The Impact of Shrimp Pond Effluent on Water Quality and Phytoplankton Biomass in a Tropical Mangrove Estuary

Water quality and phytoplankton biomass were examined over a three-year period in a mangrove estuary receiving periodic inputs of effluent from adjacent shrimp ponds, and in two adjacent, non-impacted estuaries, in north Queensland, Australia. Chlorophyll a, dissolved oxygen (DO), biological oxygen demand (BOD), pH, and salinity at the discharge site in the receiving estuary were significantly higher than in the two control estuaries. There were no significant differences between the impacted and control estuaries in total suspended solids (TSS) and dissolved nutrient concentrations. Water quality and phytoplankton biomass were within ambient levels within 1 km downstream of the discharge site, based on a comparison with long-term, pre-impact data for the estuary. Within 1–2 months after pond discharge ceased, water quality and phytoplankton biomass at the discharge site returned to levels equivalent to those in the control estuaries. The limited spatial and temporal impact suggests that the effluent was dissipated by tides and assimilated and/or mineralized by the estuarine food web. Our results imply that tidal mangrove estuaries have some capacity, at least over short spatial and temporal scales, to process intermittent inputs of pond-derived nutrients.