Research Task 12 Benthic fluxes by novel eddy correlation techniques

PhD position. Max Planck Institute Germany.
Supervisors: Dr Frank Wenzhoefer and Dr Dirk de Beer Host: MPI

The eddy correlation technique allows us to measure exchange rates across the sediment water interface in a non-invasive manner and is probably the most accurate measurement of benthic fluxes; however, so far this approach only works for oxygen.⁽¹⁾ It is a classical technique to measure CO_² and O_² exchange with atmosphere and forests, oceans and coastal areas. Essentially, local mass balances are made of turbulent eddies, perfectly simultaneous with concentration measurements in these eddies. By integrating the turbulent transport of the gasses over minutes, a flux between land and atmosphere can be calculated. In our institute the technique was refined for underwater measurements, by combining fast responding oxygen microsensors with turbulence measurements using ADV.⁽¹⁾ An autonomous instrument, suitable for deep-sea deployments, was developed and is currently used in studies of coastal oxygen cycling. The area over which it measures the exchange (the footprint) depends on the current, sediment topography and height above the sediment, and can vary from several meters to 200 meters. We propose to adapt the instrument with microsensors for other relevant compounds. Prerequisite is that the sensor responds fast, i.e. within 1-3 seconds, as it was found that most information is found in that frequency range. Sensors that can respond this fast include those for Ca_²⁺ and H_²S, but also temperature and salinity for freshwater studies; also NO^2- and NO^3- sensors are considered.

The exchange of Ca²⁺ and protons is highly relevant in view of the ongoing acidification of the oceans. Marine calcium cycling is closely linked to respiration and photosynthesis.⁽²⁾ Insights in the correlation between respiration, photosynthesis and calcium cycling are limited by lack of data. This, together with the existing O_² sensing, allows us to study the relationship between benthic photosynthesis, respiration and calcium cycling. Small-scale salinity and temperature changes can be used to identify and quantify seeps in coastal (e.g. fresh water seeps) and deep-sea (e.g. methane seeps) waters.

⁽¹⁾ Berg, P., H. et al., 2003. Mar. Ecol. Prog. Ser. 261:75-83. ⁽²⁾ Zondervan, I. 2001. PhD. University of Bremen, Bremen.