In hot water: the consequences of warming lakes, rivers, and oceans

By: Dana Sackett, PhD

With the ripples of a historic election still settling in the United States, one promise from the President-elect and Vice President-elect is resonating with the scientific community more than most. The direct promise to address climate change with science-based decisions and policies. A promise that comes as the 28th major storm of 2020 affecting the United States barrels into the Gulf of Mexico and California records its largest wildfire season in modern history. The increasing intensity and frequency of hurricanes and wildfires are the most obvious consequences to climate change because they result in immediate widespread devastation.  However, there are many less-obvious consequences to climate change that can be just as destructive and harmful to the earth and human civilization.  Rather than the thick textbook needed to discuss all the interconnected impacts of climate change, this week’s article focuses on some of the largest consequences of just one aspect: warming waters

Habitat: Warming waters can cause habitat loss that impacts aquatic life and the fisheries that many people rely on for food and employment. For example, temperature sensitive corals reefs cover ~0.2% of the ocean, are home to ~25% of marine biodiversity, are estimated to produce between $172 billion to $2.7 trillion US dollars each year, and have been dying at an alarming rate.  While there are several human-driven reasons for this decline, warming ocean waters have been a major contributor to these losses.  Another example comes from subarctic and arctic seas where temperatures have increased disproportionately faster than other areas of the world.  Sea ice cover and cooler temperatures are vital in these regions for several economically important fish species to spawn.  Consequently, scientists found that warming water, which eggs and larvae are more sensitive to, and reduced sea ice cover, which acts as nursery habitat, reduced the survival of two cod species in this region.

A figure from Dahlke et al. 2018 showing declining Atlantic cod and Polar cod egg survival at higher temperatures. Stars indicate an increased adverse temperature affect as a result of increased water carbon dioxide levels. Results of this study suggested that spawning habitat would remain suitable for these species of cod if global warming was held below an average increase of 2oC.

Another problematic trait of warmer water is that it expands.  This expansion means that water has more volume and becomes less dense. The change in volume when water is warmed is imperceptible when it is in a small glass, but when it includes all of the surface water in the oceans, the volume can increase dramatically.  Add to that melting ice near the poles and sea levels have not only been rising over the last several decades but the rate of increase is accelerating.  If left unchecked sea levels are predicted to rise dramatically over the next several decades, causing seawater to intrude into coastal areas, tidal pools, estuaries, and even freshwater rivers and lakes; all essential habitats that would be permanently altered.  

A picture showing salt water intruding into a community in the Philippines. As polar ice melts, rising sea levels can put coastal and island communities at risk of flooding and water damage. This image come from a great educational resource activity for kids in 6th to 8th grade. You can find the lesson materials at the following link. PHOTOGRAPH BY GEORGE STEINMETZ / NATIONAL GEOGRAPHIC. Source

Water cycle and currents: Warming ocean waters can also influence ocean currents driven by differences in seawater density and wind.  The most famous example of this is the system of ocean currents known as the global conveyor belt that keeps the Earth’s climate stable.  These ocean currents distribute heat, salts, dissolved gases, and nutrients across the world’s oceans.  For this conveyor belt to move, water in the north Atlantic must become cold and salty enough (from ice formation) for the water to sink. The current concern is that warming water and freshwater inputs from melting ice in the north Atlantic is causing these waters to become less dense, resulting in water sinking more slowly and slowing the entire conveyor belt.  The slowing of this vital ocean circulation system has already been noted by researchers and if it continues could bring extreme temperatures to different regions around the world. Alternatively, some scientists have suggested that the speed of several wind driven ocean currents may be accelerating, changing the way heat and nutrients are being distributed in different regions around the world.

This image come from a wonderful educational resource activity for kids in 5th to 8th grade. You can find the lesson materials at the following link. Source

Higher temperatures also increase the amount of water that evaporates.  In drier regions of the world, excessive evaporation leaves less water in streams with some drying-up entirely; fragmenting habitat and leaving fish stranded in pools of warming water.  In wetter areas of the world, increased evaporation can lead to higher amounts of rainfall resulting in increased flooding and runoff of soil and contaminants into waterways.

The water cycle is predicted to respond to climate change in different ways depending on the region. Source

Oxygen: Compounding the effects of warming is that warm water cannot hold as much oxygen as cold water.  For instance, a 10oC increase in seawater results in water that can hold ~20% less oxygen.  Some scientists have even called the ‘problem of warming waters’ the ‘problem of declining oxygen.’  Because of warming, ocean oxygen levels at the surface have been in decline since at least the 1950’s.  At mid-depths there is a zone of the ocean where microorganisms decompose sinking organic matter, using up oxygen in the process (called the oxygen minimum zone).  This region of minimal oxygen has also been expanding largely due to warming.  This decline in oxygen both at the surface and in deeper water is putting many marine species between a rock and a hard place, and many of the fisheries that rely on them in jeopardy.  

As ocean waters warm, fish are being squeezed between areas of declining oxygen at the surface and in deeper water. Source: Diagram modified from one originally published in Deep Sea Research Part I: Oceanographic Research Papers, Vol 57, Issue 4, Lothar Stramma, Sunke Schmidtko, Lisa A. Levin, & Gregory C. Johnson. Ocean oxygen minima expansions and their biological impacts, 587-595, Copyright Elsevier (2010).

Ice melt also affects oxygen levels because freshwater inputs float on the ocean surface, preventing oxygen from the atmosphere from mixing with deeper water.  This stratification ultimately limits atmospheric oxygen from reaching ocean species and decomposers that need oxygen to recycle essential nutrients back into the marine food web.  In freshwater and coastal ecosystems, warm oxygen-depleted water has also led to fish kills (for more on this see our previous article on a heat wave in Europe that led to widespread fish kills in 2018). 

The amount of dissolved oxygen water can hold is determined by temperature and atmospheric pressure with warmer waters holding less dissolved oxygen.  Source.

Energy budgets: For many aquatic species, the temperature of their surroundings dictate their body temperature (called ectotherms) and as a result their metabolism.  A higher metabolism requires more oxygen, which is scarcer in warmer water as we discussed above. Various scientists have proposed that the higher metabolic costs and limited oxygen from warming waters has resulted in the widespread decline in aquatic ectotherms body sizes. Further, balancing the increased energy budget needed in warmer water puts stress on an organism, depleting the energy reserves they would normally use to respond to or recover from other stressors such as pollution or pathogens. For example, researchers recently examined damselfly energy gains and losses in warmer waters and in the more drastic temperature fluctuations predicted to occur with climate change.  They found that changes in their energy budget caused them to be more vulnerable to pesticides. Similarly, a study modeled and validated how brown bullhead, a benthic fish species, would respond to a 4oC increase in water temperatures. They found that in their southern range bullhead would likely disappear while in their northern range increased metabolism could result in higher uptake and accumulation of contaminants. 

Changes in damselfly energy budgets in response to warming caused this species to be more vulnerable to pesticides Source: Verheyen  and Stoks 2020

Getting out of dodge: As waters warm, dissolved oxygen declines and habitats are altered from current, chemical, and structural changes, many mobile aquatic species have begun to shift their distribution to areas with more favorable conditions.  This can have major consequences for those people whose livelihoods depend on a fishery being in a certain region.  For instance, data from NOAA’s Merged Land-Ocean Surface Temperature Analysis database holds over 150 years of temperature observations that have been linked to shifting species distributions across the globe as species seek out more suitable environments.  In the Gulf of Maine, home to important lobster, shellfish, and finfish fisheries, many species distributions have begun to shift north or deeper to escape warming waters. However, not all species may be able to swim to areas with more favorable surroundings. Those that are immobile, landlocked, or have some barriers to movement are stuck.  Even those that are mobile and can move to more survivable surroundings on a day-to-day basis, may be limited by mating.

Mating: Many aquatic species have evolved to mate by meeting-up at a specific location at a specific time of year. Even if a habitat is no longer favorable for mating, it is unlikely that a new mating location will evolve quickly. To make matters worse, for some aquatic species, the temperature of the water can determine whether an individual will develop into a male or female. Therefore, a change in water temperature threatens to skew the sex of these populations relatively quickly.  Southern flounder, an economically important species in the Gulf of Mexico and southeastern United States coastal waters is one example where increasing water temperatures risk altering the sex ratio towards males and potentially affecting a fishery that relies heavily on larger females.

Juvenile southern flounder. Photo Credit: Jamie L. Honeycutt. Source

Ecosystems as a whole: All of the adverse effects mentioned here do not impact individual species in isolation. Those directly affected by warming water are interconnected with a community of other organisms and reliant on predator-prey, commensal, mutualistic, and pathogenic relationships.  They are also intimately tied to the physical environment by directly affecting habitat structure, gas exchange, and nutrient recycling (for more on the importance of an individual species see our previous article here).  One study that exemplified this fact examined the functional role of several marine mammals on their ecosystems as well as their vulnerability to warming.  They found that the potential extinction of marine mammals that were most vulnerable would have a profound impact on the functioning of marine ecosystems worldwide.

Rising river temperatures and drought put California’s native trout, salmon, and steelhead species at risk. Created by David McCarthy. Source

While the list of impacts from warming waters on aquatic ecosystems discussed here is long, it is not exhaustive. It is also important to recognize that many of the impacts mentioned here are not going to happen but are happening now.  Scientists that study chemical reactions, fish movements, habitat, ocean currents, or food webs in rivers, lakes, estuaries, coastal waters and even the deep sea have all seen the evidence of these changes.  These widespread global to local impacts from just one aspect of climate change demonstrates how important it is to make informed science-based decisions and policies that will help turn the tide of climate change. Together we can create a better and more sustainable world for future generations.

References and other reading material:

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https://e360.yale.edu/features/feeling-the-heat-warming-oceans-drive-fish-into-cooler-waters

https://www.azcentral.com/story/news/local/arizona-environment/2020/10/31/fish-scientists-climate-change-arizona/6058931002/

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