Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 337

associated with improvement of water quality, eutrophication continues to
be a very relevant issue in lakes, streams, and wetlands.
DEFINITION OF TROPHIC STATE
Classifications of the trophic state of aquatic ecosystems are useful be-
cause they allow people to compare productivity of ecosystems within and
among ecoregions and provide an initial approach for determining the extent
of cultural eutrophication. Trophic state is generally signified by the terms
oligotrophic, mesotrophic and eutrophic. Oligo means “few,” trophic means
“foods,” eutrophic means “many foods,” and mesotrophic falls between
these two categories. The three categories are only one way to characterize a
continuum of ecosystem productivity. Over the years, several systems have
been employed to describe the trophic state of lakes; trophic state classifica-
tions are not as highly developed for streams, groundwaters, and wetlands.
Early limnologists noticed that certain types of phytoplankton and
zooplankton were typically found in high-nutrient lakes and others in
nutrient-poor lakes. This observation led to extensive efforts to characterize
the trophic state of lakes with regard to their phytoplankton communities
(Hutchinson, 1967). Limnologists thus recognized the links among nutri-
ents, phytoplankton biomass and productivity, and water quality. These
links will be described quantitatively later.
Current classifications of trophic state of lakes are generally based on wa-
ter clarity, phytoplankton biomass, and nutrient concentrations (productivity
is not as easy to measure, so it is used less in trophic classification). In gen-
eral, oligotrophic lakes have low algal biomass, low algal productivity, low
nutrients, high clarity, and deep photic zones, and they may support cold-
water fisheries. Eutrophic lakes are characterized by cyanobacterial blooms,
high total nutrients, and large variation in O 2 concentrations (including
potential anoxia in the hypolimnion), and they may have frequent fish kills.
The trophic state of lakes is usually based on phytoplankton concentrations,
but shallow eutrophic lakes can have extensive macrophyte populations.
One of the commonly used classifications for lakes was constructed by
a large group of limnologists interested in eutrophication (OECD, 1982).
This classification system was constructed by combining data from many
lakes. Before the analysis, the lakes were classified by scientists as eu-
trophic, mesotrophic, or oligotrophic. The results were two classification
approaches: a probability distribution (Fig. 17.2A) and a fixed boundary
classification (Table 17.1). There are several fixed boundary classification
systems; for the most part, the boundary levels are consistent (Nürnberg,
1996). Another commonly used method of classification involves calculat-
ing a trophic index that places trophic state on an exponential scale of Sec-
chi depth, chlorophyll, and total P (Fig. 17.2B), where 10 scale units rep-
resent a doubling of algal biomass (Carlson, 1977).
Lakes may not clearly fall into an individual category in any of the
classification systems. For example, phosphorus could be high enough for
a lake to be classified as eutrophic, but light attenuation by suspended sed-
iments could keep chlorophyll levels in the mesotrophic range. Also, total
phosphorus and phytoplankton concentrations could be low in a lake with
extensive macrophyte biomass and production (Brenner et al., 1999).