The influence of stratification on the ecological response of two Western Australian embayments to nutrient enrichment
D'Adamo, N., Simpson, C., Mills, D., Imberger, J. and McComb, A.J. (1992) The influence of stratification on the ecological response of two Western Australian embayments to nutrient enrichment. Science of The Total Environment, SUPPL . pp. 829-850.
Princess Royal Harbour (PRH) and Oyster Harbour (CH) are adjacent embayments on the south coast of Western Australia. Both harbours have received similar long-term nutrient loadings although the sources are different; inputs into PRH are primarily from industry, agriculture and community whereas most nutrients enter CH via agricultural runoff in river discharge. In both harbours prolific growth of algae has resulted in widespread dieback of seagrass meadows as a result of shading and smothering. PRH contains about ten times the macroalgal biomass of OH and biological, chemical and photic factors do not appear to completely explain this anomaly. A comparison of physical measurements of the density structure in PRH and OH suggests that the contrasting hydrodynamic characteristics of these systems play dominant roles in controlling their ecological status. PRH receives nutrient inputs all year round from outfalls and drains, receives no river discharge, has weak stratification and is typically well-mixed with circulation governed by barotropic wind and tide-forced currents; residence times for nutrients are relatively large. In contrast, OH receives nutrient-rich freshwater from two rivers in winter and is salt-wedge in nature. Baroclinic transport in OH is effective in flushing nutrients in surface waters to the ocean over relatively short time scales and the stratification may restrict nutrient availability by inhibiting downward transport to the benthos. Thus, differential nutrient availability as a result of dissimilar physical processes is postulated to explain the different ecological response of these two embayments to nutrient enrichment. A Wedderburn number classification scheme clarifies the hydrodynamic response of the two systems to meteorological forcings and supports the conclusion that the eutrophication response of estuaries is strongly correlated with the strength and persistence of density stratification.
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