Coastal Gulf of Alaska Breakout Session

Discussion Leaders: Tom Royer and Anne Hollowed
Participants: Tim Baumgartner, Louis Botsford, Dan Cayan, Ted Cooney, Mike Dahlberg, Robert DeLong, Robert Francis, Nick Graham, Robert Haney, Scott Hatch, Al Hermann, Sarah Hinckley, Steve Ignell, Art Kendall, Allen Macklin, Nate Mantua, Richard Methot, Brenda Norcross, Ian Perry, Pete Rand, Michiyo Shima, Ted Strub, Dan Ware

Introduction

The coastal Gulf of Alaska (GOA) supports a complex ecosystem that includes a variety of commercially important marine resources including crab, shrimp, salmon and walleye pollock (Anon. 1993). The coastal GOA ecosystem appears to be sensitive to climate variability on time scales of several years to decades. The mix of higher trophic level species appears to have changed during the late 1970s, coincident with a major change in ocean conditions. Commercial catch for shrimp and crab declined, while many groundfish and salmon populations increased (Albers and Anderson 1985, Blau 1986, Hollowed et al 1994, Thompson and Zenger 1994, Francis and Hare 1994) . Abundance of several top trophic level predators declined in the region in the 1980s (Merrick et. al. 1987, Hatch and Sanger 1992). One of these, the Steller sea lion, is currently listed as a threatened species under the Endangered Species Act. While these changes appear to coincide with major shifts in ocean conditions, comprehensive investigations of the ecosystem response have not been conducted.

The bathymetry of the coastal GOA provides a contrast between a broad shelf region in the central and western Gulf and a narrow fjord like region off Southeast Alaska and British Columbia. The shelf is punctured by submarine canyons and frequent bays, sounds and inlets including: Yakutat Bay, Prince William Sound and Cook Inlet (Figure 3).

The major oceanographic features of the Gulf of Alaska include mesoscale eddies, strong coastal currents adjacent to a major oceanic current system, and frequent storm activity. The Shelikof Strait region supports mesoscale eddies which appear to be important for survival of larval walleye pollock (Schumacher et al., 1993; Bograd et al. 1994). The Alaska Coastal Current (ACC) is the dominant current on the shelf and is characterized as a narrow (<25 km), low salinity current driven by wind stress and freshwater input from sources along the coast (Royer 1981) (Figure 1). The Alaskan Stream current is south of the Alaska Peninsula and marks the northern boundary of the Pacific subarctic gyre (Reed and Stabeno 1993) (Figure 1). The flow of water in Shelikof Strait is composed of a two–layer, estuarine-like circulation with more saline slope water entering the sea valley in the bottom layer and ACC waters in the upper 150 m (Reed 1987).

Breakout Discussions

The coastal GOA breakout group began discussions by identifying key questions regarding three subject areas: physical forcing, lower trophic level response, and higher trophic level response. The key questions suggested by the group are outlined below. Following these discussions, the group identified potential research projects to address the key questions through retrospective studies, monitoring and modeling. While process oriented studies are considered a crucial part of a U.S. GLOBEC activity time did not permit consideration of this type of research.

Physical Questions

  1. How do changes in atmospheric forcing influence coastal circulation in the GOA?
  2. How do the Alaskan Stream and the Alaska Coastal currents interact?
  3. How do changes in precipitation and freshwater runoff influence coastal circulation in the GOA?
  4. What is the role of bottom topography in determining coastal circulation in the GOA?
  5. What is the role of tides in controlling nutrient flux in the GOA?
  6. How do the factors identified above influence the: mixed layer depth (MLD), mixed layer temperature (MLT), retention time–scales (eddies), turbidity, cross shelf transport?

Lower Trophic Level Questions

  1. What factors control primary production in the coastal Gulf of Alaska? Issues of interest include: light/nutrient/prey concentration and availability; species mix, prey quality (i.e. biochemical composition, as relates to essential amino and fatty acids).
  2. How do the above changes in transport processes (vertical and horizontal, along shore and cross shelf) due to climate variability influence the composition and production of coastal plankton communities?
  3. Is grazing/predation a major factor structuring plankton communities in this region?
  4. How would climate induced changes in the mixed layer depth (MLD) influence production at lower trophic levels?
  5. How might climate change influence trophic phasing in the coastal GOA?
  6. How would climate change influence over-wintering plankton communities and biomass -- i.e. as a baseline for the following spring bloom?
  7. How would changes in precipitation and runoff (pattern, timing, magnitude) influence plankton communities in the GOA -- either directly, or indirectly through changes in circulation?

Higher Trophic Level Questions

  1. How do changes in climate affect the distribution of predators; large scale and locally; vertically and horizontally? More specifically questions might include the following. Do physical oceanographic processes affect or possibly determine the dynamics of prey patches? Is physical forcing important in aggregating prey and making them available for efficient predation? How are these features effected by climate variability.
  2. How does climate change influence prey abundance and what role does it play in determining growth, survival and reproduction of higher trophic level species?
  3. What are the benefits of adjusting marine resource policy to track climate induced changes in marine production?
  4. How do marine organisms respond to rapid and large-scale climate changes (e.g. regime shifts)?
  5. How might climate change alter the composition of fish communities?
  6. How would climate change effect the seasonality of resources available to apex consumers?

