ROLE OF INTERNAL WAVES ON MIXING, NUTRIENT SUPPLY AND PHYTOPLANKTON COMPOSITION DURING SPRING AND NEAP TIDES IN THE RÍA DE VIGO (NW IBERIAN PENINSULA)

Abstract

The effect of internal wave activity on nutrient supply to the euphotic layer and its potential impact on phytoplankton composition and size-structure remain unexplored in the shelf edge off NW Iberian Peninsula. In August 2013, continuous measurements (yo-yo) of dissipation rates of turbulent kinetic energy, obtained with a microstructure profiler, were carried out during spring and neap tides, covering a complete tidal cycle (25 hours). Every hour the yo-yo was interrupted to collect samples for inorganic nutrient concentration. Microturbulence and nutrient concentration data were combined to compute the nutrient input into the euphotic zone driven by vertical diffusivity. Size-fractionated chlorophyll-a and pigment composition samples were taken for the determination of phytoplankton community structure. As a result of increased mixing levels, nitrate transport to the euphotic layer during spring tides was approximately 4-fold higher (34.6 mmol m-2 d-1) than during neap tides (9.4 mmol m-2 d-1). However, the phytoplankton community was always dominated by large (>20 µm) chain-forming diatoms and seemed to be mainly controlled by the upwelling conditions predominating in late August.