Role of internal waves on mixing, nutrient supply and phytoplankton community structure during spring and neap tides in the upwelling ecosystem of Ría de Vigo (NW Iberian Peninsula)

Abstract

AbstractDespite evidence of internal waves in the NW Iberian upwelling region, their action and role on nutrient supply dynamics and phytoplankton community structure remain unexplored. A multidisciplinary approach, combining analysis of Synthetic Aperture Radar (SAR) images acquired during the summer months of 2008–2011, together with high‐frequency samplings carried out in the Ría de Vigo in August 2013 during spring (CHAOS1) and neap tides (CHAOS2), was used to characterize: (1) the internal wave activity, (2) its influence on mixing and nutrient supply, and (3) its role on phytoplankton community. SAR images revealed that internal waves were more energetic during spring tides. Turbulent mixing was higher during CHAOS1‐springs (Kz =1.3 [1.0–2.0, 95% confidence interval] × 10−3 m2 s−1) compared to CHAOS2‐neaps (Kz = 0.7 [0.5–1.0] × 10−3 m2 s−1), and as a result nitrate diffusive fluxes were approximately fourfold higher (35 [17–73] mmol m−2 d−1) during CHAOS1‐springs. The sampling covered a transition from relaxation‐stratification (CHAOS1‐springs) to intensifying upwelling (CHAOS2‐neaps) conditions, resulting in nitrate supply (including both diffusive and advective fluxes) being about 50% higher during CHAOS2‐neaps. The phytoplankton community, which was overwhelmingly dominated by diatoms in both cruises, exhibited a shift in species composition, with an increase in the abundance of large Chaetoceros spp. during CHAOS2‐neaps. About 50% of the primary production in the ecosystem during periods of upwelling relaxation‐stratification could be sustained by enhanced nitrate diffusive fluxes during spring tides. Therefore, even in coastal upwelling regions, turbulent mixing driven by internal waves could play an important role in controlling phytoplankton productivity and community structure.