Date of Award
Master of Science
Eutrophication is a pervasive issue in freshwater systems. However, the effects of nutrient and piscivorous fish additions on freshwater invertebrate diversity, functional feeding groups (FFGs), and stoichiometric ratios are difficult to predict. These relationships are important to quantify in small lentic systems where nutrients like nitrogen (N) and phosphorus (P) and piscivorous fish are frequently added to enhance sport fishing. To study this, I administered 2 treatments, fertilization and Largemouth Bass (Micropterus salmoides, LMB) additions, to 28 experimental ponds in a fully crossed experimental design and estimated benthic invertebrate biomass and diversity before and after administering the treatments. I also sought to improve my understanding of the relationships between specific FFGs and nutrients by measuring the elemental composition of coarse (CPOM) and fine particulate organic matter (FPOM) and invertebrates before and after administering the treatments. Because consumer stoichiometric ratios are shaped by their environment and life histories, which can be governed by their functional role and/or evolutionary history, I was interested in determining whether my treatments and invertebrate FFG or taxonomic group better explained variation in invertebrate elemental nutrient content. I verified that fertilization altered nutrient availability in the ponds by measuring the nutrient content and biomass of FPOM and CPOM. FPOM P concentrations (μgP/L) were higher in ponds that received fertilizer (f= 4.84(1,22), P= 0.03) and LMB (f= 5.26(1,22), P= 0.04). Fertilization also increased CPOM biomass (f= 8.95(1,21), P=0.007). I found that Shannon diversity was increased or better maintained in ponds that received the fertilizer treatment (f= 6.54(1,20), P= 0.02). Predator (f= 4.47(1,21), P= 0.047) and GC (f= 5.05(1,21), P= 0.04) biomass increased after the addition of LMB to ponds. For scraper biomass, there was a significant interaction between fertilization and LMB additions such that LMB additions increased scraper biomass, but fertilization counteracted this effect (f= 4.93(1,17), P= 0.04). Fertilization increased the biomass of FCs (f= 6.64(1,9), P= 0.03) but decreased shredder biomass (f= 5.71(1,6), P= 0.004). In control ponds, SIMPER analysis revealed that a decrease in the predatory crayfish Cambaridae accounted for the greatest differences between pre and post sampling, followed by Odonates. Odonates and gastropods caused the greatest shifts in both fertilized and LMB ponds. The difference between fertilized and LMB ponds was that Coenagrionidae biomass did not increase and Dytiscidae did in fertilized ponds. Invertebrate elemental stoichiometric ratios were not affected by our treatments, which supports the assumption of homeostasis in ecological stoichiometry models. However, nutrient ratios varied across both FFG and taxonomic group, likely because of varying diets and life history strategies. Model comparison suggests that order best explained most of the variation in elemental ratios across invertebrates; however, FFG and order best explained invertebrate %P content and class and FFG best explained %C content. We found that predators, specifically Odonata and Hirudinea, have higher N content than other FFGs, and gathering-collectors (GCs), specifically Ephemeroptera, have a higher P content. Predators may have a higher N content because they feed on higher dietary N than herbivores. I determined that order was the best predictor of elemental nutrient content; however, FFG is also an acceptable predictor, which is important because identifying species to FFG is more rapid than taxonomic identification. In conclusion, humans are heavily altering the availabilities of N and P in freshwater ecosystems; thus, knowing the stoichiometric contents of individuals (at the order or FFG level) can facilitate predictions about how communities respond to nutrient additions. Furthermore, understanding how invertebrate communities respond in their functional makeup is of importance.
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