Chairperson: Ann Durbin Rapporteur: Hans Dam
If blue-water communities are stable and the sizes of animal populations within them vary little with time, then birth and death rates will be largely in balance. Knowledge of these vital rates is extremely limited for small oceanic crustaceans (e.g., Petit 1982, Dessier 1985), though not so bad for fish species (Longhurst and Pauly 1987). Thus, important goals for blue-water GLOBEC studies would be to determine vital rates (birth, death, development) of animal populations in the open ocean, and to compare these rates to those of nearshore animals at similar temperatures. The latter goal is particularly timely in light of the recent suggestion that growth rates of copepods can be estimated directly from ambient temperature (Huntley and Lopez 1992). The implication that food may not be limiting to growth in nature should be evaluated experimentally for organisms inhabiting regions of the oceans which represent the extremes of high temperature and low apparent food density -- the blue-water gyres.
Although the first step is to quantify vital rates of animal populations, the ultimate goal, for reasons of intellectual satisfaction and predictive capability, would be to understand what regulates these rates. With regard to potential effects of climate change, exposure to elevated temperatures leads generally to reductions in mean body size within populations or to changes in zooplankton community structure from larger to smaller species (Moore and Folt 1993). Climate change may also affect ocean circulation and hydrography, thus altering the growth environment (e.g., temperature, mixed layer depth, nutrient fluxes) of primary producers. This may lead to changes in production rates or phytoplankton community structure that will alter trophic transfer to higher levels, indirectly impacting the dynamics of consumer populations.
The controversy over "bottom-up" (resource competition) versus "top-down" (predation) controls of population abundances was discussed in terms of open-ocean communities. It may be difficult to determine density-dependent effects on population size when birth and mortality rates are in balance (as could be the case for stable age distributions). In the case of stable populations, birth rates may be more easily determined, and mortality assessed indirectly. The favored approach for studying the dynamics of temperate coastal zooplankton--tracking cohorts--may not be adequate in tropical and subtropical regions because species reproduce more or less continuously through time. Moreover, following cohorts through time in a spatially extensive region would be extremely difficult (but see Dessier 1985). An alternative approach would be to assume that the populations exhibit stable age distributions. Information on vital rates can then be derived from life tables constructed from snapshots of the population at a single time. It would be essential to demonstrate that the stable-age assumption is met before embarking on studies of key species in the open ocean.
Because population dynamics is by nature a field that deals with questions of demography, the first priority would be to concentrate on the study of vital rates (birth, growth, death) of populations. Quantifying and understanding physiological processes (e.g., ingestion, respiration and excretion) through the energy balance approach should be a second priority that can also aid in measurements of species growth rates (LeBorgne 1982) and can provide a mechanistic understanding of growth at the organismal level.
On the other hand, traditional incubation methods for estimating growth rates (e.g., Peterson et al. 1991) may not be adequate for fragile oceanic species. These methods are also extremely time consuming and require extraordinary care in animal handling. Furthermore, incubation methods must assume that the animals experience their natural food resources. Therefore, innovative approaches that obviate the need for incubations need to be developed. Some of the promising approaches are: the oocyte maturity index of reproduction (Runge 1987); biochemical indices of growth and metabolism (RNA:DNA, citrate synthase activity, BRDU incorporation into DNA, PCR amplification of mRNA--see reviews by Buckley and Bulow 1987, Buckley and McNamara 1993, Crawford 1993); and indices of age (otolith rings for fish and lipofuscin for invertebrates, Sheehy 1992).