Changes in the invertebrate populations, fish populations, and water chemistry of a small stream above and below two impoundments

The major impacts associated with the ponds appeared to be an alteration of energy dynamics. The alterations were seasonally dependent, and may have resulted from interception of larger particle organic matter being transported downstream and its replacement by finer organic matter. The alterations manifested themselves in changes in numbers and biomass of benthic macroinvertebrates and fish, and changes in drift patterns below the ponds. The energy dynamics alterations may have also influenced competitive interactions among benthic organisms and fish.

The direct influence of physiochemical alterations appeared to be masked by energy-related alterations; however, alterations in physiochemical factors may have caused minimal changes in species composition in aquatic communities below the ponds.

Material impacts in the system studied appeared to be negligible; however, Young (unpublished data) found significant impacts associated with the introduction of a floc of iron and iron bacteria below a bottom release impoundment. Material impacts may be of major importance in bottom release systems.

The ponds appeared to cause a general increase in stream productivity downstream. However, increased amplitude in seasonal fluctuations of physiochemical factors combined with seasonal fluctuations in energy below the ponds appeared to produce a more fluctuating environment below the ponds leading to reduced stability in downstream communities.

The impacts evidenced in this study do not appear to be as drastic as those reported in larger systems, There were no large-scale alterations in species composition or reductions in number of taxa present below the ponds. In systems containing rare species or with uniquely adapted faunas, the less stable environmental conditions, possible introduction of exotic species, and the possible disruption of competitive balance with the system may cause major impacts. These possible impacts should be dealt with in the design and planning of small pond construction.