Sediment-water Exchange Rates Research Articles

Denitrification in the mainstem Neuse River and tributaries, USA

We conducted quarterly (spring, summer, fall and winter) determinations of denitrification at representative sites in the mainstem Neuse River and tributaries, USA. Denitrification measurements were made on intact sediment cores using the N 2 /Ar and isotope pairing techniques. We measured nitrate (NO 3 - -N) and ammonium (NH 4 + -N) concentrations in the water column at the time of core collection and evaluated rates of sediment-water exchange of dissolved oxygen (O 2 ) in conjunction with denitrification rate determinations. Nitrate concentrations in the tributary streams and river water varied over an order or magnitude, from about 10 to 110 λmol/L, while NH 4 + -N concentrations were lower, varying from 0.6 to 10.8 μmol/L. Sediments consistently consumed dissolved O 2 , with rates varying from 256 to 1418 pmol m -2 h -1 . Most cores actively denitrified with rates ranging to 222 μmol N 2 -N m -2 h -1 . On average, coupled nitrification-denitrification accounted for 66 % of total denitrification. Lowest rates of denitrification were generally observed during December, when the water temperature was lowest at 4 °C. No other relationship was identified between rates of denitrification and measured physicochemical variables within or among sites or dates as sampling frequency and replication were low. Our study suggests that denitrification represents a sink of about 5 % for the NO 3 - -N load to the mainstem Neuse River and tributary streams.

The role of adsorption in sediment-water exchange of phosphate in North Sea continental margin sediments

The effect of adsorption on the sediment-water exchange of PO, was investj gated in sediments from four different types of sedimentary environments in the southern and eastern North Sea in August 1991 and February 1992. Nonlinear adsorption isotherms for oxidized sediment from eight stations indicate that North Sea sediments differ widely in their capacity to adsorb PO,. A good correlation between the value of the adsorption coefficient and NH,oxalate-extractable Fe was observed. A combination of the adsorption data with Porewater PO, profiles, solid phase results, and measured and calculated rates of sediment-water exchange of PO, for 15 stations in both August and February indicates that adsorption plays an important role in controlling sediment-water exchange of PO, during at least a part of the year in three of the four North Sea environments. At most stations, PO, adsorption constrains the flux of PO, to the overlying water. At one station in the depositional environment of the Skagcrrak, however, desorption is responsible for the maintenance of a flux of PO, to the overlying water. A one-dimensional reactiondiffusion model, describing Porewater PO, and solid phase P profiles, was de*deloped and applied to results for two stations. The model results show that both enhanced retention and enhanced release of PO, can be adequately described when simultaneous equilibrium and first-order kinetic reversible adsorptive reactions are assumed.

Composition and Behaviour of Trace Metals in Post-oxic Sediments of the Gulf of Papua, Papua New Guinea

Trace metal composition and behaviour were examined in post-oxic muds of the Gulf of Papua in January 1993. Porewater profiles and flux measurements indicate significant Fe and Mn reduction in these coastal muds. Vertical profiles, rates of sediment-water exchange, and correlation analyses imply that these silt-dominated muds mobilize Al and are a sink for Mo. Undetectable or very low porewater concentrations and flux rates of other metals (Ni, Cu, Cr, Co, Pb, Cd) indicate immobilization into solid-phase minerals. Solid-phase metal concentrations are consistent with those in average shale, suggesting pristine conditions in the gulf. Low (<1–2 μM) free sulphides, low clay content, and bioturbation are partly responsible for trace metal composition and concentrations, but several lines of evidence imply non-steady-state diagenesis in these muds: (1) the lack of a clear zonational sequence of solutes with sediment depth; (2) lack of agreement between vertical profiles of dissolved and solid-phases of most metals; (3) significant, but weak, correlations between dissolved metals and dissolved organic carbon, and between solid-phase metals and total organic carbon; and (4) lack of correlation between conservative elements (e.g. dissolved Mo and Cl −; dissolved Al and SiOH). Intense, massive physical reworking is cited as the major factor fostering apparent non-steady-state behaviour of trace metals in the surface muds of the Gulf of Papua. This scenario is very similar to that occurring on the Amazon shelf, and may be typical of other wet tropical regions with high-energy coastlines receiving large amounts of highly weathered, river-borne material.

