Nutrient and sediment dynamics in two coastal plain rivers in the Bay of Plenty

Abstract

A large amount of literature concerns eutrophication development in lakes, reservoirs, or estuaries. In contrast, literature concerning to the development of eutrophication in flowing waters is more scarce and, in New Zealand, what there is mostly concerns smaller wadable, cobble-bedded streams and not large soft-bottomed lowland rivers. Lowland rivers are at the distal end of the catchment and suffer from accumulated multiple stressors pertaining to the surrounding land use. In New Zealand coastal plain catchments, land is generally high producing pasture, urban development, or other arable practices The aim of this study was to develop a better understanding of how two major land-use related contaminants, nitrate, and suspended sediments, vary in two Bay of Plenty rivers, the Kaituna and the Rangitaiki, as they cross the coastal plain. Key goals were to determine the extent to which contaminant concentrations are attenuated or enhanced during this passage. These are two significant North Island New Zealand river systems, with intensive agricultural activities in the lowland part of the catchment. Longitudinal sampling of the lowland reaches took place between November 2020 to June 2021; additional spring samples were collected in November 2021 and early December 2021. The study included measures of nitrate, suspended sediment, turbidity chlorophyll-a, pH, and dissolved oxygen, and nitrate samples from tributary flows and a limited collection of aquatic macrophytes, to assess other aspects of water quality. The Rangitaiki and Kaituna Rivers both tended to show increasing nitrate concentration as they flowed across the coastal plain, reaching median concentrations of 0.31 and 0.54 mg/NO₃-N/L respectively. Effects of both groundwater and tributaries were evident. Suspended sediment was at relatively low concentrations within both rivers, and generally showed no substantive increase across the lowland reaches of both rivers. Chlorophyll-a and nitrate showed seasonality, and this was largely due to processes in upstream lakes or reservoirs. Little attenuation or development of phytoplankton in the river was evidenced by changes in chlorophyll concentrations, likely due to the very short residence time of water in this part of the river (<24 h). Furthermore, nitrate and suspended solids behaved differently, and there was no correlation in either river p>0.05 nor between nitrate and chlorophyll-a, p>0.05. Downstream changes in measured variables in the Rangitaiki and Kaituna rivers could ultimately be related to land use within the lower catchment. The lower Rangitaiki River nitrate concentration is largely dictated by seasonality within Lake Matahina and activities within the upper catchment, likely supplemented by groundwater influx and any minor drains discharging to the lowland river. The Kaituna River nitrate concentration is predominantly a product of the intensive agricultural catchment, with four significant tributary inflows along the lowland reach, all of which are nitrate rich. Nitrogen enrichment was sufficient to support non-native macrophyte growth, but other adverse effects associated with eutrophication were not realised in either river. This is likely due to a combination of the short residence times within both rivers, the abrasive flow preventing macrophytes accumulating sufficiently to block the channel and the turbulent, shallow water allowing ready reaeration. These results do emphasise the need for further study on lowland rivers to assess the effects of nutrient and sediment enrichment, and the extent to which existing guidelines on river are well suited to lowland reaches.