内容来源：https://www.quantamagazine.org/mixing-is-the-heartbeat-of-deep-lakes-at-crater-lake-its-slowing-down-20251114/

内容总结：

【科学前沿】全球湖泊“心跳”趋缓：气候变化下的生态警钟

在俄勒冈州火山口湖的湛蓝水域中，一场持续百年的科学观测正揭示着气候变化的残酷真相。研究人员通过缓慢沉入湖水的黑白赛奇盘测得，这个以清澈著称的湖泊能见度出现异常波动。自1886年首次测量以来，这项看似简单的观测已累积成全球最长的湖泊物理数据集，如今却成为气候危机的“记录仪”。

清澈背后的危机
尽管火山口湖仍保持着地球最清澈大型湖泊的称号，但科学家发现其水体正在变得“异常清澈”。这并非生态改善的信号，而是湖泊混合作用减弱的危险征兆。湖泊混合如同深水湖泊的“心跳”，通过季节性循环将氧气和养分输送到整个水体。随着全球变暖，湖面升温速度超过大气，导致水体分层固化，这场生命循环正面临停滞。

全球湖泊的“窒息”威胁
从意大利北部五大湖到新西兰湖泊，类似危机正在蔓延。2021年《自然》杂志研究显示，84%的温带湖泊出现分层加剧现象。意大利伊塞奥湖已有150米深水域完全缺氧，湖底生物群落彻底改变。米兰比可卡大学生态学家芭芭拉·莱奥尼指出：“这些湖泊已有20年未完全混合，可能永远无法恢复原有生态节律。”

纯净实验室的警示
作为近乎原始的生态系统，火山口湖排除农业污染等干扰因素，成为研究气候影响的理想样本。监测数据显示，湖面水温较1965年上升3摄氏度，夏季延长33天，温暖夏夜增多导致表层水体变薄。伦斯勒理工学院专家凯文·罗斯比喻道：“这就像油醋酱，分层阻力越来越大。”

生态多米诺效应
水体透明度增加源于浮游植物栖息地缩减，而冬季逆分层现象减少更阻碍深层混合。模型预测50年内火山口湖可能停止深层混合，形成缺氧区。2012年该湖首次出现沿岸藻华，入侵小龙虾种群因暖冬扩张，导致特有蝾螈濒临灭绝，展现气候变化与物种入侵的叠加效应。

科学协作的曙光与困境
全球湖泊科学家正打破孤立研究模式，通过数据共享寻找解决方案。内华达大学湖泊学家苏迪普·钱德拉强调：“我们必须认识到湖泊生物互动与气候条件的不可分割性。”然而美国国家公园局的招聘冻结使火山口湖延续40年的监测计划面临中断，首席研究员斯科特·格德纳的退休将导致关键数据收集规模缩减。

暮色中的火山口湖依旧倒映着璀璨星河，但科学家们无暇沉醉。在观测站布满签名的木屋里，研究人员用清蒸小龙虾和起泡酒告别格德纳的职业生涯。这个夜晚，蝙蝠仍掠过如镜湖面，而守护这片自然奇观的接力棒，正等待着新的传递者。

（综合《量子杂志》科学调查报告编译）

中文翻译：

七月一个阳光灿烂的午后，两位科学家将身子探出船沿，凝视着下方以清澈闻名的湛蓝湖水。他们在等待一个黑白相间、餐盘大小的塞奇盘从视野中消失的精确时刻——此刻这个圆盘正缓缓沉入俄勒冈州火山口湖的水中。

圆盘通过起重机缓缓下放，像游乐场的道具般慵懒旋转。入水约一分钟后，研究生胡安·埃斯图阿尔多·博塞尔高喊示意圆盘已不可见："看不见了！"数秒后，研究员伊娃·莱蒂随之呼应："好，我也看不见了！"操作起重机的斯科特·格德纳是位瘦高的淡水生物学家，他人生大半时光都在火山口湖国家公园度过。他记录下两次呼喊时的圆盘深度，随后缓慢提升圆盘，当年轻研究员们再次喊出"看见了"时，他又记下这些深度值。

这些读数的平均值即"塞奇深度"，自1865年意大利耶稣会牧师安杰洛·塞奇应教廷要求发明该方法以来，始终是衡量水体透明度的简易可靠指标。2025年那个午后记录的数值——约78英尺（24米），对以清澈著称的火山口湖而言显得异常浑浊——如今已成为全球延续时间最长的湖泊物理数据集组成部分。该湖首次塞奇测量可追溯至1886年，自1983年起科学家每年夏季进行月度重复测量。就湖泊健康而言，长期数据堪称无价之宝。

