Hydrological coupling between watersheds and estuaries is an important element in establishing eutrophic conditions in coastal waters. In coupled systems, nutrients derived on land enter estuaries through groundwater flow or stream runoff, resulting in increased rates of primary production. The extent of coupling is determined by precipitation, evapotransporation, and water use practices. We use stable nitrogen isotopes, salinity, and nutrient data to assess coupling in two temperate and two tropical ecosystems. In each region, we selected two estuaries, one pristine and one with considerable agricultural or urban development. In pristine systems freshwater entering estuaries had low nitrogen concentrations and low nitrogen isotopic signatures. In receiving waters of the pristine systems, there were similarly low nutrient concentrations, and the isotopic signatures in the estuary were similar to the signatures of incoming groundwater. In both disturbed sites freshwater on the watershed had high nitrogen concentrations and enriched nitrogen isotopic signatures. In the temperate system, the estuarine water had high nitrate concentrations and nitrogen isotopic signatures that were similar to that of the incoming freshwater, indicating that the watershed and estuary were tightly coupled. In contrast, in the disturbed tropical site the estuary had extremely low nitrogen concentrations, and the isotopic signatures were unrelated to the signatures found in freshwater on the adjoining watershed. These results demonstrate that under sufficiently dry conditions, watersheds and estuaries may be uncoupled.