Overview of Lakes Research at the University of Waikato, supported by Environment Bay of Plenty
Professor David Hamilton
Centre for Biodiversity and Ecology Research
Department of Biological Sciences
School of Science and Technology
The University of Waikato
5 August 2003
After one year's work our research team has made substantial progress in understanding the relative importance of processes that control water quality in the Rotorua lakes. Inputs of nutrients to lakes occur as external loads exported from lake catchments and as internal loads released from bottom sediments. Internal loads may dominate the overall nutrient load when dissolved oxygen levels are depleted from the bottom waters of a lake. Lakes have a tendency to respond in a direct cause and effect (linear) type of relationship to external nutrient loads provided there is sufficient oxygen present to avert the potential for widespread nutrient release from bottom sediments. Once bottom waters are deoxygenated, however, sediment nutrient release may trigger rapid deterioration of lake water quality, as evidenced by even greater deoxygenation of bottom waters, increased lake water nutrient concentrations and greater frequency and intensity of algal blooms.
A comparison of several Rotorua lakes in this context indicates that Lake Rotoiti has undergone the worst decline in water quality since the 1950s. Nutrient loads from the bottom sediments of Lake Rotoiti now exceed those from the catchment on an annual basis. Annual loads of nitrogen and phosphorus from the bottom sediments of Lake Rotorua also appear to be comparable to those from the catchment as a result of 3-4 periods of thermal stratification during summer, leading to deoxygenation of bottom waters and nutrient release from bottom sediments. Within the past decade, however, there has been an accelerated decline due to severe seasonal deoxygenation of bottom waters and, by implication, widespread nutrient release from bottom sediments. Lake Okaro is currently in a severe state of degradation although there were indications from bottom water oxygen levels of a previously resilient phase after its entire catchment had been converted to pasture by the 1950s. Water quality in Lake Okareka appears to have stabilised in response to the external nutrient loads, despite a relatively high percentage of agricultural development in the lake catchment. This lake and Lake Tikitapu are in a tenuous position, however, as any further deoxygenation of bottom waters is likely to lead to more widespread anoxia and potential for large increases in sediment nutrient release.
Increases in sediment nutrient release tend to be dominated by phosphorus relative to nitrogen, compared with ratios of these nutrients in the water column. Sediment nutrient release therefore tends to favour dominance of blue-green algae, particularly those with ability to ‘fix’ atmospheric nitrogen and use this nitrogen for growth requirements. Large numbers of buoyant blue-green algae may lead to a ‘surface chlorophyll maximum’, which may shade the deeper chlorophyll maximum, which is regarded as a more natural and stable state for the deeper Rotorua lakes.
Lake Tarawera is not deoxygenated substantially and is likely for the present time to respond in a linear fashion to changes in nutrient inputs from its catchment. These lakes represent the broad spectrum of water quality observed in lakes of the Rotorua region. They illustrate how lakes in general may respond in a simple cause and effect relationship to external nutrient loads while bottom waters are not deoxygenated, and how rapid deterioration in water quality may at least be partly predictable on the basis of oxygen depletion of bottom waters and consequent nutrient release.