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Rosalynn
Y. Lee
Institute of Marine and Coastal
Sciences
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Abstracts 21. Filling Gaps in LME Nitrogen Loadings Forecast for 64 LMEs. GEF/LME global project Promoting Ecosystem-based Approaches to Fisheries Conservation and Large Marine Ecosystems. Participants from 7 LMEs attended two workshops (23-25 January 2006 and 18-20 September 2006), held at UNESCO-IOC in order to receive training on the use of GIS-based models relating land use and human activities in watersheds to nutrient transport by rivers to coastal systems. Moreover, participants conducted further analyses using the software and model interface given to them during the workshop, developed science-based summary documents for their home region, provided their LME director with the model findings, as well as gave presentations to their regional government officials and at scientific meetings. This project specifically used an innovative Nutrient Export from Watersheds Model (NEWS N-Export model) to predict dissolved inorganic nitrogen (DIN) export by rivers to the coast as a function of watershed N inputs (point and diffuse sources), hydrology, and other factors. Developed by Dumont et al., [2005], NEWS-DIN has been extended to allow its application to LMEs. The model was used to examine DIN export into seven LMEs: Baltic Sea, Bay of Bengal, Benguela Current, Guinea Current, Gulf of Mexico, Humboldt Current and Yellow Sea and also from a global perspective. This Technical Report outlines the results of these two workshops, and gives a comprehensive summary of the Large Marine Ecosystem Approach developed by Kenneth Sherman.. 20. Porewater biogeochemistry and soil metabolism in dwarf red mangrove habitats (Twin Cays, Belize). Seasonal variability in biogeochemical signatures was used to elucidate the dominant pathways of soil microbial metabolism and elemental cycling in an oligotrophic mangrove system. Three interior dwarf mangrove habitats (Twin Cays, Belize) where surface soils were overlain by microbial mats were sampled during wet and dry periods of the year. Porewater equilibration meters and standard biogeochemical methods provided steady-state porewater profiles of pH, chloride, sulfate, sulfide, ammonium, nitrate/nitrite, phosphate, dissolved organic carbon, nitrogen, and phosphorus, reduced iron and manganese, dissolved inorganic carbon, methane and nitrous oxide. During the wet season, the salinity of overlying pond water and shallow porewaters decreased. Increased rainwater infiltration through soils combined with higher tidal heights appeared to result in increased organic carbon inventories and more reducing soil porewaters. During the dry season, evaporation increased both surface water and porewater salinities, while lower tidal heights resulted in less reduced soil porewaters. Rainfall strongly influenced inventories of dissolved organic carbon and nitrogen, possibly due to more rapid decay of mangrove litter during the wet season. During both times of year, high concentrations of reduced metabolites accumulated at depth, indicating substantial rates of organic matter mineralization coupled primarily to sulfate reduction. Nitrous oxide and methane concentrations were supersaturated indicating considerable rates of nitrification and/or incomplete denitrification and methanogenesis, respectively. More reducing soil conditions during the wet season promoted the production of reduced manganese. Contemporaneous activity of sulfate reduction and methanogenesis was likely fueled by the presence of noncompetitive substrates. The findings indicate that these interior dwarf areas are unique sites of nutrient and energy regeneration and may be critical to the overall persistence and productivity of mangrove-dominated islands in oligotrophic settings. 19. Scenarios of Watershed Nutrient Sources, Forms and Exports: a Millennium Ecosystem Assessment "Global NEWS" Application. Dramatic global increases in anthropogenic nutrient production on land and negative impacts on coastal systems due to export from rivers are extensively documented. Recently, the Millennium Ecosystem Assessment (MA) concluded that excessive nutrient loading of ecosystems is a major driver of global ecosystem change. Increased nutrient mobilization is expected to continue for decades in response to economic and population growth. Development of a scientific basis for actions to reverse these trends and sustain riverine and coastal ecosystem health requires quantitative models applicable at regional to global scales, sensitive to