火山口湖的规模、自然美景与超凡澄澈度——这得益于其独特环境与隔绝状态——使其成为全球最具标志性的淡水体之一。最大深度达1949英尺的它是美国最深的湖泊，也很可能是地球上最清澈的大型湖泊，其鲜明的湛蓝色调在自然界中实属罕见。

"纯净水体呈现的光学蓝色总令人惊叹。"内华达大学里诺分校的湖沼学家苏迪普·钱德拉表示，他与格德纳保持着科研合作，"这种蓝色是氢氧分子纯净结合的折射。"

但自2010年以来，格德纳与同事在塞奇数据中发现了意外变化：尽管当日读数略显浑浊，火山口湖的清澈水体实则正变得愈发透明。

这听似喜讯——每年50万游客大多为此湖玻璃般通透的质感与绚烂色泽而来。但这也可能预示着湖泊物理、化学及生态正在失衡，或将成为气候变化时代全球湖泊剧变的前兆。

随着地球变暖，夏季延长而冬夜不再严寒。许多温带深水湖的表层水温正以超越气温的速度攀升。这种表层能量流动的改变引发系列物理变化，最终导致湖泊混合机制崩溃——对于冬季不封冻的温带深水湖而言，这种周期性循环犹如心脏搏动。由风力、气温、水温及盐度驱动的湖泊混合，本应在季节或年度周期内实现水体上下循环。一旦混合停滞，氧气与养分无法遍布水层，将导致鱼类死亡，引发有害藻华，并为入侵物种提供可乘之机。

从意大利到新西兰，科学家们忧心忡忡地观察到湖泊混合减弱的现象。2021年，钱德拉与同事在《自然》发表证据：在拥有长期可靠数据的189个温带湖泊中，84%呈现日益增强的水体分层（混合减弱的标志），部分湖泊已完全停止混合。"虽各水系特性不同，但最终结局相似：这些大型深水湖都将面临混合缺失。"钱德拉指出。

在全球数百万湖泊中，火山口湖是极少数拥有超40年持续监测计划的水体。科学家正意识到这些数据集对破解湖泊物理机制及气候变化影响的关键价值。"由于当地气候年际波动显著，需多年观测才能捕捉条件变化范围并界定'常态'。"格德纳解释，"这正是长期数据集的优势所在。"

因此，火山口湖成为格德纳、钱德拉等学者开展首批湖泊系统对比研究的核心，旨在追溯生态崩溃根源，为未来预作准备，甚至规避极端影响。

"传统湖泊研究多为个案分析。"加州大学戴维斯分校太浩湖环境研究中心主任斯蒂芬妮·汉普顿指出，"鉴于形势剧变，这种孤立研究模式已难以为继。我们必须相互借鉴、整合数据以把握全球动态。"

湖中警钟

2006年，意大利北部的伊塞奥湖、科莫湖、加尔达湖、马焦雷湖与卢加诺湖这五个深水湖全面停止混合。起初科学家未予重视——自上世纪八九十年代开始监测以来，偶尔数年不发生完全混合实属常态。但随着时间推移，清澈水层顽固保持分层状态，他们开始担忧这种停滞可能永久化。

担忧似乎正成为现实。"整整二十年我们未见任何从湖面至湖底的完全混合。"米兰比可卡大学淡水生态学家芭芭拉·莱奥尼坦言，"回归往昔状态的可能性微乎其微。"

湖泊混合源于水温差异导致的水体密度变化。在温带深水湖中，这形成水体分层：较轻的暖水浮于表层，较重的冷水沉于底层。尽管影响因素众多，混合主要受季节温差、风力与波浪驱动。

由于地理环境与湖泊特性各异，混合模式并无统一公式。多数湖泊每年发生一两次完全混合，通常在春秋两季。大型湖泊的浅层混合可能按年或季周期进行，而直达湖底的完全混合或许数年一遇。通过研究不同湖泊，科学家希望找到共性规律。

意大利北部深水湖以往约每七年完成一次完全混合。夏季表层水温升高形成稳定轻质水层，随着秋冬表层降温，各水层温差缩小，在风力推动下启动混合过程，重新分配水体的热量、氧气、养分与毒素。

但如今这些湖泊已不复往昔。表层水温难以下降至触发混合的临界点，导致分层湖的底层氧气持续消失。伊塞奥湖已彻底缺氧，"150米深的水体毫无氧气。"莱奥尼表示，这使深层好氧生物灭绝并改变生物群落，"在长期缺氧的深水湖中，唯余细菌存活。"

