Introduction to Special Issue on Tropical Coastal Management

In 1992, scientists at National Estuarine Research Reserve (NERR) sites proposed establishing a national coordinated monitoring program that would attempt to identify and track short-term variability and long-term changes in representative estuarine ecosystems and coastal watersheds. Known as the NERR System-Wide Monitoring Program (SWMP), it currently consists of monitoring water quality and atmospheric variables over a range of spatial and temporal scales. Additional monitoring of ecological resources and land-water use will follow in subsequent components of the program. Water quality monitoring at NERR sites includes measurements of pH, conductivity, temperature, dissolved oxygen, turbidity, and water level. The NERR sites use data loggers that record at 30-minute intervals and relay measurements to internal memory. Standardized protocols assure that sampling, processing, and data management techniques are comparable among sites. Statistical techniques are being used to identify periodicity in water quality variables. Periodic regression analysis indicated that diel periodicity in dissolved oxygen is a larger source of variation than tidal periodicity at sites with less tidal amplitude. Tidal periodicity, however, is more important at sites with a tidal amplitude greater than 1 m. This finding suggests that natural processes controlling dissolved oxygen levels differ among sites depending upon tidal amplitude. Water quality data from the NERR SWMP have also been used to investigate occurrence of hypoxia. Results from analysis of water quality at several reserves indicate that hypoxia occurs but that the percentage of time that dissolved oxygen was less than 28% saturation varied substantially among sites and between years. Most of the hypoxic events occurred in summer but were also observed in winter and fall when low dissolved oxygen is usually not considered a problem. Without continuous monitoring by the NERR SWMP, many low dissolved oxygen events would have been missed, thus underestimating the duration and potential impact of this type of water quality variability. The NERR SWMP provides a unique opportunity to increase our understanding of how various environmental factors influence estuarine processes. Only by understanding how estuaries function and change naturally over time will we be able to predict how these systems respond to changes in climate and human-induced perturbations.