Dissolved Inorganic Nitrogen Fluxes Research Articles

Dynamics of main nutrient input to Xiangxi Bay of the Three-Gorges Reservoir

Based on routine monitoring data in Xiangxi River and its main tributary Gaolan River from September 2000 to June 2005, this paper estimates the contribution of riverine nutrients, and analyzes the monthly dynamics of concentrations and fluxes of nutrients. The results show that Xiangxi Bay annually receives 1623.49 tons of total nitrogen (TN) and 331.85 tons of total phosphorus; Xiangxi River alone accounts for 68.50% of the total nitrogen fluxes and 91.74% of the total phosphorus fluxes. In these two rivers, dissolved inorganic nitrogen (DIN) is the dominating form of nitrogen; fluxes of DIN and TN are high during summer (July), mid-spring and autumn, and relatively low in winter; non-point source pollutants that flow into rivers are the most important pathway of nitrogen. Orthophosphate is the dominating form of phosphorus in Xiangxi River, relatively low in Gaolan River; fluxes of phosphorus are high during summer and late spring, relatively low during winter and late autumn in Gaolan River, but fluctuate irregularly in Xiangxi River; phosphorus in Gaolan River is mainly caused by non-point source pollutants, while point source pollutants of phosphorus play an important role in Xiangxi River. Soil erosion probably represents the major way of non-point source pollutants, while the drainages of phosphorus diggings and factory discharges play the most important role in the point source pollutants of phosphorus. This research suggests that measures must be taken to control the point source pollutants of phosphorus in Xiangxi River in order to protect Xiangxi Bay of the Three-Gorges Reservoir.

A retrospective analysis of nutrients and phytoplankton productivity in the Mississippi River plume

Long-term patterns in riverine nutrient flux in the lower Mississippi River were examined in relationship to spatial and temporal patterns in surface nutrient concentrations, chlorophyll, and primary productivity in the outflow region in the northern Gulf of Mexico. A retrospective analysis of dissolved inorganic nutrient fluxes based on USGS water quality data and US Army Corps of Engineers discharge data from the 1950s to mid-2004 showed an increase in river-borne dissolved inorganic nitrogen (DIN) flux after 1967. Flux of DIN peaked in the early 1980s and has since fluctuated and shown a general decreasing trend since the early 1990s. Records for total phosphorus (total P) fluxes beginning in mid-1974 exhibited a variable but slight increasing trend up to 2004. The increase in fluxes during the 1970s and into the 1980s can be attributed to increases in both nutrient concentrations and river discharge. DIN concentrations since the 1980s have shown a decreasing trend. Total P concentrations exhibited large fluctuations, with no consistent long-term trend. Dissolved organic nitrogen (DON) concentrations and orthophosphate (Ortho P) peaked in the 1980s, declined relative to DIN and remained relatively low. DIN:Ortho P ratios were consistently well above the Redfield N:P ratio of 16:1. DIN:Total P ratios were variable and lower, fluctuating around the Redfield 16:1 value. Both DIN:Ortho P and DIN:Total P ratios were weakly, but significantly, correlated with river discharge and fluctuations were largely a reflection of higher DIN concentrations during high-discharge events. DIN:Ortho P ratios in surface waters of the outflow region adjacent to the birdfoot delta were higher in spring, consistent with seasonal variation in riverine DIN:Ortho P ratios. The seasonal signal diminished with increasing distance to the west of the delta, indicating a selective removal of DIN or source of Ortho P along the shelf. DIN fluxes and SeaWiFS satellite-derived chlorophyll showed seasonally elevated values during the first half of the year followed by generally lower values in late summer and fall. This seasonal signal diminished from east to west. The observed relationship between DIN flux and chlorophyll was consistent with ship-based observations of a linkage between riverine nutrient inputs and productivity. Long-term trends in river discharge were correlated with the Multivariate ENSO (El Niño Southern Oscillation) Index (MEI) ( r=−0.281, p<0.0001), evidence that river discharge was influenced by global climatic trends.

