Expected Results for the Red Head Grass Experiment
This experiment is designed to test the influence of water temperature on the growth rate of redhead grass (Potamogeton perfoliatus). The water temperature in growth chamber A will be set at 24°C (75°F) while growth chamber B will be set at 29°C (84°F) or 33°C (92°F). Both growth chambers (black tubs) are identically setup except for the water temperature.
Plant Height: All bay grasses have an optimal temperature range which maximizes growth. As temperatures deviate from this optimal range, growth rates decrease.
Maximum redhead grass growth rates typically occur between 15°C (59°F) and 26°C (79°F). As temperatures are increased above 15°C (59°F) or below 26°C (79°F), growth rates will decline until the plants eventually die. In the Chesapeake Bay, these conditions maybe found in the summer in very shallow, poorly flushed areas.
As temperatures decrease below 15°C (59°F), growth rates will decline. A decline in water temperature is a cue for redhead grass to begin preparing for the winter. In early September, redhead grass will begin producing over-wintering structures (tubers). As temperatures continue to decrease, the leaves and roots will decompose. By late November, only the seeds and tubers will remain. When water temperatures increase the next spring, the remaining seeds and tubers will produce another bed of redhead grass.
pH: The pH scale is a measure of the
acid-base balance of water, with a pH of <7 representing ‘acidic’ solutions, and
pH of >7 representing ‘basic’ solutions. Pure water would be perfectly neutral
(pH of 7), but water naturally contains a certain amount of dissolved substances
that act either as acids or as bases. If the water contains more acids (H+) than
bases, it is said to be acidic; if it contains more bases (OH-) than acids, it
is basic or alkaline. If acids and bases are present in equal amounts, the water
is said to be chemically neutral. When CO2 is in short supply, many aquatic
plants take CO2 from the hardening constituents of the water (CO3 and HCO3).
When this happens, more OH-1 molecules are generated, increasing the pH of the
water. So as more and more OH-1 molecules are generated by the shortage of CO2
and the production of more oxygen, the pH in the growth chambers will increase
over time. Because the Redhead grass in the 92°F growth chamber will be growing
slightly faster than the Redhead grass at 75°F, pH’s may be slightly higher in
the warmer tank.
pH values above 10 have been shown in lab tests to decrease photosynthesis of Redhead grass, which in turn decreases it’s growth rate. When the pH values are acidic (below 7) a lot of OH-1 is in water, and the CO2 (carbon dioxide) that plants need for photosynthesis turns into CO3, which plants can’t use. The plants have a harder time "breathing," and as a result they grow more slowly. Causes of low pH can be the result of acid rain (local rain pH is about 5) and the poor buffering capacity of the land due to variations in geology. Redhead grass can be found in pH’s ranging from 6 to 10 with a preferred pH around 8.
Nitrates: Nitrogen is one of the most
important plant nutrients (along with phosphorus). With the help of bacteria,
nitrogen goes through a cycle of chemical changes as it is absorbed, used and
then restored to a form which it can again be used. Most plants absorb nitrogen
from the sediment or water column in the form of nitrate (NO3-) or ammonium
(NH4+). Other common forms of nitrogen (nitrite (NO2), ammonia (NH3), etc)
aren’t available to plants, and can even be toxic at high levels.
In the Chesapeake Bay, areas with excess nutrients typically have large concentrations of short lived microscopic plants called phytoplankton (algae). Large concentrations of phytoplankton (algal blooms) can color the water green or brown and greatly reduce or eliminate the amount of light available for bay grasses. In addition, since algae are also plants, they produce oxygen during the day and consume it at night. A large algal bloom can remove so much oxygen in an area at night that it may kill any fish and crabs present! When phytoplankton die, they decompose (consuming oxygen) and release a large amount of nutrients back into the water which can fuel more algal blooms.
Redhead grass (and other bay grasses) are important because they consume nutrients as they grow and only slowly release them back into the water column late in the year when the water temperature has cooled below the preferred temperature of the phytoplankton.