Environmental Engineering Reference
In-Depth Information
METHOD 16.1.
Bioassay Tests for Determination of Nutrient Limitation
The most realistic way to determine nutrient limitation is to test the entire sys-
tem. However, this approach is not practical in many situations because con-
trol, multiple treatments, and replication are difficult, if not impossible, and
nutrient pollution of an entire system is undesirable. The most commonly
used alternative is in situ treatments in enclosures or small-scale treatments.
In lakes, mesocosms (limnocorrals) or containers (Goldman, 1962)
from 1 to 1000 liters have been used. Nutrients are added, and the response
of the algal biomass is usually measured after about 1 week (Fig. 16.1). En-
closing the water may lead to artifacts, such as attached algae proliferating
on the walls and lack of external nutrient inputs, but these “container ef-
fects” may be minimal in short-term studies in large enclosures.
Benthic systems are often tested with nutrient-diffusing substrata. In
these tests, nutrients are sealed inside a container, out of which they slowly
diffuse across a permeable surface that can be colonized by benthic algae.
Unglazed clay pots filled with nutrient-enriched agar are commonly used
controls the rate at which the cars can be constructed. Similarly, a living
cell requires that elements be supplied at specific ratios for growth. Liebig's
law of the minimum has been applied to nutrient limitation of primary
producer assemblages in aquatic systems, assuming that all producers have
equal nutrient requirements and nutrients are evenly distributed in space
and time in the environment (i.e., a homogeneous equilibrium condition of
nutrients exists). The law, combined with the idea of equilibrium nutrient
availability, predicts that only one nutrient will limit primary production
of a system.
How closely does this prediction match empirical data on nutrient lim-
itation? Several techniques have been used to assay nutrient limitation
(Method 16.1), and there is controversy regarding the use of such meth-
ods, but the results of the assays indicate some interesting patterns. Results
of bioassays in lakes, wetlands, and streams are considered here. Little is
known about nutrient limitation of microbial activities in groundwaters.
Surveys of tests of nutrient limitation in lakes indicate that either ni-
trogen or phosphorus most commonly limits primary production (Elser et
al., 1990a), and that many lakes are colimited by both nitrogen and phos-
phorus (Fig. 16.6A). However, CO 2 can limit phytoplankton in some cases.
Floating cyanobacteria can maintain dominance in some eutrophic lakes by
intercepting CO 2 so that it is not available to other primary producers
(Shapiro, 1997), and phytoplankton in lakes with low CO 2 can exhibit
lower photosynthetic rates (Hein, 1997). In the benthos of lakes, situations
occur in which benthic algae can be limited by CO 2 , particularly in acidi-
fied waters with low total inorganic carbon concentrations (Fairchild and
Sherman, 1990). Silicon can limit many diatom populations (Schelske and
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