Strong Influence of Baseline Respiration in an Oligotrophic Coastal Ecosystem

Abstract

Respiration is a key metabolic process in the marine environment and contemporary phytoplankton production (PhP) is commonly assumed the main driver. However, respiration in the absence of contemporary PhP, termed baseline respiration, can influence the energetics of an ecosystem and its sensitivity to hypoxia. Direct studies of baseline respiration are currently lacking. This study aims to obtain a first estimate of baseline respiration in a sub-arctic estuary and determine its contribution to plankton community respiration. Three approaches used to define baseline respiration determined the average rate to be 4.1 ± 0.1 (SE) mmol O 2 m −3 d −1. A hypsographic model at the basin scale accounting for seasonal variation estimated an annual contribution of 30% baseline respiration to planktonic respiration. There was no correlation between plankton respiration and PhP, but a significant linear dependence was found with the total carbon supply from phytoplankton and riverine input. The sum of dissolved organic carbon transported by rivers, provided by both benthic and pelagic algae, could sustain 69% of the annual plankton respiration, of which as much as 25% occurred during winter. However, only 32% of the winter season respiration was explained, indicating that unknown carbon sources exist during the winter. Nitrification had a negligible (≤2.4%) effect on baseline respiration in the system. The results show that baseline respiration accounted for a significant percentage of coastal plankton respiration when allochthonous sources dominated the carbon supply, weakening the respiration-to-PhP relationship.

Regulation of marine plankton respiration: A test of models

“…When using temperature as the main driver, the measured temperature at the time of estimation of respiration was used. Complementary data were extracted from the dBotnia database (dBotnia, 1994), which are identical to those reported by Vikström et al (2020). For the model by Eilola et al (2009), bioavailable detritus was modelled as 0.75 × [DOC] assuming 50% of marine DOC to be bioavailable and POC constituting 25% of total organic carbon (Forsgren and Jansson, 1992;Sandberg et al, 2004).…”

“…Selected equations modelling plankton respiration were tested for their predictive powers using a three-year field dataset (Vikström et al, 2020) and a study of bacterioplankton respiration (Vikström and Wikner, 2019), both collected from a boreal estuary in the northern Baltic Sea.…”

“…Estuaries will be directly influenced by input of terrestrial organic matter from rivers with some bioavailable fraction and a large humic component. The influence of riverine discharge is lacking in identified models but shown as a major driver in a Baltic Sea estuary (Vikström et al, 2020). The autochthonous productivity in meso-and eutrophic environments appear to systematically increase plankton respiration, although continued increase (Duarte and Agusti, 1998;Williams and Le, 1998) or levelling off (Hopkinson and Smith, 2005) at higher productivity rates differ between studies.…”

“…A three-year multiple season dataset of planktonic respiration that was measured with optode sensors (the OPTOCS method, n=191, Vikström et al, 2020) and bacterioplankton respiration data from two seasons (n=42, Vikström and Wikner, 2019), estimated with the same method. The two datasets are from the boreal Öre Estuary (northern Baltic Sea), with high riverine carbon input and encompasses complementary data such as primary production ( 14 CO 2 incorporation), bacterioplankton community growth rates ( 3 H-thymidine incorporation), bacterioplankton carbon contents, temperatures, dissolved organic carbon and nutrients.…”

“…Therefore, we investigated the current understanding of the regulation of plankton respiration using reported general models applied on a coastal multi-year data set of plankton respiration from the well-studied Öre Estuary located in the boreal zone (The Baltic Sea, Vikström and Wikner, 2019;Vikström et al, 2020). The boreal zone constitutes the next largest land carbon reservoir on the planet (Pan et al, 2011), and a substantial load of organic carbon is discharged to the coastal zone (Reader et al, 2014).…”