Using estuarine cores to assess changes in terrestrial-sediment loss and eutrophication in New River Estuary, Southland, New Zealand

Abstract

The New River Estuary (NRE) in Southland, New Zealand, has undergone rapid eutrophication, with rapidly declining ecosystem health, and is one of the most impacted estuaries in New Zealand by changes in land use over the last several decades. Three sediment cores from the highest and finest-grained sedimentation areas of the NRE, the Waihopai Arm and Daffodil Bay, were chosen for geochemical and isotopic characterization (major elements, nutrients, and trace elements; C, N, and S stable isotopes; and ⁷Be, ¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra, and ²²⁸Ra), 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 to 13 mm yr⁻¹ before 1935 to 20–22 mm yr⁻¹ from 2009 to 2017. The lower Waihopai Arm and Daffodil Bay have experienced increased sedimentation rates 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 (defined as extracted by a modified aqua regia digestion), which includes Fe- and Mn-oxide minerals, sulfide, readily oxidized organic-bound, or surface-adsorbed phases, have increased up to three to eight times higher than geogenic levels, which heightens vulnerability to metal mobilization in response to changes in salinity and redox state of the estuary. Increasing heavy metals and decreasing calcium loads, coupled with carbon- and nitrogen-isotopic values trending toward a terrestrial signature (δ¹³C = −28 ‰, δ¹⁵N = 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). The 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. 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). Targeting anthropogenic sources of sediments may include the use of conservation tillage, direct drilling, vegetative buffers around areas of intensive grazing, sediment traps, and treatment wetlands.