Nutrient and Organic Matter Cycling in the Nearshore Ocean and Marine Sediment of the Santa Barbara Channel

Coastal regions lie along a dual ecotone: the boundary of terrestrial and marine ecosystems and at the convergence of fresh and saltwater aquatic ecosystems. These ecotones are biogeochemically active regions that are stimulated by the supply and transport of organic material by way of their aquatic linkages. My dissertation addresses questions of organic matter and nutrient supply, retention, and transformation in the coastal regions of the Santa Barbara Channel with a focus on maintaining sufficiently high nitrogen concentrations to support primary production of kelp forests during low nutrient periods. To examine fluctuations of nitrogen concentrations in nearshore marine waters, and their relationships with physical and biological factors, I conducted intensive sampling for ammonium concentrations during the summer season and found a distinct periodicity in concentrations throughout the full water column in relationship to the tidal cycle. To determine if permeable marine sediment is a source of dissolved inorganic nitrogen to the overlying water column, I conducted a multi-year series of nutrient flux measurements using flow-through sediment bioreactors containing sediment collected near kelp forests and found that they are a source of ammonium and total dissolved nitrogen during the summer season. To investigate organic matter supply to marine sediment, I analyzed coastal sediment samples for evidence of terrestrial organic matter input before, during, and after a period with considerable rainfall that followed a 5 year drought, and I found evidence in both stream and marine sediment of terrestrial organic matter inputs becoming increasingly varied and less degraded over time. Using a Santa Barbara Coastal LTER dataset, I examined carbon and nitrogen in giant kelp tissue to evaluate patterns in nutritional content as they relate to changes in seawater temperature and larger oceanographic indices. I found that the nutritional content of giant kelp tissue collected in the Santa Barbara Channel has declined over the past 17 years, and this decline is correlated with increasing seawater temperatures and fluctuations of the North Pacific Gyre Oscillation index