The oceanic Subarctic Pacific Ocean, including all waters north of the Subarctic Boundary, is a large, productive and relatively simple ecosystem. It experiences large seasonal, interannual and decadal changes in upper ocean physics that are apparently linked to the biology of organisms. Despite the correlations between physics and biology, our understanding is limited about processes actually affecting productivity and distribution of animals (see however Miller et al. 1991; Miller 1993).
The climate of the Subarctic North Pacific Ocean changed during the late 1970s. In response to this climate change, the Aleutian Low intensified (Trenberth and Hurrell 1994); sea surface temperatures rose rapidly by several degrees (Rogers and Ruggerone 1993; Royer 1989; Graham 1995); zooplankton biomass and the catches of epipelagic nekton increased (Brodeur and Ware 1992; 1995). Salmon catches from the North Pacific increased sharply, especially in Alaska, and exceeded historical levels (Pearcy 1992; Beamish and Bouillon 1993; Francis and Hare 1994).
During this recent period of high fish production, evidence accumulated that several species of salmon, of both North American and Asian stocks, were returning as mature or maturing fish at increasingly smaller sizes (Kaeriyama 1989; Ishida et al. 1993; PICES 1993). This suggests density-dependent growth and competition for food in the ocean. Apparently, the carrying capacity of the Subarctic Pacific for salmonids, a major group of epipelagic fishes, was exceeded, even during this period of exceptionally favorable ocean conditions of the 1980s and 1990s. If the next climate shift is to cooler and less productive conditions, this problem will be exacerbated and have major economic consequences for nations along the Pacific Rim that produce wild and hatchery salmon (PICES 1993).
Besides changes in the productivity per unit area, global warming may also reduce the geographic area that is optimal for salmon growth and survival. Recent observations by Welch et al. (in prep.) suggest that salmon may undertake a reverse, northward migration during the winter. If this is true, and if global warming continues, the area of suitable habitat for salmon could be severely restricted during that season.
These pronounced large-scale climate fluctuations in the Subarctic Pacific, and their societal importance, are cogent reasons to improve our understanding of the linkages between physical and biological processes of this region.
There are other scientific reasons for expanding our research in the Subarctic Pacific. Strong physical and biological interactions occur on both seasonal and interannual time scales. Variations in the strength of the Aleutian Low result in large seasonal changes in atmospheric forcing (Wilson and Overland 1987). However, no phytoplankton bloom occurs during the spring. Is this because of lack of iron, grazing by microzooplankton or both (Miller et al. 1991)? Every spring and summer, a huge biomass of epipelagic fishes, such as Pacific pomfret and Pacific saury, migrates across the Subarctic Boundary into the Subarctic Pacific to feed (Neave and Hanavan 1960; Taniguchi 1981). What are the impacts of these seasonal migrants on the food web structure that includes salmonids? Interannually, variations in the standing stocks of zooplankton are correlated with the intensity of winter winds (Brodeur and Ware 1992), but is this related to Ekman pumping and production or changes in the structure of the food web? The details of these physical-biological processes are lacking.
Moreover, excellent background information about the Subarctic Pacific is available. This will facilitate retrospective studies of climate change and formulation of hypotheses relating physical and biological processes. Previous studies include the long time- series of physical and biological measurements at Ocean Station "P" (Fulton 1983; Frost 1983), the SUPER Project (Miller 1993), ecosystem modeling (Frost 1993), a 25 year hydrographic time series at Seward, Alaska (GAK1), and repeated biological and oceanographic measurements in conjunction with cruises by the International North Pacific Fisheries Commission and Hokkaido University. In addition, a long history of catch records is available, as are collections of salmon scales to provide long-term comparisons of ocean growth.
The approach followed was to examine a matrix of questions and research approaches:
| Question | Retrospection | Model | Process Study | Observation |
|---|---|---|---|---|
| Physical Forcing | ||||
| Lower T.L | ||||
| Higher T.L. | ||||
| Ecosystem Interactions |
The group then broke into three subgroups with the charge to identify the most promising questions for early development and implementations. The subgroups were physical forcing/lower trophic levels, higher trophic levels, and ecosystem interactions.
In the case of retrospective studies, the longest time series are for certain atmospheric conditions (air temperature, sea level pressure) which permit a general description of climate variations. Fewer data concerning abiotic conditions are available from the ocean, and even fewer for the lower trophic levels. Higher trophic level data come mostly from fishery statistics which extend back about 100 years for some North Pacific species. Cores of anoxic sediments in British Columbia and the Aleutian Islands may also provide long time series for retrospective studies of changes in fish communities. Historical data on ecosystem interactions is limited because it is dependent on information from both lower and higher trophic levels.
Whereas, modeling and process studies differ among trophic levels, there are some measurements that are required for all levels. Monitoring can be carried out from moorings, ships (especially Volunteer Observing Ships, VOS), drifters, and satellites, each being preferable for one or another type of observation.
However, it does seem desirable that a monitoring program be developed that maximizes the suite of measurements that will be made.
Priority I. Document changes in standing stocks using modern technology, e.g., long term moorings with acoustic instruments for determining zooplankton stock variation. What drives the interannual variation? To answer this question, the moorings should include measurements of temperature, salinity, incoming radiation, fluorescence. It is necessary to learn about interannual variation as a first step to understanding effects of regime shifts. East-west comparisons (eastern subarctic vs western subarctic gyre) should be made.
