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

Abstract

Bacterial abundance and biomass were extensively measured in the euphotic zone at several oligotrophic and mesotrophic sites of coastal and open Pacific ocean during during different seasons. Comparisons with phytoplankton biomass and with 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 .times. 105 ml-1 or 6 .mu.g C l-1. Consequently, with increasingly oligotrophic conditions photoautotrophic biomass may decrease well below the bacterial biomass. In oligotrophic waters bacterial biomass was commonly 2 to 3 times greater than phytoplankton biomass. In contrast, in mesotrophic to eutrophic waters bacterial biomass was generally much less than phytoplankton biomass. Comparison with total POC (which included bacterial carbon) showed that in oligotrophic waters bacterial biomass averaged 40% (range 26 to 62%) 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 oligotrophic oceans has significant implications for the food-web structure, nutrient cycling pathways, and for sinking flux of organic matter. Future studies should examine the physiological and trophic mechanisms which lead to the dominance of bacterial biomass in oligotrophic systems but of photoautotrophic biomass in the eutrophic systems.