Species richness and assemblage structure of Trichoptera in Danish streams

• 1 Species richness and assemblages of Trichoptera from 157 ‘pristine’ Danish lowland stream sites were analyzed, for dependence on geographical position of the sites and simple physical variables, using two way indicator species analysis and detrended correspondence analysis.
• 2 A total of 106 species were recorded, representing ≈ 90% of the species pool known from Danish streams. Only seven species occurred at more than half the sites, whereas an additional 11 species were found at more than a quarter of the sites.
• 3 Although sites showed significant regional differences in environmental variables (stream order, width, slope and presence/absence of riparian forest), species richness and assemblages were primarily correlated with stream order, width and slope. Maximum richness was found at the largest (5th order) stream sites.
• 4 Regional differences in species assemblages were found, with several species being absent from the islands Funen and Bornholm. Species assemblages also differed between forested and non‐forested stream sites.
• 5 We found evidence that stream temperature may be of only minor importance in determining Trichoptera species richness and assemblage composition in Danish streams compared to other size‐related physical factors.

     

Canonical variables of aquatic bryophyte combinations for predicting water trophic level

Concentrations of soluble reactive phosphorus (SRP), nitrate, and soluble reactive silicon (SRSi) were monitored in 12 streams draining small catchments (<10 km2) in the English Lake District. The catchments varied with respect to underlying geology, soil type and land cover. Average concentrations of SRP were in the range 0.5–11.2 μg P l-1, and estimated loads ranged from 0.01 to 0.14 kg P ha-1 a-1. The higher concentrations and loads were associated with catchments containing improved pasture. Mean streamwater concentrations of nitrate varied from 55 to 660 μg N l-1, while loads were in the range 0.8–9.6 kg N ha-1 a-1; no general dependence on catchment properties was discerned. Concentrations of SRSi were similar in all the streams (0.8–2 mg Si l-1), and annual loads were in the range 10–26 kg Si ha-1 a-1. Loads of all three nutrients were greatest during the winter, because of higher discharges, but in some catchments containing improved pasture, considerable transport of P also took place during the summer. Concentrations of nitrate in streams draining unimproved moorland catchments are approximately twice those reported for samples taken from similar streams in 1973 and 1974, possibly because of increased atmospheric deposition of inorganic nitrogen (ammonium and nitrate). Concentrations of SRP in such streams were similar to those reported for the earlier samples. Comparisons of stream loads of SRP and nitrate with estimated inputs suggest that catchment soils retain substantial amounts of these nutrients. Implications for surface water eutrophication of changes in P retention by soils are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The hyporheic zone as a source of dissolved organic carbon and carbon gases to a temperate forested stream

The objective of this study was to examine chemical changes in porewaters that occur over small scales (cm) as groundwater flows through the hyporheic zone and discharges to a stream in a temperate forest of northern Wisconsin. Hyporheic-zone porewaters were sampled at discrete depths of 2, 10, 15, 61, and 183 cm at three study sites in the study basin. Chemical profiles of dissolved organic carbon (DOC), CO₂, CH₄, and pH show dramatic changes between 61 cm sediment depth and the water-sediment interface. Unless discrete samples at small depth intervals are taken, these chemical profiles are not accounted for. Similar trends were observed at the three study locations, despite each site having very different hydraulic-flow regimes. Increases in DOC concentration by an order of magnitude from 61 to 15 cm depth with a corresponding decrease in pH and rapid decreases in the molecular weight of the DOC suggest that aliphatic compounds (likely organic acids) are being generated in the hyporheic zone. Estimated efflux rates of DOC, CO₂, and CH₄ to the stream are 6.2, 0.79, 0.13 moles m² d^-1, respectively, with the vast majority of these materials produced in the hyporheic zone. Very little of these materials are accounted for by sampling stream water, suggesting rapid uptake and/or volatilization.     

