Major nutrient distribution in relation to the physical structure of the Gulf of Alaska shelf

Abstract

The northern Gulf of Alaska is a biologically productive downwelling shelf. Nutrient sources supporting such productivity have not been adequately studied. Thirteen primary stations were occupied twelve times throughout 1998 and 1999 in an attempt to clarify nutrient distributions and sources. The shelf waters were warmer, fresher, lower in nitrate, and higher in phytoplankton biomass in the spring of 1998 compared to 1999. Nitrate, silicate, and phosphate were positively correlated with salinity indicating an offshore nutrient source. The largest rates of new production, estimated from nitrate drawdown in the upper layer between March and July/August, were 2.6 mmole nitrate m⁻² day⁻¹ in 1998 and 1.9 mmole nitrate m⁻² day⁻¹ in 1999. There was evidence of a summer onshore flux of dense, nutrient-rich bottom water when the downwelling regime relaxed or reversed. This seasonal flux was 20% less than the estimated nitrate flux through nearby Hinchinbrook Canyon.