Ecosystem Health

Before we begin to assess the differences and similarities of ecosystems at various levels of health, it is important to understand, firstly, what is meant by ecosystem health, and secondly, what causes a change in ecosystem health. Ecosystem ‘health’ is not judged by the degree of naturalness of a system, but rather on a system’s ability to maintain and renew itself (Bertollo 1998). Ecosystem health has been defined as including the following: homeostasis, the absence of disease, diversity or complexity, stability or resilience to stress, vigor or scope for growth and balance between system components (Costanza 1992). It is necessary to consider all or at least a majority of these aspects simultaneously in order to achieve a viable understanding

While trophic state refers purely to the amount of nutrients and subsequent plant growth in a system, each lake has a unique collection of attributes that contribute to its trophic status (Carlson 1977). The three main factors which control the trophic state of a lake includes, firstly, the rate of nutrient supply which is controlled by the bedrock geology of the watershed, type of soil, vegetation type and amount, and human landuses and management. The second factor is climate, which includes the amount of sunlight, temperature and hydrology (precipitation and lake basin turnover time). And the third and last main factor is morphometry, or lake basin shape. This includes maximum and mean depth, volume and surface area, and watershed to lake surface area ratio (Carlson

1998, Hudon et al. 2011). Mesotrophic levels of nutrients help to sustain intermediate zooplankton biomass and consequently maximum primary productivity (Hudon et al. 2011). Productivity at a given trophic level is maximised at an intermediate biomass of the trophic level above. Productivity at all trophic levels, and energy flow through the food wed, are also highest when predation pressure is intermediate at all trophic levels (Carpenter et al. 1985). When zooplankton biomass is high, productivity is restrained by grazing. While at low levels of zooplankton biomass, productivity by phytoplankton is limited by recycling (Carpenter et al.