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Ohio Sea Grant eNewsletter January 2018

OHSU-N-1541

Ohio Sea Grant eNewsletter January 2018

VOLUME: 18 ISSUE: 1
Newsletter

A Decline in Benthic Algal Production May Explain Recent Hypoxic Events in Lake Erie's Central Basin

OHSU-RS-1574
ABSTRACT:

Since the late 1990s, the central basin of Lake Erie has reputedly experienced an increase in the frequency and severity of hypoxic events. However, total phosphorus (TP) loading, in-lake TP concentrations, chlorophyll a (Chl a), and sediment oxygen demand (SOD) have all declined in the central basin since the 1970s. Water clarity in this basin has declined from the 1970s to 2000s despite the invasion of dreissenid mussels around 1990. In shallow lakes, declines in benthic primary production (PP) can generate positive feedback loops between the internal loading of nutrients/dissolved organic carbon and hypoxic/anoxic conditions in the water column. Such a hypoxia-inducing mechanism driven by declines in benthic PP has not been explored in Lake Erie. To test if a decline in benthic PP might explain hypoxic events in the central basin of Lake Erie, we calculated the inter-decadal changes in benthic and planktonic algal production in this basin from the 1970s to the 2000s. Primary production models using water column Chl a concentrations and light attenuation indicated that benthic PP represents roughly 10% of the basin’s total areal PP. However, our calculations show that benthic PP declined from approximately 540 to 200 g C/m2 y since the 1970s. We propose that a decline in benthic PP may have played a key mechanistic role in the transition from externally-induced hypoxia (i.e. watershed nutrient loading fueling phytoplankton production) in the 1970s and 80s to internally-induced hypoxia (sediment resuspension and internal loading) since the late 1990s.

DOI: 10.1016/j.jglr.2017.03.016 VOLUME: 43 ISSUE: 3 LENGTH: 5 pages
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Quantifying Emissions of Methane Derived From Anaerobic Organic Matter Respiration and Natural Gas Extraction in Lake Erie

OHSU-RS-1550
ABSTRACT:

Despite a growing awareness of the importance of inland waters in regional and global carbon © cycles, particularly as sources of the greenhouse gases carbon dioxide (CO2) and methane (CH4), very little is known about C sources and fluxes in the Laurentian Great Lakes, Earth’s largest surface freshwater system. Here, we present a study of CH4 dynamics in Lake Erie, which has large spring algae blooms linked to fertilizer runoff and followed by hypoxia, as well as an extensive network of natural gas wells and pipelines in Canadian waters. Lake Erie is a positive source of CH4 to the atmosphere in late summer, even in shallow regions without water column hypoxia. Stable isotopic measurements indicate that both biogenic and thermogenic CH4 contribute to emissions from Lake Erie. We estimate that Lake Erie emits 1.360.6 3 105 kg CH4-C d21 in late summer, with approximately 30% of CH4 derived from natural gas infrastructure. Additional work is needed to determine the spatial and temporal dynamics of CH4 emissions from Lake Erie and to confirm estimates of source contribution. Studies of the C cycle in large lakes are not as straightforward as those in smaller lakes, as, in addition to O2 availability, subsurface currents and high winds may exert significant control over dissolved CH4 patterns. If climate warming and increasing precipitation intensity lead to increased algal biomass and/or greater extent and duration of hypoxia, this may increase emissions of CH4 from Lake Erie in a positive feedback to climate change.

DOI: 10.1002/lno.10273 VOLUME: 61 ISSUE: 1 LENGTH: 10 pages
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Oxygen use by Nitrification in the Hypolimnion and Sediments of Lake Erie

OHSU-RS-1540
ABSTRACT:

