Continuous measurements of net production over a shallow reef community using a modified Eulerian approach

Abstract

[1] We made continuous measurements of the flux of dissolved oxygen over an algal-dominated reef flat community on the Kaneohe Bay Barrier Reef, Hawaii as a proxy for the net production (NP) of organic carbon. Fluxes were calculated from measurements of dissolved oxygen and current profiles taken from moored oxygen sensors and acoustic Doppler current profilers (ADCPs) located at points defining the vertices of a triangular control volume 250 m in length on each side. Surface cruciform drifters as well as dye patches were tracked across the control volume to get independent measurements of Lagrangian currents. While the dye patch speeds were highly correlated with the depth-averaged current speeds calculated from ADCP data on a 1:1 basis (r 2 = 0.86, n = 24), drifter speeds exceeded both dye and calculated current speeds by 30-100% on average. Measurements of dissolved oxygen sampled within the dye patches as it propagated across the study site were used to calculate NP following a Lagrangian approach. Lagrangian estimates of NP were highly correlated with Eulerian estimates of NP on a 1:1 basis (r 2 = 0.81, n = 7). Ratios of community production and community respiration varied from 0.7 to 1.4 on a daily basis, but approached a value of 1.0 when integrated over the entire data record.

Oceanographic forcing of nutrient uptake and release over a fringing coral reef

“…Comparatively few studies have quantified fluxes within natural reefs systems at the community scale (Crossland and Barnes 1983;Baird et al 2004;Cuet et al 2011a), largely due to the fact that uptake rates are often too slow to detect over smaller spatial scales (Atkinson and Smith 1987;Atkinson and Bilger 1992 Atkinson and Falter 2003). Although gross flux estimates have rarely been reported for natural systems Falter et al 2004;Cuet et al 2011a), the present estimates (Tables 4 and 5 Estimates of POM fluxes are even more limited than for dissolved inorganic nutrients (but see Genin et al 2009;Wyatt et al 2010;Cuet et al 2011a). Thus, although the likely significance of POM in community-and reef-scale nutrient budgets has been widely recognized for many years (Odum and Odum 1955;Ayukai 1995;Yahel et al 1998), its quantitative importance is still poorly understood.…”

“…Full definitions are provided in Falter et al (2004). Molecular diffusivity coefficients for determining Sc were 19 3 10 210 m 2 s 21 for NO 3 and 7 3 10 210 m 2 s 21 for PO 4 (Bilger and Atkinson 1992).…”

“…Maximum MTL uptake rates were also predicted for each nutrient based on a simplified version of equation 8 described in Falter et al (2004) that retains accuracy but eliminates the second-order effect of the Reynolds roughness term:…”

“…While the tight coupling between dissolved inorganic nutrient fluxes and local conditions (i.e., MTL) is well-acknowledged, the role of broader scale oceanographic processes in determining nutrient dynamics is less wellunderstood. If dissolved inorganic nutrient uptake is as close to MTL as theory and laboratory experiments suggest Atkinson and Bilger 1992;Bilger and Atkinson 1992), uptake rates are likely to depend greatly on processes that alter the incoming oceanic nutrient concentrations Baird et al 2004), as well as the hydrodynamic processes governing local benthic flow regimes (Hearn et al 2001;Falter et al 2004). Thus, nutrient dynamics on natural reefs are likely to be highly dependent on oceanographic forcing and, thus, unlikely to be as seasonally or spatially uniform as often assumed in studies of reef ecology .…”

“…Accurate estimates of uptake have required measurements over large distances (hundreds of meters) or in systems with very long residence times (Atkinson and Falter 2003). MTL theory has, thus, been challenging to investigate in the field, but several field studies have indicated MTL uptake over coral reef flats for both nitrate (Baird et al 2004) and phosphate (Falter et al 2004;Cuet et al 2011b). In support, numerous laboratory flume studies have shown that nutrient uptake increases when the boundary layer is altered due to increasing nutrient concentrations, water velocity, and surface roughness (Atkinson and Bilger 1992;Bilger and Atkinson 1995;Baird and Atkinson 1997).…”