Groundwater Discharge as an Important Land-Sea Pathway into Manila Bay, Philippines

A multidisciplinary approach was taken to assess the potential importance of groundwater seepage to nutrient inputs into Manila Bay, Philippines. Three lines of seepage meters were installed in transects along the coast at Mariveles, Bataan Province, during the period 8–10 January 2005. The overall average seepage flux was 5.1 ± 5.4 cm d−1 (n = 73) with a range of 0–26 cm d−1 and a calculated integrated shoreline flux of 12.4 m3 m−1 d−1. Additional methodologies employed included automatic seepage meters, resistivity measurements, sampling for nutrient analyses in both seepage meters and ambient seawater, and use of natural radon as a groundwater tracer. Seepage meter and tracer results provided consistent results of estimates of submarine groundwater discharge into Manila Bay. Many lines of evidence suggest that seepage fluxes are not steady state but are modulated by the tides. Resistivity profiles show that the saline-freshwater interface moves on a tidal timescale, consistent with the observed drop in salinity of the seepage waters as low tide approaches. Our results show that dissolved inorganic nitrogen (DIN) fluxes via submarine groundwater discharge are comparable in magnitude to DIN fluxes from each of the two major rivers that drain into Manila Bay.

Hydrologic and biotic control of nitrogen export during snowmelt: A combined conservative and reactive tracer approach

Dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON) stored in the snowpack are important sources of N in snow‐covered ecosystems, yet we have limited knowledge of their fate during the melt period. Our objective was to quantify the role of hydrologic and biogeochemical processes in regulating stream fluxes of DIN (NO3− + NH4+) and DON in a forest‐dominated and a wetland‐dominated catchment during the snowmelt period. We combined isotopic hydrograph separation with concurrent measurements of meltwater DIN and DON to calculate “conservative” N export (hydrologic mixing only) and compared it with “reactive” N export (i.e., observed fluxes that include biogeochemical processes). On balance, N was retained in the catchments during snowmelt because of storage of meltwater N in soils, but our N export comparison revealed N generation (mostly as DON) from the mobilization of dissolved organic matter. In contrast, NO3−, which was highly enriched in snowpack meltwater, remained below detection in streams, and both catchments were sinks for NO3−, suggesting that denitrification and/or uptake may be important at the catchment scale. Over the melt period, the forest catchment was a greater total N source because of the convergence of lateral flow and near‐stream riparian N sources in surface soils, which elevated stream DON and to a lesser extent NH4+. In contrast, preferential flow in the wetland catchment tended to dilute DIN in saturated peatland soils and in the stream, whereas DON varied little over time. These findings highlight the importance of hydrologic processes that store meltwater N in catchment soils but at the same time deliver DON from riparian sources to the stream. Further, model results suggest that biotic uptake and/or sorption effectively retain much of the meltwater DIN from the snowpack. Collectively, hydrologic storage and biogeochemical processes act to retain N that is likely important for boreal ecosystem production later in the spring and summer seasons.

Modeling nitrogen cycling in a coastal fresh water sediment

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (Norg) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the Norg deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 μmol N cm−2 yr−1. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO −3 reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 μmol N cm−2 yr−1 if DNRA is the dominant pathway. The response to salinization includes a short-term release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO −3 concentrations and greater SO 2−4 concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition.

