Arthur C. Benke

Length-Mass Relationships for Freshwater Macroinvertebrates in North America with Particular Reference to the Southeastern United States

Estimation of invertebrate biomass is a critical step in addressing many ecological questions in aquatic environments. Length-dry mass regressions are the most widely used approach for estimating benthic invertebrate biomass because they are faster and more precise than other methods. A compilation and analysis of length-mass regressions using the power model, M (mass) = a L (length)b, are presented from 30 y of data collected by the authors, primarily from the southeastern USA, along with published regressions from the rest of North America. A total of 442 new and published regressions are presented, mostly for genus or species, based on total body length or other linear measurements. The regressions include 64 families of aquatic insects and 12 families of other invertebrate groups (mostly molluscs and crustaceans). Regressions were obtained for 134 insect genera (155 species) and 153 total invertebrate genera (184 species). Regressions are provided for both body length and head width for some taxa. In some cases, regressions are provided from multiple localities for single taxa. When using body length in the equations, there were no significant differences in the mean value of the exponent b among 8 insect orders or Amphipoda. The mean value of b for insects was 2.79, ranging from only 2.69 to 2.91 among orders. The mean value of b for Decapoda (3.63), however, was significantly higher than all insects orders and amphipods. Mean values of a were not significantly different among the 8 insect orders and Amphipoda, reflecting considerable variability within orders. Reasons for potential differences in b among taxa are explained with hypothetical examples showing how b responds to changes in linear dimensions and specific gravity. When using head width as the linear dimension in the power model, the mean value of b was higher (3.11) than for body length and more variable among orders (2.8-3.3). Values of b for Ephemeroptera (3.3) were significantly higher than those for Odonata, Megaloptera, and Diptera. For those equations in which ash-free dry mass was used, % ash varied considerably among functional feeding groups (3.3-12.4%). Percent ash varied from 4.0% to 8.5% among major insect orders, but was 18.9% for snails (without shells). Family-level regressions also are presented so that they can be used when generic equations are unavailable or when organisms are only identified to the family level. It is our intention that these regressions be used by others in estimating mass from linear dimensions, but potential errors must be recognized.

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- I (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 m-2 yr-t (effective habitat). Annual pro- duction/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/M2), but biomass was very low (I 100 mg/ m2 or less). Production of primary consumers was >11 g m-2 yr- t 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-t. The muddy-substrate benthos consisted primarily of oligochaetes (Limnodrilus) and midges. Annual production was 7-10 g.m-2 yr-t 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 6/6% 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 func- tional groupings were evaluated using numbers, biomass, and production. Filtering collectors pre- dominated on snags, and gathering collectors in benthic habitats. When corrected for habitat abun- dance, 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.

Importance of Snag Habitat for Animal Production in Southeastern Streams

Abstract The Satilla River in southeastern Georgia is a low gradient coastal plain stream with large quantities of woody debris (snags) along its banks. The major objective of this study was to assess the relative importance of the snag habitat as a site of invertebrate production in comparison to benthic habitats. This was accomplished from quantitative sampling of invertebrate habitats, analysis of drifting organisms, and gut analyses of the major fish species. Invertebrate diversity, biomass, and production were considerably higher on snag surfaces than in either sandy or muddy benthic substrates. Although snags represented a relatively small habitat surface (4% of total habitat surfaces), snags supported 60% of total invertebrate biomass and 16% of the production for a stretch of river. Drift densities from night samples collected throughout the year were relatively high, and approximately 78% of drifting invertebrate biomass originated from the snags. Four of the eight major fish species obtained at ...

TROPHIC BASIS OF PRODUCTION AMONG RIVERINE CADDISFLIES: IMPLICATIONS FOR FOOD WEB ANALYSIS

We determined the trophic basis of production and quantified the food web of caddisfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Production was dominated by three net-spinning taxa (Cheuma- topsyche spp., Hydropsyche rossi, and Chimarra moselyi), comprising 97-98% of the total among at least 14 trichopteran species. Annual production (as dry mass) was among the highest estimates reported for caddisflies, ranging from 43.5 to 63.9 g/m2 of snag surface (or from 14.2 to 25.5 g/m2 of channel bottom) in two consecutive years. Although all taxa were present throughout the year, C. moselyi had much higher production in summer, H. rossi was highest from late summer to winter, and Cheumatopsyche spp. showed no pattern. Ingestion was determined from production estimates, bioenergetic efficiencies, and quan- titative gut analyses. The two macrofiltering taxa (Cheumatopsyche spp. and H. rossi) were omnivorous, with 50.6 and 64.4% of their production, respectively, due to eating animals. Somewhat lower amounts of their production (40.7 and 23.5%) were due to ingestion of amorphous detritus. Production of the microfiltering species Chimarra moselyi was pri- marily due to eating amorphous detritus (91%). Removal of amorphous detritus from the system by the major taxa was highest in summer, and diatom removal was highest in fall, whereas animals were eaten consistently throughout the year. A quantitative food web showed that while the linkages among nine taxa were complex, the ingestion pathways were dominated by amorphous detritus (total consumption = 62.5 g.m-2.yr-1) and animal prey (22.3 g.m-2.yr-1), with >99% of food resources being consumed by the three dominant filter-feeding taxa. A connectivity food web was misleading because it implied equivalence of all food resources and consumers when great variation in strength of linkages actually existed. These comparisons provide a strong argument for considering energy flow as a measure of linkage strength when evaluating food webs, and they cast doubt on the use- fulness of oversimplified connectivity webs as a basis for food web theory.