Influence of geographical extent on typology- and model-based assessments of taxonomic completeness of river macroinvertebrates

1. River InVertebrate Prediction and Classification System (RIVPACS)‐type predictive models are increasingly used to assess the biological condition of freshwaters, but management schemes may also be based on a priori groupings of similar water bodies (typologies) to control for natural variation in biota. The two approaches may lead to disagreements of the biological status of a site, depending on, for example, the spatial scale at which assessments are conducted. 2. We used data from 96 reference and 134 potentially impacted sites from Western and Central Finland to compare RIVPACS‐type models and a simple size‐based typology of rivers for the assessment of taxonomic completeness (the quotient of the Observed‐to‐Expected number of predicted taxa, O / E) of riffle macroinvertebrates. We specifically examined how geographical extent influences bioassessment performance (accuracy, precision and sensitivity to detect impact) of the two approaches. To fully examine the behaviour of the O / E‐index with the two approaches at differing spatial scales, we performed all assessments with a full range of thresholds for predicted taxa occurrence probabilities (p_t from 0+ to 0.9). 3. Both approaches performed consistently better than the corresponding null models. At the larger extent (i.e. assessment encompassing the whole study area), the RIVPACS‐approach performed in all aspects better than the typology‐approach. However, at the smaller extent (i.e. regional assessments) the RIVPACS‐type models and the typologies showed similar accuracy to predict the actual fauna (mean E), similar precision (SD) of cross‐validated O / E and similar sensitivity to detect sites with human impairment. 4. SD(O / E) decreased (i.e. precision increased) consistently with increasing p_t. However, both approaches were most sensitive at intermediate p_t:s (c. 0.2–0.6) when taxa with low predicted occurrence probabilities were excluded. 5. Our results show that RIVPACS‐type predictive models are less susceptible to variations in spatial scale, whereas the performance of a priori typologies increases with decreasing spatial extent. Thus, RIVPACS‐type models are more efficient for large‐scale bioassessments, but at restricted spatial scales, or with an otherwise biologically meaningful stratification, simple a priori classifications can be equally useful for the assessment of taxonomic completeness of river macroinvertebrates.     

Predicting fine‐scale distributions of peripheral aquatic species in headwater streams

Headwater species and peripheral populations that occupy habitat at the edge of a species range may hold an increased conservation value to managers due to their potential to maximize intraspecies diversity and species' adaptive capabilities in the context of rapid environmental change. The southern Appalachian Mountains are the southern extent of the geographic range of native Salvelinus fontinalis and naturalized Oncorhynchus mykiss and Salmo trutta in eastern North America. We predicted distributions of these peripheral, headwater wild trout populations at a fine scale to serve as a planning and management tool for resource managers to maximize resistance and resilience of these populations in the face of anthropogenic stressors. We developed correlative logistic regression models to predict occurrence of brook trout, rainbow trout, and brown trout for every interconfluence stream reach in the study area. A stream network was generated to capture a more consistent representation of headwater streams. Each of the final models had four significant metrics in common: stream order, fragmentation, precipitation, and land cover. Strahler stream order was found to be the most influential variable in two of the three final models and the second most influential variable in the other model. Greater than 70% presence accuracy was achieved for all three models. The underrepresentation of headwater streams in commonly used hydrography datasets is an important consideration that warrants close examination when forecasting headwater species distributions and range estimates. Additionally, it appears that a relative watershed position metric (e.g., stream order) is an important surrogate variable (even when elevation is included) for biotic interactions across the landscape in areas where headwater species distributions are influenced by topographical gradients.     

Leaf‐litter breakdown in pasture and deciduous woodland streams: a comparison among three European regions

1. Human land‐use has altered catchments on a large scale in most parts of the world, with one of the most profound changes relevant for streams and rivers being the widespread clearance of woody riparian vegetation to make way for livestock grazing pasture. Increasingly, environmental legislation, such as the EU Water Framework Directive (EU WFD), calls for bioassessment tools that can detect such anthropogenic impacts on ecosystem functioning. 2. We conducted a large‐scale field experiment in 30 European streams to quantify leaf‐litter breakdown, a key ecosystem process, in streams whose riparian zones and catchments had been cleared for pasture compared with those in native deciduous woodland. The study encompassed a west–east gradient, from Ireland to Switzerland to Romania, with each of the three countries representing a distinct region. We used coarse‐mesh and fine‐mesh litter bags (10 and 0.5 mm, respectively) to assess total, microbial and, by difference, macroinvertebrate‐mediated breakdown. 3. Overall, total breakdown rates did not differ between land‐use categories, but in some regions macroinvertebrate‐mediated breakdown was higher in deciduous woodland streams, whereas microbial breakdown was higher in pasture streams. This result suggests that overall ecosystem functioning is maintained by compensatory increases in microbial activity in pasture streams. 4. We suggest that simple coefficients of breakdown rates on their own often might not be powerful enough as a bioassessment tool for detecting differences related to land‐use such as riparian vegetation removal. However, shifts in the relative contributions to breakdown by microbial decomposers versus invertebrate detritivores, as revealed by the ratios of their associated breakdown rate coefficients, showed clear responses to land‐use.     

