151
discussed extensively in Chap. 4, diverse diazotrophs from heterotrophic lineages
have been identified by molecular methods in most pelagic environments (Farnelid
et al. 2011; Farnelid and Riemann 2008; Riemann et al. 2010) including the surface
waters overlying (Turk-Kubo et al. 2013), as well as within (Bonnet et al. 2013;
Fernandez et al. 2011; Löscher et al. 2014) ODZs, ocean waters below the euphotic
zone (Benavides et al. 2016, 2018a), and at latitudes outside the tropics and subtropics (where N 2 fixation is minimal or absent) (Gradoville et al. 2017).
As noted in Chap. 4 and 7, the quantitative role of N 2 -fixing heterotrophs in contributing to the input of N is currently being debated (Benavides et al. 2018a).
Heterotrophic diazotrophs have been implicated as active agents of N 2 fixation by
indirect means in several systems including through positive correlation with
organic matter pools in the subsurface waters of the Mediterranean (Benavides et al.
2016, 2018b), stimulation of N 2 fixation activity by organic substrate additions
within the ETSP (Bonnet et al. 2013; Löscher et al. 2014), in the Baltic Sea (Farnelid
et al. 2013) and in the central south Pacific gyre (Halm et al. 2012), and by detection
of the presence of nifH transcripts of heterotrophic diazotrophs in the latter two
studies (Farnelid et al. 2013; Halm et al. 2012). In any event, the contribution of
heterotrophic N 2 fixation should be captured by the geochemical proxies used for
many of the estimates.
Benthic Studies
The importance of shallow benthic systems including seagrass, salt marshes, coral
reefs, microbial mats and unvegetated sediment ecosystems to marine N 2 fixation
(Fig. 4.1) received considerable attention with respect to their biogeochemical
importance during the early burst of field research in the 1970s and 80s. Research
during that period has been summarized in several earlier reviews (Capone 1983a,
1988; Howarth et al. 1988). Available field estimates of in situ rates were aggregated
to derive averages for specific types of systems and then scaled for estimates of the
areal coverage of these systems (Capone 1983a). A global estimate of about 15 Tg
y
−1
was derived, a value that has been widely adopted (e.g. (Codispoti 2007; Gruber
2004). The review by Howarth et al. (1988), particularly focused on salt marsh systems and proposed that the input from N 2 fixation from those systems was considerably less than that previously suggested (Capone 1983b).
Aspects of N 2 fixation have been covered in more recent studies in several benthic systems with foci on contributions to the demand of primary productivity, diazotroph biodiversity and host nutrition in associations (e.g. corals and seagrasses)
(see also Chap. 4).
However, efforts on N 2 fixation in benthic habitats from a biogeochemical perspective are more limited. (Andersson et al. 2014) surveyed 60 randomly selected
sites along the west coast of Sweden and found sediment N 2 fixation to be a widespread activity, inhibited by wave action and high levels of combined N but positively correlated with salinity and PO 4
−3
concentrations. Membrane inlet mass
8.2 Basin and Global Scale Inputs
discussed extensively in Chap. 4, diverse diazotrophs from heterotrophic lineages
have been identified by molecular methods in most pelagic environments (Farnelid
et al. 2011; Farnelid and Riemann 2008; Riemann et al. 2010) including the surface
waters overlying (Turk-Kubo et al. 2013), as well as within (Bonnet et al. 2013;
Fernandez et al. 2011; Löscher et al. 2014) ODZs, ocean waters below the euphotic
zone (Benavides et al. 2016, 2018a), and at latitudes outside the tropics and subtropics (where N 2 fixation is minimal or absent) (Gradoville et al. 2017).
As noted in Chap. 4 and 7, the quantitative role of N 2 -fixing heterotrophs in contributing to the input of N is currently being debated (Benavides et al. 2018a).
Heterotrophic diazotrophs have been implicated as active agents of N 2 fixation by
indirect means in several systems including through positive correlation with
organic matter pools in the subsurface waters of the Mediterranean (Benavides et al.
2016, 2018b), stimulation of N 2 fixation activity by organic substrate additions
within the ETSP (Bonnet et al. 2013; Löscher et al. 2014), in the Baltic Sea (Farnelid
et al. 2013) and in the central south Pacific gyre (Halm et al. 2012), and by detection
of the presence of nifH transcripts of heterotrophic diazotrophs in the latter two
studies (Farnelid et al. 2013; Halm et al. 2012). In any event, the contribution of
heterotrophic N 2 fixation should be captured by the geochemical proxies used for
many of the estimates.
Benthic Studies
The importance of shallow benthic systems including seagrass, salt marshes, coral
reefs, microbial mats and unvegetated sediment ecosystems to marine N 2 fixation
(Fig. 4.1) received considerable attention with respect to their biogeochemical
importance during the early burst of field research in the 1970s and 80s. Research
during that period has been summarized in several earlier reviews (Capone 1983a,
1988; Howarth et al. 1988). Available field estimates of in situ rates were aggregated
to derive averages for specific types of systems and then scaled for estimates of the
areal coverage of these systems (Capone 1983a). A global estimate of about 15 Tg
y
−1
was derived, a value that has been widely adopted (e.g. (Codispoti 2007; Gruber
2004). The review by Howarth et al. (1988), particularly focused on salt marsh systems and proposed that the input from N 2 fixation from those systems was considerably less than that previously suggested (Capone 1983b).
Aspects of N 2 fixation have been covered in more recent studies in several benthic systems with foci on contributions to the demand of primary productivity, diazotroph biodiversity and host nutrition in associations (e.g. corals and seagrasses)
(see also Chap. 4).
However, efforts on N 2 fixation in benthic habitats from a biogeochemical perspective are more limited. (Andersson et al. 2014) surveyed 60 randomly selected
sites along the west coast of Sweden and found sediment N 2 fixation to be a widespread activity, inhibited by wave action and high levels of combined N but positively correlated with salinity and PO 4
−3
concentrations. Membrane inlet mass
8.2 Basin and Global Scale Inputs