Population Size and Summer Home Range of the Green Crab, Carcinus maenas, in Salt Marsh Tidal Creeks.

Abstract

In the 1979-1988 water budget (Fig. Ic), which is representative of current conditions, evapotranspiration (541 mm/y) and streamflow (538 mm/y) each account for about 45% of precipitation (1180 mm/y). Diversions leaving the basin (143 mm/y) are greater than diversions entering the basin (33 mm/y). The change in storage (-9 mm/y) is small. However, in this study, we ignored the absolute value of the storage component because it is highly dependent on the evapotranspiration estimate, which is the least accurate component of any large-scale water budget; only the temporal variation is considered. During the summer months (Fig. Id), the change in storage is most negative, due to increasing evapotranspiration, and it is coincident with decreasing rainfall and streamflow. Diversions remain relatively constant hroughout the year, with high groundwater pumping during the summer balanced by surface water withdrawals into reservoirs during the rest of the year. The effect of diversions should be most apparent during the summer months because streamflow is lowest at this time. With increasing water demands, diversions have become a major component of the water budget-they currently represent 15%20% of the streamflow. Our analyses of the water budget did not reveal any significant long-term trend in change in storage or in streamflow. This suggests that to understand the impact of diversions on the system we ought to reduce the time step (to daily or hourly) to examine changes in streamflow; focus the study area on the upper Ipswich basin where low flows occur and the river dries up most frequently; and look at different indices of hydrological change, such as the number of days of low flow and groundwater levels. Low streamflow is detrimental not only to the river ecosystems, but also to the downstream estuary, where alterations in salinity during the summer months could increase the stress on estuarine communities, a topic that requires further research. This research has been supported by the NSF-BUMP REU program, the Cox Charitable Trust, and the NSF grants: OCE9726921, DEB-9726862, and EAR-9807632. We thank the water departments of those towns that provided data and assistance, the Ipswich River Watershed Association, the Department of Environmental Protection, and Gil Pontius and colleagues at Clark University.