Kragh Theis, Andersen Mikkel René, Sand-Jensen Kaj
Freshwater Biological Section Biological Institute, University of Copenhagen, Universitetsparken 4, 2100, Copenhagen, Denmark.
Oecologia. 2017 Sep;185(1):157-170. doi: 10.1007/s00442-017-3931-3. Epub 2017 Aug 28.
Small, shallow lakes with dense growth of submerged macrophytes are extremely abundant worldwide, but have remained grossly understudied although open water oxygen measurements should be suitable to determine diel fluctuations and test drivers of ecosystem metabolism during the day. We measured the temporal and spatial variability of environmental conditions as well as net ecosystem production (NEP) and respiration (R) in a small, shallow Swedish lake with dense charophyte stands by collecting data from oxygen-, pH-, temperature- and light-sensors across horizontal and vertical gradients during different periods between April and June in 3 years. We found reproducible diel oxygen patterns and daily metabolic rates. The charophyte canopy accounted for almost all primary production and respiration of the ecosystem. Two novel discoveries-profound afternoon depression of production and nighttime decline of respiration-occurred on virtually every day. Extensive increase of oxygen-, temperature- and pH-levels and depletion of dissolved inorganic carbon (DIC) and CO concentrations could account for maximum NEP-rates before noon and afternoon depression with low NEP-rates. Ecosystem respiration declined during the night to 24-70% of rates at sunset, probably because of depletion of respiratory substrates. Afternoon depression of photosynthesis should be widespread in numerous habitats with dense growth of macrophytes, periphyton, or phytoplankton implying that daily photosynthesis and growth are restricted and species with efficient DIC use may have an advantage.
小型浅水湖泊中沉水大型植物生长密集，在全球范围内极为丰富，但尽管开放水域的氧气测量应该适合确定昼夜波动并测试白天生态系统代谢的驱动因素，这些湖泊仍未得到充分研究。我们通过在2014年至2016年4月至6月的不同时期，收集横跨水平和垂直梯度的氧气、pH值、温度和光传感器的数据，测量了瑞典一个小型浅水湖泊（有密集的轮藻群落）环境条件的时空变异性以及净生态系统生产（NEP）和呼吸作用（R）。我们发现了可重复的昼夜氧气模式和每日代谢率。轮藻冠层几乎占了生态系统所有的初级生产和呼吸作用。几乎每天都会出现两个新发现——生产在下午大幅下降以及呼吸作用在夜间减少。氧气、温度和pH值水平的大幅升高以及溶解无机碳（DIC）和CO浓度的消耗可以解释中午前的最大NEP率和下午低NEP率时生产的下降。夜间生态系统呼吸作用下降到日落时速率的24% - 70%，这可能是由于呼吸底物的消耗。光合作用在下午的下降可能在许多大型植物、周丛生物或浮游植物生长密集的生境中普遍存在，这意味着每日光合作用和生长受到限制，而有效利用DIC的物种可能具有优势。
