Mike R. Scarsbrook

Comparison of structural and functional approaches to determining landuse effects on grassland stream invertebrate communities

Abstract Effective catchment management in the face of landuse alteration depends on our ability to quantify ecologically significant changes and to discriminate among varying levels of impact. We compared the efficiency of traditional structural indices of change (species composition) with functional measures based on species traits (including life-history, trophic, and morphological features) in an analysis of grassland streams along a gradient of agricultural development (ungrazed native tussock, grazed tussock, extensively grazed pasture, and intensive dairy and deer farming). Streams were categorized in relation to overall agricultural intensity, and separately in terms of increasing nutrient concentrations and fine sediments on the streambed. Only 5 of 60 individual species demonstrated a significant separation across the landuse gradient, whereas 14 of 53 trait categories did so. Traits associated with population resilience (short generation time, asexual reproduction) became more prevalent with more intense agricultural pressure, reflecting predicted increases in intensity and frequency of stream disturbance. We observed a shift away from a tendency to lay unattached eggs at the water surface of the stream, reflecting the increasing likelihood of smothering by sediment, as well as increases in highly flexible and streamlined body shapes. Principal components and correspondence analyses involving species or trait composition all were able to discriminate landuse practises, but more of the overall between-landuse variance was accounted for by trait composition than species composition. All biological measures (species or trait composition) were correlated with both nutrient concentrations and sedimentation. Nutrient concentrations were better related to species and trait-category densities, whereas sedimentation was related to trait-category relative abundances. Overall, species traits successfully complemented traditional structural measures by helping to differentiate the consequences of landuse intensification in grassland stream communities. Rather than simply recording a loss or reduction of species, our functional approach was able to identify sensitive life-history characteristics linked to the functioning of ecosystems, thereby facilitating the development of targeted management actions.

Water quality in low‐elevation streams and rivers of New Zealand: Recent state and trends in contrasting land‐cover classes

Abstract River water quality in New Zealand is at great risk of impairment in low elevation catchments because of pervasive land‐use changes, yet there has been no nationwide assessment of the state of these rivers. Data from the surface‐water monitoring programmes of 15 regional councils and unitary authorities, and the National River Water Quality Network were used to assess the recent state (1998–2002) and trends (1996–2002) in water quality in low‐elevation rivers across New Zealand. Assessments were made at the national level, and within four land‐cover classes (native forest, plantation forest, pastoral, and urban). Finer‐scaled assessments were made by subdividing the large number of pastoral sites into six climate classes, and seven stream orders. At the national level, median concentrations of the faecal indicator bacterium Escherichia coli, and dissolved inorganic nitrogen and dissolved reactive phosphorus exceeded guidelines recommended for the protection of aquatic ecosystems and human health. Water quality state varied widely within land‐cover classes: E. coli and dissolved nitrogen and phosphorus concentrations in the pastoral and urban classes were 2–7 times higher than in the native and plantation forest classes, and median water clarity in the pastoral and urban classes was 40–70% lower than in the native and plantation forest classes. Water quality state in the pastoral class was not statistically different from that of the urban class, and water quality state in the plantation forest class was not statistically different from that of the native forest class. Significant trends in low‐elevation rivers were limited to four parameters: flow (trending down in all instances), and temperature, clarity, and conductivity (trending up in all instances). The trends in flow, temperature, and clarity were apparent at the national scale, and within the pastoral class. The magnitudes of these trends were very low, corresponding to changes of ≤0.5%/ year in parameter medians.

Land‐use effects on the hyporheic ecology of five small streams near Hamilton, New Zealand

Abstract Although the importance of the subsurface saturated interstitial zone (hyporheic zone) to the ecological functioning and maintenance of water quality of stream ecosystems is well known, there is little information on the impacts of different forms of land use upon this zone. Hyporheic physico‐chemistry and invertebrates were compared among small streams draining hill‐country catchments under pasture, exotic pine forest, and native forest near Hamilton, New Zealand. In streams draining native forest, the hyporheic zone harboured a relatively diverse invertebrate fauna comprising mostly taxa common in the surface benthos, although a few apparently obligate hyporheic taxa (ostracods, blind amphipods) were collected. Few individuals and taxa occupied the hyporheic zones of streams draining pasture with some groups such as water mites conspicuously absent. The hyporheos of the stream in exotic pine forest was similar in richness and abundance to that of the pasture streams. Hyporheic water temperature...

