2.3 Field Methods
65
of sampling. For some studies, additional samples may be required. These might
include samples of particular portions of the lake, littoral zone samples, or samples at different depths. Even if samples are not collected at different depths, lake
sampling should ideally be accompanied by measurements of the temperature
profile to determine if the lake is stratified and to characterize the location and
depth of the thermocline, the epilimnion, and the hypolimnion. If samples are
collected at different depths, the water temperature should be measured at each
sampled depth in conjunction with measuring the temperature profile. Water
samples cannot be determined to be epilimnetic samples unless a depth profile
is taken. Surface samples without temperature profile should be labeled as water
surface samples rather than epilimnetic samples to avoid possible confusion.
Point samples are those collected at a specific depth. Van Dorn cylinders
and Kemmerer bottles are the most common point samplers for lakes. The Van
Dorn cylinder may have some advantage in that it appears to allow better circulation of water through the sample container. In general, we recommend use of
a Van Dorn sampler for lake sampling. In either case, the sampler is difficult to
keep clean unless it is kept in a plastic bag between sites. The sampler should be
soaked in the lake prior to use. At the sample site, the sampler should be raised
and lowered several times just below the surface to further rinse the container.
It is then lowered to the desired depth, held to stabilize, and triggered, usually
with a weight that slides down the line holding the sampler. When sampling
the hypolimnion, care should be taken not to touch the bottom because this
will disturb sediment that could contaminate the sample.
Because of the drag of long lengths of rope and the sampler itself, both Van
Dorn and Kemmerer samplers are prone to sampling at shallower depths than
indicated by the length of the rope. If the boat is drifting because of current or
wind, deeper samples may be in error by a considerable margin for reported
depth. This error can be avoided by anchoring or tying to a buoy. These samplers also tend to plane while being lowered; allowing the rope to straighten
before triggering the bottle can help minimize this error.
Pumps and tubing sometimes are used to collect point samples from lakes
and streams or to integrate samples from lakes. The primary concern with
these devices is keeping the tubing clean. It is impossible to thoroughly clean
the inside of a tube to eliminate bacterial growth. Sampling tubes can generally be kept clean by storing them filled with DIW and in a black plastic bag and
avoiding disturbance of sediment while sampling. The tubing can easily block
with slush during freezing conditions. Tubing does offer the ability to collect
an integrated sample of the water column of lakes by lowering the tubing, at
a constant rate of travel while pumping, to near the bottom and back to the
surface. In general, however, lake water sampling for inventory and monitoring
of the effects of atmospheric deposition does not require collection of depthintegrated samples with a tube.
65
of sampling. For some studies, additional samples may be required. These might
include samples of particular portions of the lake, littoral zone samples, or samples at different depths. Even if samples are not collected at different depths, lake
sampling should ideally be accompanied by measurements of the temperature
profile to determine if the lake is stratified and to characterize the location and
depth of the thermocline, the epilimnion, and the hypolimnion. If samples are
collected at different depths, the water temperature should be measured at each
sampled depth in conjunction with measuring the temperature profile. Water
samples cannot be determined to be epilimnetic samples unless a depth profile
is taken. Surface samples without temperature profile should be labeled as water
surface samples rather than epilimnetic samples to avoid possible confusion.
Point samples are those collected at a specific depth. Van Dorn cylinders
and Kemmerer bottles are the most common point samplers for lakes. The Van
Dorn cylinder may have some advantage in that it appears to allow better circulation of water through the sample container. In general, we recommend use of
a Van Dorn sampler for lake sampling. In either case, the sampler is difficult to
keep clean unless it is kept in a plastic bag between sites. The sampler should be
soaked in the lake prior to use. At the sample site, the sampler should be raised
and lowered several times just below the surface to further rinse the container.
It is then lowered to the desired depth, held to stabilize, and triggered, usually
with a weight that slides down the line holding the sampler. When sampling
the hypolimnion, care should be taken not to touch the bottom because this
will disturb sediment that could contaminate the sample.
Because of the drag of long lengths of rope and the sampler itself, both Van
Dorn and Kemmerer samplers are prone to sampling at shallower depths than
indicated by the length of the rope. If the boat is drifting because of current or
wind, deeper samples may be in error by a considerable margin for reported
depth. This error can be avoided by anchoring or tying to a buoy. These samplers also tend to plane while being lowered; allowing the rope to straighten
before triggering the bottle can help minimize this error.
Pumps and tubing sometimes are used to collect point samples from lakes
and streams or to integrate samples from lakes. The primary concern with
these devices is keeping the tubing clean. It is impossible to thoroughly clean
the inside of a tube to eliminate bacterial growth. Sampling tubes can generally be kept clean by storing them filled with DIW and in a black plastic bag and
avoiding disturbance of sediment while sampling. The tubing can easily block
with slush during freezing conditions. Tubing does offer the ability to collect
an integrated sample of the water column of lakes by lowering the tubing, at
a constant rate of travel while pumping, to near the bottom and back to the
surface. In general, however, lake water sampling for inventory and monitoring
of the effects of atmospheric deposition does not require collection of depthintegrated samples with a tube.