Invasive aquatic vegetation as a sink for inorganic sediment and organic carbon in an estuarine system

Abstract:

Invasive, submerged aquatic vegetation (SAV) acts as an ecosystem engineer by altering ecosystem properties such as water chemistry and plant community composition and reducing phytoplankton productivity. Less is known regarding how invasive SAV affects long-term ecosystem processes such as carbon storage and sedimentation. In this study, we determined whether Egeria densa (Brazilian waterweed), a globally invasive species of SAV, is storing organic “blue carbon” similar to marine SAV and trapping sediment required for accretion in adjacent marshes. We studied three sites with different energy regimes in the Sacramento-San Joaquin Delta of California, a tidal freshwater region. At each site, we collected (1) five surficial push cores in sediments within and outside of E. densa patches, (2) two sediment cores on the marsh plain, and 3) two gravity cores under E. densa patches. Bulk density, % organic carbon, and loss on ignition were determined for all cores. ²¹⁰Pb, and ¹³⁷Cs were used to date marsh and gravity cores. Our results show that E. densa patches function as sinks for carbon and inorganic sediment under all three energy regimes studied. Compared to marshes, E. densa patches have greater inorganic sedimentation rates (E. densa: 1103 – 5989 g m^-2 yr^-1, marsh: 393 – 1001 g m^-2 yr^-1, p < 0.01) and vertical accretion rates (E. densa: 0.4 – 1.3 cm yr^-1, marsh: 0.3 – 0.5 cm yr^-1, p < 0.05), but similar carbon accumulation rates (E. densa: 59 – 242 g C m^-2 yr^-1, marsh: 109 – 169 g C m^-2 yr^-1, p > 0.05). Due to its many harmful traits, E. densa is clearly unsuitable for mitigating carbon pollution; however, currently invaded habitats in the Delta and elsewhere may already contain a substantial component of regional carbon storage. Our results strongly suggest that invasive estuarine SAV such as E. densa reduces the resilience of adjacent marshes to sea-level rise by decreasing the amount of sediment available for marsh accretion.