The mission of this working group was to address the question shown above. Advantage was taken of the small–group format to discuss climate change and carrying capacity in more general terms as well.
The carrying capacity of a biological population, measured in terms of the number of individuals or biomass, is the asymptotic upper limit that is controlled by population density or equivalently resource limitation (Odum, 1971). It may be determined by the availability of food, space or some other aspect of the system. For a hypothetical example of climate change affecting carrying capacity, consider a species whose territorial range shrinks because it cannot tolerate warmer water caused by global warming. As the population is forced into a smaller geographical area, its density will increase perhaps until its needs exceed some resource, e.g. food supply or breeding substrate. Then its numbers will decrease, asymptotically approaching or oscillating about a new, lower carrying capacity.
Human intervention may displace commercial fish stocks from their carrying capacity level. To increase fishery yield, walleye pollock (Theragra chalcogramma) are managed at about one-third of their pre-exploitation biomass, which might be their carrying capacity. In contrast, hatchery reared juvenile salmon introduced into their natural habitat (e.g. the North Pacific) may increase their population to levels above their carrying capacity. Compensatory poor growth and high mortality may result.
The following table provides a comparison of two types of complementary studies that are applicable to our research problem: 1) process studies, and 2) continuous, long-term measurements, i.e., monitoring.
|Lead to understanding of mechanisms||Leads to recognition of change, regime shifts|
|Relate cause to effect||Correlate changes|
|Smaller scale, shorter duration||Larger scale, longer duration|
|Best conducted at representative sites||Best conducted at pulse points|
|Harder to link to climate||Easier to link to climate|
|Easier to link to carrying capacity||Harder to link to carrying capacity|
Detailed process studies are better suited to increase the understanding of a phenomenon through studying cause-and-effect linkages, for example relating food supply to carrying capacity. Larger-scale, longer-term monitoring is better suited for observing variability correlated over space and time, e.g. climate change. One problem with trying to deduce climate effects from a process study is that an agent that may dominate on the short time scale may play little role over the longer term. Roemmich and McGowan (1995) found that the slow decline of zooplankton population abundance over 43 years in the California Current could not be extrapolated from monthly or seasonal balances.
The Subarctic Pacific and Bering Sea vary seasonally and occupy such a vast area that they could not be sampled often and finely enough by a typical five-to-seven-year PICES/GLOBEC research program. Therefore a major challenge is to choose appropriate variables to measure at key or pulse points where the short period variance is minimized and the effects of climate change on carrying capacity are indicative of large-scale changes.
The Kamchatka Current might be a pulse point for the circulation in the Bering Sea. Although inflow to the Bering Sea from the Alaskan Stream occurs through several Aleutian passes, outflow is predominantly through Kamchatka Strait (Fig. 2) in a narrow, strong current. Thus the outflow should be a good indicator of the mean cyclonic circulation for the entire basin. Moreover, the strength of the outflow from the Bering Sea in the Kamchatka Current is related to the location and strength of the Aleutian Low, the primary atmospheric forcing in this part of the ocean (Bond et al., 1994). The Alaskan Stream might be a similar pulse point for the Gulf of Alaska gyre. It is stable west of Kodiak Island to the dateline and represents part of the return flow from the northern branch of the bifurcated West Wind Drift. Chelton and Davis (1982) have hypothesized that changes in the north/south apportionment of the West Wind Drift as it nears the North American continent affect the eastern North Pacific circulation and the biomass of zooplankton in various oceanic subregions.
GLOBEC Report 3 (U.S. GLOBEC, 1991a) discussed the application of biotechnology to zooplankton field studies. Biochemical and molecular techniques exist to determine physiological rates and condition and are beginning to be applied in the marine environment. Feeding rates and dietary composition might be investigated using DNA probes and immunological and pigment identification of prey species. The possibility of using zooplankton genetics for automated real-time plankton sorting is low, but should be encouraged. Genetics can be used to identify and characterize zooplankton populations and perhaps to sense adaptation to global climate change.
A 1991 U.S. GLOBEC workshop (U.S. GLOBEC, 1991b) dealt with the existing capabilities and potential developments in acoustical and optical technology, methodology and instrumentation for measuring the distributions and assessing the behavior of marine animals. Sensors are needed that operate on continuous temporal and spatial scales in order to couple small-scale physical processes to population parameters. Acoustical and optical measurements should be integrated. Physical and biological observations in the water column should be synoptic rather than serial. It was recognized that data definition, organization, archiving, access, retrieval and display were important issues for GLOBEC. A modular acoustical instrument design approach was suggested so that common parts and algorithms could be shared as much as possible to construct instruments with different frequencies and beamwidths to measure the many size classes from zooplankton to fish. A recommendation was made for new research to advance the theory and measurements of scattering from individual organisms.
The U.S. GLOBEC Optics Technology Workshop (U.S. GLOBEC, 1993) focused on the potential to determine biomass and rate processes of zooplankton in situ. Optics with its fine resolution but limited effective range is best used on scales of microns to meters. Optics and acoustics complement one another. For example, optics can identify taxa, and acoustics can quantify size distributions. Two- and three-dimensional, non-intrusive observations should lead to a better understanding of feeding, swimming and predation rates. Optics can also be applied to quantify the distribution, abundance and types of zooplankton prey.
Two other GLOBEC publications refer to technology. Most issues of the U.S. GLOBEC News contain a Technology Forum that deals with new instruments. A section of the California Current Workshop report also (U.S. GLOBEC, 1992) considers the special technological needs of that program.
The table below lists the technologies we discussed and describes some of their advantages and disadvantages. Although computational modeling could be considered a technology, it was the subject of another breakout session and was not considered here.
Four specific technological impediments emerged that are relevant to this PICES/GLOBEC initiative.