Abstract
Riverine runoff often triggers microbial responses in coastal marine environments, including phytoplankton blooms and enhanced bacterial biomass production that drive the transformation of dissolved and particulate organic matter on its way from land to the deep ocean. We measured concentrations and characteristics of particulate organic matter (POM), concentrations of dissolved organic carbon (DOC), and bacterial community abundance and activities in the water column at three sites near the Mississippi River Delta two weeks after Hurricane Isaac made landfall in late August 2012. River plumes had salinities of >30 PSU and high levels of DOC (210-380 µM), resulting from the storm surge that pushed large quantities of marine waters upstream. Relatively high concentrations of phytoplankton POM and low levels of microbial exopolymeric particles (TEP and CSP) suggested that storm-induced riverine discharge triggered the development of phytoplankton blooms that were in their initial stages at the time of sampling. Surface water POM had C/N ratios of 5-7 and strong protein-like fluorescence signals in the base-extracted POM (BEPOM) fraction at the two sites closer to the river mouth (Stns. TE and MSP). Freshly produced POM triggered a twofold increase in heterotrophic bacterial biomass production (3H-leucine incorporation) and a fourfold increase in bacterial peptide hydrolysis (activities of leucine-aminopeptidase). In contrast, elevated DOC concentrations coincided with only moderate bacterial community activity, suggesting that heterotrophic bacterial metabolism near the Mississippi River Delta in the aftermath of Hurricane Isaac was more closely linked with autochthonous primary production.
Highlights
The nature and inventory of organic matter in the coastal ocean is mainly driven by heterotrophic microbial communities that process and transform organic matter from a myriad of potential sources, including riverine inputs, atmospheric deposition, and sediment resuspension, regulating carbon export from land to the open ocean
During that time elevated rates of primary productivity and secondary production driven by high inputs of inorganic nutrients through the Mississippi River often result in the development of seasonal hypoxia on the Louisiana Shelf (Rabalais et al, 2010; Murrell et al, 2013)
These results indicate that bacterial community activities at the Stns
Summary
The nature and inventory of organic matter in the coastal ocean is mainly driven by heterotrophic microbial communities that process and transform organic matter from a myriad of potential sources, including riverine inputs, atmospheric deposition, and sediment resuspension, regulating carbon export from land to the open ocean. In the Gulf of Mexico, most of the land-sea carbon fluxes are driven by Mississippi River runoff and subsequent biogeochemical organic matter processing that occurs on the Louisiana Shelf near the bird-foot delta, where most of the riverine discharge enters the coastal ocean (e.g., Green et al, 2006). Given its important role in transport and cycling of terrestrial carbon between the land and the ocean, numerous geochemical studies have measured carbon flux from the Mississippi River into near-shore environments (e.g., Bianchi et al, 1997; Wang et al, 2004), suggesting that microbially-driven organic matter transformation could be an important sink for terrestrial carbon on the Louisiana shelf (Benner and Opsahl, 2001; Bianchi et al, 2013). During that time elevated rates of primary productivity and secondary production driven by high inputs of inorganic nutrients through the Mississippi River often result in the development of seasonal hypoxia on the Louisiana Shelf (Rabalais et al, 2010; Murrell et al, 2013)
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