Biocatalytical Filtration and Carbon Cycling in Permeable Shelf Sediments
Markus Huettel, Joel E. Kostka, Peter Berg, Carol Arnosti.
(Funded by NSF Award OCE-0424967)
In a collaborative research project that addresses filtration and mineralization in permeable Gulf of Mexico sediments we investigate the role of advective pore water exchange on carbon mineralization.
The main goals of this project are to determine sedimentary water exchange rates, sedimentary metabolism, microbial community composition and ensuing fluxes of oxygen and nutrients in permeable coastal sands. The measurements are done at two contrasting sites with sandy sediment in the northeastern Gulf of Mexico, with one site exposed to waves and longshore currents while the other site is located in Apalachicola Bay (Fig.1).
Why do we investigate coastal sands? Sands cover a large fraction of the continental margin, most of these sands are so-called relict sands deposited by rivers on the margin during the Holocene when the sea level was lower and deltas could expand over the entire width of the present submerged shelf. In the present time, rivers and land erosion add sands to these relict sands in the coastal zone. In the nearshore environment, waves and bottom currents cause extensive sand transport, sorting and ripple formation. This results in the well sorted sand sediments that we know from Gulf and Atlantic beaches, and a common characteristic of these sands is their relatively high permeability.
When bottom currents caused by tides, wind and waves pass over the rippled sand beds, small pressure gradients develop at the sediment ripples and pump water through the upper layer of the bed (Huettel et al. 1996). In Fig. 2, this filtration is visualized by staining the pore water with red Rhodamine dye. The volume of water that is filtered through shallow shelf sands still is largely unknown, however, estimates, calculated from pore water velocities measured in the field suggest rates up to 1000 L m-2 d-1 (Reimers et al. 2004).
This sand bed filtration transports particulate and dissolved organic matter, oxygen and other solutes into the coastal sand. Particles “get stuck” in the sediment pores and oxygen is effectively used by the benthic microbial community during the degradation of the organic matter. This process suggests high decomposition rates in the coastal sands, but so far only very few studies have addressed carbon cycling in coastal sands (e.g. Jahnke et al. 1996; Marinelli et al. 1998). This is somewhat surprising as these sands are the dominant sediment type at the land-ocean interface, where input from land promotes peak primary production, highest organic deposition rates and strongest physical and biological mixing. The efficiency of the transport processes that link organic matter, oxidants and decomposers control to a large extent the rate at which organic matter is decomposed.
Regional models and budget calculations suggest that 30 to 40% of the marine primary production takes place in the shelf environment, and most of this material is re-mineralized in the shelf sediments. The finding that less than 10% of organic matter is permanently buried in the continental margin reveals that the decomposition process in the shelf sediment is very efficient. As sand is the dominant sediment type, this efficiency likely is linked to the decomposition process in the permeable sediments. However, due to the tight coupling between flow and degradation in sands it is difficult to quantify decomposition rates of such sediments in the laboratory. Likewise, relatively little is known about the microbial communities that mediate organic matter degradation in permeable sand sediments (Hunter et al. 2006).
Our integrated project approach combines the in-situ transport studies (Huettel), microbial community analyses (Kostka), decomposition rate measurements (Arnosti) and modeling (Berg). The chamber measurements (Fig. 3) revealed the autotrophic character of the sandy sediments.. Extreme production is maintained by a dense microphytobenthos population that inhabits the surface of the sands. This phytobenthos benefits from the advective percolation of the sediment, thus, the current and wave climate has a direct impact on coastal productivity. Measurements of extracellular enzymatic hydrolysis rates in surface sediments to which fluorescently labeled polysaccharides were added (Arnosti 2000) showed high hydrolysis rates supporting the in-situ measurements. We have determined the microbial community composition in the sediment, and the sequence database we have compiled will facilitate the development of improved probes and primer sets to be used in quantifying the metabolically active members of permeable sand communities.
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Hunter, E. M., H. J. Mills, and J. E. Kostka. 2006. Microbial community diversity associated with carbon and nitrogen cycling in permeable marine sediments. Appl. Environ. Microbiol. 72 5689-5701.
Jahnke, R. A., R. L. Marinelli, J. E. Eckmann, and J. R. Nelson. 1996. Pore water nutrient distributions in non-accumulating, sandy sediments of the South Atlantic Bight continenetal shelf. EOS 76: 202.
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Reimers, C. E. and others 2004. In situ measurements of advective solute transport in permeable shelf sands. Continental Shelf Research 24: 183-201.