While swimming in a lake or pond on a hot summer day, you’ve probably noticed that the temperature of the water down near your feet is much cooler than the water near the surface. This is because radiant energy from the sun is able to heat the water near the surface more efficiently. As long as the sun continues to shine on the surface, the top layer is warmer and, as a result, less dense than the cooler, heavier water below. This layering effect is known as thermal stratification. For many of us, that warm layer on the surface feels pretty good.

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Temperature in the aquatic realm affects not only human swimmers, but also aquatic plants and aquatic animals as well.

This web page introduces the reader to water temperature and its effects on aquatic plants and plant management, and temperature effects on fish and other aquatic animals. But first, let's look at how water gets its temperature.

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HOW WATER GETS ITS TEMPERATURE

Air temperature and radiant energy from the sun have the greatest influence on water temperature. Water, being semi-transparent, allows sunlight to push its energy down into the water column.

Even so, water does not heat up nearly as quickly as does land. Because land consists of a layer of opaque soil and rock, it absorbs heat from the sun and surrounding air much faster than water.

Besides direct solar transmission and air temperature, other factors contribute to water temperature.

It takes five times the energy to heat one gram of water than it does to heat one gram of soil. This is because of the "specific heat" of water. The specific heat of water explains why water temperatures fluctuate much more slowly than land temperatures do, and usually within a much smaller range.

The size and volume of a waterbody influences water temperature. For example, during hot weather, water in a small shallow lake will warm up faster than water in a large deep lake.

The color of water can influence the amount of radiant energy that is absorbed. Because of their dark coloration, redwater or blackwater lakes will be even warmer within the top few centimeters. Small deepwater lakes, like the “kettle” lakes found up north (created by melting glacial blocks of ice) and a number of Florida lakes have color because they are “fed” by low-lying swampy areas. Stratification is usually more pronounced in these lakes.

Shade also influences temperature. You can test this theory by swimming in a man-made pool that was exposed to full sunshine all day long versus one that has trees shading the water. Viva la difference!

Thermal pollution from power plants, stormwater runoff, and other sources can raise the overall water temperature of a lake or waterbody.

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AQUATIC PLANTS AND WATER TEMPERATURE

Temperature plays a major role in the life cycles of plants and conversely, plant presence or absence can have a strong influence on the temperature within an aquatic environment.

Effects temperature can have
on aquatic and wetland plants

In the spring, some aquatic plants become more buoyant due to increases in water temperature and photosynthetic activity. Both help to create an increase in gases within the plants; the gases actually push out and replace the liquid content of the plant cells allowing the plants to float to the surface.

Temperature can be the limiting factor for the depth at which some plants can grow. One study has shown that some plants are not found below certain depths even though there was plenty of light available for photosynthesis. In these cases, scientists say that plant growth is “limited” by temperature. (Dale, H.M., 1986)

The length of the growing season for some plants depends on temperature. For example, Utricularia purpurea growth depends on the length of time that (water? air?) temperatures exceed 15ºC (Moeller, 1980). Many plants have a longer growing season in Florida due to the (mostly) warm temperatures throughout the year. This may explain why so many plants are problematic in areas of the state. Winter temperatures help to kill-back or slow the growth of some nuisance tropical plants, and winter and early spring are good times to remove nuisance vegetation.

Thermal pollution from power plants, stormwater runoff, and other sources can raise the overall water temperature of a lake or waterbody. In some instances, such artificial temperature rise enables certain plant species to become dominant, whereas prior to the artificial rise in water temperature, the same plant did not do so well (Moss 1959 and Allen and Gorham, 1973). A good site about "thermal pollution": University of Georgia at Athens

Tiny free-floating plants known as algae also seek the warmer sunlight-rich zone near the surface; the surface gives them all the sunlight energy they need to produce their own food.

Effects plants can have on water temperature

Large areas of floating plants can cast significant shadows on the water below. This reduces temperature in much the same way that tree canopies shade and cool the forest floor and the air in-between. Shading can have a profound influence on daily temperature fluctuations within a waterbody. In one study, field observations showed that during clear weather, very little light energy was able to penetrate through floating mats of Lemnaceae (duckweed) and as a result, temperatures were 4-11º C higher at the surface than in areas of deeper water with sparse plant coverage (H.M. Dale and T.J. Gillespie (1976). This type of temperature gradient is known as thermal stratification.

In some instances, the shading effect created by floating plants can create such a “strong layers" that oxygen from the top layer of water is not able to penetrate through the thermocline to the cooler layer underneath, resulting in low oxygen problems for many bottom dwelling organisms (fish, invertebrates, etc.).

Another study showed that a similar dynamic occurs in lakes with large “crops” of submersed plants. This time researchers noted that water temperature gradients were the most extreme (between surface waters and layers underneath) in areas with especially dense plant growth (Dale and Gillespie 1977). Algae blooms have demonstrated a similar effect (Idso and Foster, 1974).

An abundance of plants (floating, submersed, emersed) can lessen wind effects and/or inhibit water currents. With limited water flow or wave action, surface waters surrounding the plants are “calmer” and can warm considerably.

Plant presence also decreases evaporation from the water’s surface; more evaporation occurs in open water.

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AQUATIC ANIMALS AND WATER TEMPERATURE

Water temperature can have a tremendous influence on aquatic animals: where they can be found within the habitat, how they’ll behave or if they’ll survive, from the biggest, bossiest bass down to the tiniest shrimp larva. This is due to the fact most aquatic organisms lack the ability to internally regulate their core body temperature. As a result, water temperature influences virtually every aspect of their life including the availability of dissolved oxygen; their distribution, energy, growth rate and metabolism (appetite and food requirements); and their respiration (breathing), reproduction and many other processes.

