TUGGERAH LAKES ESTUARY PROCESS STUDY I I February 2000 WYONG SHIRE COUNCIL I I WYONG SHIRE COUNCIL • TUGGERAH LAKES ESTUARY PROCESS STUDY I Prepared by D. E. Roberts Marine and Estuarine Ecologist I Strategic Planning Department Wyong Shire Council, February 2000 I Contributing Authors I Section Author(s) Catchment and Landuse S Witkowski, T Dickinson (Wyong Shire Council) I Dlstnbutlon of Sediments T Dickinson (Wyong Shire Council) Sediment Dynamics T Dlckrnson (Wyong Shire Council) I HydrodynamiCS D van Senden (Manly Hydraulics Laboratory) Flood Behaviour T Wallace (Wyong Shire Council) I Catchment Water Quality M Lincoln Smith (The Ecology Lab) Estuanne Water Quality S Cummrns (Wyong Shire CounCil) Groundwater B Kerry (University of Newcastle) I Recreational Water Quality R McKenzie (Wyong Shire CounCil) Nutnent Fluxes S Bourgues, M Carpenter, G Coade, P Scanes, K Koop (EPA) I Nutnent Budget F Garofalow (Wyong Shire CounCil) Fnnglng Wetlands G Salnty (Sarnty & Associates) I Macroalgae S Cummins, H Daley (Wyong Shire CounCil) Phytoplankton S Cummins (Wyong Shire CounCil) I Terrestnal Macrofauna C Wallbndge (University of Newcastle) Birds P Mackey (AquatiC Ecological Investigations) BenthiC Invertebrates S Cummins (Wyong Shire CounCil) I Zooplankton A Redden, K Blacklock (University of Newcastle) Scope of Management Study G Walkerden (Wyong Shire Council) I Weed Harvester K Casey (University of Newcastle) I I I I 2 I EXECUTIVE SUMMARY The Tuggerah Lakes are a series of three interconnected shallow coastal barrier lagoons that are open to the sea at The Entrance. An estuary process study was done as part of the NSW state governments estuary management program. The estuary process study was to describe the physical, chemical and biological patterns and processes operating within the estuary and identify management issues that would be the focus of the subsequent I management study. In the past, community concerns about deteriorating environmental quality within the estuary have led to scientific studies and a number of management I actions. Many of the studies were not done at appropriate spatial and/or temporal scales and were generally focused on issues surrounding the operation of the Munmorah Power I Station and its effects on the estuary. The Tuggerah Lakes restoration program was a direct result of community and political pressure to do something about eutrophication within the I estuary. The results of that particular management action have and continue to be questioned by the community and technical experts from various disciplines. he akes I restoration program may be considered a bandaid solution" because it only treated the syrn~oms and not the ac ual cau e of the roblem. hl I VJ f, (h " J (~ The Tuggerah Lakes Estuary Process Study was to identify data gaps and key estuarine I' processes so that we had a clear understanding of how the estuary actually "worked". All the relevant scientific studies that had been done were examined so that a generalised I description of the processes within the estuary could be used to assist with managing environmental issues. It became clear early in the study that many of the previous works were inadequate for describing estuary processes because appropriate spatial and or I temporal scales were never examined. To help overcome this, a number of studies were done aimed at describing the estuary in terms of its important physical, chemical and I biological components. Many' of these studies are ongoing and are funded by Wyong Shire Council independent of external funding (e.g. water quality, macroalgae and phytoplankton I monitoring). These programs will also be reported in annual state of the environment and technical reports and in the relevant scientific literature. I The estuary was formed some 5,000 years ago when sea levels rose after the last ice age. I Most of the geomorphological features of the estuary are relic i.e. they are no longer active except for the river deltas of Wyong and Ourimbah Creeks and the tidal delta at The I Entrance. Sedimentary processes within the estuary were found to be slow, with no evidence for depth changes since the early bathymetry studies in the 1970's. There were I however small-scale changes with some places becoming shallower around inflows whereas other places had become deeper due to the effects of mine subsidence. The Tuggerah I I I 3 estuary is one of the slowest infilling estuaries along the NSW coast and at current rates I would take over 1000 years to completely fill. The flushing and mixing characteristics of the estuary were examined and tidal flushing contributes very little to its circulation and mixing I patterns. Overall the bottom sediments within the estuary were relatively "healthy" apart from some small-scale problems in some urbanised