意大利深水湖的"心脏"已停止搏动，养分循环中断，它们昭示着湖泊混合停滞的后果。火山口湖则提供另一种研究契机：精确解析气候变暖如何破坏湖泊的基础物理机制。

混合异变

夏日从古老火山口边缘俯瞰，火山口湖如完美镜面倒映流云与天光。但在平静湖面之下，动态过程持续上演。

相较于全球多数大型湖泊，火山口湖几近原始。它被荒野环抱，受国家公园保护，上空多为太平洋来风，周边几无污染城市或工业。没有河流溪涧携外界污染物汇入，湖水全然依赖雨雪补给。七月间，格德纳与钱德拉用湖水装满两个大型水桶——足以供约13人的科考团队（包括访学科学家、学生、国家公园职员及随行记者摄影师）彻夜饮用。湖水口感如瓶装水般纯净，在烈日下保持天然清冽。

水质纯净不仅提供优质饮水，更使火山口湖成为研究气候影响的理想样本。"没有农业、污水、地表径流及取水等干扰因素，"格德纳指出，"更容易凸显气候变化的影响。"

格德纳自1995年起在此工作，长期主持监测项目。他常告诫团队：记录变化远远不够，必须理解驱动因素及其对湖泊物理、化学与生物的影响。为此，每晚八点，管状剖面仪会沿锚定钢缆从585米深处升至湖面再返回，每秒两次检测水体电导率、温度、含氧量与盐度，其他传感器通过光线测量叶绿素荧光与浮游植物颗粒密度。

这些数据集讲述着火山口湖的健康编年史。与全球绝大多数湖泊类似，它正在变暖：自1965年以来表层水平均温度上升3摄氏度。夏季夜间气温增速超越白昼，最凉的夏夜不再如昔寒冷。夏季也在延长：过去60年间，随着春季不断提早，火山口湖的夏季延长了33天。

往昔当夏夜转凉，湖泊会释放日间蓄积的热量，导致表层水密度增大而下沉。这种现象驱动着夏季的浅层混合。但随着夜晚变暖，此过程减弱，混合速度放缓。

有悖直觉的是，表层暖水层在变暖的同时也在变薄。"如今夏季浮于表层的暖水平均厚度仅为1971年的一半。"格德纳表示。这加剧了与下层冷水的密度差异，进而需要更强风力才能突破分层实现混合。

"好比油醋酱，"与格德纳及钱德拉合作的伦斯勒理工学院淡水生态学家凯文·罗斯比喻道，"存在混合阻力。"

这与湖水愈发清澈有何关联？答案在于生物学。火山口湖的温暖表层水中栖息着浮游植物群落。较薄的暖水层意味着栖息地缩减，浮游植物减少导致水中散射光线的颗粒递减，从而提升整体透明度与光线穿透深度。

使湖水直抵湖底完全混合的冬季过程，同样经历着深刻变革。这涉及"逆分层"现象减弱：当严寒冬季空气冷却表层形成极冷冰水层，覆盖在约4摄氏度（水密度最大时的温度）的较暖层之上时（低于此温度水分子开始形成较轻的冰晶），强风推动超冷表层水横向流动，部分水体在靠近湖岸处被迫下沉。若下潜足够深，压力增大使其密度超越4度水层，数小时内沉入湖底产生混合效应。

历史上火山口湖约80%-90%的冬季会出现逆分层。随着冬季变暖，此现象日益罕见。"火山口湖正处于临界状态，已非常接近无法形成逆分层。"格德纳警告。

这对湖泊未来混合并非吉兆。当格德纳的同事运用其数据模拟不同气候情景时，模型预测约50年内逆分层将变得稀少。若该过程完全停止，火山口湖将不再实现湖底混合。数十年间，缺氧区将逐渐形成——类似意大利北部湖泊的状况。这不仅带来重大生态风险，更可能积累有毒化合物，一旦湖泊再次混合将翻滚至表层。

火山口湖刚踏上这条剧变之路。数百英里外另一个标志性湖泊预示着可能的前景。

连锁效应

横跨加州与内华达州的美国第二深湖——太浩湖，曾以清澈度比肩火山口湖。十九世纪时，岩石透过晶莹湖水熠熠生辉。随着二十世纪五十年代人口激增带来污染，近岸藻类开始滋生。近年来这些藻类向浅水区蔓延。塞奇盘数据显示，自1967年以来太浩湖透明度下降近40英尺，昔日浓郁的湛蓝在某些区域已然褪色。

