Strong Influence of Baseline Respiration in an Oligotrophic Coastal Ecosystem

Kevin Vikström,Jan Karlsson,Johan Wikner,Ines Bartl

doi:10.3389/fmars.2020.572070

Abstract

Respiration is a key metabolic process in the marine environment and contemporary phytoplankton production is commonly assumed the main driver. However, respiration in the absence of contemporary phytoplankton production, 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. Four approaches used to define baseline respiration determined the average rate to be 4.1 ± 0.1 (SE) mmol O2 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 phytoplankton production, 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-phytoplankton production relationship.

Highlights

The presence of sufficient oxygen is essential for maintaining healthy aquatic ecosystems
phytoplankton production (PhP) was higher in the autumn than in the winter (Mann–Whitney U test, p < 0.01) when values were below the detection limit (Figures 2, 3)
Measurements of pelagic respiration during winter are rare in the literature and were unexpectedly high in the studied estuary (4.2 mmol O2 m−3 d−1), most likely due to the high influence of allochthonous carbon (Table 3)

Summary

Introduction

The presence of sufficient oxygen is essential for maintaining healthy aquatic ecosystems. Respiration in lakes and rivers can be supported both by autochthonous carbon derived from primary production, and allochthonous carbon from terrestrial sources (del Giorgio et al, 1999; Prairie et al, 2002; Karlsson et al, 2007; McCallister and del Giorgio, 2008). A study of Chesapeake Bay shows that respiration is predominately supported by phytoplankton generated organic carbon, with only minor contribution of allochthonous carbon (Smith and Kemp, 2001). Both studies are markedly influenced by environments with high primary production rates (the former benthic production rates). Neither of them resolve the relationship at low values in oligotrophic water bodies, which are common in many marine environments where pelagic production dominates (Table 1)

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Results

Conclusion