LAKE MODEWARRE WATER QUALITY 2004–05 Publication 1018 December 2005 1. SUMMARY EPA has found the lake continues to support an abundance of water bugs and bird life. Given EPA Victoria has investigated the water quality of Lake sufficient rainfall, Lake Modewarre should recover Modewarre following reports of two large fish kills and be able to support freshwater fish in the future. (carp and eels) in the summer of 2004–05. Eels in healthy condition were caught by EPA did not find evidence of fish disease or pollution in recreational anglers in Lake Modewarre in late 2005. Lake Modewarre and has concluded that water quality EPA will continue to monitor the health of Lake in Lake Modewarre has been affected by an extended Modewarre along with a number of lakes it is period of below-average rainfall. monitoring in the Western District. The results will Low rainfall and changes in the catchment have resulted contribute to the establishment of ecological in low water levels, rising salinity and high summer guidelines for assessing the environmental quality water temperatures. The combined effects of these of Victoria’s lakes. drought-related impacts appear to have made the lake 2. LAKE MODEWARRE unfavourable to some aquatic life and are the most likely cause of the 2004–05 carp deaths in the lake. Lake Modewarre is situated 30 km south-west of Geelong. It is a relatively shallow crater lake: in 2005 The eel deaths may also have been related to drought, the lake was around two metres deep. However, it or to eel deaths seen in other waterways (see EPA has reportedly varied between being 11 metres deep Publication 1013, Eel Deaths in Victoria, 2004–05). and completely dry. Low water level at Lake Modewarre pier, August 2005 1 LAKE MODEWARRE WATER QUALITY 2004–05 Lake Modewarre has been home to a range of fish species and has been an important recreational fishery. It has been stocked with brown and rainbow trout since 1953. Since 1986 Lake Modewarre has been annually stocked with around 5,000 rainbow and 5,000 brown trout. DPI Fisheries Victoria report that the lake has had a history of marginal conditions and poor angling during drought periods. Carp lining the shores of Lake Modewarre in Like many lakes in the Western District, Lake December 2004 (photo: Brian Gane, landholder) Modewarre is saline (with a salt concentration EPA has investigated the following water quality currently around one-third that of seawater) and parameters in Lake Modewarre: nutrient-rich. It has a relatively small catchment area, with agricultural and rural residential land uses. • drought-related impacts (rising salinity and high Agricultural activities include cattle grazing and water temperature) cropping. • algae and aquatic plant growth (nutrients, algae There is limited historical information on water quality and dissolved oxygen) in the lake. EPA has made use of studies by Pollard • fish health (including bacteria and viruses) (1971), Conder (1998), Corangamite Catchment • ecological condition (aquatic Management Authority (2003), Barwon Water/Surf macroinvertebrates [waterbugs] and birds) Coast Shire (unpublished) and Barr (unpublished) for this investigation. • possible pollutants (pesticides and metals). EPA’s investigations into the water quality of Lake 3. INVESTIGATION Modewarre following the fish kills have revealed an aquatic system under stress from a number of The number of fish caught in Lake Modewarre drought-related impacts. reportedly declined in November 2004. EPA received Trout may also have been affected, though reports of the fish kills in December 2004 (carp) and significant trout mortalities were not reported to January 2005 (eels). It has been estimated that some EPA. A series of fish surveys by DPI Fisheries Victoria 30,000 carp and 5,000 eels died in the lake during the did not find any trout, despite the lake being summer of 2004–05. There were five weeks between stocked with around 19,000 in 2004. the two reports. There is a chance that there was some connection between the events. Levels of salinity at Lake Modewarre had increased to nearly one-third that of seawater in December 2004, a level which has been shown to be harmful EPA Victoria 2 LAKE MODEWARRE WATER QUALITY 2004–05 to carp1. EPA has recorded large carp kills in other Salinity Victorian waterways at salinity levels similar to that Falling water levels affect the salinity of Lake recorded at Lake Modewarre. Other factors in the lake Modewarre because, as the water evaporates, the (fluctuating dissolved oxygen and high pH) may also salt becomes more concentrated. have contributed to the deaths. When rainfall is low, groundwater plays an It is not clear whether eels were affected by drought increasingly important role in the water level and conditions, or whether the deaths were related to the salinity of the lake2. According to a 2003 technical other eel deaths seen through much of south-eastern report by Corangamite Catchment Management Victoria and NSW. Authority, the groundwater level at Lake Modewarre is very close to the surface and is naturally saline 4. FINDINGS (one-fifth that of seawater)3. 