"随着气候变化加剧，这些趋势很可能持续。"加州大学圣迭戈分校斯克里普斯海洋研究所水文气候学家迈克尔·德廷杰指出。当太浩湖混合机制崩溃、夏季水温升高且持续时间延长，浮游植物生长季延长导致水体浑浊。未来一个世纪，更频繁强烈的风暴预计增加入湖水量，可能带入"巨量"沉积物与养分。野火烟雾沉降的颗粒物也会改变湖泊的光照结构与营养组成。

钱德拉强调，这类事件可影响湖泊数十年发展轨迹。与改变的湖泊混合机制叠加后，将形成恶性生态循环。

藻华便是这些扰动的产物之一。分层湖中积聚的贫氧富营养水体（尤其当含有径流与野火带来的额外养分时）可能渗向湖岸，刺激近岸藻类生长，形成环绕清澈湖心的"浴缸环"。"这是我们推测太浩湖现状的工作假说之一。"钱德拉表示。

火山口湖在2021年遭遇首次岸缘藻华。"仿佛有人用巨型亮绿荧光笔沿湖岸涂抹。"格德纳回忆。由于当时疫情关闭湖滨旅游，未引发公众抗议。若藻华发生在2025年7月这般游船如织的正常夏季，游客惊叹周遭碧蓝深渊之时，此事必成全国头条。

绿色环带出现时，格德纳团队措手不及。起初他们不解突发藻华成因，随后注意到关键细节：最绿区域正是入侵螯虾高密度区。螯虾入侵使摄食藻类的昆虫幼虫等水生无脊椎动物数量锐减约95%。"它们彻底摧毁了昆虫种群。"格德纳指出。实验显示，有螯虾区域的藻类生长量是无螯虾区域的七倍。

但格德纳怀疑尚有其他诱因。虽然这种入侵掠食者早在1915年便被引入湖泊，但整整百年间未曾发生重大藻华。他与同事最终发现，2021年异常炎夏的破纪录水温助长了藻类繁殖，螯虾只是推波助澜。

温和冬季使螯虾种群扩张至湖域新区，进一步破坏生态。全球仅存于此的火山口湖蝾螈亚种已近乎绝迹。螯虾不仅争夺无脊椎猎物，更用螯肢捕捉蝾螈，活生生吞食这些不幸的两栖动物。

类似气候驱动的入侵物种模式已现身其他湖泊。钱德拉指出，这些连锁影响印证了湖泊状况与气候本质相联："我们无法将湖泊生物组成及相互作用与区域气候条件割裂。"

厘清火山口湖的气候、湖泊混合与生态互动，将为全球研究团队提供持续变暖世界的前瞻蓝图，或成为规避最坏情形的关键。

未卜前途

去年，钱德拉、莱奥尼等学者围坐伊塞奥湖畔的咖啡馆交流各自湖泊的气候变化，店主突然插话："我们何必知晓这些？既然无能为力，何必费心？"

钱德拉常遇此类观点。但他仍怀希望：某些湖泊影响或可延缓或避免。虽个人无法阻挡气候变化的巨轮，但地方干预措施能有所作为。策略需因地制宜，可能包括平衡湖泊营养、控制入侵物种、治理污染或修复湖周森林湿地。

"若某湖泊已有类似经历，就能制定更佳策略。"米兰比可卡大学淡水生态学博士后研究员维罗妮卡·纳瓦表示。汉普顿认为团队协作"正是淡水科学的未来走向"。但此类努力刚起步——学者近几年才开始考虑大型湖泊生态系统的比较研究。此刻美国科研经费的削减正动摇新兴合作。"研究经费削减将重创大型合作项目。"汉普顿坦言。

即便如火山口湖这般典范科研项目也前途未卜。格德纳年末即将退休，他几乎将整个职业生涯奉献于此。联邦政府冻结国家公园管理局招聘，他的职位将无限期空缺。期待同事独立维持同等科研产出并不现实。"我们必须缩减工作范围。"他坦言。

在此之前，他们专注于分内之事：为火山口湖历史再添一年数据。忙碌终日，格德纳驾船返回巫师岛码头——这座火山渣锥犹如尖顶帽耸立湖中。杂乱船库的木墙上布满数十年来的签名与素描，见证着为认知此湖贡献力量的学生与学者。钱德拉将入侵螯虾煮至鲜嫩，佐以辣酱供众人分享。他们传递着几瓶普罗塞克起泡酒，为格德纳退休举杯。

