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 is producing 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).
How are we sampling?
We sample the Seward line in early May 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 now 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. 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 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, each of which lasts for multiple years (see figure). The winters of 2014 and 2015 were particularly warm, with temperatures at some locations as much as 2.6° C above the long-term average.
The Seward Line time series is approaching sufficient duration that we might soon extract longer-term trends underlying these shorter-term cycles, as is now possible for the sampling station GAK 1 and for Prince William Sound monitoring.