Environmental Engineering Reference
In-Depth Information
[41], and it involves incubation of sediment cores with different proportions
of
14
NO
3
−
and
15
NO
3
−
in the overlying water, at a constant total nitrate con-
centration. Such an approach would circumvent potential effects of sediment
homogenization on the rates, as often observed for other microbial processes
(e.g., [28]), but the technique with its underlying assumptions awaits practical
testing.
3.2 Anoxic Fjords and Basins
Golfo Dulce.
Golfo Dulce is a 50 km long and 10 km wide embayment on
the Pacific coast of Costa Rica with a 200 m-deep inner basin and a sill at
60
m [79]. The water chemistry, first described by Richards and coworkers [49],
is characterized by a broad anoxic, NO
3
−
-containing zone reaching from
∼
100
m depth to near the bottom with hydrogen sulfide detected only in the deepest
∼
∼
20 m [11, 49, 68]. Low-oxygen NO
3
−
-rich waters of the Eastern Tropical
North Pacific oxygen minimum zone appear regularly to spill over the sill and
disturb the stratification in the outer bay, whereas deep oxygenation has not
been observed at the head of the bay (refs. above; B. Thamdrup, T. Dalsgaard,
M.M. Jensen, J. Acu na-Gonzales, unpublished results).
Richards and coworkers [49] noted both ammonium and DIN deficiencies
in the waters of Golfo Dulce and attributed this to denitrification and anaerobic
ammonium oxidation with nitrate. These observations were repeated in 2001
and the activity of denitrification and of anammox as a sink for ammonium
was directly demonstrated through incubations [11]. Anammox was detected
at all four depths investigated between 100 and 180 m at each of two stations
(Fig. 3). Rates varied between 2 and 20 nM h
−
1
with no clear trends with
depth or between stations. In contrast, denitrification rates tended to increase
with water depth with particularly high rates at 180 m, and rates were higher
at the inner than at the outer station. As a result, the relative contribution of
anammox to N
2
production varied between 13 and 68% for individual depths.
The depth-integrated contributions from anammox were 35 and 19% at the outer
and inner station, respectively. The high rates of denitrification in the deepest
waters, which lowered the depth-integrated contributions from anammox, were
suggested to be coupled to sulfide oxidation. Except for the deepest part of
the water column, ammonium concentrations were near the detection limit,
and ammonium-limitation of anammox was indicated by 2 - 4-fold increases
in rates after ammonium addition. Thus, anammox efficiently scavenged the
ammonium released from mineralization in the water column.
Except for the sulfidic near-bottom water, the water-column chemistry of
Golfo Dulce resembles that of the oceanic oxygen deficiency zones, with el-
evated concentrations of NO
2
−
(up to 2 µM in Golfo Dulce) and very low
ammonium concentrations. The finding and significance of anammox in Golfo
