, physical parameters, Fangar Bay. Acknowledgements: This work has been funded by the I+D+i project ECO-BAYS (PID2020-115924RB-I00) financed by MCIN/AEI/10.13039/501100011033; and REST-COAST (H2020-101037097-REST-COAST) under the European Union’s Horizon 2020 program.