Field Process Studies
Understanding the coupling of physical and biological processes in the
sea is the core of U.S. GLOBEC. This understanding has been limited by
our ability to sample, process, and analyze biological data on scales
commensurate with physical data. To accomplish this, U.S. GLOBEC
emphasizes improved rapid discrete sampling, continuous in situ
measurement, and remote sensing in ongoing and future field studies.
The emphasis is on sampling marine populations on appropriate time
scales and with sufficient spatial resolution to compare with the
concomitant physical data. In addition, process studies are necessary
to elucidate the actual mechanisms coupling biology and physics. Only
by understanding mechanism can we extrapolate, generalize, and make
predictions about the influences of global climate change on marine
ecosystems.
Banks, Shelves and Shallow Seas
These environments are the home of many of the world's most important
fisheries. The first U.S. GLOBEC field study is occurring in the
Northwest Atlantic on Georges Bank-the site of oceanographic and
fisheries studies for more than a century (Backus, 1987). The cod,
haddock and other groundfish stocks of Georges Bank have historically
been important to the economy of New England. In recent years the
stocks of these species have declined, in part due to overfishing. One
challenge that U.S. GLOBEC faces on Georges Bank (and elsewhere) is that
of deciphering natural population fluctuations from anthropogenic
impacts. Georges Bank is thought to be highly sensitive to climatic
change because it is positioned in a faunal, climatic, and oceanic
boundary region. Moreover, model results indicate that the Georges Bank
region will be more heavily impacted by climate change than other areas
in the North Atlantic Ocean (Manabe et al. 1991). Finally, Georges Bank
is an excellent site for a study of the population biology of marine
animals because it is of sufficient size and has a physical circulation
pattern resulting in distinct, trackable populations that persist for
long periods amenable to time-series study. The focus of the Georges
Bank study is to determine how biological and physical processes
interact to control the population dynamics and retention of specified
target species on the Bank (see Appendix A.1). The information provided
by the Georges Bank field studies will permit assessment of the
potential fate of these zooplankton and fish populations under various
plausible global climate change scenarios. More information can be
found in U.S. GLOBEC reports Nos. 2 and 6 and Appendix A.1.
Eastern Boundary Currents
Eastern boundary current (EBC) systems are particularly appropriate for
examining both high and low frequency components of climate variability.
Biological and physical responses to forcing at interannual (e.g., ENSO
events) to decadal time scales (e.g., regime shifts) are known to be
very strong. The local biological response almost certainly involves
coupling to a variety of physical processes. Some of these prominent
physical processes are: wind speed, direction and wind stress curl;
pycnocline depth; alongshore and cross-shore advection; and buoyancy
inputs. These are influenced by larger-scale (basin-wide) oceanic and
atmospheric circulation. Lower frequency components of biological
variability (decades to centuries) are clearly evident in
reconstructions from historical and sedimentary data. Equally
important, several EBCs, including the California Current System (CCS),
have long biological and physical records, making them ideal for
examination of long-term changes, such as might occur due to gradual
global climate change. In the CCS, quantitative surveys of zooplankton
and fish have been conducted off British Columbia, Washington, Oregon,
California, and Baja California (Mexico). The best of these is perhaps
the CalCOFI investigations of the past 40 years. Data and scientific
insights developed from one EBC may be applicable to others, providing
global significance to such studies. EBCs are oceanographically and
ecologically distinctive; the dominant life history patterns and trophic
pathways contrast with those of other continental shelf ecosystems.
Moreover, in the CCS, some species extend over a broad latitudinal range
and are exposed to large differences in the intensity and timing of
seasonal circulation patterns. Conversely, other planktonic, benthic
and fish species are restricted to smaller regions, perhaps due to the
population's responses to the differing mesoscale physical variability
in different regions. Finally, eastern boundary current systems are
important economically-approximately 35% of the global marine fish catch
is taken from the EBC systems of the Atlantic and Pacific, and the U.S.
west coast fishery in 1992 produced an impact of $4 billion on the
economies of California, Oregon and Washington (U.S. Department of
Commerce, 1992). More information can be obtained from U.S. GLOBEC
Reports No. 7 and No. 11 (U.S. GLOBEC 1992, 1994) and Appendix A.2.
