John J. Hutchens

Ecosystem linkages between southern Appalachian headwater streams and their banks: Leaf litter breakdown and invertebrate assemblages

We examined red maple (Acer rubrum L.) leaf litter breakdown in streams and riparian zones at two sites in the southern Appalachian Mountains to understand how differences in abiotic and biotic factors influence leaf breakdown rates. Litterbags were placed in three riparian habitats differing in litter layer moisture: stream > bank > upland. Invertebrates colonizing litterbags at one site were also examined to determine how variations in community and functional structure affect breakdown rates. Leaves broke down fastest in streams and slowest in upland habitats, whereas bank habitats were intermediate and characterized by high variability. Faster leaf breakdown rates in streams appeared to be a function of greater moisture availability, a more stable thermal regime, and a higher biomass of leaf-shredding invertebrates, especially the stonefly Tallaperla. In addition, patterns of leaf breakdown and invertebrate community structure provided evidence for a stronger than expected ecological connection between the stream and the bank. Overall, detritus processing within this narrow riparian ecosystem varied considerably depending on the availability of moisture. Results from this study show that stream channel–floodplain interactions in riparian ecosystems of steep forested mountains are analogous to ones in larger downstream or low-gradient systems. Riparian zones throughout a river network display a remarkable heterogeneity in their ability to process organic matter, which is ultimately driven by changes in hydrological conditions.

Role of Podostemum ceratophyllum Michx. in structuring benthic macroinvertebrate assemblages in a southern Appalachian river

Abstract Podostemum ceratophyllum Michx. has been associated with extremely high secondary production of benthic macroinvertebrates in open-canopy rapids. We conducted an experiment in the 7th-order Little Tennessee River, North Carolina, to test whether varying amounts of Podostemum influenced macroinvertebrate abundance, biomass, community composition, and functional feeding group structure. The experiment consisted of 3 treatments in which P. ceratophyllum was completely, partially, or not removed from portions of 4 bedrock outcrops at 2 sites. Macroinvertebrates were sampled at 0, 3, and 6 wk post treatment. Complete removal of P. ceratophyllum greatly reduced overall macroinvertebrate abundance and biomass and altered assemblage structure, but had relatively little effect on functional structure. The lack of change in functional feeding group structure was probably a result of the importance of P. ceratophyllum as a substrate for epiphytic algae, and the availability of nearby colonists in undisturbed habitats. We found a strong positive relationship between surface area of Podostemum and total macroinvertebrate abundance and biomass. We estimated that P. ceratophyllum increased surface area by 3 to 4 times over bare bedrock. Podostemum ceratophyllum in the Little Tennessee River serves as an important habitat supporting high abundance and biomass of macroinvertebrates.

LIFE HISTORY AND TROPHIC BASIS OF PRODUCTION OF THE MAYFLY CALLIBAETIS FLUCTUANS (WALSH) (EPHEMEROPTERA: BAETIDAE) IN A MITIGATED WETLAND, WEST VIRGINIA, USA

We determined the life history, secondary production, and trophic basis of production for Callibaetis fluctuans in a mitigated wetland at the Green Bottom Wildlife Management Area, Cabell County, West Virginia, USA. Quantitative benthic samples were collected from three sites covering a range of water depths and vegetation types from January to December 1994. Length-frequency histograms were constructed for each sample date, and secondary production was calculated using the size-frequency method. Foregut analyses were conducted seasonally to determine diet. Callibaetis fluctuans had a multivoltine life cycle with at least three cohorts. Fast larval growth occurred during the spring and fall, whereas an overwintering cohort had slower growth. Total annual production was 208.0 mg·m−2·yr−1 (AFDM) with a production-to-biomass ratio of 12.6. Amorphous detritus comprised >80% of the diet in all seasons and accounted for 70% of total production. To account for this production, C. fluctuans consumed 3675 mg·m−2·yr−1 of detritus. This is one of the first studies to provide production estimates for an aquatic insect in a permanently inundated wetland.

Temporal variability of stream macroinvertebrate abundance and biomass following pesticide disturbance

