Though temperature seems so simple, it is an important indicator. Many other characteristics of water depend on temperature.

• Cold water holds more oxygen than warm water.
• The rates of metabolism and photosynthesis increase as water temperature rises.
• Organisms are limited to certain temperature ranges and may be more susceptible to disease or poisoning from toxins when the temperature rises.
• Human activities can upset the temperature balance in an estuary.
• Many industries use water for cooling purposes and release large amounts of water temperature. Though they may not add chemical pollutants their thermal pollution may be just as harmful to organisms.

You'll find that many of the other monitoring procedures require measuring temperature. Because cold water is denser than warm water, many bodies of water have distinct layers of water of different temperatures, with the warmer water on top and the colder water on the bottom. It is important to take temperature readings from several depths at any location because water with density differences resists vertical mixing.

1. Obtain 3 water samples using a commercially produced sampling device or equivalent,
   • 0.5 m from the surface
   • 0.5 m from the bottom
   • mid-depth.
2. The sample temperature may change quickly. Immediately insert an alcohol-filled Centigrade thermometer.
3. Wait about 2 minutes for the thermometer to stabilize and read it.

Saltiness is the most obvious characteristic of sea water. Because estuaries always have freshwater flowing into them they are less salty than the ocean, but how salty they are depends upon the size of the river, the season, and even the tide. It may not make much of a difference to us, but to a plant or animal living in the intertidal zone, the salinity level can be the difference between life and death.

In the ocean, most organisms have the same concentration of salts inside their cells as in the outside water. Osmosis (molecules moving through a membrane from an area of higher concentration to an area of lower concentration) regulates this balance. In fresh water, organisms rely on physiological adaptations to keep the salt levels inside their cells higher than the fresh water outside. Though most organisms can adjust to small changes in salinity, a drastic change can be lethal.

Estuary organisms face a constant fluctuation in salinity as tides and fresh water flow interact. Salt water is denser than fresh water, so the organisms often have to deal with "layers" of different saltiness. Because of this stratification, it is important to take samples from varying depths when testing for salinity.

Salinity is usually measured in parts per thousand. Sea water has about 35 grams of salt for every 1000 grams of water. The symbol 0/00 means parts per thousand. (The symbol % which means parts per hundred, or percent.)

Humans can affect the salinity of bodies of water as they develop and utilize shoreline areas. Dikes and tidegates decrease the salinity of sloughs that were once saltmarshes. Taking water from a river for irrigation can reduce the fresh water flow into an estuary, making it saltier. Water wells that pump large amounts of fresh water from underground aquifers too near the shoreline may cause sea water intrusion. This has threatened many fresh drinking water supplies as salt water moves into empty space left by the freshwater that was pumped out.

Most researchers use a salinity meter to measure salinity most accurately. Since salt water conducts electricity better than fresh water, a salinity meter can determine the conductivity of a solution, and then convert conductivity to salinity. These meters can be expensive but are reliable and easy to use. A refractometer is a less expensive option, and is also reliable and easy to use. If you need an inexpensive, low-tech method, you can also calculate salinity using a hydrometer, available at aquarium stores.

A hydrometer is a hollow glass tube with a scale printed on the top. It works on the principle that increased salinity results in increased density. Things are more buoyant (they float higher) in saltier water. The hydrometer will float higher in saltier water and the water surface will be lower on the printed scale. Unfortunately, cold water is also denser than warm water, so the temperature affects the buoyancy at the same time.

To determine the salinity of your sample you will need to take the measurement on the hydrometer and correct it for temperature.

1. Obtain water samples from various depths.
2. Pour about 450 ml of your sample into a container such as a 500 ml graduated cylinder.
3. Measure and record temperature (centigrade).
4. Measure density with the hydrometer. (Look at the point where the waterline crosses the scale.)
5. Determine salinity using the appropriate conversion charts. (Available in The Field Manual for Water Quality Monitoring (Mitchell and Stapp, 1995).

For estuary organisms that breathe, oxygen (O2) dissolved in water is vital. Oxygen levels often determine which organisms can live in an area. Oxygen can enter the water directly from the air by diffusion, or is supplied by the photosynthesis of plants. Many things affect the amount of dissolved O2 in water:

• Temperature - - Cold water holds more O2 than warm water,
• Aeration - - Water stirred by currents and winds picks up extra O2 ,
• Photosynthesis - - Plants and algae produce oxygen daily as they turn sunlight into food.
• Plankton growth - - Plankton blooms caused by excess nutrients deplete the O2 when the microorganisms die and decay,
• Organic matter - - Decomposing organic matter takes O2 from the water. Organic matter can come from:
  • decaying organisms,
  • urban and agricultural runoff,
  • sewage treatment plants,
  • industrial wastes (dairies, fish pens, lumber mills, paper plants, food processing plants, etc.).

