IMPLICATIONS OF SEASONAL AND REGIONAL ABUNDANCE PATTERNS OF DAPHNIA ON SURFACE-WATER MONITORING AND ASSESSMENT

The seasonal dynamics of Daphnia populations vary regionally throughou t the United States. Within the general pattern, Daphnia increase in a bundance after the initiation of the spring algal bloom in all lakes, but their subsequent seasonal patterns differ in various climatic regi ons. Lakes in regions with cooler summers have large-bodied Daphnia po pulations that tend to persist throughout the summer, although the spe cies dominance may shift. Regions with warmer summers tend to have lar ge-bodied Daphnia populations that decline or are absent through much of the summer. Still warmer water bodies tend to have medium-to small- bodied species that are abundant during spring, but absent most of the summer. Many central Florida lakes lack Daphnia; if Daphnia species a re present, they tend to be small-bodied. Daphnia abundance in these w ater bodies varies, but seems to be independent of temperature. If sur face water (lake, pond) sampling is done in all regions during July an d August, the impression will be that Daphnia are absent from large se gments of the United States. This would be erroneous, because Daphnia are important earlier during the spring and early summer but are likel y to be absent during midsummer in some U.S. regions. Year-to-year var iation will be superimposed on this regional pattern. Because there ar e differences in the dates when spring and summer occur, it would be u seful to have an index period that would standardize the start of the growing season. The use of the terrestrial onset of greenness, based o n remote sensing of the Normalized Difference Vegetation Index, is sug gested as a possible index set point.