Spatial Abundance Quantiles as a Tool for Assessing Habitat Compression in Motile Estuarine Organisms

Estuarine zooplankton, hyperbenthos and nekton often move upstream in response to reduced freshwater inflows. In estuaries with drowned-river-mouth morphologies, this upstream movement repositions organisms within estuarine reaches that have reduced volumes. The movement into more volumetrically confined habitats and into areas near water-control structures raises concerns about habitat compression and associated reductions in estuarine carrying capacity, particularly if this trend is associated with excessive freshwater withdrawal. We present a simple method for identifying habitat compression along the longitudinal estuarine axis and apply it to faunal transect surveys of several west-central Florida tidal rivers. Regression was used to compare the length (km) of the interdecile range (10th–90th cumulative abundance percentiles) under variable freshwater inflow conditions. The use of quantiles frees the analysis from assumptions about the shape of longitudinal abundance distributions. We demonstrate how his approach can also be used to identify sampling artifacts and to quantify the extent of impingement on estuarine dams. In estuaries that have strong seasonal variation in inflow, useful results can be obtained withavailable.