The exploitation of variable nutrient pools by aquatic predators in tidal freshwater and deep chemosynthetic communities: A multiple stable isotope and compound-specific approach

Aquatic predators are often mobile and have opportunities to exploit prey arising from different nutrient sources within the same ecosystem. Likewise, prey animals may be mobile or migratory and may only be available to predators at certain times. Food choice, availability and degree of utilization among predators is an important aspect of ecosystem function. Predators may preferentially utilize some resources relative to others, selectively influencing prey populations which has throughout the system. In this dissertation, the stable isotopes of carbon, sulfur and nitrogen are used to determine the nutrient sources and trophic relationships among predators in two different systems. In one system it is shown that anadromous fish (Alosa sp.) are vectors for marine nutrient delivery directly to freshwater piscivores in the tidal freshwater of Virginia. These freshwater predators are non-indigenous (introduced) blue catfish (Ictalurus furcatus) and are the apex predator in a system with few top predators. The migrating resource is heavily preyed upon when available and approximately 42% of the catfish biomass carbon and sulfur is derived from Alosa. This percentage is a long-term integrated average from years of feeding. The fact that this introduced predator derives a significant amount of its nutrition from the anadromous Alosa has important conservation implications because I. furcatus is an introduced species and also because the Alosa are economically and ecologically important. This dissertation also shows that predators associated with the chemosynthetic communities in the Gulf of Mexico derive a significant proportion of their carbon and sulfur from chemosynthetic production. The chemosynthetic communities have been viewed as potential oases of biomass (food) for predators in the otherwise nutrient-poor deep ocean; and if so then they should be heavily grazed. Smaller predators such as gastropods, crabs and various worm-like forms derive almost 100% of their nutrition from chemosynthesis. Larger predators, such as fish, giant spider crabs and giant isopods, derive significantly less (generally less than 20%) of their nutrition from chemosynthetic production compared to smaller, less mobile predators. However, there were significant exceptions among these larger fauna and some showed more than 50% of their biomass carbon or sulfur was derived from chemosynthetic sources.