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.
