Publications

Sub-tropical seagrass ecosystem metabolism measured by eddy covariance

Matthew H. Long, Peter Berg, Karen McGlathery, Joseph C. Zieman

The metabolism of seagrass ecosystems was examined at 4 sites under in-situ conditions using the eddy correlation technique in south Florida, USA. Three sites were located across a phosphorus-driven productivity gradient to examine the combined effects of dynamic variables (irradiance and flow velocity) and more static variables (sediment, phosphorus and organic content, seagrass biomass) on ecosystem metabolism and trophic status. Gross primary production and respiration rates varied significantly across Florida Bay in the summer of 2012 with the lowest rates (64 and -53 mmol O₂ m^-2 d^-1, respectively) in low-phosphorus sediments in the northeast and the highest (287 and -212 mmol O₂ m^-2 d^-1, respectively) in the southwest where sediment phosphorus, organic matter, and seagrass biomass are higher. Seagrass ecosystems offshore of the Florida Keys had larger daily production and respiration rates (397 and -217 mmol O₂ m^-2 d^-1, respectively) and were influenced by flow through the permeable offshore sediments. Across all sites, net ecosystem metabolism rates indicated that the seagrass ecosystems were autotrophic in the summertime. Substantial day-to-day variability in metabolic rates was found due to fast-changing environmental conditions such as the irradiance and flow velocity. At all sites the relationship between photosynthesis and irradiance was linear and did not approach saturating conditions over the entire irradiance range (up to 1400 µmol photons m^-2 s^-1). This was likely due to the efficient light utilization by the large photosynthetic surface area of the seagrass canopy and because sampling was done in-situ, which integrated across all autotrophs in the seagrass ecosystem.

Seagrass metabolism across a productivity gradient using the eddy covariance, Eulerian control volume, and biomass addition techniques

Matthew H. Long, Peter Berg, James L. Falter

The net ecosystem metabolism of the seagrass Thalassia testudinum was studied across a nutrient and productivity gradient in Florida Bay, Florida, using the Eulerian control volume, eddy correlation, and biomass addition techniques. In situ oxygen fluxes were determined by a triangular Eulerian control volume with sides 250 m long and by eddy correlation instrumentation at its center. The biomass addition technique was done within the control volume and evaluated the aboveground seagrass productivity through the net biomass added. The spatial and temporal resolutions, accuracies, and applicability of each method were compared. The eddy correlation technique better resolved the short-term flux rates and the productivity gradient across the bay, which was consistent with the long-term measurements from the biomass addition technique. The net primary production rates from the biomass addition technique, which excluded belowground production and sediment metabolism, were 71, 53, and 30 mmol carbon m^-2 d^-1 at 3 sites across the bay. The net ecosystem metabolism was 35, 25, and 11 mmol oxygen m^-2 d^-1 from the eddy correlation technique and 10, -103, and 14 mmol oxygen m^-2 d^-1 from the Eulerian control volume across the same sites, respectively. The low-flow conditions in the shallow bays allowed for periodic stratification and long residence times within the Eulerian control volume that likely limited its precision. Overall, the eddy correlation technique had the highest temporal resolution while producing accurate long-term flux rates that surpassed the capabilities of the biomass addition and Eulerian control volume techniques in these shallow coastal bays.

In situ coral reef oxygen metabolism: An eddy correlation study

Matthew H. Long , Peter Berg, Dirk de Beer, Joseph C. Zieman

Quantitative studies of coral reefs are challenged by the three-dimensional hard structure of reefs and the high spatial variability and temporal dynamics of their metabolism. We used the non-invasive eddy correlation technique to examine respiration and photosynthesis rates, through O2 fluxes, from reef crests and reef slopes in the Florida Keys, USA. We assessed how the photosynthesis and respiration of different reef habitats is controlled by light and hydrodynamics. Numerous fluxes (over a 0.25 h period) were as high as 4500 mmol O2 m−2 d−1, which can only be explained by efficient light utilization by the phototrophic community and the complex canopy structure of the reef, having a many-fold larger surface area than its horizontal projection. Over diel cycles, the reef crest was net autotrophic, whereas on the reef slope oxygen production and respiration were balanced. The autotrophic nature of the shallow reef crests implies that the export of organics is an important source of primary production for the larger area. Net oxygen production on the reef crest was proportional to the light intensity, up to 1750 µmol photons m−2 s−1 and decreased thereafter as respiration was stimulated by high current velocities coincident with peak light levels. Nighttime respiration rates were also stimulated by the current velocity, through enhanced ventilation of the porous framework of the reef. Respiration rates were the highest directly after sunset, and then decreased during the night suggesting that highly labile photosynthates produced during the day fueled early-night respiration. The reef framework was also important to the acquisition of nutrients as the ambient nitrogen stock in the water had sufficient capacity to support these high production rates across the entire reef width. These direct measurements of complex reefs systems yielded high metabolic rates and dynamics that can only be determined through in situ, high temporal resolution measurements.

Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light

Peter Berg, Matthew H. Long, Markus Huettel, Jennie E. Rheuban, Karen J. McGlathery, Robert W. Howarth, Kenneth H. Foreman, Anne E. Giblin, Roxanne Marino

Based on noninvasive eddy correlation measurements at a marine and a freshwater site, this study documents the control that current flow and light have on sediment-water oxygen fluxes in permeable sediments. The marine sediment was exposed to tidal-driven current and light, and the oxygen flux varied from night to day between −29 and 78 mmol m⁻² d⁻¹. A fitting model, assuming a linear increase in oxygen respiration with current flow, and a photosynthesis-irradiance curve for light-controlled production reproduced measured fluxes well (R² = 0.992) and revealed a 4-fold increase in oxygen uptake when current velocity increased from ∼ 0 to 20 cm s⁻¹. Application of the model to a week-long measured record of current velocity and light showed that net ecosystem metabolism varied substantially among days, between −27 and 31 mmol m⁻² d⁻¹, due to variations in light and current flow. This variation is likely typical of many shallow-water systems and highlights the need for long-term flux integrations to determine system metabolism accurately. At the freshwater river site, the sediment-water oxygen flux ranged from −360 to 137 mmol m⁻² d⁻¹. A direct comparison during nighttime with concurrent benthic chamber incubations revealed a 4.1 times larger eddy flux than that obtained with chambers. The current velocity during this comparison was 31 cm s⁻¹, and the large discrepancy was likely caused by poor imitation by the chambers of the natural pore-water flushing at this high current velocity. These results emphasize the need for more noninvasive oxygen flux measurements in permeable sediments to accurately assess their role in local and global carbon budgets.

Oxygen exchange and ice melt measured at the ice-water interface by eddy correlation

M. H. Long, D. Koopmans, P. Berg, S. Rysgaard, R. N. Glud, and D. H. Søgaard

This study examined fluxes across the ice-water interface utilizing the eddy correlation technique. Temperature eddy correlation systems were used to determine rates of ice melting and freezing, and O₂ eddy correlation systems were used to examine O₂ exchange rates driven by biological and physical processes. The study was conducted below 0.7 m thick sea-ice in mid-March 2010 in a southwest Greenland fjord and revealed low rates of ice melt at a maximum of 0.80 mm d⁻¹. The O₂ flux associated with release of O₂ depleted melt water was less than 13 % of the average daily O₂ respiration rate. Ice melt and insufficient vertical turbulent mixing due to low current velocities caused periodic stratification immediately below the ice. This prevented the determination of fluxes 61 % of the deployment time. These time intervals were identified by examining the velocity and the linearity and stability of the cumulative flux. The examination of unstratified conditions through vertical velocity and O₂ spectra and their cospectra revealed characteristic fingerprints of well-developed turbulence. From the measured O₂ fluxes a photosynthesis/irradiance curve was established by least-squares fitting. This relation showed that light limitation of net photosynthesis began at 4.2 μmol photons m⁻² s⁻¹, and that algal communities were well-adapted to low-light conditions as they were light saturated for 75 % of the day during this early spring period. However, the sea-ice associated microbial and algal community was net heterotrophic with a daily gross primary production of 0.69 mmol O₂ m⁻² d⁻¹ and a respiration rate of −2.13 mmol O₂ m⁻² d⁻¹ leading to a net ecosystem metabolism of −1.45 mmol O₂ m⁻² d⁻¹. This application of the eddy correlation technique produced high temporal resolution O₂ fluxes and ice melt rates that were measured without disturbing the in situ environmental conditions while integrating over an area of approximately 50 m² which incorporated the highly variable activity and spatial distributions of sea-ice communities.

