On phosphate pumping into the surface layer of the eastern Gotland Basin by upwelling

• Published in: Journal of marine systems 2010-02, Vol.80 (1), p.71-89
• Main Authors: Lass, Hans-Ulrich, Mohrholz, Volker, Nausch, Günther, Siegel, Herbert
• Format: Article
• Language: English
• Subjects: Baltic Sea; Basins; Boundary layer; Coastal; Coastal environments; Depletion; Eastern Gotland Basin; Intermediate winter water; Marine; Mesoscale eddies; Nitrate; Particulate phosphorus; Phosphate; Phosphates; Phytoplankton; Surface layer; Upwelling; Vertical turbulent diffusion; Winter
• ISSN: 0924-7963; 1879-1573
• DOI: 10.1016/j.jmarsys.2009.10.002

Hydrographic, current, and nutrient measurements were performed on board of the R/V Poseidon in the eastern Gotland Basin from 12 to 21 July 2007, complemented by satellite measurements of sea surface temperature and chlorophyll-a and by measurements of turbulent dissipation. The main aim of the investigations was to elucidate the pathways of phosphate from the intermediate winter water into the surface mixed layer and how this phosphate flux fosters phytoplankto growth. The deep water of the Gotland Basin was exposed to an enduring stagnation phase resulting in a nitrogen to phosphorus ratio very much lower than the Redfield ratio. The intermediate winter water was completely depleted of nitrate but contained a phosphate pool of about 0.40 µM. The surface mixed layer had a mean depth of about 18 m and water temperature of 16 °C and was depleted of both nitrate and phosphate. The average of photosynthetically active radiation during the cruise was about 700 µE/(m²s) at the sea surface. Three pulses of westerly wind with maximum wind speed of about 15 m/s forced upwelling during the cruise at the east coast of Gotland and downwelling at the Latvian coast. The corresponding eastward Ekman offshore transport moved surface water within about three days through the coastal boundary layer and in roughly 50 days across the Gotland Basin. Two mesoscale eddies were observed off the south eastern coast of Gotland which supported upwelling filaments near the southern tip of this island. Within the coastal boundary layer, the upwelling transported phosphate from the intermediate winter water into the surface layer off Gotland where it was mixed with the phosphate depleted surface water. Generally, the upwelled phosphate was taken up by the plankton community of the surface mixed layer already within the coastal boundary layer and transformed completely into the particulate and dissolved organic phase. However, at sites where upwelling filaments develop, dissolved phosphate is transported over a distance of about three times the internal Rossby radius in offshore direction. The phosphate transport into the surface mixed layer of the eastern Gotland Basin by upwelling exceeded the corresponding transport by turbulent mixing through the seasonal thermocline by about one order of magnitude during typical summerlike wind conditions.