Short-term high-resolution physical-chemical-biological coupled observations on the inner shelf of the Northern Margin of the Gulf of Cadiz

Coastal systems are productive zones where the understanding of coupled physical-chemical and biological processes is of utmost relevance. Deployments of high-resolution observational systems in these zones, preferentially for long periods, play a key role contributing to bridging the knowledge gaps of the processes involved. In this context, the relationship between currents shifts at an artificial reef in shallow inner-shelf waters at the Northern Margin of the Gulf of Cadiz (NMGoC) and the impact on phytoplankton development was examined through a high-intensity observational experiment lasting for 12 days during April 2022. A wave-powered vertical profiler continuously logged an average of ~120 high-resolution profiles (2 Hz) every hour through the ~20 m deep water column, achieving an unprecedented description of the changing water column properties (temperature, salinity, dissolved oxygen, turbidity, and chlorophyll-a, Chl-a). Additionally, hourly current and local wind velocities were respectively retrieved from an Acoustic Doppler Current Profiler (ADCP) moored nearby and from the ERA 5 Reanalysis database.

During these 12 days of observation, three different periods were identified with direct changes in phytoplankton activity. Phytoplankton concentration varied greatly depending on the physical forcing, which included current velocity and direction, wind, which also comprised local land breeze, cross-shelf transport, and tidal conditions. While a 6-day upwelling event was observed, with a colder and less saline water signature, the intensity of the surface current was the highest (0.4 m s-1), with a mixed water column, and the phytoplankton concentration was at its lowest (< 1.5 mg m^-3). High spring seasonal levels of Chl-a (3-4 mg m^-3) were observed during a 4-day event of Coastal Counter Current (CCC) and a 20h current inversion to a westward alongshore flow, both driven by wind relaxation. The CCC setup was synonym of a reduction of the dominant eastward alongshore flow (0.2 m s^-1 at surface), when an intermittent thermal stratification was found. This corresponded to a vertical thermal gradient of up to 2.5 °C along the water column, which partially resulted in the localised increase in phytoplankton biomass underneath the warmer strata, along with afternoon land-breezes enhancing cross-shelf mixing. Strong midday irradiance inhibited phytoplankton development, that systematically dropped at midday and increased only after 3 pm. The Chl-a concentration appeared homogenously mixed in the water column only when the alongshore current reversed to a westward flow, suggesting the westward advection of phytoplankton-rich waters from the retention “shadow” area in the vicinity of the Guadiana River.

While upwelling may be an important source of nutrients enabling phytoplankton development, rapid dynamic changes in biomass leave open speculation on the importance of current relaxation and longer residence time. Both enhanced thermal stratification and water column stability, favouring phytoplankton growth. Furthermore, diurnal local wind shifts played a role in cross-shelf mixing and the advection of near-shore and mid-shelf communities. These data demonstrate the importance of high-resolution observational systems in productive coastal areas, contributing to better understanding the processes involved.