We determined the extent of macroinvertebrate recovery in a former pesticide-treated stream (FTS) relative to a reference stream (RS) by examining macroinvertebrates colonizing red maple (Acer rubrum L.) litter bags between 5 to 10 y following pesticide treatment. Mean abundance and biomass, variability in abundance and biomass (using the coefficient of variation [CV]), and assemblage structure were compared both within and among years to assess recovery. The 5 y of study included 3 drought years followed by 2 wet years. Mean total abundance and biomass of macroinvertebrates, and that of most functional feeding groups (FFG) did not significantly differ between streams during this study, nor did within-year variability of these means, indicating macroinvertebrates in FTS had recovered relative to RS. Some exceptions to the above patterns (shredder abundance and mean annual gatherer biomass) resulted from the dynamics of a single taxon in each group (Leuctra and Paraleptophlebia, respectively). Macroinvertebrate assemblage structure in litter bags was similar between streams throughout the study as shown by their similar ordination scores; hence, assemblage structure had also recovered. In each stream, mean annual abundance and biomass of total macroinvertebrates and of each FFG, aside from shredder abundance, differed significantly among years. However, assemblage structure was generally similar among years. Among-year CVs were usually lower than within-year CVs because macroinvertebrate abundance and biomass fluctuated more during a year than it did from year to year, and different processes apparently contributed to the variation observed at these 2 time scales. For example, juvenile development time influenced within-year CVs, indicating that life-history characteristics affected temporal variability of macroinvertebrate abundance and biomass. Examination of both the means and their variances was useful for determining the extent of recovery and how macroinvertebrates responded to natural environmental variability. The detailed analysis of temporal dynamics at different time scales afforded by the CV supported our contention that FTS had recovered from the pesticide application relative to RS.

GASTROPOD ABUNDANCE AND BIOMASS RELATIONSHIPS WITH SALT MARSH VEGETATION WITHIN OCEAN-DOMINATED SOUTH CAROLINA, USA ESTUARIES

Abstract Molluscs, both gastropods and bivalves, appear to disproportionately influence the dynamics of salt marsh habitats. Bivalves typically are facilitators positively affecting the growth and survival of marshes, but gastropods may have unanticipated top-down effects through direct consumption of the predominant marsh vegetation, Spartina alterniflora. Natural, elevational differences in S. alterniflora density and morphology also may exert an influence on the spatial distributions of marsh snails. We examined the abundance of both plants and snails across an elevation gradient to determine if similarities or differences existed in the observed distribution patterns. Plant, mainly S. alterniflora, and snail, Littoraria irrorata (Say) and Melampus bidentatus Say, densities were measured along tidal creek to forest transects within the intertidal marshes of three marine-dominated South Carolina inlets. Significant differences in S. alterniflora density among marsh zones were paralleled by similar L. irrorata density differences. A consistent, unimodal pattern with peak L. irrorata density in the “short,” dense S. alterniflora mid marsh occurred within each inlet. In contrast, M. bidentatus was restricted almost entirely to the high marsh across all transects and inlets. Densities of L. irrorata and S. alterniflora were positively correlated in the low marsh consistent with the potential importance of culms as a food source and refuge from predators. Littoraria irrorata biomass was significantly different among zones in only one inlet and increased in a shoreward direction in two inlets. Although biomass patterns previously have not been reported for Southeastern US marshes, amounts were consistently greater than expected within the high-marsh zone. In SC, L. irrorata appears to be a conspicuous resident across the entire vegetated marsh and our results suggest previous studies focusing on the marsh near tidal creeks could underestimate appreciably the total population and overall significance of L. irrorata within coastal marshes.

Prey capture in the Venus flytrap: collection or selection?

Charles Darwin first proposed that the Venus flytrap (Dionaea muscipula Ellis) functions optimally by capturing and digesting large prey, the small prey escaping through openings at the trap margins. This hypothesis, although intui- tively sound in the context of trap mechanics or plant allocation theory, has not been tested adequately with populations of plants growing in the field. Here, with traps collected in the endemic habitat over 9 months, we show that prey capture in the Venus flytrap is opportunistic rather than selective. While there was no effect of trap size on prey capture success, there was a significant but weak positive relationship between trap length and prey length. Prey sizes were well below the theoretical maximum holding capacities of traps and relatively small insects were represented across the range of trap sizes. Our results show that prey capture was not biased toward large invertebrates. Instead, we suggest that nonselective prey capture across the observed range of trap sizes is the best-fit explanation of trap function in the context of relatively limited ability to change allocation in response to sudden increases in resource availability.

The ecological boundaries of six Carolina bays: Community composition and ecotone distribution

Community and environmental gradients within the ecological boundaries of Carolina bay wetlands may provide important information on the interaction between Carolina bays and associated uplands, and may also provide guidance for improved management. We established twelve 30-m transects on the sloping rims of each of six Carolina bays in northeastern South Carolina to characterize the community gradient, as well as important environmental factors producing this gradient. Mid-points of the transects were placed on jurisdictional wetland boundaries. Hydrology, soil properties, and plant species composition were measured within these transects. On average, transects included an elevation change of 0.6 m that corresponded with gradients of hydrology, soil properties, and community characteristics. Decreasing surface soil moisture (i.e., fewer flood events) and decreasing soil nutrients were associated with a shift from shrub-bog vegetation with relatively low alpha diversity and prominence of evergreens to a relatively diverse and heterogeneous community characterized by grasses, herbs, low shrubs, and vines. Ecotones, identified by abrupt changes in community composition, were more frequently found outside jurisdictional wetland boundaries. Likewise, five near-endemic and endemic plant species were found outside the wetland boundaries. Our data reinforce the need for better understanding of how Carolina bays interact with adjacent landscape elements, and specifically how ecological boundaries are influenced by this interaction.

