Environ. Sci. Technol. 2008, 42, 5112–5117
Combined Gel Probes for the In Situ Determination of Dissolved Reactive Phosphorus in Porewaters and Characterization of Sediment Reactivity P H I L M O N B E T , * ,† I A N D . M C K E L V I E , AND PAUL. J. WORSFOLD† Water Studies Centre, School of Chemistry, Monash University, Clayton 3800, Victoria, Australia
Received January 17, 2008. Revised manuscript received May 1, 2008. Accepted May 7, 2008.
Diffusive equilibrium in thin films (DET) and diffusive gradients in thin films (DGT) were applied in situ to obtain high spatial resolution dissolved reactive phosphorus (DRP) sediment porewater profiles in two lagoons of the Gippsland Lakes (SE Australia) during summer. Although the DRP depth profiles were different in each lake, highlighting the sensitivity of DRP to the redox state of the sediment, spatial DRP variations obtained from DET and DGT showed striking similarities with depth in each lake. Comparison of DRP concentrations obtained from DGT and DET allows an assessment of the reactivity of the sediment. A dynamic numerical model of DGT-induced flux in sediments (DIFS) quantified reactivity kinetics. Sediment response time (Tc) related to sedimentary phosphorus resupply resulting from DGT-induced lowering of the porewater DRP concentration was calculated. Values of Tc ranged from 4128 to 183 400 s and from 55 to 149 400 s for Lakes Victoria and Wellington, respectively. These results indicate the limited capacity of the sediment to quickly resupply DRP to the porewater, especially in surface sediment, which was always characterized by the highest Tc. Adsorption and desorption rate constants were also calculated from DIFS with values ranging from 2 × 10-3 to 2 × 10-2 day-1 and from 0.3 to 21 day-1 for Lake Victoria and from 2 × 10-4 to 0.8 day-1 and from 0.6 to 1558 day-1 for Lake Wellington, respectively. Diffusive fluxes estimated from the in situ DRP gradient at the sediment-water interface by DET were 558 and 1.2 µmol m-2 day-1 in lakes Victoria and Wellington, respectively. Despite only a single measurement, these fluxes highlight a substantial contribution of P from the sediment to the water column in summer for Lake Victoria which is likely to contribute to the recurring blooms of blue-green algae. These calculated fluxes obtained with minimal sediment disturbance and high spatial resolution were substantially different (by a factor of 6-180) from other reported values using more conventional quantitative assessments such as diffusive fluxes (from core slicing) and benthic chambers.
* Corresponding author phone: +44 (0)1752 233006; fax: +44 (0)1752 233009; e-mail:
[email protected]. † Current address: Biogeochemistry & Environmental Analytical Chemistry Group, School of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth PL4 8AA, United Kingdom. 5112
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 14, 2008
Introduction Phosphorus (P) is involved in biogeochemical processes that occur in sediment during early diagenesis. Microbial reactions decompose organic matter, resulting in a release of inorganic P to the sediment pore water. Depending on the environmental conditions in the sediment, the nutrients produced can be transported back to the water column, where they can fuel new primary production (1), or they can be adsorbed to sediment grain surfaces, coprecipitated with iron oxyhydroxides, or eventually transformed in situ to mineralized forms within the sediments. These processes can generate steep chemical gradients at the sediment-water interface (2). Understanding of remobilization processes and flux calculations to and from the sediment relies on the ability to determine these gradients with high spatial resolution. Conventional techniques (coring-slicing-centrifuging, squeezing, and dialysis) have recently improved significantly (3, 4) and different approaches, such as benthic landers/ chambers (5) and two-dimensional analysis of P fluxes (6, 7), have been used to determine benthic fluxes and gain insight into early diagenesis. Emerging techniques such as DGT (diffusive gradients in thin films) and DET (diffusive equilibration in thin films) provide high spatial resolution data on porewater sediment profiles for metals (3, 8, 9). Surprisingly nutrients, especially phosphorus, have received less attention. Only one paper describes DGT for measuring DRP in natural waters and sediments (10) and recently Jarvie et al. (11) measured DRP profiles and fluxes in riverine sediments using DET probes. This work describes the first use of a combination of DET and DGT gel probes for measurement of porewater nutrient (i.e., DRP) profiles and fluxes in sediments in a manner similar to that used by Metzger et al. (12) for metals. The Gippsland Lakes in southeast Australia are a group of shallow estuarine lagoons separated from Bass Strait by broad sandy barriers. The Lakes suffer from major water quality concerns including harmful algal blooms (Nodularia, Anabaena, Microsystis). Remobilization of P from the sediment is a substantial source of phosphorus in the overlying water column of the lakes (13). Although this latter study has described the processes controlling storage of phosphorus in sediments, none have investigated the reactivity of the sediments. In this context, DET and DGT were deployed in situ in shallow waters in Lakes Wellington and Victoria. This paper presents new data for porewater DRP with high spatial resolution and emphasizes the complementary nature of DGT and DET. Fluxes of DRP at the sediment-water interface were calculated, and the reactivity of the sediment was assessed by means of a sediment reactivity coefficient (R) derived from the combination of DGT and DET. This approach coupled with simple modeling also enables quantitative characterization of the kinetics of the resupply process.
2. Experimental Section 2.1. Study Area. The Gippsland Lakes are 69 km long and 10 km at their widest point and are located in southeast Australia. They are large, shallow (