watershed anthropogenic forcings, and capable of predicting changes in element ratios and nutrient forms (dissolved, particulate, organic, inorganic) which modulate the impacts of nutrient loading on marine ecosystems. The Global Nutrient Export from Watersheds (NEWS) system of models was designed to meet these requirements and was previously applied to contemporary (1995) forcings. We will present preliminary results from an application to the four MA scenarios thru 2050. These scenarios integrate economic, social, and ecosystem processes, and represent possible futures with contrasting degrees of global cooperation and of sustainability of ecosystem services. Consistent global datasets of land nutrient fluxes (diffuse and point sources) and hydrological alterations were developed for this application. 18. Past, Current and Future Trajectories of Watershed Nutrient Sources, Forms and Exports: a Global NEWS application to the Millennium Ecosystem Assessment scenarios. Dramatic global increase in anthropogenic nutrient production on land and negative impacts on coastal systems due to export from rivers are extensively documented. Recently, the comprehensive Millennium Ecosystem Assessment (MA) concluded that excessive nutrient loading of ecosystems is one of the major drivers of global ecosystem change. Increased nutrient mobilization is expected to continue for decades in response to economic and population growth. Development of a scientific basis for actions to reverse these trends and sustain riverine and coastal ecosystem health requires quantitative models applicable at regional to global scales, sensitive to changes in watershed anthropogenic forcings, and capable of predicting changes in element ratios and nutrient forms (dissolved vs. particulate, organic vs. inorganic) which have been shown to modulate the impacts of nutrient loading on marine ecosystems. The Global Nutrient Export from Watersheds (NEWS) system of models was designed to meet these requirements and was previously applied to contemporary (1995) forcings. We will present preliminary results from an application to past (1970) and current (2000) conditions, and compare them to four MA future scenarios thru 2050. These scenarios integrate economic, social, and ecosystem processes, and represent plausible futures with contrasting degrees of global cooperation and of sustainability of ecosystem services. Consistent global datasets of land nutrient fluxes (diffuse and point sources) and hydrological alterations were developed for this Global NEWS effort. 17.
Primary production, nitrogen cycling and the ecosystem role of mangrove
microbial mats on Twin Cays, Belize.
The seasonal variability of porewater nutrient concentrations, metabolism,
nitrogen cycling (denitrification and nitrogen fixation), and primary production
(oxygenic and anoxygenic photosynthesis and chemoautotrophy) was examined in benthic
mangrove environments on Twin Cays, Belize. Twin Cays mangroves exhibit a tree-height
gradient from tall seaward fringe trees, through a transition of intermediate tree
heights to short dwarf trees surrounding interior ponds and lagoons. Detailed
investigations of steady state porewater profiles of nutrients and terminal metabolic
products in dwarf mangrove soils revealed strong seasonal differences in salinity,
organic carbon and nitrogen inventories, redox conditions and reduced manganese
concentrations. Substantial rates of organic matter remineralization were coupled
primarily to sulfate reduction. Redox conditions contributed to variability in mat
nitrogen fixation and denitrification response to nutrient addition, while dissolved
organic carbon did not. Nitrogen fixation was controlled primarily by the sensitivity
of nitrogenase to oxygen inhibition, whereas denitrification was limited by nitrate
availability. 16. CDOM production by mangrove leaf litter and Sargassum colonies in Florida Keys coastal waters. We have investigated the importance of leaf litter from red mangroves (Rhizophora mangle) and living Sargassum plants as sources of chromophoric dissolved organic matter (CDOM) to the coastal ocean waters and coral reef system of the Florida Keys. The magnitude of UVB exposure to coral reefs in the Florida Keys exhibits substantial temporal variability controlled almost exclusively by the level of CDOM present in the water column. Previous research indicated that seagrasses are important local sources of CDOM, while tidally-exchanged Florida Bay water is the primary allochthonous source. Our data indicate that CDOM (using absorption coefficient at 305 nm as proxy) is released from both mangrove