Abstract
Phytoplankton primary production in coastal bays and estuaries is influenced by multiple physical variables, such as wind, tides, freshwater inputs or light availability. In a short-term perspective these factors may influence the composition of biological variables such as phytoplankton biomass, as well as the amount of nutrients within the waterbody. Observations in Fangar Bay, a small, shallow, stratified and micro-tidal bay in the Ebro Delta (NW Mediterranean Sea), have shown that during wind episodes the biological variables undergo sudden variations in terms of concentration and distribution within the bay. The Regional Ocean Model System (ROMS) coupled with a nitrogen-based nutrient, phytoplankton, zooplankton, and detritus (NPZD) model has been applied to understand this spatio-temporal variability of phytoplankton biomass in Fangar Bay. Idealised simulations prove that during weak wind events (< 6 m·s−1), the stratification is maintained and therefore there is not dynamic connection between surface and bottom layers, penalizing phytoplankton growth in the whole water column. Conversely, during intense wind events (> 10 m·s−1) water column mixing occurs, homogenising the concentration of nutrients throughout the column, and increasing phytoplankton biomass in the bottom layers. In addition, shifts in the wind direction generate different phytoplankton biomass distributions within the bay, in accordance with the dispersion of freshwater plumes from existing irrigation canals. Thus, the numerical results prove the influence of the freshwater plume evolution on the phytoplankton biomass distribution, which is consistent with remote sensing observations. The complexity of the wind-driven circulation due to the bathymetric characteristics and the modulation of the stratification implies that the phytoplankton biomass differs depending on the prevailing wind direction, leading to sharp Chl a gradients and complex patterns.
Highlights
The intense biological activity of estuaries and coastal bays and their importance as a source of resources and socio-economic services is well known
Four points within the bay have been chosen to investigate the temporal evolution of the biological variables obtained from the NPZD model: in the mouth area (M1), in the centre of the bay (M2), in a coastal area in front of the IM discharge point (M3) and in the innermost part of the bay (M4)
Substantial differences of phytoplankton biomass between surface and bottom layers are evident in M1, where the stratification tends to be larger in contrast to the shallowest point (M4)
Summary
The intense biological activity of estuaries and coastal bays and their importance as a source of resources and socio-economic services is well known. Freshwater inputs can have a considerable effect on the water circulation (Cerralbo et al, 2014) and, acting as fluvial nutrient suppliers, can determine the temporal and spatial variability of phytoplankton biomass (Geyer et al, 2018; Jiang et al, 2020) In this sense, the use of coupled physico-biological numerical models as a tool to understand the complexity of the phytoplankton regulatory mechanism in estuaries has increased in recent years, complementing in situ data and satellite imagery (Llebot et al, 2010; Artigas et al, 2014; Jiang et al, 2020). The use of coupled physico-biological numerical models as a tool to understand the complexity of the phytoplankton regulatory mechanism in estuaries has increased in recent years, complementing in situ data and satellite imagery (Llebot et al, 2010; Artigas et al, 2014; Jiang et al, 2020) These numerical models can provide information on the current state of the estuary, create hypotheses and numerical experimentation, and predict certain events and ecosystem responses (Stow et al, 2009; Llebot et al, 2010)
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