The impact of increasing saline penetration upon estuarine and riverine benthic macroinvertebrates

Coastal and estuarine systems worldwide are under threat from future global climate change, with potential consequences including increased penetration of tidal driven salt water into estuarine surface waters. In coastal climate change research this issue has been neglected, despite increases in salinity potentially detrimentally impacting upper estuarine and riverine ecosystem function worldwide. In this research the first direct attempt is made at predicting the impact of future climate-driven increases in saline penetration upon estuarine and riverine benthic macroinvertebrate communities through the acute salinity tolerances of selected species. Two study estuary-river systems were selected based upon their perceived susceptibility to future increases in saline penetration. These estuaries exhibited dynamic tide and salinity profiles with large salinity ranges recorded over a tidal cycle and significant differences in saline penetration extents between low (summer) and high (winter) freshwater river discharge conditions. Salinity was shown to be the dominant environmental variable driving benthic macroinvertebrate species distributions in both estuaries; however additional environmental factors were shown to have locally dominant effects (i.e. sediment grain size). Laboratory and field based salinity toxicity experiments suggested that the tolerance of euryhaline-marine and brackish water species to reductions in salinity corresponded well to tolerance values in published literature. In contrast limnic derived species exhibited greater salinity tolerance under laboratory and field tidal cycle conditions than those published. For all test species, actual field distributions did not reflect distributions anticipated by saline tolerances alone, likely due to the effects of additional biotic and abiotic factors experienced under field conditions. The macroinvertebrate species salinity tolerances did not account for actual field distributions with sufficient accuracy to allow for precise prediction of future distribution patterns under projected saline penetration profiles due to the influence of additional environmental factors. Under the high greenhouse gas emissions climate scenario (SRES A1FI) for the years 2020, 2050 and 2080, projected relative sea level rise was shown to result in an increase in both the upstream extent of saline penetration and gradient of maximum salinity zones in both estuaries. However these increases were moderate even under worst-case conditions (0.32 km and 0.15 km) and unlikely to result in large-scale changes to the benthic macroinvertebrate community. However, in addition to relative sea level rise, predicted changes to freshwater river discharge (climatic and anthropogenic induced) and channel morphology could result in significant increases in the upstream extent of saline penetration predicted for projected sea level rise alone. This could result in critical consequences for estuarine and riverine ecology and ecosystem function across all trophic levels. A conceptual model exploring the potential ecological effects of both increases in saline penetration and changes to the estuarine system (anthropogenic and climatic) was developed, and implications for the future management of estuarine and riverine environments were identified

Is the hyporheic zone a refugium for aquatic macroinvertebrates during severe low flow conditions?

The potential role of the hyporheic zone as a refugium for stream invertebrates during hydrological perturbations was acknowledged more than five decades ago. However, field evidence to support the hyporheic refuge hypothesis during periods of flow recession and severe low flow remains equivocal. Some studies report fauna using the hyporheic zone during periods of flow cessation whilst others have recorded little or no refuge use due to limited habitat availability or harsh abiotic conditions. We assessed aquatic macroinvertebrate community changes associated with severe low flow conditions during a severe supra-seasonal drought on the Little Stour River (UK). Paired benthic and hyporheic samples were collected from four sites (two perennial, two intermittent) on the upper reaches of the river. The number of benthic taxa and the proportion of benthos (particularly the amphipod Gammarus pulex) within the hyporheic zone relative to those in the benthic samples increased significantly during the latter stages of the drought at all sites. These changes coincided with elevated benthic and hyporheic water temperatures rather than a reduction in river discharge alone. The abundance of obligate hypogean macroinvertebrates also increased during the latter stages of the event, suggesting that hypogean taxa may also utilise the shallow hyporheic zone during adverse environmental conditions. Our results, based on paired surface-hyporheic field samples at multiple sites, support the hyporheic refuge hypothesis within a temperate groundwater-dominated stream during severe drought. The results also clearly demonstrate the importance of considering surface-subsurface linkages when assessing responses to disturbance in streams

How freshwater biomonitoring tools vary sub‐seasonally reflects temporary river flow regimes

Characterizing temporary river ecosystem responses to flow regimes is vital for conserving their biodiversity and the services they provide to society. However, freshwater biomonitoring tools rarely reflect community responses to hydrological variations or flow cessation events, and those available have not been widely tested within temporary rivers. This study examines two invertebrate biomonitoring tools characterizing community responses to different flow-related properties: the “Drought Effect of Habitat Loss on Invertebrates” (DEHLI) and “Lotic-invertebrate Index for Flow Evaluation” (LIFE), which, respectively reflect community responses to habitat and hydraulic properties associated with changing flow conditions. Sub-seasonal (monthly) variations of LIFE and DEHLI were explored within two groundwater-fed intermittent rivers, one dries sporadically (a flashy, karstic hydrology—River Lathkill) and the other dries seasonally (a highly buffered flow regime—South Winterbourne). Biomonitoring tools were highly sensitive to channel drying and also responded to reduced discharges in permanently flowing reaches. Biomonitoring tools captured ecological recovery patterns in the Lathkill following a supra-seasonal drought. Some unexpected results were observed in the South Winterbourne where LIFE and DEHLI indicated relatively high-flow conditions despite low discharges occurring during some summer months. This probably reflected macrophyte encroachment, which benefitted certain invertebrates (e.g., marginal-dwelling taxa) and highlights the importance of considering instream habitat conditions when interpreting flow regime influences on biomonitoring tools. Although LIFE and DEHLI were positively correlated, the latter responded more clearly to drying events, highlighting that communities respond strongly to the disconnection of instream habitats as flows recede. The results highlighted short-term ecological responses to hydrological variations and the value in adopting sub-seasonal sampling strategies within temporary rivers. Findings from this study indicate the importance of establishing flow response guilds which group taxa that respond comparably to flow cessation events. Such information could be adopted within biomonitoring practices to better characterize temporary river ecosystem responses to hydrological variations