Progress Report: April 1999
Project Title: The Gulf of Alaska GLOBEC LTOP Program
Investigators: Thomas Weingartner (PI), A. J. Paul (PI), Ken Coyle (PI), Lew Haldorson (PI), Dean Stockwell (PI), Terry Whitledge (PI), Jennifer Boldt, Seth Danielson, Alexei Pinchuk, and Amy Ruehs (All at University of Alaska), and Tom Royer (PI; Old Dominion Univ.)
The LTOP monitoring in the Gulf of Alaska completed its first year of sampling which included week-long cruises using the Alpha Helix in October 1997, and March, April, May, July, October, and December 1998. Sampling on all cruises included occupying stations along the Seward Line, which extends from the inner shelf to the edge of the continental slope. At each station CTD profiles and water samples for nutrients, chlorophyll and zooplankton nauplii were collected. Additional sampling includes MOCNESS tows at each of these stations with the tow depths chosen on the basis of backscatter from a 4- frequency, split-beam towed transducer array. The MOCNESS samples allow verification of the acoustical results and provide samples for species identification and additional analyses. The transducer is towed continually along the line and in conjunction with underway measurements of sea surface temperature, salinity, fluorescence and ADCP data provide data at very high spatial resolution. This standard sampling was supplemented during the July and October cruises with coincidental trawls from a charterd fishing vessel and surface gill nets from the Alpha Helix. We have also provided samples and/or sampling opportunities to other scientists for measurements of stable isotope composition of biota, dissolved carbon, bacteria, microzooplankton, seabird and marine mammal, and circulation.
The data suggest that the shelf is organized into three distinct dynamic and biological regimes. The innermost portion of the shelf consists of the Alaska Coastal Current, which is a high speed, persistent, dilute coastal current which is generally low in nutrients and chlorophyll. The inner shelf is segregated from the midshelf region by a salinity front. Nutrient concentrations, chlorophyll biomass, zooplankton, fish and seabird abundances appear to be consistently high just offshore of this front. However, it is not clear if this region of locally high biological productivity is associated with frontal processes or with the locally complicated bathymetry and coastline curvature. Over the midshelf region the circulation is weak and variable and biological abundances appear to be patchy in time and space. This region of the shelf often contains eddies having a diamter of ~50km. A third regime straddles the shelfbreak salinity front. Flow here is also swift (and often reflects shoreward movement of the Alaskan Stream) and nutrient, chlorophyll and zooplankton conentrations tend to be relatively high. Fish, seabird, and mammal abundances are also relatively high in this region. There is some suggestion that the vertebrate community structure here differs from that of the inner shelf.
Time permitting we augment the sampling along the Seward Line with additional alongshore transects. While these are relatively few in number they suggest that biological production is highly patchy owing perhaps to very complicated bathymetry.
The first year of sampling occurred during an El Nino year and the physical effects of this phenomenom included unusually warm (~1-2C above normal) and fresh (.15 psu) waters over the uper 200 m of the shelf. By summer, deep water temperatures were returning to normal and deep water salinities were above normal. The reason for the latter is not known but it implies either abnormally high upwelling rates or changes in the the composition of the offshore water masses that annually flow inshore. Higher salinities imply higher nutrient concentrations. However, it is not clear that changes in deep nutrient concentrations in the deep water on the shelf affect surface production.
Additional information about the GLOBEC LTOP in the Coastal Gulf of Alaska, including plots of cruise results, station locations, etc., can be obtained at: http://www.ims.uaf.edu/GLOBEC/
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