Inorganic carbon and nutrient fluxes on the Arctic Shelf

Historic data from the Russian-American Hydrochemical Atlas of Arctic Ocean together with data from the TRANSDRIFT II 1994 and TUNDRA 1994 cruises have been used to assess the spatial and inter-annual variability of carbon and nutrient fluxes, as well as air–sea CO 2 exchange in the Laptev and western East Siberian Seas during the summer season. Budget computations using summer data of dissolved inorganic phosphate (DIP), dissolved inorganic nitrogen (DIN) and dissolved inorganic carbon (DIC) gives that the Laptev Sea shelf is a net sink of DIP and DIN of 2.5×10 6, 23.2×10 6 mol d −1, respectively, while it is a net source of DIC (excluding air–sea exchange) of 1249×10 6 mol d −1. In the East Siberian Seas the budget computations give 0.5×10 6, −11.4×10 6 and −173×10 6 mol d −1 (minus being a sink) for DIP, DIN, and DIC, respectively. In summers, the Laptev Sea Shelf is net autotrophic while the East-Siberian Sea Shelf is net heterotrophic, and both systems are weak net denitrifying. The Laptev Sea Shelf takes up 2.1 mmol CO 2 m −2 d −1 from atmosphere, whereas the western part of the East-Siberian Sea Shelf loose 0.3 mmol CO 2 m −2 d −1 to the atmosphere. The variability of DIP, DIN and DIC fluxes during summer in the different regions of the Laptev and East Siberian Seas depends on bottom topography, river runoff, exchange with surrounding seas and wind field.

Hydrologic forcing of submarine groundwater discharge: Insight from a seasonal study of radium isotopes in a groundwater‐dominated salt marsh estuary

A seasonal study of radium‐derived submarine groundwater discharge (SGD) and associated nitrogen fluxes was carried out in a salt marsh estuary between 2001 and 2003 (Pamet River Estuary, Massachusetts). Twelvehour time series of salinity and radium at the estuary inlet were used to determine the relative importance of fresh versus saline SGD, respectively. The distinct radium (228Ra : 226Ra) isotopic signature of marsh peat pore water and aquifer‐derived brackish groundwater was used to further partition the Ra‐derived SGD estimate. Of these three groundwater sources, only the marsh‐derived groundwater was constant across time. The ratio of brackish to fresh SGD was inversely correlated with water table elevation in the aquifer, suggesting that Ra‐derived SGD was enhanced during dry periods. The various SGD fluxes were responsible for an average annual dissolved inorganic nitrogen (DIN) input of between 1.7 mol m−2 yr−1 and 7.1 mol m−2 yr−1 and a soluble reactive phosphate (SRP) flux of 0.13‐0.54 mol m−2 yr−1. Approximately 30% of the SGD‐derived DIN and SRP flux is exported to coastal waters (Cape Cod Bay), whereas 70% is retained by the salt marsh ecosystem.

Influences of natural and anthropogenic processes on the nitrogen and phosphorus fluxes of the Yangtze Estuary, China

Nutrient flux to the sea through the estuary is important to the health of the sea. Combining natural processes with anthropogenic activities, we discuss the influence on the nitrogen and phosphorus fluxes to the Yangtze River basin, to the estuary and to the sea. The fluxes of dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) to the estuary through the river/estuary interface are obviously higher than those to the sea through to the estuary/sea interface of the Yangtze estuary. The changes in nutrient fluxes through different interfaces are largely due to the estuarine hydrological and biogeochemical processes. Household, livestock and agricultural runoff are major sources of nitrogen from human activities, and household and livestock contribute to an increase in the anthropogenic phosphorus. The fluxes of DIN and DIP from economic activities account for about one-third of DIN and DIP fluxes to the sea through the Yangtze estuary.

