One of the marvelous things about the lower basin of the St. Johns River is the diversity of life that can exist within its banks.
In addition to the charismatic megafauna, those large animals that everyone loves, like dolphins and manatees, there is also huge diversity of fresh water, estuarine and marine species. Some are so small they require a microscope, others are large like the manatee, with everything in between.
Perhaps the one thing they all have in common is that they live in water. Sounds pretty simple, but it isn’t.
Water is unique in so many ways that it is hard to remember just how important it is to life as we know it. It is our standard on how we measure things, such as volume, weight and temperature, plus a few more.
Most of the values we understand were created by humans to measure quantities. So that 1 gram of water is also 1 cubic centimeter, or 1/100th of a meter cubed. We link the weight or mass to a specific volume of water to create a set of useful ways to measure the size and weight of things. Of course, people can never seem to agree. We have pounds and ounces in one method and grams and liters in another.
We have done the same thing to temperature.
Water freezes, or changes from liquid to solid at one point, or changes from liquid to gas at another. In the centigrade or Celsius scale, we separate those two points by 100 units to get our temperature measurement units. But in the Fahrenheit scale, we divide the two points by 180 units to get our temperature measurement units.
You can convert from one to the other using some simple math. Most people seem to know and remember that freezing is 32 degrees F or 0 degrees C, while boiling is 212 degrees F or 100 degrees C. A couple of other benchmarks to remember are that a cool but comfortable 68 degrees F is 20 degrees C, and that your body temperature of 98.6 degrees F is 37 degrees C.
To make it even more complex, there are at least six other temperature scales to help keep us confused. All these scales have a purpose beyond just knowing whether to wear a jacket today.
Another property of water is the amount of heat energy it takes to change forms, or go from gas to liquid to solid. We actually measure that energy in calories, where a calorie is the amount of energy needed to change the temperature of 1 gram or 1 milliliter (same as 1 cubic centimeter; confused yet?) 1 degree C. Now to keep you confused, I need to point out that the Calories (spelled with a capital “C”) on your cereal box are actually 1,000 calories (spelled with a lowercase “c”).
It takes 540 calories for liquid water to become a gas, and only 80 calories to convert ice to liquid water. We term this heat capacity, or the amount of energy taken up or given off by a substance, in this case water, before it changes state (goes from gas to liquid to solid).
All of this becomes very important to aquatic life. Water slowly changes temperature depending on the amount of sunlight. So when days are longer, and water can absorb more energy from the sun, the water warms somewhat slowly. As days shorten, and nighttime lengthens, the water temperature drops, again somewhat slowly.
People close to larger bodies of water see this in cooler temperatures during the summer day and warmer temperatures during the winter night. It freezes less at the beach than inland around Jacksonville.
ASK RIVER LIFE
Why do manatees like the springs in the winter, yet when I swim in the springs in the summer they feel cold?
It’s all relative. During the winter, the 72 degrees of the springs seems relatively warm to the manatee when surrounding waters may be in the 50s. In a similar fashion, that 72 degrees feels cool to you compared to the 80-degree or 90-degree air temperature. Water also conducts heat away from our bodies nine times faster than air. So 72-degree water feels much cooler than 72-degree air.
River Life runs the last Friday of each month in The Florida Times-Union. E-mail A. Quinton White, executive director of Jacksonville University’s Marine Science Research Institute, with questions about our waterways at firstname.lastname@example.org. For more on the MSRI, visit ju.edu/msri.