McGlathery Karen J.

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McGlathery
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Karen J.
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  • Article
    Sub-tropical seagrass ecosystem metabolism measured by eddy covariance
    (Inter-Research, 2015-06-08) Long, Matthew H. ; Berg, Peter ; McGlathery, Karen J. ; Zieman, Joseph C.
    The metabolism of seagrass ecosystems was examined at 4 sites in south Florida, USA, using the eddy covariance technique under in situ conditions. Three sites were located across a phosphorus-driven productivity gradient to examine the combined effects of dynamic variables (irradiance, flow velocity) and state 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 similar large daily production and respiration rates (397 and –17 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 variations in irradiance and flow velocity. At all sites the relationship between photosynthesis and irradiance was linear and did not show any sign of saturation over the entire irradiance range (up to 1400 µmol photons m–2 s–1). This was likely due to the efficient use of light by the large photosynthetic surface area of the seagrass canopy, an effect which can only be examined by in situ measurements that integrate across all autotrophs in the seagrass ecosystem.
  • Article
    Multiple timescale processes drive ecosystem metabolism in eelgrass (Zostera marina) meadows
    (Inter-Research, 2014-07-17) Rheuban, Jennie E. ; Berg, Peter ; McGlathery, Karen J.
    The oxygen flux between benthic ecosystems and the overlying water column is a measure of metabolic status and a commonly used proxy for carbon cycling. In this study, oxygen flux was measured seasonally using the eddy correlation technique in a restored eelgrass (Zostera marina L.) meadow in the Virginia coastal bays (USA). In 5 intensive field campaigns, we covered seasonal variation in oxygen metabolism and biomass with overlap in late summer to observe interannual variability. The high-resolution measurements allowed identification of the drivers of metabolism at multiple timescales: minute to hourly, daily, and monthly to seasonally. There was a strong correlation between nighttime hourly fluxes and current velocity that varied seasonally with seagrass shoot density and temperature. No similar relationship was observed during the day. A hysteresis effect in oxygen flux throughout the day was observed during October and August that was most likely due to increased respiration (R) in the afternoon. In October, net community production was 90% lower in the afternoon than in the morning at the same irradiance. From this hysteresis, we calculated that daytime R may be up to 2.5-fold larger than nighttime R. The magnitudes of daily gross primary production (GPP) and R were well correlated throughout the year with close to a 1:1 ratio that reflected a tight coupling between GPP and R on daily to seasonal timescales. Our results document the dynamic nature of oxygen fluxes that, when integrated over time, translate into highly variable rates of ecosystem metabolism over daily to seasonal timescales. This variation must be incorporated to accurately determine trophic status.
  • Article
    Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light
    (Association for the Sciences of Limnology and Oceanography, 2013-07) Berg, Peter ; Long, Matthew H. ; Huettel, Markus ; Rheuban, Jennie E. ; McGlathery, Karen J. ; Howarth, Robert W. ; Foreman, Kenneth H. ; Giblin, Anne E. ; Marino, Roxanne
    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.
  • Article
    Nitrogen fixation associated with epiphytes on the seagrass Zostera marina in a temperate lagoon with moderate to high nitrogen loads
    (Springer, 2023-10-09) Marino, Roxanne ; Hayn, Melanie ; Howarth, Robert W. ; Giblin, Anne E. ; McGlathery, Karen J. ; Berg, Peter
    As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N2 fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N2 fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N2 fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of 15N2 into biomass and a novel application of the N2:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N2 fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m−2 d−1), with the highest rates (2.6 mmol N m−2 d−1) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N2 fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m−2 d−1), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N2 fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow.