Population Characteristics and Genetics

Chairperson: 	Ann Bucklin
Rapporteur:	Michael Miller

ISSUES:

CONCLUSIONS:

Text of Chair/Rapporteur's Report:

The Issues

Open ocean regions, like the central Pacific gyres, are environments of high spatial homogeneity and temporal stability. Zooplankton communities in such regions are extraordinarily diverse, with a preponderance of rare species. Mechanisms controlling the dynamics of these communities are poorly understood, and, in fact, extremely difficult to investigate. For instance, numerous salp species co-occur in time and space; feeding nonselectively and not obviously food limited. An analogous situation exists for copepods (Hayward and McGowan, 1979). How can competition for food be demonstrated in a stable system without evidence of habitat partitioning? The role of predation in stabilizing such systems is also unclear. Testable hypotheses for the regulation of population size and community structure of open ocean zooplankton do not currently exist.

The "sensitivity" of open ocean organisms to variations in the physical environment and climatic fluctuations was discussed at length Our initial thought was that populations from environments that normally experience little variation in physical parameters should respond strongly to environmental perturbations. Enormous changes in the census size of Antarctic euphausiids, for instance, follow relatively small changes in sea surface temperature (SST) (Quetin and Ross, 1984). On the other hand, plankton in the central Pacific gyre appear to be markedly insensitive to temperature fluctuations (McGowan and Walker, 1985). Moreover, since vertical gradients in temperature overwhelm horizontal and temporal variations in such systems, it is difficult to hypothesize circumstances in which small changes in SST will significantly impact the population dynamics of vertically migrating species.

We also considered the influence of oceanic circulation on diversity and abundance of animal plankton in the open ocean. Although large scale circulation patterns are clearly a dominant influence in oceanic plankton distributions and dynamics, logistical difficulties preclude any attempt to characterize gyre-scale patterns of ocean circulation. Since circulation patterns directly affect and determine patterns of sea surface temperature, we agreed that temperature was a useful indicator of environmental fluctuation, and that the relationship between temperature and the dynamics of open ocean populations and communities should be examined.

With regard to the genetics of open ocean plankton and fish, the primary question is one of partitioning. It seems unlikely that genetically distinct sub-populations remain isolated within gyres. However, gyre populations may be ecologically partitioned as individuals become physiologically and reproductively suited to local conditions over short time periods. Physiological variation may be highly significant in ecological terms even if genetic homogeneity of the gyre population is maintained by periodic mixing. Appropriate markers of non-genetic variation within species populations include: functional differences in enzymes that are not reflected in allozymic variation (see Graves et al., 1983) and variations in regulatory genes (e.g., cytochrome P450) that may be important in adapting individuals to local conditions. There may also be genetic determinates of behavior that allow behavioral switching according to local conditions. Studies might focus on vertical migration, which may be under both genetic and behavioral control (Bollens and Frost , 1989).

Although gyre populations are likely to be genetically homogeneous, there may be significant genetic structuring at larger spatial scales (e.g., between populations in the North and South Pacific Central gyres). The genetic cohesiveness of amphitropical species should be examined. Molecular clock approaches may be cautiously applied for estimating time-since-divergence, and to investigate the likelihood of gene flow between populations in different gyres during periods of global warming and cooling.

Hypotheses

Recommended Approaches

Time-series measurements are highly desirable, but their shiptime requirements may be prohibitive. GLOBEC can achieve a modified time-series approach by repeating the cruise tracks of previous field studies in the North Pacific and North Atlantic. In the North Pacific, a transect between Hawaii and Kodiak, Alaska was studied in 1960, 1963 and 1980. Another analysis of this transect would provide the basis for a comparative study and evaluation of decadal variations in population abundance, species diversity, and dominance structure. In the North Atlantic, a similar result could be achieved by repeating a transect between the Canary Islands and Iceland.

Methodological Hurdles/Needs

GLOBEC will require taxonomists trained to discriminate widely distributed species groups so that introduced species can be recognized. Geographic locale cannot be used as a taxonomic character, in the face of increasingly frequent, especially anthropogenic, species exchanges between ocean basins. New methods of taxonomic discrimination will also be required, to facilitate rapid identification and quantification of species abundances in oceanographic samples. GLOBEC should investigate all possible ways of automating zooplankton enumeration, particularly for larval and juvenile stages that cannot be easily resolved morphologically. Biochemical, molecular, optical and acoustic approaches should be examined.

Direct observation of the behaviors of open ocean plankton will be required to understand population and community dynamics, since these organisms generally behave unnaturally in contained, experimental systems. GLOBEC should continue to encourage development of in situ observation techniques, including video imaging and photography.

Conceptual Problems

U.S. GLOBEC strategies for the study of nearshore planktonic ecosystems may not be appropriate for the open oceans. In particular, identification of "key" species will be problematic. In a community of numerous rare species, few can impact community and trophic interactions by numerical fluctuations. How do we select target species in such systems?

Opportunities

The approach of JGOFS is particularly useful for integration with open ocean GLOBEC studies, because of the time-series analyses at fixed sites near Bermuda and Hawaii. Hydrographic, meteorological data, and samples of zooplankton, and perhaps fish, from these sites should be examined to describe temporal patterns of variation in open ocean environments.

Comparisons among open ocean, margin, and coastal areas may reveal unique characteristics and dynamics of open ocean ecosystems. Three regional GLOBEC studies border central gyres--the Northwest Atlantic (Georges Bank) Study, the Eastern Boundary (California) Current Study, and the Nordic Seas (Mare Cognitum) Study--and will provide useful information for comparisons.

A semi-submersible Deep-Sea Observatory (DSO) may also be useful for time-series observations in the open ocean (Wiebe et al. 1993). The DSO would provide a platform for blue-water diving for observational studies. Collection of biological samples could be achieved by trawling from work boats sited at the Observatory.

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