Soil Characteristics and Sediment Deposition Rates Across Intertidal Habitats in Morro Bay, California

Assessing the spatial variation in accretion rates and soil properties helps increase understanding of tidal wetland processes and can be used to calibrate model of long-term wetland resilience. Nine deep soil cores (50-100 centimeters) were sampled across tidal flat, tidal marsh, and upland transition areas across Morro Bay estuary. Short (7 centimeters) soil cores were also sampled across select tidal marsh locations, associated with a sediment deposition and marker horizon surface accretion study. A gouge auger was used to extract the sediments, and then cores were sectioned in the field. Wet and dry bulk density was calculated, as well as percent organic matter via loss-on-ignition. The marsh soil cores that were positioned adjacent to long-term surface elevation table monitoring stations were used to calculated mineral and organic matter accumulation rates. Accretion rates from the marker horizon plots were measured biannually from 2023-2025. Grain size classifications, coarse sand, fine sand, and mud, from a subset of deep soil cores as well as from opportunistic samples across the tidal flat were determined by passing ground samples through two sieves.

Sediment deposition in tidal marshes can vary substantially across space and time. We deployed sediment traps in 2023 to better understand deposition patterns across tidal marshes in Morro Bay, California. Traps consisted of quantitative pads attached to a ceramic tile with rubber bands. Traps were distributed in transects perpendicular to tidal creeks and spaced at 0.5, 2, 6, 12, and 24 meters from the sediment source; two replicate traps were deployed at each distance. Eight transects were deployed across Morro Bay tidal marshes (Morro Bay State Park and Sweet Springs), capturing variation in elevation and vegetation type. The study was split into winter (January-April) and summer (June-August) sampling periods and traps were replaced with a new, pre-weighed pad every two weeks (winter) or monthly (summer). The daily flux of mineral and organic sediment was calculated. In addition, feldspar marker horizon plots were established at the 2, 12, and 24 m distances to determine longer-term rates of deposition. Vertical accretion was measured periodically 2023-2024. To estimate the potential accretion from the sediment traps, shallow soil cores were sampled adjacent to a subset of the transect locations (2, 12, 24 meters) and processed for bulk density and percent organic matter. Together, these data provide valuable insights into the spatial and temporal patterns of sediment deposition in Morro Bay tidal marshes.