Nitrification is an oxygen consumptive process, consuming 2 mol of oxygen permol of ammonium oxidized. Hypolimnion
and sediment sampleswere collected during the summers of 2008–2010 in Lake Erie to determine the
total oxygen consumption and oxygen consumption fromnitrification by blocking nitrification with selective inhibitors.
Oxygen consumption by nitrification in the hypolimnion was 3.7 ± 2.9 (mean ± 1 SD) μmol O2/L/d,
with nitrification accounting for 32.6 ± 22.1% of the total oxygen consumption. Nitrification in the hypolimnion
contributed more to oxygen consumption in the eastern sites than western sites and was lowest in September.
The nitrification rate did not correlatewith environmental factors such as oxygen, nitrate or ammonium, or nitrifier
numbers. Oxygen consumption by nitrification in sediment slurries was 7.1 ± 5.8 μmol O2/g/d, with nitrification
accounting for 27.0 ± 19.2% of the total oxygen consumption with the lowest rates in July and the lowest
percentages in June. Oxygen consumption by nitrification in intact sediment coreswas 682 ± 61.1 μmol O2/m/d
with nitrification accounting for 30.4 ± 10.7% of the total oxygen consumption. Nitrification rates in intact cores
were generally highest in September. The proportion of oxygen consumed by nitrification corresponds closely
with what would be predicted from complete oxidation of a Redfield molecule (23%).While nitrification is unlikely
to be the dominant oxygen consumptive process, the rates observed in Lake Eriewere sufficient to theoretically
deplete a large portion of the hypolimnetic oxygen pool during the stratified period.

VOLUME: 40 ISSUE: 1 LENGTH: 5 pages
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Seasonal Changes in Microbial Community Structure and Activity Imply Winter Production is Linked to Summer Hypoxia in a Large Lake

OHSU-RS-1566
ABSTRACT:

Carbon and nutrient cycles in large temperate lakes such as Lake Erie are primarily driven by phototrophic and heterotrophic microorganisms, although our understanding of these is often constrained to late spring through summer due to logistical constraints. During periods of > 90% ice cover in February of 2008, 2009, and 2010, we collected samples from an icebreaker for an examination of bacterial production as well as microbial community structure. In comparison with summer months (August 2002 and 2010), we tested hypotheses concerning seasonal changes in microbial community diversity and production. Bacterial production estimates were c. 2 orders of magnitude higher (volume normalized) in summer relative to winter. Our observations further demonstrate that the microbial community, including single-celled phototrophs, varied in composition between August and February. Sediment traps deployed and collected over a 3 year period (2008–2011) confirmed that carbon export was ongoing and not limiting winter production. The results support the notion that active primary producers in winter months export carbon to the sediments that is not consumed until the warmer seasons. The establishment of this linkage is a critical observation in efforts to understand the extent and severity of annual summertime formations of a zone of regional hypoxia in Lake Erie.

DOI: 10.1111/1574-6941.12238 VOLUME: 87 ISSUE: 2 LENGTH: 10 pages
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Transient Social–Ecological Stability: the Effects of Invasive Species and Ecosystem Restoration on Nutrient Management Compromise in Lake Erie

OHSU-RS-1546
ABSTRACT:

Together, lake ecosystems and local human activity form complex social–ecological systems (SESs) characterized by feedback loops and discontinuous change. Researchers in diverse fields have suggested that complex systems do not have single stable equilibria in the long term because of inevitable perturbation. During this study, we sought to address the general question of whether or not stable social–ecological equilibria exist in highly stressed and managed lacustrine systems. Using an integrated human–biophysical model, we investigated the impacts of a species invasion and ecosystem restoration on SES equilibrium, defined here as a compromise in phosphorus management among opposing stakeholders, in western Lake Erie. Our integrated model is composed of a calibrated ecological submodel representing Sandusky Bay, and a phosphorus management submodel that reflects the societal benefits and costs of phosphorus regulation. These two submodels together form a dynamic feedback loop that includes freshwater ecology, ecosystem services, and phosphorus management. We found that the invasion of dreissenid mussels decreased ecosystem resistance to eutrophication, necessitating increased phosphorus management to preserve ecosystem services and thus creating the potential for a shift in social–ecological equilibrium. Additionally, our results suggest that net benefits in the region following the invasion of dreissenids may never again reach the pre-invasion level if on-site phosphorus control is the sole management lever. Further demonstrating transient system stability, large-scale wetland restoration shifted points of management compromise to states characterized by less on-site phosphorus management and higher environmental quality, resulting in a significant increase in net benefits in the region. We conclude that lacustrine SESs are open and dynamic, and we recommend that future models of these systems emphasize site-specific perturbation over equilibrium, thereby aiding the development of management plans for building system resistance to undesirable change that are both flexible and sustainable in an unknowable future.

VOLUME: 15 ISSUE: 1 LENGTH: 28 pages
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