California Sea Grant College Program

Comprehensive primary productivity measurements of the impact of invasive species are important environmental indicators. This study obtained measurements of the impact of invasive Spartina densiflora on the primary productivity of a Humboldt Bay (California) salt marsh using above and belowground biomass measurements coupled with paired closed-chamber carbon dioxide flux measurements. Compared to samples dominated by native vegetation, samples dominated by S. densiflora displayed higher aboveground net primary productivity, lower belowground net primary productivity, lower net primary productivity, and lower net ecosystem exchange measurements; thus, S. densiflora colonization reduced primary productivity. Benthic macroalgal cover was a strong predictor of net ecosystem exchange. In plots dominated by S. densiflora, less benthic macroalgae was present and less photosynthetically active radiation reached the substratum. Therefore, increased shading of the sediment surface in plots dominated by S. densiflora contributed to lower net ecosystem exchange measurements. These results greatly improve our understanding of the impact of S. densiflora on the primary productivity of Humboldt Bay salt marsh ecosystems.