Lead Organizer: Theodore Packard, University of Las Palmas Gran Canaria (theodore.packard@ulpgc.es)

Co-organizers:

Barbara Campbell, Clemson University (bcampb7@clemson.edu)

Presentations

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10:30 AM

SMALL-SCALE SPATIAL AND TEMPORAL VARIABILITY OF GREENHOUSE GAS EMISSIONS FROM A LARGE GERMAN RIVER (5397)

Primary Presenter: Matthias Koschorreck, Helmholtz Centre For Environmental Research - UFZ (matthias.koschorreck@ufz.de)

Streams and rivers are globally relevant emitters of greenhouse gases (GHG). Emission estimates are typically based on field data with limited temporal and spatial resolution. However, GHG emissions may differ substantially between different habitat types, especially in larger rivers. We measured GHG emissions in different habitat types at river Elbe in Germany, to assess the effect of intra-system variability as well as diurnal variability on GHG emission upscaling. CH₄ emissions near the shore and in groyne fields were higher than in the middle of the stream due to CH₄CH₄ production in the sediments. In contrast, CO₂ emissions were highest in the middle of the stream, where gas transfer coefficients were higher. Thus, both spatial variability of dissolved gas concentrations and gas transfer velocity contributed to spatial variability. Day-night variations in aquatic CO₂ emissions were larger than the differences measured between sites, while spatial differences compared to temporal variability predominated for CH₄ emissions. We observed considerable spatiotemporal variability of GHG emissions from dry sites and diurnal fluctuations of CO₂ emissions were larger at dry than aquatic sites. CH₄ flux measurements at dry fallen sediments revealed a heterogeneous pattern of sources and sinks depending on different controlling factors. Thus, both small-scale spatial and temporal patterns were different for the two considered gases. Our results imply that in large rivers, CH₄CH₄ and CO₂ call for different measuring strategies in order to obtain optimal whole-system emission estimates.

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10:45 AM

Stratification and drought impair water quality and enhance carbon emissions of a Mediterranean reservoir (6480)

Primary Presenter: Jorge Montes-Pérez, University of Malaga (jmontesp@uma.es)

The number of reservoirs is in expansion due to population growth, linked to an increase in water and energy demand. Besides that, climate change and direct anthropogenic pressures trigger alterations in hydrology and thermal structure which could modify carbon fluxes and water quality in these ecosystems. This study tackles the effect of hydrological and thermal structure changes in carbon fluxes in a Mediterranean reservoir (El Gergal, Spain) measuring CO₂ and CH₄ fluxes from water surface, CO₂ fluxes from drawdown areas and the accumulation of CO₂ and CH₄ in the hypolimnion. The results showed that hydrology and thermal structure have an impact on carbon fluxes in the reservoir affecting CO₂ and CH₄ fluxes. During stratification, water level is lower, increasing drawdown areas and, therefore, rising CO₂ emissions from these surfaces. In addition, stratification and lower water renewal enhance anoxia in the bottom of the reservoir promoting anaerobic respiration of organic matter. This leads to higher production of CH4 that together with lower water level will increase diffusive and ebullitive fluxes of CH₄. Furthermore, anaerobic processes produce reduced substances (e.g. HS^-), impairing water quality, and accumulate CO₂ and CH₄ in the hypolimnion that could be released to the atmosphere during fall turnover. These results suggest that longer stratification periods with more frequent and intense droughts, which lead to low water episodes with higher residence time and higher risk of anoxia, will impair water quality and enhance the carbon emissions of reservoirs.

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11:00 AM

AIR-SEA CARBON DIOXIDE FLUXES IN THE HIGH LATITUDE NORTH ATLANTIC OCEAN (5790)

Primary Presenter: Jon Olafsson, University of Iceland (jo@hi.is)

The surface water masses in the North Atlantic region of Iceland are of contrasting origins and character, on the one hand the northward-flowing North Atlantic Drift, a Gulf Stream offspring, on the other hand southward moving cold low salinity Polar and Arctic Waters with signatures from Arctic freshwater sources. We have studied by observations, the CO₂  air-sea flux of the relevant water masses in the vicinity of Iceland in all seasons and in different years. Here we report on the flux variations associated with the three main water masses. We examine all seasons, the importance of wind strength and relations to biological nutrient cycles. The results reveal distinct differences between the three water masses. The Polar Water originating from the Arctic is a strong CO₂ sink and so is also the Arctic Water in the Iceland Sea. The annual mean flux of the Atlantic Water south and west of Iceland shows it to be a weak or neutral CO₂ sink. We discuss the Polar Water influence on the North Atlantic CO₂ sink in light of the connection to developments in the rapidly warming and changing Arctic.

