Water renewal in semi‐enclosed basins: A high resolution Lagrangian approach with application to the Bay of Algeciras, Strait of Gibraltar

Abstract

Lagrangian experiments of particle tracking were carried out in the semi-enclosed Bay of Algeciras attached to the Strait of Gibraltar in order to investigate the flushing patterns. A high resolution three-domain-nested hydrodynamic model provided the velocity fields from a 61-d hindcast, with the aim of analyzing the flushing efficiency of eight different docks under a variety of external conditions, namely, tide phase and strength, and winds. The tracking algorithm was specifically developed to exploit the high spatial resolution of the model that reproduces the local dynamics accurately. Winds are the dominant agent, with westerlies featuring e-folding times one order of magnitude lower than easterlies. Fortnightly tidal modulation causes a counter-intuitive effect, with spring tides promoting higher accumulation of particles inside the docks and higher e-folding times than neap tides. Additionally to high resolution details on the flushing patterns of the Algeciras Port, the model also confirms the Bay as a potential feeder of floating tracers for the nearby Alboran Sea. The proposed approach is easily scalable and exportable to other similar locations worldwide.

Time Variability Patterns of Eutrophication Indicators in the Bay of Algeciras (South Spain)

“…Nevertheless, overall, salinity trended to be slightly higher in BA compared to ST, implying that certain local features of the BA would facilitate the mixing of SAW and MW. Among these features, tidal regime would play an important role as it also affects the exchange of SAW between inside and outside BA [14].…”

“…Furthermore, some samples collected during 2014 and 2015 showed high salinity and phosphate according to their scores for the third PC extracted from the PCA, but nitrate did not contribute to this variance component. A possible explanation for this weak correlation between nutrients and salinity is that nutrients were strongly impacted by phytoplankton consumption due to the expected relatively long residence time of the water masses within the Bay (10-20 days according to [14]). In fact, the PCA results indicate that Chl T a was negatively correlated with silicate and phosphate.…”

“…According to these authors' calculations, the flushing time of the whole BA water ranges from 10 to 20 days. A recent study based on a Lagrangian model for surface particle dispersion [14] supports that renewal times are fairly variable among different areas within the BA depending on wind conditions. The studies published on contaminant dispersion and their effects in the BA have focused on sediment [15,16] and benthic communities [17,18].…”

“…The geographical location of the stations was based on a thorough analysis of a time series of chlorophyll a satellite images for the period 2002-2010 (data not shown). This strategy allowed us to cover adequately the chlorophyll a gradient observed in the BA, normally located from the northern coast to the centre of the bay, and to avoid the influence of coastal structures (ports and docks), which modify the surface circulation pattern at a small scale [14]. Additionally, three stations located close to the BA in the northern Alboran Sea (Sotogrande; ST) in Figure 1) were sampled one day after visiting the BA.…”

“…According to Periáñez [12] and Sánchez-Garrido [13], the horizontal circulation patterns in BA are mainly regulated by the Atlantic Jet (AJ) entering the Alboran Sea through the Strait of Gibraltar. Changes in the position of the AJ with respect to the northern shoreline, due to atmospheric forcing, modify the circulation modes inside BA [14]. On the other hand, the wind regime regulates the amount of SAW and MW in the BA and affects the ascension of MW towards the euphotic layer.…”