Environmental Engineering Reference
In-Depth Information
diazotrophs supply 48-530 µmolNm
−
2
d
−
1
to the upper ocean of the Atlantic
[43, 45, 79] and 93-137 µmol N m
−
2
d
−
1
to the Pacific [26, 59, 75].
5.2 N
2
Fixation in the Baltic Sea
The Baltic Sea is an estuarine environment that has anoxic basins in restricted
areas. The natural water column stratification in the Baltic Sea includes ther-
mal stratification in the summer at ca. 10-20 m depth, and permanent salinity
stratification, a halocline, that both separate the upper water masses from the
deeper (
>
60-80 m) more saline waters. During periods of several years up to
decades when the input of ocean water is limited, the water beneath the halocline
can become hypoxic or anoxic. This occurs as a result of oxygen consumed
by organic matter decomposition in the deep waters [36]. The topography of
the Baltic Sea basin defines the areas that become anoxic. The most perma-
nent anoxic areas include a few deep basins, including Gotland and Landsort
Deeps.
Widespread N
2
-fixing cyanobacterial blooms, dominated by the genera
No-
dularia
,
Aphanizomenon
, and
Anabaena,
are an annual occurrence. N
2
-fixing
cyanobacterial blooms dominated by
Nodularia
spp. and
Aphanizomenon
sp.
typically follow the approximate boundaries of the halocline. N
2
fixers in
the Baltic Sea may also include unicellular species [132]. Sediment pigment
records indicate that cyanobacterial blooms have been present in the Baltic Sea
throughout the past 8000 years [9]. The size of the blooms vary from year
to year, and although some evidence suggests blooms have intensified during
the past decades fueled by anthropogenic eutrophication [31], their periodic
occurrence appears to form a natural component of this ecosystem.
Cyanobacterial blooms in the Baltic Sea can benefit from or be dependent
upon sources of nutrients and trace elements in anoxic layers. The main con-
nection between presence of anoxic bottom layers and cyanobacterial surface
bloom intensities in the Baltic Sea lies in the supply of bioavailable P and low
N:P ratio water from anoxic layers to the euphotic zone. In the late summer,
primary productivity in the open Baltic Sea is primarily N-limited, although P
is often the secondary limiting nutrient for productivity [42, 66]. This situation
leads to a competitive advantage for the N
2
-fixing cyanobacteria. In contrast to
the overall phytoplankton productivity which is N- or N+P limited, the growth
of cyanobacteria and their rate of N
2
fixation most often appear P-limited
[81].
In oxygenated Baltic Sea water, P has a tendency to precipitate with ferric
hydroxides, while in the absence of oxygen, P is solubilized and released
from sediments as phosphate [23, 85]. Certain upwelling and frontal zones
that act by introducing PO
4
3
−
from the deep water to the euphotic zone, such
as the entrance to the Gulf of Finland, serve as seed areas for cyanobacterial
∼
