Sediment Water Nutrient Exchange Research Articles

Temporal Variation in Resuspension Potential and Associated Nutrient Dynamics in Shallow Coastal Environments

Sediment resuspension may play a major role in sediment-water exchange of nutrients, matter and energy in coastal areas where waves and currents dominate sediment transport. Biogeochemical sediment properties regulate sediment erodibility, but there is only limited knowledge of how temporal variability in environmental variables is reflected in the resuspension potential, especially for subtidal habitats. Further, the significance of resuspension on nutrient fluxes in coastal environments has remained unclear as contradicting results have been reported. Here we quantified the temporal variation in resuspension potential metrics (erosion threshold (τc; N m−2) and erosion constant (me; g N−1 s−1)) and associated nutrient fluxes from three sites in the Hanko archipelago (Finland) using a core-based erosion device (EROMES). The sites were sampled bi-monthly from April to December. We also quantified the temporal variation in biogeochemical sediment properties at each site. The τc exhibited the clearest temporal pattern in muddy sediment, where the coefficient of variation (= 67) was two to three times higher than the mixed (= 29) and sandy (= 16) sediments. Dry bulk density was the best predictor for sediment erodibility at all sites explaining 26–46% of the temporal variation in τc despite its limited variability at sandier sites. In addition, temporal variations in the macrofaunal community were important predictors of muddy sediment erodibility and therefore community dynamics need to be considered in sediment transport studies. All sites were potential nutrient sources, yet the overall role of sediment resuspension on nutrient release from the sediments was small.

Potential mobility of inorganic nutrients and its controls at the sediment-water interface in the main path of Kuroshio Current off eastern Taiwan

Surface sediments were collected from the main path of Kuroshio Current (KC) off eastern Taiwan (ETW), and laboratory incubation was conducted for the study of the nutrient exchange at the sediment-water interface. Exchange fluxes of NO3−-N, NO2−-N, NH4+-N, PO43−-P and SiO32−-Si under simulated site condition were −0.0866, 0.00146, 0.0634, −0.00812 and −0.181mmolm−2d−1, respectively. This indicated that sediments in the studied area might act as a source of NO2−-N and NH4+-N and a sink of NO3−-N, PO43−-P and SiO32−-Si. Factors affecting nutrient exchange fluxes were studied by changing incubation conditions. Compared with some coastal areas in China, the main path of KC off ETW showed relatively low nutrient exchange fluxes at the sediment-water interface. It could be inferred that the sediment-water exchange of nutrients in the main path of KC off ETW had little influence on the downstream nutrient transportation by KC year-round based on the results of this research.

Effects of water level fluctuation and sediment–water nutrient exchange on phosphorus biogeochemistry in two coastal wetlands

Freshwater coastal wetlands are sensitive to drying and wetting events (DWE), associated with water level fluctuations. Prior studies have shown that DWE influence sediment–water nutrient exchange, but the fate of these nutrients has received much less attention. To address this information gap, we investigated how microbial (i.e., bacteria and phytoplankton) structure and function respond to DWE-induced release of nutrients from the sediment of mesotrophic and eutrophic coastal wetlands. Our approach was three-fold: (1) sediment from each wetland was subjected to a desiccation (2 month) and re-wetting (2 days) period in the laboratory; (2) the overlying water column from the DWE was measured for nutrient concentration; and (3) we conducted a microbial response experiment using the water from either DWE experiment (containing the nutrients released during the DWE) or with added nutrients obtained from commercially available sources (but at similar concentrations to those released from the sediments), to test the effect of nutrient concentration and source on microbes inhabiting the water column and their role in the fate of the released nutrients, in particular phosphate (Pi). In both coastal wetlands, the microbial community structure in the water column (absolute and relative abundance of different microbial groups) was modified by bacteria resuspended from the sediment after re-wetting. However, results from the microbial response experiments showed that the microbial communities did not grow following inputs of commercially available nutrients or release of sediment nutrients. In Pi amended treatments, Pi uptake rates increased but not enough to reduce Pi turnover time to values measured in the control. As a result, Pi concentration and turnover remained high. In coastal wetlands with limited hydrologic exchange, these elevated nutrients will accumulate and lead to further eutrophication. However, in open coastal wetlands, with substantial hydrologic exchange, advected nutrients may influence biological activity in the nearshore zone.

