Award: OCE-1623590

Material exchange between ecosystems is increasingly being recognized as an important aspect of many ecological patterns and processes. These exchanges are especially evident in the coastal zone, which receives terrestrial particulate organic (POM) matter through stream and river discharge. In semi-arid regions such as southern California, the delivery of terrestrial POM to the nearshore largely occurs during winter storms that are intensified during El Niño years of above average rainfall. The processing and fate of this material is not well known, yet results from stable isotope analyses conducted by the Santa Barbara Coastal Long Term Ecological Research program suggest that terrestrial POM may serve as a reservoir of nitrogen that helps to sustain nearshore productivity during months and years when ocean nitrate concentrations are low. In this project we used the opportunity of a prolonged drought coupled with anticipated above normal rainfall associated with the 2015-2016 El Niño to examine the biogeochemical properties of POM in coastal marine sediments of the Santa Barbara Channel during drought conditions and immediately aftera winter with substantial stream discharge. We also sought to determine whether the dissolved organic matter released from POM in marine sediments was in a form that could be readily processed by marine microbes. The high rainfall predicted for southern California during the 2015-2016 El Niño did not happen, extending the severe drought conditions and delaying our project. However, above average rainfall did occur in the winter of 2017 and a one-year no-cost extension of our RAPID award enabled us to collect samples from streams, estuaries and shallow coastal reefs before and after a period of substantial stream runoff. Analyses of these samples is ongoing at the time of this report and have thus far focused on determining the amount of lignins and particulate organic nitrogen in marine sediments. Lignins are a constituent of the cell walls of terrestrial plants that can be used as a tracer for the dispersion of terrestrial POM in marine sediments, while information on the nitrogen content of marine sediments provides much needed insight into the role of marine sediments in serving as a reservoir of nitrogen. Results of analyses of sediments collected during drought conditions identified lignins in marine sediments and showed that they were higher in sediments collected near stream mouths compared to those collected far from stream mouths. Ongoing analyses of lignin and other sediment properties (e.g., grain size, bulk POM, carbon, stable isotope ratios of carbon and nitrogen, and degradation rates of dissolved matter released from POM) collected in March and June 2017 after substantial runoff will enable us to more fully characterize the dispersion and processing of terrestrial POM in marine sediments before and after periods of stream runoff. The nitrogen content of marine sediments collected during the drought was higher than that collected in sediments after substantial stream runoff suggesting that runoff does not contribute significantly to the amount of nitrogen stored in coastal marine sediments. Concentrations of inorganic and organic dissolved nitrogen in the porewater of marine sediments was substantial and the flushing of this porewater was rapid, suggesting that particulate nitrogen is rapidly processed by microbes and released into bottom waters. The broader impacts resulting from this award at the time of this report include three fully documented datasets that are contributing to two PhD dissertations and several planned articles that will be submitted to scientific journals for publication. The project provided research training and mentoring to one postdoctoral researcher, three graduate students and two recently graduated undergraduate students. Information on the project can be found on the BCO DMO website https://www.bco-dmo.org/project/675166 where the datasets listed below are publically available. 1. Marine sediment porewater ammonium and urea concentrations 2. Time series of marine sediment temperatures by depth 3. Particulate organic matter content and composition of stream, estuarine, and marine sediments Last Modified: 04/02/2018 Submitted by: Daniel C Reed