The use of benthic organisms to indicate ecological integrity is beneficial for several reasons. Benthos are indicators of many environmental stressors because they are able to integrate spatial and temporal changes in ecosystem factors (Smith et al. 2001). Another rational for using benthos is that they cannot travel large distances when ecosystem condition change. This is important because abundance, diversity, and biomass changes can be measured over long periods of time at established sampling points and more accurately reflect the changes in ecosystem condition. Other marine organisms such as fish are more mobile and changes in biomass, diversity, and abundance can be the result of other biological responses including avoidance.

For the domino theory, the species used to determine the ideal freshwater inflow regime for a given estuarine system that will restore or maintain a sustainable estuary are that estuary’s benthos. Benthos are ideal indicator species for many reasons, namely their ability to reflect changes in salinity. Although benthos have a wide salinity range, they often only function within a minute range (Montagna et al. 2013). Table 4 shows the salinity tolerance range of several estuarine system indicator species.

Benthos samples are taken using cores. Cores are tubes that take replicate samples at each study site at the surface of the bottom substrate and are then taken back to the lab where the substrate is removed and benthic organisms can then be studied to determine species composition, abundance, biomass and diversity. Replicates are multiple samples from the same location at the same time to increase confidence in data measurements.

Changes in benthos are determined by measures of their status and are referred to as indices. Therefore, qualifying and quantifying a species using indices can be used to determine the integrity of estuarine resources. For the purpose of understanding the relationship of freshwater inflows with the estuary, and as discussed earlier, indices that are studied here are of the indicator species of interest. The quantifiable indices include the indicator species composition, abundance, biomass, and diversity. In this way, indicator species indices can provide a measure of ecological changes in the environment.  Salinity effects on selected estuarine macrobenthic and epibenthic organisms.

Salinity tolerance of estuarine system indicator species. Source: Montagna et al. 2013

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Access further information on species integrity at the Ordination Methods for Ecologists' Abundance of Species in Ordination.