Geochemistry and isotopic composition of sediment cores to understand the lithological and anthropogenic controls on eutrophication in the New River Estuary, Southland, New Zealand

Abstract

The New River Estuary (NRE) in Southland, New Zealand is highly eutrophic and has rapidly declining ecosystem health. Historic estuarine reclamation, extensive catchment drainage, and waterway modification have increased the susceptibility of the estuary to degradation. In addition, more recent (post-1984) agricultural intensification and a shift in primary land use from sheep to dairy farming have increased the fine-sediment and associated pollutant loss to the catchment. Extensive macroalgae cover in the NRE reflects the ecosystem's response to eutrophication, where opportunistic species outcompete native plants in the nutrientenriched environment. Three sediment cores from the primary depositional areas in the NRE, the Waihopai Arm and Daffodil Bay, were geochemically characterized, including stable and radiogenic isotopes, to assess changes in the rate of sediment, nutrient, and heavy metal accumulation. The sedimentation rate in the upper Waihopai Arm has increased from 7.3-13 mm yr-1 before 1935 to a very high rate of 20-22 mm yr-1 from 2009-2017. The lower Waihopai Arm and Daffodil Bay have increased to a high rate of sedimentation in the last decade from 5.9 to 17.5 mm yr-1 and 5.5-7 to 10.3 mm yr-1 , respectively. Phosphorus and trace metal concentrations in the bioavailable sediment fraction, which includes Fe- and Mn-oxides, sulfide, organic, or surface-adsorbed phases, have increased up to three and eight times higher than geogenic levels, respectively, which heightens their vulnerability to mobilization in response to changes in salinity and redox state. Increasing heavy metals and decreasing calcium loads, coupled with carbon- and nitrogen-isotopic values trending toward a terrestrial signature (𝛿 13C = -28‰, 𝛿 15N = 8‰), delineates a transition in sediment source from marine-dominated (pre-1935) to terrestrial-dominated (post-1985). The composition of fallout radionuclides also indicates a change in the delivery of terrestrial sediment from channel bank collapse and subsoil erosion (pre-1965) to sheet erosion of surface, likely pasture, soils (post-1997). This study highlights the importance of differentiating the natural sediment signatures from the anthropogenic sources of pollutants to assess the proportion of low-quality sediment (i.e., high nutrient and/or metal concentration) for which a targeted mitigation approach should be applied.