Part 3 - Results of Sampling Program to Address Critical Knowledge Gaps identified in Assessing the Environmental Condition of Lake Mulwala

"September 2005".

Project Number: Lake Mulwala Water Quality Study.

MDFRC item.

Part 3 of 4 part report (see Executive summary: Lake Mulwala Water quality study), (Part 1: A review of the existing studies on Lake Mulwala and its catchment), (Part 2: Sampling program to address critical knowledge gaps in Lake Mulwala),and (Part 4: Developing a monitoring strategy for Lake Mulwala).

Following a review of existing studies on Lake Mulwala and its catchment (Howitt et al. 2004) five key knowledge gaps were identified: sediment quality, whether or not the Lake undergoes thermal stratification, the distribution of algal blooms in the lake, the condition of vegetation in the lake and surrounding riparian zones and estimates of the loads of nutrients entering and leaving the lake. A preliminary investigation into the first 4 of these knowledge gaps was undertaken from November 2004 – June 2005. During the study period the lake did not experience a large algal bloom so it was not possible to determine the factors affecting the spatial distribution of blooms in the lake. This document reports on outcomes of studies into the other three knowledge gaps and will be used to help design a comprehensive monitoring program for Lake Mulwala. Sediment Quality An assessment of organochlorine pesticides, organophosphorus pesticides, polychlorinated biphenyls, metal/metalloids (As, Cd, Cu, Pb, Hg, Se and Zn) and hydrocarbon contamination in Lake Mulwala was undertaken. The purposes of the project were firstly to determine the extent of contamination in the sediments (gauged against Australian and international sediment quality guidelines) and secondly, attempt to identify sources of contamination to the Lake. The levels of organochlorine pesticides, organophosphorus pesticides, polychlorinated biphenyls were beneath the detection limit of the analytical technique used., With the exception of a few samples containing slightly elevated levels of As and/or Hg, the levels of all metals in the lake were below the Australian Sediment Quality Guidelines lower trigger level for further investigation for most contaminants. High molecular weight hydrocarbons (up to 700 mg/kg) were found in most sediments. Non-metric statistical analysis clearly showed that the distribution of contaminants were different in different parts of the Lake, with lowest concentrations generally found at the influent to the lake. While no definitive source of contamination could be identified, we argue that the distribution of contaminants is consistent with most contamination coming from upstream sources. Thermal Stratification A thermistor chain consisting of 10 tiny-tag thermistors was deployed at the deepest part of the Lake for mid-January to mid-March. Thermal stratification occurred during deployment for periods up to 10 days at a time. The implications for thermal stratification on the biogeochemistry of the Lake, particularly the cycling of nutrients, are discussed. Vegetation Distribution and Condition A program for monitoring vegetation associated with Lake Mulwala was developed based on the methods of Baldwin et al (2004). This report presents the fist (base line) assessment to characterise the riparian and aquatic vegetation communities present in and around the lake in order to quantify changes in the distribution, abundance and condition of vegetation over time. Apart for the ‘Everglades’, the distribution of native riparian vegetation surrounding the lake is quite poor. Similarly, appreciable communities of submerged aquatic vegetation were only recorded in the Everglades region of the Lake It was also noted that canopy condition (based on in-situ chlorophyll fluorescence) of the riparian was consistent with the macrophytes being stressed. Although the quality of vegetation associated with Lake Mulwala is generally poor, the eastern end, and in particular the extensive system of islands within the Everglades, represents an area of high ecological value requiring conservation. Further clearing of native vegetation in this region should be strictly controlled. It is also important to protect the in-channel vegetation from detrimental impacts from surrounding land-use by improving riparian vegetation in this region. Generally revegetation of native plants within the riparian zone and lake edge of the whole lake is required; however it would be most sensible and effective to focus on the Everglades and Central Lake regions. One of the major improvements to be recommended is to increase the width of selected riparian vegetation patches – it is preferable to progress via a series of connected blocks of vegetation than attempt to increase the riparian width around the whole lake (e.g. with a double row of trees).