Surface gas exchange determined from an aquatic eddy covariance floating platform

M.H. Long, D.P. Nicholson

We present a new approach to quantifying air-water flux and gas transfer velocity (k600) from underwater eddy covariance (EC) of dissolved oxygen. EC fluxes were measured 35 cm below the air-water interface using an acoustic Doppler velocimeter (ADV) coupled with a fast-responding dissolved oxygen optode probe, integrated on a floating platform. A micro-electro-mechanical system (MEMS)-based inertial motion unit integrated with the ADV enabled compensation of measured velocities for platform motion and changing sensor orientation. Deployments of 16 h and 30 h were conducted under low to moderate wind conditions in Sage Lot Pond, a small estuarine embayment. We evaluate air-sea flux parameterizations based on wind speed, current speed, and turbulent kinetic energy dissipation rate compared to our directly measured EC flux. Total kinetic energy was linearly correlated with EC-determined k600, indicative of a more direct relationship to near surface turbulence compared to wind and current speed based parameterizations, which were subject to biases attributable to directional differences in fetch and low current speed. Our observations, which encompassed a period of low turbulent energy, suggest that existing parameterizations are not well constrained for these conditions. This new aquatic EC technique is highly advantageous for the determination of air-water exchange, especially in dynamic near-shore and inland systems. The system presented here has the potential for wide applicability for lake, riverine, and open-ocean air-water exchange and the results can be extrapolated for use with a wide range of biogeochemically relevant gases.


Extreme low oxygen and decreased pH conditions naturally occur within developing squid egg capsules

M.H. Long, T. Aran Mooney, Casey Zakroff

Young animals are the foundation of future cohorts and populations, but are often particularly susceptible to environmental changes. This raises concerns that future conditions, influenced by anthropogenic changes such as ocean acidification and increasing oxygen minimum zones, will greatly affect ecosystems by impacting developing larvae. Understanding these potential impacts requires addressing present tolerances and current conditions in which animals develop. Here, we examined changes in oxygen and pH adjacent to and within normally-developing squid egg capsules, providing the first observations that the egg capsules, housing hundreds of embryos, have extremely low internal pH (7.34) and oxygen concentrations (1.9 µmol l-1). While early-stage egg capsules had pH and oxygen levels significantly lower than the surrounding seawater, late-stage capsules dropped dramatically to levels considered metabolically stressful even for adults. The structure of squid egg capsules results in a closely packed unit of respiring embryos, which likely contributes to the oxygen-poor and CO2-rich local environment. These conditions rival the extremes found in the squids’ natural environment, suggesting they may already be near their metabolic limit, and that these conditions may induce a hatching cue. While squid may be adapted to these conditions currently, further climate change could place young, keystone squid outside of their physiological limits.

Oxygen metabolism and pH in coastal ecosystems: Eddy Covariance Hydrogen ion and Oxygen Exchange System (ECHOES)

Matthew H. Long, Matthew H. Charette, William R. Martin, Daniel C. McCorkle

An aquatic eddy covariance (EC) system was developed to measure the exchange of oxygen (O2) and hydrogen ions (H+) across the sediment-water interface. The system employs O2 optodes and a newly developed micro-flow cell H+ ion selective field effect transistor; these sensors displayed sufficient precision and response times required for measuring turbulent fluctuations. Discrete samples of total alkalinity and dissolved inorganic carbon (DIC) were used to determine the carbonate equilibria of the water column and relate the O2 and H+ fluxes to benthic processes. The ECOHES system was deployed in a eutrophic estuary (Waquoit Bay, Massachusetts, USA), and revealed that the benthic processes were a sink for acidity during the day and a source of acidity during the night, with H+ and O2 fluxes of ±0.0001 and ± 10 mmol m-2 h-1, respectively. H+ and O2fluxes were also determined using benthic flux chambers, for comparison with the EC rates. Benthic chamber measurements co-varied with EC measurements but were of ~4 times lower magnitude, likely due to chamber artifacts on hydrodynamics, and the depletion or elevation of DIC and O2 within the enclosed chamber. The individual H+ and O2 fluxes were highly correlated in each data set (EC and chambers), and both methods yielded H+ fluxes that were not explained by O2 metabolism alone. This ECHOES system provides a new tool for the determining the influence of benthic biogeochemical cycling on coastal ocean acidification and carbon cycling.

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.