Microbial production and particle flux in the upper 350 m of the Black Sea

Abstract

A field experiment, designed to evaluate several aspects of the microbially mediated organic carbon cycle, was conducted in the western basin of the Black Sea during May 1988. Measurements included standing stocks of particulate and dissolved carbon, rates of microbial production and downward particle flux. Photoautotrophic production was restricted to the upper 55 m of the water column. Values ranged from 0.7–15 mg Cm −3 day −1; total integrated primary production was 575 mg C m −2 day −1. Euphotic zone heterotrophic bacterial production was estimated to be 260 mg C m −2 day −1, a value equivalent to 45% of the contemporaneous photoautotrophic production. Beneath the euphotic zone, and across the oxic-to-anoxic boundary (55–95 m), bacterial production rates were low but measurable (≤6 mg C m −3 day −1). Heterotrophic bacterial productivity was approximately equal to the rate of chemoautotrophy in the 55–95-m depth interval. No bacterial biomass or production peaks were observed in the chemocline region. POC and PON fluxes from the base of the euphotic zone (60 m) were 140 and 11.6 mg m −2 day −1, respectively. Beneath 60 m, POC and PON fluxes decreased rapidly with depth to 39 and 3.3 mg m −2 day −1 at 80 m, followed by a more gradual decline to 24 and 2.0 mg m −2 day −1 at 350 m. The fluxes of particulate ATP and fecal pellets also exhibited order of magnitude decreases in the 60–80-m depth interval with minimal losses thereafter. Our results indicate a rapid and efficient recycling of particulate organic matter in the sub-oxic portion of the water column (60–80 m) and relatively low rates of decomposition in the permanently anoxic zone.