Photosynthetic carbon reduction: high rates in the sea-surface microlayer

The ocean is a significant sink for increasing anthropogenic carbon dioxide emissions. The sea-surface microlayer (upper 50 m layer) serves as the primary point for exchange of materials, including carbon dioxide, between the atmosphere and hydrosphere. We determined microalgal standing stocks and activities in microlayer and subsurface water from areas with (S) and without (NS) visible natural surface slicks in Sequim Bay, Washington, USA, in July 1984. Enrichment ratios (microlayer concentration: subsurface bulkwater concentration) were: phytoneuston population abundance, 37 (NS) to 154 (S); total chlorophylls, 1.3 (NS) to 18 (S); particulate carbon fixation, 2 (NS) to 52 (S); and dissolved carbon excretion, 17 (NS) to 63 (S). Photoinhibition of 36 to 89% occurred in phytoneuston under natural summer light-intensities. Slick samples had greater standing stocks and rates of primary production than non-slick samples. The species composition of phytoneuston was distinctly different from that of phytoplankton. These results suggest that phytoneuston form a unique community. Although carbon fixation in summer was less per individual in phytoneuston than in phytoplankton, because of their overall abundance at the air-sea interface, phytoneuston may play an important role in the transfer of CO₂ from the atmosphere into ocean surface-water.