Primary production and nutrient dynamics of the southeastern Bering Sea shelf

Abstract

Understanding the relationships between the distributions of organisms and oceanographic conditions was one of the major goals of the Southeastern Bering Sea Carrying Capacity (SEBSCC) study. As a part of SEBSCC, this study focused on the response of nutrients and primary production to the variations of physical conditions, the general distribution of primary production, and the dynamics of phytoplankton growth, and nutrient utilization over the middle shelf and shelf break regions. The concentration of nutrients and primary productivity were measured over the shelf during 1997-1999. Shipboard nutrient and iron addition experiments were conducted over the middle shelf and shelf break region of the southeastern Bering Sea shelf during 2000-2001. The variations in physical conditions strongly affected the distribution of nutrients in the surface euphotic layer as well as in the deep layer. The offshore transport of the middle shelf water at mid-depth over the outer shelf may playa very important role in the export of materials, including regenerated iron, from the middle shelf to the shelf break. There were large seasonal and spatial variations in the development of the spring phytoplankton bloom due to the strength of upwelling and the slope of the front at the shelf break. However, annual primary production, estimated by combining carbon uptake data of the PROBES study and this study, were similar over the inner (133 g C m⁻² y⁻¹), middle (144 g C m⁻² y⁻¹) and outer (138 g C m⁻² y⁻¹) shelves and the shelfbreak (143 g C m⁻² y⁻¹). Nutrient addition studies showed that nitrogen availability was essential to continuous phytoplankton growth during summer, and that the interaction between ammonium and nitrate may play an important role in the dynamics of nutrient utilization. The iron addition study suggested that lack of iron did not affect the growth of phytoplankton over the middle shelf, but slightly suppressed growth at the outside edge of the shelf break region.