Influence of water-level fluctuation duration and magnitude on sediment–water nutrient exchange in coastal wetlands

This study examined the influence of water-level fluctuation (WLF) on sediment–water nutrient exchange in the Laurentian Great Lakes. Water levels in the Laurentian Great Lakes have been below the long-term mean for the past 15 years, causing the exposure of sediments that previously have been either continuously inundated or periodically exposed. The magnitude, duration, and frequency of WLF, as well as land-use history, each can influence the amount and type of sediment–water nutrient exchange. We collected sediment cores from relatively pristine coastal wetlands located on Beaver and Garden Islands in northern Lake Michigan. Sediment cores were taken from different water depths to simulate WLF magnitude; desiccation time was experimentally manipulated to simulate WLF duration. At these relatively pristine wetlands, desiccation time and water depth significantly influenced flux. However, nutrient exchange did not behave in a consistent fashion; phosphorus, nitrate, ammonium, and sulfate flux varied based on sediment exposure history and desiccation time. Sediment–water nutrient exchange rates also were compared to prior measurements taken from more impacted coastal wetlands in southern Lake Michigan and Saginaw Bay in Lake Huron. This comparison revealed a stronger influence of anthropogenic stress than desiccation time, with impacted wetland sediments releasing more soluble reactive phosphorus, sulfate, and ammonium, and retaining more nitrate, than pristine wetlands. Our results indicate that WLFs have the potential to influence sediment–water nutrient exchange, which may influence system productivity, but environmental context can override this influence.

Water level fluctuation and sediment–water nutrient exchange in Great Lakes coastal wetlands

The influence of water level fluctuation on sediment–water nutrient exchange in coastal wetlands of Lakes Michigan and Huron was investigated using controlled, laboratory experiments. At each wetland, sediment cores were collected from 5 locations along a transect perpendicular to the shoreline, desiccated for 8weeks, and then re-wetted with original site water for 24h to simulate water level fluctuation. Soluble reactive phosphorus release declined exponentially along transects, with highest release rates from sediments collected at the ordinary high water mark (OHWM), and lowest rates from sediments underlying water >0.25m in depth. Nitrate exchange showed no obvious pattern in the Lake Michigan wetlands but nitrate was lost at all locations in the Lake Huron wetlands, suggesting denitrification. Ammonium was released at all sites, but with no obvious pattern along transects. Sulfate release was low at the OHWM locations and increased in a lakeward direction, plateauing by the 0.25m water depth.Based on phosphorus (P) release rates, we modeled water level scenarios based on climate change projections to estimate the effects of 3 different water level recessions and 1 water level increase on system-level P exchange. The impact of the P release could be ecologically significant in the localized area of release given the high P concentrations measured in the overlying water during sediment incubations (>500μg/L). The long-term ecological significance of sediment nutrient flux under fluctuating water levels deserves further attention.

Low boundary layer response and temperature dependence of nitrogen and phosphorus releases from oxic sediments of an oligotrophic lake

The effects of temperature, diffusive boundary-layer thickness, and sediment composition on fluxes of inorganic N and P were estimated for sediment cores with oxidized surfaces from nearshore waters (2–10 m) of a montane oligotrophic lake. Fluxes of N and P were not affected by diffusive boundary-layer thickness but were strongly affected by temperature. Below 16 °C, sediments sequestered small amounts of P and released small amounts of N. Above 16 °C, the seasonal maximum water temperature, sediments were substantial sources of N (NH4 +–N = 2–24 mg m−2 d−1; NO3 − + NO2 −–N = 2–5 mg m−2 d−1) and P (0.1–0.4 mg m−2 d−1), indicating potential responsiveness of sediment–water nutrient exchange, and of corresponding phytoplankton growth, to synoptic warming.

