Primary Production

One of the most important biological variables monitored in limnological studies is the rate of photosynthetic carbon fixation, known as primary productivity (PPr). The total amount of PPr in a given aquatic system, as well as the ratio of PPr contributions from planktonic algae, aqatic macrophytes (large aquatic plants), and peryphyton (bottom dwelling algae living on rocks and sediment) is interwoven with a whole series of other variables commonly studied by limnologists. During photosynthesis, CO₂ is combined with water and light energy from the sun to form the sugars the photosynthetic organism depends upon and releases oxygen as a byproduct:

6 CO₂ + 12 H₂O (+ sunlight) --> C₆H₁₂O₆ + 6 O₂ + 6 H₂O

As such, light penetration, and amount of Dissolved inorganic carbon (DIC) from CO₂ all directly affect the reaction. Water temperature is also important as temperature affects reaction speed. As phytoplankton and plants often require other nutrients to survive, levels of chemicals such as Ammonium and Nitrate can also impact the amount of photosynthesis occurring.

Primary Production setup Rate and source of PPr are also closely tied to the structure of the food webs in aquatic systems. Since one of the products of photosynthesis is oxygen, all organisms depending on oxygen for respiration are affected by the amount photosynthesis occurring around them. Oxygen producing planktonic algae and aquatic plants also serve as the food source for many other organisms and the rate at which they are consumed can affect the rate of PPr as well. For example, If the amount of zooplankton increases, their rate of oxygen uptake may increase as may the rate at which they consume phytoplankton. So an increase in the zooplankton community might result in a decrease in PPr. In this way, PPr and resulting amount of oxygen is tied to the food web and may vary as the food web varies in different parts of an aquatic ecosystem.

There are several ways of measuring the rate of PPr commonly used among Limnologists. At Castle Lake, PPr is measured using the ¹⁴C method. This technique employs a carbon tracer that gets taken up during photosynthesis along with Carbon available from CO₂. Water samples are taken at depths spanning the water column. For each depth there is a dark bottle sample and a light bottle sample (one in which photosynthesis can occur and one in which it cannot). Samples are incubated for a fixed amount of time at the depth they were collected, so that light amount and temperature remains constant for that depth. After incubation, the samples are taken to the laboratory and run through a very fine filter. The rate of photosynthesis at a particular depth can be determined by comparing the amount of tracer on the filter from the light bottle with that of the filter from the dark bottle.