夕阳西沉，疲惫的科学家在码头铺开睡袋。格德纳在岛上度过无数夜晚（他坦言远超前妻所愿），这将是最后时光之一。天际柔和的粉橙金渐次暗去，银河闪烁显现。人声渐杳，蝙蝠掠过如镜湖面。湖泊前途犹未可知，但守护这片自然奇观的紧迫性，却从未如此清晰。

英文来源：

Katie Falkenberg for Quanta Magazine
On a radiant July afternoon, a pair of scientists hung their heads off the side of a boat and peered into the brilliant blue water of a lake known for its clarity. They were watching for the exact moment when a black-and-white, dinner plate–sized object called a Secchi disc disappeared from view in the water column of Crater Lake in Oregon.
The disc was being slowly lowered by crane, spinning lazily like a carnival prop. A minute or so after it hit the water, graduate student Juan Estuardo Bocel gave a shout to indicate that he could no longer see the disc: “I am out!”
Seconds later, researcher Eva Laiti echoed: “OK, I’m out!”
The crane operator, Scott Girdner, a lanky freshwater biologist who has spent most of his adult life at Crater Lake National Park, recorded the disc depth for each call. Then he slowly raised it until the junior researchers piped up again when it was back in view, and he recorded those depths, too.
The mean of those readings, known as the Secchi depth, has been used as a simple and dependable measure of water clarity since 1865, when the Italian Jesuit priest Angelo Secchi invented it at the behest of the papacy. The value recorded that afternoon in 2025 — about 78 feet (24 meters), an unusually cloudy reading for Crater Lake — is now part of one of the world’s longest-running datasets on lake physics. The lake’s first Secchi reading was taken in 1886, and in 1983 scientists began to repeat the procedure several times per month every summer. When it comes to lake health, long-term data is treasure.
Crater Lake’s size, natural beauty and otherworldly clarity — a reflection of its setting and isolation — make it one of the world’s most iconic freshwater bodies. With a maximum depth of 1,949 feet, it is the deepest lake in the United States. It’s also very likely the clearest large lake on Earth, with a vivid blue hue seldom encountered in nature.
“People are just amazed and wowed at the optical blue that you see from pure water itself,” said Sudeep Chandra, a limnologist at the University of Nevada, Reno, who collaborates with Girdner. “That blueness is the reflection of the hydrogen and oxygen hanging out together without any material in it.”
Since 2010, however, Girdner and his colleagues have noticed an unexpected change in the Secchi data: Despite the day’s slightly cloudy reading, Crater Lake’s clear water is getting even clearer.
This might sound like a good thing. After all, the lake’s remarkable, glasslike transparency and brilliant hue are major draws for the half-million tourists who visit every year. But it might also indicate that something is going wrong with the lake’s physics, chemistry and ecology, and it could be a harbinger of changes to lakes across the world in the age of climate change.
As the planet warms, summers are growing longer and winter nights aren’t getting as cold as they used to. As a result, the surfaces of many deep, temperate lakes are warming even faster than the air. This shift to the energy flux of the top layer of water can set in motion a series of physical changes that add up to a breakdown of lake mixing — a fundamental process that acts like a heartbeat for deep, temperate lakes that don’t freeze in winter. Lake mixing is driven by physical properties such as wind, air temperature, water temperature and salinity, and on seasonal or annual cycles it circulates water between the surface and the depths. When mixing stops, oxygen and nutrients don’t get distributed throughout the water column, which can kill fish, trigger unsightly and dangerous algal blooms and invite invasive species to take over.
From Italy to New Zealand and beyond, scientists have been alarmed to observe reduced lake mixing. In 2021, Chandra and his colleagues published evidence in Nature of greater stratification in the water column over time — an indicator of weaker mixing — in 84% of 189 temperate lakes for which they could find sufficiently long and robust datasets. Some lakes had stopped mixing altogether. “While each system is unique, the endgame is generally the same: a lack of mixing for these large, deep lakes,” Chandra said.
Of the world’s millions of lakes, Crater Lake is one of very few with a monitoring program that stretches back more than 40 years. Scientists are now beginning to realize how crucial those datasets are for unraveling lake physics and how climate change is altering it. “Because local weather can be extremely variable from year to year, it takes many years to capture the range in conditions and measure ‘normal,’” Girdner said. “Hence the advantage of long-term datasets.”
Crater Lake is therefore at the center of the first efforts by researchers, including Girdner and Chandra, to compare lake systems to get to the bottom of their breakdown, so they can prepare for the future and perhaps even ward off the most extreme impacts.
“Historically, people have studied lakes one at a time,” said Stephanie Hampton, director of the Tahoe Environmental Research Center at the University of California, Davis. In light of how quickly things are changing, that siloed approach no longer works, she said. “We need to learn from each other and synthesize these data to understand what’s happening globally.”