Phosphorus release from sediments in a treatment wetland: Contrast between DET and EPC 0 methodologies

Wetlands are capable of reducing nutrient loadings to receiving water bodies, and hence many artificial wetlands have been constructed for wastewater nutrient removal. In this study, diffusive equilibrium in thin films (DETs) and equilibrium phosphorus concentration (EPC 0) analysis were used to examine the role of sediment as a nutrient source or sink in a constructed treatment wetland in summer. The effect of dredging on sediment-water nutrient exchange was also studied. Soluble reactive phosphorus (SRP), ammonium (NH 4 +) and sulphate (SO 4 2−) concentration profiles were measured by DET across the sediment-water interface (SWI) in both a settling pond and iris reed bed within the wetland. The SRP concentrations in the sediment pore-waters of the settling pond were extremely high (up to 29,500 μg l −1) near the SWI. This is over an order of magnitude higher than the levels found in the water column, which in turn are over an order of magnitude higher than environmental levels proposed to limit eutrophication in rivers. The profiles demonstrated an average net release of SRP and NH 4 + from the settling pond sediment to the overlying water of 58 mg m −2 d −1 (±32 mg m −2 d −1 (1 sd)) and 16 mg m −2 d −1 (±25 mg m −2 d −1 (1 sd)), respectively. The DET SO 4 2− concentration profiles revealed that the sediment was anoxic within 2 cm of the SWI. Dredging of the reed bed made no significant difference to the P release characteristics across the SWI. The EPC 0s were much lower than the SRP concentration of the overlying water, indicating that the sediment had the potential to act as a phosphate sink. The apparent contradiction of the DET and EPC 0 results is attributed to the fact that DET measurements are made in situ, where as EPC 0 measurements are ex situ. These results show that substantial releases of P can occur from wetland sediments, and also highlight the need for caution when interpreting ex situ EPC 0 analytical results.

Simple parameterisations for describing N and P diagenetic processes: Application in the North Sea

An appropriate description of nutrient exchanges at the sediment–water interface is prerequisite to model the eutrophication process in shallow coastal waters and its response to nutrient reduction policy. Most existing eutrophication models lack such a description due to the complexity of diagenetic processes. In this paper we use a two-layer, analytic diagenetic model applied in the North Sea coastal waters to derive simple mathematical relationships linking benthic N and P fluxes at the sediment water interface to organic matter inputs to the sediment. The two-layer, analytic diagenetic model describes organic matter mineralization, the resulting C, O 2, N and P diagenetic processes and the sediment–water nutrient exchange fluxes as a function of bottom water nutrient concentration, total sediment organic matter mineralization, temperature and the sediment characteristics (porosity, tortuosity, bioturbation). The model was first verified by comparing nutrient and oxygen depth profiles and mineralization features along an eutrophication gradient with results obtained under the same conditions with a dynamic, vertically resolved model (OMEXDIA). The model was then applied to several stations in the North Sea characterized by different sediments and the results were compared with available data (NO 3, NH 4, PO 4 and O 2 sediment profiles and sediment–water fluxes). Finally, we performed numerous Monte-Carlo model simulations with different parameter values to derive simplified mathematical relationships of benthic N and P fluxes at the sediment water interface.

Sediment–water inorganic nutrient exchange and nitrogen budgets in the Colne Estuary, UK

