Anthropogenic nutrient enrichment of ecosystems is a global phenomenon that often negatively affects ecological communities leading to lower biodiversity. Elevated nutrients can alter the proportion of nutritional elements such as carbon and phosphorus in food resources and can influence the diversity and type of species present in ecosystems. Many aquatic species have evolved to use low quality leaf litter as food resources (i.e., detritivores); these species are adapted to a diet that has a high ratio of carbon relative to phosphorus. When phosphorus levels in food resources are elevated, as is often the case in human-enriched aquatic ecosystems, the growth of some detritivore species may be negatively impacted due to an increased energetic cost of eliminating excess phosphorus from their bodies. This counter-intuitive, negative effect of elevated nutrients on consumer growth may lead to the loss of detritivore species from aquatic ecosystems. This project will determine whether changes in the ratio of carbon to phosphorus in detrital resources result in predictable effects on growth of insect detritivore species across an enrichment gradient in Ozark Highland streams. The research will combine laboratory experiments with an analysis of a regional data set on stream ecosystems to determine whether anthropogenic phosphorus enrichment of detrital resources is having direct negative effects on aquatic detritivore species diversity.
Results from this project will provide a more thorough understanding of the ways that nutrient enrichment can impact stream systems, information that is useful to improved management of these ecosystems. Through outreach to watershed groups and state agencies, the results of this research will be communicated to the local community and natural resource managers. The proposed activity will support research training and career development of graduate and undergraduate students, including members of under-represented groups. This project represents a collaboration of two beginning investigators from a research university and a primarily undergraduate university.
Intellectual Merit: Human-derived nutrient [nitrogen (N) and phosphorus (P)] enrichment of ecosystems is a global phenomenon that often negatively affects ecological communities leading to lower biodiversity. Elevated nutrients can alter the proportion of nutritional elements such as carbon (C) and P in food resources and can possibly influence the number and type of species present in ecosystems. Water quality and biological condition are reduced in approximately 50% of US streams due to nutrient enrichment. Often, negative impacts on freshwaters are linked to excessive algal growth and toxic algal blooms. The main objective of the proposed research was to determine whether nutrient enrichment had negative impacts on stream biota via changes in detrital food resources, which are important resource subsidies in these systems. Specifically, we wanted to determine if changes in detrital resource ratio (C:P) result in negative impacts on detritivore growth and predictable losses of insect detritivore species loss across a dissolved P gradient in Ozark Highland streams (median threshold declines in richness=60mg DIP/L). Stream insect detritivores are charismatic fauna including stoneflies and caddisflies that channel leaf litter from the watershed into stream production at higher trophic levels. Experimental manipulations of dissolved inorganic P (DIP) provided dose-response relationships between oak and maple litter C:P and decomposition rates. Decomposition rates increased and then saturated in the laboratory at approximately 8 and 20 mg DIP/L for oak and maple, respectively. Litter C:P declined and became stable at approximately 80 mg DIP/L for both species; oak had a greater C:P than maple across the DIP concentration gradient. Similar negative relationships between total P and mixed leaf litter detritus C:P in Ozark Highlands streams were found suggesting laboratory results were representative of field conditions. Combined, laboratory and field studies suggest rapid changes in detrital quantity and quality are occurring at concentrations associated with previously published threshold declines in detritivore richness. Ecologists increasingly use threshold elemental ratios (TERs), which is the food resource ratio where growth switches from one element to another, to predict organism responses to altered food resource ratios. TER calculations are grounded in diet-dependent growth, but growth data are limited for most taxa. Thus, TERs are derived instead from bioenergetics models that rely on simplifying assumptions, such as fixed organism C:P (i.e., strict homeostasis) and no P excretion at peak growth. Our study provides growth data across a gradient of oak and maple C:P for 9 detritivorous insect taxa. Many of the insect taxa showed strict homeostasis during the laboratory feeding experiments. Most taxa whose growth varied considerably with diet C:P (Pycnopsyche sp., Amphinemura sp., Tipula sp., Micropsectra sp.), maintained less strict homeostasis than taxa that grew similarly across the food resource gradient. Further, taxa whose growth varied across diet C:P lost most of the material they assimilated through respiration and excretion (i.e., they had low C and P gross growth efficiencies) regardless of leaf type or stoichiometry. Therefore, the most sensitive taxa to diet C:P violated common assumptions of bioenergetics models that are often used to calculate TERs; we suggest using a growth-based approach instead of models. Low C and P gross growth efficiencies in most taxa underline the importance of how detritivore consumption and post-assimilatory processing could influence whole-stream material storage and nutrient cycling in detrital based ecosystems. Our results suggest an important but species-specific role of leaf litter type in mediating the growth response of diverse shredders to phosphorus enrichment. Caddisfly taxa and the Diptera, Tipula sp. and Micropsectra sp., exhibited lower growth on maple diets. Other taxa, such as the stoneflies Amphinemura and Allocapnia did not respond as strongly to diet litter type. In feeding studies, oak leaf litter had a greater C:P than did maple leaf litter; litter C:N and fungal biomass measured as ergosterol content, were similar between species. Greater growth on oak compared to maple litter challenges the paradigm that food quality is positively related to nutrient content. In addition, stream riparian restorations may be more beneficial for detritivorous insects if they included re-planting oak rather than maple species. Broader Impacts: This research has provided a basic stoichiometric framework for predicting effects on stream insects that are needed to help manage detritus-based stream ecosystems for maintenance of designated uses. Dose-response relationships between leaf litter detritus C:P and decomposition were quantified and detritivorous insect growth patterns across a litter C:P gradient were examined. This project supported 28 presentations at scientific conferences and at local watershed meetings, 6 peer-reviewed journal articles, and 1 feature article. Several more articles are in preparation for peer-review. The proposed activity supported the career development of 6 graduate and 10 undergraduate students (60% under-represented minorities and females) research projects. The principle investigators received tenure and promotion to Associate Professor during the active period for this grant. Further, the principle investigators served as technical advisors for local watershed groups.