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11:15 AM

Coastal acidification and carbon sequestration driven by inorganic carbon export from tidal wetlands (5689)

Primary Presenter: Gloria Reithmaier, University of Gothenburg (gloria.reithmaier@gu.se)

Coastal ecosystems are under threat from ocean acidification. Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and biogeochemical processes altering carbonate chemistry. Mangroves and saltmarshes are global biogeochemical hotspots sequestering large amounts of carbon in sediments and in the ocean following lateral carbon export (outwelling). Here, we investigate whether mangroves and saltmarshes drive or buffer coastal waters against acidification and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of tidal wetlands export more DIC than alkalinity, enhancing pH declines of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol/m²/d in mangroves and 57 ± 104 mmol/m²/d in saltmarshes) was the major fate of plant production. However, substantial amounts of fixed carbon remain unaccounted for in budgets. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant carbon sequestration mechanism enhancing the overall value of tidal wetlands as a nature-based solution to climate change.

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11:30 AM

MECHANISTIC INFERENCE SUGGESTS PERIODIC DOMINANCE OF INTERNAL HETEROTROPHIC RECYCLING IN THE WESTERN ENGLISH CHANNEL CARBON CYCLE (5482)

Primary Presenter: Falk Eigemann, Technical University Berlin (falkeigemann@gmail.com)

Fluxes between phytoplankton and heterotrophic bacteria maintain aquatic carbon cycles, but mechanistic understanding and quantitative determinations of fluxes remain scarce. We applied the mechanistic inference method FLUXNET to a seven-year time-series of the Western English Channel (WEC), and estimated time-variable carbon fluxes between 135 phytoplankton operational taxonomical units (OTUs, 18S) and 157 bacterial amplicon sequences variants (ASVs, 16S), as well as bacterial utilization patterns of DOC species for phytoplankton spring and summer blooms, as well as bacterial summer blooms. In spring blooms, fluxes from phytoplankton to bacteria dominated, and a succession from phytoplankton to consecutive bacterial ASVs occurred. Vice versa, successions of abundant bacterial ASVs in bacterial summer blooms were followed by an increase of phytoplankton, culminating in phytoplankton summer blooms. Both types of summer blooms, however, were characterized by high bacteria to bacteria carbon fluxes, with the same magnitude as fluxes from phytoplankton to bacteria in bacterial summer blooms. Utilization patterns of hypothetical DOC species suggest that carbon processing in periods with high internal heterotrophic recycling are dominated by generalist utilization, whereas in periods with high phytoplankton to bacteria fluxes, specialist strategies dominate. In conclusion, we propose that the mostly neglected carbon flux between different heterotrophic bacteria periodically exceeds the flux between phytoplankton and bacteria, and substantially contributes to the WEC carbon cycle.

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11:45 AM

ATLANTIC WATER COLUMN REMINERALIZATION, NUTRIENT RETENTION EFFICIENCY, AND BENTHIC REMINERALIZATION (5744)

Primary Presenter: Ted Packard, University of Las Palmas Gran Canaria (theodore.packard@ulpgc.es)

Atlantic ocean rates of carbon-flux, and carbon remineralization can be calculated, one from the other, and when combined can be used to calculate ocean water-column nutrient retention efficiency. Here, data from helium-tritium dating, advection-diffusion modeling, apparent oxygen utilization (AOU), respiratory electron transport activity (ETS), and three different types of sediment traps were used to calculate metabolic-based rates of carbon-flux, and carbon remineralization in the ocean water-column. Carbon remineralization (Jc), at 100m, ranged from 0.4 to 109 millimol m^-3 yr^-1and at 1000m, it ranged  more than two orders of magnitude lower, from 0.001 to 4.3 millimol m^-3 yr^-1. We then used Jc and sediment trap data to calculate carbon flux (Fc) profiles. Fc ranged from 1.5 to 17.8 millimol m^-2 yr^-1 at 100m and to 0.03 to 12.1 millimol  m^-2 yr^-1 at 1000m. Integrating Jc from the bottom of the euphotic zone to the ocean bottom yielded a North Atlantic export carbon production range of 0.07 to 23.3 mol m^-2 yr^-1. The ratio, Jc/Fc, as a percent, was the nutrient retention efficiency that predicts regenerated production. NRE is inversely related to carbon-flux transfer efficiency (Teff) and both NRE and Teff are related to b_J and b_F, the different attenuation exponents of the Jc and Fc power functions. For a 50m water column centered at 125m, NRE ranged from 51 to 27% while Teff ranged from 49 to 73%. Finally, benthic Jc was calculated from Fc, using different limits of integration. For the North Atlantic, we found that benthic Jc ranged from 2.1 to 7040.0  millimol m^-2 yr^-1.  

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