Environmental Engineering Reference
In-Depth Information
than 3 mg l
−
1
. Associated nutrient data indicated that the dominant form of
nitrogen shifts from nitrate to ammonium as anoxia is approached (as in Fig.
10). The lower denitrification rates where oxygen was less than 1 mg l
−
1
may be
due to nitrate limitation or an increase in the competitive advantage of microor-
ganisms capable of dissimilatory nitrate reduction to ammonium. Suppression
of denitrification at low oxygen concentrations will increase the residence time
of bioavailable nitrogen and could act as a positive feedback mechanism in the
formation of hypoxic bottom waters.
Limited data from benthic flux chambers deployed in hypoxic waters in
Louisiana indicated a net flux of dissolved inorganic nitrogen into the sediments
as a result of consumption of nitrate and nitrite (-1.2 mmol (NO
3
+NO
2
)m
−
2
d
−
1
) and an efflux of ammonium (2.5 mmol NH
4
+
m
−
2
d
−
1
) [41]. The overall
net efflux of DIN was higher in waters with dissolved oxygen concentrations
greater than 2 mg l
−
1
but still below saturation.
Preliminary measurements of nitrous oxide in a range of dissolved oxygen
concentrations in the hypoxic area of the Louisiana shelf indicate that it is
produced in low oxygen waters (M.B. Westley et al. unpubl. data). N
2
Owas
sometimes present in waters at 5 µMO
2
or less, and its concentration was
sometimes as high as two to three times higher than saturation levels. The
concentration of N
2
O increased with depth through a 20-m water column as
dissolved oxygen declined with depth. On the other hand, Childs et al. [7] found
no nitrous oxide production in any of their experimental overlying water.
Higher phosphate and silicate concentrations at the lower dissolved oxygen
levels are consistent with strong fluxes from the sediments as they become
anoxic or near anoxic. Higher dissolved silicate fluxes from the sediments
under hypoxic bottom waters in Chesapeake Bay were related to the flux of
organic matter from surface waters, but only one month after deposition events
(shown as elevated sediment chlorophyll
a
concentrations) [4]. One expects
then that the stations with higher silicate concentrations in bottom waters (Fig.
10) received a greater flux of silicate-based organic material in prior months.
Reduced forms of metals are often released from sediments during severe
hypoxia. Mallini [25] found that dissolved Mn concentrations rapidly increased
as oxygen concentrations fell below 63 µM. Extremely high dissolved Mn
concentrations (1000 to 3200 µM) occurred in oxygen concentrations from 10
µM to anoxia.
The regeneration of nutrients in the lower water column or from the sediments
could contribute to further nutrient-enhanced production in the upper mixed
layer, if they diffuse or mix in significant amounts across the strong pycnocline
present much of the year. The rates of these processes, if they occur, are not
known, nor have they been estimated. Preliminary examination of vertical
nutrient profiles does not indicate a transfer of high nutrients in hypoxic bottom
waters into the overlying water column on the Louisiana shelf (N.N. Rabalais