Dragonfly Production and Prey Turnover

Annual production was calculated for three dominant larval odonate populations (Ladona deplanata, Epitheca spp., and Celithemis fasciata) coexisting in the littoral zone of an abandoned farm pond. Dragonfly populations and their prey were collected simultaneously with an Ekman grab at 2-wk to 1-mo intervals. Production for each dragonfly species was calculated using both the Allen curve method and the removal-summation method. Annual production for the three dominant species was 6 g/m2 (dry wt), and total odonate production was estimated as 8 g/m2. Weekly odonate production was consistently > 0.3 g/m2 from July through September. With a mean annual standing stock of 2 g/m2, the odonate annual turnover ratio (TR) is 4. Weekly TRs were highest during summer months, generally declining as individual size increased. From back calculations of the amount of food required to feed the odonates, it was esti- mated that a minimal value of annual prey production was 18 g/m2 and a minimal estimate of weekly prey production in the summer months was almost 1 g/m2. With an observed prey standing stock of 0.6 g/m2, this would necessitate an annual prey TR of 30 and a weekly summer TR > 1. Since such an indirect method of calculating prey production does not account for other sources of prey mortality such as fish predation and emergence, the TRs are considered to be minimum values, perhaps only half of actual values. These relatively high TR values agree quite well with estimates based upon the amount of food required to sustain fish predators. The significance of these estimates made using indirect methods is that they are considerably higher than estimates made using more traditional direct methods. An attempt is made to resolve this discrepancy by examining sources of error associated with both approaches. Furthermore, since odonates have a standing stock frequently two to three times that of their prey and a large daily consumption capacity, I suggest that prey are saved from annihilation because they can find sufficient refuges.

PRODUCTION DYNAMICS OF RIVERINE CHIRONOMIDS: EXTREMELY HIGH BIOMASS TURNOVER RATES OF PRIMARY CONSUMERS

A critical step in understanding food webs and trophic dynamics of communities is quantification of the role of the primary consumers, and a major aspect of such quantification is determination of their production. Annual production and biomass turnover were estimated for the larval chironomid (midge) assemblage found on the submerged woody (snag) habitat of a Coastal Plain blackwater river. Temperature-specific growth rate equations, generated from field growth studies, were applied to the biomass values from quantitative field samples to obtain mean daily production on a monthly basis throughout the year. The most productive genera were the filtering collectors, Rheotanytarsus (31.1 g dry mass·m−2·yr−1 for snag surface area) and the gathering collectors, Polypedilum (11.3 g·m−2·yr−1) and Rheocricotopus (9.8 g·m−2·yr−1). Total production was 65.4 g·m−2·yr−1 based on summing values of individual taxa, and 69.9 g·m−2·yr−1 when applying a family-level equation to biomass of all taxa. When converted to produ...

URBANIZATION AFFECTS STREAM ECOSYSTEM FUNCTION BY ALTERING HYDROLOGY, CHEMISTRY, AND BIOTIC RICHNESS

Catchment urbanization can alter physical, chemical, and biological attributes of stream ecosystems. In particular, changes in land use may affect the dynamics of organic matter decomposition, a measure of ecosystem function. We examined leaf-litter decomposition in 18 tributaries of the St. Johns River, Florida, USA. Land use in all 18 catchments ranged from 0% to 93% urban which translated to 0% to 66% total impervious area (TIA). Using a litter-bag technique, we measured mass loss, fungal biomass, and macroinvertebrate biomass for two leaf species (red maple [Acer rubrum] and sweetgum [Liquidambar styraciflua]). Rates of litter mass loss, which ranged from 0.01 to 0.05 per day for red maple and 0.006 to 0.018 per day for sweetgum, increased with impervious catchment area to levels of approximately 30-40% TIA and then decreased as impervious catchment area exceeded 40% TIA. Fungal biomass was also highest in streams draining catchments with intermediate levels of TIA. Macroinvertebrate biomass ranged from 17 to 354 mg/bag for red maple and from 15 to 399 mg/bag for sweetgum. Snail biomass and snail and total invertebrate richness were strongly related to breakdown rates among streams regardless of leaf species. Land-use and physical, chemical, and biological variables were highly intercorrelated. Principal-components analysis was therefore used to reduce the variables into several orthogonal axes. Using stepwise regression, we found that flow regime, snail biomass, snail and total invertebrate richness, and metal and nutrient content (which varied in a nonlinear manner with impervious surface area) were likely factors affecting litter breakdown rates in these streams.