A comparison and correction of light intensity loggers to photosynthetically active radiation sensors

M.H. Long, Jennie E. Rheuban, Peter Berg, Joseph C. Zieman

Accurate light measurements are important in the analysis of photosynthetic systems. Many commercial instruments are available to determine light; however, the comparison of light estimates between studies is difficult due to the differences in sensor types and their calibrations. The measurement of underwater irradiance is also complicated by the scattering and attenuation of light due to interactions with particulates, molecules, and the bottom. Here, three sensor types are compared to evaluate the calibration of light intensity loggers to estimate photosynthetically active radiation (PAR). We present a simple calibration of light intensity loggers that agree within 3.8% to factory-calibrated scalar PAR sensors under a wide range of environmental conditions. Under the same range of conditions, two identical factory-calibrated PAR sensors showed a similar difference of 3.7%. The light intensity loggers were calibrated to a high-quality PAR sensor using an exponential fit (r² = 0.983) that accounts for differences in sensor types with respect to the angle of incoming light, scattering, and attenuation. The light loggers are small, robust, and simple to operate and install, and thus well-suited for typical subsurface research. They are also useful for small-scale measurements, when broad spatial coverage is needed, or in research requiring multiple sensors. Many studies have used these simple light intensity sensors to estimate PAR, yet their limitations and advantages in mimicking PAR have not been well defined previously. We present these small and user-friendly loggers as an excellent alternative to more sophisticated scalar PAR sensors.

Development of an in situ underwater particle image velocimetry (UWPIV) system

Qian Liao, Harvey A. Bootsma, Jianen Xiao, J. Val Klump, Andrew Hume, Matthew H. Long, Peter Berg

A low-cost self-contained underwater particle image velocimetry (UWPIV) system has been developed to measure small-scale turbulent flow structures in situ. The UWPIV employs a compact continuous-wave laser and an optical scanner to deliver a light sheet that illuminates naturally occurring particles. Particle images are taken by a CCD camera along with an ultra-compact PC. The nontethered and compact design can be fit in two small underwater housings with all components powered by batteries: an ideal design for a variety of in situ deployments. The system has been field-tested in the coastal zone of Lake Michigan. Turbulent flow structures of the wave-current bottom boundary layer are measured right above the nearshore lakebed, which is densely covered by quagga mussels (Dreissena bugensis). Vertical profiles of mean velocity, Reynolds stresses, dissipation rate of the turbulent kinetic energy, turbulent viscosity, plankton particle concentration, and the turbulent flux of particles are presented and discussed.

The role of organic acid exudates in liberating phosphorus from seagrass-vegetated carbonate sediments

M.H. Long, Karen J. McGlathery, Joseph C. Zieman, Peter Berg

Sediment-bound phosphorus (P) is a potential nutrient source for P-limited seagrasses inhabiting carbonate sediments. We explored the role of organic acid (OA) exudation by seagrasses in liberating mineral P from carbonate sediments. Organic acids can act to increase available P by dissolving carbonate sediment, competing with P for binding sites and complexing dissolution end products, and also by fueling microbial processes that change pore-water pH. We used dialysis tubing placed around individual roots in situ to quantify dissolved species immediately adjacent to roots (root zone) and compared these to bulk pore-water concentrations in vegetated and nonvegetated sediments. Total OA concentrations were highest in the root zone (29.8 ± 1.8 [mol L⁻¹) compared to bulk measures of 15.5 ± 1.9 and 7.5 ± 0.6 [mol L⁻¹ in vegetated and nonvegetated sediments, respectively. Phosphate concentrations were also highest in the root zone and were linearly related to OA concentrations (R² = 0.63). Organic acid concentrations increased along a seagrass productivity gradient, and ratios of OA concentrations to productivity showed a significant response to a gradient in P-limitation of seagrasses. Organic acid concentrations found in and around roots, compared to those found in bulk sediment measures, indicate that seagrasses are a significant source of OA. Sampling at small spatial scales (mm) immediately adjacent to the roots is critical, because bulk sediment pore-water measures did not capture the observed fluctuations caused by the rapid reaction and consumption of OA in the sediment. Root-zone processes can liberate considerable quantities of P, and OA exudates likely contribute significantly to the success of T. testudinum in P-limited environments.