Resuspension and estuarine nutrient cycling: insights from the Neuse River Estuary

D. R. Corbett

doi:10.5194/bg-7-3289-2010

Abstract

Abstract. For at least the past several decades, North Carolina's Neuse River Estuary (NRE) has been subject to water quality problems relating to increased eutrophication. Research initiated in the past several years have addressed the nutrient processes of the water column and the passive diffusion processes of the benthic sedimentary environment. Resuspension of bottom sediments, by bioturbation, tides, or winds, may also have a significant effect on the flux of nutrients in an estuarine system These processes can result in the advective transport of sediment porewater, rich with nitrogen, phosphorus and carbon, into the water column. Thus, estimates of nutrient and carbon inputs from the sediments may be too low. This study focused on the potential change in bottom water nutrient concentrations associated with measured resuspension events. Previous research used short-lived radionuclides and meteorological data to characterize the sediment dynamics of the benthic system of the estuary. These techniques in conjunction with the presented porewater inventories allowed evaluation of the depth to which sediments have been disturbed and the advective flux of nutrients to the water column. The largest removal episode occurred in the lower NRE as the result of a wind event and was estimated that the top 2.2 cm of sediment and corresponding porewater were removed. NH4+ advective flux (resuspended) was 2 to 6 times greater than simply diffusion. Phosphate fluxes were estimated to be 15 times greater than the benthic diffusive flux. Bottom water conditions with elevated NH4+ and PO43− indicate that nutrients stored in the sediments continue to play an important role in overall water quality and this study suggests that the advective flux of nutrients to the water column is critical to understand estuarine nutrient cycling.

Highlights

In most estuaries, the amount of nutrients supplied by external sources has consistently been shown to supply less than that required by primary producers (Dugdale and Goering, 1967; Haines, 1976; Windom et al, 1975; Nixon, 1981; Stanley and Hobbie, 1981; Fisher et al, 1982; Boyer et al, 1988)
Bottom water conditions with elevated NH+4 and PO34− indicate that nutrients stored in the sediments continue to play an important role in overall water quality and this study suggests that the advective flux of nutrients to the water column is critical to understand estuarine nutrient cycling
This study provides an estimate of the additional flux of nutrients that are released to the water column associated with sediment resuspension events in the Neuse River Estuary (NRE), North Carolina

Summary

Introduction

The amount of nutrients supplied by external sources (e.g. atmospheric deposition, river runoff, nitrogen fixation) has consistently been shown to supply less than that required by primary producers (Dugdale and Goering, 1967; Haines, 1976; Windom et al, 1975; Nixon, 1981; Stanley and Hobbie, 1981; Fisher et al, 1982; Boyer et al, 1988). The benthic environment is important due to the large portion of organic matter that reaches the sediment surface after settling out of the water column This organic matter is remineralized, a process during the decomposition of organic matter where inorganic nutrients, such as nitrogen and phosphorus, are released, increasing their concentrations in the interstitial waters. In most shallow-water systems, surficial sediments and the overlying water are continually interacting, exchanging and redistributing particles and solutes, making this recycling process extremely important for understanding nutrient dynamics in the estuarine environment (Wells and Kim, 1989; Rizzo, 1993; Rizzo and Christian, 1996; Luettich et al, 2000). Winds can break down stratification and force nutrient-rich bottom water to the surface, promoting algal blooms Resuspended sediment in these shallow estuaries has other impacts, through light limitation and consequent reductions in primary production

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Conclusion