Source, chemical composition and bacterial utilization of dissolved labile organic carbon in the Potomac River estuary

Bacterial abundance in the mesohaline Potomac River is extremely high, predominately free-living and greatest during summer when anoxia is observed. Bacterioplankton respiration is the main process driving seasonal anoxia. We hypothesize that dissolved microbially labile organic carbon (DiMLOC) produced in situ by phytoplankton fuels this process.; In a monthly study conducted between March and November 2002 an antibiotic cocktail was used to inhibit bacterial assimilation of DiMLOC (as dissolved monosaccharides, (DM), dissolved free amino acids (DFAA), glycolic acid (GA) and biochemcial oxygen demand (BOD)) so accumulation could be tracked during a diel interval. Quadruplicate samples were collected from a cross-river transect in the mesohaline Potomac River. Two samples were treated with antibiotics (AB) and two controls were untreated. Of each treatment (AB or control) one was blackened and one exposed to ambient irradiance and held at in situ conditions for up to 30 hours. Bacterial production ( 3H-thymidine incorporation), metabolism (14C-glucose uptake), particulate and dissolved primary production rates (PPR) ( 14C-Sodium Bicarbonate incorporation) and DiMLOC parameters, were tracked every three hours over the diel period.; Bacterial metabolism and production in AB samples was reduced by 89% compared to controls. In controls, DM averaged 0.2 mg C l−1, and DFAA 0.4 mg C l−1 AB samples contained 6–25 times more DM and DFAA than controls suggesting that differences in concentration between AB samples and controls were that consumed by bacteria. DFAA, DM and BOD peaked during summer along with PPR; on average, 90% of PPR was as dissolved. These data indicate that bacterioplankton rapidly utilize DiMLOC and that the source of DiMLOC was phytoplankton.; These findings suggest that the food web in the Potomac River differs significantly from the grazer model in that much of the energy trapped by primary producers is not transferred through herbivores up the food web, but instead is shunted to a large bacterial community, which reaches enormous populations. This study also demonstrates the importance of tracking DiMLOC and bacterial parameters in order to better understand oxygen consumption in the Chesapeake Bay and the proximate factors which ultimately lead to anoxia.