Primary production of a waterbody is the rate of conversion of solar energy to chemical energy (organic compounds) by green plants. We usually distinguish between gross primary production and net primary production. Gross primary production is the rate of all photosynthetic processes regardless of the fate of the compounds which are produced during photosynthesis. Part of the photosynthate is diverted to growth (i.e., increase in biomass) and part is reconverted to C02 and water in the process of respiration, which provides the energy to run the metabolic machinery of the cells. Net production is the total amount of photosynthesis which is diverted toward growth, and is thus always less than gross production. The relationship between gross production, net production and respiration is as follows:
Gross primary production = Net primary production + Respiration.
In studying natural freshwater habitats, it is most useful to know all three components of the above equation. Typically only two components are measured directly, and the third is obtained by subtraction. The oxygen difference technique for determining primary production is based on this principle.
To determine primary production at a particular depth in any water body, three sets of bottles are required: the initial bottles, the light bottles (i.e., those that admit light), and the dark bottles (i.e., bottles which are darkened with foil or tape so that they admit no light). A fresh sample is taken from the depth of interest with a water sampler and is added to each of the three types of bottles. Usually each bottle type is duplicated or triplicated. The amount of time the bottles spend in the direct sun is absolutely minimized while the bottles are being filled. Excessive exposure of the bottles to sunlight may damage the plankton and thus result in erroneous data. The light and dark bottles are attached to a suspension device and lowered back to the level from which the sample was taken, where they are allowed to incubate for several hours. As soon as possible after these bottles are in place, the initial bottles are treated with Winkler reagents. The initial bottles will provide a measure of the oxygen content of the water at the beginning of the experiment.
The length of the incubation is determined by the productivity of the waterbody which is being studied. In very productive waters, short incubations produce enough oxygen in the light bottle to be detected by the Winkler technique. Short incubations usually span three to four hours. In less productive lakes longer incubations may be necessary, but incubations longer than 8 hours are highly undesirable because the artificial conditions inside the bottle resemble conditions in the lake less and less as time goes by. In some oligotrophic or unproductive waters, such as mountain lakes, it may not even be possible to use the oxygen difference method for detecting photosynthesis and respiration. In this case the Carbon-14 method must be used because of its greater sensitivity to low production rates.
The start of the incubation period must be noted carefully. After the incubation has been in progress for a predetermined number of hours, the bottles are removed from the lake and the time is noted once again. The bottles are immediately treated with Winkler reagents to stop all biological processes and fix the oxygen.
All of the bottles are returned to the laboratory and titrated carefully for oxygen determination. Typically two different titrations are done on each bottle. The dark bottles will show a decline in the amount of dissolved oxygen from the initial condition. Since photosynthesis is impossible in a dark bottle, only respiration will have occurred in the dark bottles. The decline of oxygen in the dark bottles is a quantitative measure of the amount of respiration which occurs over the incubation period. It should be noted, however, that this respiration includes that not only by plants but also animals and bacteria. Fortunately, the respiration of plants (algae) usually predominates in plankton systems and the respiration in the bottle is a pretty good approximation of the total respiration by plants.
In the light bottles, both respiration and photosynthesis will be occurring simultaneously. Since total photosynthesis minus respiration gives net photosynthesis, these light bottles will indicate the amount of net primary production. If the lake is quite productive, the amount of oxygen in the light bottles will go up by a considerable amount over the incubation period. If the lake is unproductive, or if the incubation is done at a depth where the illumination is very weak, the amount of increase in oxygen may be very small, negligible, or even negative. Since the increase or decrease of oxygen in the light and dark bottles with reference to the initial oxygen conditions gives us a measure of respiration and net primary productivity, it is possible to calculate gross primary productivity using the equation given earlier. In the calculations it is customary to report respiration, net production, and gross production all in terms of milligrams of oxygen per liter per incubation period.
