Environmental Engineering Reference
In-Depth Information
the stream is about equal. Two main processes reduce the DOC concentration in the soil-
sorption to minerals and respiration by microbes (
McDowell and Wood 1984; Bernhardt
and McDowell 2008
).
A Tidal, Freshwater River
The Hudson River originates at Lake Tear of the Clouds in the Adirondack Mountains
of New York state. The Mohawk River, which drains part of central New York, joins the
Hudson near Troy, NY. After flowing over a small dam at Green Island, basically on the
site of the original fall line of this river, the Hudson estuary flows south 240 km where it
joins the sea at the southern tip of Manhattan Island. The Hudson estuary is navigable
and at sea level for its length and is influenced by tides up to the dam at Green Island.
Only the final southern 40 km of the river contain brackish (meaning salinity from 0.5 to
30 psu) water most the time. Occasionally, during dry summers, the salt front is pushed
farther north, but most of the time the upper 150 to 200 km of the Hudson is tidal, fresh-
water, and relatively well mixed top to bottom in the 8 to 9 m deep (on average) water
column (
Cole et al. 1992
).
Unlike the forest at Hubbard Brook, the C inputs to the Hudson River from its water-
shed (650 g C m
2
2
y
2
1
;
Howarth et al. 1996; Caraco and Cole 2004
) dominate the organic
C balance and exceed riverine primary production by about two-fold. The major primary
producers within this system are phytoplankton and macrophytes. Phytoplankton GPP
(prior to the recent introduction of the exotic zebra mussel) was about 330 g C m
2
2
y
2
1
and a large fraction of this (280 g C m
2
2
y
2
1
or 85%) was respired by the phytoplankton
themselves. So the net input (or NPP) of phytoplankton was about 50 g C m
2
2
y
2
1
and co-equal to the NPP of the two dominant macrophytes (Vallisneria and Trapa)of30
to 40 g C m
2
2
y
2
1
. Bacteria dominated the respiratory output (116 g C m
2
2
y
2
1
)andactu-
ally respired more organic C than the combined inputs from NPP of phytoplankton and
macrophytes. Benthic invertebrates (again prior to the zebra mussel invasion) accounted
for only about 10% of bacterial respiration (9 g C m
2
2
y
2
1
). NEP for this ecosystem was
negative,
43 g C m
2
2
y
2
1
, and there was a large export of the organic C not consumed
by respiration (607 g C m
2
2
y
2
1
).
The appearance of the invasive zebra mussel (Dreissena polymorpha) in 1992 had a large
effect on the C balance. The respiration of zebra mussels added some 83 g C m
2
2
y
2
1
to R
h
and greatly reduced, by its huge filtration rates, the standing stock and primary produc-
tion of phytoplankton (to NPP of about 12 g C m
2
2
y
2
1
if we correct for 24-hour algal
respiration). Thus, following the zebra mussel
invasion in 1982, NEP of the Hudson
153 g C m
2
2
y
2
1
) and downstream export somewhat
smaller (497 g C m
2
2
y
2
1
;
Caraco et al. 1997; Caraco and Cole 2004
).
In summary, compared to the Hubbard Brook Experimental Forest, this river has much
less autochthonous primary production and much larger allochthonous inputs. Further, to
a first approximation the river does not store C in its sediments, apart from some small
depositional areas covering much less than 1% of the river's bottom. The forest is net auto-
trophic; GPP
became even more negative (to
R and some organic C is sequestered and net exported. The river is
.