4.1 DROUGHT-RELATED IMPACTS Table 1: Salinity and water temperature in Lake Modewarre Water levels Salinity in parts per Water temp. Date Rainfall in much of the State has been below average thousand (ppt) (°C) over the last ten years. Rainfall in the Lake Modewarre 4 Feb 19642 4.8 25 area has been very low, with annual rainfalls in the 2 Dec 19642 3.1 18 lowest 10 per cent of records. As a result, the levels of 8 Aug 19974 4.1 9 many lakes in the Western District have fallen during 13 Mar 19984 8.2 20 this period. 7 Aug 19984 4.9 9 14 Dec 19995 * 6.5 Not recorded At Lake Modewarre this is demonstrated by the boat 19 Dec 20005 * 7.6 Not recorded ramp and jetty, which are now some distance from the 3 Dec 20015 * 5.6 Not recorded water’s edge (see picture on page 1). 10 Dec 20025 * 6.4 Not recorded The falling water levels in Lake Modewarre may also 14 Dec 20036 8.8 19 have been influenced by changes in the catchment 22 Dec 2004* 10.7 25 affecting water run-off. Below-average rainfalls and 31 Jan 2005 11.5 22–23 falling water levels in Lake Modewarre have caused 26 May 2005 12.4 12 11 Aug 2005 12.2 7 rising salinity levels and high summer water 24 Nov 2005* 12.8 19 temperatures (see Table 1). In addition, low water Seawater 35 − levels are known to result in a higher concentration of Groundwater near 7 − nutrients and heavy metal pollutants in water because Lake Modewarre3 of greater wind disturbance of bottom sediments (see * Readings taken at lake edge sections 4.2 and 4.3). EPA has recorded high salinity (up to 13 parts per thousand or ppt) at Lake Modewarre (see Table 1). Information Bulletin 3 LAKE MODEWARRE WATER QUALITY 2004–05 Although hardy fish, carp are not tolerant of high EPA collected algae samples from the lake (see salinity. Researchers have shown that a salinity of Table 2) and the results are discussed below. between 9 and 17 ppt can kill them1. Eels and trout are Green algae more tolerant of high salinity. Green algae are a normal component of nutrient-rich Water temperature lakes, particularly during summer, when they often bloom with high water temperature and nutrient One effect of falling water levels is an increase in water levels. temperatures during summer, as the smaller volume in the lake heats up more readily. EPA recorded an increase in green algae numbers from 22 December 2004 (750 cells/mL) to 8 February EPA recorded the water temperature in the centre of 2005 (25,500 cells/mL). Such an increase is not Lake Modewarre in January 2005 at 22–23 °C (see unusual in summer. Table 1), a temperature level unfavourable to trout. High water temperatures, in conjunction with excess Blue-green algae nutrients and calm conditions, can also lead to blooms Like green algae, blue-green algae can be present in of algae and aquatic plants. large numbers in nutrient-rich lakes. Many blue- green algae do not produce toxins. Moderately high 4.2. ALGAE AND AQUATIC PLANT concentrations of a non-toxic blue-green algae were GROWTH recorded in one sample in February 2005. Although a normal part of lake ecology, some blue- Nutrients green algae can lead to fish deaths through the EPA recorded high nutrient levels in Lake Modewarre, production of toxins. Levels of these varieties of which could lead to excess plant growth. Phosphorus blue-green algae were low when compared to levels recorded by EPA in May 2005 (0.4–0.9 mg/L) Department of Sustainability and Environment have increased since 1997–98 (<0.1 mg/L). guidelines. While high nutrient levels are a common characteristic Golden algae of Western District lakes, the elevated nutrient levels in Golden algae have been shown to lead to fish the water are probably due to a combination of land deaths through the production of toxins. Golden use in the catchment, the falling water level and algae were not found in any samples. increased mixing of lake water and bottom sediments. Dissolved oxygen Algae and aquatic plant growth In warmer weather, growth of aquatic plants and While no algal blooms were reported at Lake algae can cause large fluctuations in dissolved Modewarre during the summer of 2004–05, large algal oxygen levels in water, leading to there being too blooms have previously been recorded in the lake. little or too much oxygen for fish to survive.