leaves (0.002-0.067 m-1 g-1 h-1) and Sargassum (0.004-0.012 m-1 g-1 h-1) at similar rates and that CDOM production increases sharply with temperature. Measured CDOM fluxes from both the mangrove litter and Sargassum plants were comparable to previous flux measurements for seagrasses in the Florida Keys. Release from mangrove litter depended greatly on the stage of senescence of the leaf as freshly fallen leaves exhibited the highest CDOM production while older decayed leaves released very little CDOM. Production of CDOM by Sargassum was highest in plants exposed to natural sunlight indicating that large Sargassum colonies may be important sources of CDOM to open ocean waters. The photobleaching half-life of CDOM produced from mangrove litter was approximately 24 h. We are currently investigating the photobleaching rates of CDOM released by the Sargassum colonies. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy. 15. Seasonal patterns of nitrogen fixation and denitrification in oceanic mangrove habitats. Mangrove peat soils are home to a variety of microbial communities that may play a vital role in system-level elemental cycling. We examined rates of nitrogen fixation and denitrification in benthic microbial mats on Twin Cays, Belize, a pair of oceanic mangrove islands. A tree-height gradient across the islands created distinct habitats for benthic microbes. Seawater flushing of the benthos and tree height decreased landward from tall, dense trees on the island fringe through a transition zone of high elevation and intermediate tree heights. In the center of the islands, microbial mats with dense communities of cyanobacteria and purple sulfur bacteria covered the benthic surface of shallow ponds and around dwarf trees. Wet-dry seasonality, tidal cycles and elevation controlled the extent of mat exposure to desiccation and UV radiation. Nitrogen fixation was controlled primarily by the sensitivity of nitrogenase to oxygen inhibition, whereas denitrification was limited by oxidant (nitrate) availability. Diel patterns of nitrogen fixation varied with the type of cyanobacteria dominant in each mat. Dissolved inorganic nitrogen concentration influenced both nitrogen fixation and denitrification rates. Redox conditions contributed to variability in mat nitrogen fixation and denitrification response to nutrient addition, while dissolved organic carbon did not. Microbial mat nitrogen cycling likely contributes to the nutrient (nitrogen and phosphorus) limitation patterns observed in the mangrove trees; in dwarf habitats, mats serve as a source of nitrogen via nitrogen fixation, while in fringe and transition habitats, mats compete with the trees for nitrogen via denitrification. 14. Primary production and nutrient fluxes in mangrove soils and microbial mats. We studied microbial mats from Belize and Panama mangrove habitats to determine rates of primary production and nutrient transformations. The morphology of microbial mats in Belize and Panama differed, with mats from Belize occurring as thin (< 1 cm) surface films and mats from Panama occurring as thicker (1-3 cm) mushroom-shaped discs that either floated freely or were attached to the soil surface or to prop roots. In Belize, a diverse array of cyanobacteria as well as purple sulfur bacteria and heterotrophic bacteria were important components of microbial mat communities. In Panama, the cyanobacterial community was dominated by Lyngbya spp. Sediment chlorophyll a concentrations at both sites (up to 900 mg m-2) illustrated high photosynthetic biomass. Rates of primary carbon fixation, measured as gross oxygenic photosynthesis, and nitrogen fixation and denitrification, measured using specific metabolic inhibitors, were determined in field incubations. Primary production rates were similarly high at both sites. In Belize, nitrogen fixation was higher than denitrification, while the opposite was true in Panama. Collectively, the data show that microbial mats are a significant source of fixed carbon and nitrogen in mangroves and suggest that mats may serve as an important component of the food web in tropical mangrove wetlands. 13.
Primary production and respiration rates of microbial mats in
an oceanic mangrove ecosystem. Microbial mats are dynamic contributors
to ecosystem primary production and elemental cycling. We investigated
microbial mat carbon (C) and oxygen (O) cycling in mangrove soils on
Twin Cays, a pair of tropical oceanic mangrove islands in the oligotrophic
waters offshore of Belize. Twin Cays mangroves grow along a tree-height
gradient, with the tallest trees on the ocean-fringing edge of the islands.