The exchange of dissolved inorganic nutrients (NO3 - , NO2 - NH4 + , PO4 3- and SiO3 - ) was measured across the sediment-water interface at 4 sites along the Colne Estuary at monthly intervals between August 1996 and March 1998 in the dark and under illumination. Fluxes across the sedi- ment-water interface were scaled up to calculate nutrient budgets for the whole estuary, accounting for the area of sediment exposed to illumination. Sediments were a net sink of -16.28 Mmol N yr -1 for oxidized inorganic nitrogen (NO3 - + NO2 - ). Mean nitrate fluxes across the sediment-water interface were -329 and -91 µmol m -2 h -1 in the dark and light respectively. Colne sediments were a source of ammonium in the dark (12.4 Mmol N yr -1 ). Illumination reduced ammonium flux into the overlying water or affected uptake due to assimilation by the microphytobenthos and a stimulation of nitrifi- cation. Mean ammonium fluxes across the sediment-water interface were 399 and -169 µmol m -2 h -1 in the dark and light, respectively. Consequently, sediments were a net sink for ammonium in the light (-0.31 Mmol N yr -1 ). Similarly, illumination affected silicate exchange across the sediment- water interface, with silicate flux into the overlying water reduced or reversed under illumination compared to the dark. The estuarine sediments were a net source of silicate (6.09 Mmol yr -1 ) and soluble reactive phosphate (1.91 Mmol yr -1 ). Uptake of inorganic oxidized nitrogen was approxi- mately balanced by adding together ammonium flux and denitrification supported by nitrate in the overlying water. This mass balance indicates that dissimilatory nitrate reduction to ammonium (DNRA) was a significant process in the sediments of the Colne Estuary. Fluxes in the light accounted for less than 6% of the total sediment-water exchange of both ammonium and nitrate. However, a large proportion of the -15.49 Mmol N yr -1 of oxidized inorganic nitrogen taken up by the sediment in the dark was cancelled out by a flux of 12.4 Mmol N yr -1 into the overlying water as ammonium. Net dissolved inorganic nitrogen (DIN) fluxes were -3.09 Mmol N yr -1 in the dark and -1.17 Mmol Ny r -1 in the light, as fluxes of all nitrogen species were negative under illumination. Therefore fluxes under illumination accounted for 28% of the net uptake of DIN by estuarine sediments in the Colne Estuary, despite the short time periods and small area of sediment under illumination. In macrotidal estuaries such as the Colne, where microphytobenthos are abundant, light plays an important role in moderating sediment-water nutrient exchange.

Benthic nutrient recycling in shallow coastal waters of the Bohai Sea

Sediment-water fluxes of N and P species in the Bohai Sea were investigated in September-October 1998 and April–May 1999. The benthic fluxes of nutrient species were determined by incubating sediment core samples with bottom seawater bubbled with air or nitrogen. NO2−, NH4+, dissolved organic nitrogen (DON) and phosphorus (DOP), total dissolved nitrogen (TDN) and phosphorus (TDP), and PO43− showed a net exchange flux from seawater to sediment, while NO3−, dissolved inorganic nitrogen (DIN) and SiO32− were released from sediment to seawater in the Bohai Sea. Sediment-water nutrient exchange increases DIN and reduces the phosphorus load in the Bohai Sea. The release of silicate from sediment to overlying seawater reduces potential silicate limitation of primary production resulted from decrease of riverine discharge. The exchange flux of nutrients showed no obvious seasonal variation. The present study showed that the concentrations and composition of nutrients in the water column were affected by suspended sediment, and that not all the exchangeable phosphate in sediment could be released via sediment resuspension.

Changes in sediment processes across the western Irish Sea front

Sediment characteristics, sediment respiration (oxygen uptake and sulphate reduction) and sediment–water nutrient exchange, in conjunction with water column structure and phytoplankton biomass were measured at five stations across the western Irish Sea front in August 2000. The transition from thermally stratified (surface to bottom temperature difference of 2.3 °C) to isothermal water (14.3 °C) occurred over a distance of 13 km. The influence of the front on phytoplankton biomass was limited to a small region of elevated near surface chlorophyll (2.23 mg m −3; 50% > biomass in mixed waters). The front clearly marked the boundary between depositional sediments (silt/clays) with elevated sediment pigment levels (≈60 mg m −2) on the western side, to pigment impoverished (<5 mg m −2) sand, through to coarse sand and shell fragments on the eastern side. Maximal rates of sedimentary respiration on the western stratified side of the front e.g. oxygen uptake S2 (852 μmol O 2 m −2 h −1) and sulphate reduction at S1 (149 μmol SO 4 2− m −2 h −1), coupled to significant efflux of nitrate and silicate at the western stations indicate closer benthic–pelagic coupling in the western Irish Sea. Whether this simply reflects the input of phytodetritus from the overlying water column or entrapment and settlement of pelagic production from other regions of the Irish Sea cannot yet be resolved.

