Source of Organics and Water Quality
From: Andy Soos, ENN.com
It is not unusual that
when it rains, it will dissolve surface materials or carny it off as suspended
materials into steams and such. Each time it rains, runoff carries an earthy
tea steeped from leaf litter, crop residue, soil, and other organic materials
into the storm drains and streams that feed Chesapeake Bay or many other bodies
of water.
Apparently some sources of organics are worse than others. A new study led by researchers at William & Mary's Virginia Institute of Marine Science reveals that land use in the watersheds from which this dissolved organic matter originates has important implications for Bay water quality, with the organic carbon in runoff from urbanized or heavily farmed landscapes more likely to persist as it is carried downstream, thus contributing energy to fuel low-oxygen dead zones in coastal waters.
Apparently some sources of organics are worse than others. A new study led by researchers at William & Mary's Virginia Institute of Marine Science reveals that land use in the watersheds from which this dissolved organic matter originates has important implications for Bay water quality, with the organic carbon in runoff from urbanized or heavily farmed landscapes more likely to persist as it is carried downstream, thus contributing energy to fuel low-oxygen dead zones in coastal waters.
The new study appears in this month's issue of the Journal of Geophysical Research, and was highlighted by the journal's publisher, the American Geophysical Union, as an AGU Research Spotlight in their print and online channels.
Low-oxygen dead zones
are a growing problem in Chesapeake Bay and coastal ecosystems worldwide. While
most management practices focus on reducing inputs of nitrogen and other
nutrients known to fuel dead zones, Canuel says "organic matter from the
watershed may also contribute. One goal of our study was to examine the quality
of organic matter derived from streams and its potential to contribute to
dead-zone formation."
In the 1970s, the
Chesapeake Bay was discovered to contain one of the planet's first identified
marine dead zones, where hypoxic waters were so depleted of oxygen that they
were unable to support life, resulting in massive fish kills. This results in
part from large algal blooms, which are nourished by the runoff of residential,
farm and industrial waste throughout the watershed.
As streams and rivers
carry dissolved organic matter downstream, bacteria or sunlight can modify it
into compounds and forms that are more difficult for organisms to use. While
the team's research showed no significant difference in bacterial degradation
of organic matter from cleared or forested watersheds, Canuel says it did show
that "organic carbon in runoff from watersheds affected by human activity
is less susceptible to solar degradation than that from forested
watersheds."
Urban organics thus
remain at higher levels longer, says Canuel, "delivering more organic
material to the river mouth and increasing the likelihood that low-oxygen
conditions will develop in downstream locations such as estuaries and the
coastal ocean."
The authors aren't yet
sure why the organic carbon from the more developed watersheds is less
vulnerable to breakdown by sunlight in rivers and streams, but suggest that it
might be because it has already been exposed to appreciable sunlight in the
less shady urban and agricultural environment.
Says Canuel, "Urban
organics may persist downstream because their more photoreactive compounds have
already been degraded due to greater light exposure in urban areas, farm
fields, and pastures, leaving only the more photo-resistant, refractory
compounds to wash into the coastal zone."
"Our results show
that future studies should assess not only the quantity of dissolved organic
carbon entering our rivers and streams, but also its source," says Canuel.
"Understanding how organic matter from developed and undeveloped
watersheds behaves in the aquatic environment will contribute to the
development of more effective watershed management practices and hopefully more
successful efforts to reduce the number, extent, and duration of low-oxygen
dead zones."
For further information
see Carbon Sources.