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Figure 5: Impacts of imperviousness on macroinvertebrate communities in the headwater streams of the Anacostia River (Schueler and Galll, 1992) 40 30 20 10 0 It is clear that fe urban streams ca Metric Values 0 Metric Values are based upon the sum oC scores assigned for the following categories: EPTC balance. EPT' lndez, Average Generic Diversity, Chvonomid Abundance, Taxonomic Richness (Family and Generic) Klein"` was one of the first to note that macroinvertebrate diversity drops sharply in urban streams in Maryland. Diversity consistently became poor when watershed imperviousness ~~ exceeded 10 to 15 percent. The same w, if any, basic threshold has been reported by n support all other research studies that have diverse benthle communities at looked at macroinvertebratedivcrsity moderate to high levels of in urban streams (Table 2). imperviousness (25 or more). In each study, sensitive macroin- ~4®q vertebrates were replaced by ones that were more tolerant of pollution and hydrologic stress. Species such as stoneflies, mayflies, and caddisflies largely disappeared and were replaced by chironomids, tubificid worms, amphipods, and snails. Species that employ specialized feeding strate- gies-shredding leaf litter, grazing rock surfaces, fil- tering organic matter that flows by, and preying on other insects-were lost. A typical example of the relationship between imperviousness and macroinvertebrate diversity is shown in Figure 5. The graph summarizes diversity trend for 23 sampling stations in headwater streams of the Anacostia watershed.24 While good. to fair diversity was noted in all headwater streams with less than 10% imperviousness, nearly all stations with 12% or more impervious cover recorded poor diversity. The same sharp drop in macroinvertebrate diversity at around 12 50 to 15% imperviousness was also observed in streams in the coastal plain and piedmont of Delaware.zs Other studies have utilized other indicators to mea- sure the impacts of urbanization on stream insect communities. For example, Jones and Clark'S moni- tored 22 stations in Northern Virginia and concluded thatbenthic insect diversity composition changed mark- edly after watershed population density exceeded four or more individuals per acre. The population density roughly translates to half-acre or one acre lot residen- tial use, or perhaps 10 to 20 percent imperviousness. Steedman27 evaluated 208 Ontario stream sites, and concluded that benthic diversity shifted from fair to poor at about 35% urban land use. Since "urban land" includes both pervious and impervious areas, the ac- tual threshold in the Ontario study may well be closer to 7 to 10% imperviousness.' Steedman also reported that urban streams with intact riparian forests had higher diversity than those that did not, for the same level of urbanization. While the exact point at which stream insect diver- sityshifts from fair to poor is not known with absolute precision, it is clear that few, if any, urban streams can support diverse benthic communities at moderate to high levels of imperviousness (25% or more). Four different studies alI failed to find stream insect commu- nities with good or excellent diversity in any highiy urban sueam.'•21•'= 104 ~ ~"_1f4~~x-C~+ ±a~{~°~ • ti,i~~si~i>iv_~ ~y~G~ S~.;I!~tk_ci ^ !~I~ ~~~!~,~ - ~-