The trophic ecology of phreatic karst aquifers
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The trophic structure of groundwater communities is profoundly influenced by the availability of resources derived from allochthonous, photosynthetic detritus or autochthonous, chemolithoautotrophic production. The four themes of the research presented here, corresponding to the four chapters of this dissertation were: 1) quantification of temporal and spatial variability in organic matter within a biodiverse phreatic karst aquifer, 2) identification of historical and ecological factors that influence trophic length in groundwater systems on a global scale, 3) identification of mechanisms by which sympatric stygobionts partition food resources, and 4) quantification of how the relative importance of photosynthetic and chemolithoautotrophic organic matter to stygobiont communities change in response to hydrogeochemical conditions. These themes are summarized below. Although many of the results of this research specifically relate to the Edwards Aquifer of Central Texas, many of the implications are relevant to other groundwater systems and food webs in general. Chapter 1. δ13C values for fine particulate OM (FPOM) in streams recharging the Edwards Aquifer decreased during regional drought between fall 2010 and spring 2012 and were positively related to FPOM C:N ratios, possibly due to an increasing contribution of periphyton. Along the freshwater-saline water interface of the Edwards Aquifer (FWSWI), δ13CFPOM¬ values were positively related to δ13C values for dissolved inorganic carbon (δ13CDIC) and were depleted relative to δ13CDIC values by 28.44‰, similar to fractionation values attributed to chemolithoautotrophic carbon fixation pathways using DIC as the substrate. δ13CFPOM¬ values also became enriched through time, and δ13CDIC values and δ13CFPOM values at FWSWI sites increased with distance along the southwest-northeast flowpath of the aquifer. Spatial variability in FWSWI δ13CDIC values is likely due to variable sources of acidity driving carbonate dissolution, and the temporal relationship is explained by changes to recharge and aquifer levels that affected transport of chemolithoautotrophic OM across the FWSWI. Chapter 2: Trophic level and uncertainty in trophic level was estimated for 19 stygobiont species from two geochemically distinct sites in the Edwards Aquifer. Additionally, historical and environmental determinates of food chain length (FCL) were assessed using stable nitrogen isotope data from published studies of global groundwater habitats. Despite uncertainty associated with intraspecific δ15N variability and low sample sizes, species averages span 9‰ and strongly suggest the presence of 2° predators. Ecosystem age and, to a lesser extent, ecosystem size and the presence of vertebrates are all positively correlated with FCL. However, incomplete sampling of taxa for isotope analysis obfuscates the strength of these relationships. Chapter 3: Isotopic and mouthpart morphometric data were used to investigate feeding strategies of seven sympatric subterranean amphipods. Amphipods occupied significantly different regions of isotopic space, suggesting utilization of different food resources and trophic specialization. Trophic position, measured as δ15N, was significantly negatively associated with planar area of the mandible and number of molar ridges and significantly positively associated with incisor width. These morphologies are associated with predatory feeding strategies in non-subterranean amphipods. δ13C exhibited weaker relationships with morphometrics, but was significantly negatively correlated with the number of denticles on the setae of the distal margin of the 2nd maxilla. Morphologic and isotopic data suggest the presence of specific scraping and filter feeding food chains. Species showed moderate to absent ontogenetic shifts in trophic position, and body size had little to no effect on trophic position. Chapter 4: I present isotopic and geochemical evidence of a groundwater food chain in which primary consumers show morphologic specializations for scraper/benthic foraging and filter feeding. Specialization is an adaptation to the presence of two disparate food sources: chemolithoautotrophic production by epilithic biofilm and photosynthetic organic matter, the relative prevalence of which varies as a function of hydrological proximity to geographically separated chemolithoautotrophic and photosynthetic organic matter inputs. Horizontal trophic diversity resulting from scraping/ chemolithoautotrophic and filtering/ photosynthetic food chains increases biomass available to support higher trophic levels, including secondary predators. Within the aquifer, species richness decreases with increasing distance from chemolithoautotrophic sources, indicating that chemolithoautotrophy is fundamental for supporting this trophic complexity, especially during periods of decreased photosynthetic production and groundwater recharge during the mid-Holocene altithermal period.