Microbial Food-Webs and Nutrient Cycling in Lakes: Changing Perspectives

Abstract

within the last decade,there has been a profound change in our perception of nutrient flux and trophic cycling in the oceans. some (but not all) of the considerations leading to our present views are: 1) frequently,most of thephotosynthesi s in open waters is due to picoplanktonic algae. there may have been some underestimates of net primary productivities in open oceans. in some environments,carbon fixation by bacterial autotrophs could be significant.2) bacteria may be actively growing on photosynthetically derived organic material,but both they and the picoplanktonic algae are probably being effectively grazed mainly by nanoplanktonic protozoans. as a result, bacterial numbers remainrelatively constant although considerable material flux may still occur. 3) heterotrophic,phagocytotic protists are efficient grazers and recyclers of nutrients. these organisms may themselves be an important link inmetazoan nutrition. 4) mixotrophic organisms able to photosynthesize or to grow as heterotrophs are sometimes common constituents of the plankton. thus,the simple idea of orderly trophic level transfers of carbon, phosphorus,andnitrogen from primary producers to metazoans and upward has become inadequate. as yet,the extent to which the marine paradigm can be extended to lake ecosystems is unclear,largely because of the dearth of appropriate observation andexperimental evidence. this review presents some of the existing information from marine and freshwaters and suggests future areas for investigation.