Biogeochemical significance of bacterial biomass in the ocean's euphotic zone

Bacterial abundance and biomass were extensively measured in the euphotic zone at several oligotrophic and mesotrophlc sites of coastal and open Pacific ocean during different seasons. Cornpansons with phytoplankton biomass and l a t h particulate organic carbon (POC) were made in order to determine the quantitative significance of bacteria as a carbon and nitrogen pool in the euphotic zone. Analysis of our data and that from the literature showed that bacterial abundance in the euphotic zone has a lower threshold of ca 3 X 105 1111-' or 6 pg C 1-' Consequently, with increasingly oligotrophic condit~ons photoautotrophic b ion~ass may decrease well below the bacterial b~omass . In oligotrophic waters bacterial biomass was comn~only 2 to 3 times greater than phytoplankton biomass. In contrast, In mesotrophic to eutrophic waters bactenal biomass was generally much less than phytoplankton biomass. Cornpanson w ~ t h total POC (which included bacterial carbon) showed that in o l~go t roph~c waters bacterial b~omass averaged 40 % (range 26 to 62 '6) of POC inexplicably, the sum of bacterial carbon and phytoplankton carbon was always about one-half of POC The dominance of bacterial biomass over phytoplankton biomass in oligotrophlc oceans has sign~ficant impl~cations for the food-web structure, nut r~ent cycling pathways, and for s i n l n g flux of organic matter Future studies should examine the physiological and trophic mechan~sms which lead to the dominance of bacterial biomass in oligotrophlc systems but of photoautotrophic biomass In the eu t roph~c systems

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