Canary in the Lake
In 2006, five deep lakes in northern Italy — Iseo, Como, Garda, Maggiore and Lugano — stopped fully mixing. At first, scientists didn’t think much of it. They had been monitoring the lakes since the 1980s and 1990s, and it was normal for a few years to go by without complete mixing. But as time passed and the clear waters remained stubbornly in place, they began to fear that the pause might be permanent.
Their fears seem to have been borne out. “It’s been 20 years that we haven’t observed any full mixing from the top to the bottom,” said Barbara Leoni, a freshwater ecologist at the University of Milan-Bicocca. “I don’t know that it will be possible to return to the past behavior.”
Lake mixing is a function of the fact that water has different densities at different temperatures. In deep temperate lakes, this creates stratification in the water column: Lighter, warmer water floats on top, and colder, denser water sinks below. Any number of factors can influence mixing, but it is primarily driven by seasonal temperature changes, wind and waves.
Because these features vary from place to place and from lake to lake, mixing does not follow a single formula. In many lakes, complete mixing occurs once or twice a year, usually in spring and fall. In very large lakes, mixing might happen in the shallow upper waters on annual or seasonal cycles, while full mixing to the deepest bottom layer may occur only every few years. By studying different lakes, scientists are hoping to find shared rules.
Italy’s deep northern lakes previously achieved complete mixing on an approximately seven-year cycle. During the summer, the lake water would maintain distinct layers as surface waters warmed and remained light and in place. As surface temperatures dropped in autumn and winter, the layers would become closer in temperature; with a push from the wind, the lake would begin to mix. This redistributed heat, oxygen, nutrients and toxins throughout the water column.
That’s not how the Italian lakes work anymore, however. Now, the surface waters fail to get cool enough to sink and trigger mixing. As a result, oxygen is disappearing from the bottom of the stratified lake. It has already been depleted entirely in Lake Iseo. “We have 150 meters of water without oxygen,” Leoni said. This kills off oxygen-breathing life at depth and transforms the biological community. “In lakes where the deep waters have been oxygen-free for a long time, only bacteria survive,” she said.
The hearts of Italy’s deep lakes have stopped and are no longer circulating nutrients; they show what can happen when lakes stop mixing. Crater Lake offers a different opportunity: to study how, exactly, warming temperatures can break the fundamental physics of a lake.
Mixing Mix-Up
On summer days, viewed from the rim of the ancient caldera that holds it, Crater Lake is a perfect mirror reflecting the procession of clouds and colors of the sky above. But beneath that glassy surface, dynamic processes are underway.
Compared to many other large lakes around the world, Crater Lake is close to pristine. It is surrounded by wilderness and protected as a national park. The air above it is mostly wind blowing off the Pacific Ocean, with few polluting cities or industries nearby. The lake lacks any rivers or streams emptying into it that could bring in pollution from elsewhere; it is filled by rain and melting snow. In July, Girdner and Chandra filled two large water coolers with lake water — enough to keep the team of around 13 visiting scientists, students and National Park employees, plus a journalist and photographer, hydrated overnight. The lake’s water tasted as pure as bottled water, and it maintained a natural, refreshing temperature under the blazing summer sun.
The water purity does more than provide good drinking: It makes Crater Lake an ideal system for studying climate impacts. Without the confounding factors of agriculture, sewage, parking lot runoff and water withdrawals that tend to affect other lakes, Girdner said, “it’s easier to see the influence of climate change.”
Girdner started working at Crater Lake in 1995 and has overseen the long-term monitoring program ever since. He often tells his staff that it’s not enough to just record change; they must also understand its drivers and its implications for the lake’s physics, chemistry and biology. To that end, every night at 8 p.m., a tube-shaped profiler instrument crawls along an anchored metal cable from a depth of 585 meters to Crater Lake’s surface and back down again. On this round trip, it tests twice a second for water conductivity, temperature, oxygen and salinity. Other sensors use light to measure chlorophyll fluorescence and phytoplankton particle density.
That dataset and others tell the story of Crater Lake’s health across time. Like virtually all lakes around the world, it’s getting warmer: Average surface water temperatures have increased by 3 degrees Celsius since 1965. In summer, nighttime air temperatures are increasing faster than daytime ones; the coldest summer nights are not as cold as they used to be. And there are more summer nights: Crater Lake has gained 33 additional days of summer weather over the past 60 years, as spring arrives earlier and earlier.
