Late Holocene Environmental History of the Los Osos Watershed, Morro Bay, CA

A comprehensive understanding of past changes in wetland ecosystems is integral for creating policies for modern land use practices. The Morro Bay salt marsh is home to a large wetland that has experienced significant environmental impacts in the last few centuries. In this study, sediment cores from the Morro Bay salt marsh were analyzed to discern changes in environment since the time of European contact, which occurred in 1772. The marsh is fed by two creeks (Chorro and Los Osos) and their associated watersheds. Sediment cores taken from a portion of the marsh fed by Los Osos creek were analyzed and the results compared to those from previous studies on cores taken from the Chorro and Los Osos portions of the marsh. Magnetic susceptibility, loss on ignition, pollen, radiocarbon, and X-ray fluorescence (XRF) analyses were conducted. An age-depth model was established for the Los Osos cores using two radiocarbon dates, as well as Erodium cicutarium as a chronological marker. Preliminary pollen analysis from Chorro marsh cores indicates vegetation shifts at the time of contact, when the salt marsh formed. Magnetic susceptibility and XRF data indicate dramatically increased rates of erosion from the time of contact consistently until the present. Influx of non-carbonate inorganic material also indicates a rapid increase in sedimentation in the marsh starting at the time of contact. Comparison of sedimentation rates between the two creeks suggests that differences in watershed geomorphology and land use practices have had pronounced impacts on erosional processes. Over the last decade, the Morro Bay National Estuary Program (MBNEP) has taken more measures to reduce erosion and sedimentation rates in the Chorro watershed, as is reflected by reduced sedimentation rates in MBNEP data collected within the last few years. Our study helps to elucidate the impacts of anthropogenic land use change on wetland systems, and provides much needed data to policy makers seeking to understand and attenuate anthropogenic impacts on sensitive coastal ecosystems.