Eelgrass (Zostera marina) meadows provide many ecosystem goods and services but high rates of carbon sequestration may not be one of them

Abstract

Coastal wetlands are known to be efficient carbon sinks due to carbon burial by mineral sediments, high rates of primary productivity, and low rates of decomposition. Of the three coastal wetland types: tidal marshes, tidal forests, and seagrass meadows, carbon burial by seagrasses is relatively under-studied, with reported rates ranging widely from 45 to 190 g C m-2 yr-1. Additionally, most of these seagrass data are from the species Posidonia oceanica and not from Zostera marina, the species common to the Pacific Northwest. In this study, we measured sediment organic matter and long-term accretion rates to estimate carbon stocks and sequestration rates for a Z. marina meadow in Padilla Bay, a U.S. National Estuarine Research Reserve in the Salish Sea. We found rates of carbon sequestration to be quite low, averaging 20 g C m-2 yr-1, due to both low sediment organic content and low rates of accretion. We postulate here that Padilla Bay’s low carbon sequestration capacity may be representative of most Z. marina meadows rather than an outlier, and that Z. marina meadows have an inherently low carbon sequestration capacity due to the species’ low tolerance for suspended sediment (which limits light availability) and sediment organic content (which leads to toxic sulfide levels). We note here that we are reporting only on the rates of carbon sequestration and not the standing stock, which can still be quite high despite low rates of sequestration. As a next step, research should focus on measuring carbon sequestration rates from other Z. marina meadows, particularly from sites that exhibit, a-priory, a potential for higher rates of carbon sequestration (i.e., existing beds in active depositional zones, if such a thing exists)