Part II. Biological Modeling.
Introduction
In the western basin of the Arctic Ocean there occur several species of endemic and expatriate mesozooplanktonic copepods. These populations survive as a product of the influence of general/regional circulation patterns coupled to the internal physiological response to the external environment. A few of these species hibernate (diapause) in the deep basin during the nonproductive season and emerge from hibernation as conditions favor sufficient primary production allowing for positive growth in copepod individuals and populations (Figure 1). Due to the interannual and intra-annual variability in climatic conditions, the success/failure of planktonic populations to propogate their populations depends on the species' ability to adapt to varying conditions.
Success of diapausing populations under the conditions experienced in the Arcitc Ocean requires the population to synchronize the timing of diapause with the nonproductive season and to actively grow and reproduce during the productive season. During the growing season, individuals need to accumulate enough energy reserves for maintanence during the nonfeeding period with enough energy left over for gamete production, especially for those species that release propagules as nonfeeding adults. The main energy reserve in these copepod species is in the form of wax esters. In this study we examine the amount of lipid accumulation by individuals based on available resources and calculate remaining lipid reserves when individuals are expected to emerge. Energy expenditure is calculated from known catabolic rates (respiration and excretion) in relation to the life cycle (Figure 2).
Conceptualizing the mechanism for diapause.
The exact mechanism that determines the onset and termination of diapause is not known. Though there have been various hypotheses formulated to explain the underlying mechanism (Miller et al. 1991; Carlisle and Pitman 1961). These range from purely environmental cues to purely internally driven cues, likely the actual mechanism is a combination of environmental and endogenous cues contributing to varying degrees and may differ from species to species. An environmental cue hypothesized to induce and terminate diapause is light penetration (Miller et al. 1991). An endogenous cue that has been connected to diapause is the hormonal control of molting (Carlisle and Pitman 1961). For our hypothesis, we exclude light and food availability as cues for the initiation and termination of diapause. First, it seems unlikely that light variation could play a major role beneath the ice at 500-1000 m in early spring when diapausing copepods emerge and reproduce. Second, if the presence of food, in the form of phytoplankton, creates a chemical cue within the vicinity of diapausing copepods, it is probably too late for the new generation to take full advantage of the year's phytoplankton bloom for substantial growth. Instead our hypothesis focuses on the interaction between ambient water temperature, lipid accumulation, and the ecdysal hormonal complex (Figure 3). If we assume the diapausing stage to be the fifth copepodite stage (CV), prior to diapause, when the individual molts to the CV stage it is still actively feeding. During the intermolt phase of the molt cycle, the individual accumulates up to 65-70% of body weight as lipids, afterwhich it will enter diapause, or if ambient water temperature exceeds 5°C the individual will enter diapause in order to retain accumulated lipids. After a period of time, either when lipid content falls to 35% of body weight, or when ecdysone exceeds the molt inhibiting hormone (MIH) titer, the individual will emerge from diapause. Thus, there will be a combination of factors to induce and terminate diapause, which is reflected in the variability of diapause duration.
We include temperature as a possible factor to induce diapause since lipid accumulation can be controlled by temperature through the differential catabolism of lipids and proteins. The metabolic response of different species varies with temperature, but in general the O:N ratio increases with temperature for copepods in the Arctic Ocean (Figures 4 and 5). The increase in O:N ratio indicates greater lipid catabolism and reduces the capacity to retain lipids as temperatures increase. This general trend is not observed for Calanus hyperboreus in the North Atlantic where an increase in temperature leads to a lower O:N ratio (Figure 6). The lower O:N ratio is not due to a decrease in oxygen consumption, but rather a significant increase in ammonia excretion. In the North Pacific copepod, Neocalanus plumchrus, the slopes of the regressions of respiration and excretion versus temperature are not significantly different from each other, implying that the O:N ratio does not change significantly with temperature (Figure 7). Metabolic response to temperature has not been measured for Neocalanus spp. in the Arctic Ocean for comparison to other indiginous Arctic species.
Modeling diapause under different metabolic conditions and available energy for egg production.
Literature data indicate a wide range in measured metabolic rates as well as a wide range in lipid content during diapause. Based on a 210 day diapause period we simulate metabolic loss and available lipid after the diapause period using C. glacialis as an example (Figure 8). Under conditions of low metabolic expenditure, individuals are able to remain in diapause for the 210 days without dropping down to the critical lipid mass. In the case of individuals starting diapause with a high lipid content, the amount of remaining lipid produces 400 eggs, whereas individuals starting with a low lipid content, are able to produce 240 eggs at the end of the 210 day period. Under conditions of high metabolic expenditure, none of the individuals are able to sustain diapause for 210 days before reaching the critical lipid mass.
References
Carlisle, D., and W. Pitman, 1961: Diapause, neurosecretion and hormones in copepoda. Nature, 190 (4778), 827-828.
Miller, CB, TJ Cowles, PH Wiebe, NJ Copley, and H Grigg. 1991. Phenology in Calanus finmarchicus; hypotheses about control mechanisms. Mar. Ecol. Prog. Ser. 72: 79-91.