127
heterogeneous distributions of N 2 -fixing species were due to nutrient (P) intrusions
associated with entrainment of deep water in mesoscale features (Robidart et al.
2014). In another study, an autonomous vehicle equipped with holographic imaging
(LISST-Holo) and a pulse amplitude modulated (PAM) fluorometer suggested that
local blooms of symbiotic diatoms in the remote Eastern Tropical North Pacific were
not limited by Fe availability (Anderson et al. 2018). The development of instrumentation will help to increase the spatial resolution of data on diazotrophs at all scales.
7.5 Coastal and Upwelling Regions
In early field studies, N 2 fixation was generally not considered and is not as welldescribed in coastal and upwelling regions, perhaps because of the enriched nutrients (Chap. 3) and sometimes lower temperatures. However, UCYN-A nifH
sequences and cells visualized by FISH (or CARD-FISH) have now been reported
in diverse geographic areas in addition to the open ocean, including the mouth of the
Chesapeake Bay (Short and Zehr 2007) and coastal waters off eastern North America
(Mulholland et al. 2012, 2019; Tang et al. 2019), the California Coastal Current
(Hamersley et al. 2011; Needoba et al. 2007) and the Danish Strait (Bentzon-Tilia
et al. 2015). UCYN-A cells are actively fixing N 2 in nitrate-enriched coastal
California waters as determined using nanoSIMS (Mills et al. 2020). Low rates of
N 2 fixation along with nifH genes were found in areas of active upwelling in the
eastern (Moreira-Coello et al. 2017) and the equatorial Atlantic Ocean (Subramaniam
et al. 2013). Diazotrophy occurs in several major river plumes (for example, the
Amazon and Mekong Rivers) (Bombar et al. 2011; Foster et al. 2007a; Grosse et al.
2010; Subramaniam et al. 2008). The diazotrophs found in these coastal regions are
generally dominated by UCYN-A and NCDs, although Trichodesmium and diatom
symbionts are common in the more saline reaches of the Amazon River (Foster et al.
2007b) and Mekong (Bombar et al. 2011) river plumes.
Better resolving the extent of N 2 fixation in coastal waters is an important area for
current and future research. New approaches to sampling rates at very high spatial resolution are already providing unprecedented coverage in these regions (Tang et al. 2019).
7.6 Seasonality
The seasonality of climate and ocean conditions that affect water temperature and
nutrients increases as one moves away from the tropics, with dynamic shifts in
microbial assemblages, including the diazotrophs (Fig. 7.7). Temporal patterns of
N 2 fixation and diazotrophs associated with seasonal cycles are poorly characterized because of the logistical challenges and expense of sampling the ocean frequently. Thus, there are few long-term data sets. Perhaps the best existing records
for seasonal oceanic N 2 fixation are those collected at the time-series Station
ALOHA off of Hawaii (Böttjer et al. 2017; Church et al. 2005, 2009) where
7.6 Seasonality
heterogeneous distributions of N 2 -fixing species were due to nutrient (P) intrusions
associated with entrainment of deep water in mesoscale features (Robidart et al.
2014). In another study, an autonomous vehicle equipped with holographic imaging
(LISST-Holo) and a pulse amplitude modulated (PAM) fluorometer suggested that
local blooms of symbiotic diatoms in the remote Eastern Tropical North Pacific were
not limited by Fe availability (Anderson et al. 2018). The development of instrumentation will help to increase the spatial resolution of data on diazotrophs at all scales.
7.5 Coastal and Upwelling Regions
In early field studies, N 2 fixation was generally not considered and is not as welldescribed in coastal and upwelling regions, perhaps because of the enriched nutrients (Chap. 3) and sometimes lower temperatures. However, UCYN-A nifH
sequences and cells visualized by FISH (or CARD-FISH) have now been reported
in diverse geographic areas in addition to the open ocean, including the mouth of the
Chesapeake Bay (Short and Zehr 2007) and coastal waters off eastern North America
(Mulholland et al. 2012, 2019; Tang et al. 2019), the California Coastal Current
(Hamersley et al. 2011; Needoba et al. 2007) and the Danish Strait (Bentzon-Tilia
et al. 2015). UCYN-A cells are actively fixing N 2 in nitrate-enriched coastal
California waters as determined using nanoSIMS (Mills et al. 2020). Low rates of
N 2 fixation along with nifH genes were found in areas of active upwelling in the
eastern (Moreira-Coello et al. 2017) and the equatorial Atlantic Ocean (Subramaniam
et al. 2013). Diazotrophy occurs in several major river plumes (for example, the
Amazon and Mekong Rivers) (Bombar et al. 2011; Foster et al. 2007a; Grosse et al.
2010; Subramaniam et al. 2008). The diazotrophs found in these coastal regions are
generally dominated by UCYN-A and NCDs, although Trichodesmium and diatom
symbionts are common in the more saline reaches of the Amazon River (Foster et al.
2007b) and Mekong (Bombar et al. 2011) river plumes.
Better resolving the extent of N 2 fixation in coastal waters is an important area for
current and future research. New approaches to sampling rates at very high spatial resolution are already providing unprecedented coverage in these regions (Tang et al. 2019).
7.6 Seasonality
The seasonality of climate and ocean conditions that affect water temperature and
nutrients increases as one moves away from the tropics, with dynamic shifts in
microbial assemblages, including the diazotrophs (Fig. 7.7). Temporal patterns of
N 2 fixation and diazotrophs associated with seasonal cycles are poorly characterized because of the logistical challenges and expense of sampling the ocean frequently. Thus, there are few long-term data sets. Perhaps the best existing records
for seasonal oceanic N 2 fixation are those collected at the time-series Station
ALOHA off of Hawaii (Böttjer et al. 2017; Church et al. 2005, 2009) where
7.6 Seasonality