Coastal connectivity: structure and function of recipient beach ecosystems respond to variation in kelp subsidies
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Coastal connectivity: structure and function of recipient beach ecosystems respond to variation in kelp subsidies

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Many ecosystems can be described as dynamic, open, and connected systems that are intrinsically linked to the attributes and processes of neighboring ecosystems. Connectivity among these ecosystems is critically important across multiple levels of biological organization. The cross-ecosystem exchange of organic matter is a well-described example of high variability in ecosystem connectivity. Subsidies of material and energy from donor ecosystems can significantly influence the structure and dynamics of recipient communities and food webs. In systems with pulsed resource subsidies variability in subsidy type, amount, and frequency affects consumer populations, species interactions, and food web complexity causing them to differ from systems with consistent in situ production. The largest magnitude in observed cross-ecosystem fluxes are from marine to terrestrial systems. An exceptional example of this flux is the substantial subsidy of organic matter exported by highly productive nearshore kelp forests to sandy beaches.In the marine realm, canopy forming kelps are considered foundation species that structure the surrounding nearshore reef community. Kelp forests provide many ecosystem functions including food and habitat provisioning, nutrient cycling, increased biodiversity, and subsidies to other habitats. The high turnover of biomass in kelp forests leads to the export of the majority of their net primary production as detritus to adjacent marine ecosystems. A large fraction of that kelp detritus from the donor ecosystem washes ashore on nearby sandy beaches, the recipient ecosystem. Beach ecosystems are characterized by frequent disturbance, low in situ primary productivity, and a reliance on marine subsidies. The inputs of wrack to beaches are strongly linked to their community and food web structure and to ecological functioning. Wrack subsidies from kelp forests to beaches are highly variable across space and time. This variation can affect species and populations directly via changes in habitat and food, indirectly through changes in species interactions, and functionally through effects on ecological processes on the beach. In my first chapter, I examine the role of habitat partitioning in reducing potential negative interspecific interactions across space and time among four co-habiting species of wrack detritivores, talitrid amphipods (Megalorchestia spp.), and how this niche partitioning is mediated by tide phase using field studies and mesocosm experiments. In my second chapter, I evaluate the hypothesis that biodiversity promotes ecosystem function by testing the effects of consumer species diversity and identity on kelp wrack processing rates using laboratory experiments and field consumption assays. In my third chapter, I further explore ecosystem functioning on sandy beaches by comparing in situ fluxes of CO2 from intertidal sediment to wrack and detritivore abundance and in relation to laboratory measured respiration rates of detritivore species. In my final chapter, I explore how wrack subsidies structure the sandy beach macroinvertebrate community, enhance species richness and abundance across multiple trophic levels, and stimulate multiple ecological and biogeochemical ecosystem functions. While the important role of linkages between donor and recipient ecosystems is widely known, there is much to learn with respect to how these links function across spatial and temporal scales and across species. Overall, my findings suggest that the inputs of wrack from kelp forests to beaches is a critical driver of important biotic and abiotic processes that interact to shape the community and promote ecosystem functioning. Four congeners of talitrid species reduce possible negative biotic interactions by partitioning their habitat and surface-active periods and the magnitude of these separations is mediated by tide phase. For the key ecosystem function of kelp wrack processing, consumption rates were not dependent on species richness in the laboratory or field trials. Those rates are body-size dependent and pointed to species identity as a significant factor in this ecosystem function, especially the presence of the two large-bodied species. Expanding on sandy beach ecosystem functioning, I found that wrack abundance across a natural range in variability is a significant driver of sediment CO2 flux and that wrack consumer respiration rates, determined by laboratory measurements, are a notable component (10.5%) of the net CO2 flux. The role of wrack in the sandy beach ecosystem is multifaceted, as demonstrated by my field survey of sites across a large natural range of wrack inputs. Wrack abundance structures the detritivore, predator and full wrack-associated macroinvertebrate communities, increases redundancy within the invertebrate food web, enhances shorebird diversity and abundance, and increases ecosystem multifunctionality. My results demonstrate how the dynamic coupling between sandy beach ecosystems and nearshore giant kelp forests is reflected in the role of kelp wrack subsidies in mediating ecosystem diversity, community structure, and ecosystem functioning of beaches.

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