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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.