Effects of nutrient loading on the trophic state of Lake Brunner

Abstract

Abstract. Lake Brunner, an oligotrophic monomictic lake on the West Coast of the South Island of New Zealand, is under pressure from urban expansion and increased farming activity, which has led to concern for the effects on water quality in the lake. Epilimnetic nitrogen, phosphorus and chlorophyll a concentrations have increased since 1992, and Secchi depth decreased. This suggests an increased algal productivity caused by increased nutrient inputs, further supported by increased hypolimnetic oxygen depletion since 1992. These observations are likely to have resulted from enhancement of pasture drainage and effluent inputs from expanding dairy farms. The Vollenweider model predicted a mean phosphorus concentration in the lake close to that observed, from estimated catchment loading, suggesting that the Vollenweider model adequately estimated the retention of phosphorous. With the Vollenweider model the effects of potential future loading scenarios were explored. Modelling suggested that a 70% increase in phosphorus loading could turn the lake into a mesotrophic state. Trend analysis of total phosphorus suggests that, with present land uses in the catchment (intensive dairy farming) continuing to develop at the same rate using the same land management practises, this transition to a mesotrophic state will occur by 2040.

Managing pollutant inputs from pastoral dairy farming to maintain water quality of a lake in a high-rainfall catchment

“…Productivity in Lake Brunner is limited by the availability of P as demonstrated by very low concentrations of inorganic P relative to inorganic N. The DIN-to-FRP mass ratio in the epilimnion is 131 (Verburg et al 2013). Lake Brunner is vulnerable to changes in P loading because of the large difference between the N and P concentrations relative to the stoichiometric demand of phytoplankton productivity.…”

“…Annual rainfall in the Lake Brunner catchment is highly variable, ranging from 3 m year À1 at the northern end of the lake to 4.8 (maximum 6.8) m year À1 at Inchbonnie, 10 km away at the southern end of the lake. Lake Brunner is oligotrophic (total phosphorus (TP) ,10 mg m À3 ) but Secchi depth transparency has decreased since the early 1990s from ,6.5 to 5.5 m (Verburg et al 2013). The decrease in transparency corresponds to an observed increase in chlorophyll a, an algal pigment that is a good proxy for algal abundance in the lake.…”

“…The increase in productivity is a result of increased nutrient loading to the lake as suggested by increased concentrations of total nitrogen (TN) and TP. The loading of nitrogen has increased faster than that of phosphorus, as indicated by an increase in the TN : TP ratio in the lake (Verburg et al 2013). Lakes, such as Brunner, that are used extensively for recreation during summer may be adversely affected by inputs of faecal matter from the intensively farmed parts of the catchment (Macpherson 2005;Donnison and Ross 2009) Other components of farm runoff (e.g.…”

Nutrients and eutrophication: introduction

“…The study by Wilcock et al (2013b) examined catchment sources and pathways for water-quality contaminants. The study by Verburg et al (2013) assessed the relative of these contaminants on an oligotrophic lake (Brunner). Wilcock et al (2013b) found that while trends were decreasing (i.e.…”

“…Moreover, at the farm scale the measured loads closely matched modelled loads, indicating that attenuation in the stream network was limited and that mitigation should be directed mostly at the source. Verburg et al (2013) then went onto assess the relative impact of the loads of N and P, finding that the Vollenweider model predicted a mean P concentration close to the observed concentration and that algal productivity was most strongly stimulated by additional inputs of P to the lake. They indicated using the Vollenweider model that at the present rate of increasing agricultural intensification and P loading from the catchment, Lake Brunner would likely change from oligotrophic to mesotrophic.…”