Nutrient early diagenesis in the sandy sediments of the dogger bank area, north sea: Pore water results

Nutrient pore water profiles from the upper 100 mm of the sandy and organic-poor sediments of 12 stations in the Dogger Bank area (North Sea) were obtained during July – August 1988. O 2 penetration, as measured with microelectrodes, varied between 4 and 15 mm. Silicate, ammonium, nitrate, and phosphate concentrations were low compared with values reported in the literature for other marine interstitial waters. Nitrate showed clear nitrification peaks in the upper 10 mm of the sediment, while the other constituents reached their maximum levels between 20 and 40 mm. In some cores the latter concentrations declined again below these depths. Averaged over all stations, benthic respiration, measured in whole-core incubations, was 12 mmol(O 2)·m −2·day −1, while measured nutrient sediment-water exchange rates were 333 (Si), 167 (NH 4 +), 161 (NO 3 −) and 32 (P) μmol·m −2·day −1. Compared with the daily phytoplankton nutrient requirements, these fluxes amount to less than 10% for Si, ∼2% for N, and ∼ 3% for P. This suggests that nutrient regeneration largely occurs in the water-column. The pore-water profiles were evaluated quantitatively, using one-dimensional steady-state diagenetical models in which the apparent diffusion coefficient is calibrated against experimentally obtained fluxes. Results indicate that apparent diffusion is 1 to 6 times molecular diffusion. For Si the first-order dissolution rate estimated from the Si model is 0.2 to 9.3·10 −6·s −1 at the sediment surface, declining to less than 5% of these values at 7.5 to 37.5 mm. The combined ammonium - nitrate model revealed that mineralization of organic nitrogen into ammonium is restricted to the upper 6 to 59 mm. First-order nitrification and denitrification rates were estimated at 0.3 to 16.5·10 −4 and 0.06 to 5.5·10 −4·s −1, respectively. Total nitrification was calculated at 155 to 1930 μmol·m −2·day −1, total denitrification at 54 to 546 μmol·m −2·day −1. Denitrification is completely fuelled by nitrate produced during nitrification in the upper mm's. According to the model, 70 to 80% of the organic N being mineralized within the sediment is released to the water column, largely as nitrate. The P model includes both first-order sorption in the upper oxidized sediment layer and first-order phosphate removal in the zone below. Oxic sorption rate is estimated at 1 to 120·10 −6·s −1, sorption equilibrium concentration is ∼1.7 mmol·m −3. For the removal rate below oxygen penetration, two situations were observed. At 8 stations, removal rate in the reduced zone was 3 orders of magnitude less than oxic sorption. At 4 stations, both rates and equilibria were equal to those in the oxic layers, probably indicating the absence of substantial Fe(III) reduction in the upper cm's at these stations. Average benthic P-mineralization was calculated at 80 μmol·m −2·day −1, of which ∼ 60 μmol·m −2·day −1 was at least temporarily removed from the pore water by sorption. This implies that ∼25% of the organic P mineralized within the sediment is directly regenerated to the water column.

The sedimentary flux of nutrients at a Delaware salt marsh site: A geochemical perspective

A basic geochemical approach has been used to study the diagenetic cycling and sediment-water exchange rates of essential nutrients at a site in a Delaware salt marsh. The pore water nutrients Si (OH)4, NH4+, PO4−3, and NO3− were analyzed and compared seasonally, and potential diffusive fluxes were calculated from the corresponding pore water concentration gradients. Concurrent direct flux measurements were also made using closed chambers deployed on the salt marsh surface under ambient and controlled conditions. The differences in these two approaches allows for estimation of nutrient production and consumption in the root zone and at the salt marsh surface. Silicate and phosphate appear to be primarily geochemically controlled, while the nitrogen species undergo dynamic seasonal redox fluctuations resulting from microbial mediation (e.g., nitrification and denitrification).