Aquatic invertebrate community structure along an intermittence gradient: Selwyn River, New Zealand

Abstract Changes in community structure and life-history traits of benthic invertebrates were examined along a longitudinal intermittence gradient in an alluvial river. The gradient was characterized with modeled and measured hydrologic, chemical, and physical environmental variables. The invertebrates were collected in the Selwyn River, southeastern New Zealand, at multiple sites in each of 4 river sections with distinct hydrological conditions (perennial-losing, ephemeral, intermittent, perennial-gaining). Values of hydrological metrics for each site were generated with an empirical model developed for the Selwyn River. The metrics included 4 that characterized intermittent flow (flow permanence, flow duration, drying frequency, distance to nearest perennial site). Most invertebrate richness and density metrics were significantly higher in the perennial-losing and perennial-gaining sections than in the ephemeral and intermittent sections. A principle components analysis (PCA) separated invertebrate samples from the 4 sections along 2 primary factors. Nine of 13 hydrological metrics, including the 4 intermittence metrics, were correlated with the PCA site scores. Linear regressions indicated that most taxon-richness metrics and some density metrics were related to flow permanence, flow duration, or both. Based on the regression analysis, we predicted that 1.9 taxa/m2 are added with each 10% increase in flow permanence, and 0.5 taxa/m2 are added with each 10-d increase in flow duration. Results from a nestedness analysis indicated that communities at ephemeral and intermittent sites were nested subsets of the communities at perennial sites, and the nesting order of sites was related to both flow permanence and flow duration. Assemblages of taxa with particular life-history traits (life span, fecundity, maximum size, and voltinism) varied linearly with flow permanence and flow duration. The variation in invertebrate communities along the Selwyn River was primarily the result of progressive removal of desiccation-sensitive taxa with increasing intermittence, not to selection for desiccation-resistant specialists. Quantitative intermittence–ecology relationships are needed to predict the consequences of future changes in flow intermittence, but such relationships are rare. The univariate relationships reported in our study contribute to a small but growing array of intermittence–ecology relationships.

Transition from pasture to native forest land-use along stream continua: Effects on stream ecosystems and implications for restoration

Abstract Three first‐order, hill country, pasture streams in Waikato, New Zealand, were chosen to investigate the effects of patches of late‐succession indigenous riparian forest on water quality, epilithon, stream morphology, and aquatic macro‐invertebrates. Sites were situated in open pasture and at two distances (c. 50 and 300 m) into a forest remnant on each stream. Shade, channel width, and epilithon biomass were restored to conditions similar to a native forest control site within 300 m of the streams entering the native forest remnants, whereas water chemistry and levels of surficial fine sediment changed more slowly. Invertebrate community composition showed shifts towards the native forest condition just 50 m into the forest remnants, and full recovery had occurred within 300 m. Results from this study suggest that discontinuous restoration of riparian margins could mitigate some changes associated with pastoral land use, but sediment and water quality problems may not be solved.

Riparian protection and on-farm best management practices for restoration of a lowland stream in an intensive dairy farming catchment: a case study.

Abstract Poor water quality (high concentrations of nitrogen (N), phosphorus (P), suspended solids (SS), and faecal bacteria) in Waiokura Stream, southern Taranaki, New Zealand, is attributed to diffuse and point source (PS) inputs from dairy farming. Trend analysis of concentration time‐series data (2001–2008) and annual yields (i.e., stream load divided by catchment area) showed that significant improvements occurring since 2001 may be attributed to changes in farming practices and riparian management. Yields of filterable reactive P, total P and SS declined by 25–40% as a result of increased riparian protection, a reduction in dairy shed effluent (DSE) pond discharges from 8 to 6 with conversion to land irrigation, and a 25% reduction in the average application rate of P fertiliser. Median annual Escherichia coli concentrations declined at a rate of 116 per 100 ml per year, as a result of fewer PS discharges and improved riparian management. Thus, improvements in stream water quality were attributed to adoption of on‐farm best management practices, fewer DSE discharges and riparian management involving permanent livestock exclusion from stream banks and riparian planting to mitigate runoff from pasture. During 2001–06, N fertiliser use increased by 30% and, with a 130% increase in supplementary cattle feed during 2003–08, led to an increase in average milk solids production 1021 to 1262 kg ha−1 during 2001–06 with the increased production likely associated with increased N leaching losses. Total N and nitrate‐N concentrations and yields increased during 2001–07 as a result of the intensification in land use and increased N cycling. Stream invertebrate surveys using the macroinvertebrate community index (MCI) metric showed little improvement in MCI during 2002–07, probably because of the relatively short timeframe of this study and because water temperatures were not a limiting factor for invertebrate communities. The absence of native forest streams in the proximity of Waiokura Stream that might act as sources of sensitive species to recolonise the restored stream should also be considered as a constraint to improvements in biological community structure.

Controlled release experiments to determine the effects of shade and plants on nutrient retention in a lowland stream

