While many of the linkages involving zooplankton in the Hudson food web are known, uncertainty remains concerning their importance to populations. Are predator-prey interactions tightly coupled to the respective dynamics of individual populations or are these associations "loose" in the sense that specific trophic interactions have little affect on dynamics? Loose linkages could be the case if populations are highly omnivorous. In this view the estuary represents a dynamic feeding tableau with sufficient resources to support population growth. Exploitation is a matter of ever changing opportunities, and resources have little influence on populations. Alternatively, the highly repeatable dynamics of some populations (for example, Bosmina freyi) suggest there are structured interactions that determine population processes and that some of these are related to trophic effects. The Hudson probably contains both loose and tight trophic interactions and sorting the causes and consequences of these relationships remains a goal for future study.
Zooplankton by definition are animals in the water column and are often studied in isolation from interactions with benthic processes. Mero-planktonic animals, however, arise from the benthos and return there after a brief pelagic phase. More permanent members of the plankton have resting stages that reside in the bottom and shallows of the Hudson. These stages likely seed the blooms and population increases of many species. Further, planktonic animals retreat to the bottom or near-bottom environment as part of daily migrations probably to avoid visual predators but also to maintain position and retention within the estuary in association with tides. These few examples of benthic-pelagic interactions point to an area of limited knowledge. A more fully integrated understanding of zooplanktonrequires abetter appreciation of how benthic-pelagic interactions influence populations and food web interactions.
We have argued that zooplankton are a critical trophic linkage to fish in the Hudson and much of their functional significance in the ecosystem derives from this role. Fish populations are, however, quite variable. Large interannual variations are observed. In addition, populations vary substantially in time and space within a year and hence predation from fish on zooplankton may be highly episodic. The consequences of this presumably variable interaction between fish and zooplankton remain poorly understood. Means for measuring predatory mortality and variation in these rates are critical to testing many of the ideas posed in this chapter.
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