Causes and Consequences of Highly Variable CO2 in Aquatic Ecosystems

9317698 Caraco通过对二氧化碳分压的直接测量，我们和其他人已经发现，就二氧化碳而言，湖泊的表层水很少与大气平衡。地表水中含水二氧化碳浓度（CO2aq）随季节而变化，各湖泊的含水二氧化碳浓度从不到平衡浓度的十分之一到十倍以上不等，而大气中的二氧化碳浓度则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变化的原因和后果都没有得到很好的理解。% % %的原因。相对于大气，湖泊表层水的二氧化碳含量可能低于饱和或过高。严重欠饱和的原因相对来说已经很清楚了。来自流域的过量养分输入（氮和磷）增加了初级生产和碳埋藏，从而降低了二氧化碳浓度。虽然这个模型是合理的，但它只在少数实验富营养化湖泊中进行了测试，目前尚不清楚它的普遍适用性。二氧化碳超饱和的原因还没有得到很好的调查，但更有趣的是，数据表明，事实上，大多数湖泊在大部分时间都处于超饱和状态。二氧化碳含量过饱和的湖泊必然是大气中二氧化碳的净来源。我们的初步数据和审查表明，大多数湖泊在年周期内都是净二氧化碳源，许多湖泊甚至在夏季生长季节也是二氧化碳源。这种从水生系统中净吸收二氧化碳的现象表明，湖泊与其流域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的陆源有机物质，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通量是湖泊碳预算的一个重要组成部分，值得进一步研究。我们假设湖泊与其流域的联系方式驱动了水中二氧化碳的极端变化。% % %的后果。大气中CO2相对较小的变化（2倍）可以对陆生植物产生很大的影响，包括生产力的变化和植物材料元素组成的改变。对水生系统中二氧化碳的大得多的变化所产生的影响，特别是对浮游植物的影响，人们所知甚少。我们假设，自然水体系统中超过100倍的CO2浓度范围可能会影响浮游植物的种类组成，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食物的价值。目标：1)确定在什么条件下湖泊是大气CO2的源或汇；2)确定单个湖泊（镜像L）中过量二氧化碳的来源，我们知道该湖泊在一年中大部分时间都在排放二氧化碳；3)利用操纵实验研究这一剧烈变化的CO2浓度对浮游植物种类组成的影响。

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.