Light dependent sediment-water exchange of dissolved reactive phosphorus and silicon in a producing microflora mat

Light dependent sediment-water exchange rates of dissolved reactive silicon (DRSi) and phosphorus (DRP) were studied on field station Archipel (3 m water depth) in Lake Grevelingen (SW Netherlands). Bell jars, either light or darkened, were fixed permanently over a productive microflora mat of mainly Navicula spp.; sediment-water exchange was monitored over an 11 days period. Gross primary production values in the mat amounted to 1000 mg C·m−2·day−1. In the dark bell jar, DRSi and DRP release rates from the sediment were ca. 275 and 85 mg·m−2·day−1, respectively. Release rates in the light bell jars were on average only 15% of these values. Parallel bell jar experiments under different environmental conditions indicate a direct relationship between the primary production figures and nutrient sediment-water exchange rates.

Benthic-pelagic coupling and nutrient cycling across an experimental eutrophication gradient

Sediment-water exchange rates of dissolved inorganic nitrogen (NH:, NO;, NO;) and phosphorus (PO:-), water column concentrations of both dissolved and particulate forms of N and P, and net primary production (14C) were measured during a summer period in large (13 m3) experimental mesocosms that had been subjected to continuous daily nutrient additions (N, P, Si) for over 1 yr. The concentrat~on of combined dissolved inorganic plus particulate nutrient forms (N, P) was linearly related to the nutrient input rate across a loading range from 596 to 34,100 mmols N m-2 yr-l. Benthic nutrient regeneration generally increased, although not uniformly, with loading. Average summer fluxes for treatments ranged from about 150 to 1200 pmols NH: m-2 h' , 188 to 989 pmols DIN m-2 h' , and 8 to 76 pmols PO:m-2 h' , with highest rates being I-ecorded at the 2 highest loading levels. Benthic nutrient regeneration d ~ d not increase in direct proportion to loading, but appeared strongly related to net primary production rates across the enrichment gradient. Comparison, across the experimental gradient, of the external nutrient supply rate with the benthic feedback of dissolved nutrients to the water column indicated that beyond a loading of about 2000 to 5000 mmols N m-? yr-' the water column nutrient dynamics became dominated by the external supply. Results suggest that the importance of benthic-pelagic interactions to biogeochemical and ecological cycles of coastal areas may decrease markedly with nutrient enrichments which exceed the needs of the autotrophic components of the ecosystem.

The rates of sediment-water exchange of oxygen and sediment bioturbation in Lough Neagh, Northern Ireland

Oxygen is transported 30 mm into the sediment at an 8 m depth site in eutrophic Lough Neagh by the irrigational activities of the benthic fauna. Faunal activity also mixes the upper 20 mm of sediment. Sediment oxygen uptake rate, redox potential-depth profile and the chlorophylla concentration were measured in the upper sediment layers from February to November 1979. Chlorophylla input to the sediment, following the Spring phytoplankton maximum, remained in the 0–1 cm sediment layer but did cause the redox potential profile to change from one with potentials around 400 mV in the upper 50 mm to one with a strong gradient over the 0–30 mm region. The start of benthic faunal activity in May caused the chlorophylla to be mixed into the 1–2 cm layer and also caused oxygen to be transported into the sediment at a rate sufficient to change the redox potential back to its initial state. The biodiffusion coefficient for solids in the upper 20 mm was estimated to be 6 × 10−8 cm2 s−1. Oxygen transport in the pore, waters of the upper sediment layers was considered to be best described as advection, caused by the irrigational activities of the benthic fauna.

The rates of sediment-water exchange of oxygen and sediment bioturbation in Lough Neagh, Northern Ireland

The rates of sediment-water exchange of oxygen and sediment bioturbation in Lough Neagh, Northern Ireland

The rates of sediment-water exchange of oxygen and sediment bioturbation in Lough Neagh, Northern Ireland

The rates of sediment-water exchange of oxygen and sediment bioturbation in Lough Neagh, Northern Ireland