Thresholds in macroinvertebrate biodiversity and stoichiometry across water-quality gradients in Central Plains (USA) streams

Abstract

Abstract. N and P often limit primary and secondary production in ecosystems, but they also can cause eutrophication and negatively influence sensitive species above a certain level or threshold point. Aquatic biodiversity can have negative threshold relationships with water-quality variables at large scales, but the specific mechanism(s) driving these threshold relationships are not well established. We hypothesized that resource quality (i.e., C:P) might partly drive primary consumer (grazer and detritivore) richness thresholds by altering competitive interactions among species with differing resource demands, but might have less influence on predator richness. We estimated total N (TN), total P (TP), and turbidity thresholds for macroinvertebrate richness across trophic levels and feeding groups in Central Plains (USA) streams. We also determined if mean taxon body C:P of groups with diversity losses were negatively related to TP, a pattern that would suggest that eutrophic communities were dominated by a few species with high dietary P demands. Primary consumers were more sensitive to TN and TP (threshold mean = 1.0 mg N/L and 0.06 mg P/L) than secondary consumers (threshold mean = 0.09 mg P/L), a result supporting the resource quality hypothesis. Turbidity reduced richness regardless of feeding mode (threshold mean = 4.7 NTU), a result suggesting that turbidity and nutrient thresholds were driven by different factors. The TP-richness threshold could be driven partially by changes in food quality because the mean body C:P of shredding and collector-gathering taxa declined as TP increased (threshold mean = 0.07 and 0.75 mg P/L, respectively). Mean scraper C:P was not related to TP, a result indicating other factors might be responsible for the scraper richness threshold. Our results suggest that changes in resource quality could contribute to large-scale losses in biodiversity in nutrient-enriched lotic ecosystems. Within shredder and collectorgatherer macroinvertebrate feeding groups, P-rich food might allow faster growing taxa with high body P demands to out-compete slower growing taxa adapted to lower quality food resources. This pattern suggests that biotic integrity is directly linked to nutrients in streams and that toxicity, low dissolved O 2 , and increased turbidity might not be the only mechanisms leading to reductions in diversity as nutrient concentrations increase.

Considerations for analyzing ecological community thresholds in response to anthropogenic environmental gradients

“…We used EPT because it and other taxonomic richness metrics are among the most widely used metrics of stream biological condition and have been used in recent threshold analyses (e.g., Evans-White et al 2009). We used B-IBI because it combines several univariate community metrics into an aggregate index and is directly applicable to management because it is used in Maryland to classify streams according to biological condition (Klauda et al 1998).…”

Leaf-litter stoichiometry is affected by streamwater phosphorus concentrations and litter type

“…This difference in elemental composition between resource and consumer can limit consumer productivity and provides important selection pressure to promote species diversity in streams , Evans-White et al 2009). Some investigators have explored the effects of variable water-column nutrient availability on leaflitter-associated microbial biomass and decomposition Suberkropp 2003, Baldy et al 2007) and of detrital stoichiometry on the trophic response of invertebrates (Hladyz et al 2009, Small et al 2011, but broad assumptions often are made concerning the chemical alterations in the basal food resources.…”

An Operational Framework for the Advancement of a Molecule-to-Biosphere Stoichiometry Theory

“…Combination of multiple community ecology theories would certainly improve our understanding of community stoichiometry, since competition theory alone does not explain the high biodiversity found in, for example, phytoplankton communities (Passarge et al, 2006). Species diversity is a factor that has received special interest recently, both as a factor affecting (Striebel et al, 2009;Abbas et al, 2013) and being affected by stoichiometry (Evans-White et al, 2009;Lewandowska et al, 2016). The study by Guiz et al (2016) is unique in its attempts at causally explaining both the mean and variation in stoichiometry within local communities and suggests an intriguing convergence in the mean and variance of C:N ratios between plant communities established on different soil fertilities.…”