Data_Sheet_1_Strong Influence of Baseline Respiration in an Oligotrophic Coastal Ecosystem.xlsx

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 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 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.

呼吸作用是海洋环境中的关键代谢过程，当前学界普遍认为当代浮游植物生产量（Phytoplankton Production，PhP）是其主要驱动因素。然而，在缺乏当代浮游植物生产量的条件下发生的呼吸作用，被称为基线呼吸作用（baseline respiration），可影响生态系统的能量动力学及其对低氧环境的敏感性。目前针对基线呼吸作用的直接研究仍较为匮乏。本研究旨在首次估算亚北极河口（sub-arctic estuary）的基线呼吸作用水平，并明确其对浮游生物群落呼吸作用的贡献。本研究采用三种方法界定基线呼吸作用，测得其平均速率为4.1±0.1（标准误，Standard Error，SE）毫摩尔·立方米⁻¹·天⁻¹。基于盆地尺度的水深面积模型（hypsographic model）并考虑季节变化，研究估算得到基线呼吸作用对浮游生物呼吸作用的年贡献率为30%。研究发现，浮游生物呼吸作用与PhP并无显著相关性，但与浮游植物和河流输入提供的总碳供给存在显著线性依赖关系。由底栖与浮游藻类产生、经河流输送的溶解有机碳（dissolved organic carbon）总和，可支撑69%的年浮游生物呼吸作用，其中多达25%的呼吸作用发生在冬季。然而，该总和仅能解释32%的冬季呼吸作用总量，表明冬季仍存在未被探明的碳源。硝化作用（nitrification）对该系统的基线呼吸作用影响可忽略不计（≤2.4%）。研究结果表明，当外源碳源（allochthonous sources）主导碳供给时，基线呼吸作用在沿海浮游生物呼吸作用中占比可观，削弱了呼吸作用与PhP之间的关联。