Tree height decreases through a transition ecotone of intermediate-height
stands towards the center of the islands, where dwarf-height trees proliferate
around treeless ponds and lagoons. The tree-height gradient provides
a natural environment in which to study the relationship between soil
light availability and benthic primary production. Filamentous and heterocystous
cyanobacteria, purple sulfur bacteria, diatoms and algae coat the soil
surface and dominate photosynthesis in microbial mats. Chlorophyll a
concentrations were highest in the dwarf, intermediate in the fringe
and lowest in the transition ecotone, but exhibited large within-site
variability. Detailed HPLC pigment analysis revealed that transition
and fringe sediments were dominated by diatoms and anoxygenic photosynthetic
bacteria, with minimal cyanobacterial presence. Dwarf sediments, however,
were dominated by cyanobacterial pigments, though diatoms and anoxygenic
photosynthetic bacteria were also present (up to 30.9 mg fucoxanthin
m-2 and 102.6 mg bacteriochlorophyll a m-2,
compared to a maximum chlorophyll a of 993 mg m-2).
12. Environmental controls on denitrification in temperate and tropical shallow sediments. Denitrification is an important sink for inorganic nitrogen (nitrate) and may serve to ameliorate nutrient loading to aquatic ecosystems. We examined spatial and temporal patterns of activity and used environmental data and laboratory experiments to elucidate the environmental controls on denitrification in temperate (Georgia, South Carolina) and tropical (Twin Cays, Belize) coastal environments. In situ rates of denitrification were higher in temperate environments, likely because these environments experienced higher N loading rates from land. Potential denitrification rates were comparable in temperate and tropical sediments, suggesting that N-deplete tropical sediments may support substantial activity when conditions are suitable. Temperature and dissolved oxygen concentration influenced denitrification rates but other factors were more important. Rates of benthic primary production influenced denitrification by altering oxygen gradients, stimulating nitrification, and competing for inorganic nitrogen. Nitrate availability frequently limited denitrification rates in all sites, while reductant (e.g. glucose) addition rarely stimulated activity. Concentrations of hydrogen sulfide exerted a strong control on total denitrification rates, the production of nitrous oxide versus dinitrogen, and on the relative rates of denitrification relative to dissimilatory nitrate reduction to ammonium. 11. Benthic microbial mats: Important sources of fixed nitrogen and carbon to the Twin Cays, Belize ecosystem. We collected surface microbial mats at sites on Twin Cays, Belize to determine rates of primary production and nitrogen transformations. A diverse array of cyanobacteria, including filamentous, coccoidal and heterocystous cyanobacteria, as well as purple sulfur bacteria and heterotrophic bacteria were important components of microbial mat communities. Sediment chlorophyll a concentrations illustrated high photosynthetic biomass in surface sediments. Rates of primary carbon fixation, measured as gross oxygenic photosynthesis, and nitrogen fixation and denitrification, measured using specific metabolic inhibitors, were determined during both day and night. Primary production rates were similarly high across different mat types. Nitrogen fixation rates were substantial under in situ conditions, with nighttime activity frequently exceeding daytime activity, while denitrification rates were negligible during the day and night. In the presence of added nitrate, denitrification rates increased during day and night time incubations such that they equaled or exceeded nitrogen fixation rates. Collectively, our data show that microbial mats are a significant source of fixed carbon and nitrogen to the Twin Cays ecosystem and suggest that mats may serve as an important component of the food web. 10. Diel and seasonal patterns of benthic fluxes of nutrients, gases and dissolved organics from temperate intertidal sediments of Georgia and South Carolina. Incubations of sediment cores were used to quantify benthic fluxes of nutrients, dissolved gases, dissolved organics, and reduced constituents in temperate intertidal sediments in Georgia and South Carolina. Sediment flux cores (n=2-4) were collected seasonally at each site, incubated at in situ temperatures for at least 72 hours, and sampled at 12 hour