Freshwater invertebrate responses to fine sediment stress: A multi-continent perspective

Abstract

Excessive fine sediment (particles <2 mm) deposition in freshwater systems is a pervasive stressor worldwide. However, understanding of ecological response to excess fine sediment in river systems at the global scale is limited. Here, we aim to address whether there is a consistent response to increasing levels of deposited fine sediment by freshwater invertebrates across multiple geographic regions (Australia, Brazil, New Zealand and the UK). Results indicate ecological responses are not globally consistent and are instead dependent on both the region and the facet of invertebrate diversity considered, that is, taxonomic or functional trait structure. Invertebrate communities of Australia were most sensitive to deposited fine sediment, with the greatest rate of change in communities occurring when fine sediment cover was low (below 25% of the reach). Communities in the UK displayed a greater tolerance with most compositional change occurring between 30% and 60% cover. In both New Zealand and Brazil, which included the most heavily sedimented sampled streams, the communities were more tolerant or demonstrated ambiguous responses, likely due to historic environmental filtering of invertebrate communities. We conclude that ecological responses to fine sediment are not generalisable globally and are dependent on landscape filters with regional context and historic land management playing important roles.

Keywords: aquatic biodiversity; community composition; conservation; ecological threshold; ecosystem function; global scale.

FIGURE 1

Taxonomic (taxa) and functional (traits) change in invertebrate communities across a deposited fine sediment gradient (% visual cover) as identified by Gradient Forest analysis. Split density importance (grey bars) shown on secondary y axis (right hand side). Points along the gradient where the ratio of densities >1 indicate areas where compositional change is highest compared with the turnover occurring elsewhere across the gradient, thus indicating community threshold points.

FIGURE 2

Observed sumz− (red) and sumz+ (blue) maxima (i.e., change points) identified by Threshold Indicator Taxa ANalysis (TITAN) for taxonomic (taxa) and functional (traits) measures of communities. Peak change points indicated as circles, with 5th and 95th percentile distributions as horizontal lines. Change points are filtered to include only pure and reliable taxa/traits.

FIGURE 3

Taxonomic (taxa) and functional (traits) community change identified by Threshold Indicator Taxa ANalysis (TITAN) shown as density plots of sumz for positively responding taxa (blue circles) and negatively responding taxa (red circles) across fine sediment (%) gradient for Australia, Brazil, New Zealand and the UK. Peaks in values across the sumz gradient indicate points of large amounts of community change. The gradient of the cumulative frequency distribution of sumz− and sumz+ indicates the certainty of change point locations with vertical lines indicating higher confidence relative to shallow gradient lines. Sumz are filtered to include only pure and reliable taxa.

FIGURE 4

Correlation matrix for taxonomic and functional indices of community composition with deposited fine sediment (% visual cover) for each country. Colour ramp indicates Spearman's rank correlation coefficient. Only significant pairwise correlations (p < .05) are presented (Holm–Bonferroni corrected).