Effects of experimental acidification on macroinvertebrate drift diversity in a mountain stream

A small stream (Norris Brook) within the Hubbard Brook Experimental Forest was acidified to determine what effect elevated H⁺ stress may have on the ecology of a mountain stream. The experiment was designed to simulate a pH level (4.0) that can occur during initial snowmelt (acute period) and during longer term (chronic period) acidification. Daily macroinvertebrate drift samples were collected from treatment and reference areas of Norris Brook. Drift diversity at the generic level was calculated using Brillouin's formula and partitioned hierarchically following macroinvertebrate classifications based on taxonomy (orders) and feeding strategies (functional groups or guilds). The rate of movement of individuals and genera was significantly greater for those organisms leaving the acid-stressed area during the first five days than for those entering, whereas no difference between the rate of macroinvertebrates entering or leaving the acid-stressed area was apparent for either numbers or genera over the remaining 25-day study period. For the acute period (first five days), the increased macroinvertebrate drift leaving the acidified area was significantly more diverse at the levels of aquatic insect orders and functional groups but less diverse at the generic level than the drift entering. For the chronic period (25-day period) no significant differences were detected in either major taxa, functional group (with the exception of collectors), or generic diversity between the drift entering and leaving the treatment reach. Mayflies and probably chironomids leaving the acid-stressed area during the acute period were generically more diverse than those entering, whereas stoneflies drifting out of the acidified reach were generically less diverse than those drifting in. The overall change in the normal pattern of spatial and temporal variation in drift rate and diversity provides quantitative evidence that H⁺ stress significantly altered the structure and function of the macrobenthic community.