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 ecosystems 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 bc 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 marinc systems compared with temperate lakes. The data suggest that this more intense yield is not due to the size or interconnection of marine areas or to higher primary production. Rather, the efficiency of transfer of primary production to fish appears to be greater. Tropical lakes 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.