intervals. Light/dark incubations were performed to evaluate the impact of benthic primary production on benthic fluxes. Benthic chlorophyll concentrations and rates of gross photosynthesis illustrated that sediments were very productive. Benthic phototroph activity limited the efflux of both N and P. Oxygen concentrations in the overlying water decreased over the incubation (dark treatments) and fluxes of PO43-, Fe2+, and NOx exhibited a strong correlation to oxygen concentration. During dark incubations, there was little nutrient efflux from sediments with a microalgal cap and even during prolonged dark incubations, efflux occurs only after sediments become anoxic. N2 and N2O fluxes illustrated high denitrification rates that were correlated with photosynthesis activity and redox (oxic/anoxic) conditions. During the winter and summer of 2002, 15NO3- tracer studies were used to evaluate the fate of NO3- at these sites. NO3- availability appeared to limit benthic primary production as shown by a large uptake of Si following NO3- addition. Additional evidence suggests that the NO3- in the dark treatments was not only utilized by denitrifiers, but also by microorganisms carrying out dissimilatory nitrate reduction to ammonium. These results demonstrate the dynamic nature of materials fluxes on both large (seasonal) and small (diel) time scales and underscores the role of benthic primary producers in influencing benthic flux patterns. 9. Benthic microalgal production and nutrient dynamics in intertidal sediments. Intertidal sediments are a dynamic and important component of coastal environments that function as a reactive interface with the overlying water column at high tide and with the atmosphere at low tide. The activity of benthic microalgae, in terms of oxygen production and nutrient uptake, influences the rates and magnitude of sediment metabolism. We determined the distribution of benthic chlorophyll, measured benthic primary production and pore water oxygen microprofiles, and quantified sediment-water fluxes of nutrients and organic materials during laboratory incubations at several sites along the coast of Georgia and South Carolina. Our results demonstrate the dynamic nature of materials fluxes on seasonal and daily (diel) scales and underscore the importance of benthic phototrophs in influencing benthic nutrient exchange in intertidal sediments. Benthic photosynthesis may also alter the relative importance of various nitrogen cycling processes, such as denitrification and dissimilatory nitrate reduction to ammonium, by altering redox gradients and competing with nitrogen cycling bacteria for available substrates. 8. The importance of microbial mats in the C and N cycle of mangrove ecosystems. Twin Cays is a pair of tropical oceanic mangrove islands in the oligotrophic waters offshore of Belize. Microbial mats are a common but poorly studied component of the Twin Cays ecosystem. Microbial mats often support high rates of primary production and are sites of rapid elemental cycling. Diverse microbial mat communities, dominated by filamentous or heterocystous cyanobacteria or by purple sulfur bacteria, are present throughout Twin Cays. Diel nitrogen, oxygen, and carbon cycling was examined using acetylene reduction, acetylene block, microelectrodes, and stable isotope tracer techniques. Sediment chlorophyll and pore water nutrient concentrations illustrated high photosynthetic biomass and reduced conditions. In the presence of added nitrate, rates of denitrification exceeded rates of nitrogen fixation by orders of magnitude. At in situ nitrate concentrations, nitrogen fixation dominated the diel N cycle. Primary production rates were high (up to 45 mmol C m-2 h-1 and 35 mmol O2 m-2 h-1 gross) across different mat types. In the Twin Cays system, microbial mats account for a significant amount of primary production and nutrient cycling and may serve as an important component of the food web. 7. Thermal evidence of water exchange through a coastal aquifer: Implications for nutrient fluxes. We report the discovery of a semi-confined, high permeability zone (HPZ) 2 meters below the sea bed where exchanges between coastal aquifers and the ocean occur. A temperature probe placed in the HPZ recorded a 1oC semidiurnal cycle. The cycle was in phase with the tide, indicating tidal pumping was driving water exchange through the HPZ. We use these temperature variations to quantify water exchange between the HPZ and the ocean. Water in the HPZ is enriched in nutrients and radium; both total dissolved nitrogen and phosphorus correlate strongly with dissolved 226Ra. We use these correlations and previously quantified subterranean fluxes of 226Ra to demonstrate that this system may be a significant source of nutrients to the coastal ocean. 