Modelling and visualizing the fate of shrimp pond effluent in a mangrove-fringed tidal creek

Field and model studies were carried out of the flushing and processing of shrimp pond effluent in a mangrove-fringed tidal creek, Port Douglas, Austr alia. Tidal flushing results in the concentration of dissolved and particul ate nutrients decreasing with distance downstream. Flushing is slow in this 6 km long mangrove creek, with a residence time varying between four days at spring tides and 10-15 days at neap tides. Since spring and neap tides a lternate at seven days intervals, the system is never at equilibrium. As a result, occasional along-channel sampling of sediment and nutrients provide little information on the fate of the effluent. A quantitative estimate of the physical and biological processes of nutrients and suspended sediments in the creek was made possible by comparing their discharge from the farm with the net, tidal-averaged discharge in the creek at a point 1.3 km downs tream. Significant changes in discharges of nutrients occurred in the creek ; chi a and TDN increasing in the creek, and DON, PC, TDN, TDP and suspende d sediments being removed from the creek. The results were very sensitive t o the natural, small-scale variability in the creek, and this variability m ay be under-sampled by triplicate samples. High concentrations of plankton downstream of the discharge interact with suspended particulate matter (mud , detritus from mangroves and pond effluent) and generate muddy marine snow . Muddy marine snow is not present in non-impacted creeks. Several processe s may limit eutrophication. These are tidal flushing, secondly, selective s ettling of low nutrient particles, thirdly, dilution with original creek mu d, and finally, bacteria mineralizing nutrients on the muddy marine snow. ( C) 2000 Academic Press.