How
is a Secchi Disk Used?
Secchi
readings are taken on the shady side of
a boat preferably between 10 a.m. and 2
p.m. The Secchi disk is lowered into the
lake water with a calibrated line until
it can be no longer seen by the volunteer.
This depth is recorded. The disk is then
lifted until it reappears and the second
depth is recorded. The average of these
two depths is called the Secchi depth and is a measure of the lake water transparency.
The
Secchi depth will depend partly on the eyesight
of the viewer, the reflectivity of the disk,
the contrast between the disk and the surrounding
water and the disk's diameter. The water
transparency itself can be affected by the
lake's color, the amount of algae present,
and suspended sediments. In general, transparency
diminishes as these other factors increase.
In many Nova Scotian lakes the Secchi depths
are quite shallow because the water is stained
brown by decaying plant matter and humic
acids. In fact, some say that our lakes
are the color of tea.
Algae are small, green aquatic plants whose
numbers are related to the quantity of plant
nutrients, especially phosphorus. As a result,
transparency can be affected by the amount
of plant nutrients coming into the lake
from sources such as cottage septic systems
and lawn and agricultural fertilizer.
Suspended sediments enter the water column
from sources such as construction sites,
agricultural fields, and re-suspension of
the lake bottom.
Transparency
is an indicator of the impact of human activity
on the lakeshore. If transparency is measured
annually or monthly, trends become apparent.
Transparency can serve as an early-warning
signal that activities on the land are having
a negative effect on a lake. The Kings
County Volunteer Lake Monitoring Program
records Secchi depths at ten Gaspereau watershed
lakes on a monthly basis between May and
October, so that small changes in lake water
quality won't go unnoticed.
Look for Secchi
depths of local lakes in the Program
Archives section. |
