Lake Metabolism at North Temperate Lakes LTER 2000

Recent literature suggests that for many lakes and rivers, the respiratory breakdown of organic matter (R) exceeds production of organic matter by photosynthesis (gross primary production; GPP) within the water body. This metabolic balance (GPP less than R; heterotrophy ) implies that allochthonous organic matter supports a portion of the aquatic ecosystems respiration. Evidence that many lakes are heterotrophic comes from diverse approaches, and debate remains over the circumstances in which heterotrophy exists. The methods used to estimate GPP and R and the limited extent of lake types studied, especially with respect to dissolved organic carbon (DOC) and total phosphorus (TP) concentrations, are two reasons for differing conclusions. In this study, O2 and CO2 sondes were deployed during July and August, 2000 to measure diel gas dynamics in the surface waters of 25 lakes in the Northern Highland Lake district of Wisconsin and the Upper Peninsula of Michigan. The lakes were chosen to span wide and orthogonal ranges in DOC and TP concentrations. From these data, we calculated GPP, R and net ecosystem production (NEP=GPP-R). Over the broad range in TP and DOC among the lakes, diel CO2 and O2 changed on a near 1:1 molar ratio. Metabolism estimates from the two gases were comparable, except at high pH. Most lakes in our data set had -NEP, but GPP and R appeared to be controlled by different factors. TP correlated strongly with GPP, whereas DOC correlated with R. At low DOC concentrations, GPP and R were nearly equal, but at higher DOC, GPP and R uncoupled and lakes had -NEP. Strong correlations between lake metabolism and landscape related variables suggest that allochthonous carbon influences lake metabolism. Sampling Frequency: Chemical parameters and physical properties sampled from 1 to 4 times during the summer. Time series data step is 30 minutes. Number of sites: Time series data for 25 lakes. Chemical and physical data from 31 lakes.近期研究表明，在多数湖泊与河流中，水体内部的有机质呼吸分解过程（respiratory breakdown of organic matter, R）超过了通过光合作用产生的有机质总量——总初级生产力（gross primary production, GPP）。这种代谢平衡（GPP小于R，即异养型（heterotrophy））表明，外源有机质支撑了水生生态系统的部分呼吸消耗。诸多湖泊呈现异养型的结论来自多种研究方法，但关于异养型存在的具体条件，学界仍存在争议。用于估算GPP和R的方法，以及所研究湖泊类型的局限性（尤其是在溶解有机碳（dissolved organic carbon, DOC）与总磷（total phosphorus, TP）浓度维度上），是导致研究结论存在分歧的两大原因。本研究于2000年7月至8月部署了氧气（O₂）与二氧化碳（CO₂）传感器，用于监测美国威斯康星州北部高地湖区以及密歇根州上半岛的25个湖泊表层水体的昼夜气体动态变化。所选湖泊在DOC与TP浓度上覆盖了宽泛且相互独立的梯度范围。基于上述数据，我们计算得到了GPP、R以及生态系统净生产力（net ecosystem production, NEP=GPP-R）。在这些湖泊覆盖的宽泛TP与DOC浓度范围内，昼夜尺度下CO₂与O₂的变化摩尔比接近1:1。除高pH环境外，基于两种气体得到的代谢速率估算结果具有可比性。本数据集内的多数湖泊呈现负的生态系统净生产力（-NEP），但GPP与R的调控因素存在差异：TP与GPP呈现显著正相关，而DOC则与R显著相关。当DOC浓度较低时，GPP与R近乎相等；但当DOC浓度升高时，GPP与R出现解耦，湖泊呈现负的生态系统净生产力。湖泊代谢与流域相关变量间的强相关性表明，外源碳对湖泊代谢过程具有调控作用。采样频率：夏季期间对化学参数与物理性质进行1~4次采样；时间序列数据的采样步长为30分钟。采样点位：25个湖泊拥有时间序列数据，31个湖泊提供了化学与物理性质数据。