Land-ocean fluxes in the Paranaguá Bay estuarine system, southern Brazil

A worldwide modeling effort has been proposed by the LOICZ (Land-Ocean Interactions in the Coastal Zone) Program to foster the acquisition of intercomparable data on land-ocean fluxes in estuaries and continental shelf ecosystems from all continental margins. As part of the South American component of this initiative, we present flux estimates of water, salt, dissolved inorganic phosphorus (DIP), dissolved inorganic nitrogen (DIN) and plankton for the estuarine system of Paranaguá Bay, southern Brazil, based on the LOICZ modeling approach and local data obtained during the 1990's. This system is strongly influenced by a seasonal meteorological cycle, represented by the rainy/summer and dry/winter periods. Semi-diurnal tides of up to the 2.7-m range are responsible for the short time-scale dynamics. The model indicated a potential water export to the adjacent coast of up to 7 x 10(6) m³ d-1 in the dry season, and 28 x 10(6) m³ d-1 during the rainy season. The system exhibits seasonal and spatial variations in DIP and DIN fluxes. "DIP amounted to +2.3 x 10(6) mol P yr-1 and "DIN to -2.7 x 10(6) mol N yr-1, suggesting that net production of phosphate and consumption of inorganic nitrogen predominate throughout in the system. Fluxes and therefore export of DIN and eespecially of DIP are higher in the rainy season. Stoichiometric estimates based on the C:N:P ratios of the reacting particulate organic matter (mangrove and plankton detritus) suggest that net denitrification predominates all over the bay, with values between -24.3 and -10.6 x 10(6) mol N year-1. Estimated seaward outflows had little effect upon the fate of the phyto- and zooplankton biomass in different sectors of the bay. This is exemplified by the low net export of algal production from the upper to the middle sectors of the estuary.

Evaluation of benthic nutrient fluxes and their importance in the pelagic nutrient cycles in Suo Nada, Japan

The benthic nutrient fluxes in Suo Nada, in the western part of Seto Inland Sea, Japan were estimated seasonally in 2002, and their contribution to the pelagic nutrient cycle was evaluated. The horizontal distributions of inorganic phosphorus (DIP), ammonia and dissolved silica (DSi) concentrations in the pore water of the surface (0–3 cm) sediments were high in the western part, indicating a eutrophic condition relative to the other parts. Seasonally, the highest nutrient concentration in the pore water was observed in summer, as reflected by the increasing decomposition rate of organic matter. Multiple regression analyses revealed significant positive effects of acid volatile sulfide (AVS) and temperature on the DIP flux and also significant positive effects of ignition loss (IL) along with these two parameters on the dissolved inorganic nitrogen (DIN) flux. Temperature, IL and redox potential (Eh) were effective on DSi. The estimated DIP, DIN and DSi fluxes from the sediments were relatively high or equivalent to the total inputs from rains and rivers in annual average, with high values in summer of 2–3-fold higher than the other sources (rain plus rivers). Thus, it is concluded that the benthic nutrient fluxes play an important role in the pelagic nutrient cycle of the Suo Nada ecosystem.

Modelling agricultural nitrogen contributions to the Jiulong River estuary and coastal water

The geographical setting of the Jiulong River estuary determines that the estuary receives wastes from both riverine input and adjacent urban sewage. However, the dominant nitrogen (N) source remains unclear. A nutrient mass-balance model and a preliminary LOICZ (Land–Ocean Interactions in the Coastal Zone) biogeochemical model were linked to evaluate agricultural N contributions from the Jiulong River catchment to the estuary and coastal water. Results showed that agricultural N surplus was the largest N source in the catchment, contributing 60.87% of the total Nitrogen (N) and 68.63% of the dissolved inorganic nitrogen (DIN). Household wastes and other sources followed. Riverine DIN fluxes were about 20.3% of exportable DIN and 14.4% of exportable total N, but approximately 9.7% of DIN inputs, and 7.3% of total N inputs to the Jiulong River catchment. The model system clearly showed that agricultural and anthropogenic activities in the catchment were the major N sources of the estuary and coastal water, and riverine N fluxes from these sources substantially impacted the estuary and coastal water quality and biogeochemical processes.

