Arved C. Schwendel

Linking disturbance and stream invertebrate communities: how best to measure bed stability

Abstract Substrate stability is a key determinant of stream invertebrate community composition, but its measurement can be problematic. Stream ecologists often use different approaches and techniques to quantify bed stability, and this variability makes comparison among studies difficult. We examined the link between 6 reach-scale measures of substrate stability and invertebrate community metrics in 12 New Zealand mountain streams. The strength of the link varied with the method used to define substrate stability. We used morphological budgeting to measure spatial patterns and volumes of scour and fill. We found that as erosion of sediments increased, invertebrate diversity declined exponentially. In particular, increases in the volume of scour reduced taxonomic richness, whereas deposition of coarse sediments was less relevant for invertebrate communities. Overall, the distance travelled by in-situ-marked tracer stones was most strongly linked with all invertebrate community metrics, whereas the bottom component of the Pfankuch Index related very well to diversity. Both metrics showed near-linear declines in diversity with decreasing stability. In contrast, the link between invertebrate communities and the proportion of bed area affected by entrainment was weak. Therefore, we propose tracer-based indices and the Pfankuch bottom component as the most suitable measures for research involving invertebrate–substrate-stability relationships. Measures derived from in-situ-marked tracer stones reflected only entrainment and transport of particles. In contrast, the bottom component of the Pfankuch Index encompassed the widest range of bed-stability characteristics but is prone to observer bias. An objective method that combines the efficiency of the Pfankuch Index with the characteristics measured using tracer stones could serve as a powerful explanatory tool in stream ecology.

A macroinvertebrate index to assess stream-bed stability

Biotic indices based on community composition and calculated from sensitivity scores assigned to individual taxa are commonly used as indicators for ecological integrity of fluvial ecosystems. Macroinvertebrate indices can assess water quality but invertebrate community composition also responds to other environmental factors including stream bed disturbance. This study presents a biotic community index that assesses stream bed stability in stony riffles. This Macroinvertebrate Index of Bed Stability is calibrated on transport and entrainment of in situ-marked tracer stones in 46 streams in New Zealand’s North Island, representing a wide range of substrate stability. Scores were investigated for 67 common invertebrate taxa using Indicator Species Analysis based on taxa abundance at varying levels of substrate stability. The resulting site score, weighted by taxa abundance, improved a predictive model of bed stability, generated with model trees, when added to the pool of habitat variables and explained 69% of the variation in bed stability. Site scores were strongly correlated with measured bed stability at the development sites, but not at eight independent validation sites, suggesting the need for further testing on a larger dataset including streams in other regions of New Zealand, and overseas.

Tracing of particulate organic C sources across the terrestrial-aquatic continuum, a case study at the catchment scale (Carminowe Creek, southwest England)

Soils deliver crucial ecosystem services, such as climate regulation through carbon (C) storage and food security, both of which are threatened by climate and land use change. While soils are important stores of terrestrial C, anthropogenic impact on the lateral fluxes of C from land to water remains poorly quantified and not well represented in Earth system models. In this study, we tested a novel framework for tracing and quantifying lateral C fluxes from the terrestrial to the aquatic environment at a catchment scale. The combined use of conservative plant-derived geochemical biomarkers n-alkanes and bulk stable δ13C and δ15N isotopes of soils and sediments allowed us to distinguish between particulate organic C sources from different land uses (i.e. arable and temporary grassland vs. permanent grassland vs. riparian woodland vs. river bed sediments) (p<0.001), showing an enhanced ability to distinguish between land use sources as compared to using just n-alkanes alone. The terrestrial-aquatic proxy (TAR) ratio derived from n-alkane signatures indicated an increased input of terrestrial-derived organic matter (OM) to lake sediments over the past 60years, with an increasing contribution of woody vegetation shown by the C27/C31 ratio. This may be related to agricultural intensification, leading to enhanced soil erosion, but also an increase in riparian woodland that may disconnect OM inputs from arable land uses in the upper parts of the study catchment. Spatial variability of geochemical proxies showed a close coupling between OM provenance and riparian land use, supporting the new conceptualization of river corridors (active river channel and riparian zone) as critical zones linking the terrestrial and aquatic C fluxes. Further testing of this novel tracing technique shows promise in terms of quantification of lateral C fluxes as well as targeting of effective land management measures to reduce soil erosion and promote OM conservation in river catchments.

Measurement of Stream Bed Stability Characteristics Relevant to Lotic Ecosystems

Climate change and anthropogenic pressure have led to significant impacts on rivers and streams in most parts of the world (Wohl, 2006; Vorosmarty et al., 2000). Changes in rainfall patterns and altered land use may affect the flow regime, catchment erosion and thus sediment supply to river systems. Human influence via damming, water abstraction and channel modification additionally impacts on flow regime and sediment dynamics (Wang et al., 2001). These alterations directly affect stream bed stability which is a key habitat parameter for lotic ecosystems (Jowett, 2003). As a response to changes in substrate stability the species composition and functionality of these ecosystems may adjust. Thus it is important for ecologists to be able to identify relevant characteristics of stream bed stability and to quantify them.