The macrobenthos of a new reservoir, Lake Anna, Louisa County, Virginia

The macrobenthos of a new reservoir in central Virginia, Lake Anna, was studied for the first three years after impoundment, 1972-1975. Prior to this, extensive macrobenthic collections had been made in the river destined for impoundment, the North Anna River. The biota of this river had been seriously affected by acid mine drainage for over 100 years.

Macrobenthos consists of the organisms living on the bottom or the solid-liquid interface. The occurrence of these organisms is significant, because they are major items in the diet of many fish, and are thus important in the flow of energy through an ecosystem. The basic objectives of this study were to determine the changes in macrobenthic community structure brought about by impoundment, and then to observe the successional changes which occurred in the following years. This information was then compared to a hypothetical model predicting the productivity of new reservoirs.

It was discovered that traditional grab sampling was not reliable in new reservoirs because of the abundant submerged terrestrial vegetation. An original sampling method was developed which involved the use of SCUBA to place and retrieve artificial substrate samplers. This SCUBA method was compared to grab sampling, and found to more reliably estimate macrobenthic community structure.

Following impoundment, there was an immediate change in macrobenthic community structure. Colonization of the new reservoir occurred very quickly, especially during the first summer period. The acid mine drainage did not affect the distribution of macrobenthos in the new reservoir, probably because of dilution. Identification of over 525,000 organisms revealed that they could be classified into three groups: first colonizers, omnipresent species, and second colonizers. A review of the food habits of these organisms indicated that there were four trophic functional groups: micropredators, macropredators, collector-microgatherers, and collector-microfilterers. The first colonizers consisted of three species, one of which was completely dominant in each of the first three functional groups the first year. The second colonizers consisted of many additional species, several of which shared dominance in each of the same three functional groups the second and third years. The omnipresent group consisted of midge larvae which were dominant in the fourth functional group, collector-microfilterers, in all three years.

This information, in conjunction with analysis of the horizontal, vertical, and temporal distribution, indicated that the development of macrobenthic community structure in Lake Anna followed the general trends expected in ecological succession. These trends are increased number of species, increased equitability of species composition, and better organization of distribution. A comparison of the development of macrobenthos in new reservoirs with a model of expected productivity indicated that the abundance of macrobenthos. may also exhibit the same trends as productivity. These trends are an initial increase in productivity for several years, followed by a sharp decline for several years, to be finally followed by a small increase and stabilization. This sequence of events probably requires at least 15 years for macrobenthos.