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1998-01-22_AgendaPacket
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1998-01-22_AgendaPacket
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Grass Lake WMO
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Agenda/Packet
Commission/Committee - Meeting Date
1/22/1998
Commission/Committee - Meeting Type
Regular
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Figure 5: Impacts of imperviousness on macroinvertebrate communities in the headwater <br />streams of the Anacostia River (Schueler and Galll, 1992) <br />40 <br />30 <br />20 <br />10 <br />0 <br />It is clear that fe <br />urban streams ca <br />Metric Values <br />0 <br />Metric Values are based upon the sum oC scores assigned for the following categories: EPTC balance. EPT' lndez, Average <br />Generic Diversity, Chvonomid Abundance, Taxonomic Richness (Family and Generic) <br />Klein"` was one of the first to note that <br />macroinvertebrate diversity drops sharply in urban <br />streams in Maryland. Diversity consistently became <br />poor when watershed imperviousness <br />~~ exceeded 10 to 15 percent. The same <br />w, if any, basic threshold has been reported by <br />n support all other research studies that have <br />diverse benthle communities at looked at macroinvertebratedivcrsity <br />moderate to high levels of in urban streams (Table 2). <br />imperviousness (25 <br />or more). In each study, sensitive macroin- <br />~4®q vertebrates were replaced by ones that <br />were more tolerant of pollution and <br />hydrologic stress. Species such as stoneflies, mayflies, <br />and caddisflies largely disappeared and were replaced <br />by chironomids, tubificid worms, amphipods, and <br />snails. Species that employ specialized feeding strate- <br />gies-shredding leaf litter, grazing rock surfaces, fil- <br />tering organic matter that flows by, and preying on <br />other insects-were lost. <br />A typical example of the relationship between <br />imperviousness and macroinvertebrate diversity is <br />shown in Figure 5. The graph summarizes diversity <br />trend for 23 sampling stations in headwater streams of <br />the Anacostia watershed.24 While good. to fair diversity <br />was noted in all headwater streams with less than 10% <br />imperviousness, nearly all stations with 12% or more <br />impervious cover recorded poor diversity. The same <br />sharp drop in macroinvertebrate diversity at around 12 <br />50 <br />to 15% imperviousness was also observed in streams in <br />the coastal plain and piedmont of Delaware.zs <br />Other studies have utilized other indicators to mea- <br />sure the impacts of urbanization on stream insect <br />communities. For example, Jones and Clark'S moni- <br />tored 22 stations in Northern Virginia and concluded <br />thatbenthic insect diversity composition changed mark- <br />edly after watershed population density exceeded four <br />or more individuals per acre. The population density <br />roughly translates to half-acre or one acre lot residen- <br />tial use, or perhaps 10 to 20 percent imperviousness. <br />Steedman27 evaluated 208 Ontario stream sites, and <br />concluded that benthic diversity shifted from fair to <br />poor at about 35% urban land use. Since "urban land" <br />includes both pervious and impervious areas, the ac- <br />tual threshold in the Ontario study may well be closer <br />to 7 to 10% imperviousness.' Steedman also reported <br />that urban streams with intact riparian forests had <br />higher diversity than those that did not, for the same <br />level of urbanization. <br />While the exact point at which stream insect diver- <br />sityshifts from fair to poor is not known with absolute <br />precision, it is clear that few, if any, urban streams can <br />support diverse benthic communities at moderate to <br />high levels of imperviousness (25% or more). Four <br />different studies alI failed to find stream insect commu- <br />nities with good or excellent diversity in any highiy <br />urban sueam.'•21•'= <br />104 ~ ~"_1f4~~x-C~+ ±a~{~°~ • ti,i~~si~i>iv_~ ~y~G~ S~.;I!~tk_ci ^ !~I~ ~~~!~,~ - <br />~- <br />
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