Organic matter in the estuary is decomposed by organisms such as bacteria and fungi. This decomposition process removes oxygen from the water. If too much organic matter comes in, oxygen levels can be depleted to the point that aquatic organisms die. Fish kills are often the result of the sudden discharge of too much organic matter that, as it decomposes, uses up all the available oxygen in the water. Continually low dissolved oxygen levels in a body of water can eliminate all breathing organisms.

There are many ways to measure dissolved oxygen (DO). The method you choose may depend on your budget and time. Researchers use dissolved oxygen meters because of their accuracy, but they are expensive. Many companies offer an inexpensive dissolved oxygen test kit. Be sure to carefully follow the maufacturer's instructions.

1. Sample several depths at each site
   • 0.5 meters below the surface
   • mid-column
   • 0.5 meters above the bottom.
2. When collecting samples, try to keep extra oxygen out of the sample (i.e. no air bubbles). Siphon the water from one container - don't pour it- and fill the bottle to overflow to eliminate any air spaces. Place the stopper or lid on carefully to remove any remaining air bubbles. Note: The DO of your estuary is always changing, and is related to water temperature, salinity, sample depth, weather, tide, and season.

Biochemical Oxygen Demand (BOD) is a measure of how much oxygen will be used up over a period of time by microorganisms as they consume organic matter. This standard test is used at sewage treatment plants and some industries to make sure they aren't "polluting" the water with too much organic matter. If during your monitoring you discover a site with unusually low DO, you may wish to test the nearest water source for BOD. The source may be a stream, river, or drain pipe which is contributing organic matter to the estuary.

You can perform a BOD test by filling 2 bottles with sampled water. Determine the DO for one bottle. Completely wrap the second bottle with black tape and incubate it in a dark place for 5 days (68oF). After 5 days, determine the DO for the "black" bottle.

BOD = DO1 minus DO2
DO1 = DO of first sample
DO2 = DO of incubated sample

A healthy, balanced estuary is teeming with bacteria, minute organisms that enter into the many biological processes from digestion to nutrient cycling. Bacteria are essential members of a functioning ecosystem. Too many of the wrong kinds of bacteria, however, can indicate trouble. A sudden input of large numbers of bacteria can deplete the oxygen supply affecting sensitive animals such as fish. Certain pathogenic (disease causing) bacteria can accumulate in invertebrates such as shellfish and cause human illnesses. Coliform bacteria normally grow in the intestinal tracts of mammals, humans included. They are present in large numbers in human sewage and have thus become an indicator of contaminated water. They can enter water systems in several ways: directly from boats, from septic tanks which are not working properly, from sewage treatment plants, and from pets and livestock (cows in a stream or inadequate storage and spreading of manure). The presence of coliform bacteria is not a problem in itself. In fact, coliform bacteria generally do not reproduce outside of the digestive tract. They may indicate problems, however, since they indicate the possible presence of harmful bacteria and the contamination of water by sewage or livestock. That contamination is often accompanied by a concentration of toxic, heavy metals and an overload of organic matter.

Eelgrass, an economically important species. cannot grow at the mouth of a large river where turbidity is too high. High turbidity can have several causes. Rivers naturally carry large amounts of silt, clay, and detritus from their watersheds.

Moving water carries more suspended solids than still water, waves and tides can cause turbidity. Human activities such as logging, dredging, farming, and construction result in more particles entering the water. Large numbers of phytoplankton can cause turbidity -- which may be a sign of a healthy, productive environment, or a sign of over-fertilization.

Turbidity is often measured with a Secchi disc. It consists of a weighted disc that is lowered into the water until it disappears. This "vanishing point" is where there are so many suspended particles between the disc and the viewer that the disc can no longer be seen. This will be deeper in less turbid water and shallower in more turbid water.

The Secchi disc has two sides -- one white and one with alternate white and black quadrants. The side you choose is a matter of personal preference. Try them both and decide which is easier for you to see. The rope should be marked at 0.25 m intervals and should not stretch.

• 1. Lower the disc slowly into the water and record the depth when the disc disappears.
• 2. Slowly raise the disc and record the depth at which it reappears.
• 3. Record both observations and calculate the average.

Note: In currents, the disc may hang at an angle, resulting in the line actually being longer than the depth. Make your readings with the line hanging straight down. Direct sunlight may obscure your view of the disc, so try to lower the disc into a shaded spot. If the waters you are testing are so shallow or so clear that you can't get an accurate secchi disc reading, you can also use a test kit (Jackson Tube method) to determine turbidity.