Importance of bacterial maintenance respiration and baseline respiration for development of coastal hypoxia

Sammanfattning: 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.