Evaluating benthic flux measurements from a gradient flux system

Abstract

Multiple methods exist to measure the benthic flux of dissolved oxygen (DO), but many are limited by short deployments and provide only a snapshot of the processes occurring at the sediment–water interface. The gradient flux (GF) method measures near bed gradients of DO and estimates the eddy diffusivity from existing turbulence closure methods to solve for the benthic flux. This study compares measurements at a seagrass, reef, and sand environment with measurements from two other methods, eddy covariance and benthic chambers, to highlight the strengths, weaknesses, and uncertainty of measurements being made. The results show three major areas of primary importance when using the GF method: (1) a sufficient DO gradient is critical to use this method and is limited by the DO sensor precision and gradient variability; (2) it is important to use similar methods when comparing across sites or time, as many of the methods showed good agreement but were often biased larger or smaller based on the method; and (3) in complex bottom types, estimates of the length scale and placement of the DO sensors can lead to large sources of error. Careful consideration of these potential errors is needed when using the GF method, but when properly addressed, this method showed high agreement with the other methods and may prove a useful tool for measuring long-term benthic fluxes of DO or other chemical sensors or constituents of interest that are incompatible with other methods.