6. Diel and seasonal patterns of benthic fluxes of nutrients, gases, and dissolved organics from temperate intertidal sediments. Sediment flux cores were used to quantify benthic fluxes of nutrients, dissolved gases, dissolved organics, and reduced constituents in temperate saltmarsh sediments collected from sites in Georgia and South Carolina. Sediment flux cores (n=2-4) were collected seasonally at each site, incubated at in situ temperatures for 72 hours, and sampled at 12 hour intervals. Light-dark incubations were performed to evaluate the impact of benthic primary production on benthic fluxes. Benthic chlorophyll concentrations and rates of gross photosynthesis (determined using microelectrodes) illustrated that these sediments were very productive. Oxygen concentrations in the overlying water decreased over the incubation and fluxes of PO4, Fe, and NOx exhibited a strong correlation to oxygen concentration. The flux of dissolved organics exhibited a seasonal pattern with increased efflux of DOC, DON, and DOP during summer. N2 and N2O fluxes showed high rates of denitrification that were correlated with photosynthesis activity and redox (oxic/anoxic) conditions. These results demonstrate the dynamic nature of materials fluxes on both large (seasonal) and small (diel) time scales and underscore the role of benthic primary producers in influencing benthic flux patterns. 5. Ecosystem controls on nitrogen, carbon, and oxygen cycling in temperate and tropical intertidal sediments. We examined diel patterns of nitrogen and carbon fixation, denitrification, and oxygenic photosynthesis in intertidal sediments on a tropical oceanic mangrove island (Twin Cays, Belize) and in a temperate salt marsh (Sapelo Island, Georgia). Rates of these processes were determined during a series of 4-12 hour incubations over a 36-48 hour period to capture diel variations in rates of processing and in pore water oxygen concentrations. Diel integrated rates of nitrogen fixation exceeded rates of denitrification in the tropical system while the reverse held true in the temperate system. Rates of both nitrogen fixation and denitrification were influenced by pore water concentrations in dissolved inorganic nitrogen, organic carbon, and oxygen. Chlorophyll a concentrations, rates of carbon fixation and rates of oxygenic photosynthesis were higher in cyanobacterially-dominated tropical versus temperate sediments. However, chlorophyll a specific rates of oxygenic photosynthesis were similar in the two environments. Nitrogen fixation contributed a significant portion of the nitrogen required to support primary production in tropical sediments, but not in temperate sediments. The temperate and tropical systems clearly function differently in terms of C and N cycling in sediments, due to their eutrophic or oligotrophic natures. 4. Salt water intrusion into shallow coastal aquifers: Impact on sediment nitrogen and phosphorus pools and potential for altering nutrient fluxes. The flux of groundwater to coastal environments represents an important source term for nutrients and organics at the land-sea margin. Increased demand for water due to development activities in coastal regions often results in withdrawals exceeding recharge, and as a consequence, the intrusion of salt-water into previously fresh aquifers. We investigated the impact of salinity alteration on nutrient distributions in fresh water and salt marsh sediments in Georgia and South Carolina using laboratory experiments. When fresh water sediment (0 ppt) was exposed to small increases in salinity (0 to 2 ppt), significant desorption of DIN (largely ammonium) and DIP occurred. Further increases in salinity (5 - 40 ppt) led to substantial desorption of DON (but not DOP) from sediments. Sediments from high salinity (25 ppt) environments exhibited DON and DOP release when exposed to zero salinity but release of DIN and DIP was not salinity-driven. Saltwater intrusion may increase inorganic and organic nutrient fluxes from coastal sediments and could contribute to eutrophication in coastal ecosystems, particularly in highly populated areas with elevated water demands. 