Report: TR 2002/11

Author: Grant Barnes

Abstract

The collection and analysis of water quality data is an important tool in assessing the ecological condition of lakes and compliments ecological studies on fish, invertebrate and plant communities. Trend analysis using proven statistical provides a powerful tool to determine whether a waterbody improved, degraded or has remained unchanged. The current shallow lakes water quality monitoring was initiated in 1995 following a review of the previous programme.

Of the eight shallow lakes monitored by Waikato Regional Council, Lakes Rotomanuka North, Rotomanuka South and Waikare have deteriorated since water quality monitoring commenced. No change was detected in Lakes Mangahia, Ngaroto, Rotokauri and Waahi. Water quality monitoring of Whangape by the National Institute of Water and Research (NIWA) between 1992 and 1996 detected a significant improvement in water quality.

Many shallow lakes within the Waikato region are within pastoral catchments with little or no indigenous vegetation. Inputs of contaminants from intensive land uses have contributed to an overall decline in water quality, and many cases have contributed to a loss of indigenous biodiversity. Improvement in the water quality of many eutrophied lakes be difficult under current landuse activities. The input contaminants from diffuse sources can be reduced by implementing riparian protection strategies. However, scale changes will be required to ensure sufficient reduction external nutrient loads.

Submerged aquatic plants are known to regulate the ecology shallow lakes by influencing trophic state and by buffering impact of external nutrient loads. However, many shallow lakes are now devegetated following the collapse of invasive species.

The protection of remaining submerged plant communities encouraging the re-establishment of collapsed populations important for buffering the impact of landuse intensification for maintaining and enhancing in-lake biodiversity.

Water quality trends in selected shallow lakes in the Waikato Region, 1995-2001 [PDF, 227 KB]