TheEffects of Sewage Discharges on the Ecological Processes involving
MarineFlora in Southern Australian Waters
ReportNo WSAA 140
Theproductivity of nearshore marine environments depends on the growth ofmacroalgae and seagrasses which are dominant components of marine systems.Nutrient enrichment is an important problem in many of these systems especiallywhen a limiting nutrient is supplied (whether through natural or anthropogenicinfluences) to such an extent that excessive growth of opportunistic algalspecies occurs. This can lead to eutrophication of the receiving environmentand a reduced abundance and diversity of other marine plant an animal species.Nutrient loads from sewage outfalls into nearshore marine environments canpromote the growth of opportunistic nuisance macroalgae. This reportinvestigated the relationship between nutrient availability and growth forthree species of fast growing macroalgae, Hincksiasordida Clayton, Polysiphoniadecipiens Montagne and Ulva sp.All are common to southern Australian marine waters. It also examined the toxiceffects of sewage effluent on the germination and growth of early life stagesof the common kelp species Eckloniaradiata (C. Agardh) J. Agardh.
Astudy 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 correspondsto observed higher growth rates of these taxa in the field. Nutrientavailability limited photosynthetic performance of Hincksia sordida. Conversely, limiting factors on thephotosynthetic performance of the other two species appeared to be light andtemperature. Temperature is most likely the critical factor determiningseasonal changes in photosynthesis in Ulvasp. and Polysiphonia decipiens.
Eachof the species examined were found to have the capacity to increase its tissuenitrogen reserves without concomitant increase in growth, suggesting thatnutrients may be stored in tissue for later use when supply is low. Therelationship between growth and tissue nutrient storage capacity provided afunctional explanation to observed growth strategies of species and providedinformation on the critical tissue nitrogen threshold limits for growth.
Theeffect of primary and secondary treated sewage effluents on reproductive phasesof the life cycle of the marine macrophyte (kelp) Ecklonia radiata were also investigated. Ecklonia radiata is a major primary producer in nearshore medium tohigh energy habitats and as a benthic organism is subject to long term chronicexposure to coastal effluent. Inhibition of germination and reduction in growthrate of gametophytes (early microscopic life stage) are used as end points inbioassays. For primary treated effluent germination, inhibition wassignificantly inhibited at less than 1% effluent while growth responsesindicated significant inhibition at 4%, and enhanced growth at 2%. Responses tosecondary treated effluent indicated that for both assay forms the principaleffect was due to reduced salinity. For one of the secondary treated effluentforms there was no significant difference in response to salinity reductionemploying both assays. For the other type of secondary effluent, toxicity wasexpressed at 40% to 60% effluent. The primary effect of reduced salinityappears to take effect below 25 PPS. Both assays offer great potential for theroutine screening of effluent quality. Both assays are simple to conduct, showhigh reproducibility and are ecologically relevant.
Productivityand kinetic uptake data developed for each species can be used as input data tothe COASEC model developed to predict the effects of wastewater discharges onnearshore marine environments This data complements information on water columnprocesses (water borne concentrations of phytoplankton, particulate organicsand dissolved inorganic nitrogen) for particular discharges that is alsorequired as input data for the COASEC model. This model may provide WaterAuthorities with environmental threshold limits for nutrient concentrations above which the growth ofopportunistic macroalgae is promoted. This information can be utilised by localWater Authorities to predict the effects of their coastal discharges on thegrowth of opportunistic species that lead to eutrophication.
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