Dissolved Oxygen - If we were to communicate with a school of fish or a cluster of clams, they would probably tell us that the most important thing to remember about temperature is that it determines the amount of oxygen that water is able to hold. They would tell us that if dissolved oxygen concentrations should become too high or too low, they can become severely stressed, or even die (as in a fish kill).

The general rule for temperature and oxygen in water:

warm water contains less dissolved oxygen
than does cool water.

For example, water that is 90º F can only hold 7.4 mg/L of dissolved oxygen (at saturation) while cooler water at 45º F can hold 11.9 mg/L, which is about 60% more oxygen.

Distribution - A host of aquatic life including shrimp, crayfish, clams, fish, insects, microscopic zooplankton, and even plants, seek specific temperature regions within a lake, pond, river, etc. based on the temperature of the water, which can vary by several degrees depending on the location.

Some species prefer the cooler temperatures near the bottom while others prefer the warmer waters of the shallow areas along the shoreline or the surface waters out in the middle of a lake, just under floating mats of vegetation. Some species, like the northern coldwater trout, will never be found in a Florida lake because they simply can’t tolerate our warm temperatures. The tendency in animals and plants to seek optimal environmental conditions is referred to by scientists as species distribution.

Growth Rate and Metabolism - Like all cold-blooded animals, Florida’s warmwater fish species become less active as water temperatures cool each autumn and winter. They eat less and, as a result, grow less during this time. Such a close connection with water temperature, especially when water temperatures begin to warm in the spring, means that the metabolism and general activity rates for most aquatic animals is increased.

Respiration - Just as in humans, when fish and other animals become more active, they breathe faster. Breathing is "respiration".

In some aquatic animals, respiration rates will increase by 10% or more per degree Celsius (Giller, Paul S. and Bjorn Malmqvist. "The Biology of Streams and Rivers." 1998.) Respiration increases can cause great stress in fish and other aquatic animals. Increased respiration means an increase in oxygen demand from the water. If the oxygen supply is limited for some reason (e.g., from several consecutive days of hot, calm weather and overcast skies, or a sudden die-off of plants or algae), the delicate balancing act can be knocked off kilter and the chances of an oxygen-related fish kill increase dramatically.

Some good news: water temperature can be used as a predictor for potential oxygen problems in ponds and small lakes and steps can be taken to try to prevent an oxygen “crash.”

Reproduction - Water temperature is also critical to the reproductive cycles of aquatic animals. Temperature changes are one of the main triggers for spawning activity. A rapid increase or decrease in temperature can cause fatalities for most fish species during their reproductive period as eggs or newly hatched fry will die from a dramatic change. Sometimes changes in temperature will cause adult fish to abandon their nests. Of course, every aquatic organism is affected a little differently.

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FISH AND WATER TEMPERATURE

Ask any serious angler and he or she will tell you how important water temperature is to their fishing success. They know that temperature often determines the actual depth and location for fish to be found and once located, they know water temperature can have a tremendous influence on a fish’s appetite and behavior. Even a difference of one or two degrees can determine whether or not a fish will bite.

Of course, there is much more to know about the relationship between fish and water temperature. Here are a few basics:

Water temperature determines the type of fish found in a specific aquatic environment or geographic region. For example, coldwater trout wouldn’t last long in Florida’s shallow warmwater lakes in July and conversely, the Florida largemouth bass subspecies Micropterus salmoides floridanus would never survive a typical winter in northern latitudes.

This knowledge has led scientists to classify fish into three basic categories:

Coldwater fish such as steelhead trout, salmon and char don’t usually survive in water temperatures above 21 degrees Celsius (69º F). FYI: 21 degrees C is considered to be the defining “limit” for coldwater fish.

Coolwater fish include sturgeon, walleye, pike and striped bass and their comfort zone is a little more flexible, preferring water temperatures of 22 – 23 degrees C (71-73º F).

Warmwater fish like the largemouth bass, crappie, bluegill and other sunfish fare best when water temperatures are between 25-28 degrees Celsius (77-80º Fahrenheit). However, some species in the catfish family can endure temperatures as high as 90º F.

Warmwater lakes tend to support greater species diversity than coldwater lakes. In some instances, a lake may even support more than one category of fish, with warmwater fish staying near the surface and coolwater or coldwater fish staying in the bottom portion of a lake. Of course, this usually doesn’t apply to Florida lakes, which are almost entirely warmwater lakes, with the exception of a few spring-fed lakes.

More than 100 native warmwater species thrive in Florida’s freshwater habitats accompanied by at least 73 non-native species. Most of the non-natives are from the tropics and have been introduced accidentally from people emptying their aquariums into various canals, lakes, and rivers. Fortunately, freezing winter temperatures in central and north Florida provide an effective climate barrier, preventing the further spread of many of these fish.

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Other helpful temperature links:

Our Fish Web Page

Temperature and Oxygen
Florida LAKEWATCH Information Circular 109.

WOW's temperature site

For more on fish and dissolved oxygen, see LAKEWATCH Information 107 - Understanding Fish Kills in Florida’s Freshwater Systems.

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This page was authored by Amy Richard
with assistance from Alison Moss.
Data is from the APIRS database.
Layout was by Alison Moss.

Vic Ramey is the editor.

DEP review is by Jeff Schardt and Judy Ludlow.

MAIN INDEX

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This project is a collaboration of
the Center for Aquatic and Invasive Plants, University of Florida,
and the Bureau of Invasive Plant Management, Florida Department of Environmental Protection

CAIP-WEBSITE@ufl.edu
Copyright 2005 University of Florida