After five months in Odense (Denmark), I have completed my stay at the University of Southern Denmark (SDU), where, in addition to learning how Aquatic Eddy Covariance (AEC) works, I conducted two experiments that provide key information on its application in intertidal zones. Although AEC is an innovative method for measuring O₂ fluxes and studying various dynamics in aquatic ecosystems, its use remains limited due to its complexity. Internationally, only a few laboratories have adopted this technique, and in Spain, only one research institution has implemented it to date.

Aquatic Eddy covariance. A bit of an introduction….

The AEC is a non-invasive method that allows the measurement of benthic biogeochemical fluxes with a high resolution, during days or weeks continuously. This method is a relatively new technique (Berg et al., 2003). Over the last two decades, the quality of benthic O₂ flux measurements by AEC has improved by incorporating new rapid response systems that measure O₂.   Due to their complexity, O₂ measurement systems used for AEC must undergo rigorous testing under well-defined laboratory and field conditions to assess performance, reliability and potential limitations (Berg et al., 2016; Chipman et al., 2012; McGinnis et al., 2011).

An example of a new O₂ measurement system is the AquapHOx-LX logger (PyroScience, GmbH; referred to as AquapHOx). It is an optical submersible meter, which has been designed to perform high resolution O₂ measurements underwater, making it a suitable option for collecting robust AEC data.

In intertidal zones, the use of the AquapHOx system can present challenges due to temperature variations between the sediment and the water column. This can occur because, during low tide, the sediment is directly exposed to solar radiation, which causes significant heating of the sediment surface. When the tide rises and the sediment returns underwater, the temperature difference can generate heat fluxes from the seabed to the water column. These thermal fluctuations, however small, can affect the O₂ signal. This potential error in the O₂ signal can alter the interpretation of biogeochemical fluxes in these areas.

Since AEC is a relatively new technique (Berg et al., 2003), there are hardly any studies in intertidal zones (Volaric et al., 2020) that quantify the importance of heat fluxes between the sediment and the water column. Therefore, it is necessary to test:

• how much the O₂ signal is affected by temperature, i.e., what percentage of the oxygen fluctuations are driven by natural oxygen changes versus temperature effects, and

• how long it takes for the sediment surface and water to equilibrate in temperature.

To verify this, we performed two short experiments. The first consisted of quantifying the effect of temperature on the O₂ signal; the second consisted in quantifying the time it takes for the sediment surface temperature to equilibrate with the water column.

Test 1. Evaluating the response of the O₂ signal to temperature changes.

The sensitivity of the O₂ sensor to thermal variations was evaluated following the procedure described by Granville et al. (2023).

Test 2. Quantifying the time, it takes for the sediment surface temperature to equilibrate with the water column after the sediment surface was exposed to high temperature

This experiment aimed to measure how long sediment takes to reach equilibrium with the water column after being exposed to high temperatures, simulating low tide conditions. Additionally, it also examined whether sediment color affects heat absorption.

To do that, five sediment cores of different color were collected from Odense Fjord, Denmark, acclimatized for a day. After that, we performed different temperature profiles, with and without a water column, to obtain initial conditions.

The sediment surfaces were then exposed to high light intensity (simulating solar radiation) for 2 hours without water. Afterward, temperature profiles were measured every 20 minutes using a temperature probe until the temperatures returned to initial levels. The process was then repeated for each core in the presence of a water column.

After these experiment, we can conclude that, although, the sediment surface may heat up more in the absence of the water column and in darker sediments, the recovery time showed no observable differences. Furthermore, considering that the temperature of the water column and of the uppermost layers of the sediment quickly returned to their initial conditions, it can be concluded that AquapHOx can be used in intertidal zones.

From 1 May until 1 October 2024, Sandra Rizzo, one of the pre-doctoral students from our laboratory, is visiting the University of Southern Denmark (SDU, Odense) on an international stay to learn the use of the novel Aquatic Eddy covariance (AEC) technique. Sandra will work under the supervision of Profs. Karl Attard and Ronnie Glud.

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AEC is a direct and non-invasive method that allows the quantification of benthic metabolism rates by measuring oxygen fluxes with a high resolution, during days or weeks and continuously.

The stay at SDU will allow us to transfer the know how to our laboratory, which only a few laboratories at international level have and extend our collaboration with our Danish colleagues. Furthermore, in practical terms, the use of the AEC method will facilitate the monitoring of benthic metabolism rates in different habitats in our area and to study the effect of extreme wind events, without the logistical problems associated with sampling under such conditions.  Our new project MULTIFLUX will focus heavily on this aspect.