Recent changes in the near-surface oceanography and productivity of Prince William Sound

Prince William Sound (PWS) is a large and complicated estuarine-fjord system with numerous subbasins around its margins. The marine ecosystem in the PWS region is extremely productive, and supports several groundfish and salmon fisheries that are of considerable importance to the local economy. The region is experiencing a long-term warming trend, and is rapidly losing ice mass; it also experienced marine heat waves in 2014 and 2019. Vessel surveys and high frequency observations from a moored profiler, put into context with a 50 year climatology for the region suggests that increased surface temperatures, along with enhanced freshwater inputs, have increased near-surface stability which has resulted in a thinner seasonal mixed layer. A thinner mixed layer results in less near-surface nitrate (the limiting nutrient in Alaskan coastal waters), and overall phytoplankton productivity has shown a declining trend over the last 25 years. The 4 lowest values in the time series have all occurred in the past 5 years, with the lowest observed in 2023. Zooplankton abundances did not change to a great degree in recent years, but the species assemblages shifted, with the canonical large subarctic taxa declining and smaller subtropical species more common far to the south increasing in relative abundance during heat wave years. The observed changes in lower trophic levels can be expected to continue to cascade upwards throughout the food web in the northern Gulf of Alaska and management of the resources of the region will benefit from a continued monitoring effort.