Septic Tank Impacts on Ground Water Quality and Nearshore Sediment Nutrient Flux

AbstractField studies were conducted at three coastal plain sites to characterize inorganic nutrient and fecal coliform bacteria ground water quality and intertidal sediment nutrient fluxes under the influence of residential septic tank effluent. Mean drainfield DIP (dissolved inorganic phosphorus) and DIN (dissolved inorganic nitrogen) concentrations varied between 294 to 336 and 4494 to 5391 μmol/L, respectively, with mean fecal coliform bacteria densities ranging from 105.04 to 106.29 MPN (most probable number) /100 mL. DIP and fecal coliform bacteria exhibited a high degree of attenuation with shoreline concentrations at or near background levels. In contrast, septic tank nitrogen loadings to shallow ground water were significant and resulted in mean shoreline DIN concentrations ∼50 to 100 times greater than adjacent surface water concentrations. Mean site sediment DIP and DIN flux to surface waters varied from 1.1 to 1.6 and 52 to 135 μmol/m2/hr, respectively. Whereas DIP sediment fluxes were similar to reported values for sites adjacent to forested and agricultural lands, DIN fluxes were elevated compared to forested lands and near the lower end of the range reported for agricultural lands within the southern Chesapeake Bay region. Maximum measured sediment DIN flux was 1514 μmol/m2/hr. Estimated waste water nitrogen loading to shallow ground water (5.7 to 10.7 kg/household/yr) was significant and with 0.5 to 1 acre lot sizes, comparable to water table nitrogen loadings from dominant row‐crop land use in the mid‐Atlantic Coastal Plain. Given the trends in population growth in the Chesapeake Bay watershed and other coastal regions, nitrogen loads from septic tanks to these systems should also be expected to increase.

Fluid expulsion from the Dvurechenskii mud volcano (Black Sea)Part I. Fluid sources and relevance to Li, B, Sr, I and dissolved inorganic nitrogen cycles

Significant sediment–ocean chemical fluxes are produced by the expulsion of sedimentary fluids at continental margins. Although such fluxes could play a role in global geochemical cycles, few quantitative estimates of their global, or even regional, significance exist. We carried out a pore water geochemical study of fluids expelled from the Dvurechenskii mud volcano (DMV) in the Black Sea, with the aim of understanding the role played by mud volcanoes in Black Sea geochemical cycles. The DMV is presently expelling highly saline fluids particularly enriched in geochemically important species such as Li + (1.5 mM), B (2.17 mM), Ba 2+ (0.57 mM), Sr 2+ (0.79 mM), I (0.4 mM) and dissolved inorganic nitrogen (DIN) (22 mM). A combination of geochemical indicators shows that this geochemical signature was acquired via organic matter and silicate alteration processes in the subsurface down to 3-km depth and near-surface gas hydrate formation. We used a simple transport model to estimate the benthic fluxes of these solutes at the DMV. Our results show that the DMV is expelling fluids at a rather low seepage rate (8–25 cm year −1 ) resulting in a total water flux of 9.4×10 −5 km 3 year −1 . This gentle regime of fluid expulsion results in Li + , B, Sr 2+ , I and DIN fluxes between 3.8×10 4 and 2.1×10 6 mol year −1 . Surface biogeochemical processes affect the benthic fluxes of Ba 2+ such that the deep Ba 2+ flux is completely consumed through the precipitation of authigenic barite (BaSO 4 ) in surface sediments. The Black Sea I cycle is likely to be affected by mud volcanism, if the 50 known Black Sea mud volcanoes share the rather sluggish activity of the DMV. Mud volcano fluxes of Li, B, Sr and DIN, instead, are too small to affect Black Sea geochemical cycles. On a global scale, mud volcanism could play a role in the marine cycles of Li, B, Sr, I and DIN if current estimates of mud volcano abundance are correct.

Fluid expulsion from the Dvurechenskii mud volcano (Black Sea): Part I. Fluid sources and relevance to Li, B, Sr, I and dissolved inorganic nitrogen cycles

