Why are we sampling?
The Gulf of Alaska supports a diverse ecosystem that includes several commercially important fishes, as well as culturally and economically important mammals and plants. Historic observations suggest a connection between the Gulf of Alaska ecosystems and climate variations that range from interannual to interdecadal; the specific mechanisms by which climate variation causes ecosystem changes, however, are poorly understood. Sampling along the Seward line produces a multi-year data set that will lead to a better understanding of the seasonal cycle and the variability that occurs from year to year in environmental conditions and biological productivity in the Gulf of Alaska.
Where are we sampling?
The Seward line is a transect of oceanographic survey stations that begins at the GAK1 mooring at the mouth of Resurrection Bay in the Gulf of Alaska (59°50.7’N, 149°28.0’W) and continues south across the Gulf of Alaska to the outer edge of the continental Shelf, or “shelf break” (58°5.9’N, 147° W). It is now embedded in the Northern Gulf of Alaska Long-term Ecological Research (NGA-LTER) program that adds additional lines from the Copper River to past Middleton Island, and from Kodiak’s Albatross bank to offshore waters.
How are we sampling?
We sample the Seward line in early May, mid-summer (July), and mid-September from a research vessel and collect data about a variety of environmental and biological conditions from salinity and temperature to plankton. The water column is sampled using a CTD (conductivity, temperature, depth) profiling instrument, plankton nets, and instruments that sample the water from different depths to evaluate carbonates for ocean acidification monitoring. While physical data are read directly from instruments, chemical and biological data require considerable post-cruise laboratory analyses.
What are we finding?
The Seward Line represents the most detailed multi-disciplinary long-term oceanographic sampling program in the northern Gulf of Alaska, with sampling occurring early each May and mid-September for over 20 years. It is coordinated with other Gulf Watch Alaska oceanography projects that focus on Cook Inlet and Kachemak Bay, the Alaska Coastal Current, Prince William Sound, and still more broadly across the Gulf of Alaska through the CPR project. Among these projects, it is the only one that measures ocean productivity, fluxes and carbonate chemistry for ocean acidification monitoring. The Seward Line monitoring has allowed us to recognize that the Gulf of Alaska shelf undergoes alternating periods of warm and cold springs, some of which lasts for multiple years. The winters of 2014 and 2015 were particularly warm, with spring temperatures at some locations as much as 2.6° C above the long-term average. The northern Gulf of Alaska ecosystem does not respond in a predictable way to intermittent basin-scale events such as El Niño or to longer-term regime shifts such as the Pacific Decadal Oscillation, perhaps because the ecosystem is highly adapted to great variability. Nonetheless, it is profoundly affected by warmer years, fresher years, and light conditions in spring that influence the timing and magnitude of planktonic processes. After a return to average temperatures during May 2017 and 2018, temperatures during spring 2019 were above average (0.64°C) representing the 5th warmest in the time series, and summer surface temperatures were in excess of 15°C.
Warmer years result in reduced body size and typically lower lipid storage by the large-bodied spring copepods (i.e., Neocalanus spp.). In contrast to spring, temperature has been much less variable during late summer, although biological communities continue to show high variability, including increased prevalence of southern species during warmer years such as those during the marine heatwave. While southern species returned to more normal absences during spring 2017 and spring 2018, during late summer several of these species persisted at low abundances. During 2019 many of these southern species rebounded during summer and early fall.