Photo Credit: Bill Rome

Ocean and Climate Forces

Overview

This aspect of the project focuses on the physical and biological oceanography that influences the survival of the five focal groundfish species (Pacific cod, pollock, Pacific ocean perch, sablefish, and arrowtooth flounder) during their first year of life.  Oceanographers are testing the hypothesis that cross-shelf and along-shelf transport of nutrients and plankton differs in the central and southeast Gulf of Alaska and that the mechanisms controlling primary production differ as a result.  They are also testing the hypothesis that the food webs leading to larval and juvenile fish differ between these regions.

Courtesy GOAIERP Research Team

Cross-Shelf Transport

Cross-shelf transport is the process by which nutrient-rich water from the deep ocean basin is carried by currents or upwelling into the shallower waters over the continental shelf. Nutrients collect in deeper waters when organisms die and sink to the sea floor. When they are brought up into shallower waters, nutrients act as the fertilizer that planktonic algae need to grow. When these planktonic plants are exposed to sunlight in the presence of nutrients, they can grow very quickly in what is known as a plankton bloom and form the base of the marine food web. This project will contribute to our understanding of the processes controlling when and where these blooms occur and how that influences the food web in the Gulf of Alaska.

Along-Shelf Transport

Along-shelf transport is the movement of water along the coast over the continental shelf. In the Gulf of Alaska, coastal water typically flows counter-clockwise, northward along the coast of Southeast Alaska, westward along the central Gulf of Alaska coast, and southwest as it moves past Kodiak Island toward the Aleutian Island chain. This project is describing this movement of water and how it changes both seasonally and inter-annually to better understand how currents may affect the transport of zooplankton and larval fish from their offshore spawning areas to nearshore nursery areas. It is also providing information about the transport of iron, zooplankton, and fish off of the continental shelf into the deeper waters of the basin.

Data Collection & Sampling

Field data collection is conducted in spring and fall throughout the study region (to see maps of the sampling sites, please visit the Study Region page under the About the Project menu). A variety of data are collected aboard the oceanographic vessels. An instrument is lowered at each sampling station to collect infomation about salinity, temperature, and depth to create a profile of the water column. Water samples are also collected at depth.

Iron sampling is conducted aboard the oceanographic vessels as well. Iron is necessary for primary production to occur, and iron is typically input into coastal Gulf of Alaska waters via terrestrial freshwater runoff. Understanding the processes that concentrate iron in the ocean, such as eddies (circular currents), will further our understanding of primary production and allow us to better predict when and where plankton blooms are likely to occur.

Satellite-tracked drifters are deployed from the oceanographic vessels to illustrate the actual movement of water around the Gulf of Alaska. Researchers watch the movment of these drifters over time to learn about the passive transport of larval fish and how their trajectories may change based on climatic conditions.

Bongo nets are used to catch fish eggs, zooplankton and larval fish to describe the base of the food web and how it differs with geographic region. Information about when and where fish eggs and larval fish of each of the five focal groundfish species are found is being used to initialize ecological models that will simulate the transport of larval fish from their offshore spawning areas to nearshore nursery areas.

Biophysical moorings are deployed throughout the Gulf of Alaska in February and are recovered in October. These moorings collect detailed information about water properties, currents, and phytoplankton and zooplankton concentrations in localized areas. These data represent time-series that provide important insights into how these factors vary seasonally and inter-annually. Little was known about the dynamics of water flow in Southeast Alaska prior to this study and this project is advancing our knowledge of the oceanography in the region considerably.