Microphytobenthos production potential and contribution to bottom layer oxygen dynamics on the inner Louisiana continental shelf

Abstract

To investigate the relative importance of microphytobenthos (MPB) oxygen (O2) production on a river-dominated shelf, we made sediment core incubation measurements of MPB O2 production and sediment O2 consumption, and compared these to water-column measures of primary production and respiration during one spring and two summer cruises. Sites were located across a light attenuation (Kd) gradient on the inner Louisiana shelf (<25 m depth) with three sites on the eastern shelf and one site on the western shelf. MPB production rates were highest (range = 0.7-33.4 mmol O2 m-2 d-1) at the western site where light attenuation was lowest (range = 0.13-0.16 m-1). Near the river plumes, where Kd was higher (range = 0.17-0.64 m-1), MPB production was lower (range = 0-2.0 mmol m-2 d-1). MPB production influenced the magnitude and direction of sediment O2 exchanges at the western site and was observed to be a function of light availability. Overall, though, in comparison with integrated bottom layer production and respiration rates, MPB photosynthetic O2 production as a percentage was small (range of means = 0%-2%) at the eastern shelf sites. At the western site, MPB production was larger, and more variable, 39% (SE 34%) of bottom layer O2 production. Though based on a small data set, results suggest distinct differences between eastern and western shelf Kd and MPB contributions to bottom water O2 due to proximity to the freshwater discharges of the Mississippi and Atchafalaya rivers.