The sea-surface microlayer: phytoneuston productivity and effects of atmospheric particulate matter

Abstract

The sea-surface microlayer, the upper 50 μm of the ocean surface, provides a habitat for an important biota (the neuston), an interface for exchange of gases between the atmosphere and oceans and a site for deposition of anthropogenic metals and other materials from the atmosphere. Several recent studies have suggested that biochemical processes, including photosynthesis, in the microlayer are inhibited relative to the bulk seawater. We compared the biomass, species composition and productivity of phytoneuston to that of phytoplankton in Sequim Bay, Washington State, USA. Mean enrichment ratios (microlayer: bulk water concentrations) for bacteria, microalgae, chlorophyll pigments and photosynthesis (estimated gross) were 2 444, 380, 12 and 40, respectively. Compared to the bulk water, the microlayer had a unique assemblage of microalgae with a higher concentration of chlorophyll c. When exposed to high light intensities (summer) or metalrich urban atmospheric particulate matter, radiocarbonmeasured photosynthesis was lower in phytoneuston than in phytoplankton. Deposition of atmospheric particulate matter at rates similar to those occurring in urban coastal areas resulted in a significant (P<0.01) reduction in radiocarbon-measured photosynthesis in the sea-surface microlayer. These apparent decreases in photosynthesis are believed to result from the extracellular release of 14C as glycolate or other soluble compounds and may not reflect a true decrease in gross primary productivity in the microlayer. Further measurements of the degree of extracellular carbon release will be necessary to quantify gross photosynthetic rates in the microlayer.