Abstract
The European Arctic is rapidly changing where increasing water temperatures and rapid loss of sea-ice will likely influence the structure and functioning of the entire ecosystem. This study aimed to describe the taxonomic composition and spatial distribution of early summer (2015-2016) nano- and microplanktonic protists in the Nordic (Norwegian, Greenland) Seas and the Fram Strait (70.99°N to 78.84°N; 1.52°E to 19.90°E) and to determine the distribution patterns of the communities from the aspect of hydrography, as deduced from in situ measurements. Here we identify some generalized regularity in the protistan distribution, indicating the two separated domains at the 6 °C threshold. While Phaeocystis seemed to be a fairly conservative representative of the colder area ( 6 °C) may vary significantly between successive summers: from mostly Bacillariophyceae-dominated communities in 2015 to flagellate-dominated in 2016. Based on our results, we hypothesised that the more intense phototrophs development in the area, as deduced from higher remotely sensed chlorophyll a concentrations in 2016, i.e., record warm year in the observational period, could lead to faster depletion of nutrients, and thus, an earlier shift into the post-bloom community stage. Taking into account the possible phenological shift towards early summer domination of flagellates in a warmer year, as well as a higher number of heterotrophic protists associated with the warmer domain in two evaluated summers, it is highly likely that climatic warming of this region will have an impact on energy transfer to higher trophic levels. Although generalized patterns could be elucidated, more information is needed to predict and understand how the changing Arctic will alter protistan communities, and thus, higher-order consumers.
Highlights
The Nordic Seas (Greenland, Iceland, and Norwegian Seas) cover a large area south and north of the Arctic Circle
The second branch, the West Spitsbergen Current (WSC), continues northward along the continental slope. This flow is called the eastern branch of the WSC or the core of the WSC; correspondingly, the continuation of the Faroe branch linked with the oceanic frontal system is called the western branch of the WSC (Walczowski, 2013). These two currents and hydrological fronts related to both branches of the WSC create the dynamic borders of the region occupied by the Atlantic water (AW)—the Atlantic domain of the Nordic Seas
The analysis of the CTD data from the upper (50 m) water layer demonstrated that the mean seawater temperature and salinity for the whole region were slightly higher in 2016 than in 2015 (+0.29◦C and +0.045, respectively); the warmer conditions in 2016 were reflected in the less sea ice concentrations in this area (Figures 1, 2A,B)
Summary
The Nordic Seas (Greenland, Iceland, and Norwegian Seas) cover a large area south and north of the Arctic Circle. In situ determined relationships between environmental factors and protists are well-documented mainly for the West Spitsbergen fjords and the adjacent waters (e.g., Keck et al, 1999; Wiktor, 1999; Owrid et al, 2000; Piwosz et al, 2009, 2015; Hegseth and Tverberg, 2013; Kubiszyn et al, 2014, 2017; Smoła et al, 2017; Hegseth et al, 2019) Those studies have indicated several potential responses of protist communities to the ongoing environmental changes regarding shifts in species sizes toward pico- and nanoplanktonic organisms, promoting flagellates and reducing the share of microplanktonic primary producers during spring bloom period (Hegseth and Tverberg, 2013; Piwosz et al, 2015; Kubiszyn et al, 2017); functional structure with the prevalence of microplanktonic grazers (especially ciliates and dinoflagellates: Seuthe et al, 2011; Mayzaud et al, 2013; Kubiszyn et al, 2014); biogeographical patterns with the northward expansion of boreal species such as coccolithophores (Hegseth and Sundfjord, 2008; Dylmer et al, 2015); and phenology due to earlier ice breakup and onset of the phytoplankton bloom (Søreide et al, 2010; Hodal et al, 2012). This is all the more important that the recent studies revealed potential climate-generated changes in the productivity of the Atlantic–Arctic sector, including slight trends toward higher productivity in the eastern Fram Strait (Nöthig et al, 2015), a considerable influence of AW advected phytoplankton on the local in situ primary production (Vernet et al, 2019), and a shift in the summer months plankton composition from large diatoms toward Phaeocystis pouchetii and other small flagellates (e.g., Lalande et al, 2013; Nöthig et al, 2015)
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