Physical Energy Inputs and the Comparative Ecology of Lake and Marine Ecosystems

Although freshwater and marine systems both receive light and heat energy from the sun and are mixed by the wind, only marine systems receive additional mechanical energy from the tide. This input is very small relative to the flux of solar energy but may exceed that from wind. Some obvious physical consequences of this additional energy input include the development of intertidal habitats, the presence of stronger currents, and more vigorous vertical mixing. It is argued that these (and perhaps other) consequences lead to coastal marine cosystems which differ in a number of important ways from temperate lakes. There is some evidence that coastal marine systems generally maintain a larger standing crop of benthic animals and that these fauna are more effective in mixing the bottom sediments. As a result of better sediment mixing (and perhaps warmer and better oxygenated bottom water), organic matter deposited on the bottom of coastal marine areas may be more completely metabolized and less C, N, and P retained than in lake sediments. Materials that are more tightly bound to particles, like many metals, may behave similarly in lake and marine sediments. Although many lakes are strong sinks for nutrients and metals, marine bays and estuaries may be much less effective in retaining nutrients. A major consequence of the input of tidal energy appears to be a more intensive yield of fish from marine systems compared with temperate lakes. The data suggest hat this more intense yield is not due to the size or interconnection f marine areas or to higher primary production. Rtther, the efficiency oftransfer of primary production to fish appears to be greater. Tropical akes appear more like marine systems in this regard, and this may be related to lower thermal stability and more efficient wind energy transfer because of a small Coriolis effect at low latitudes. It is an editor's reward to have the chance to make some closing observations and speculations. I cannot claim that the following pages will synthesize or even summarize in any systematic and comprehensive manner all the papers that have come before. Certainly, my own thoughts have been helped greatly by reading all of the preceding reviews, but it would be unfair for me to put this chapter forward as anything more than my own contribution tothe subject of this volume. Much of it is speculative, perhaps much of it will prove to be wrong. In any case, I am confident this last paper will not be the final word on the subject of the comparative cology of freshwater and marine ecosystems. This review carried me into waters both saltier and fresher than is my normal habitat and I was often over my head in both. In addition to those who prepared the reviews in this volume, many others tried to keep me afloat by providing insights, criticisms, data, and references. Among those I must thank are H. T. Odum, Michael Pilson, Virginia Lee, John King, Saran Twombly, Saul Saila, Jeff Frithsen, Jim McKenna, Candace Oviatt, Nelson Hairston, Rebecca Schneider, Ed Herdendorf, Anne Jones, Norb Jaworski, and Michael Prager. Figures were prepared by the URI Bay Campus illustration group. Dolores Smith prepared various versions of the manuscript and helped greatly with references. Holly Turton helped with editing and proofreading. Support for much of my work in comparative ecology has come from the Sea Grant Program, NOAA. Comparative size and energy input Surface area and depth-It seems useful to begin with some simple comments about the relative surface area and depth of freshwater, estuarine, and marine systems for two reasons. First, because many oceanographers-even those who work in coastal areas-seem to think of all lakes as small and shallow. Second, because the rest of this paper is devoted to the hypothesis that it is a difference in physical energy input that ultimately leads to some of the most profound, interesting, and important differ-