Daphne Donis

An assessment of the precision and confidence of aquatic eddy correlation measurements

The quantification of benthic fluxes with the aquatic eddy correlation (EC) technique is based on simultaneous measurement of the current velocity and a targeted bottom water parameter (e.g., O2, temperature). High-frequency measurements (64Hz) are performed at a single point above the seafloor using an acoustic Doppler velocimeter (ADV) and a fast-responding sensor. The advantages of aquatic EC technique are that 1) it is noninvasive, 2) it integratesfluxes over a large area, and 3) it accounts for in situ hydrodynamics. The aquatic EC has gained acceptance as a powerful technique; however, an accurate assessment of the errors introduced by the spatial alignment of velocity and water constituent measurements and by their different response times is still needed. Here, this paper discusses uncertainties and biases in the data treatment based on oxygen ECflux measurements in alarge-scaleflume facility with well-constrained hydrodynamics.Theseobservations areusedto reviewdata processing proceduresandtorecommendimproveddeploymentmethods,thusimprovingtheprecision,reliability,andconfidence of EC measurements. Specifically, this study demonstrates that 1) the alignmentofthetimeseriesbasedonmaximum cross correlation improved the precision of EC flux estimations; 2) an oxygen sensor with a response time of ,0.4s facilitatesaccurateECfluxesestimatesinturbulenceregimescorrespondingtohorizontalvelocities,11cms 21 ;and3) the smallest possible distance (,1cm) between the oxygen sensor and the ADV’s sampling volume is important for accurate EC flux estimates, especially when the flow direction is perpendicular to the sensor’s orientation.

Factors controlling interannual variability of vertical organic matter export and phytoplankton bloom dynamics – a numerical case-study for the NW Mediterranean Sea

Mid-latitude spring blooms of phytoplankton show considerable year-to-year variability in timing, spatial extent and intensity. It is still unclear to what degree the bloom variability is connected to the magnitude of the vertical flux of organic matter. A coupled three-dimensional hydrodynamic-biogeochemical model is used to relate interannual variability in phytoplankton spring-bloom dynamics to variability in the vertical export of organic matter in the NW Mediterranean Sea. Simulation results from 2001 to 2010, validated against remote-sensing chlorophyll, show marked interannual variability in both timing and shape of the bloom. Model results show a tendency for the bloom to start later after cold and windy winters. However, the onset of the bloom occurs often when the mixed layer is still several hundred metres deep while the heat flux is already approaching zero and turbulent mixing is low. Frequency and intensity of wind episodes control both the timing and development of the bloom and the consequent export flux of organic matter. The wintertime flux is greater than zero and shows relatively low interannual variability. The magnitude of the interannual variability is mainly determined in March when the frequency of windy days positively correlates with the export flux. Frequent wind-driven mixing episodes act to increase the export flux and, at the same time, to interrupt the bloom. Perhaps counterintuitively, our analysis shows that years with discontinuous, low-chlorophyll blooms are likely to have higher export flux than years with intense uninterrupted blooms. The NW Mediterranean shows strong analogy with the North Atlantic section within the same latitude range. Hence, our results may also be applicable to this quantitatively more important area of the world ocean.

Full-scale evaluation of methane production under oxic conditions in a mesotrophic lake

Oxic lake surface waters are frequently oversaturated with methane (CH4). The contribution to the global CH4 cycle is significant, thus leading to an increasing number of studies and stimulating debates. Here we show, using a mass balance, on a temperate, mesotrophic lake, that ~90% of CH4 emissions to the atmosphere is due to CH4 produced within the oxic surface mixed layer (SML) during the stratified period, while the often observed CH4 maximum at the thermocline represents only a physically driven accumulation. Negligible surface CH4 oxidation suggests that the produced 110 ± 60 nmol CH4 L−1 d−1 efficiently escapes to the atmosphere. Stable carbon isotope ratios indicate that CH4 in the SML is distinct from sedimentary CH4 production, suggesting alternative pathways and precursors. Our approach reveals CH4 production in the epilimnion that is currently overlooked, and that research on possible mechanisms behind the methane paradox should additionally focus on the lake surface layer.The contribution of methane (CH4) produced in oxic freshwaters to the global atmospheric CH4 budget is poorly constrained. Here, using a mass balance and in-situ incubations, the authors show that significant CH4 emissions are supported by CH4 produced in the oxic surface mixed layer in Lake Hallwil.

Determining Near‐Bottom Fluxes of Passive Tracers in Aquatic Environments

In aquatic systems, the eddy correlation method (ECM) provides vertical flux measurements near the sediment-water interface. The ECM independently measures the turbulent vertical velocities w′ and the turbulent tracer concentration c′ at a high sampling rate (> 1 Hz) to obtain the vertical flux w′c′ from their time-averaged covariance. This method requires identifying and resolving all the flow-dependent time (and length) scales contributing to w′c′. With increasingly energetic flows, we demonstrate that the ECM’s current technology precludes resolving the smallest flux-contributing scales. To avoid these difficulties, we show that for passive tracers such as dissolved oxygen, w′c′ can be measured from estimates of two scalar quantities: the rate of turbulent kinetic energy dissipation ε and the rate of tracer variance dissipation χc. Applying this approach to both laboratory and field observations demonstrates that w′c′ is well resolved by the new method and can provide flux estimates in more energetic flows where the ECM cannot be used.