Macroinvertebrates Associated with Water Hyacinth Roots and a Root Analog

Abstract: The ecological effects of water hyacinth (Eichhornia crassipes), an introduced macrophyte, in freshwater systems depend on the growth and extent of floating mats. We studied macroinvertebrates associated with roots of water hyacinth in the Waccamaw River, a blackwater, tidal river in northeastern South Carolina, USA. In this system, water hyacinth is limited to a few protected bays and backwaters where the ecological effect is unknown. Our goal was to assess whether water hyacinth roots provided unique habitat. Plants representing ambient conditions, plants with defaunated roots, and a root analog (cotton mop strands = mop) were secured to floating frames in open water adjacent to water hyacinth mats. Samples were collected every 2 wk for 2 mo, and invertebrates were identified and quantified. Colonization of defaunated roots began within 2 wk, and invertebrate assemblages differed between roots and mops. The most common taxa on water hyacinth roots were Branchiopoda, Oligochaeta, Talitridae, and Chironomidae (Diptera), whereas Oligochaeta and Chironomidae were predominant on mops. Berosus sp.(Hydrophilidae) was the top-ranked taxon by proportional biomass on roots and mops. Total abundance and taxon richness of macroinvertebrates were greater on roots than on mops. Collector-gatherers were the most abundant functional feeding group (FFG) on mops, whereas distributions of abundance were relatively even among FFGs on ambient and defaunated roots. Predators dominated invertebrate biomass of all treatments, and shredder biomass was higher on roots than on mops. These data suggest that water hyacinth roots provide habitat for a diverse assemblage of macroinvertebrates, a function that should be weighed and assessed with other impacts before management actions are initiated.

Effects of Shoreline Development on Composition and Physical Structure of Plants in a South Carolina High Marsh

Increased freshwater and nutrient runoff associated with coastal development is implicated in dramatically altering estuarine communities along eastern US shorelines. We examined effects of three categories of shoreline development on high-marsh environments within Murrells Inlet, South Carolina, USA by measuring sediment nutrients, porewater salinity, plant species diversity, and above- and belowground plant biomass. Effects on new plant growth also were examined in plot clearing and transplantation experiments. Greater nutrient availability in sediments along developed shorelines was reflected in greater aboveground biomass and nitrogen storage in Juncus roemerianus plant tissue. Plant species composition was not significantly different among levels of shoreline development. Zinc concentrations were greater in sediments from developed shorelines and may represent an easily measured indicator of shoreline development. Recently accelerating shoreline development in the southeastern USA may alter plant production, nitrogen storage, and sediment metal content in salt marshes.

Local-scale characteristics of high-marsh communities next to developed and undeveloped shorelines in an ocean-dominated estuary, Murrells Inlet, SC

Anthropogenic alteration of terrestrial shorelines can have pronounced effects on marine environments at the upland-marsh boundary. Possible terrestrial development effects on several physical and biological variables of high-marsh habitats were examined along developed and undeveloped shorelines in an ocean-dominated, southeastern US estuary. Analyses of sediment characteristics suggested development of the upland boundary affected physical conditions within the high-marsh. For example, pore water salinities were greater along undeveloped shorelines during a non-drought period even after rain events. Significant floral and faunal differences also existed between shoreline treatments. Black needle rush stems were significantly taller and marsh periwinkle densities significantly greater, but eastern coffee bean snail densities were significantly reduced along developed shorelines. Benthic infaunal community abundance and composition also were significantly different between shoreline treatments with sand fly larvae, human pest precursors, either only present or present in greater densities along developed shorelines. Sediment respirometry experiments indicated significant differences in heterotrophic and autotrophic processes occurring between shoreline treatments. Greater sediment surface temperatures along developed shorelines provided one possible mechanism driving high-marsh responses to boundary alteration. The history and extent of shoreline development along with a tendency in ocean-dominated southeastern marshes to resist change likely influenced current ecological conditions within our high-marsh study areas. A greater understanding of the driving mechanisms producing localized effects on salt marshes and recognizing regional differences in marsh resistance to change will facilitate predictions of shoreline development consequences and help in proposing effective management strategies for coastal boundaries.