Significant sediment–ocean chemical fluxes are produced by the expulsion of sedimentary fluids at continental margins. Although such fluxes could play a role in global geochemical cycles, few quantitative estimates of their global, or even regional, significance exist. We carried out a pore water geochemical study of fluids expelled from the Dvurechenskii mud volcano (DMV) in the Black Sea, with the aim of understanding the role played by mud volcanoes in Black Sea geochemical cycles. The DMV is presently expelling highly saline fluids particularly enriched in geochemically important species such as Li + (1.5 mM), B (2.17 mM), Ba 2+ (0.57 mM), Sr 2+ (0.79 mM), I (0.4 mM) and dissolved inorganic nitrogen (DIN) (22 mM). A combination of geochemical indicators shows that this geochemical signature was acquired via organic matter and silicate alteration processes in the subsurface down to 3-km depth and near-surface gas hydrate formation. We used a simple transport model to estimate the benthic fluxes of these solutes at the DMV. Our results show that the DMV is expelling fluids at a rather low seepage rate (8–25 cm year −1) resulting in a total water flux of 9.4×10 −5 km 3 year −1. This gentle regime of fluid expulsion results in Li +, B, Sr 2+, I and DIN fluxes between 3.8×10 4 and 2.1×10 6 mol year −1. Surface biogeochemical processes affect the benthic fluxes of Ba 2+ such that the deep Ba 2+ flux is completely consumed through the precipitation of authigenic barite (BaSO 4) in surface sediments. The Black Sea I cycle is likely to be affected by mud volcanism, if the 50 known Black Sea mud volcanoes share the rather sluggish activity of the DMV. Mud volcano fluxes of Li, B, Sr and DIN, instead, are too small to affect Black Sea geochemical cycles. On a global scale, mud volcanism could play a role in the marine cycles of Li, B, Sr, I and DIN if current estimates of mud volcano abundance are correct.

Temporal changes and spatial variation of soil oxygen consumption, nitrification and denitrification rates in a tidal salt marsh of the Lagoon of Venice, Italy

The aim of the present study was to investigate seasonal and spatial patterns of soil oxygen consumption, nitrification, denitrification and fluxes of dissolved inorganic nitrogen (DIN) in a tidal salt marsh of the Lagoon of Venice, Italy. In the salt marsh, intact soil cores including overlying water were collected monthly at high tide from April to October in salt marsh creeks and in areas covered by the dominant vegetation, Limonium serotinum. In May, cores were also collected in areas with vegetation dominated by Juncus maritimus and Halimione portulacoides. In laboratory incubations at in situ temperature in the dark, flux rates of oxygen and DIN were monitored in the overlying water of the intact cores. 15N-nitrate was added to the overlying water and nitrification and denitrification were measured using isotope-dilution and -pairing techniques. The results show that highest soil oxygen consumption coincided with the highest water temperature in June and July. The highest denitrification rates were recorded in spring and autumn coinciding with the highest nitrate concentrations. Soil oxygen consumption and nitrification rates differed between sampling sites, but denitrification rates were similar among the different vegetation types. The highest rates were recorded in areas covered with L. serotinum. Burrowing soil macrofauna enhanced oxygen consumption, nitrification and denitrification in April and May. The data presented in this study indicate high temporal as well as spatial variations in the flux of oxygen and DIN, and nitrogen transformations in the tidal salt marshes of the Venice lagoon during the growth season. The results identify the salt marshes of the Venice lagoon as being metabolically very active ecosystems with a high capacity to process nitrogen.

Humans, Hydrology, and the Distribution of Inorganic Nutrient Loading to the Ocean

Most modern estimates of dissolved nitrogen and phosphorus delivery to the ocean use Meybeck's estimates from approximately 30 large rivers. We have derived an extended database of approximately 165 sites with nutrient loads. For both dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP), the logarithmic yields (log [load/area]) can be parameterized as functions of log (population density) and log (runoff/area) (R2 for DIN and DIP approximately 0.6). Landscape production of DIN and DIP is largely assimilated. Even though DIN and DIP follow substantially different biogeochemical cycles, loading for DIN and DIP is tightly coupled (R2 for log DIN versus log DIP approximately 0.8), with a constant loading ratio of about 18:1. Estimates of DIN and DIP fluxes are distributed globally around the world coastlines by using basin population density and runoff at 0.5° increments of latitude and longitude. We estimate that total loads for the 1990s are about three times Meybeck's estimates for the 1970s.