Global climate change is predicted to be greatest at high latitudes, with
dominant effects being increased temperature and changes in ocean
circulation. The Antarctic has a high negative radiation budget; its
immense masses of continental ice and annual sea-ice act as a
refrigerator buffering seasonal and multiannual changes in temperature.
The extent of sea-ice in the Southern Ocean is not, however, constant
from year to year. The fluctuations in sea-ice extent may represent one
of the most dramatic manifestations of climate change in the Southern
Hemisphere. Recent paleoclimate studies indicate that changes in
atmospheric greenhouse gas concentrations may have already affected the
extent of sea-ice.
Were atmospheric warming in the Antarctic to reduce the areal extent of
sea-ice, this would almost certainly reduce photosynthetic carbon
fixation, destroy habitats, and disrupt the life cycles of marine
animals. Marine zooplankton, like krill, and higher trophic level
animals, whose present-day biogeographic ranges are directly related to
the extent of sea-ice coverage, might be most seriously impacted. On
the other hand, increased meltwater input from the continental ice sheet
might have a compensatory effect by altering water column stability,
stratification, and extending the high production zone further from
shore. It has been suggested that because of the tight linkages between
trophic levels (producerÐherbivoreÐcarnivore) in the Antarctic,
long-term studies focusing on predator-prey relationships and their
environment are an efficient way to monitor the effects of man-induced
perturbations on the entire regional ecosystem (Croxall et al. 1988).
The focus of a U.S. GLOBEC Southern Ocean study will be how variability
in sea-ice extent determines variability in the population dynamics of
the target species. To understand the mechanisms responsible for
changes in resource levels for the higher trophic level consumers
requires knowledge of the many inter-related factors affecting krill
abundance and availability. These include water mass distribution,
reproductive and recruitment success, and food availability, which may
depend directly or indirectly on ice cover. Further information about
the Southern Ocean GLOBEC study can be found in U.S. GLOBEC Report No.
5, GLOBEC International Report No. 5, and Appendix A.3.
The physical and biological environment in the open ocean differs
dramatically from that found in coastal, polar, and marginal seas. So
too may the coupling of physical and biological processes. We presently
do not know whether open ocean ecosystems will be resistant to climatic
variations, or shift dramatically under such potential forcing. For
example, there may be less variability in physical forcing over time
scales of days to several months and over both small (e.g., meters to
km) and large (10's to 1000's of km) spatial scales in the open ocean
than is commonly found in the coastal ocean. Further, the frequency
distribution of numbers of individuals in a species in much of the open
ocean appears to be more stable year-to-year than in coastal
environments. If the apparent stability of open ocean biological
communities is due to internal biological checks and balances, then the
biomass and food-web relations of oceanic communities might be
relatively resistant to changing physics caused by climate change.
Conversely, if the stability of the open ocean biological communities is
due to the near constancy of the physical environment, then oceanic
communities may be more susceptible to physical perturbations resulting
from global climate change. Some open ocean environments, for example
the Arabian Sea (U.S. GLOBEC Report 9), do not fit the paradigm. The
Arabian Sea is subject to extremely strong physical forcing due to
seasonal reverses of monsoonal winds. Quite likely the biological
community is also strongly seasonally variable.
Comparison of zooplankton vital rates and population dynamics of various
oceanic and coastal habitats will be essential. In fact, several
circumglobally distributed species might be targeted for study. In this
way some of the inherent variability associated with different habitats
in different regions of the world's oceans might be minimized and the
biological responses to global climate change extracted from records of
seasonal and interannual variability. Possible U.S. GLOBEC studies in
the open ocean were discussed at a workshop held in September 1993 and a
report of the workshop discussions and recommendations is under review.
Possible U.S. GLOBEC studies in the Arabian Sea were discussed at a
workshop in June 1992. Recommendations and further information can be
found in U.S. GLOBEC Report 9.