early part of the 20th century, lakes have been classified according
to their trophic state.
nutrition or growth.
A eutrophic ("well-nourished")
lake has high nutrients and high plant growth.
lake has low nutrient concentrations and low plant
lakes fall somewhere in between eutrophic and oligotrophic lakes. While
lakes may be lumped into a few trophic classes, each lake has a unique
constellation of attributes that contribute to its trophic status. Three
main factors regulate the trophic state of a lake:
geology of the watershed
landuses and management
of lake basin (morphometry)
(maximum and mean)
and surface area
to lake surface area ratio (Aw : Ao)
status is a useful means of classifying lakes and describing lake processes
in terms of the productivity of the system. Basins with infertile soils
release relatively little nitrogen and phosphorus leading to less productive
lakes, classified as oligotrophic or
mesotrophic. Watersheds with rich organic
soils, or agricultural regions enriched with fertilizers, yield much
higher nutrient loads, resulting in more productive, eutrophic
(even hyper-eutrophic) lakes.
the progress of a lake toward a eutrophic condition, is often discussed
in terms of lake history. A typical lake is said to age from a young,
oligotrophic lake to an older, eutrophic
lake. Geological events, such
as glaciation, created lakes in uneven land surfaces and depressions.
The landscapes surrounding lakes were often infertile, and thus many
lakes were oligotrophic. Eventually some of the shoreline and shallow
areas supported colonizing organisms that decomposed unconsolidated
materials into reasonably fertile sediments. Active biological communities
developed and lake basins became shallower and more eutrophic as decaying
plant and animal material accumulated on the bottom. Shallow lakes tend
to be more productive than deep lakes, in part because they do not stratify,
thereby allowing nutrients to remain in circulation and accessible to
plants. They also tend to have a smaller lake volume, so nutrient loading
from their watershed has a larger impact. There are undoubtedly exceptions
to this typical progression from oligotrophy to eutrophy,where geology,
topography, and lake morphology caused eutrophic conditions from the
of lake aging has unfortunately been interpreted by some as an inevitable
and irreversible process whereby a lake eventually "dies."
In fact, many oligotrophic lakes have persisted as such since the last
glaciation and some ultra-oligotrophic lakes, such as Lake Tahoe may
have been unproductive for millions of years. Furthermore, research
in paleolimnology has provided evidence that contradicts the idealized
version of a lake becoming more and more eutrophic as it ages. Studies
of sediment cores have suggested that the algal productivity of Minnesota
lakes actually may have fluctuated a great deal during the past 12 -
14,000 years (the period since the last glaciation). Changes in climate
and watershed vegetation seem to have both increased and decreased lake
productivity over this period. Some lakes probably experienced high
rates of photosynthesis fairly soon after glacial retreat and then became
less productive until recent times. It is also possible that water sources
for some lakes have changed over the past thousands of years through
diversions of stream flow, for example. In such cases water supplies
to a lake (and therefore nutrient supplies) could have changed, leading
to changes in the lake's productivity.
lakes may be culturally eutrophied by accelerating their natural rate
of nutrient inflow. This occurs through poor management of the watershed
and introduction of human wastes through failing septic systems. Such
changes may occur over periods of only decades and are reversible if
nutrient loading can be
controlled. In the 1960s this was a serious issue, exemplified by the
hyper-eutrophic condition of Lake Erie. Although it was pronounced "dead,"
it eventually returned to less eutrophic conditions, when major point
sources of phosphorus were controlled in the early 1970s (by spending
millions of dollars to build advanced wastewater treatment plants).
America, most of the problems associated with the direct discharge of
domestic wastewater have been successfully mitigated. Now the regulatory
focus is on the much more difficult problem of
sources (NPS) of nutrient pollution such as agricultural drainage,
stormwater runoff, and inadequate on-site septic systems. NPS pollution
is particularly difficult to address because it is diffuse, not attributable
to a small number of polluters, and associated with fundamental changes
in the landscape, such as agriculture, urbanization and shoreline development.
excellent discussion of the factors and issues relating to natural
versus cultural eutrophication is a paper called:
Algal Bowl- A Faustian View of Eutrophication, (by J.R. Vallentyne,
1972, Federation Proceedings, Vol 32 (7), pp1754-7. American Society
of Biological Chemists Symposium on Man and his Environment at
the 56th Annual Meeting of the Federation of American Societies
for Experimental Biology, Atlantic City, NJ, USA, April 10, 1972).