Be sure to record the manufacturer's name and model number of the instruments we use. You will have to include this information in the methods section of your report on Falls Lake. The techniques described in this chapter do not have to be repeated in your methods section; simply reference this guide.
The name implies that this simple tube samples the euphotic zone directly, but the thickness of the euphotic zone varies between stations on one date, and between dates at one station. Usually it extends deeper than the depth sampled by the tube. A better name for the tube would be "surface water sampler."
We will use a Van Dorn water bottle mounted horizontally to collect deepwater samples. Cock the two end stoppers open by attaching cable loops to the trigger pegs, then lower the sampler to a point 30-50 cm from the bottom. Locate this depth by raising the bottle after it contacts the sediments. Gently swing it back and forth for about 30 seconds to flush out any water taken in at shallower depths or mud picked up from the bottom, then send the "messenger" weight down the line to release the end stoppers. Retrieve the sampler and open one stopper to let the water drain into the "deep" bucket.
Van Dorn bottles have a spring valve and air vent which allow water to be transferred to a closed bottle without exposure to air, but most of our measurements are not affected by aeration, so we will not use this feature.
The surface water sample from each station is subsampled to measure
pH, alkalinity, turbidity, phytoplankton identities, and chlorophyll and
nutrient concentrations. The bottom water sample is subsampled to measure
pH, alkalinity, turbidity, and nutrient concentrations.
Turbidity is measured in nephelometric turbidity units (NTU) in a portable nephelometer, or turbidity meter. It measures the sideways scatter of light by particles suspended in water using a photoreceptor at right angles to a light source.
Before the first measurement, calibrate the instrument with the enclosed standard. Polish and place the standard vial into the well and slowly rotate it until a minimum reading is obtained, then adjust the scale to 0.02 with the calibration knob.
Fill the screw-capped nephelometer cuvette with water from the lake,
carefully wipe the outer surface dry and clean with an absorbent, lint-free
cloth, and place it in the well. Allow the sample to stand for a minute
or two to allow bubbles to dissipate and the largest particles to settle
out before taking the reading. Slowly rotate the vial until you obtain
the minimum reading, and record it.
Initial pH readings can be combined with alkalinity titration. Rinse the probe thoroughly with water from the sample before taking a measurement. Immerse the end of the probe completely in a subsample and allow the meter to stabilize. Record this first reading as the current sample's pH.
Typical pH in our local waters ranges widely, from above 10 in highly
productive waters on warm, still days, to less than 6 in deep, anaerobic
water during summer stratification. Because our waters are poorly
buffered, fluxes of CO2 associated with higher rates of photosynthesis
and respiration can drive pH of surface water layers through a wide daily
cycle. pH in fresh samples can change rapidly if the water is aerated,
allowing CO2 concentrations to equilibrate with the atmosphere.
If you initially measure a pH of 9.0 in Yates Millpond surface water on
a warm, still day, you should be able to watch it decline over several
minutes as carbon dioxide diffuses into the water from air. Similarly,
water from near the bottom at pH < 6.0 might gradually rise toward as
excess (supersaturated) carbon dioxide diffuses out into the air.
Take initial pH measurements as soon as possible after collecting water
samples and with a minimu o fstirring. These changes in pH will not
affect alkalinity, however.
During the titration, pH will change very slowly until you pass 5.0, but then drop quickly with the addition of only a little more acid. Take note of the pH after each drop below 5.5 so you won't overshoot the end point. After you have done a sample or two, you may add about 75% of the expected acid volume quickly, then titrate to the endpoint more carefully. In Yates Millpond, the endpoint should be reached after 2 - 9 mL of acid has been added.
For more precise estimates, you may use the "Gran titration" method. This method requires you to record the pH after each additional drop of acid throughout the titration. Calculate, for each drop of acid, the value of the index:
F1 = [H+] x (Va + Vw)
where,
F1 = a pH index
[H+] = molar concentration of hydrogen ion
Va = cumulative volume of acid added, and
Vw = volume of the initial lake water sample.
Convert pH to [H+] by taking 10-pH.
Plot Va versus F1 on logarithmic paper (or log F1
versus Va on linear graph paper). The resulting curve
should be horizontal at first, then beyond a point where the water's pH
has dropped below 5, the line should bend upward and continue straight
at that angle. Calculate the slope of the rising portion of the line and
rearrange the formula for a straight line to calculate the value of X for
Y = 0.
That X intercept is the exact volume of acid required to reach the end point.
From the sample volume (200 mL), the acid normality (20 meq L-1), and the mL of acid used to reach the final end point (including any acid necessary to titrate to pH 8.3), calculate alkalinity with the formula:
AT = (Va x Na)/Vw
where,
AT = alkalinity, in milliequivalents per liter (meq L-1)The value for alkalinity in meq L-1 may be converted to commonly used units of "mg CaCO3 L-1." The molecular weight of calcium carbonate is about 100 and there are 2 equivalents per mole of this salt, so the conversion factor is 50. Multiply alkalinity in meq L-1 by 50 to obtain alkalinity in mg CaCO3 L-1. Remember, however, that simple acid titration does not determine how much, if any, of the alkalinity was actually accounted for by calcium compounds. Do not use this number as an estimate of actual calcium concentration in the water.
