Using information on spatial variability of small estuaries in designing large-scale estuarine monitoring programs

In the early 1990s, EPA's Environmental Monitoring and Assessment Program ( EMAP) documented the ecological condition of the overall population of smal l estuaries along the mid-Atlantic coast of the United States. However, the Program did not provide detailed information on the condition of individua l estuaries less than 260 km(2) in surface area, a group of estuaries of co ncern to environmental managers. To address the needs of environmental mana gers, when EMAP returned to the region in summer 1997, it included a study of the spatial variability of ecological indicators within individual small estuaries. At 127 probability-based sites in 10 estuaries, EMAP measured a variety of parameters of water quality and sediments, including dissolved oxygen (DO), nutrients, grain size of sediments, contaminants in sediments, and community structures of benthic macroinvertebrates. From this informat ion the ecological condition (e.g., percent area with DO concentrations bel ow 5mg L-1) for each estuary, along with 90% confidence interval, was deter mined. The width of the confidence interval was then recalculated for sampl e sizes ranging from two stations to the total number of stations sampled i n that estuary. Confidence interval widths were then plotted against sample size. These plots can be useful in designing future regional monitoring pr ograms with a goal of describing conditions in individual systems as well a s broad geographic regions. Results illustrate that beyond five stations pe r estuary, the reduction in the width of the confidence interval with incre asing sampling intensity is relatively small; however, individual program m anagers need to determine "how small is small enough."