Victorian index of estuary condition: Implementation trial final report

To improve the management of Victoria’s estuaries there is a need for a consistent method for assessing their environmental condition to optimise resource allocation for threat mitigation and asset protection. The Index of Stream Condition (ISC) has been used for decades in flowing freshwater systems for this purpose but is not suitable for estuaries. Arundel et al. (2009), after extensive review, consultation and expert panel workshops, recommended a Victorian Index of Estuary Condition (IEC), using the approach of the ISC, based on existing knowledge and identified conceptual links between estuary assets, threats and ecological responses. The recommended IEC included six themes (physical form, hydrology, water quality, sediment, flora, and fauna) that represent different aspects of ecological condition. A range of measures within these themes were identified to ensure all aspects of ecological condition were comprehensively represented, and in total 18 measures were recommended. Based on existing knowledge these were thought to have links with current condition, changes to condition and to potential manageable threats. For estuaries, the main threats to estuarine condition are likely to be catchment land-use patterns, flow regime, urban and coastal development around the estuary, recreational and commercial use, climate change, pest species and estuary entrance management. The recommended IEC uses a referential approach to assess estuary condition, however the high degree of existing disturbance in Victoria’s estuaries and lack of pre-disturbance data will mean that reference or baseline equates often to best available. The Department of Sustainability and Environment (now Environment, Land, Water and Planning, DELWP) funded a major recommendation from Arundel et al. (2009) to trial the implementation of the recommended IEC methods. Melbourne Water contributed to additional trialling in its jurisdiction. This report presents the results of the IEC implementation trial with its three objectives of data derivation, baseline condition development, and assessment and refinement of the final measures. Data derivation focused on refining sampling protocols, including spatial and temporal replication, collecting and collating data, and refining data recording methods. Baseline condition development included calibrating the scoring ranges to establish the scoring tables which calibrate the condition of each of the measures into five condition bands. Assessment and refinement of the final measures involved developing and refining aggregation methods to calculate condition scores and index scores. In doing this the trial assesses the proposed sampling methodologies from Arundel et al. (2009) and whether the methods were applicable state-wide. It assesses and delineates zonation and subestuaries for practicality and efficiency of sampling. It also establishes or confirms score baselines, scoring methods and score confidence metrics for each measure using existing and new data. The implementation trial represents over four years of effort in collating and interpreting existing data, three intensive summer field sampling seasons across the Victorian coastline, and a number of measure-specific pilot studies. The majority of the trial was conducted by researchers at Deakin University, with the fish and bird measures in the fauna theme developed and evaluated by two research teams from the Arthur Rylah Institute. The results of the faunal research are summarised in this report. This report presents the approaches used in determining baselines, scoring and score confidence for 16 measures in five themes (physical form, hydrology, water quality, sediment and flora). It also presents the approach used to aggregate measure scores within and between all six themes, including fauna (fish), and presents some insights into the current condition of Victorian estuaries. Score confidence is an important addition to the IEC which is not currently part of the ISC. It allows the spatial and temporal suitability of the data used to score a measure to be assessed as either high, moderate or low confidence. This allows users of the IEC to interpret the reliability of the score and relative risk of making decisions based on it. At the beginning of the implementation trial the Deakin team visited and interviewed staff at the coastal Catchment Management Authorities (CMA’s) and Melbourne Water to discuss the data needs of the trial, and to identify and locate any suitable existing data. The information gathered through this process is included as an appendix in this report. Unfortunately but not unexpectedly, there were little existing suitable data across multiple estuaries let alone across the entire coast. One of the best sources of multi-estuary data was from the now discontinued Glenelg Hopkins CMA Estuary Water Quality Monitoring Program, where the water column was sampled in multiple sites in five estuaries monthly for seven years. In addition, estuary water depth was recorded with depth loggers in these estuaries from 2005, adding considerable value to the staff and volunteer mouth-state observations. Melbourne Water also sampled surface waters within six estuaries. Estuary mouth-state observations from volunteers in the EstuaryWatch program in the Corangamite CMA were also useful along with their water quality monitoring in four estuaries. For the implementation trial one hundred and one estuary mouths, current and natural heads, and fluvial and estuarine catchments have been spatially defined. Field sampling in 55 estuaries, and their subestuaries, allowed the definition of the upper, middle and lower estuarine zones, as well as the collection of depositional sediment size data and water depths. It also allowed the comparison of water clarity in 55; surface dissolved oxygen in 53; diurnal oxygen sags in 45; bank erosion in 48; microphytobenthos biomass in 41; and phytoplankton biomass in 44 estuaries. Data collected and collated as part of the trial allowed the comparison of modification of estuary extent in 67, modified freshwater flow in 101, sediment load in 56, and marine exchange in 85 estuaries. Two workshops were held during the trial to help identify baseline conditions and scoring approaches. Baseline condition, also known as reference condition, is a conceptual model of how an estuary should be under realistic minimal human activity. Numerous ways were used to define baselines which reflected the level of knowledge or availability of data for a particular measure. In order of decreasing levels of knowledge or data, baselines were derived from: natural condition (pre-European); historical data; current state-wide data (best available); current individual estuary data; modelling of past conditions; and expert opinion. Identifying a baseline condition of each measure allowed condition bands or score thresholds to be identified or derived. This was done based on available data and known conceptual threat/impact models or ecological thresholds from the literature. An individual measure is scored based on deviation from the baseline into five categorical condition bands (5 = excellent, 4 = good, 3 = moderate, 2 = poor and 1 = very poor). Thirteen measures across the six themes have been recommended for the first formal IEC program. These measures are Sediment Load, Upstream Barriers and Lateral Connectivity in the Physical Form theme with Changed Bathymetry not being recommended for the first formal IEC as it needs substantial development. Marine Exchange and Freshwater Flow modification measures are recommended for the Hydrology theme. There needs to be substantial development before there is a viable method to measure modification of the Salinity Regime throughout an estuary even though this is seen as a fundamental measure for the IEC. Both Water Clarity and Dissolved Oxygen measures were recommended for the Water Quality theme and their scoring informed by the Riverine Estuary Objectives developed by EPA. Sediment Particle Size and Bank Erosion were recommended for the Sediment theme although both need some further work. The Sediment Respiration Rate measure was trialled in four estuaries but not recommended as suitable for the IEC due to its high cost and level of expertise needed. Aquatic plants extent change and blooms are recommended together as two parts of an Aquatic Plants measure, Fringing Macrophyte extent and condition as another and Phytoplankton biomass as the final measure in the Flora theme. Microphytobenthos biomass was not recommended for the first formal IEC as substantial more work needs to be done to establish baselines and relevant scoring. The temporal requirements for undertaking data collection or derivation for a formal IEC assessment are not arduous for most measures. Ten of the recommended measures need to be sampled or derived once every eight year assessment period. One additional measure, Aquatic Macrophytes, needs to be sampled in the late summer-early autumn twice in the eight year assessment period. It is suggested that macroalgae component of this measure could be sampled at the same time but this needs more development. Three measures need to be sampled regularly throughout the entire IEC assessment period. Both measures in the water quality theme, Turbidity and Dissolved Oxygen need to be sampled monthly at multiple sites within the estuary. This is also the case for the Phytoplankton biomass in the flora theme. One measure from the hydrology theme, Marine Exchange a) mouth openings needs to be monitored continually and depth loggers have been recommended. The other part of the Marine Exchange measure, b) structures and behaviours, in permanently open estuaries requires recording dredge volume and frequency. It is recommended that the data should be obtained through the authority that commissions the dredging through a formal arrangement. The spatial sampling requirement within an estua