The ecology of the Tuggerah Lakes system. A review: with special reference to the impact of the Munmorah Power station. Stage 1- Hydrology, Aquatic macrophytes, Heavy metals, Nutrient dynamics

"July 1990".

Project Number: MDFRC Consultancy Folio Number M/04/221.

MDFRC item.

At the request of the Electricity Commission of New South Wales, the Wyong Shire Council and the State Pollution Control Commission, this review of the impact of the Munmorah Power Station on the Tuggerah lakes system on the NSW central coast has been undertaken by a multidisciplinary CSIRO team whose expertise included aquatic plants, heavy metals, nutrient dynamics and hydrology. A range of issues were specifically indentified for examination by the team. This report covers the first of two stages of the investigations; the second stage will require investigations by experts in marine biology. The Tuggerah Lakes system consists of three interconnected shallow coastal lagoons, Budgewoi Lake, Lake Munmorah and Tuggerah Lake connected to the sea by a narrow channel from Tuggerah Lake. There is limited interchange of water between the lakes and the sea through this channel. The lakes support a diversity of aquatic plants and animals and are an important aesthetic and recreational resource. Human utilisation of the catchment has progressed from logging and agriculture to residential housing, industry, recreation and tourism. The resident population has increased markedly in recent years to about 96,000 in 1989. The Munmorah Power Station was commissioned in 1967 and has a maximum generating capacity of 1400 MW. It obtains its cooling water from the uppermost lake, Lake Munmorah, and discharges it to the central lake, Budgewoi Lake; setting up an artificial flow between the two lakes which is the reverse of the natural flow. The hydrology and hydrodynamics of the lake system and the influence of the power station have been deduced from limited salinity and temperature data and water balance estimates. The lakes naturally are characterised by large temporal variations in salinity and temperature. They are well mixed, largely as a result of wind shear effects, though stratification can occur in Tuggerah Lake. Major inflows lead to significant flushing and reduced salinities. Tuggerah Lake is assessed to be virtually unchanged by the operation of the power station. Cooling water circulation has ensured that the surface water of Budgewoi Lake and all the waters of Munmorah Lake have a common composition. The cooling field plume in Budgewoi Lake induces a semi-permanent stratification above bottom water closely related to Tuggerah Lake. Heating effects due to the power station are most perceptible in Budgewoi Lake. Temperatures appear to have risen less in Lake Munmorah and Tuggerah Lake generally appears to be unaffected. However, the additional hear causes further evaporation averaging an increase of about one-third more than would naturally occur from the upper two lakes. The power station has a marginal impact on the two upper lakes with the additional evaporation being such that for the lake system as a whole, the maximum increase in salinity cannot exceed 3 ppt. Instead, local rainfall and consequent runoff remains the dominant factor in salinity variation in the lakes. This understanding of the hydrology and hydrodynamics of the system set out above has provided a valuable background to the interpretation of plant growth, nutrient and heavy metal data. The larger aquatic plants in the lakes belong to two groups: seagrasses and macroalgae. Some of the latter are not attached to the lake bottom and can be blown by the wind into large accumulations. The growth of these plants is affected by a number of environmental factors which may also interact with one another so that the situation is complex and dynamic. Various methods have been used to chart the distribution and measure the extent of these aquatic plant populations. Unfortunately the plant populations show a high degree of natural variability, both temporally and spatially. Furthermore it is a difficult and time consuming exercise to collect statistically rigorous measurements. Notwithstanding these difficulties, the data showed that seagrass populations around the near-shore perimeter of the lakes increased from the mid-1950s to the mid-1960s and have fluctuated in area and species composition since then. More recently there has been a marked increase in the amount of macroalgae in waters near the shore where they can constitute the bulk of the plant matter in some localities. There is circumstantial evidence that this process has been encouraged by enrichment of the lakes by plant nutrients in dissolved forms and in sediments from the catchment. The hydrology and morphology of the lakes also make them especially susceptible to eutrophication processes. There are insufficient data to make categorical statements regarding the effect of elevated temperatures caused by power station cooling water on the growth of seagrasses in Tuggerah Lakes and experiments designed specifically to investigate this are required. Other factors such as light penetration, salinity and sediment type affect the species composition and biomass of aquatic macrophytes but these have not been markedly changed by the construction and operation of the power station. However, a coherent, well designed monitoring program should be established with the aim of giving early warning of any undesirable effects that may occur. The nutrient dynamics of the system is closely connected with the hydrodynamics of the system. Flushing flows of freshwater following rains and tidal exchanges through The Entrance are clearly important, the former resulting in a net loss of nutrients from the system and the latter a net gain in phosphate-phosphorus. Because of the beds of submerged vegetation in shallow inshore waters, exchange between inshore and mid-lake waters is often impeded. This is important as there has been an increase in organic matter in the sediments near the lake shore which is now covered with 5-8 cm of an organic ooze (2-6% organic matter). Phosphorus in the ooze is in excess of that accounted for by known inputs including urban runoff, especially in the northern lakes. The discrepancy in the phosphorus balance is of concern and all the sources of phosphorus have yet to be identified. The flux of nutrients from the sediment is increased by anoxia and Eutrophication. It is possible that the nutrient enrichment of the inshore waters has occurred because of the deposition of ooze onto sediments which were previously white, aerobic and mesotrophic. Increased surface water temperature in Budgewoi Lake, brought about by the power station, could result indirectly in increased nutrient release rates from the sediment. Particulate ash from the power station may represent a potential source of phosphorus, but more detailed monitoring is needed to define the phosphorus budget to the lakes, and quantify all nutrient sources. An analysis of sediments revealed the presence of ash, as high as 12% in Lake Munmorah. Surface enrichments of zinc, lead, copper and antimony were also greatest in this lake and decreased with distance south. This is consistent with the elevated concentrations of these elements in precipitator ash, although bottom ash was also evident. Both dissolved and particulate sources of these elements were found and most evidence implicated the power station. The depth of the enrichments was used to estimate sedimentation rates, which in Lake Munmorah were found to be high. It was not possible to fully assess the contribution of the power station to the increased sedimentation, which may indeed have occurred in the early operation of the station, when operational constraints were less stringent. It was clear that despite enrichments of some metals in sediments there was no evidence of any excessive accumulation by seagrasses or other sediment-dwelling biota. Other studies have shown that, while seagrass species are reasonably tolerant of heavy metal contamination, many of the animals that live in seagrass communities are not. There is however no need for any concern about the dissolved metal concentrations, which are currently not enriched significantly above background, with the reservation that any small difference between the dissolved concentrations in the power station inlet and outlet waters may represent a significant mass input to the lakes, given the high water throughput. A number of additional research needs and recommendations concerning lake management have been identified.