A robust numerical procedure for biogeochemical interpretation and analysis of measured concentration profiles of solutes in sediment pore water has been developed. Assuming that the concentration-depth profile represents a steady state, the rate of net production or consumption as a function of depth can be calculated, together with the flux across the sediment-water interface. Three kinds of vertical transport can be included in the analysis: molecular diffusion, bioturbation, and irrigation. The procedure involves finding a series of least square fits to the measured concentration profile, followed by comparisons of these fits through statistical F-testing. This approach leads to an objective selection of the simplest production-consumption profile that reproduces the concentration profile. Because the numerical procedure is optimized with respect to speed, one prediction can typically be done in a few minutes or less on a personal computer. The technique has been tested successfully against analytical solutions describing the transport and consumption of 0, in sediment pore water. In other tests, measured concentration profiles of O,, NO;, , NH:, and ZCO, have been interpreted using the new procedure.

This paper presents a new non-invasive technique for measuring sediment O 2 uptake that, in its concept, differs fundamentally from other methods used to date. In almost all natural aquatic environments, the vertical transport of O 2 through the water column toward the sediment surface is facilitated by turbulent motion. The new technique relies on measuring 2 parameters simultaneously and at the same point in the water above the sediment: the fluctuating vertical velocity using an acoustic Doppler velocimeter and the fluctuating O 2 concentration using an O 2 microelectrode. From these 2 parameters, which typically are measured 10 to 50 cm above the sediment surface for a period of 10 to 20 min and at a frequency of 15 to 25 Hz, the vertical flux of O 2 toward the sediment surface is derived. Based on measurements performed under actual field conditions and comparisons with in situ flux-chamber measurements, we believe that this new technique is the optimal approach for determining O 2 uptake by sediments. The technique is superior to conventional methods as measurements are done under true in situ conditions, i.e. without any disturbance of the sediment and under the natural hydrodynamic conditions. Furthermore, this technique can be used for bio-irrigated or highly permeable sediments, such as sands, where traditional methods often fail. While this paper only focuses on O 2 uptake by sediments, the technique can also be applied to other solutes that can be measured at a sufficiently high temporal resolution. KEY WORDS: Oxygen uptake · Sediment · Eddy-correlation measurementsResale or republication not permitted without written consent of the publisher

The sandy sediments that blanket the inner shelf are situated in a zone where nutrient input from land and strong mixing produce maximum primary production and tight coupling between water column and sedimentary processes. The high permeability of the shelf sands renders them susceptible to pressure gradients generated by hydrodynamic and biological forces that modulate spatial and temporal patterns of water circulation through these sediments. The resulting dynamic three-dimensional patterns of particle and solute distribution generate a broad spectrum of biogeochemical reaction zones that facilitate effective decomposition of the pelagic and benthic primary production products. The intricate coupling between the water column and sediment makes it challenging to quantify the production and decomposition processes and the resultant fluxes in permeable shelf sands. Recent technical developments have led to insights into the high biogeochemical and biological activity of these permeable sediments and their role in the global cycles of matter.

The eddy correlation technique is rapidly becoming an established method for resolving dissolved oxygen fluxes in natural aquatic systems. This direct and noninvasive determination of oxygen fluxes close to the sediment by simultaneously measuring the velocity and the dissolved oxygen fluctuations has considerable advantages compared to traditional methods. This paper describes the measurement principle and analyzes the spatial and temporal scales of those fluctuations as a function of turbulence levels. The magnitudes and spectral structure of the expected fluctuations provide the required sensor specifications and define practical boundary conditions for the eddy correlation instrumentation and its deployment. In addition, data analysis and spectral corrections are proposed for the usual nonideal conditions, such as the time shift between the sensor pair and the limited frequency response of the oxygen sensor. The consistency of the eddy correlation measurements in a riverine reservoir has been confirmed-observing a night-day transition from oxygen respiration to net oxygen production, ranging from 220 to 15 mmol m 22 day 21 -by comparing two physically independent, eddy correlation instruments deployed side by side. The natural variability of the fluctuations calls for at least ;1 h of flux data record to achieve a relative accuracy of better than ;20%. Although various aspects still need improvement, eddy correlation is seen as a promising and soon-to-be widely applied method in natural waters.

Dissolved oxygen (DO) fluxes were measured by eddy correlation to estimate net ecosystem metabolism (NEM) during summer in a restored eelgrass (Zostera marina) meadow and a nearby, unvegetated sediment. This technique measures benthic fluxes under true in situ light and hydrodynamic conditions, integrates over a large area (typically . 100 m 2 ), and captures short-term variations. DO fluxes measured through eight 24-h periods showed pronounced temporal variation driven by light and local hydrodynamics on multiple scales: hour-to-hour, within each daily cycle, and between deployments. The magnitude of variation between hours during single deployments equaled that between deployments, indicating that short-term variation must be included for metabolism estimates to be accurate. DO flux variability was significantly correlated to mean current velocity for the seagrass site and to significant wave height for the unvegetated site. Fluxes measured in low-flow conditions analogous to many chamber and core incubations underestimated those measured in higher-flow conditions typical of in situ conditions by a factor of 2-6. Rates of gross primary production (GPP), respiration (R), and NEM varied substantially between individual deployments, reflecting variations in light and hydrodynamic conditions, and daily values of GPP and R for individual deployments were tightly linked. Average daily NEM of the seagrass site was higher than that of the unvegetated site; the seagrass site was in metabolic balance, and the unvegetated site showed a tendency toward net heterotrophy during this midsummer period.