Rep16 - Sub. Pac. Monitoring Studies

## Monitoring Studies

We make the following recommendations with regard to monitoring activities in the subarctic Pacific:

1. Continue or establish time-series to address the following questions:

   - How does the Aleutian Low drive physical forcing? Specifically, how do variations in the strength and position of the low affect alongshore and cross-shelf transport, and the circulation in mesoscale features?

   - How does physical forcing affect the availability and production of prey and the abundance of predators of juvenile salmon in the coastal Gulf of Alaska? This might require observations of physical and biological parameters along a few key cross-shelf transects (sampled every other month, or more frequently if feasible). Bimonthly sampling is inadequate to resolve the dynamics of the shorter lived zooplankton that are potential prey of the juvenile salmon. The detailed transect data should be complemented with continuous time series ADCP, acoustics, bio-optics, and physics measured from a small number of moorings, to prevent aliasing of the data, and to capture large amplitude events that occur during the interval between transects.

   - How does physical forcing affect the production and availability of salmon prey as indicated by zooplankton in the open ocean (deep water of the Gulf of Alaska)? This question relates to the growth of salmon during their oceanic phase (H2 above).

   We recommend that several (we suggest 3) deep-water moorings be placed in the Alaskan Gyre. These moorings should be located (1) off the shelf in deep water, but adjacent to the coastal region selected for detailed process studies; (2) near the center of the Alaskan Gyre; and, (3) at an intermediate location between (1) and (2). The intent is to use the data collected from these three moorings to monitor the abundance and distribution of potential salmon prey. The locations are suggested by the observations of Brodeur and Ware (1992) that zooplankton abundance increased most markedly over decadal scale periods along the margins of the Gulf of Alaska--more so than in the central Gulf. If an important feature of the decadal change observed by Brodeur and Ware is a shift in the distribution of zooplankton from the central to marginal regions of the gyre, then it is important for a U.S. GLOBEC study to include an effort to capture the transport of subarctic zooplankton to the coastal zone. Multiple moorings, spanning the central gyre to the margin, would provide the data necessary to document such shifts. The moorings should consist of mostly biological instrumentation, but with some physical observation capability. It is most important that this mooring be equipped to measure acoustic backscatter, preferably at multiple frequencies, to provide an estimate of zooplankton biomass (and perhaps size), and light and fluorescence sensors (to measure phytoplankton stocks). An appropriate bio-optical model could be used to estimate primary production from light and phytoplankton biomass estimates. In the Prince William Sound region, the three deep-water moorings would complement existing or planned moorings near Seal Rock on the shelf proper, and two moorings within PWS.

   Ships of opportunity should be used to expand geographic coverage in the Alaskan gyre beyond that of the mooring locations. For example, it was noted at the workshop that ships routinely cross the gyre enroute from Valdez, AK to Hawaii. This would be a valuable route for towing a high-speed undulating instrument. There may be other routes as well.

2. Develop methods to measure cross-shelf exchange, perhaps using chemical or biological tracers. This should be an initial activity--to begin as soon as funding becomes available for a U.S. GLOBEC North Pacific study.

3. Large-scale monitoring is needed to evaluate how variability in atmospheric forcing and variability in the position and strength of the west wind drift affect the circulation and water mass characteristics of the Alaskan Gyre. This is needed to document the effects of these large-scale forcings on the productivity of the Alaskan Coastal Current, and on the distribution, growth and survival of salmon and their prey in the open ocean. Monitoring of the entire North Pacific Basin should be conducted as a coordinated multinational effort. We recommend that U.S. GLOBEC monitor the circulation and characteristics of the Alaskan Gyre and the bifurcation of the west wind drift as it nears North America by a combination of remote sensing (including altimetry), a few strategically placed moorings and/or transects, and atmospheric models. Temperature and salinity profiles of the Subarctic Basin could be obtained using PALACE floats. PALACE floats are programmed to sit at depth on a density surface (perhaps at 800-1000 m depth). Periodically, perhaps at weekly to biweekly intervals, they collect high-quality temperature, salinity and pressure profiles as they rise to the surface. They remain at the surface (order 16-32 hours) long enough to transmit their hydrographic data via ARGOS transmitters. During their time at the surface, they also provide data on surface currents in a Lagrangian sense. Their estimated lifetime is two years.