Hyperoxic boundary layers inhabited by the epiphytic meiofauna of Fucus serratus

Abstract

Opto-chemical microsensors were used to gather information about oxygen levels close to the thallus surface of Fucus serratus L. Oxygen tensions as high as 560% air saturation were recorded at the seaweed surface in seawater under static conditions at a light intensity of 229.5 μmol photon m-2 s-1. This level of oxygen supersaturation was restricted to a thin layer directly above the seaweed surface and a steep oxygen gradient was observed through the boundary layer, which extended to a distance of 1 to 2 mm in a static water body. Although remaining considerably elevated relative to the oxygen tension in the surrounding water, oxygen tensions at the seaweed surface were reduced under flow conditions. The thickness of the oxygen-supersaturated boundary layer decreased to less than 1 mm under flow conditions. However, flow rate itself did not have a significant effect on oxygen tensions at the seaweed surface or on the depth of the boundary layer. A rapid reaction of oxygen tension within the boundary layer was observed in response to initiation or cessation of flow. In darkness, under static conditions, the oxygen tension at the seaweed surface decreased to as little as 40% air saturation. Under these conditions, oxygen tension increased through the boundary layer, reaching levels similar to the surrounding water in less than 1 mm. These data indicate that small epiphytic animals (copepods, nematodes etc.) encounter highly hyperoxic conditions during daytime, even when there is water flow over seaweed. They are therefore exposed to the possibility of hyperoxic/free radical damage, even though living in an apparently benign environment.