Metabolism of a tide-dominated reef platform subject to extreme diel temperature and oxygen variations

Abstract

Benthic dissolved oxygen fluxes were measured on the reef flat of Tallon Island, an intertidal reef platform in the Kimberley region of northwestern Australia, for periods of 2 weeks in the wet and dry seasons. This reef flat is strongly tidally forced by semidiurnal tides (spring range > 8 m) and experiences highly asymmetric water level variability, with ebb durations lasting ∼10 h; this results in diel variations in water temperature and dissolved oxygen (DO) concentration (up to ∼11°C and 440 μM, respectively) that are among the most extreme recorded for reefs worldwide. Given the consistent tidal flow patterns, a one-dimensional control volume approach was used to make continuous Eulerian measurements of net production and community respiration from observed changes in DO within two zones: an inner zone dominated by seagrass and an outer zone dominated by macroalgae. Community respiration (R) was controlled primarily by DO concentration; however, fluxes approached the limits of DO mass transfer at low flow speeds. Estimates of gross primary production (P) suggested that reef communities were able to fix carbon at rates comparable to other tropical seagrass and mixed reef flat communities despite short-term (∼hours) extremes in light (up to 1800 μmol m−2 s−1) and temperature (> 35°C). Daily net community production fluctuated between net autotrophy and heterotrophy over a ∼15 d period depending on the phase difference between the solar and tidal cycles but was nonetheless metabolically balanced on time scales greater than weeks (P : R = 1.0–1.1).