Retrospective Studies

Several data sets and their sources were identified that could be used to conduct retrospective studies of the Gulf of Alaska ecosystem (Table 8). The group also identified several types of retrospective analyses that could be conducted to address the questions above. These retrospective questions are not listed in order of priority and should not be considered the only types of retrospective questions that could be addressed in the region.

Recommended retrospective analyses regarding forcing questions

  1. Compare time series of coastal meteorology station data, atmospheric pressure data and current measurements made at GAK1 and FOCI line 8 to examine the impact of atmospheric forcing on coastal circulation (addresses physical question 1) .
  2. Compare time series of precipitation, runoff, and NODC temperature and salinity profiles with current measurements from GAK1 and FOCI line 8 to measure the influence of freshwater on coastal circulation (addresses physical question 3).
  3. Conduct spatial analysis of NMFS and NODC temperature and salinity profiles, ships of opportunity data, and remote sensing data to identify major physical features and to map their spatial and temporal patterns (addresses physical question 6).
  4. Compare time series of nutrient concentrations from stations in the eastern and western Gulf of Alaska for evidence of differences in nutrient flux. Examine the data for evidence of tidal influences (addresses physical question 5).
  5. Construct time series of mixed layer depth, mixed layer temperature, eddies (retention), turbidity, and cross-shelf transport and conduct simulation studies and multivariate analyses to explore potential relationships of these times series to atmospheric and large scale climate variability (addresses physical question 6).

Recommended retrospective analyses regarding lower trophic level response questions

  1. Analyze species composition of existing zooplankton samples from three regions of the Gulf of Alaska: Southeast Alaska or La Perouse Bank {although note that La Perouse Bank is more representative of the California Upwelling domain} , Prince William Sound, and Shelikof Strait. Compare indices of mixed layer transport (vertical and horizontal, along shore and cross shelf) with time series of species composition and production of coastal plankton communities.
  2. Compare time series of physical variables such as the mixed layer depth, precipitation or runoff with time series of plankton abundance identified by taxonomic group or species.

Recommended retrospective analyses regarding higher trophic level and ecosystem response questions

  1. Conduct spatial analyses of the distribution of predators, identify major oceanographic features that influence the distributions of higher trophic level predators and their prey.
  2. Conduct multivariate analyses of time series of physical oceanographic, atmospheric and recruitment (survival indices) of higher trophic level species.
  3. Examine historical information on the growth of higher trophic level species and compare historical information with indices of climate variability and prey abundance.
  4. Conduct simulations to explore the impact of adjusting marine resource policy to track climate induced changes in marine production
  5. Identify how organisms adapt to a new environment and how successful the adaptation strategy is for survival in a new regime.
  6. Construct a simulation model using past climate trends and potential climate change effects on the seasonality of resources available to apex consumers and compare the results against observed distributions and abundance.

Potential Monitoring Activities

This group discussed several types of monitoring activities that would be useful in a U.S. GLOBEC program in the Gulf of Alaska (Table 9). These included existing and proposed monitoring activities. For example, the group recommended that volunteer ships and the Alaskan Ferries could provide valuable physical, chemical and biological samples if they were properly equipped. Several pulse points were identified where attempts should be made to initiate, continue, reinstate or expand monitoring activities: FOCI line 8, GAK1, line P, and the flow through Unimak Pass.

Recommendations for Future Modeling

There are modeling activities on–going for various regions of the Gulf of Alaska, most notably in Shelikof Strait and the Western Gulf of Alaska, Prince William Sound, and La Perouse Bank (although the latter more correctly represents an upstream boundary for the Gulf of Alaska System). Modeling activities need to integrate physical and biological responses (from physical forcing to lower to higher trophic levels) to climate-induced variability in along- and cross-shelf circulation, to vertical mixing processes, and to variations in upstream conditions--e.g., variations in the intensity and location of the North Pacific Current and its bifurcation at the coast of North America. An effort to nest regional models of Shelikof Strait, Prince William Sound, and Southeast Alaska into a large-scale bio-physical model of the Gulf was recommended. Existing output from large-scale physical simulations of the North Pacific might be employed for this purpose, serving as boundary conditions on the regional models. True nesting, with feedback from the regional to the larger scale model, is preferable, but one-way coupling could be fruitful for comparing regional responses to large-scale climate change. If such a modeling exercise was undertaken the physical model should have some mixed layer physics, to get the lower trophic levels adequately. Existing physical model output could be used to drive a suitable biological model. A broad-scale biological model of the Gulf might include the following: phytoplankton and protozoa, euphausiids and copepods, jellyfish, salmon, herring, and pollock.