Analysis of primary biomass dynamics in a micro-tidal estuary: The case of Fangar Bay.

Primary production in coastal bays and estuaries is shaped by various physical factors, including wind, tides, freshwater inflows, and light availability. Over short timeframes, these factors can affect other key variables, such as phytoplankton biomass and nutrient levels in the water. Research conducted in Fangar Bay—a small, shallow, microtidal bay in the northwest Mediterranean Sea—has highlighted spatial and temporal variations in phytoplankton biomass linked to different wind patterns. This bay is characterized by regular sea breezes and occasional episodes of intense northwesterly winds, which can mix the water column for varying durations.

To analyse both temporal and spatial variability, the Regional Ocean Modelling System (ROMS), integrated with a nitrogen-based NPZD (nutrient, phytoplankton, zooplankton, and detritus) model, was used to simulate eco-hydrodynamics in Fangar Bay. Four sampling points were selected to observe changes in phytoplankton concentration across the bay. During sea breeze events (wind speeds of 6 m·s⁻¹), vertical stratification dominates, leading to higher phytoplankton concentrations near the sea surface compared to the bottom. Conversely, during strong NW wind episodes (exceeding 10 m·s⁻¹), the water column becomes fully mixed, equalizing nutrient distribution and increasing phytoplankton biomass in deeper layers.

Additionally, the dispersion of freshwater plumes from irrigation channels significantly influences the spatial distribution of phytoplankton biomass within the bay. Numerical simulations confirm the role of freshwater plume dynamics in shaping the distribution of both phytoplankton and other water properties, aligning with observations from remote sensing. These findings offer valuable quantitative insights for developing more sustainable management strategies for such environmentally sensitive coastal areas.

The analysis of phytoplankton biomass dynamics in the Fangar Bay provides essential insights for implementing nature-based solutions (NBS) aimed at improving water quality. This knowledge enables the optimisation of freshwater discharge management and its interaction with prevailing winds, promoting natural processes such as water mixing and renewal. These strategies, inspired by the bay’s ecological cycles, can help reduce excessive phytoplankton growth and mitigate issues like eutrophication, contributing to the sustainability of the coastal ecosystem.

Keywords: phytoplankton biomass, ROMS-NPZD model, wind, biological parameters, chlorophyll-a, Fangar Bay.

 

Funding:  This work has been funded by the I+D+i project ECO-BAYS (PID2020-115924RB-I00) financed by MCIN/AEI/10.13039/501100011033

Acknowledgements: We would like to thank the REST-COAST project (H2020-101037097-REST-COAST) European Union’s Horizon 2020 program. As a group, we would like to thank the Departament de Recerca i Universitats de la Generalitat de Catalunya (2021GR0060)