Stability and resilience in benthic macroinvertebrate assemblages - Impactof physical disturbance over twenty-five years

Benthic macroinvertebrate assemblages of Bushkill Creek, Northampton County , PA were studied at the same site during the 1970s (11 dates) and 1990s (8 dates) to evaluate stability and resilience. In the 1970s, a Surber sample r was used, and in the 1990s, a Hess sampler. Assemblages appeared stable o ver a wide range of environmental stresses with the exception of 1994-1995 when total numbers and total Trichoptera decreased. Taxa richness and EPT i ndices varied little in 1994-1995 from other sampled years. By July 1996, a ll metrics (Trichoptera numbers, total numbers, taxa richness, EPT index, B ray-Curtis Index) resembled the 1970s exception for lower wet weight. Bray- Curtis indices and taxa composition were similar in July 1972 and July 1996 , suggesting assemblage stability over 25 years. The Trichoptera, Psychomyi a (Psychomiidae) and Leucotrichia (Hydroptilidae), decreased during the 199 0s and never rebounded to 1970s numbers. During winter 1994, the coldest te mperatures and greatest cumulative snowfall occurred in the region. These c onditions probably stressed the assemblage with low temperatures, anchor/fr azil ice and ice break-up. The assemblage was then exposed to four bankful floods in winter/spring 1994 and five bankful floods in winter/spring 1996. Recovery time following these multiple disturbances was 27 months. Previou s recovery times from winter and flood disturbances were considerably short er (2-5 months). The 1990s recovery time (5-9 times previusly recorded) for this assemblage was apparently extended by multiple physical disturbances, outside the predicted range. The assemblages had not been previously expos ed to such severe conditions and, therefore, recovery time was extended. De spite severe weather conditions, the assemblage recovered and exhibited bot h stability and resilience in its return to an assemblage similar to the 19 70s.