Appendix K. Possible biases involving faunal interactions.
Unknown differences in faunal interactions between experimental and ambient conditions might have influenced the experimental outcome, such as predation on small grazers by grass shrimp or direct feeding on Ruppia by pinfish. Large grazers as well as potential predators of grass shrimp were excluded by the cylinders. Small mesograzers were not excluded from microcosms, although their movements would have been restricted. Although other mesograzers such as amphipods undoubtedly occurred within the Ruppia habitat; they were not very abundant (D. Drury McCall, personal observation). Nevertheless, epiphyte chl [a] concentrations within the Medium Shrimp treatment mimicked those in the ambient environment in all three experiments, possibly reflecting comparable epiphyte grazing pressure. A conservative estimate of natural density of grass shrimp in Experiment 1 (i.e., 196 m2) compared well with the effective density (i.e., 180 m2) in the Medium Shrimp treatment. Moreover, orthogonal contrasts revealed very little evidence of enclosure artifacts among the response variables (Appendix H). Other studies have found ambient grazing intensity to be effective in abating epiphyte overgrowth of SAV (Lin et al. 1996; Worm et al. 2000). Conceivably, heightened grass shrimp grazing resulting from lowered predation pressure (Quiñones-Rivera and Fleeger 2005) compensated for artificial deficits in epiphyte grazing by other organisms.
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