Accretion rates of low intertidal salt marshes in the Pacific Northwest

Abstract

Salt-marsh accretion rate was investigated at sites that spanned a gradient in relative rate of sealevel rise in Washington and Oregon. Mean accretion rate over all sites was 3.6 mm yr−1 (95% CI=2.4 to 4.8 mm yr1), which exceeded present mean sea-level-rise rate (1.3 mm yr1; sd=0.6). However, a mean rise rate of 5.5 mm yr−1 (sd=1.9) predicted by a moderate sea-level-change scenario to occur by the year 2050 exceeds mean accretion rate. Marshes with adequate sediment input seemed to have the capacity to keep pace with an increased sea-level-rise rate. Lowest accretion rates were recorded at sites with the least sediment supply. Accretion rate showed a weak negative correlation with sediment organic matter (measured as volatile solids) and marsh standing stock. The data suggest that moderate and high rise-rate scenarios would threaten the existence of salt marshes in the region in the absence of increased sediment supply. A better understanding is required of marsh accretion and predicted rate of sea-level rise to refine predictions of the effects of sea-level rise on Pacific Northwest salt marshes.