Phosphate uptake (P-uptake) into coral reef communities has been hypothesized to be mass-transfer limited. One method of demonstrating mass-transfer limitation of P-uptake is to show dependence of P-uptake on water velocity. Water velocity across reef flats varies with tides and swell; thus, we measured P-uptake over the entire reef flat on eight different days, representing a range in water velocities. P-uptake was calculated from changes in P concentration of the water column. Changes in P concentration were measured by water sampling at six sites along a 300-m cross-reef transect while simultaneously measuring water velocity. To smooth the variability in phosphate concentrations, peristaltic pumps were used to get time-integrated water samples for 4–6 h at each site. Water velocities were measured in the middle of the transect using an acoustic Doppler current profiler and were averaged to match the time-integrated water sampling. Depth-averaged cross-reef water velocities were 0.031 ± 0.013 m s−1 (mean ± SD), while the root-mean-square water velocities, accounting for oscillatory flow, averaged 3.3 times higher, 0.101 ± 0.021 m s−1 (mean ± SD). Phosphate decreased along all transects. The first-order rate constant for P-uptake (S) was 8.5 ± 2.4 m d−1 (mean ± SD) and increased linearly with root-mean-square water velocity. The Stanton number derived from oscillatory flow, the ratio of the first-order rate constant for P-uptake to the root-mean-square water velocity (S/U rms), was (9.4 ± 1.2) × 10−4 (mean ± SD). P-uptake ranged from 0.2 to 1.1 mmol P m−2 d−1, demonstrating that P-uptake is variable on short time scales and is directly related to P concentration and water velocity.
相似文献Productivity, nutrient input, nutrient uptake, and release rates were determined for a coral-dominated reef flat at La Réunion, France, to assess the influence of groundwater nitrogen on carbon and nutrient budgets. Water samples were collected offshore in the ocean, at the reef crest and back reef for nutrients, picoplankton, pH, and total alkalinity. Volume transport of ocean water across the reef flat was measured using both current meters and drogues. Groundwater advected onto the reef flat and mixed with incoming ocean water. Metabolic rates for the reef community were determined to be: gross primary production = 1,000 mmol C m−2 d−1, community respiration = 960 mmol C m−2 d−1, and community calcification = 210 mmol C m−2 d−1. Across the reef flat, silicate behaved conservatively, there was net uptake of phosphate (0.06 mmol P m−2 d−1) and net release of nitrate, ammonia, dissolved and particulate organic nitrogen (total 7.0 mmol N m−2 d−1). Groundwater nitrate contributed 37% of the increase in nitrate plus ammonia. The first-order mass transfer coefficient of phosphate was 3.3 m d−1, and for nitrate plus ammonia, 5.9 m d−1. Gross N and P uptake from estimates of mass transfer and uptake of particles were 0.37 mmol P m−2 d−1 and 7.2 mmol N m−2 d−1, respectively giving an N:P uptake ratio of 20:1. Thus, the elevation of nitrogen across the reef flat maintains a high N:P flux, enhancing algal growth downstream of the transect. We conclude that net community production (40 mmol C m−2 d−1) was sustained by net uptake of phosphate from the ocean and net uptake of new nitrogen from groundwater.
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