In the past, when summer nights grew cold, the lake released the day’s accumulated heat, causing surface water to become denser and sink. This phenomenon drives the shallow mixing that occurs in summer. As nights have warmed, however, this process has weakened, and mixing has slowed.
Counterintuitively, as the layer of surface water has become warmer, it has also become thinner. “In the summer, there is half as much warm water floating on the surface now, on average, than there was in 1971,” Girdner said. This creates a sharper density difference with the cold water below, which in turn increases the amount of wind energy required to break through and mix the layers.
“I think about it like a vinaigrette,” said Kevin Rose, a freshwater ecologist at Rensselaer Polytechnic Institute in New York who collaborates with Girdner and Chandra. “There’s resistance to mixing.”
So what does all of this have to do with the fact that the lake is getting clearer? That’s where biology comes in. In Crater Lake’s warm surface water lives a community of phytoplankton. A thinner warm surface layer means less habitat, so there are fewer phytoplankton, which means fewer particles in the water to scatter light. This boosts the water’s clarity overall and the depth to which light can penetrate.
Crater Lake’s winter processes, which mix the lake all the way to the bottom, are undergoing their own profound changes. These transformations involve the weakening of a phenomenon called reverse stratification, in which a layer of very cold water, cooled by frigid winter air, forms on top of a slightly warmer layer that is around 4 degrees Celsius, the temperature at which water is heaviest. (At temperatures below that, water molecules begin to organize into lighter ice crystals.) When strong wind pushes the extra-cold surface water horizontally, as it approaches the lake’s edge some of it is forced down. If it is pushed down far enough, the increased pressure causes it to become denser than the 4-degree water layer. It then sinks to the bottom in a matter of hours, creating a mixing effect.
Historically, reverse stratification occurred during 80% to 90% of Crater Lake winters. As winters warm, it is becoming less common. “Crater Lake is sitting on a knife edge where it’s already really close to not being able to form reverse stratification,” Girdner said.
This does not bode well for the lake’s future mixing. When Girdner’s colleagues used his data to simulate what might happen under a range of climate scenarios, the model predicted that reverse stratification will become rare within about 50 years. If the process stops entirely, Crater Lake will no longer mix to the bottom at all. Over decades, an oxygen dead zone will begin to form — similar to the ones in the northern Italian lakes. This risks significant ecological impacts, as well as a buildup of toxic compounds that could billow up to the surface if the lake does mix again.
Crater Lake is just starting on the path toward such dramatic changes. Another iconic lake a few hundred miles away suggests what might happen next.
A Trickle-Down Effect
Lake Tahoe, the second-deepest lake in the United States, on the California-Nevada border, once rivaled Crater Lake in its clarity. In the 19th century, rocks glistened through its crystal-clear water. Then, rapid population growth in the 1950s polluted the water, causing algae to start growing offshore. In recent years, those algae have advanced into shallower waters. Secchi disc readings show that, since 1967, clarity in Lake Tahoe has been reduced by nearly 40 feet. The lake’s formerly rich blue hue is now diminished in some places.
These trends will likely continue as climate change advances, said Michael Dettinger, a hydroclimatologist at Scripps Institution of Oceanography at the University of California, San Diego. As Lake Tahoe’s mixing breaks down and summer waters get warmer and linger longer, phytoplankton enjoy an enhanced growing season and cloud the water. Over the next century, more intense and frequent storms are projected to increase water inflows, likely bringing “enormous spikes” of sediments and nutrients into the lake, Dettinger said. Smoke from wildfires also deposits particles, which can change the light structure and nutrient composition of the lake.
Such events can affect a lake’s trajectory for years, Chandra said. When combined with altered lake mixing, they create a vicious ecological cycle.
Algae blooms are a product of these and other disruptions. In addition to killing fish, the accumulation of oxygen-poor, nutrient-rich water that builds up in a stratified lake — especially one loaded with extra nutrients from runoff and wildfires — can leak to the shoreline, triggering nearshore algae growth that forms a green bathtub ring surrounding a clear center. “That’s one of the working hypotheses for what we think is happening in Lake Tahoe,” Chandra said.
