Assessing remote sensing as a tool to monitor hydrological stress in Irish catchments with Freshwater Pearl Mussel populations

Mathias Kuemmerlen, Evelyn A. Moorkens, Jeremy J. Piggott
Trinity Centre for the Environment, School of Natural Sciences, Department of Zoology, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland

-Potential of remote sensing imagery to assess hydrological variables yet un-tapped.
-MSI is a suitable surrogate for soil moisture; NDVI for evapotranspiration.
-Indices show seasonal patterns for both land-cover types investigated.
-Reaction to severe drought diverges among indices and land-cover types.
-Open peat habitats’ regulating function is essential for functional FPM populations.

The West Coast of Ireland hosts many of the few populations of Freshwater Pearl Mussels (FPM) left in Europe. The decline of this keystone species is strongly related to deteriorating hydrological conditions, specifically to the threat of low flows during dry summers. Populations still capable of reproducing require a minimum discharge and flow velocity to support juvenile mussels, or else stress builds up and an entire generation may be lost.

Monitoring environmental and hydrological conditions in small and remote FPM catchments is difficult due to the lack of infrastructure. Indices derived from remote sensing imagery can be used to assess hydrological variables at the catchment scale. Here, five indices are tested as possible surrogates for soil moisture and evapotranspiration, based on two relevant land-cover types: open peat habitats (OPH) and forestry.

Selected indices are then assessed in their ability to reproduce seasonal patterns and in their response to a severe drought event. The moisture stress index (MSI) and normalized difference vegetation index (NDVI) were found to be the best surrogates for soil moisture and evapotranspiration respectively. Both indices showed seasonality patterns in the two landcover types, although the variability of MSI was significantly higher.

During the 2018 drought, MSI visibly increased only in OPH, while NDVI rose only for forestry. The results suggest that OPH enhances the long-term hydrological resilience of a catchment by conserving water in the peat substrate, while industrial forestry plantations exacerbate the pressure on water during drier periods. This has consequences for river discharge, freshwater biodiversity and specifically for FPM. Implementing these surrogates have the potential to identify land-use management strategies that reduce and even avert the effects of drought on FPM.

Such strategies are increasingly necessary in a climate change context, as recurring summer droughts are expected in most of Europe.

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