Invertebrate Productivity in a Subtropical Blackwater River: The Importance of Habitat and Life History

Habitat and life history are critical elements in assessing the production dynamics of invertebrates and their role in aquatic ecosystems. We studied invertebrate productivity at two sites in a subtropical blackwater river (the Satilla) in the Lower Coastal Plain of Georgia, USA, and found that submerged wooden substrates, or snags, are heavily colonized by aquatic insects. We compared invertebrate productivity on the snag habitat with productivity in the sandy benthic habitat of the main channel, and the muddy benthic habitat of the backwaters. The size—frequency method was applied to individual taxa in order to determine total invertebrate productivity. Emphasis was placed on the importance of the length of larval life, or the cohort production interval, in determining biomass turnover rates. The diversity of taxa was much higher on the snag habitat than in either of the benthic habitats. Filter—feeding caddisflies (especially Hydropsyche spp.) and black flies (Simulium spp.) were the major consumers on the snag habitat. Several species of midges, mayflies, and beetles also were abundant. Total densities, standing stock biomass, and production were very high for primary consumers on snags. Annual production was 51.9 and 67.1 g°m — 2 °yr — 1 (dry mass per surface area of snag, or effective habitat) for the two sites. Hellgrammites, dragonflies, and stoneflies were the major insect predators colonizing snags, and their production was 5.5 and 5.2 g@mm — 2 °yr — 1 (effective habitat). Annual production/biomass ratios (P/B) were usually 5—10 for insects that had univoltine or bivoltine life cycles. Annual P/B estimates were very high for midges (>100) and black flies (>70), since length of larval life was estimated to be very short. The sandy—substrate benthos consisted almost exclusively of very small midges with oligochaetes of lesser abundance. Densities were quite high (>20 000/m 2 ), but biomass was very low (° 100 mg/m 2 or less). Production of primary consumers was >11 g°m — 2 °yr — 1 with a very high estimate of annual P/B (166—227). The major predators were Ceratopogonidae (biting midges) larvae with an annual production of 1.6—2.6 g°m — 2 °yr — 1 . The muddy—substrate benthos consisted primarily of oligochaetes (Limnodrilus) and midges. Annual production was °7—10 g°m — 2 °yr — 1 for primary consumers. The major predators were larger Tanypodinae midges. On a substrate surface area basis, standing stock biomass on snags was 20—50 times higher than in the sandy habitat and 5—10 times higher than in the muddy habitat. Production on snags was only 3—4 times higher than production in the benthic habitats, with higher annual P/B in the latter. The production estimates for the snag habitat are among the highest yet reported for lotic ecosystems, and it appears that production on snags is limited by available substrate. Habitat areas per length of shoreline were estimated so that we could approximate relative amounts of biomass and production for a stretch of river. Although the snag habitat accounted for only °6% of the effective habitat substrate over a stretch of river, it was responsible for over half of invertebrate biomass, and °15—16% of production. Taxa within each habitat were categorized to functional feeding groups, and habitat—specific functional groupings were evaluated using numbers, biomass, and production. Filtering collectors predominated on snags, and gathering collectors in benthic habitats. When corrected for habitat abundance, the distribution of biomass among filtering collectors, gathering collectors, and predators was very close. However, the distribution of production was °12% filtering collectors, 71% gathering collectors, and 17% predators. We suggest that production is the most meaningful parameter to consider in functional group analysis and that the use of numbers or biomass alone can sometimes result in misleading conclusions. As a middle order (5th—6th) stream, the distribution of production or biomass among functional groups in the Satilla River differs considerably from that predicted by the river continuum concept, predicting a high percentage of grazing consumers.