Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 107

water from lakes (Lind, 1974). The Van Dorn bottle consists of a tube with
covers over each end. The messenger releases a catch so the stretched rub-
ber connectors can pull the covers onto the tube, sealing the water into it.
Kemmerer bottles operate similarly, but rather than using rubber connec-
tors to pull ends onto a tube, gravity is used (Fig. 6.13).
Additional devices include pumps to remove water from depth, bottles
with strings attached to stoppers that can be unplugged at depth, and pipes
that allow water to flow up to containers that displace surface waters (Fig.
6.13).
The choice of device depends on the type of sample that is required.
Toxic materials generally are to be avoided, and if chemical analysis on
metals is to be done metal samplers should not be used. The violent clo-
sure of some samplers can harm some organisms that are susceptible to
pressure shock. Zooplankton may avoid an opaque sampler more than a
clear one because of their predation avoidance behaviors.
on the bottom and freshwater on the top. Saline springs on the bottom of
lakes have also formed stable layers. Some of the dry-valley lakes in Antarc-
tica have such stable layers (see Chapter 15).
The biological and biogeochemical effects of stratification on lake or-
ganisms are strong. Molecular diffusion rates that dominate movement of
dissolved materials across the metalimnion are slow enough that a signifi-
cant depletion of O 2 in the hypolimnion will lead to anoxia during the sum-
mer. In turn, O 2 loss from the hypolimnion means that biogeochemical cy-
cling and lake productivity are altered. Anoxia and the biogeochemistry are
discussed in detail in Chapters 11-13. Given the very complex chemical and
physical characteristics in many stratified lakes, several methods for sam-
pling lake waters from different depths have been developed (Method 6.1).
WATER MOVEMENT AND CURRENTS IN LAKES
The movement of wind is generally the main cause of waves across
lakes, although motorboat activity can cause significant wave action. Wave
action is important partially because it is associated with surface mixing
and erosion of the shoreline. Lakeshore erosion can lead to habitat de-
struction and large financial losses associated with property damage; many
environmental engineering firms specialize in controlling erosion. Wave ac-
tion can influence which species can successfully inhabit the different
depths of the shallow benthos (littoral zone). The two main determinants
of wave height are the strength (speed and duration) of the wind and the
length of lake on which the wind acts. The influence of the wave also is
dependent on the geometry and materials that make up the shoreline.
The length of lake on which the wind acts is called the fetch (Fig. 6.14).
The longer the fetch, the higher the waves (Fig. 6.14). A perfectly round lake
would be affected similarly by wind from any direction. On an irregularly
shaped lake, certain wind directions lead to the largest waves. In smaller