The Effects of Sewage Discharges on the Ecological Processes involving Marine Flora in Southern Australian Waters
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
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Report no. UWRAA 140
October 1998
Synopsis
The productivity of nearshore marine environments depends on the growth of macroalgae and seagrasses which are dominant components of marine systems. Nutrient enrichment is an important problem in many of these systems especially when a limiting nutrient is supplied (whether through natural or anthropogenic influences) to such an extent that excessive growth of opportunistic algal species occurs. This can lead to eutrophication of the receiving environment and a reduced abundance and diversity of other marine plant an animal species. Nutrient loads from sewage outfalls into nearshore marine environments can promote the growth of opportunistic nuisance macroalgae. This report investigated the relationship between nutrient availability and growth for three species of fast growing macroalgae, Hincksiasordida Clayton, Polysiphonia decipiens Montagne and Ulva sp. All are common to southern Australian marine waters. It also examined the toxic effects of sewage effluent on the germination and growth of early life stages of the common kelp species Eckloniaradiata (C. Agardh) J. Agardh.
A study on the photosynthetic characteristics (measured via an oxygen electrode)of each species over an annual cycle showed that Hincksia sordida and Ulvasp. exhibited 2 to 3 fold higher rates of photosynthesis than Polysiphonia decipiens. This corresponds to observed higher growth rates of these taxa in the field. Nutrient availability limited photosynthetic performance of Hincksia sordida. Conversely, limiting factors on the photosynthetic performance of the other two species appeared to be light and temperature. Temperature is most likely the critical factor determining seasonal changes in photosynthesis in Ulvasp. and Polysiphonia decipiens.
Each of the species examined were found to have the capacity to increase its tissue nitrogen reserves without concomitant increase in growth, suggesting that nutrients may be stored in tissue for later use when supply is low. The relationship between growth and tissue nutrient storage capacity provided afunctional explanation to observed growth strategies of species and provided information on the critical tissue nitrogen threshold limits for growth.
The effect of primary and secondary treated sewage effluents on reproductive phases of the life cycle of the marine macrophyte (kelp) Ecklonia radiata were also investigated. Ecklonia radiata is a major primary producer in nearshore medium to high energy habitats and as a benthic organism is subject to long term chronic exposure to coastal effluent. Inhibition of germination and reduction in growth rate of gametophytes (early microscopic life stage) are used as end points in bioassays. For primary treated effluent germination, inhibition was significantly inhibited at less than 1% effluent while growth responses indicated significant inhibition at 4%, and enhanced growth at 2%. Responses to secondary treated effluent indicated that for both assay forms the principal effect was due to reduced salinity. For one of the secondary treated effluent forms there was no significant difference in response to salinity reduction employing both assays. For the other type of secondary effluent, toxicity was expressed at 40% to 60% effluent. The primary effect of reduced salinity appears to take effect below 25 PPS. Both assays offer great potential for the routine screening of effluent quality. Both assays are simple to conduct, show high reproducibility and are ecologically relevant.
Productivity and kinetic uptake data developed for each species can be used as input data to the COASEC model developed to predict the effects of wastewater discharges on nearshore marine environments This data complements information on water column processes (water borne concentrations of phytoplankton, particulate organics and dissolved inorganic nitrogen) for particular discharges that is also required as input data for the COASEC model. This model may provide Water Authorities with environmental threshold limits for nutrient concentrations above which the growth of opportunistic macroalgae is promoted. This information can be utilised by local Water Authorities to predict the effects of their coastal discharges on the growth of opportunistic species that lead to eutrophication.
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