Howard
Evan M.
Howard
Evan M.
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ArticleQuantifying photosynthetic rates of microphytobenthos using the triple isotope composition of dissolved oxygen(Association for the Sciences of Limnology and Oceanography, 2013-07) Stanley, Rachel H. R. ; Howard, Evan M.Microphytobenthos are important mediators of nutrient and carbon fluxes in coastal environments. However, quantifying production rates by microphytobenthos is difficult, and existing methods necessitate perhaps erroneous assumptions that dark respiration equals light respiration. Here we present a new method for quantifying photosynthetic rates of microphytobenthos, i.e., gross primary production, by using the triple isotope composition of dissolved oxygen in benthic flux chambers. Because the triple oxygen isotope signature is sensitive to photosynthesis, but not to respiration, this method allows quantification of gross photosynthetic oxygen fluxes by microphytobenthos without assumptions about respiration. We present results from field experiments in Waquoit Bay, Massachusetts, that illustrate the method.
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ArticleShallow ponds are heterogeneous habitats within a temperate salt marsh ecosystem(John Wiley & Sons, 2017-06-15) Spivak, Amanda C. ; Gosselin, Kelsey M. ; Howard, Evan M. ; Mariotti, Giulio ; Forbrich, Inke ; Stanley, Rachel H. R. ; Sylva, Sean P.Integrating spatial heterogeneity into assessments of salt marsh biogeochemistry is becoming increasingly important because disturbances that reduce plant productivity and soil drainage may contribute to an expansion of shallow ponds. These permanently inundated and sometimes prominent landscape features can exist for decades, yet little is known about pond biogeochemistry or their role in marsh ecosystem functioning. We characterized three ponds in a temperate salt marsh (MA, USA) over alternating periods of tidal isolation and flushing, during summer and fall, by evaluating the composition of plant communities and organic matter pools and measuring surface water oxygen, temperature, and conductivity. The ponds were located in the high marsh and had similar depths, temperatures, and salinities. Despite this, they had different levels of suspended particulate, dissolved, and sediment organic matter and abundances of phytoplankton, macroalgae, and Ruppia maritima. Differences in plant communities were reflected in pond metabolism rates, which ranged from autotrophic to heterotrophic. Integrating ponds into landcover-based estimates of marsh metabolism resulted in slower rates of net production (−8.1 ± 0.3 to −15.7 ± 0.9%) and respiration (−2.9 ± 0.5 to −10.0 ± 0.4%), compared to rates based on emergent grasses alone. Seasonality had a greater effect on pond water chemistry, organic matter pools, and algal abundances than tidal connectivity. Alternating stretches of tidal isolation and flushing did not affect pond salinities or algal communities, suggesting that exchange between ponds and nearby creeks was limited. Overall, we found that ponds are heterogeneous habitats and future expansion could reduce landscape connectivity and the ability of marshes to capture and store carbon.
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ArticleNutrient enrichment induces dormancy and decreases diversity of active bacteria in salt marsh sediments(Nature Publishing Group, 2016-09-26) Kearns, Patrick J. ; Angell, John H. ; Howard, Evan M. ; Deegan, Linda A. ; Stanley, Rachel H. R. ; Bowen, Jennifer L.Microorganisms control key biogeochemical pathways, thus changes in microbial diversity, community structure and activity can affect ecosystem response to environmental drivers. Understanding factors that control the proportion of active microbes in the environment and how they vary when perturbed is critical to anticipating ecosystem response to global change. Increasing supplies of anthropogenic nitrogen to ecosystems globally makes it imperative that we understand how nutrient supply alters active microbial communities. Here we show that nitrogen additions to salt marshes cause a shift in the active microbial community despite no change in the total community. The active community shift causes the proportion of dormant microbial taxa to double, from 45 to 90%, and induces diversity loss in the active portion of the community. Our results suggest that perturbations to salt marshes can drastically alter active microbial communities, however these communities may remain resilient by protecting total diversity through increased dormancy.
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ArticleAn inland sea high nitrate-low chlorophyll (HNLC) region with naturally high pCO2(John Wiley & Sons, 2015-02-19) Murray, James W. ; Roberts, Emily ; Howard, Evan M. ; O'Donnell, Michael ; Bantam, Cory ; Carrington, Emily ; Foy, Mike ; Paul, Barbara ; Fay, AmandaWe present a time series of data for temperature, salinity, nitrate, and carbonate chemistry from September 2011 to July 2013 at the University of Washington's Friday Harbor Laboratories. Samples were collected at the Friday Harbor dock and pump house. Seawater conditions at Friday Harbor were high nitrate-low chlorophyll, with average nitrate and pCO2 concentrations of ∼ 25 ± 5 μmol L−1 and ∼ 700 ± 103 μatm (pH 7.80 ± 0.06). Transient decreases in surface water nitrate and pCO2 corresponded with the timing of a spring bloom (April through June). The high nitrate and pCO2 originate from the high values for these parameters in the source waters to the Salish Sea from the California Undercurrent (CU). These properties are due to natural aerobic respiration in the region where the CU originates, which is the oxygen minimum zone in the eastern tropical North Pacific. Alkalinity varies little so the increase in pCO2 is due to inputs of dissolved inorganic carbon (DIC). This increase in DIC can come from both natural aerobic respiration within the ocean and input of anthropogenic CO2 from the atmosphere when the water was last at the sea surface. We calculated that the anthropogenic “ocean acidification” contribution to DIC in the source waters of the CU was 36 μmol L−1. This contribution ranged from 13% to 22% of the total increase in DIC, depending on which stoichiometry was used for C/O2 ratio (Redfield vs. Hedges). The remaining increase in DIC was due to natural aerobic respiration.