Some further calculations are possible with the data. Typically a pyrheliometer is operating in the field during the incubation period. It is possible to simply mark off the incubation period on the chart from the pyrheliometer and determine by use of a planimeter how much sunlight fell on the waterbody during the incubation period. Since the amount of photosynthesis at a given depth in the lake is proportional to the amount of light present, the amount of light during the incubation period as a fraction of the total amount of light for the day can be used to calculate the total amount of net primary production as follows:
(Total light for the day / Total light for the incubation period) X
(Net primary production for the incubation period) =
Total estimated net primary production for the day.
This allows the results to be expressed in terms of milligrams of oxygen per liter per day.
Respiration is not directly related to sunlight, so it is assumed that the amount of respiration can be determined by the length of the incubation period. We may thus divide the total amount of respiration by the total length of the incubation period and obtain respiration in terms of milligrams of oxygen per liter per hour. When this figure is multiplied by 24, we obtain an estimate- of the total respiration in a day, expressed as mg/02/day. This procedure actually involves some simplification because the rate of respiration is not perfectly constant over a 24 hour period, but it is a reasonable approximation. Now that the net production and respiration per day are known, it is a simple matter to compute the gross production expressed on a daily basis by adding the two together.
Production can be expressed as carbon instead of oxygen provided that the ratio of carbon to oxygen in photosynthesis is known. The molar ratio of carbon to oxygen in the synthesis of simple glucose would be 1.0, but the synthesis inside cells involves production of proteins and lipids as well. For this reason the molar ratio is actually nearer to 1.2 on average than 1.0. Using this value, and taking into account the difference in weight of carbon and oxygen, the following conversion can be made:
(mg 02/liter/day) X (312.5) = mg C/m3/day
The values of production can also be converted to dry weight after the conversion to carbon is made. To convert carbon to dry weight, simply multiply by two, since approximately half the dry weight of protoplasm is carbon. To get production in terms of wet weight of protoplasm, multiply the dry weight figure by ten. To convert to energy, multiply mg C by ten and the result will be calories.
Up to this point the discussion has been of primary production measurements made at a single point in a water body. Typically measurements are made simultaneously at several points in the water column. This is actually essential for a complete understanding of the production of a water body because production varies greatly with depth. Primary production is usually suppressed just at the surface because the light is too strong there, but reaches a maximum just below the surface because this is where the light optimum occurs. Below the light optimum, production declines rapidly with light. Usually bottles are incubated at the surface and at four or five depths in the water column all at the same time so that the amount of primary production can be graphed as a function of depth. Once such a graph is available, a planimeter (or integration by counting squares) can be used to obtain the total amount of primary production under a unit of lake surface area. The results are expressed in terms of mg C/m2/day for a particular water body.
Exercises
Water samples from several depths at Third Sister Lake will be incubated during the morning and then collected during lab in the afternoon. Each sample will then be titrated to determine the amount of dissolved oxygen present in milligrams per liter (bring your handout on oxygen to lab as a reminder of the procedure and calculations). Taking the means of duplicate samples or titrations, respiration, net photosynthesis and gross photosynthesis can then be calculated as follows:
where,
Solar radiation data will be provided, and from that you should calculate respiration, net photosynthesis, and gross photosynthesis in terms of mg O2 per liter per day as explained in the 'background' section of this handout.
Next calculate the net and gross photosynthesis in terms of mg C/m3/day, again as explained earlier. (If you had to, could you explain how the 312.5 multiplication factor is derived?)
From this information it is possible to plot photosynthesis in mg C/m3/day on the x-axis (across the top of the graph) against depth in the water column in meters on the y-axis (O meters should be at the upper left of the graph, with Z at the lower left). You will make only one graph, using your net photosynthesis data only. The graph paper selected must be finely divided (i.e., lots of squares; 12 x 12 to the inch or an equivalent).
This graph can now be integrated to provide production values on an areal basis (mg C/m2/day). First determine the value of each square on the graph in mg C/m2/day. Then simply determine the number of squares present under the curve in an area marked off by a certain depth, and repeat this for each successive depth, adding the sum of the previous values to each newly calculated value. This is identical to integration of the entire curve by planimetry .
Questions