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Tadpole - sediment interactions of the western toad, Bufo boreas, in a temperate-lentic system Wood, Sylvia Louise Rebecca

Abstract

Sediment and nutrient loading in freshwater systems are leading causes of aquatic habitat degradation in North America. The impacts of fine-sediment and nutrient additions on the growth and survival of Bufo boreas tadpoles and emergent metamorphs was investigated in mesocosm and exclosure experiments. Mesocosm tanks received weekly pulses of fine, organic-rich (8% -9%) sediments to create initial concentrations of 0, 130 and 260 mg/L of sediment and bi-weekly additions of nutrients (N-160 μg/L, P-10 μg/L) in a factorial design. Within mesocosms, tadpole exclosures allowed for quantification of tadpole grazing pressure on periphyton biomass, chlorophyll α and sediment deposition. Tadpoles receiving sediment additions experienced slower growth rates and reduced survival to metamorphosis, though no effects of treatment were detected on metamorphic size or timing. Nutrient additions also lowered survival, but had no impact on other measured parameters. Dissections and gut content analysis revealed that tadpoles ingested sediment in large quantities and scanning electron microscopy showed particles were also found in their gill tissues. Together these results suggest that though organic-rich sediments were readily consumed, tadpoles derived little or no net benefit from these materials. Measures from tiles within the exclosures in the mesocosm experiment demonstrated that tadpoles were able to reduce the standing stocks of periphyton by 35-80% and to clear virtually 100% of all deposited sediment from grazing surfaces. Sediment clearing activities via ingestion acted to restructure the benthic abiotic habitat, but at tadpole densities used in the experiment did not have a beneficial effect on underlying periphyton growth. Under natural conditions, such grazing pressure and sediment removal activities could lead to changes in the algal community and consequent shifts in invertebrate grazers. Together, these results highlight a potential role for Bufo boreas tadpoles as ecosystem engineers in temperate pond habitats.

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