Beach Wrack Nutrient Leaching and Community Composition on Gravel Beaches

Abstract

Beaches world-­‐wide have low primary production due to a stressful environment and action of strong hydrodynamic forces. For this reason, input from marine wrack as well as terrestrial debris is an important source of production in food webs. Wrack can also provide food and habitat for meiofauna that help transfer nutrients up the food web. These interactions have been examined on sandy beaches, but not on gravel beaches. I examined the relationship between beach wrack type, nutrient leaching, and meiofauna colonization on a gravel beach in San Juan County, WA. I used three different types of wrack: Fucus disticus, Zostera marina, and leaves from Prunus avium (Cherry). I conducted two different experiments: a nutrient leaching experiment by measuring nitrate and phosphate leaching from wrack over seven weeks relative to seawater controls with daily tidal simulation, and a community composition experiment using wrack-­‐filled packets placed on a gravel beach. Both nitrate and phosphate showed similar patterns across all wrack types. Nitrate was absorbed by all wrack types at the beginning of the experiment, released in the middle, then absorbed again at the end. Phosphate was absorbed by Zostera and Fucus (though released by cherry) at the beginning, then released by all types of wrack, then all types had similar nutrients as seawater. Diversity of meiofauna was significantly correlated with final weight of the wrack inside the packets. Diversity of meiofauna changed significantly across type of wrack by date sampled. My data suggest that external factors such as bacteria, weather, and decomposition are more important than wrack type in nutrient leaching and community composition.