UVA & Ifremer

« Coupled dynamics of benthic metabolism and pCO2 in a shallow seagrass meadow (Eastern Shore of Virginia, USA) »

Marie Lise Delgard1, Pierre Polsenaere2, Peter Berg1, Karen McGlathery1

1Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
2Resources and Environment Laboratory, IFREMER, L’Houmeau, France

Study context and main objectives

 

Seagrass meadows are significant contributors to marine carbon sequestration due to high rates of both net primary production (Duarte et al., 2010) and carbon burial (Duarte et al., 2011; Mcleod et al., 2011; Fourqurean et al., 2012). High rates of primary production (Barrón et al., 2004; Eyre et al., 2011; Hume et al., 2011) are supported by high nutrient uptake rates, and seagrass meadows act as a coastal filter that improves local water quality (McGlathery et al., 2007). Thus a detailed understanding of their metabolism and relevant controlling factors is important as a key component of carbon cycling over coastal systems (Rheuban et al., 2014-b).

 

The coastal lagoons of the Virginia Coast Reserve (VCR) have been the site of a successful large-scale seagrass restoration project (McGlathery et al., 2012; Orth et al., 2012). These coastal bays historically were colonized by eelgrass (Zostera marina), but underwent an ecosystem state change in the late 1920’s and early 1930’s when Z. marina was completely removed from the system as a result of disease (slime mold Labrynthula zosterae) and a hurricane in 1933 (Cottam and Munro, 1954). Until the late 1990’s, the lagoons were devoid of Z. marina, and between 2001 and 2008, a systematic restoration via seeding occurred and meadows have continued to expand by natural recruitment to cover over 17 km2 by 2012 (Orth et al., 2012).

 

Previous studies aimed to investigate benthic O2 fluxes and processes occurring at multiple timescales that influence ecosystem-scale metabolism of seagrass meadows of the eastern shore of Virginia (Hume et al., 2011; Rheuban et al., 2014-a and -b). The aquatic Eddy Covariance (EC) technique (Berg et al., 2003) was used in this way as it represents a powerful and innovative approach to measure ecosystem metabolism and provides the most direct and accurate measure of in situ oxygen fluxes. Simultaneous water partial pressure of CO2 (pCO2) measurements were realized through this UVA/IFREMER collaboration to (i) evaluate the interaction between in situ seagrass metabolism and water pCO2 over diurnal/seasonal cycles at the ecosystem scale (ii) characterize to what extent seagrass metabolism and associated environmental factors (see figure) drive the dynamic of water pCO2 and air-sea CO2 fluxes and (iii) study the potential influence of natural water pCO2 fluctuations on seagrass photosynthesis.

 

Materials and Methods

South Bay is a shallow lagoon with no riverine inputs and high water quality (McGlathery et al., 2012; Cole, 2011). It is bound by narrow tidal channels and surrounded by a barrier island and salt marshes. It is located close to an inlet that allows exchange with the Atlantic Ocean water (Fig. 1, Safak et al., 2015). The site is approximately 1.3 m deep at mean sea level with a 1 m tidal range, and is relatively protected from waves by the barrier islands.

Sensors of the aquatic Eddy Covariance technique and water pCO2 deployed at South Bay over the subtidal Zostera marina seagrass meadow.

Measurements were made in a dense restored eelgrass, Zostera marina meadow of over 6 km2 as of 2015 (see figure). During 12 days in late April - early May 2015 and 14 days in June 2015, benthic O2 fluxes were measured with two AEC systems (Berg et al., 2003) deployed simultaneously ~10m apart. Each system consisted of an acoustic Doppler velocimeter (ADV, a Vector from Nortek-AS) coupled to a fast responding (90% response <0.4 s) Clark-type O2 microsensor (Unisense), via a submersible pico-amplifier (see McGinnis et al., 2011). pCO2 were measured in the water column just above the canopy (at 30 cm above the sediment surface) using an autonomous pCO2 underwater sensor (Mini-Pro CO2™, Pro-Oceanus System Inc.). It has an oil resistant advanced semi-permeable membrane through which the senor equilibrates with surrounding waters, and this allows rapid diffusion of gas from liquid to a non-dispersive infrared detector (NDIR). The measurement range is 0-2000 ppmv with a resolution of 1 ppmv, an absolute accuracy of 40 ppmv and an equilibration time of 3 min (t63%). In order to evaluate the importance of precipitation/carbonate dissolution processes in the seagrass meadow, pH was measured during three 48-hour deployments in the water column just above the canopy (at 30 cm above the sediment surface).pH was recorded every 15 min with a digital, combination pH probes with a non-refillable, gel-filled single-junction reference and built-in temperature sensor (Hach Systems, IntelliCALTM PHC10105). The resolution and accuracy of this pH probe are estimated to be 0.001 and +/- 0.02 pH units respectively. At last, associated environmental parameters (current velocity, direction, depth, O2, turbidity, PAR….) as well as seagrass characteristics were also measured as expected to affect pCO2 and seagrass metabolism were also measured during deployments.

Links and additional information

Delgard M.L., Polsenaere P., Berg P. and McGlathery K.J. Coupled dynamic of benthic metabolism and pCO2 in a shallow seagrass meadow: an in situ investigation by aquatic eddy-covariance. Research paper in prep.

Polsenaere P., Delgard M.L., Berg P. and McGlathery K. (2016) Coupled dynamics of benthic metabolism and partial pressure of CO2 in a shallow seagrass meadow (Eastern Shore of Virginia, USA). Journées ODE. 8-9 novembre 2016, Centre Nautique de Moulin-mer, Logonna Daoulas, Finistère. Oral com.

Delgard M.L., Polsenaere P., Berg P. and McGlathery K.J. (2015) Drivers of seagrass metabolism determined by aquatic Eddy Covariance, CERF 2015, 8-12 November, Portland, OR, USA. Oral com.

Polsenaere P., Delgard M.L., Berg P. and McGlathery K.J. (2015) Collaborative research project between the University of Virginia and Ifremer – « Seagrass meadow metabolism (Zostera marina): Simultaneous aquatic Eddy Covariance and water pCO2 measurements » 1st field campaign report (14/04-10/05/2015, Eastern shore of Virginia), 04/06/2015, L’Houmeau, France. Oral com.

Polsenaere P., Delgard M.L., Berg P. and McGlathery K. (2015) Short- and long-term collaborative research project between the University of Virginia (Department of Environmental Sciences) and Ifremer (LER/PC): “Biogeochemical flux (carbon, oxygen and nutrients) characterization over two temperate coastal zones”, 20 pp.