Interactions between pH and nutrients on benthic algae in streams and consequences for ecological status assessment and species richness patterns

Abstract

Eutrophication and acidification are among the major stressors on freshwater ecosystems in northern Europe and North America, but possible consequences of interactions between pH and nutrients on ecological status assessment and species richness patterns have not previously been assessed. Using data from 52 river sites throughout Norway, we investigated the combined effects of pH and nutrients on benthic algae assemblages, specifically 1) taxaspecific couplings between nutrient and acidity traits, 2) the degree of consistency between different biotic indices, separately for nutrients and acid conditions, 3) the impact of pH on nutrient indices and phosphorus on indices of acid conditions, and 4) the impact of pH and phosphorus supply on diatom and non-diatom taxon richness. We found that 1) acid-tolerant taxa are generally associated with nutrient-poor conditions, with only a few exceptions; this is probably more a consequence of habitat availability than reflecting true ecological niches; 2) correlation coefficients between nutrient indices and TP, as well as acid conditions indices and pH were barely affected when the confounding factor was removed; 3) the association of acid-tolerant taxa with nutrient-poor conditions means that the lowest possible nutrient index at a site, as indicated by benthic algae, is lower at acid than at circumneutral sites. Although 2 this may be an artefact of the datasets from which taxa-specific indicator values were derived, it could lead to a drift in nutrient indices with recovery from acidification; 4) the response of non-diatom taxon richness follows a complex pattern with a synergistic interaction between nutrient supply and pH. In contrast, diatom richness follows a simple additive pattern; this suggests structural differences between diatoms and non-diatom benthic algae in their response to nutrient supply and pH; diatom taxon richness tended to increase with nutrient supply, while non-diatom richness decreased.

Stream order-dependent diversity metrics of epilithic diatom assemblages

“…The relationship between diatom diversity metrics and productivity has remained unclear either because of complex environmental influences or because other variables than nutrients determine diatom diversity (Soininen, 2009). Archibald (1972) found linear negative and Lavoie et al (2008) found positive relationships between diversity and nutrients, Schneider et al (2013) between species richness and nutrient (TP), while Bellinger et al (2006) did not find any significant patterns between them, similar to our results. This is the reason why some authors did not suggest these indices for evaluating ecological conditions (Blanco et al, 2012).…”

“…3 A self- percent organic matter, and chlorophyll showed a clear, significant increase with the stream order until the sixth-order streams and then slightly decreased (Naiman, 1983). Diatom species and genus number showed hump-shaped pattern along the pH (Schneider et al, 2013). Neither the above pattern nor the humped-shaped relationship appeared in case of benthic diatoms along stream order in the study of Minshall et al (1983).…”

Comparing aspirations: intercalibration of ecological status concepts across European lakes for littoral diatoms

“…1a). Schneider et al (2013) demonstrate the problems of evaluating nutrient status in the presence of a strong acid pressure and although this paper deals with rivers rather than lakes, the principles should be transferable. Juggins (2013) points out other issues associated with interpretation of univariate responses in the presence of confounding variables.…”

Benthic soft-bodied algae as bioindicators of stream water quality

“…Some of these studies (e.g., Lavoie et al, 2004;Kelly et al, 2008;Potapova and Carlisle, 2011) contribute to the impression that soft-bodied algae did not improve stressor responsiveness of diatoms alone, but they were based on taxonomy methods that allowed mainly genus-level or coarser identifications of soft-bodied algae which may account for the conclusions. In contrast, studies designed to explore the full potential of soft-bodied algae demonstrated that they enhance bioassessment power along the following lines of consideration: (1) multimetric indices based on entire algal communities, created in southern Californian streams, showed better responsiveness to anthropogenic stress over indices based either on diatoms or soft-bodied algae assemblages alone (Fetscher et al, 2014); (2) the best performing softbodied algal index exhibited greater discriminatory power than its diatom counterpart near the higher end of the range of anthropogenic disturbance (Fetscher et al, 2014); (3) differences in diatom and soft-bodied algal biotic indices were detected in ecosystems which are subject to changing environmental conditions; these differences could provide indications related to ecosystem stability (Schneider et al, 2012); (4) diatom and soft-bodied algal communities respond to nutrient supply and pH differently, with diatom taxon richness generally increasing with nutrient availability in contrast to decreasing soft-bodied algae richness (Schneider et al, 2013); (5) diatoms in conjunction with soft-bodied algae provide a more robust assessment of nutrient conditions, inferring nitrogen (N) limitation in 20% more sites than monitoring with either algal group alone (Stancheva et al, 2013b).…”