Seasonal Changes in Productivity in the Copper River Plume and Coastal Gulf of Alaska

The Copper River is the largest point source of fresh water to the northern Gulf of Alaska, but the hydrography, biogeochemistry, and biology of the region are not well described. Oceanographic surveys were conducted in April, May, June and July 2010, along a transect line stretching from the mouth of the Copper River, across the plume, to well past the shelf break. In April, the water column was well mixed, with a very low freshwater signal nearshore; phytoplankton biomass was low, and macronutrient (nitrate, silicate, and phosphate) concentrations were high. The zooplankton was dominated by relatively scarce small bodied copepods inshore, and large pteropods and abundant large bodied copepods (Neocalanus sp.) offshore. By early May, weak temperature and salinity stratification were present, and phytoplankton biomass had increased, while nutrients showed indications of being drawn down. Although the spring bloom was at an early stage, phytoplankton biomass and nutrient drawdown were more pronounced onshore, indicating an earlier onset to production in the near shore region. Zooplankton abundance was low, and was mostly small bodied copepods and early stages of Neocalanus. By June, the spring bloom was completed, and all nutrients were depleted in the surface water, except for silicate, which was elevated in the less saline water in the vicinity of the plume. Small bodied copepods dominated the plankton at all stations, and small pteropods were abundant offshore. Surface trawls for forage fish along the transect line caught none in April and gravid Eulachon were collected near shore in May. Juvenile salmon (primarily coho and sockeye) and herring were collected in June, and abundances were low near shore, largest immediately at the river plume front, and smallest offshore of the front. No salmon were caught in July, and large numbers of stickleback were captured at the shelf break.