LTREB Renewal: Climate driven acidification in lowland Neotropical streams: building on a 30-year study of groundwater-surface water interactions

哥斯达黎加河流正在进行的长期研究发现，干旱对水化学和邻近森林土壤之间的关系有很大影响。先前的环境生物学长期研究发现，在这些低地溪流中发生了酸化事件。酸化事件是指水变得中等酸性(低pH值)的短时间段，从几小时到几天不等。它们的出现是溪流与周围森林相互作用的结果，也是降雨量变化的结果。在旱季，有机物质积累在森林地面上，当降雨开始分解时，这些物质会释放出二氧化碳，随着水进入溪流，在溪流中创造酸性条件(或降低水的PH值)。酸化事件在厄尔尼诺期间变得极端，在哥斯达黎加，这几年的季节比正常情况更干燥、更潮湿。酸化的变化会影响河流的功能和相关的河流生物多样性。预计与全球气候变化有关的降雨量变化可能加剧酸化事件对河流生态系统的影响。之前对哥斯达黎加低地溪流的研究表明，溪流中的无脊椎动物和藻类可以承受中等酸性的条件，但在极端或更频繁的条件下会受到影响。这项长期环境生物学研究(LTREB)奖将调查酸化对哥斯达黎加低地河流生态系统的影响。研究将在哥斯达黎加加勒比海斜坡上的La Selva生物站进行，那里的溪流已经研究了30多年。该奖项的主要目的是了解降雨模式变化对热带溪流酸度和生物反应的机制和后果。这项研究将为研究生提供培训机会，并向公众推广。该奖项将探索时断时续的酸化事件的后果，以及通过地下径流途径流入土壤来源的二氧化碳导致pH下降的假设，这表明降雨、陆地和水生生态系统之间存在紧密耦合。气候驱动的酸化事件在溶质贫乏、缓冲较差的溪流中比在溶质丰富、缓冲良好的溪流中更强烈、更常见，这是研究环境的另一个特征。该奖项将通过解决三个主要目标来继续评估这一假设：(1)评估溪水化学的日、季、年和年代际模式与主要气候事件(例如，厄尔尼诺和拉尼娜-ENSO)之间的关系，利用高频采样重点关注极端事件。(2)评估生物对气候驱动的酸化的反应，重点是实验室条件下的事件持续时间。最后，(3)用个别分类群的实验室实验来补充低溶质河流的实验缓冲。对溪流大型无脊椎动物长期记录的分析表明，昆虫的总体数量下降，与溶质丰富、缓冲良好的溪流相比，溶质贫乏、缓冲较差的溪流在组合组成上存在显著差异。气候趋势和全球环流模型预测了极端天气的频率和强度的变化。对于中美洲，模型预测了更大的季节性，类似于在ENSO年观察到的模式。在哥斯达黎加的加勒比海低地，厄尔尼诺事件导致旱季降水量异常低，而拉尼娜事件导致降水量高。因此，该奖项将促进对极端气候事件对热带低地河流生态系统影响的理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Ongoing long-term studies in Costa Rica streams have found strong implications of drought on the relationship between the water chemistry and the adjacent forest soils. Prior long-term studies in environmental biology uncovered the occurrence of acidification events in these lowland streams. Acidification events are short periods, from hours to days, when the water becomes moderately acid (low pH values). Their occurrence is the result of interactions between the stream and the surrounding forest and result from changes in rainfall. During the dry season organic materials accumulate on the forest floor, when the rains start these materials breakdown, releasing carbon dioxide that moves with the water into streams, creating acid conditions in streams (or lowering the water pH). Acidification events become extreme during El Niño years, which in Costa Rica has drier and wetter seasons than normal. Changes in acidification affect how streams function and the associated stream biodiversity. Projected changes in rainfall associated with global climate change could exacerbate the effects of acidification events on stream ecosystems. Prior research in lowland streams in Costa Rica indicate that stream invertebrates and algae can withstand moderately acid conditions, but are affected when conditions are extreme or more frequent. This Long-term Research in Environmental Biology (LTREB) award will investigate acidification effects on stream ecosystems in lowland Costa Rica. Research will take place at La Selva Biological Station, on the Caribbean slope of Costa Rica, where streams have been under study for more than 30 years. The main objective of this award is to understand the mechanisms and consequences of changing rainfall patterns on the acidity and biotic response of tropical streams. This study would provide training opportunities for graduate students as well as outreach to the public.This award will explore the consequences of the episodic acidification events and the hypothesis that an influx of soil-derived CO2 via subsurface flow paths contributes to those pH declines, signaling a tight coupling among rainfall, terrestrial, and aquatic ecosystems. Climate-driven acidification events are stronger and more common in solute-poor, poorly-buffered, streams than in solute-rich, well-buffered, streams - another characteristic of the study landscape. The award will continue evaluating this hypothesis by addressing three main objectives: (1) to assess relations among daily, seasonal, annual, and decadal patterns in stream water chemistry with major climate events (e.g., El Niño and La Niña - ENSO), focusing on extreme events using high frequency sampling. (2) Assess biotic responses to climate-driven acidification, focusing on event duration under laboratory conditions. Finally, (3) to supplement the experimental buffering of a low-solute stream with laboratory experiments of individual taxa. An analysis of the long-term records of stream macroinvertebrates indicates an overall decline in insects and strong differences in assemblage composition between solute-poor, poorly-buffered, streams compared to solute-rich, well-buffered, streams. Climate trends and global circulation models predict changes in the frequency and intensity of weather extremes. For Central America, models predict greater seasonality, similar to patterns observed during ENSO years. In the Caribbean lowlands of Costa Rica, El Niño events result in abnormally low precipitation during the dry season, while La Niña results is high precipitation. Thus, the award will advance the understanding of the consequences of extreme climate events on tropical lowland stream ecosystems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.