Some Aspects of Structure and Function of Benthic Macroinvertebrate Populations in a Spring

Structure and function of populations of benthic macro- invertebrates in an open-water area and a vegetation-choked area of a spring were examined. Although more species were collected from the vegetated area, more individuals were taken from the open area. Stic- tochironomus annulicrus and Limnodrilits sp. comprised 93 % of the indi- viduals and of the biomass in the open area, and 78% of the individuals and 73% of the biomass in the vegetation. Although Stictochironomus and Limnodrilus were similar in numbers, they were considerably differ- ent in biomass/individual and therefore in total biomass. Species diversity was analyzed with the index, d-_ - i log2 i. Species diversity was less in the open area of the spring than in the vege- tation-choked area. In the open area, d increased from May through August when numbers of individuals were used and decreased when biomass units were used. Nasturtium officinale was i-mportant in influencing the structure of the benthic community, but was unimportant as a source of energy. Mean monthly standing crop of algae in the entire spring was 3500 kcal, while detritus averaged 15,900 kcal. Mean standing crop of benthic herbivores was 3800 kcal. The size of the benthic community was prob- ably limited by space but not by food. Caloric content of fourth-instar Stictochironomus larvae was 5.50 kcal/ash-free g and respiratory rate was 0.765 ml 02/ash-free g per hr. Caloric content of Limnodrilus was 5.61 kcal/ash-free g and respiratory rate was 0.810 02/ash-free g per hr. Benthic macroinvertebrate respira- tion was almost 25% of community respiration in the open area and less than 2% in vegetation.