Annual cycle of organic matter partitioning and its availability to bacteria across the Santa Barbara Channel continental shelf

Abstract

Seasonal trends in organic matter (OM) partitioning (between the dissolved and particulate phases), dissolved inorganic nitrogen (DIN) distributions, and bacterioplankton dynamics were examined across a 3 km stretch of the Santa Barbara Channel continental shelf from January 2008 to April 2009. OM partitioning was assessed as the percentage of 14 C-primary production (PP) released as 14 C-labeled dissolved organic carbon (DOC), i.e. percent extracellular release (PER), and as the ratio of DOC and particulate organic carbon (POC) concentrations. Spring upwelling raised surface DIN concentrations, stimulating phytoplankton blooms and OM accumulation. During upwelling, PER accounted for 10 to 30%, with ~60% of OM accumulating as POC versus ~40% as DOC. After stratification and macronutrient depletion, PER increased to between 35 and 45%, and the proportion of seasonally produced DOC to total accumulated organic carbon increased to > 50%. Experiments revealed a seasonal maximum in DOC bioavailability to heterotrophic bacterioplankton during upwelling: 9 to 18% of DOC was consumed in <1 wk at bacterial growth efficiencies (BGE) of 5 to 12%. Throughout the stratified period, 5% of DOC was consumed in <1 wk, and the BGE increased to between 12 and 52%. Bacterial carbon demand (BCD) ranged from 0.2 to 24 mol C l -1 d -1 , and for most of the sampling period, PP was sufficient to meet BCD. However, at times, BCD exceeded phytoplankton PP, indicating that sources of DOC other than daily PP were used to support BCD. Similar to other coastal systems, OM is partitioned mainly into the particulate phase under nutrient-replete conditions and into the dissolved phase following stratification and the onset of oligotrophic conditions. We discuss how the partitioning of OM has consequences for bacterial metabolism and carbon cycling of this coastal system.