Mass balance constraints on nutrient cycling in tropical seagrass beds

A relatively simple mass balance model is presented to study the cycling of nutrients (nitrogen and phosphorus) in tropical seagrass beds. The model is based on quantitative data on nutrient availability, seagrass primary production, community oxygen metabolism, seagrass tissue nutrient contents, sediment-water nutrient exchange rates, and seasonal dynamics in seagrass and environmental variables for two contrasting seagrass meadows from a single study area in South Sulawesi (Indonesia). Possible pathways of nutrient acquisition and nutrient cycling are described and quantified, and implications for leaf/root uptake and autotrophy/heterotrophy are discussed. It is concluded that, on an annual basis, the tropical seagrass ecosystems in the study area are more or less equally balanced between net autotrophy and net heterotrophy. Root uptake may potentially account for 66–98% of total nutrient uptake from external sources. Model simulation based on 50% internal nutrient resorption (withdrawal of nutrients from old plant parts and translocation to new growing parts) shifted the ratio between leaf vs. root-uptake, increasing the importance of leaf uptake. The results of these mass balance calculations indicate that root and leaf uptake contribute about equally to total plant nutrient uptake in tropical seagrass beds.

Application of SWITCH, a model for sediment-water exchange of nutrients, to Lake Veluwe in The Netherlands

Application of SWITCH, a model for sediment-water exchange of nutrients, to Lake Veluwe in The Netherlands

Nutrient Dynamics in Great Lakes Coastal Wetlands: Future Directions

The most comprehensive investigations of N and P dynamics in Great Lakes coastal wetlands have been done at Old Woman Creek National Estuarine Research Reserve (OWC); whether OWC is a good general model of coastal freshwater wetlands remains to be shown. This wetland is probably a nutrient sink, storing P in sediments (at least temporarily) and releasing N by dissimilatory denitrification. Also, its biotic community transforms dissolved inorganic N and P inputs into organic dissolved and particulate outputs, thereby altering nutrient availability to Lake Erie communities. Nutrient dynamics in coastal wetlands probably differs greatly from that of inland marshes (where slow decomposition rates permit peat to accumulate as a nutrient sink) and estuaries (where high salinity alters sediment nutrient dynamics). A conceptual model specific for coastal wetlands is presented that accounts for the wide range of redox potentials encountered over the short vertical span of the shallow OWC wetland ecosystem. Future studies need to be conducted within the context of testable hypotheses generated from this model. Future investigations should focus attention on annualized nutrient budgets, sediment-water nutrient exchanges and their dependence on organic matter generated within the ecosystem.

Measured and modelled effects of temperature, dissolved oxygen and nutrient concentration on sediment-water nutrient exchange

Empirical models of sediment-water fluxes of NH4 +, NO3 − were and PO4 3− were formed based on published reports. The models were revised and parameters evaluated based on laboratory incubations of sediments collected from Gunston Cove, VA. Observed fluxes ranged from — 18 (sediments uptake) to 276 (sediment release) mg NH4 + m−2 day−1, −17 to −509 mg NO3 − m−2 day−1, and −16.4 to 8.9 mg PO4 3− m−2 day−1. The model and observations indicated release of NH4 + was enhanced by high temperature and by low DO. Uptake of NO3 − was enhanced primarily by high NO3 − concentration and to a lesser extent by high temperature and by low DO. Direction of PO4 3− flux depended on concentration in the water. Release was enhanced by low DO. No effect of temperature on PO4 3− flux was observed.

A eutrophication model for a lake

Different lake models were tested against measured values. It was found that a model based upon phytoplankton population dynamics gave a better description than a model based upon Monod's kinetics. The calibration of the model showed that it was necessary to use a recently developed submodel for sediment—water nutrient exchange. Furthermore, the work showed that it was essential to include in the model three trophic levels, denitrification, a time dependent nitrogen input and Steele's expression for the grazing rate. A prediction for different waste-water treatment alternatives was worked out.