areas. Initial investigations on pollutants I within the sediments indicated very low levels of pesticides whilst some heavy metals were found in Lake Budgewoi although these levels were well below levels found to cause adverse ecological effects. The sediments within the estuary had high nutrient I concentrations and studies on sediment nutrient fluxes indicated positive flux rates. Ambient nutrient concentrations within the water column were found to be above the ANZECC I guidelines and the estuary could be classed as mesotrophic ·.e meaium nutrient status}. I Phytoplankton populations within the estuary were low with occasional blooms whilst macroalgae assemblages experience periodic small-scale blooms (generally around I developed foreshores). Macroalgae blooms during the 1980's and early 1990's were consistent with the estuary being eutrophic. With projected increased population pressure in I the Wyong catchment these macroalgae blooms could return if appropriate management action is not taken. One of the major ecological issues in the estuary has been the I defragmentation and loss of up to 85% of fringing saltmarsh and land vegetation. Saltmarsh and fringing wetlands play an important role in nutrient recycling processes and I their removal has probably altered the way seagrass and macroalgae wrack are recycled through the estuary. Ecologically sustainable foreshore management will need to be I addressed as part of any management plan for the estuary. Contrary to public opinion there has been a significan deer a e en 0 seagrasses within tne e uary by at least 50 Yo since the 1960's, probably due to increased turbidity within the lakes and by physical I disturbance. Descriptions of ecologically sensitive habitats that will need to be considered in I the management plan are also included. I I I I I I 4 ! Estuary management Issues were Identified In conjunction with the Tuggerah Lakes Estuary Management Subcommittee workshops and through community consultation The following I key Issues will require further exploration In the estuary management study I • Ecologically sustainable management of estuarine beaches and foreshores 'I • Protection of ecologically sensitive habitats, eg Budgewol Sandmass and Tuggerah Bay I Managing potentially elevated nutrient and sediment loads to the estuary from ,I Increased urbanisation and development In the catchments Identification and management of those nutrients responsible for excessive aquatic I plant growth ,I Implications of periodiC dredging of rivers and the tidal delta at The Entrance I • Practical and ecological Implications of a second entrance and/or break walls to alter eXisting estuarine mixing and flushing 1,1 • The management of both recreational and commercial fisheries I· • Potential ecological effects of mine subsidence ·1 • The feaSibility of uSing "Blolndlcators" to quantify whether management targets for I the estuary are attained I , I I I I I I I I 5 CONTENTS I 1. INTRODUCTION .................................................................................................................................. 7 1 1 THE TuGGERAH LAKES ESTUARY 7 ,I 1 2 CATCHMENT ANDLANDUSE 7 13 FRAMEWORK FOR THE ESTUARY MANAGEMENT PLAN 10 14 ADAPTIVE ENVIRONMENTAL ASSESSMENT AND MANAGEMENT 11 1 5 PEER REVIEW AND COMMUNITY CONSULTATION 11 I 1 6 LITERATURE AND DATA REVIEW 11 1 7 OBJECTIVES OF THE PROCESS STUDY 12 1 8 SCOPE OF THE PROCESS STUDY 12 I 1 9 PATTERNS AND PROCESSES IN THE ECOLOGY OF ESTUARIES 13 2. PHYSICAL PATTERNS AND PROCESSES ........................................................................................ 15 2 1 INTRODUCTION 15 I 22 SEDIMENTARY PROCESSES 16 221 The Geomorphology ofthe Tuggerah Lakes Estuary 17 222 DIstributIOn ofSedIments m the Tuggerah Lakes Estuary 24 I 223 SedIment DynamIcs wlthm the Tuggerah Lakes Estuary 30 23 HYDRODYNAMICS 35 231 The Clrculallon of Water m the Estuary 35 232 MIXmg wlthm the Estuary 38 I 233 Flushmg of the Tuggerah Lakes Estuary 39 234 The BehavIOur ofFloods m the Estuary 41 I 3 CHEMICAL PROCESSES AND INTERACTIONS .............................................•.•..................... .44 3 1 WATER QUALITY 44 311 Summary of Catchment Water Qualzty 46 312 Summary of Water Qualzty m the Estuary (1963-1997) 47 I 313 PhYSIco-chemIcal Water Qualzty Program 50 314 Water Qualzty DynamIcs 59 315 The Flow ofNutrients through the Estuary 71 I 316 Groundwater ContrzbutlOns to the Estuary 74 317 RecreatIOnal Water Qualzty Momtormg 76 32 CHEMICAL INTERACTIONS WITH ESTUARINE SEDIMENTS 79 321 Nutrlents m SedIments 79 I 322 SedIment-water Fluxes of Oxygen and Nutrlents 84 323 Heavy Metals and Organochlorznes m SedIments 84 324 Potentzal ACId Sulfate SOIls 88 I 4. PATTERNS IN THE BIOLOGICAL ASSEMBLAGES .........................................................................