12 J. Aquat. Plant Manage. 39: 2001. J. Aquat. Plant Manage. 39: 12-17 Managing Submersed Aquatic Plants in the Sydney International Regatta Centre, Australia D. E. ROBERTS 1, 2 , G. R. SAINTY 1 , S. P. CUMMINS 1, 3 , G. J. HUNTER 1 AND L. W. J. ANDERSON 4 ABSTRACT The Sydney International Regatta Centre is a $A40 mil- lion dollar facility located at Penrith NSW, Australia. It con- sists of two interconnected shallow lakes, with a capacity of around 3,000 megalitres. The competition lake is 2.3 km long and 170 m wide with an average depth of 5 m, while the warm-up lake has an average depth of 4 m and a convoluted shoreline. The two lakes are used for training, rowing, and canoeing events up to and during the Sydney 2000 Olympic games and form the rowing heart of Sydney. Adjoining these two lakes and drawing its water from them is the white water slalom course for the Olympics. When the rowing course was originally constructed, the lakes were planted with ribbon- weed ( Vallisneria americana var. americana Michaux), with the intent to establish a basis for a healthy aquatic plant assem- blage, which was capable of “out-competing” invasive macro- phyte species and nuisance blue-green algae. Over the past few years, the aquatic plant assemblages have flourished, with six additional native species colonizing the lakes. In March 1997, ribbonweed began to break away at its base and float to the surface. This defoliation caused major problems for management of the lakes and its primary users, as float- ing leaves interfered with rowing and swimming events. To alleviate the problem, ribbonweed was physically removed, and mechanical harvesting has been the primary manage- ment tool used to keep the plants under control. A program of monitoring was established in 1997, where spatial and temporal patterns in the distribution and abundance of sub- mersed aquatic macrophytes were measured over two years. This program found that harvesting was ineffective, in terms of keeping plant biomass to a manageable level. In this paper we report the results of the monitoring program and present some preliminary data on a management experiment, which used the herbicide fluridone to help reduce plant growth in conjunction with physical and mechanical removal. Key Words: macrophyte, artificial lake, Potamogeton spp., Vallisneria americana , Hydrilla verticillata , harvesting, herbi- cide, fluridone. 1 Sainty and Associates, P.O. Box 1219, Potts Point, NSW Australia, 2011. 2 Wyong Shire Council, P.O. Box 20, Wyong, NSW Australia, 2259. E-mail: Dannio@bigpond.com.au. 3 Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories A11, University of Sydney, NSW Australia, 2006. 4 Exotic and Invasive Weed Research Unit, U.S. Department of Agricul- ture, UC Davis, 1 Shields Ave., Davis CA 95616. Received for publication July 25, 2000 and in revised form October 29, 2001. INTRODUCTION Submersed and emergent plants are essential components of aquatic ecosystems and play vital structural and functional roles within these habitats (Westlake 1975). Eutrophication of waterways and invasion by exotic species can lead to exces- sive growth of macrophytes (Roberts et al. 1999). Major problems associated with the growth of nuisance aquatic weeds have occurred to lakes and rivers throughout the world and are generally related to conflicts over the intend- ed use or perceived function of a body of water (Clayton 1996). To be able to manage aquatic plants effectively within any body of water, a fundamental understanding of the na- ture of scale and the patterns and processes operating within the system are essential (Farmer and Adams 1991). The Sydney International Regatta Centre (SIRC) near Penrith, Australia (Figure 1) was constructed as part of the Penrith Lakes Scheme (Carter et al. 1994). The competition lake is 2.3 km long and 170 m wide with an average depth of 5 m. The substratum at the edge of the rowing lake consists of a 5-10 cm diameter gravel bed overlaying clay, while the deeper parts of the lake are shale with a silt covering. The warm-up lake has an average depth of 4 m and a convoluted shoreline. The two lakes are used for water sports and will be used during the Sydney 2000 games. When the SIRC was originally constructed, the lakes were planted with ribbon- weed ( Vallisneria americana var. americana Michaux) (Jacobs and Frank 1997) in an effort to alleviate the potential prob- lems caused by algae and exotic macrophytes 5 , and to estab- lish a “balanced aquatic ecosystem” with a healthy aquatic plant assemblage, capable of out-competing nuisance algae and “exotic” macrophytes (McCreary 1991). Over the past few years, the aquatic plant assemblages in the lakes have flourished, which is not surprising given the shallow depth and clear water. In March 1997, ribbonweed began to break away at its base and float to the surface 5 . The actual cause of the defoliation was never established but was probably due to rising water levels, increased turbidity, strati- fication and low dissolved oxygen levels 6 . Further investiga- tions indicated that blue-green algae ( Oscillatoria sp.) growing at the base of the ribbonweed could be associated with the detachment 6 . This defoliation caused major prob- lems for management of the lakes and its primary users, as 5 AWT. 1997. An assessment of the current status of Penrith Lakes macro- phytes. AWT EnSight report No. 97/099 for Penrith Lakes Development Corporation Ltd. 6 Sainty, G. R. 1997. Sydney International Regatta Centre: submerged plants—rowing and warm-up lake. Report to the Olympic Co-ordination Authority. Sainty and Associates, Sydney.