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Both the Bering Sea and the Gulf of Alaska are productive marine ecosystems supporting commercially important fisheries. Both regions experience inter-annual and inter-decadal variability in productivity and ecosystem dynamics. We use ecosystem models as a tool to explore the mechanisms behind these changes.

Use the links on the left to find out more about our project to explore mechanisms of zooplankton transport onto the continental shelves of the Bering Sea and the Gulf of Alaska.

Funding for this project was provided by North Pacific Research Board.

Project Synopses

Background…

Oceanic copepods are large-bodied high-energy crustaceans that are an important food source for juvenile stages of commercially important fish such as herring, pollock, capelin and salmon in the Gulf of Alaska and the Bering Sea. The copepods require deep water to successfully reproduce so overwinter in oceanic and shelf-break habitats. Annual differences in year class strength of fish stocks may therefore depend on climatic and oceanographic conditions promoting cross-shelf transport of the oceanic copepods onto the continental shelves in these two regions.

Why we did it…

The mechanisms promoting successful on-shelf transport of oceanic zooplankton was poorly understood. We conducted this study to better understand the mechanisms promoting or suppressing on and cross-shelf transport in the Gulf of Alaska and Bering Sea.

How we did it…

A three-dimensional oceanographic model was coupled to a float tracking model with simple vertical migration rules to represent the oceanic zooplankton. The zooplankton floats were initialized in deep oceanic water in winter. The floats trajectories resulting from alternate climate forcing were compared to determine which environmental conditions resulted in enhanced cross-shelf transport.

What we discovered…

Cross-shelf transport of oceanic zooplankton was far more prevalent in the Gulf of Alaska where the rough topography and deep shelf prevent the development of persistent frontal structures. In this region wind is the primary driver of on-shelf transport. The greatest on-shelf transport was seen with strong northwards wind. In the Bering Sea, with its broad shallow shelf, the shelf break front was very persistent and hindered on-shelf transport. The strength of the front was dependent not only on the seasonal heating in a given year but on the water temperature of the shelf from the preceding winter. As in the Gulf of Alaska, the largest cross-shelf transport in the Bering Sea was at times with strong northwards wind. Increasing the wind mixing promoted the cross-shelf transport of the zooplankton by shifting the location of the front further onto the shelf rather than diminishing the strength of the front.

What’s next?

Our findings suggest that in the Bering Sea the winter water temperature on the shelf plays a large role in determining the strength of the frontal structures that impede cross-shelf transport the following spring. Extending our research into additional years, especially ones known for cold/warm winter conditions to clarify the role that winter conditions have on cross shelf transport would be very insightful. If a clear relationship between winter temperatures and spring on-shelf transport can be established it would enable early prediction of the extent of oceanic zooplankton on the shelf and thus the likely availability of food for higher trophic levels.

 

 This is a movie showing model simulations of zooplankton transport in the Eastern Bering Sea. For more information on our findings visit the Results page.