Environmental Engineering Reference
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MERL
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FIGURE 5.14
MERL nutrient and solid enrichment experiment and pollutant loading.
what loading rates were associated with extreme ecosystem degradation
(Oviatt et al. 1987, Oviatt et al. 1986 and Maughan and Oviatt 1993). Thus
it was possible to identify the impact significant criteria for the loading of
many pollutants with respect to the bright line between acceptable and
unacceptable impact on estuarine systems. For some pairings of pollut-
ant loading rates and responses, there was a predictable gradient and it
was possible to identify a series of quantifiable significant impact criteria
(in terms of loading rates of a pollutant) associated with various levels of
impact.
The relationship of the benthic community to organic carbon loading from
the combination of carbon in the solids and from the increased primary
production stimulated by nutrient additions is an example of a quantifiable
gradient response (Maughan and Oviatt 1993). At low organic carbon load-
ing rates, the benthic community characteristics were similar to those in the
control mesocosms and lower Narragansett Bay, which received no nutrient
additions. However at the highest loading rates, the community was signifi-
cantly different from the controls with a total density of benthic organisms
up to four times the density measured in controls, domination by pollution-
tolerant species with such species occurring at over 300 times the density
seen in controls, and at the end of the experiment the assimilative capacity
of the ecosystem was exceeded and mass mortality of the benthic commu-
nity occurred. Some of the pollution-tolerant species showed a strong linear
correlation with the combined organic carbon input to the mesocosm (cor-
relation coefficient greater than 0.9), which permitted prediction of benthic
community characteristics at given organic loading rates.
The MERL experiments gave solid scientific backing for the identification of
pollution loading rates associated with various levels of environmental impact
from municipal wastewater treatment and discharge approaches (i.e., impact
significant criteria). However, there was a degree of uncertainty with the results
because of inherent limitation with mesocosms and concerns with applicability
to ecosystems different from Narragansett Bay. These concerns were addressed
by a comparison of MERL experimental results to field investigations reported
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