Population Characteristics and Genetics
Chairperson: Ann Bucklin
Rapporteur: Michael Miller
- How do food supply, competition and predation interact to maintain species diversity and community structure?
- Are oceanic species sensitive or resistant to environmental variation?
- Are oceanic populations genetically uniform, or are there ecological sub-populations with differing characteristics?
- Does diversity or community structure differ between the North Pacific and North Atlantic gyres?
- Can target species be selected for study from highly diverse communities?
- Studies of populations in the subtropical gyres should be conducted for comparison with results from GLOBEC investigations in adjacent coastal regions.
- New methodologies are needed to address problems of taxonomy and genetics in the open oceans.
- Behavior-oriented studies are essential for understanding the dynamics of species in diverse assemblages.
Text of Chair/Rapporteur's Report:
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
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.
- The recently recorded change (rise) in temperature in the North Pacific has altered population sizes, dominance structure (rank order of abundance), and diversity in the Central Pacific gyre.
- Populations in systems having continuity of circulation and stability of environmental variables are genetically unstructured. However, species may exhibit significant variations among "ecological" populations in physiological and functional characteristics driven by natural selection.
- Species diversity, population sizes, and dominance structure differ between the North Atlantic and North Pacific Oceans.
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.
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.
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
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.