3. Groundwater geochemistry at the island-estuary interface at perturbed and pristine sites on a Georgia Bight barrier island. We conducted cation, anion, and nutrient analyses of groundwater and surface water samples from two sites (pristine and low salinity vs. anthropogenically perturbed and high salinity) on a sand-dominated barrier island (Sapelo Island) in the Georgia Bight. To the best of our knowledge, this is the first complete suite of groundwater and surface water geochemical analyses on samples obtained at the critical interface between the island uplands and the tidal creek-marsh-estuary complex in Georgia. Groundwater samples were obtained from shallow (4-5.5 m deep) monitoring wells and discrete multilevel sampling wells that penetrated the surficial aquifer. Additional analyses were conducted on tidal creek waters, oceanwater, and rainwater. Field (pH, DO, temperature, and conductivity) and laboratory (Na, Mg, Ca, K, Fe, Al, Cl, SO4, NO3, NO2, NH4, DON, PO4, H2S, SiO2, alkalinity) data provide important insights about the degree of lateral intrusion of saltwater into the island uplands, microbial degradation reactions, and eutrophication. At the low salinity site, adjacent to a tributary of the Duplin River, vertical conductivity logs and geochemical analyses indicate marked freshening (decreasing concentration in all major seawater components) within < 5 m of the creek bank. Samples collected from multilevel discrete sampling wells reveal the presence of more saline waters at depth. Together, these data imply not only lateral intrusion of seawater, but also vertical mixing of the freshwater and underlying saline water, a result confirmed by coincident noninvasive geophysical surveys. Analyses of groundwater collected at the high salinity, anthropogenically perturbed site adjacent to the Duplin River indicate lateral intrusion of saltwater to a distance of at least 20 m from the tidal creek. Nutrient analyses at this site show great variability extending over orders of magnitude with respect to NH4, NO3, DON, PO4 and particulate N and P, with dominant forms of N and P as NH4, DON and particulate P. 2. Seasonal variability of and nutrient controls on denitrification in freshwater and brackish water sediments. Estuarine and coastal systems receive the highest rates of anthropogenic nutrient loading in the world. Denitrification, the microbially-mediated conversion of nitrate (NO3) to gaseous end products, occurs in estuarine sediments, which may provide a nitrogen (N) sink that reduces land-derived N loading to coastal waters, particularly when groundwater is an important N source. Denitrification can sometimes result in the production and release of nitrous oxide (N2O), a potent greenhouse gas, as well as dinitrogen (N2). Rates of denitrification were measured using the acetylene inhibition technique and compared to the rates measured directly. The ratio of N2O to N2 production was also measured and compared to the variation in each site’s regional N-loading regimes, where estuarine sediments receiving higher NO3 loading supported higher rates of denitrification. 1. Release
of N2 and N2O from salt marsh sediments subject
to different land-derived N loads.
Coastal systems receive the highest loading of anthropogenic nutrients
in the world. Denitrification (DNF) occurs in sediments of salt marshes
fringing estuaries, and provides a nitrogen sink that reduces land-derived
N loading to estuarine waters, but can also release N2O,
a trace greenhouse gas that can consume stratospheric ozone. We used
the acetylene inhibition technique to measure rates of DNF in three
estuaries in the Waquoit Bay watershed which receive variable N loads
and hence provide the opportunity to see if DNF varies in salt marshes
exposed to different regional-scale regimes of land-derived N inputs.
The Michaelis-Menten saturation rates (Vmax) of both total potential
DNF and N2O production increased as the land-derived N load
increased. The half saturation constant (Ks) from total potential DNF
also increased with loading, whereas the Ks of N2O production
did not change. The N2O:N2 ratios at in situ NO3
concentrations of our most highly loaded estuary were consistent with
literature values. Our ratios for the lesser loaded estuaries, indicated
greater release of N2O relative to N2 at lower
N loads. As loading increases, any local increases in NO3
concentrations could release more N2O into the atmosphere,
while supply of NO3 determined by regional loading regimes
will lower N2O produced by denitrification compared to N2
release. |
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