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Coastal filter effect by microbial mineralization of riverine DOC in a sub-arctic river-estuary gradient
Umeå University, Faculty of Science and Technology, Umeå Marine Sciences Centre (UMF). Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The existence of a coastal filtering effect was studied in a 16 km boreal river-estuary system (RES) during contrasting low and high production conditions. Marked transformations occurred within 5 km (salinity 3) from the river mouth for many of the variables during high productive conditions. During the less productive season changes were small and occurred closer to the river mouth. Active transformation dominated the patterns detected in August (53 %), while mixing of river and coastal marine water was the dominant process in April. Bacterial community respiration was similar during both seasons averaging 2.6 μmol O2 dm-3 d-1, indicating efficient remineralization of riverine dissolved organic carbon (DOC). This was further reflected in a low and variable bacterial growth efficiency (BGE) with a mean of 12 % in the surface water at high productivity, but only 3 % at low productivity conditions. Bacterial community growth (BCG) showed strong relationship to water temperature at both seasons indicating energy limitation. Some indication of removal of nitrogen was observed, while no removal of phosphorus could be demonstrated. Phosphorus concentration showed a strong reciprocal power-function relationship to BCG, suggesting efficient assimilation of the limiting nutrient at carbon sufficiency. Marked spatial changes in diversity of phytoplankton, protozoa and bacterioplankton occurred at low salinities within 1 km from the river mouth. We conclude that this subarctic estuary acts as a coastal filter mainly by remineralizing riverine DOC to CO2.

National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-152583OAI: oai:DiVA.org:umu-152583DiVA, id: diva2:1255805
Available from: 2018-10-15 Created: 2018-10-15 Last updated: 2018-10-16
In thesis
1. Importance of bacterial maintenance respiration and baseline respiration for development of coastal hypoxia
Open this publication in new window or tab >>Importance of bacterial maintenance respiration and baseline respiration for development of coastal hypoxia
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reduced oxygen concentrations and increasing hypoxic zones havebecome more common in the sea due to climate change andeutrophication. The main cause of oxygen loss in oxygenatedenvironments is respiration. Respiration rates can be estimated usingoptode methodologies which utilize dynamic luminescence quenching toestimate the oxygen concentration declines in dark incubations. Apublished optode methodology was improved by using optodes withtitanium housing instead of plastic housing plausibly trapping oxygen.Drift was highly reduced by the titanium casings leading to a higherprecision and lower detection limit of 0.97 mmol O2 m-3 d-1. 28% ofmeasurements were shown to have non-linear oxygen concentrationdeclines. The rate of oxygen change was derived with a 2nd degreepolynomial at 1 hour from the incubation start. The majority of non-lineardeclines were concave and due to carbon substrate limitation. Analyzingnon-linear trends linearly, a common practice, leads to anunderestimation of respiration by up to 64%.

Bacterial maintenance respiration (Rm) was studied using anecophysiological model unverified in natural environments. The modelwas applicable at high productivities but a quadratic model wasdemonstrated to give a better fit. Rm was found to represent a significantpart in the sub-arctic estuary contributing to 58% of the annual specificbacterial respiration. Therefore, Rm may be more important in nature thanpreviously recognized. The ecophysiological model is driven solely by thebacterial specific growth rate (μ) where the relative influence of Rm iselevated as μ decreases. As a consequence, I hypothesize that a reductionin nutrients may not decrease the oxygen consumption but rather shiftbacterial growth based respiration to Rm as μ approaches zero.Baseline respiration (Rbl), defined as ecosystem respiration disconnectedfrom contemporary primary produced carbon, was also studied. Rbl wasshown to be largely supplied by allochthonous carbon in a coastalecosystem and had a contribution of 50% to the annual planktoncommunity respiration in the sub-arctic estuary studied. I claim that Rbland Rm are crucial to include for understanding and managingdevelopment of aquatic hypoxia in an effective and economic manner.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2018. p. 40
Keywords
Ecology, Respiration, Estuarine, Allochthonous, Maintenance, primary production, bacterial production
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-152587 (URN)978-91-7601-927-6 (ISBN)
Public defence
2018-11-09, N440, Naturvetarhuset, Umeå, 09:00 (English)
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Available from: 2018-10-19 Created: 2018-10-15 Last updated: 2019-03-19Bibliographically approved

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Wikner, JohanVikström, Kevin

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