Causes and Consequences of Highly Variable CO2 in Aquatic Ecosystems

9317698卡拉科通过直接测量二氧化碳分压，我们和其他人发现，湖泊表面的水很少与大气中的二氧化碳保持平衡。表层水中的二氧化碳浓度随季节变化，各湖泊之间的二氧化碳浓度不到平衡浓度的十分之一到十倍以上，大气中的二氧化碳浓度不到平衡浓度的两倍。水生生态系统中二氧化碳浓度的巨大差异的原因和后果都没有得到很好的理解。%原因。相对于大气而言，湖泊的表层水可能是二氧化碳饱和不足，也可能是过饱和。严重饱和的原因是相对较好的理解。来自分水岭的过量养分输入(氮和磷)增加了初级生产力和碳埋藏，从而降低了二氧化碳浓度。尽管这个模型是合理的，但它只在几个实验中富营养化的湖泊中进行了测试，还不知道它的普遍适用性有多大。二氧化碳过饱和的原因还没有得到很好的研究，但更耐人寻味的是，数据表明，大多数湖泊实际上大部分时间都是过饱和的。一个二氧化碳过饱和的湖泊肯定是大气中二氧化碳的净来源。我们的初步数据和回顾表明，大多数湖泊在一年的周期内都是净二氧化碳来源，许多湖泊甚至在夏季生长季也是来源。这种从水生系统中净规避二氧化碳意味着湖泊和其分水岭之间存在着一种重要的、相对未被探索的联系。这些湖泊要么输入和代谢大量陆源有机物质，要么从流入地下水的土壤呼吸中输入二氧化碳。无论哪种方式，来自水生系统的二氧化碳流量是湖泊碳收支的一个很大组成部分，值得进一步研究。我们假设，湖泊与其分水岭的联系方式导致了水中二氧化碳的这些极端变化。%后果。大气二氧化碳相对较小的变化(2倍)可以对陆地植物产生很大影响，包括生产力的变化和植物材料的元素组成的变化。水生系统中二氧化碳变化大得多的影响不太为人所知，特别是对浮游植物。我们推测，自然水生系统中二氧化碳浓度超过100倍的范围可能会影响浮游植物的物种组成、浮游植物的C：P和C：N比率，并可能深刻改变浮游植物作为消费生物的食物的价值。%目标。1)确定在什么条件下湖泊是大气二氧化碳的源或汇；2)确定单个湖泊(L镜)中过量二氧化碳的来源，我们知道这个湖在一年中的大部分时间都会释放出二氧化碳；3)使用操纵实验来调查二氧化碳浓度的这种巨大变化对浮游植物物种组成的影响。

9317698 Caraco Using direct measurements of the partial pressure of CO2, we and others have been finding that the surface waters of lakes are rarely in equilibrium with the atmosphere with respect to CO2. Aqueous carbon dioxide concentrations CO2aq in surface waters vary seasonally and across lakes from less than one-tenth to more than ten-fold of their equilibrium concentration with the atmospheric CO2 of less than 2-fold. Neither the causes nor the consequences of the enormous range in CO2 concentrations in aquatic ecosystems is well understood. %%% Causes. The surface waters of lakes can be under-or over saturated in CO2 with respect to the atmosphere. The causes of severe under saturation are relatively well understood. Excess nutrient inputs (nitrogen and phosphorus) from the watershed allow increased primary production and carbon burial which draws down the CO2 concentration. Although this model is reasonable, it has only been tested in a few experimentally eutrophied lakes and it is not known how generally applicable it is. The causes of CO2 super saturation are much less well investigated and all the more intriguing because the data suggest that a majority of lakes are, in fact, super saturated much of the time. A lake supersaturated in CO2 must be a net source of CO2 to the atmosphere. Our preliminary data and review suggest that most lakes are net CO2 sources over an annual cycle and many are sources even during the summer growing season. This net evasion of CO2 from aquatic systems implies an important and relatively unexplored linkage between the lake and its watershed. These lakes must either import and metabolize significant amounts of terrestrially-derived organic matter or they import CO2 from soil respiration in inflowing ground water. Either way, the flux of CO2 from aquatic systems is a large component of a lake's carbon budgets and warrants further study. We hypothesize that the way a lake is linked to its watershed drives these extreme variations in aquati c CO2. %%% Consequences. Relatively small variations in atmospheric CO2 (2-fold) can have large effects on terrestrial plants, including changes in the rate of productivity, and alteration of the elemental composition of plant material. The effects of the much larger variations in CO2 in aquatic systems are less well known, especially for phytoplankton. We hypothesize that the more than 100-fold range in CO2 concentrations in natural aquatic systems may affect phytoplankton species composition, C:P and C:N ratios of phytoplankton and may alter profoundly the value of phytoplankton as food for consumer organisms. %%% Goals. 1) To determine under what conditions lakes are sources or sinks for atmospheric CO2; 2) To identify the sources of excess CO2 in a single lake(Mirror L) which we know outgasses CO2 during most of the year; and 3) To investigate, using manipulative experiments, the effect this dramatic range in CO2 concentrations has on phytoplankton species composition.