Watershed Geology You should be able to read the basic information from a geologic map. (You may have to do a little additional digging then to gain information about the chemical composition, hardness, and weathering characteristics of the particular rock type(s) involved). However, the basic thing we are most often concerned with is the extent to which the rocks contribute carbonates to the water supply. Carbonates are important to us for four reasons: 1. they form the “backbone” of the acid neutralizing capacity of the water. If the water is “well buffered” there will be little pH change in the system regardless of input, and thus the system is not likely to suffer from limited productivity due to high (or more likely) low pH’s. 2. The input of CO32- from dissolving limestone/marble (as CaCO3) in the watershed drives the equilibrium of the carbonate buffer system in the direction of CO2 improving the overall availability of CO2 for photosynthesis. 3. High levels of alkalinity (the concentration of the HCO3- and the CO32- ions) keep pHs on the high side of neutral. These basic (or “alkaline”) pH’s support chemical reaction conditions that result in the regeneration of phosphorus from sediments…thus high alkalinity can improve phosphorus availability. 4. the bicarbonate ion itself (HCO3-) can substitute for CO2 as a carbon source for some photosynthetic organisms, thus aquatic ecosystems with a good amount of alkalinity will tend to be more productive that a system with lower levels with all else being equal.
Factors we measure/collect data on:
Temperature: This may be one of the simplest things we measure but its importance can’t be overestimated. All aquatic life (i.e., phytoplankton, zooplankton, fishes, insects, etc…with the exception of aquatic mammals) are what are known as thermoconformers – their body temperature is equal to the temperature of the ambient environment. Since rates of chemical reactions are proportional to temperature, the rates of the chemical reactions essential to life are largely controlled by the temperature of the surrounding environment where thermoconformers are concerned. Because of this, aquatic organisms do everything more slowly when the temperature is colder. Thus, in comparing two lakes where every thing else is held equal, a warm lake will be more productive than a cold lake. You can extend this to think about the same lake at different times of the year, again, holding everything else constant, a lake should be more productive in the summertime than in the late fall or early spring based on temperature differences alone!
Oxygen saturation: (see this website for a nice discussion of oxygen saturation) http://www.esr.pdx.edu/pub/biology/limnology/limn-11.htm
We always look at dissolved oxygen concentrations (in mg/L) and think about how that may be affecting what kinds of organisms might live where in the volume of water, but we don’t always talk about the oxygen saturation value, a quantity which we measure in percent. You can quantify the amount of a gas (e.g., oxygen) that can be dissolved in water at a given temperature and pressure (this relationship is called Henry’s law). The
filename: A Limnological Guide to Productivity Indicators.doc
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