Species displacement and diversity loss
Ecotype-level changes observed in this study did not only suggest
changing structures in community composition and ecosystem functioning
but also in the biogeography and distribution of some species, such as
the northward advancement of the warm-adapted chlorophytes and E.
huxleyi . These data provided new insights and information on the
potential loss in diversity and species displacement brought about by
the changes occurring in the polar region. It is not clear however if
the change in the abundant ecotype was more associated with the
increased transport of the warm water mass or the absence or loss of
ice, or both. Neukermans et al. (2018) showed a strong correlation
between the blooms of E. huxleyi in the Arctic and the increasing
sea surface temperature. Such observations are consistent with the
‘Atlantification’ event, where physico-chemical conditions in the
Eurasian Arctic are becoming more similar with that of the Atlantic
(i.e., Polyakov et al., 2017; Lind, Ingvaldsen, & Furevik, 2018).
Changes in physico-chemical regimes would also mean more favorable
conditions for Atlantic-type species to thrive in the Arctic region.
This has been documented for many macroorganisms including amphipods
(Kraft et al., 2013), the avian black legged kittiwakes (Rissa
tridactyla; Vihtakari et al., 2018), boreal fishes (Fossheim et al.,
2015) but rarely reported for microorganisms (Neukermans et al., 2018).
In the long term, these changes in microbial communities would have
profound implications to ecosystem functionality and services since
cold- or ice-associated communities have been estimated to contribute to
as high as 60% in high Arctic primary productivity (Fernández-Gómez,
Montserrat Sala, & Pedrós-Alió, 2014). Functionally, it is interesting
to note that the replacing taxa or those proliferating more during warm
anomaly has similar biogenic composition as those they replaced (e.g.,M. polaris with M. commoda ). This has significant
implications on our understanding of the biogenic matter fluxes in the
Arctic in the wake of the changing climate. Recent ice loss has also
been implicated with increased frequencies of ‘fall blooms’ based on
satellite images (Ardyna et al., 2014; Renaut, Devred, & Babin, 2018).
However, these studies were not able to identify the blooming species,
which would be important when organic matter transport is being
considered. For example, smaller cells are thought to be less efficient
in sinking, and thus, would be highly retained in the upper trophic
waters where they are regenerated resulting in less particulate
transport. Interestingly, in this study, we found high abundance of
chlorophytes and haptophytes in the sediment traps, indicating that
cells associated with these taxa could actually sink at least to the
depths of 200 to 300 m. The underlying mechanisms for such transport
however remain little explored, and roles of trophic upgrading through
biotic interactions are still unknown.
Since the phytoplankton serve as the foundation of food webs especially
in oceanic systems such as the Arctic Ocean, understanding of their
diversity, distribution, and biogeography are critical to gaining
insights into their roles and responses to the changing environment.
Here, we demonstrated that HTS-generated data could actually contribute
significant information not available through conventional means.
However, we also highlighted the importance of primer efficiency and
sensitivity in exploring and realizing such goals and further emphasized
to take caution in interpreting environmental data from gene-based
surveys.