Patterns of macrophyte wrack deposition on sandy beaches of the Pacific Northwest coast, U.S.A. Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rf55zb339

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  • Dislodged macroalgae and seagrasses, also known as marine wrack, frequently wash into coastal ecosystems from the ocean and are potentially important ecological resources for biological communities. These!nutrient and organic matter subsidies may be especially important on sandy beaches, where little in situ primary productivity exists for higher trophic levels. To better understand the prevalence of marine wrack as a coastal subsidy!to nutrient-poor sand and beachgrass ecosystems, we investigated the pattern of deposition and composition of wrack that washes onto Pacific Northwest sandy beach ecosystems. We conducted a large-scale, observational study at 12 coastal sites located from southern Washington to northern California to estimate the patch density, biomass, total biomass, and species composition of wrack on each beach. All sites were surveyed in both early summer and early fall to understand how the temporal patterns of certain processes (e.g., storms and algal senescence) affect wrack presence. Linear mixed-effect models were used to determine the effect of site location and season, as well as a suite of environmental predictors including cross-shore and alongshore upwelling and shoreline change rate (SCR). At all sites,!marine wrack increased in patch density and biomass on beaches in the fall suggesting that productivity over the summer, storm activity, and wave action in the fall may have affected the delivery of wrack. In terms of spatial distribution, wrack patch density (per m²) was greatest in the northern region (Greys Harbor to Sand Lake) and wrack biomass (per m²) was greatest in the southern region (Bandon to Cape Mendocino). However, overall total wrack biomass (per block), an integrative measure of patch density and biomass, was greatest in the south (Bandon to Cape Mendocino), intermediate in the north (Greys Harbor to Sand Lake), and lowest in the central region (South Beach to North Spit), although Greys Harbor and Cape Lookout in the north had similar amounts to sites in the southern region. Further analysis showed that cross-shore upwelling had a positive effect on wrack patch density and wrack biomass, while total biomass was not correlated with the environmental variables we tested. Composition varied predictably across the coast, with eelgrass present in greater proportions at northern sites and kelp present in greater proportions at central and southern sites. Kelp and eelgrass biomass were also strongly negatively influenced by distance from rocky reefs and estuaries, their respective source habitats (kelp biomass decreased further from rocky reefs, while eelgrass biomass decreased further from estuaries). In addition, kelp biomass was positively affected by SCR, while eelgrass biomass was negatively affected by cross-shore upwelling and positively affected by alongshore upwelling. This study, one of the first to examine large biogeographic scale patterns of wrack deposition, suggests that upwelling, outwelling (the movement of debris from estuaries to the open coast), proximity to source habitat, and beach geomorphology interact to determine the distribution and abundance of wrack onshore. Wrack plays an important role in structuring beach community and food web dynamics, and may also influence beach grass productivity, and thus dune structure and function. Further research on how wrack subsidies affect beach and dune ecosystems, both as a food resource and a nutrient resource, would provide greater insight to its role in these systems, and could have potential implications for dune restoration and conservation.
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  • description.provenance : Submitted by Jessica Reimer (reimerje@onid.orst.edu) on 2014-10-08T18:21:20Z No. of bitstreams: 2 license_rdf: 1223 bytes, checksum: d127a3413712d6c6e962d5d436c463fc (MD5) ReimerJessicaN2014.pdf: 2573510 bytes, checksum: 89ee605f27013411bd2d18a716fd2fac (MD5)
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