Water and nutrients transfer in the ria-ocean interface and phytoplankton patterns of two winter upwelling events in a Galician ria

Abstract

The consequences of two upwelling events in mid (MW) and late (LW) winter on the oceanographic patterns were studied in the Pontevedra Ria and compared with patterns measured under typical winter conditions and under a summer upwelling event. Thermohaline properties measured during MW revealed the intrusion of saltier seawater into the ria associated to the Iberian Poleward Current, however in LW the upwelled seawater showed characteristics of Eastern North Atlantic Central Water mass. In both cases, the water residence time measured, 4 days and 10 days respectively, was related to both meteorological and fluvial forcing. During MW-up, the ria becomes poor in nutrients (exported 3.4 molDIN·s-1 in comparison with 6.9 molDIN·s-1 of non-upwelling) and phytoplankton showed an uniform distribution throughout the ria, as during unfavorable upwelling conditions. The phytoplankton distribution was characterized by the dominance of diatoms, mainly Nitzschia longissima and Skeletonema costatum. During LW-up, a nutrient depletion in the photic layer also occurred but as a result of a phytoplankton spring bloom developing at this time. The ria was a nutrient trap where 4.1 molDIN·s-1 were processed by photosynthesis. The ria became more productive (three times higher than under non-upwelling conditions) although no as productive as during a summer upwelling event (9.9 molDIN·s-1). The taxonomic composition of phytoplankton community did not change, varying only the relative proportions. Diatoms were always the dominating microphytoplankton community with Pseudo-nitschia pungens, Thalassionema nitzschioides and several species of Chaetoceros as characteristic taxons