Benthic Fluxes of Dissolved Inorganic Nitrogen in a Coastal Lagoon of the Northern Adriatic Sea: an Interpretation of Spatial Variability Based on Sediment Features and Infauna Activity

Abstract. Dark respiration rates, dissolved inorganic nitrogen (DIN) fluxes and nitrification rates were measured at two sites in the microtidal Sacca di Goro lagoon in September 2000. DIN fluxes correlated with the biomass of the dominant macrofauna species (the amphipod Corophium spp. at station Giralda and the polychaete Neanthes spp. at station Faro). Respiration (&gt; 6 mmol O2 m−2h−1) and ammonium fluxes (&gt; 80μmol N m−2h−1) were higher at station Giralda despite the lower organic matter content (4.5 %) and lower macrofauna biomass (4 g AFDW m−2). At both sites ammonium fluxes were significantly correlated with the biomass of the benthic infauna, but Corophium stimulated ammonium NH4+ fluxes 3‐fold compared to Neanthes. The amphipod also enhanced nitrification rates (&gt; 300 μmol N m−2h−1) due to the high density of its burrows, the higher NH4+ regeneration rates and the enhanced oxygen supply to the bacteria.

Factors affecting microphytobenthic biomass, species composition and production in the Colne Estuary (UK)

Biomass, species composition and primary production of the microphytobenthos (MPB) were investigated along environmental gradients in the Colne Estuary between August 1996 and March 1998. Mean monthly sediment chlorophyll a (chl a) concentrations varied between 6 and 241 mg m -2 . There was no seasonal pattern in chl a distribution. Low chl a concentrations were asso- ciated with the estuary mouth, which was characterised by exposed sandy sediments and low water column nutrient concentrations. Position on the shore affected biomass, with greater chl a concentra- tions on the high shore. Chl a concentrations followed the same pattern at both high and low shore positions. There was a significant relationship between concentrations of sediment colloidal carbo- hydrate and chl a. Epipelic diatoms dominated the MPB, with cluster analysis identifying 10 distinct species assemblages. Navicula spp. were an important component of all assemblages. Peaks in chl a biomass occurred when a single species made up >65% of the cell numbers. The distribution of the diatom assemblages was related to salinity, temperature and dissolved inorganic nitrogen (DIN) con- centrations. Net production of oxygen occurred in sediments under illumination at all sites on almost all occasions. Oxygen uptake by sediment in the dark was positively correlated with temperature at the head of the estuary, but not at any other site. Mean annual chl a concentrations correlated posi- tively with total annual dark oxygen uptake. Annual primary production was estimated by 4 models using chl a concentrations and a model based on gross O2 fluxes across the sediment-water interface. Production was estimated to be in the range of 25 to 1199 gC m -2 yr -1 , depending on site and model. There was greater production on the high shore than on the low shore, due to higher chl a concen- trations and longer emersion periods. Total production of MPB in the estuary was estimated to be between 117 and 852 tons C yr -1 . Assimilation by the MPB provided a temporary sink for DIN, equiv- alent to 12.8% of the annual 1996 load to the Colne Estuary. N assimilation rates by the MPB were in the same order of magnitude as denitrification, indicating that assimilation by the MPB is an impor- tant process affecting the flux of DIN through this mesotidal estuary.

Utility of radium isotopes for evaluating the input and transport of groundwater‐derived nitrogen to a Cape Cod estuary

Because of rapid increases in population, anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape Cod. A major source of “new” nitrogen to these estuaries is groundwater, which intercepts septic tank fields in its flow path to the coastline. Many attempts have been made to quantify this process; however, groundwater discharge is often patchy in nature and is therefore difficult to study by use of traditional techniques such as seepage meters. In Waquoit Bay, MA, we tested an approach based on radium, which is naturally enriched in aquifer fluids and has four isotopes with half‐lives ranging from 4 d to 1600 yr. Groundwater entering the bay was low in salinity and contained several orders of magnitude greater radium and dissolved inorganic nitrogen (DIN) relative to ambient bay water. Using a mass‐balance approach for radium, we calculated a submarine groundwater flux of ~37,000 m3 d−1, which compared well with aquifer recharge rates calculated from rainfall. From the DIN content of the groundwater, we estimated that ~2100 mol N d−1 was directly input to the estuary. However, this nitrogen flux was small in comparison with literature values for DIN fluxes from the heavily populated subestuaries. Furthermore, our results suggest that groundwater flux of DIN was assimilated by plant biomass during the summer but may be exported from the embayment to coastal waters during the winter months.

