Providing critical information on ecosystem health and carbon flows
Researchers at the University of Virginia and Florida State University are quantifying the exchange of oxygen between the seafloor and the water above using the aquatic eddy covariance technique – a relatively new approach that enables, for the first time, measurements of this exchange without disturbing the seafloor and flow environment. The Nortek Vector has become the central instrument used in this technique due to its ability to record external sensor outputs at high frequency – in this case oxygen – along with the current velocity data.
Data on the oxygen exchange provide important information on ecosystem health and carbon flows and help address “big questions” on global carbon budgets, global warming trends, and ocean acidification.
Overall, the results of the University of Virginia and Florida State University research project demonstrate the high activity of shelf sands with respect to carbon transformation and provide critically needed information on carbon cycling.
Advantages of the aquatic eddy covariance technique The aquatic eddy covariance technique for oxygen exchange measurements at the seafloor was developed by Peter Berg and the Max Planck Institute for Marine Microbiology, Germany. The technique relies on measuring turbulent-driven fluctuations of the vertical flow velocity and associated oxygen concentration right above the seafloor. Through integration of time series of these data, the oxygen exchange can be derived. The first proof-of-concept paper was published in 2003 (Berg et al. 2003).
Since then, the number of studies using the approach has been steadily increasing because the technique has a number of significant advantages. Commonly used chamber incubations, for example, isolate a small area on the seafloor and exclude the naturally occurring flow (currents and wave motions). This exclusion is particularly problematic for permeable sands because it limits the exchange of water between the open pore space in the sand and the water above. This exchange, or flushing, can reach many centimeters into the sand and under natural conditions supplies degradable organic substances and oxygen. Therefore, oxygen consumption (and production at shallow locations exposed to light) measured with such chambers may not be correct.
More information on the aquatic eddy covariance technique and its potential can be found in a recently published comprehensive review at: http://onlinelibrary.wiley.com. The eddy covariance technique has been used in the atmospheric boundary layer for decades, and today is by far the most common approach for measuring exchange between land and air. A similar widespread use of the technique is expected for aquatic environments as more experience is gained with the technique and more sensors are developed.
Addressing the lack of data on oxygen exchange from permeable sands In a large-scale project, funded by the National Science Foundation (USA), Peter Berg (University of Virginia) and Markus Huettel (Florida State University) are using the aquatic eddy covariance technique to study oxygen exchange between highly permeable continental shelf sands and the water above. Although the continental shelf covers only about 8% of the global seafloor, the shelf sediments may be responsible for up to 90% of the biogeochemical breakdown of organic matter on the floor of the ocean.
About half of the shelf sediments are composed of sands and are located in the nearshore zone where organic matter production and decomposition activities are highest. Because there is a tight (and known) coupling between organic matter production and decomposition at the seafloor and its exchange of oxygen with the water above, this oxygen exchange is a highly attractive variable to measure in aquatic systems. Although permeable sands have been recognized for playing an important role in global carbon cycling, available data on their oxygen exchange are remarkably sparse, and their dynamics and controls are not well understood. This lack of data is partly due to limitations of traditional methods for measuring this oxygen exchange.
Carbonate and silicate sands have notably high oxygen consumption and production In this study that relies on the Nortek Vector for velocity measurements and parallel recording of oxygen concentrations, the exchange of oxygen of two main types of shelf sands is investigated.
At the first study site in the Florida Keys in the Gulf of Mexico, oxygen exchange in carbonate sands is investigated, whereas measurements at the second site, located off the Virginia coast, address oxygen exchange in sediment composed of silicate sands.
The results of the aquatic eddy covariance measurements reveal that the carbonate and silicate sands have notably high oxygen production and consumption that match the rates reported for muddy shelf sediments. Previously, muddy sediments were considered to be more active due to their higher content of organic matter.
The subtropical study site in the Florida Keys is located in an environment that is characterized by nutrient-depleted waters with relatively low organic-matter content, which explains the high water clarity and coral growth at this site. Nevertheless, the carbonate sands are highly active and the algae at the seafloor produce more oxygen during daytime than can be consumed by the organic matter degradation for the entire day.
In comparison, the site off the Virginia coast receives more nutrients and also organic matter; nevertheless, the activities of these silicate sediments are similar to those measured in the Florida Keys. This apparently contradictory result for the Virginia sands can be explained by the lower light intensities, lower temperatures, and lower specific surface area of the silicate sand (i.e. the surface area per unit mass of sand).
The lower light intensities limit the algal growth at the seafloor, which is also affected by the lower clarity of the water in the relatively nutrient-rich environment off the Virginia coast. Also, microbial activity typically scales with temperature, and lower temperatures at this site reduce the activity of bacteria that degrade organic matter. Finally, the specific surface area of the sand defines the area that is available for colonizing microbes that degrade organic matter.
Instruments in use
Vector - 300 m
Sample 3D velocity at up to 64 Hz for small-scale research in coastal areas