Importance of flood regime to invertebrate habitat in an unregulated river-floodplain ecosystem

Unlike most large rivers of the northern hemisphere, several medium-sized rivers in the southeastern USA Coastal Plain remain unregulated. These smaller rivers possess 2 habitat types (snags and floodplain) that were historically important for invertebrate assemblages in many rivers and are strongly dependent on flood regime. I reviewed and compared 2 models of habitat inundation (snags and floodplain) that were developed for the Ogeechee River (Georgia, USA) to understand the ecological significance of these habitats. These models showed that snag habitat surfaces varied from only ∼20 to 50% of channel bottom surfaces, but floodplain inundation varied from 0 to 37 times the width of the channel at 100% inundation. Long-term analysis of inundation patterns from a 58-y record of discharge demonstrated that substantial flooding occurred almost annually for 1 to 2 mo/y. Habitat-specific invertebrate biomass was highest on snags (mostly aquatic insects), followed by the main channel (dominated by Corbicula), and then the floodplain (oligochaetes, crustaceans, aquatic insects). After correction for total amount of habitat surface area, invertebrate biomass contributions were highest in the floodplain > main channel > snag. However, arthropods and oligochaetes, the most likely prey of higher trophic levels, were clearly dominant on snags and in the floodplain. In many rivers around the world, invertebrate productivity from snags and floodplains is likely to have been significantly diminished because of snag removal, channelization, and floodplain drainage for >2 centuries. Understanding the interaction between flood regime and invertebrate habitat in unregulated rivers like the Ogeechee River can serve as a benchmark in restoration efforts.

Rapid growth of snag-dwelling chironomids in a blackwater river : the influence of temperature and discharge

Growth rates were measured for chironomid larvae throughout the year in chambers simulating natural conditions at a field site on the bank of the Ogeechee River, a blackwater river in the Georgia Coastal Plain. Growth rates were among the highest ever reported for chironomids in laboratory or field studies, averaging 0.60, 0.43, and 0.44 mg mg-1 d-1 for Chironomini, Tanytarsini, and Orthocladiinae, respectively. The relationship of growth with temperature was best described with second-order polynomial curves for Chironomini, Tanytarsini, and total Chironomidae. Most curves suggest an x-axis intercept (zero growth) at 4-7°C and a growth maximum at 21-24°C. Growth rates were significantly lower and the relationship of growth with temperature was no longer significant when river discharge was well below the mean. Growth equations were applied to natural environmental conditions, predicting maximum growth during spring and fall, with an annual growth rate (an approximation of annual P/B ratio) ranging from 222 for Chironomini to 182 for Tanytarsini. Predicted growth rates were high throughout all seasons, indicating that the bioenergetic role of chironomids in invertebrate communities is substantially greater than can be assumed based on their standing stock biomass alone.

Benthic invertebrate production—facilitating answers to ecological riddles in freshwater ecosystems

Abstract Invertebrate secondary production, or the formation of invertebrate biomass through time, has been estimated in many freshwater benthic habitats. It has been a major research theme for the North American Benthological Society (NABS), and many of its members have made significant contributions to the subject, both before and during the existence of J-NABS. Although some benthic production work occurred before 1960, the major methods were developed primarily during the 1960s and 1970s. Most of these methods also were applied in terrestrial and marine environments. The main focus of our paper is how secondary production has been used as an essential variable in facilitating answers to a wide variety of ecological questions. Benthic freshwater production studies before the inception of J-NABS were primarily related to life history, interpopulation comparisons, niche overlap/competition, predator–prey relationships, differences in production/biomass (P/B), energy flow, the trophic basis of production, habitat-specific microdistributions, effects of pollution and dams, and quantification of aquatic–terrestrial linkages. Since that time, new applications have been related to habitat-specific macrodistributions, quantitative food webs, experimental and tracer-based studies of trophic resources, chemical flows/stoichiometric relationships, diversity/function relationships, influence of nonnative species and landuse changes, implications of metabolic theory, and the importance of meiofauna vs macrofauna. J-NABS has been a major outlet for many of these applications, has probably included a higher fraction of papers incorporating secondary production analysis than any other journal, and probably will continue to be a leader in this area.