Periphyton collectors as a tool to measure environmental performance of ocean outlets

Abstract

Brown and Root has participated in extensive investigations of the effects of the discharges from Perth's Ocean Outlets, as part of the Perth Long-Term Ocean Outlet Monitoring (PLOOM) Programme (1995 to 2001). The major environmental concern with these discharges is the potential for nutrients in the wastewater to stimulate excess primary production in the sea. PLOOM, and its predecessor, the Perth Coastal Waters Study, have been instrumental in developing parameters for the measurement of the performance of Perth's ocean outlets. These parameters are currently being integrated in the development of Environmental Quality Criteria (EQC) for the Perth region. EQC play an important role in the management framework by providing the quantitative benchmarks for measuring success in achieving the environmental quality objectives. PLOOM has monitored a range of environmental parameters in the Perth Metropolitan area, including water quality, nutrient levels, water circulation and plume dilution, levels of metals and pesticides present in the marine environment, and the environmental health of benthic communities, in particular of temperate reef systems. During the PLOOM studies, a valuable tool was being developed to monitor outlet performance. Artificial reef structures ("periphyton collectors") were placed in the plume trajectory. Here, periphyton is defined as: the microalgae (diatoms and microscopic filamentous forms), algal propagules, bacteria, microfauna and particulate material that are found in a mucous-like layer commonly coating seagrass leaves, and that initially colonise artificial surfaces. The advantage of periphyton collectors is that these largely remove the effects of natural variability, can be placed at any depth and distance from a potential nutrient source, provide an easy, cost effective measure of environmental impact, integrated over an extended period (one month), and produce tangible results which can be interpreted by the wider community, as well as legislative authorities and by outlet managers. In addition, outlet performance can be measured by means of these tools, and tested against accepted environmental criteria. Between 1995-2001, periphyton collectors, consisting of 15 x 15 cm PVC plates attached to moorings at fixed depths (2, 4 and 8 m), were deployed for one month during spring, summer and autumn, at increasing distance from the source (250 and 500 m distance to the north, east, west, and south, and at 1,000, 2,000, 4,000 m to the north and south only). After retrieval, the collectors were analysed for total biomass (g AFDW m(-2)), calcium carbonate content (% AFDW) and chlorophyll levels (chlorophyll a m(-2)). The results confirmed the predictions made by hydrodynamic modelling (e.g. Zic and Gondinoudis, 2002) and are in accordance with measured nutrient and chlorophyll a levels around the outlets, and demonstrated that the zone of influence was strongly determined by the prevailing currents (to the north), and largely restricted to surface layers (2-4 m depth). Both biomass and chlorophyll content proved reliable parameters, which have the potential to be used as Environmental Quality Criteria (EQC's) for the management of Perth's coastal waters. These EQC's were developed in collaboration with legislative authorities, as part of draft criteria, in accordance with national guidelines: ANZECC/ARMCANZ (2000) Australian and New Zealand Guidelines for Fresh and Marine Water Quality.