Va = volume of acid used to titrate to the endpoint in liters (calculate as mL/1000)
Na = normality of acid used in milliequivalents per liter (meq L-1)
Vw = volume of lake water titrated in liters
Take readings just below the water surface (0.05 m), then every 0.5 m to a maximum of 2.0 m. The meter is neither stable nor accurate at readings below about 25 micro-einst.
The photoreceptor is sensitive only to wavelengths which are absorbed and used by chlorophyll and other photosynthetic pigments, called "PAR" (Photosynthetically Active Radiation). Some limnologists still use foot-candles or lumens as units of light. Others prefer to convert units to the amount of energy, as calories or joules, per unit surface area (usually 1 cm2) and time (usually sec). Our meter gives data in micro-einsteins (micro-einst) cm-2 s-1, which is a measure of the photon flux adjusted for varying quantum energies of photons at different wave frequencies.
e = (lnI0 - lnIz)/z
where,
e = the extinction coefficientThe higher the extinction coefficient, the more rapidly light is attenuated in the lake. Much of the excess of light attenuation over that caused by pure water is due to suspended clay in our area.
I0 = light intensity at a shallower depth (micro-einst cm-2 s-1)
Iz = light intensity at a greater depth, and
z = depth interval between shallower and deeper measurements (m)
Although the Secchi is a generally reliable and widely used method of measurement of light attenuation, several errors and technical problems must be considered. People differ in visual acuity, so some variation is inevitable with so many different people taking measurements. Reflection of light off the surface interferes with ability to see the disk. This effect is worst in full sunlight and when the sun is at low angles. For greatest repeatability, readings should be made between 10:00 a.m. and 2:00 p.m., local standard time, and be made in shadow.
In use, the probe of the meter is lowered to the surface (0.05 m), 0.25 m, 0.5 m, 0.75 m, 1.0 m, then at 0.5 meter intervals to the bottom. Readings for temperature, specific conductance (the second of the sequential conductivity readings on the YSI Model 85 meter we are using), oxygen concentration, and oxygen % saturation are recorded at each depth.
The oxygen probe takes about 15 minutes to reach electrical equilibrium after it is turned on, so turn on the meter a little while before you plan to take measurements in the pond.
The oxygen reading will quickly drop to 0 when the probe hits bottom, but also when the probe passes into an anaerobic water layer. Learn to tell the difference when the line goes slack and the probe has hit bottom.
% = ([O2]/[Osat]) x 100
where,
% = percent saturationSaturation levels indicate how much biological processes have affected the water recently. At the beginning of summer stratification, or perhaps even during the previous night, oxygen concentrations at mixed depths were exactly saturated, that is, in equilibrium with the air. Water just at the surface of lakes should be at or close to 100% saturation because exchange of oxygen with the air by diffusion and wave turbulence usually occurs at a faster rate than photosynthesis or respiration. However, in enriched or contaminated lakes, changes due to biological activity may exceed physical exchange rates even at the surface, and saturation levels may be much higher or lower than 100% at any depth, even near the surface. Supersaturation is frequently observed near the surface of Yates Millpond on hot, still days.
[O2] = observed oxygen concentration, and
[Osat] = saturated concentration of oxygen at the local temperature at sea level.
The OCT meter calculates % saturation of oxygen for you. The only potential error comes from variations in air pressure at the lake surface, which can change under the influence of weather systems and altitude. Yates Millpond's surface altitude is about 75 m, which, on the average, requires a correction in the value for 100% oxygen saturation by - 0.1 mg L-1. The most precise % saturation values require detailed information on barometric pressure at the time of measurement. We will not go to that effort, since the important patterns are apparent without the greater accuracy.
Salinities in the brackish range (> 0.5 g L-1) also decrease oxygen's solubility, but Yates Millpond is completely freshwater, not brackish. You must take that into consideration if you are measuring estuarine or marine waters.
Units of specific conductance are mSiemens, which is similar to the unit formerly used in the United States, micro-mho cm-1. Our local surface waters have relatively low conductivity (45 - 90 micro-S) , but higher values can occur when waste effluents, groundwater, or salts dissolved from sediments are present.
Salinity is the weight of inorganic salts in solution. In the oceans, the composition of salts is constant in proportion, and changes in salinity apply proportionally to all component salts. This allows oceanographers to make a simple conversion from specific conductance to salinity. But a direct conversion is not possible in inland waters because many different ions can occur in widely varying proportions. For that reason, salinity is rarely reported for fresh or non-estuarine, brackish waters.
Near the sediments biological processes indirectly have a major effect on specific conductance. First, CO2 from bacterial respiration at the mud surface builds up, lowering pH and dissolving carbonates from the sediments which increase both conductance and alkalinity. Then, when oxygen is entirely depleted by microbial respiration and anaerobic bacteria take over, major chemical changes begin to occur. Iron, manganese, and other metals are reduced and become more soluble. You may expect to see higher specific conductances as the probe nears the bottom, and highest specific conductances where there is no oxygen in the water near the mud.
Wetzel, R. G. and G. E. Likens. 1989. Limnological Analysis. Saunders Publ. Co., New York. 357 p.
Last modified on August 7, 2002