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A male copepod - the base of
the tidal food pyramid
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Plankton Goals and Chlorophyll-a Criteria
The ever-increasing human population of the Chesapeake area has
used tributary and tidal waters of the watershed to dilute and ultimately
absorb or bury its waste since cities and farms were established
over three centuries ago. Those same waters were also relied on
to provide ever-larger harvests of fish and shellfish. Mid-twentieth
century, the Chesapeake ecosystem’s ability to absorb these
impacts and fulfill these functions began to fail. Algal blooms,
once a local problem, became widespread. Summer anoxia in bottom
waters extended farther and lasted longer. Turbidity increased,
killing underwater grasses and altering patterns of algal photosynthesis
and production. Recruitment of new living resources declined. Disposed
chemicals from multiple sources were accumulating to harmful levels.
As restoration efforts are made in Chesapeake Bay and its tributaries,
including the Potomac, a suite of ecological goals must be created
in order to give direction to the efforts and to provide methods
for quantitatively measuring restoration effectiveness and ecosystem
relevance. Has the ecosystem really been improved by our efforts?
What further efforts are needed to regain the fish and bird populations
we want? What do we really want, and are we willing to make the
required effort?
Plankton Goals Project
Plankton are essential members of the Chesapeake open water food
web, and link nutrients with many fish, birds and mammals. The term
plankton encompasses all bacteria and all microscopic plants and
animals that drift in open water habitats. Bay-wide plankton monitoring
programs were instituted by Chesapeake Bay Program (CBP) partners
in the mid 1980's in recognition of the plankton’s important
ecosystem roles. The CBP Monitoring and Living Resources subcommittees
subsequently supported development of plankton-based indicators
of Chesapeake Bay “health.” The plankton indicators are
currently used to interpret and communicate monitoring results.
They have highlighted deteriorating trends in the food web that
are linked to nutrient enrichment, excess sediment, low dissolved
oxygen, and over-harvesting of certain living resources. The plankton
monitoring data and indicators could also be effective means for
justifying nutrient and sediment reductions and multispecies management.
The purpose of the Plankton Goals project was to develop that justification.
In the last two years, ICPRB has lead a team of researchers to 1)
establish a Plankton Reference Community that can be measured with
the plankton indicators, and use it to assess Chesapeake plankton
“health,” and 2) identify the water quality conditions
needed to support the Plankton Reference Community. These CBP-sponsored
efforts are nearly completed. The results have already provided
much of the basis for a chlorophyll-a criteria for Chesapeake Bay
water quality standards.
Chlorophyll-a Criteria
Chlorophyll-a is a useful expression of phytoplankton biomass and
is arguably the single most responsive indicator of N [nitrogen]
and P [phosphorus] enrichment in aquatic systems, including Chesapeake
Bay.” Chlorophyll-a is the light-sensitive chemical produced
by all plants, including phytoplankton, that enables them to photosynthesize.
By its very nature, chlorophyll-a is both an integrated biological
measure of production of the primary food source of the entire Bay
food web and a critical indicator of water quality by reducing light
penetration and fueling bacterial processes leading to low dissolved
oxygen levels. Compelling evidence indicates that water clarity
and dissolved oxygen improve when excess phytoplankton, or “blooms,”
measured as chlorophyll-a, are significantly reduced. To attain
the Chesapeake Bay dissolved oxygen and water clarity criteria will
require reductions in chlorophyll-a concentrations through reduced
nutrient and sediment loadings. chlorophyll-a reductions will also
improve phytoplankton food quality for upper trophic levels. Algal
blooms and the presence of harmful species degrade phytoplankton
food quality, and lower an ecosystem “carrying capacity,”
or ability to produce and maintain a diverse array of living resources.
Restoration of living resources in Chesapeake Bay depends in large
part on how well plankton meet the nutritional needs of their fish
and bottom-dwelling consumers.
ICPRB is participating in the CBP-lead Chlorophyll Criteria Team,
and is performing many of the data analyses used by the group to
develop and quantify chlorophyll-a criteria for the new Chesapeake
Bay water quality standards. The question is “How good is good
enough?”
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