Macrofaunal Effects on Nitrification and its Coupling to Denitrification in Coastal Marine Sediments

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This study examines the interaction between benthic macrofauna and sediment nitrification and denitrification in the coastal marine environment. Dr. Kemp and his co-inves- tigator Dr. Boynton will make direct measurements of macrofaunal influence on nitrification and coupled nitrification - denitrification using a modified 15N isotope technique for intact coastal marine sediments with manipulated animal densities. Three levels of macrofaunal abundance will be used for representative species (e.g. amphipod, bivalve, polychaete). Alternative mechanisms by which macrofauna may stimulate nitrification (e.g. increased supplies of oxygen vs. ammonium) will be examined separately using artificial tubes will be compared using microelectrode techniques. Three possible mechanisms for macrofaunal enhancement of nitrification - denitrification coupling will be tested with the isotopic procedures including: 1) effects of radial diffusion geometry; 2) effects of alternating oxic- anoxic conditions; 3) role of anoxic microsites in surface deposited fecal pellets. Dissolved nitrogen compounds such as nitrate and ammonium are important nutrients for biological production. However, the oxidation of ammonium (nitrification) coupled to the reduction of nitrate to nitrogen gas (denitrification) results in the decrease of nitrogen in biologically active pools and may serve one of the important mechanisms controlling the nutrient levels in sea water. This study addresses the coupling of these processes and their relation to the larger organisms living in the coastal sediments, an environment in which nitrification/denitrication is particularly active. The work will test quantitatively the notion that the sediment burrows these organisms construct (along with the activities of these animals) facilitates the microbiological nitrification/denitrification activity in the surrounding sediment. The work include novel experiments using sediment samples with varying numbers of organisms and laboratory experiments using artificial burrows. Isotopic labelling experiments will be used to measure the actual rates of nitrification/denitrification and to address the mechanisms by which the larger sediment organisms influence these microbiological processess.