Environmental Engineering Reference
In-Depth Information
in phosphorus in lakes and estuaries (associated with phosphates in detergents and fertili-
zers) can stimulate high rates of population growth by N-fixing blue-green algae (cyano-
bacteria). These N-fixers are often unpalatable to zooplankton, and despite high rates of
primary production, this shift in algal species composition can lead to less available edible
food for consumers. In oceans, increases in N from riverine inputs associated with fertil-
izer application and human wastewater can lead to algal blooms as well. When these algal
blooms die and sink to deeper layers and decompose, the increased decomposition can
lead to very low concentrations of oxygen, which can be harmful to fishes and other
aquatic organisms. Loss of the aerobic conditions above the sediments also cuts off the
coupled nitrification
denitrification process that operates in sediments with an aerobic
surface layer. Loss of this process creates a positive feedback as ammonium produced by
mineralization (which occurs under both aerobic and anaerobic conditions) diffuses back
into the water column where it can contribute to further increases in primary production.
This switch from aerobic to anaerobic conditions can occur seasonally; in summer, decom-
position of organic material produced during spring “bloom” conditions can deplete oxy-
gen at depth in the water column leading to completely anaerobic sediments (
Figure 7.8
).
Sediments can also be made anaerobic by direct additions of organic matter to open water
FIGURE 7.8
Sediment N dynam-
ics in the mesohaline portion of the
Chesapeake Bay. (a) Seasonal varia-
tion in net fluxes of ammonium and
nitrate across the sediment
SEDIMENT-WATER FLUXES
(a)
400
water
interface. (b) Seasonal variation in
net nitrification and denitrification
in sediments. Nitrogen dynamics are
driven by oxygen availability, which
is low in summer. (From
Kemp et al.
1990
.)
300
200
Ammonium
100
0
Nitrate
-100
MAM J J A SON
Month of year
N PROCESSES
(b)
60
Nitrification
40
Denitrification
20
0
MAM J J A SON
Month of year
