130
7.8 Estuaries and Inland Seas
N 2 fixation has been found in a relatively limited range of estuarine or brackish waters,
and has generally been restricted to the lower salinity reaches of those estuaries (<10 on
the practical salinity scale) (Howarth et al. 1988; Smith 1990). However, it has been
shown that Nodularia can fix N 2 at full salinity and that it is not strictly the factor that
constrains the distribution of N 2 -fixing cyanobacteria in estuaries (Moisander and Paerl
2000; Paerl et al. 1996).
As mentioned above, N 2 fixation is a major fraction of the N budget of the low
salinity Baltic (Larsson et al. 2001), driven by seasonal N 2 fixation supported by P
inputs coupled with lower salinities (Wasmund et al. 2001). In the Baltic, Nodularia,
Aphanizomenon and Dolichospermum form dense blooms and account for most of
the N 2 fixation there (Olofsson et al. 2020) (Fig. 4.3a). Some bloom-forming cyanobacterial species in the Baltic can be toxic (Sivonen et al. 1989). It has been hypothesized that the blooms are stimulated by P that enters the Baltic from the ocean and
from its release from anoxic sediments (Bianchi et al. 2000).
The history of blooms in the Baltic has been an area of some debate. It has been
argued that heterocyst-forming cyanobacteria have long been a characteristic feature of
the Baltic Sea since its reopening to the ocean after the last glaciation (Bianchi et al.
2000). Others contend that the blooms are a much more recent phenomenon and have
recently intensified due to increasing anthropogenic inputs since the industrial age
(Finni et al. 2001). Similarly, N 2 fixation could account for about 17% of the N inputs
to the Peel-Harvey estuary in southwest Australia (Huber 1986) where Nodularia
spumigena formed large blooms over several summers in the late 1970s and early 1980s.
Less is known of the distribution of non-heterocyst-forming N 2 -fixing cyanobacteria (Wasmund et al. 2001) in estuaries and brackish waters. UCYN-A has now
been reported in the Great Belt at the entrance to the Baltic (Bentzon-Tilia et al.
2015). UCYN-A was also found in the Spencer Gulf inverse estuary in Southern
Australia (Messer et al. 2015a).
In addition to N 2 -fixing cyanobacteria, NCDs are diverse and abundant in estuaries such as the Chesapeake Bay and the Neuse River (Affourtit et al. 2001; Jenkins
et al. 2004) and in the Baltic (Farnelid et al. 2009, 2013) even though N 2 fixation
rates by these diazotrophs can be low. Particles may be important in providing a
suitable low O 2 environment (Pedersen et al. 2018). N 2 fixation has been reported in
estuarine sediments associated with NCDs (Newell et al. 2016; Burns et al. 2002),
although intriguingly nifH transcripts of the cyanobacteria UCYN-A were also
reported (Brown and Jenkins 2014).
7.9 High Latitudes Including the Arctic
As mentioned above, it was generally held that oceanic N 2 fixation was constrained to
warm oligotrophic waters until symbiotic UCYN-A and NCD nifH gene sequences
were detected in cooler waters and at higher latitudes of the North and South Pacific
7 Biogeography of N 2 Fixation in the Surface Ocean
7.8 Estuaries and Inland Seas
N 2 fixation has been found in a relatively limited range of estuarine or brackish waters,
and has generally been restricted to the lower salinity reaches of those estuaries (<10 on
the practical salinity scale) (Howarth et al. 1988; Smith 1990). However, it has been
shown that Nodularia can fix N 2 at full salinity and that it is not strictly the factor that
constrains the distribution of N 2 -fixing cyanobacteria in estuaries (Moisander and Paerl
2000; Paerl et al. 1996).
As mentioned above, N 2 fixation is a major fraction of the N budget of the low
salinity Baltic (Larsson et al. 2001), driven by seasonal N 2 fixation supported by P
inputs coupled with lower salinities (Wasmund et al. 2001). In the Baltic, Nodularia,
Aphanizomenon and Dolichospermum form dense blooms and account for most of
the N 2 fixation there (Olofsson et al. 2020) (Fig. 4.3a). Some bloom-forming cyanobacterial species in the Baltic can be toxic (Sivonen et al. 1989). It has been hypothesized that the blooms are stimulated by P that enters the Baltic from the ocean and
from its release from anoxic sediments (Bianchi et al. 2000).
The history of blooms in the Baltic has been an area of some debate. It has been
argued that heterocyst-forming cyanobacteria have long been a characteristic feature of
the Baltic Sea since its reopening to the ocean after the last glaciation (Bianchi et al.
2000). Others contend that the blooms are a much more recent phenomenon and have
recently intensified due to increasing anthropogenic inputs since the industrial age
(Finni et al. 2001). Similarly, N 2 fixation could account for about 17% of the N inputs
to the Peel-Harvey estuary in southwest Australia (Huber 1986) where Nodularia
spumigena formed large blooms over several summers in the late 1970s and early 1980s.
Less is known of the distribution of non-heterocyst-forming N 2 -fixing cyanobacteria (Wasmund et al. 2001) in estuaries and brackish waters. UCYN-A has now
been reported in the Great Belt at the entrance to the Baltic (Bentzon-Tilia et al.
2015). UCYN-A was also found in the Spencer Gulf inverse estuary in Southern
Australia (Messer et al. 2015a).
In addition to N 2 -fixing cyanobacteria, NCDs are diverse and abundant in estuaries such as the Chesapeake Bay and the Neuse River (Affourtit et al. 2001; Jenkins
et al. 2004) and in the Baltic (Farnelid et al. 2009, 2013) even though N 2 fixation
rates by these diazotrophs can be low. Particles may be important in providing a
suitable low O 2 environment (Pedersen et al. 2018). N 2 fixation has been reported in
estuarine sediments associated with NCDs (Newell et al. 2016; Burns et al. 2002),
although intriguingly nifH transcripts of the cyanobacteria UCYN-A were also
reported (Brown and Jenkins 2014).
7.9 High Latitudes Including the Arctic
As mentioned above, it was generally held that oceanic N 2 fixation was constrained to
warm oligotrophic waters until symbiotic UCYN-A and NCD nifH gene sequences
were detected in cooler waters and at higher latitudes of the North and South Pacific
7 Biogeography of N 2 Fixation in the Surface Ocean