Anthropogenic phosphorus loads to Lake Rotorua

Abstract

Past wastewater disposal practices and ongoing catchment land-use intensification have resulted in the eutrophication of Lake Rotorua. These changes have been associated with elevated nutrient loads to the lake from point and diffuse sources. Phosphorus and nitrogen are important macronutrients limiting phytoplankton growth. Bioassay studies in Lake Rotorua have shown temporal and spatial variability of phytoplankton growth limitation by either or both of these nutrients, hence the importance of controlling both phosphorus and nitrogen loads to the lake. This report was commissioned by the Bay of Plenty Regional Council to quantify sources of anthropogenic (i.e., associated with human activity) phosphorus in the catchment, so that these sources can be managed with the aim of reducing phytoplankton biomass in Lake Rotorua. This will form part of an integrated framework of catchment management which also aims to reduce the nitrogen load to the lake. Determination of anthropogenic and natural (baseline) sources of phosphorus is essential for managing lake eutrophication. Natural concentrations (indicative of baseline or reference levels) of dissolved reactive phosphorus (DRP) and total phosphorus (TP) can be modelled from observed concentrations in undisturbed catchments. This model can then be used to estimate anthropogenic phosphorus in other catchments as the difference between natural and observed concentrations. Mean annual discharge (MAD) was estimated individually for nine major stream sub-catchments of Lake Rotorua, and a combined MAD was estimated for other minor and ungauged stream sub-catchments. Natural stream phosphorus concentrations were derived from published values based on the River Environment Classification (REC) land classes within the lake catchment. Natural groundwater phosphorus concentrations were derived from published relationships between groundwater age and phosphorus concentrations in the Rotorua sub-catchments. Total phosphorus and DRP loads were calculated for each inflow for the period 2007-2014 (‘contemporary loads’) using the daily discharge estimates and time series of estimated daily mean concentrations of each nutrient species (TP, DRP) for each inflow. Daily loads were then summed to calculate annual loads. Natural loads for each inflow were subtracted from contemporary loads to estimate anthropogenic loads. Anthropogenic phosphorus loads to Lake Rotorua were estimated to be 22% (expressed as a catchment areal rate of 0.12 kg DRP ha-1 y-1) of the total DRP load and 48% (0.47 kg P ha-1 y-1) of the TP load. Relative to other New Zealand catchments, anthropogenic phosphorus was a relatively low proportion of the total load, given that the Rotorua catchment is highly developed. This can be attributed to the large volume of groundwater discharge to the lake, which is DRP-enriched due to gradual dissolution of phosphorus in the rhyolitic pumice bedrock. In sub-catchments where groundwater discharge to the lake was volumetrically dominant, e.g., Hamurana (89.3%) and Awahou (79.6%), the natural contribution to DRP loads was high, despite extensive agricultural land-use. By contrast, sub-catchments dominated by surface water discharges, e.g., Ngongotaha (34% groundwater) and Waiteti (14.5% groundwater), had comparatively high anthropogenic TP loads and elevated concentrations of particulate phosphorus. The minor and ungauged catchments, representing drains, small streams, overland flow, groundwater discharge to the bed of the lake, discharge below gauged stream sites and a residual term, were estimated to contribute the largest percentage (33%) of anthropogenic particulate phosphorus loads to Lake Rotorua despite being groundwater dominated.