Macroecological limits of heterotrophic bacterial abundance in the ocean

Abstract The global association between heterotrophic bacteria and phytoplankton in the open ocean was examined to identify the macroecological limits of bacterial abundance. A consolidated dataset was constructed using 13,973 paired measurements of bacterial abundance and chlorophyll concentration from 15 years of observation in various biogeochemical provinces covering the Atlantic, Pacific, Arctic and Antarctic Oceans as well as the Arabian Sea. The bivariate relationship between bacteria and chlorophyll is a filled polygon whose upper boundary undergoes a change in slope from positive to negative at about 1 mgChl m −3 , suggesting a transition from bottom-up to top-down control of bacteria along a resource supply gradient. The upper limit of bacterial abundance in the ocean is everywhere set by phytoplankton, but the limit is not realized in productive waters because of mortality losses. A carrying capacity of about 7 trillion bacteria for 1 mg of chlorophyll is indicated by empirical observations, consistent with theoretical considerations and predictions from food web modelling.