Effect of climate and human induced changes on the seagrasses in Lake Illawarra

Abstract

Lake Illawarra normally has over 5 km2 of seagrasses, including large meadows of Zostera capricorni and Ruppia megacarpa, and smaller patches of Halophila ovalis and H. decipiens. The seagrasses are regionally significant as a fish and wildlife habitat and provide a very large proportion of the primary productivity of the Lake. At some locations these seagrasses, particularly the beds of Ruppia, are associated with macro-algae blooms and can be responsible for odours and public access issues. As a result, the recognised need to conserve the seagrasses often conflicts with the public pressure for lake “management”. There is some evidence for long-term (decadal) permanent loss in seagrass meadows from some specific sites around the Lake, some due to lake management (e.g., dredging and reclamation). However, a massive dieback in seagrasses (and many other shallow water communities) occurred during the 2003 El Nino event. A drop in lake water level, combined with severe westerly winds and winter dieback, were responsible for the loss of virtually all above-ground evidence of seagrasses in the lake. The falling lake water levels were a result of drought conditions that affected a large proportion of eastern Australia. Such climatic events can have dramatic impacts on intermittently open and closed coastal lakes. Recovery of the seagrasses in Lake Illawarra after such a large loss will depend on the overall health of the lake, particularly in respect to key environmental factors important for seagrass growth, such as water clarity.