Benthic metabolism and nutrient regeneration on the continental shelf of Eastern Massachusetts, USA

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 224:1-19 (2001) - doi:10.3354/meps224001 Benthic metabolism and nutrient regeneration on the continental shelf of Eastern Massachusetts, USA Charles S. Hopkinson Jr.*, Anne E. Giblin, Jane Tucker The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA *E-mail: chopkins@mbl.edu ABSTRACT: Benthic metabolism and nutrient cycling were examined in depositional sediments of Broad Sound, Massachusetts Bay, Stellwagen Basin and Cape Cod Bay between 1990 and 1994 in water 16 to 76 m deep. Bottom water nitrate and dissolved oxygen concentrations were typically <10 µM and >70% of saturation. Sediment organic content was uniformly low at all sites ranging from 1.3 to 2.1% carbon and 0.1 to 0.3% nitrogen. Sediment chlorophyll a and phaeopigment concentrations averaged ca 2 and 15 ug cm-3 respectively. Porewater nutrients and sediment-to-water fluxes of O2, dissolved inorganic carbon (DIC), NH4+, NO2- + NO3-, urea, PO43-, N2O and dissolved silicon (DSi), were measured in cores from 4 stations, 4 to 6 times a year, for 2 yr, in order to examine seasonal and annual patterns. An additional 8 sites were examined over several years in late summer in order to examine spatial patterns. Average benthic community respiration ranged from 10.6 to 14.3 mmol O2 m-2 d-1or 12.1 to 29.8 mmol C m-2 d-1 . Within regions, respiration was highest in Broad Sound, followed by Massachusetts Bay, Cape Cod Bay and Stellwagen Basin. Sediments were sources of inorganic N. Annual average dissolved inorganic nitrogen (DIN) fluxes ranged from 0.5 to 1.8 mmol N m-2 d-1. For all stations, the flux of DIN was dominated by NH4+. The importance of NO3- fluxes relative to NH4+ increased with increasing depth. For example, in late summer the relative importance of NO3- fluxes increased from 0 to 44% of total DIN flux between Broad Sound (<24 m) and Stellwagen Basin (75 m). Fluxes of N2O and urea were always extremely small (<0.1% of DIN). Seasonal cycles, which in general tracked bottom-water temperature and phytoplankton sedimentation, were very strong for respiration (O2 and DIC) but not for fluxes of NO3-, urea, N2O, and PO43-. Annual variation in benthic respiration was ca 25%, similar to that for primary production. Inorganic nutrient fluxes often varied in excess of 100% between years. Sediment mixing or porewater advection is thought to contribute to high nutrient flux variability. C, N, and P flux stoichiometry deviated substantially from the Redfield ratios, suggesting strong phosphorus retention by sediments and large nitrogen removal via denitrification. DSi fluxes were relatively high, which we attribute to focusing of biogenic material in these depositional sediments. Denitrification exceeded total DIN flux, averaging 1.9 and 1.3 mmol N m-2 d-1 in Massachusetts Bay and Stellwagen Basin respectively. The percentage of remineralized N that was denitrified increased with increasing water depth (from 60 to 72% for 35 and 75 m water). Denitrification was supported almost exclusively by coupled nitrification and denitrification. The benthic community decomposed 12 to 22% of overlying water primary production but only provided 3 to 8% of the phytoplankton inorganic N requirement. KEY WORDS: Benthic metabolism · Benthic nutrient regeneration · Porewater nutrients · Denitrification · Cape Cod Bay · Gulf of Maine · Massachusetts Bay Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 224. Online publication date: December 18, 2001 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2001 Inter-Research.