Phosphorus and nitrogen loads in waters associated with the River Murray near Albury-Wodonga, and their effects on photoplankton populations

Abstract

This is part of a survey carried out to assess the likely impact on the River Murray of urban development at Albury-Wodonga. The study area included two impoundments, Lake Hume and Lake Mulwala, the tributary Mitta Mitta, Kiewa and Ovens Rivers, and the River Murray.

Phosphorus and nitrogen loads were examined over 4 years, incorporating a high flow period, 1974-75, and a low flow period, 1976-77. Loads varied directly with stream flow, although loadings to the two impoundments were generally high and above levels considered indicative of eutrophy elsewhere. Phytoplankton biomasses did not reflect this situation, howeyer, and averaged below 2000 mm³ m^-3 Phytoplankton growth, and hence nutrient assimilation, probably were restricted by low light penetration resulting from high turbidities. This is supported by preliminary laboratory experiments involving nutrient enrichments over a range of turbidities.

Under low flow conditions, nutrient inputs to the impoundments were reduced and underwater light penetration increased, due to settling of abiogenic suspended material. Nutrient enrichment experiments in Lake Mulwala supported the proposition that availability of nutrients, particularly phosphorus, could limit phytoplankton biomass under those conditions. The results generally indicate that pro rata inputs from Albury-Wodonga need to be reduced as the urban population grows, if the risk of nuisance algal blooms during low flow periods is to be minimized.