Priority II. What observations are needed to distinguish the effects of iron, Ekman pumping, cloud variation, etc. on primary production biomass? Two approaches seem feasible: (a) An Ironex experiment utilizing pairs of bio-optical drifters, one drifter receiving iron additions, the other not. Both drifters would have fluorometers and spectral radiometers to estimate changes in standing stock or production rates. (b) A process study of microherbivore control of phytoplankton stocks, with shipboard perturbation experiments to examine the feeding responses of microherbivores. The experiments would involve both dilution and enrichment.
Priority III. The Chelton-Davis hypothesis on the split of the west wind drift as it nears North America should be studied by deploying multiple drifters and with radar altimetry (TOPEX Satellite). The hypothesis predicts that the intensity of the flow of the Alaska and California currents is out of phase and that zooplankton biomass in the California Current is high when much of the west wind drift turns south and vice versa. Chelton et al. (1982) and McGowan (1989) found zooplankton volume at central and southern California was correlated with transport in the same region. This suggests that changes in the West Wind Drift may affect zooplankton biomass in the California Current System. This should be tested and compared to transport and zooplankton biomass in the Gulf of Alaska.
Retrospection -- Examine history of upper ocean thermal structure, position and width of transition zone, MLD and ENSO.
Models -- Basin models, MLD, Ekman pumping, ENSO, Kuroshio/Oyashio.
Process studies -- Circulation studies with idealized and real forcing functions.
Observing Systems -- Moorings (T, S, MLD, acoustics, chlorophyll); Volunteer Observing Ships (T, S, fluorescence); Oshoro Maru (CTD, ADCP); drifters (CTD, fluorescence, acoustics); R/V; satellites (chlorophyll, currents, cloud cover); AXCTD (aircraft-dropped), and Autonomous Underwater Vehicles (AUV).
Note: the technology is available to instrument the region. Despite the high cost, it should be done, along with a strengthened VOS program.
Lower Trophic Level Response -- How do primary and secondary producers respond in productivity, and in species and size composition, to climate variability in different ecosystems of the Subarctic Pacific? Do these changes differ from regime to regime? Why is there no spring bloom? Has phytoplankton, and zooplankton production changed with the regime shift? How do the lower trophic levels respond to perturbations in, for example, iron, or microzooplankton?
Retrospection -- Examine history of nutrient data, chlorophyll, zooplankton species composition, secchi disk; MLD and light (cloud cover) as related to timing of bloom.
Models -- Model zooplankton production (up to euphausiids), seasonal migration (copepods), diel migration experiments; nitrogen flux, size spectrum of phytoplankton.
Process Studies -- Feeding behavior of microzooplankton, macrozooplankton feeding selectivity, field and on-board experiment with iron introduction in Gulf of Alaska (ironex) (late summer?) This study can expand on work already conducted through the SUPER program. Biological coefficients of uptake; role of gelatinous zooplankton.
Observation Systems -- In addition to items under forcing (q.v.), SAR, rainfall (related to iron supply?) zooplankton abundance, species composition.
Higher Trophic Level Response -- How do regime changes affect the life history patterns (distribution, vital rates and population dynamics) of higher trophic level species, through competition, and through direct response to changes in the physical environment?
Note: Effects of regime shifts may include changes in life history and behavior, migrations and distributions, growth, survival, species composition, food habits, carrying capacity, growth and size-at-age, survival, diet shifts, predation rates, reproduction success of marine mammals and sea birds (see report of subgroup 3 on carrying capacity).
Retrospection -- Salmon size at maturity, scales (size-at-age), (studies underway in U.S., Japan, Canada, and Russia), life history summaries with species composition, northern fur seals (same information as for salmon), whale and sea bird feeding, distribution (see OCSEAP data), 15N in fur seals and whales, and fish scales.
Models -- University of British Columbia simulated currents, temperature, salmon (sockeye) migrations, bioenergetics, growth; fur seal pup survival vs temperature.
Process Studies -- Salmon growth vs age-at-maturity (Auke Bay), diel feeding studies, thermal limits (large scale), small scale distribution.
Observation Systems -- Japan, Canada, and USA studies of winter distribution of large nekton squids, gillnet comparisons (Oshoro Maru), food habits (diet shifts), vertical distribution (salmon) - acoustic tags. Non-commercial forage fishes (myctophids, smelts, atka mackerel), stock distribution of salmon (hatchery, wild), distributions and migrations of nekton and marine mammals, age and growth of nekton, changes in species composition and abundance, foraging behavior, reproductive success of sea birds and mammals.
Ecosystem Interactions -- How are Subarctic Pacific ecosystems structured? Do higher trophic levels respond to climate variability solely as a consequence of bottom up forcing? Are there significant intra-trophic level and top down effects on lower trophic level production and on energy transfer efficiencies?
Note: Issues include the microbial loop, competition, predation rates (including human predation), food-web structure and efficiency, and particle fluxes. Of particular importance are comparisons between events and processes in the Alaska Current and those in the Subarctic Gyre, and between the east and west gyres.
Retrospection -- Change in species composition and shifts in species dominance before and after regime changes, N03 in scales, teeth, baleen.
Model -- model salmon trophodynamics, linkage with lower and higher trophic levels, size spectrum theory, production/biomass (P/B) ratios, marine mammal food web, Sverdrup theory on initiation of spring bloom, re-visit Laevastu model, European Regions Seas model (ERSEM).
Process Studies -- Establish biological coefficients for models, coastal studies on juvenile salmon predation, sediment trap-15N, role of seasonal migrants, isotope studies (Cs-K) of trophic levels.
Monitoring -- add sediment traps.