Understanding nutrient uptake and retention in streams remains an important challenge for lotic scientists. In this study a series of pulse and continuous releases of dissolved nutrients were made to shaded and unshaded (reference) reaches of a small lowland stream to determine whether suppression of macrophyte growth by riparian shade impaired nutrient retention. The nutrients were dissolved reactive phosphorus (DRP), total ammoniacal nitrogen (NH4–N) and nitrate nitrogen (NO3–N). Nutrient reductions ranged from 100% of DRP when stream water was anoxic, to 5–10% for NH4–N and NO3–N in the reference reach. Nutrient removals were affected by travel times in each reach. Percentage removals of NH4–N (46 ± 10) and NO3–N (52 ± 14) were higher in the shaded reach than in the swifter moving reference reach (15 ± 8 and 16 ± 10, respectively). DRP (%) removals were 75± 7 and 57 ± 12 for the shaded and reference reaches, respectively. The presence of emergent marginal macrophytes (Persicaria hydropiper) increased stream velocity in the reference reach by reducing the effective channel cross-section area. Shading reduced plant biomass, increased the channel cross-section and lowered velocity in the experimental reach, effecting dramatic reductions in nutrient concentrations over short distances. The opposite effect is more typical for larger, swifter streams having dense stands of submerged macrophytes, where lowering channel plant biomass will cause increased velocities and lower relative nutrient losses. Riparian shade does not necessarily impair nutrient uptake from small streams. Where invasive marginal species such as P. hydropiper dominate headwater streams shade may be beneficial to the protection of downstream waters from eutrophication. Where reduction of nutrient fluxes from small streams is a key objective for protection of downstream waters, active management of streams should seek to increase travel times, allowing greater potential for nutrient uptake. This will need to be weighed against the need for effective drainage in pastoral areas where reduced travel times are usually sought.

Detecting patterns in hyporheic community structure: Does sampling method alter the story?

Abstract Hyporheic invertebrates were sampled in six small headwater streams draining pine, pasture, and native forest catchments. Sites were sampled in autumn and spring using three different hyporheic sampling methods (colonisation pots, pump sampling, and freeze‐coring). Total invertebrate abundance and taxon richness differed significantly among sites on both sampling occasions for pump samples, whereas freeze‐coring did not discriminate among sites, and pot samples showed significant differences in autumn, but not in spring. Differences among sites with respect to community composition were not consistent between sampling methods. Community ordination indicated that samples from colonisation pots tended to group together, whereas pump and freeze core samples were widely separated. Differences across land uses were also apparent (native forest sites formed a distinct cluster separated from pine and pasture sites). Results suggest that colonisation pots may provide a pathway for the entry of epigean organisms into the hyporheic zone even when hydraulic connectivity between surface and hyporheic zones is low. Invertebrate abundance was very low in pump samples, and this method also captured the fewest taxa of the three methods. The size bias known to exist with pump sampling techniques may restrict the usefulness of the method when comparing streams of differing hydraulic conductivity.

Factors controlling litter input dynamics in streams draining pasture, pine, and native forest catchments

Abstract The effects of varying land use on the inputs of litter to streams were investigated in nine small Waikato, New Zealand, hill country streams between June 1995 and October 1997. Mass, timing, and composition of both vertical and lateral litter inputs were measured. Litter inputs to pasture streams were lower than those to streams in native or exotic pine forest. Litter inputs to native forest streams peaked in summer, with leaf material forming the dominant litter‐type throughout the year. The pine forest sites showed a winter minimum, with a pulse of reproductive litter (pollen heads) in spring. One pasture site, where riparian vegetation included willow and poplar showed an autumnal peak, with low inputs at other times of the year. Climate variables (air temperature, rainfall, and windrun) varied in their power of prediction of litter inputs. Mean air temperature showed a strong positive relationship with monthly litter input at the most intensively sampled native forest site. Annual litter inputs were positively related to canopy cover, although canopy‐type modified this relationship. At several sites lateral inputs of litter showed a positive relationship with the slope of the contributing area. Overall, lateral inputs were positively related to % unvegetated groundcover. In open pastures the combination of a lack of riparian trees, and the potential litter‐trapping capacity of pasture grasses, severely limits inputs of coarse particulate organic matter to streams.

Effects of small ponds on stream water quality and macroinvertebrate communities

Abstract Six small constructed ponds (surface area 500–7500 m2, catchment area 28–158 ha) in rural and native forest catchments in the Auckland region had poorer water quality than the streams they replaced. Temperature (24°C) and dissolved oxygen (DO) (4 mg/litre) criteria were exceeded for up to 46% and 84% of days, respectively, during a critical 40‐day summer period. The poor conditions found in ponds, even within undeveloped native forest catchments, indicated that the physical characteristics of ponds (e.g., lack of shade, organic sediments) affected water quality independently of other factors (e.g., land use, riparian protection). The frequency and severity of the exceedences were related to pond size, retention time, and catchment land use; the most degraded conditions were found in rural ponds with largest surface areas and longest retention times. Ponds affected water quality and macroinvertebrate communities downstream. Exceedences of temperature and DO criteria occurred more frequently and were more severe downstream than upstream of ponds. Ponds in rural catchments increased mean daily stream temperatures 3.1–6.6°C during the critical summer period, and temperature differences were three times higher than those in bush catchments (0.8–2.0°C). Elevated temperatures were observed for hundreds of metres downstream owing to the slow rate of cooling (1°C/ 100 m), expanding the extent of adverse effects well beyond the “footprint” of the pond. Macroinvertebrate community composition (sample area 1–3 m2) and values of four commonly used metrics appeared to be significantly affected by ponds in rural and native forest catchments. These finding have important management implications that should lead to modifications (e.g., breaching dams) of the estimated 4500 existing ponds in the Auckland region, where possible, and restrictions on proposals for new “on‐line” ponds.