EFFECTS OF CLIMATE-CHANGE ON FRESH-WATER ECOSYSTEMS OF THE SOUTH-EASTERN UNITED-STATES AND THE GULF-COAST OF MEXICO

The south-eastern United States and Gulf Coast of Mexico is physiograp hically diverse, although dominated by a broad coastal plain. Much of the region has a humid, warm temperate climate with little seasonality in precipitation but strong seasonality in runoff owing to high rates of summer evapotranspiration. The climate of southern Florida and eas tern Mexico is subtropical with a distinct summer wet season and winte r dry season. Regional climate models suggest that climate change resu lting from a doubling of the pre-industrial levels of atmospheric CO2 may increase annual air temperatures by 3-4 degrees C. Changes in prec ipitation are highly uncertain, but the most probable scenario shows h igher levels over all but the northern, interior portions of the regio n, with increases primarily occurring in summer and occurring as more intense or clustered storms. Despite the increases in precipitation, r unoff is likely to decline over much of the region owing to increases in evapotranspiration exceeding increases in precipitation. Only in Fl orida and the Gulf Coast areas of the US and Mexico are precipitation increases likely to exceed evapotranspiration increases, producing an increase in runoff. However, increases in storm intensity and clusteri ng are likely to result in more extreme hydrographs, with larger peaks in flow but lower baseflows and longer periods of drought. The ecolog ical effects of climate change on freshwaters of the region include: ( 1) a general increase in rates of primary production, organic matter d ecomposition and nutrient cycling as a result of higher temperatures a nd longer growing seasons: (2) reduction in habitat for cool water spe cies, particularly fish and macroinvertebrates in Appalachian streams; (3) reduction in water quality and in suitable habitat in summer owin g to lower baseflows and intensification of the temperature-dissolved oxygen squeeze in many rivers and reservoirs; (4) reduction in organic matter storage and loss of organisms during more intense flushing eve nts in some streams and wetlands; (5) shorter periods of inundation of riparian wetlands and greater drying of wetland soils, particularly i n northern and inland areas; (6) expansion of subtropical species nort hwards, including several non-native nuisance species currently confin ed to southern Florida; (7) expansion of wetlands in Florida and coast al Mexico, but increase in eutrophication of Florida lakes as a result of greater runoff from urban and agricultural areas; and (8) changes in the flushing rate of estuaries that would alter their salinity regi mes, stratification and water quality as well as influence productivit y in the Gulf of Mexico. Many of the expected climate change effects w ill exacerbate current anthropogenic stresses on the region's freshwat er systems, including increasing demands for water, increasing waste h eat loadings and land use changes that alter the quantity and quality of runoff to streams and reservoirs. Research is needed especially in several critical areas: long-term monitoring of key hydrological, chem ical and biological properties (particularly water balances in small, forested catchments and temperature-sensitive species); experimental s tudies of the effects of warming on organisms and ecosystem processes under realistic conditions (e.g. in situ heating experiments): studies of the effects of natural hydrological variation on biological commun ities; and assessment of the effects of water management activities on organisms and ecosystem processes, including development and testing of management and restoration strategies designed to counteract change s in climate. (C) 1997 by John Wiley & Sons, Ltd.