Appendix L. Alternative interpretations of Experiment 3.
Various explanations can be proposed for the lack of an apparent grazing effect in Experiment 3. Notwithstanding the lack of detectable Shrimp effects on epiphyte standing crop, grazing might still have played a role in Experiment 3; for example, if epiphyte removal was balanced by the rate of replacement. Such a balance could exist if nonselective grazing enhanced the relative amounts of faster growing epiphytes during early succession. Based on phytopigments, Quiñones-Rivera and Fleeger (2005) found that grass shrimp non-selectively graze all types of epiphytes growing on Spartina. Another way that a balance between grazing and growth rates could be achieved would be through the facilitation of rapid epiphyte colonization through grazing. Such conditioning of detritus surfaces by grass shrimp does occur (Welsh 1975). Also, shrimp grazing could have altered the algal canopy architecture in a way that increased photosynthetic efficiency for Ruppia without decreasing epiphyte biomass, as has been shown for Littorina littorea (Sommer 1999).
The lack of a real grazing effect might signify that the standing crop or nutrient content of epiphytic algae was below some threshold level of profitability on newly emergent SAV in Spring. The lack of an interaction effect between Shrimp and Nutrient Factors in Experiment 3 suggested the lack of a threshold response within the Nutrient addition treatment; and reinforces the interpretation that grazing was relatively unimportant during this experiment. Epiphyte loading is initially low during emergence of new plant growth in Spring, after which epiphyte production subsequently increases with water temperature and day length (Quiñones-Rivera and Fleeger 2005). Seasonal profitability of epiphytes also might increase as algal succession unfolds or as seasonally abundant associated meiofaunal organisms increase. Omnivorous grass shrimp can consume many alternative food sources besides epiphytes, although grass shrimp may rely on epiphytes when they are abundant (Morgan 1980).
Grazing intensity was likely lower during Experiment 3 when cooler water temperatures prevailed relative to the other two experiments. Estimated grass shrimp grazing rates could have been as much as 1.5 fold lower at 26° C (Experiment 3) than at 30°C (Experiments 1 and 2) (Morgan 1980). Finally, the inconsistent epiphyte response in Experiment 3 might reflect the high loss of grass shrimp (55%) within the High Shrimp treatment, possibly from starvation or other density-dependent effects (see Appendix J).
Morgan, M. D. 1980. Grazing and predation of the grass shrimp Palaemonetes pugio. Limnology and Oceanography 25:896902.
Quiñones-Rivera, Z. J., and J. W. Fleeger. 2005. The grazing effects of grass shrimp, Palaemonetes pugio, on epiphytic microalgae associated with Spartina alterniflora. Estuaries 28:274285.
Sommer, U. 1999. Periphyton architecture and susceptibility to grazing by periwinkles (Littorina littorea, Gastropoda). International Revue Hydrobiologie 84:197204.
Welsh, B. L. 1975. The role of grass shrimp, Palaemonetes pugio, in a tidal marsh ecosystem. Ecology 56:51330.