Funding: ARC LIEF
Project Team: Bradley Eyre, Damien Maher, Perran Cook, Ronnie Glud, Peter Berg
The Centre for Coastal Biogeochemistry has three eddy correlation instruments funded from an ARC LIEF Grant (LE100100013). This eddy array is one of a few in the world. We are using this new cutting-edge non-invasive instrumentation to give new insights on the effect of flow and scale on the productivity and respiration of benthic communities in tropical coastal systems.
Organic matter production, respiration and burial are important controls on the concentrations of O2 and CO2 in the global biosphere. Phoytosynthesis produces organic matter and oxygen, but this organic matter is unstable and quickly decomposes (respiration). Coastal ecosystems are a significant component of the global carbon balance due to large amounts of organic matter production, respiration and burial (Borges et al., 2005; Duarte et al., 2005).
The primary production of an ecosystem minus its respiration (net ecosystem metabolism or NEM) is a measure of how much organic matter is available for higher trophic levels (e.g. fisheries production), burial, and export to adjacent aquatic systems, or, if negative, how much CO2 is released to the atmosphere. In shallow coastal systems light can reach much of the seafloor and, as such, the majority of primary production and respiration occurs in the benthos, with the ratio of benthic to pelagic production strongly dependent on the water column depth (Maher and Eyre, submitted).
Humans are modifying global nitrogen (N) and phosphorus (P) cycles at an alarming rate. Anthropogenic rates of N2 fixation now exceed natural (pre-industrial) rates (Galloway et al. 2008) and the mining of P is rapidly accelerating. The release of this fixed N and mined P into the environment drives the production of excess organic matter (eutrophication), which is one of the greatest threats to coastal ecosystems worldwide (Howarth, 2008).
An understanding of the benthic production and respiration of coastal ecosystems is central to several of the current major environmental problems facing society such as global carbon emissions and budgets, coastal eutrophication and fisheries production. However, all previous measurements of benthic production and respiration in coastal ecosystems have been taken using ex-situ cores (e.g. Eyre and Ferguson, 2005), in-situ benthic chambers (e.g. Webb and Eyre, 2004a), oxygen microsensors (e.g. Glud et al., 2002) or planar optodes (e.g. Glud et al., 2001). These measurements physically disturb the benthic community, change overlying oxygen (and other) conditions, exclude or modify water flow, and integrate over only small spatial scales. We hypothesise that because of the disturbance, containment and scaling effects of previous methods most of the existing productivity and respiration (metabolism) rate measurements of benthic communities may be quite different from the 'true' in-situ rates.
The ProblemOxygen fluxes (productivity, respiration) in benthic communities are typically measured using ex-situ cores and in-situ benthic chambers, which enclose a known volume of water and benthic community; the benthic oxygen flux rate is calculated from the change in O2 in the water column over time (e.g. Eyre and Ferguson, 2005). Alternatively, benthic oxygen fluxes can be calculated using oxygen microsensors or planar optodes, which are inserted into the surface of the benthic community to measure the change in oxygen with depth, from which the flux rate can be calculated (e.g. Glud et al., 2001, 2002).
All of the aforementioned techniques are invasive and potentially modify the benthic flux of oxygen. For example, the oxygen concentrations within a benthic chamber or core decrease over the course of an incubation. A decrease in oxygen can modify faunal activity and chemical reactions (e.g. sulfide oxidation) with an associated change in the benthic flux of oxygen. Nutrient concentrations also change within the enclosed core or chamber, which may change the productivity and respiration of the benthic community, and it is difficult, or not possible, to use these techniques in benthic communities with large 3-D structures (e.g. seagrass, coral reefs, mangrove pneumatophores, large macrofauna borrows etc.). In addition, the techniques only integrate over small spatial scales (1 to 10s cm2), which are difficult to scale up to whole ecosystem estimates. Most importantly, the enclosure techniques exclude external modifying processes such as water currents and wave oscillations resulting in measured benthic oxygen flux rates that may be very different from the true in-situ rate (i.e., artificial).
The Solution - Eddy Correlation The eddy correlation technique has been used in atmospheric research for nearly 60 years to measure land-air exchanges (e.g. Swinbank, 1951). Only recently has it been used for quantifying benthic oxygen fluxes in aquatic systems (Berg et al., 2003). The technique involves simultaneously measuring vertical flow and dissolved oxygen concentrations (and potentially other parameters) in-situ at a point above the benthic surface. It is based on the assumption that turbulent motion is responsible for all the O2 transported vertically away from the sediment surface (Berg et al., 2003).
The eddy correlation instrument consists of an acoustic Doppler velocimeter (ADV), an O2 microelectrode (although the latest generation eddy will have an O2 optode) connected to a purpose-built amplifier, and a battery, all housed on a triangular frame (Figure 1). Instruments that measure light, temperature, depth etc. are typically also attached to the eddy instrument frame. The footprint of the measured benthic flux of oxygen is typically about 40 m2 area upstream of the eddy correlation instrument.
The eddy correlation technique is truly non-invasive and, as such, overcomes all of the limitations of previous methods for measuring benthic metabolism (see above). Most importantly it integrates over a large surface area (up to 40m2) and can be used in highly heterogeneous benthic communities giving vastly improved estimates of whole system metabolism. As such, the eddy correlation technique will revolutionise our understanding of the functioning of benthic communities in aquatic ecosystems, including muds with benthic microalgae, muds with large macrofauna, permeable sands, seagrass communities, coral reefs, and mangroves.
(A) Collaborator Ronnie Glud's eddy correlation instrument undertaking some preliminary measurements at a permeable carbonate sands site on a Heron Island reef flat during the December 2009 Eyre and Glud DP0878683 field campaign
(B) Close-up of the (a) acoustic doppler velocimeter (ADV) sensor head, and (b) O2 microelectrode.