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ArticleBiological production, export efficiency, and phytoplankton communities across 8000 km of the South Atlantic(John Wiley & Sons, 2017-07-11) Howard, Evan M. ; Durkin, Colleen A. ; Hennon, Gwenn ; Ribalet, François ; Stanley, Rachel H. R.In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March–May 2013. We used observations of picophytoplankton, nanophytoplankton, and microphytoplankton to evaluate community structure and diversity and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O2 m−2 d−1, gross primary production (GPP) between 40 and 100 mmol O2 m−2 d−1, and NCP/GPP, a measure of export efficiency, ranging from 0.1 to 0.2 (0.05–0.1 in carbon units). However, notable exceptions to this basin-scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of dissolved organic carbon export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.
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ArticleUsing noble gases to compare parameterizations of air‐water gas exchange and to constrain oxygen losses by ebullition in a shallow aquatic environment(John Wiley & Sons, 2018-09-07) Howard, Evan M. ; Forbrich, Inke ; Giblin, Anne E. ; Lott, Dempsey E. ; Cahill, Kevin L. ; Stanley, Rachel H. R.Accurate determination of air‐water gas exchange fluxes is critically important for calculating ecosystem metabolism rates from dissolved oxygen in shallow aquatic environments. We present a unique data set of the noble gases neon, argon, krypton, and xenon in a salt marsh pond to demonstrate how the dissolved noble gases can be used to quantify gas transfer processes and evaluate gas exchange parameterizations in shallow, near‐shore environments. These noble gases are sensitive to a variety of physical processes, including bubbling. We thus additionally use this data set to demonstrate how dissolved noble gases can be used to assess the contribution of bubbling from the sediments (ebullition) to gas fluxes. We find that while literature gas exchange parameterizations do well in modeling more soluble gases, ebullition must be accounted for in order to correctly calculate fluxes of the lighter noble gases. In particular, for neon and argon, the ebullition flux is larger than the differences in the diffusive gas exchange flux estimated by four different wind speed‐based parameterizations for gas exchange. We present an application of noble gas derived ebullition rates to improve estimates of oxygen metabolic fluxes in this shallow pond environment. Up to 21% of daily net oxygen production by photosynthesis may be lost from the pond via ebullition during some periods of biologically and physically produced supersaturation. Ebullition could be an important flux of oxygen and other gases that is measurable with noble gases in other shallow aquatic environments.
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ThesisEcosystem metabolism in salt marsh tidal creeks and ponds : applying triple oxygen isotopes and other gas tracers to novel environments(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2017-02) Howard, Evan M. ; Kujawinski, ElizabethSalt marshes are physically, chemically, and biologically dynamic environments found globally at temperate latitudes. Tidal creeks and marshtop ponds may expand at the expense of productive grass-covered marsh platform. It is therefore important to understand the present magnitude and drivers of production and respiration in these submerged environments in order to evaluate the future role of salt marshes as a carbon sink. This thesis describes new methods to apply the triple oxygen isotope tracer of photosynthetic production in a salt marsh. Additionally, noble gases are applied to constrain air-water exchange processes which affect metabolism tracers. These stable, natural abundance tracers complement traditional techniques for measuring metabolism. In particular, they highlight the potential importance of daytime oxygen sinks besides aerobic respiration, such as rising bubbles. In tidal creeks, increasing nutrients may increase both production and respiration, without any apparent change in the net metabolism. In ponds, daytime production and respiration are also tightly coupled, but there is high background respiration regardless of changes in daytime production. Both tidal creeks and ponds have higher respiration rates and lower production rates than the marsh platform, suggesting that expansion of these submerged environments could limit the ability of salt marshes to sequester carbon.
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ArticleRevising estimates of aquatic gross oxygen production by the triple oxygen isotope method to incorporate the local isotopic composition of water(John Wiley & Sons, 2017-10-25) Manning, Cara C. ; Howard, Evan M. ; Nicholson, David P. ; Ji, Brenda Y. ; Sandwith, Zoe O. ; Stanley, Rachel H. R.Measurement of the triple oxygen isotope (TOI) composition of O2 is an established method for quantifying gross oxygen production (GOP) in natural waters. A standard assumption to this method is that the isotopic composition of H2O, the substrate for photosynthetic O2, is equivalent to Vienna standard mean ocean water (VSMOW). We present and validate a method for estimating the TOI composition of H2O based on mixing of local meteoric water and seawater H2O end-members, and incorporating the TOI composition of H2O into GOP estimates. In the ocean, GOP estimates based on assuming the H2O is equivalent to VSMOW can have systematic errors of up to 48% and in low-salinity systems, errors can be a factor of 2 or greater. In future TOI-based GOP studies, TOI measurements of O2 and H2O should be paired when the H2O isotopic composition is expected to differ from VSMOW.