Crater Lake suffered its first bloom of shoreline algae in 2021. “It looked like someone took a massive bright green highlighter along the shore,” Girdner said. Because lake tours were closed due to the Covid-19 pandemic that summer, there was no public outcry. Had the bloom occurred during a normal summer — like July 2025, when tourists crowded the lake in passenger boats to marvel at the seemingly bottomless blue abyss around them — the situation might have made national headlines.
When the green ring appeared, Girdner and his colleagues felt overwhelmed. At first they had no idea what could be driving the sudden growth. Then they noticed a telling detail: The greenest places were those with the highest numbers of invasive crayfish. When crayfish move into an area, the population of insect larvae and other aquatic invertebrates that graze on algae declines by about 95%. “They just hammer the insects,” Girdner said. In experiments, Girdner and his colleagues found that about seven times more algae grow in areas with crayfish compared to those without.
Yet Girdner suspected there was more than crayfish at work. Those invasive predators had regrettably been introduced to the lake in 1915, but in the intervening century, no other major algae blooms had occurred. He and his colleagues found, instead, that record-breaking water temperatures during the exceptionally hot summer of 2021 had fueled the algae growth. Crayfish had just given it a boost.
Milder winters have let the crayfish population grow and spread to new areas of the lake, further disrupting ecosystems. The Mazama newt (or Crater Lake newt), a subspecies found nowhere else in the world, has virtually disappeared. In addition to competing for the same invertebrate prey, the crayfish also capture newts in their pincers and devour the hapless amphibians alive.
Similar climate-driven invasive species patterns have been seen in other lakes. These cascading impacts exemplify the fact that lake conditions are inherently and intimately tied to climate, Chandra said. “We cannot divorce the biological composition and interactions within a lake from the climatic conditions within the landscape.”
Teasing out the interactions between climate, lake mixing and ecology at Crater Lake will give research teams around the globe a blueprint for what to expect as the world continues to warm, and could be key to averting worst-case scenarios.
An Uncertain Future
Last year, Chandra, Leoni and other researchers were sitting in a cafe near Lake Iseo, comparing notes about climate change at their lakes, when the cafe owner interrupted. “Why do we even need to know this?” Chandra recalled him asking. “There’s not much we can do about it, so why even care?”
It’s a sentiment that Chandra often encounters. He harbors hope, however, that some impacts to lakes can be slowed or avoided. While individuals cannot stop the juggernaut of climate change, he said, local interventions could make a difference. Those strategies would be context-dependent, but they could include working to balance a lake’s nutrients, controlling invasive species, cleaning up pollution, or restoring the forests and wetlands surrounding lakes.
Collaborations between different groups of scientists could enhance such interventions, said Veronica Nava, a postdoctoral researcher in freshwater ecology at the University of Milan-Bicocca. “If one lake has already experienced what you’re observing, you can come up with better strategies,” she said.
Teamwork “is really where freshwater science is moving,” Hampton said. But such efforts are in their early days, as researchers have only started to think about comparing large lake ecosystems over the last few years. Now threats to U.S. research are rattling their newfound collaboration. “The cuts to research funding are going to hit large collaborations pretty hard,” Hampton said.
The future of even Crater Lake’s exemplary scientific program is in jeopardy. After spending nearly his entire career at the lake, Girdner is retiring at the end of the year. The federal government has frozen hiring for the National Park Service, so his position will remain unfilled indefinitely. It’s unrealistic, he said, to expect his colleagues to continue the same research output on their own. “We’re going to have to pare down what we’re doing,” he said.
Until then, they’re focused on what they can do: adding another year’s data to Crater Lake’s history. After a busy day, Girdner steered the vessel back to the dock at Wizard Island, a volcanic cinder cone that juts out of Crater Lake like a pointy hat. In the cluttered boathouse, decades of signatures and sketches coated the wooden walls, bearing witness to the students and scientists who had made some contribution to a better understanding of the lake. Chandra boiled a few invasive crayfish until they were delectably tender, and the group ate them with dabs of hot sauce. They passed around a few bottles of prosecco to toast Girdner’s retirement.
As the sun dipped low, the exhausted scientists unrolled sleeping bags on the dock. Girdner had spent countless nights on the island (more than his ex-wife had liked, he admitted). This would be one of his last. The sky’s soft gradient of pink, orange and gold slowly darkened, and the Milky Way twinkled into view. Voices faded, while bats skimmed the water’s still surface. The lake’s future was uncertain. But the urgency of protecting its natural splendor could not have been clearer.