Primary production in Rattlesnake Springs,a cold desert spring-stream

Primary productivity and respiration were measured in Rattlesnake Springs, Washington, using the upstream-downstream diel pH-CO₂ curve and harvest methods.Daily P_g and P_n rates averaged 8.7 and 0.9, 0.6 and 0.3, and 9.3 and 1.2 g C m^–2 d^–1 for periphyton, watercress, and total community, respectively. Average photosynthetic efficiencies (%, P _n Lt^–1) were approximately 0.22 and 0.07 for periphyton and watercress, respectively. Annual community P_g was 2 700 g C m^–2 a^–1 and was highest for periphyton (2 526 g C m^–2 a^–1). Periphyton P_n (356 g C m^–2 a^–1) exceeded that of watercress (87 g C m^–2 a^–1). Community R was 2 257 g C m^–2 a^–1, and was highest for periphyton (2 170 g C m² a^–1).Desert streams appear to be enigmas in terms of their relationship between autotrophy and heterotrophy and their ability to be net importers or exporters of organic matter. The fact that they can be autotrophic and net importers of organic matter is probably related to the characteristic flash-flooding of desert streams, and emphasizes the necessity of examining these systems over more than a single annual cycle.     

Factors influencing nitrate depletion in a rural stream

A mass balance procedure was used to analyze rates of nitrate depletion in three adjacent reaches of West Duffin Creek, Ontario, Canada. Daily nitrate losses in individual reaches were highly variable (0.5–24 kg N) during low and moderate stream flows in May–October, 1982–1985. Nitrate removal efficiency (nitrate loss as a % of nitrate input) showed a rapid exponential decline with increased nitrate inputs to each reach. Nitrate losses and nitrate removal efficiency also had a significant negative correlation with stream discharge. The association of large nitrate loads with high stream discharge reduced the nitrate removal capacity of the stream because of shorter residence times and a higher ratio of water volume to stream bed area. Water temperature exhibited a significant positive correlation with nitrate loss which may reflect increased denitrification at higher temperatures.Variations in nitrate losses and nitrate removal efficiency between the three reaches were highly influenced by differences in water residence time. Standarized nitrate losses with respect to water residence time revealed a longitudinal decline in nitrate depletion between the reaches which was associated with a downstream decrease in stream nitrate concentration and in the organic carbon content of fine textured sediments from pool habitats.     

Vegetative characteristics of three low-lying Florida coastal rivers in relation to flow, light, salinity and nutrients

The Chassahowitzka, Homosassa and Crystal rivers along the central Gulf coast of Florida were studied from 1998 to 2000 to identify factors controlling the abundance and distribution of submersed aquatic vegetation (SAV). Each of these three low-lying coastal rivers are spring-fed and exhibit low to moderate absolute flow rates (flows in either direction because of tidal influences, 0.06–0.46ms^–1) with only 14 of the stations sampled for SAV having flow rates in excess of 0.25ms^–1. At those stations where flow rates exceeded 0.25ms–1, the substrate was generally comprised of exposed limestone outcroppings and did not provide a favorable habitat for either submersed macrophytes or macroalgae. The remaining sampling stations, where flow rates were less than 0.25ms–1, had suitable substrates (e.g. mud, mud/sand, and sand) for the colonization and subsequent growth of SAV. Light availability and salinity were determined to be major factors affecting the distribution and abundance of SAV. Sampling stations, where the percent of incident light at the surface reaching the substrate was less than 10, had little or no SAV biomass. Low SAV biomass was also linked to sites where annual average salinities exceeded 3.5. Nutrient loads and nutrient concentrations accounted for little variance in SAV biomass after accounting for flow and related substrate type, light and salinity. These latter factors control the distribution and abundance of SAV in these three Florida coastal rivers.