A systematic characterization of Southeast Alaska’s oceanography

Abstract

Continued anthropogenic warming of the world’s surface oceans will lead to increased thermal stratification and decreased vertical nutrient supply, resulting in an expected decline in the rate of photosynthetic carbon production. Decreased primary productivity will alter the marine food web, and may impact subsistence, commercial, and recreational fisheries in Southeast Alaska (SEAK), which are important sources of food and income for local communities. Historic oceanographic data in SEAK are spatiotemporally sparse, and consistent measurements are necessary to understand present patterns and predict long term change. We analyzed the physical, chemical, and biological oceanography of SEAK from 2022-2024, initiating what is set to be consistent, longitudinal data collection in this region. We characterized local environmental parameters with systematic vertical profiles (n > 800) of temperature, salinity, and chlorophyll, alongside discrete water samples (n > 600) for macronutrient analysis. Results show spatiotemporal (both seasonal and interannual) variability in all parameters, with 2022 showing generally warmer, fresher, and more nutrient rich conditions in near surface waters (< 30 m). Evidence of nitrification was detected from the timing of peak concentrations of ammonium, nitrite, and nitrate at 30 m, suggesting that Southeast Alaska resembles a more closed system than the Gulf of Alaska. Surface nutrients were also compared to the physical and geographical parameters that may influence variability and it is evident that strongly stratified regions with high volumes of freshwater input have greater differences between surface and deep nutrient concentrations. The oceanographic setting of SEAK underpins local primary productivity, and subsequently the spatiotemporal distribution of fish and megafauna. This work represents a critical first step in the establishment of a comprehensive baseline understanding of SEAK oceanography, and may help to inform local communities about the mechanisms that impact the ecosystems upon which they rely, helping them prepare for future change.