Renee Gruber
Reef organisms are strongly dependent on the physical and chemical conditions of surrounding ocean waters, which influence rates of key metabolic and biogeochemical processes such as primary production, respiration, nutrient uptake, and grazing on suspended particulate matter. Flows driven by currents and waves interact with reef topography to determine the delivery rate of dissolved and particulate material to the surfaces of reef organisms. Reef circulation also controls the temperature and
more » ... e temperature and light availability at the reef surface, which in turn influence broader-scale ecological processes, especially rates of community primary production. Tide-dominated reefs are reefs that experience mean tidal ranges in excess of local mean significant wave heights. Despite being common (~one third of all reefs worldwide), almost no studies have focused on how the large tidal forcing of these systems controls the physical and chemical properties of overlying waters and, in turn, community ecological processes. This study was conducted in the remote and near-pristine Kimberley region of northwest Australia at Tallon Island. This reef platform is strongly forced by semidiurnal tides (spring range >8 m) and experiences highly asymmetric tidal phase durations, with ebb tides lasting ~10 hours. The reef itself has distinct benthic zonation, with a seagrass-dominated community inhabiting the inner portion of the platform and a macroalgal-dominated community (including small corals, ~5-10% cover) on the outer seaward zone. Three field experiments (each lasting 2-3 weeks) were conducted in Oct 2013 (dry season), Feb 2014 (wet season), and Apr 2014 (late wet season). Hydrodynamic instrumentation, in situ calibrated fluorometers, and dissolved oxygen (DO) loggers deployed on the reef platform provided continuous measurements of flow iv speed, chlorophyll a (chl a), DO, and water temperature, while concentrations of dissolved and particulate nutrients were repeatedly measured with strategic water sampling. A one-dimensional control volume approach was used to estimate: 1) community production and respiration; 2) benthic chl a and particulate nutrient fluxes; and 3) dissolved nutrient uptake and release within the two major reef zones. Temperature and DO variations measured on the reef platform were among the most extreme recorded for reefs worldwide, with diel ranges of ~11°C and 440 µM DO. Hypoxic conditions (DO < 63 µM) occurred when low water depth coincided with nighttime, and could last for up to 7 hours each night. Estimates of productivity suggest that benthic communities were not negatively affected by short-term (~hours) extremes in light (up to 1800 µmol m -2 s -1 ) and temperature (>35°C), and were moderately productive compared to similar communities worldwide. The reef alternated between net autotrophy and heterotrophy over a ~15 day cycle, which was related to the phase difference between the solar and tidal cycles. Suspended particulate nutrients and chl a, food sources for benthic filter-feeders, were depleted towards the end of ebb tide. Flood tides then 'refreshed' these pools by delivering a pulse of oceanic water onto the reef platform. Rates of benthic chl a uptake measured on Tallon were lower than previous studies of many reef communities, and the reef was a net source of particulate nutrients to the surrounding ocean waters, with particulate organic carbon exports comprising ~3% of the reef's daily production. Potential uptake rates of dissolved inorganic nutrient species varied over a tidal cycle as a function of flow conditions and water column nutrient concentrations, with fluxes varying by an order of magnitude over the range of flow speeds naturally occurring on this reef platform. Fluxes were also forced by offshore nutrient concentrations, which v increased during the wet season. A net release of inorganic nutrients was balanced by an uptake of dissolved organic nutrients on the reef platform, suggesting that organic species may be an important nutrient source. vi
doi:10.4225/23/59a77fae6ce0b fatcat:izxv5kp25fdilgsfikwejczgsm