Why is relating plankton community structure to pelagic production so problematic?

The conceptual framework for quantitative marine ecology is attributable to Victor Hensen (1887), who proposed that quantitative studies of plant and animal production in the sea would permit predictions of annual fish yields. Hensen was strongly influenced by concurrent conceptual developments in agriculture, in which crop production was being predicted from knowledge of physiology and its relationship to environmental variables. As fish were being "harvested" by man, it was argued that relationships similar to agriculture existed between primary production and fish yield. Thus was born the notion that pelagic ecosystems were structured from the "bottom-up", or resource-limited. Subsequent refinements argued that, from basic knowledge of how vertical mixing regulates primary production, and assuming certain features of food-chain length and efficiency, one can estimate fish yields. Fundamental to these arguments are assumptions concerning resource limitation which appear to be uncertain as generic marine pelagic characteristics, primarily that trophic levels are nutrient/food limited and respond to increased resource availability by elevated standing stocks. Whereas this conceptual model explains certain features of energy flow, it fails to describe how marine pelagic foodwebs are structured and why they function as they do. Rather, it appears that certain taxa are better than others at integrating their environments and regulating the flux of materials through the foodweb, and that predation is as important as resource limitation. There appears to be a distinct need in pelagic research to focus on predation, not as a rate process so much as a mechanism responsible for organism behaviour, morphology, life history and community structure.