WHAT COASTAL MARSH SEDIMENTS REVEAL ABOUT LAND USE CHANGE; BODEGA BAY, CA

Coastal wetlands may serve as an important terrestrial carbon sink due to high primary productivity and anoxic sediments. Land use change in California plays a large role in the destruction of coastal wetlands where over 80% were filled or diked for agriculture, urban development, and salt production. This study explores the effects of land use change on sedimentation, biological productivity, and carbon storage in coastal wetlands in Bodega Bay, CA.

The construction of Westside Road in 1963 separated the Rail Ponds coastal marsh from Bodega Harbor, CA. The marsh is now brackish and has dense vegetation. Nine short (<20 cm) gravity cores collected from the Rail Ponds and Bodega Harbor in July 2008 were analyzed to assess the impacts of recent land use changes on the marsh. Grain size analysis on 1 cm depth increment samples indicates silt and clay fractions in harbor cores range between 10-15%, while Rail Pond silt and clay fractions vary from 12-32% in the top 7 cm to 6-11% deeper in the core. Low silt and clay in the harbor suggests higher wave action. The transition from low to high silt and clay in Rail Ponds cores occurs between 5 and 16 cm depth. This transition indicates a changing depositional environment from higher energy with direct connection to the harbor to lower energy, semi-isolated marshes.

Carbon and nitrogen analyses indicate the harbor sediments contain <1% C and N at all depths sampled, while the Rail Ponds cores contain 1-3% C above the grain size transition. Rail Ponds cores contain <1% C in deeper sediments and <1-2% N throughout. High organic C values near the top of Rail Ponds cores suggest an environmental transition that enabled greater C preservation than when these marshes were directly connected to the harbor. Grain size and C and N content results suggest the Rail Ponds record a transition from low carbon/nitrogen storage and predominantly sand size grains to higher carbon/nitrogen storage and increased silt and clay fractions at approximately 10 cm. This transition may be connected to the construction of Westside Road and land use change in the area. Lower wave energy and increased vegetation may be key factors in increased C storage in the Rail Ponds. Results indicate land use change may impact biogeochemical cycling in coastal wetlands and suggest coastal wetlands are favorable environments for carbon storage.