Bottom-up and top-down forces in tidepools : the influence of nutrients, herbivores, and wave exposure on community structure Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/k930c1670

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  • The relationship between nutrients and community structure is poorly understood in open-coast habitats. I created a system of artificial tidepools, of identical age and physical dimensions, at two sites that differed in wave exposure, and manipulated nutrient levels and the abundance of herbivores. Using these unique field mesocosms, I explored the role of changes in nutrient dynamics and tested two predictive models of community structure in a rocky intertidal community. I modified a simple food-chain model to include the effect of hydrodynamics on nutrient delivery rates and herbivore foraging efficiency. Field experiments demonstrated that nutrients had strong effects on the abundance and productivity of seaweeds. Algal productivity was negatively influenced by herbivory, contrary to model predictions, because species with the potential to increase growth rates when given additional nutrients were virtually eliminated in the presence of herbivores. The effects of both nutrients and herbivory varied in a manner consistent with predicted effects of hydrodynamic forces. Contrary to simple food-chain models, herbivores did not respond to nutrient additions. I assessed nutrient dynamics during low tide, demonstrating that nutrients were rapidly depleted from tidepools. I also examined variation in nutrient uptake rates relative to the experimental treatments described above, for both whole pools and on a biomass-specific basis. Nutrients were almost always removed from pools at the same rate dispensers added them. Uptake rates were significantly correlated with the abundance of fleshy seaweeds. Synthesizing the results of these and other studies, I proposed that the abundance of tidepool seaweeds can be modeled as a function of pool volume, degree of tidal isolation, water flow at high tide, and herbivory. I tested the predictions of a functional group model and evaluated the validity of equating physical and biological disturbances by examining algal diversity and abundance patterns in tidepools along gradients of potential productivity, herbivory, scour and wave exposure. The abundance of functional groups varied along environmental gradients, but not always in a manner consistent with predictions. I suggested that physical and biological processes must be modeled separately, and that better operational definitions of environmental potentials will aid in development of these models.
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