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.