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, Australia. Tidal flushing results in the concentration of dissolved and particulate 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 alternate 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 downstream. Significant changes in discharges of nutrients occurred in the creek; chi a and TDN increasing in the creek, and DON, PC, TDN, TDP and suspended sediments being removed from the creek. The results were very sensitive to the natural, small-scale variability in the creek, and this variability may 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 processes may limit eutrophication. These are tidal flushing, secondly, selective settling of low nutrient particles, thirdly, dilution with original creek mud, and finally, bacteria mineralizing nutrients on the muddy marine snow. (C) 2000 Academic Press.