Effect of the 1997-1998 ENSO-related drought on hydrology and salinity in a Micronesian wetland complex

The potential effects of global climate change on coastal ecosystems have a ttracted considerable attention, but the impacts of shorter-term climate pe rturbations such as ENSO (El Nino-Southern Oscillation) are lesser known. I n this study, we determined the effects of the 1997-1998 ENSO-related droug ht on the hydrology and salinity of a Micronesian mangrove ecosystem and an adjacent freshwater swamp. A network of 9 piezometer clusters installed at the study site served as sampling points for continuous and manual measure ments of salinity and water level. During the drought period from January t hrough April 1998, mean water table levels in the mangroves and freshwater swamp were approximately 12 and 54 cm lower, respectively, than during May through December when precipitation returned to near normal levels. At the peak of the drought (February 1998), the most dramatic result was a reversa l in groundwater flow that sent groundwater from the mangroves upstream tow ard the freshwater swamp. Flow nets constructed for this period and immedia tely after illustrate the strong hydrological linkage between the two syste ms. This linkage was also illustrated by measurements of groundwater salini ty in the piezometer network. Ninety-six percent of the salinity measuremen ts taken in the mangroves during the study were at least 10 parts per thous and less than the salinity of sea water, indicating that the mangroves were consistently receiving freshwater flows. An analysis of variance of ground water salinity measurements during and after the drought showed that salini ty levels in the 0.5 and 1.0 m depth piezometers were greater during than a fter the drought. In a comparison of salinity values in 0.5-m wells during low tide, mean salinity was approximately twice as high during the drought than after (14.7 parts per thousand versus 6.2 parts per thousand, respecti vely). This study demonstrates that short-term climate perturbations such a s ENSO can disrupt important coastal processes. Over repeated drought cycle s, such perturbations have the potential to affect the structure and functi on of mangrove forests and upstream ecosystems.