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General Introduction UvA-DARE (Digital Academic Repository) Polar phytoplankton dynamics in relation to virus and zooplankton predators Biggs, T.E.G. Publication date 2020 Document Version Other version License Other Link to publication Citation for published version (APA): Biggs, T. E. G. (2020). Polar phytoplankton dynamics in relation to virus and zooplankton predators. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:01 Oct 2021 Chapter 1 General Introduction Chapter 1 Due to their small size but numerical dominance, marine microbes contribute > 70% to total living biomass in the ocean (Bar-On et al. 2018) and the unicellular photosynthetic organisms (phytoplankton) fraction is responsible for roughly half of global net primary production (Field et al. 1998). Phytoplankton serve as the foundation of most ocean food webs, fuelling the flow of energy and matter to higher trophic levels, as well as carbon sequestration to the deep ocean (i.e. process of capturing and storing atmospheric carbon dioxide). Primary production is controlled by ‘bottom-up’ factors such as light, nutrients and temperature, and the accumulation of phytoplankton biomass (standing stock) is regulated by so called ‘top-down’ predation factors (traditionally zooplankton grazing). Not only the sum, but also the timing of primary (and secondary) production determines food availability for consumers and influences the ecosystem carrying capacity. As an alternative loss factor to zooplankton grazing, viral lysis of phytoplankton has been shown quantitatively (mortality rates) and qualitatively (selective infection) important in temperate and (sub)tropical oceans and coastal seas (Wilson et al. 2002; Brussaard 2004; Baudoux et al. 2007; Brussaard and Martínez 2008; Mojica 2015). The ecological importance of viral lysis is, however, highly understudied in cold polar waters. Even the environmental variables underlying phytoplankton dynamics are still not fully understood. An improved insight into the regulatory factors of polar phytoplankton production, as well as the most dominant type of mortality factor, is highly relevant as these processes affect trophic transfer efficiency and consequently biogoechemical cycling very differently. The low temperature polar waters are particularly sensitive to global warming (Montes-Hugo et al. 2009; Stammerjohn et al. 2012; Constable et al. 2014) and as such it is utterly warranted to obtain a better understanding of the functioning of the lower food web to predict more accurately how the oceans will respond to environmental change (Dinasquet et al. 2018). This introductory chapter provides a brief overview of the key players of the polar marine environment and their ecological roles, while providing knowledge gaps that underlie the research questions of the current thesis. 10 General Introduction 1.1 Phytoplankton Phytoplankton rely on light for photosynthesis and as such seasonality in phytoplankton production is observed in temperate and especially high latitude 1 regions (Ma et al. 2014). Environmental drivers of net primary production such as light, nutrients and temperature are, in turn, influenced by oceanic and atmospheric processes such as cloud cover, wind speed, vertical stratification strength and, in polar seas, also ice (Boyd et al. 2014). In polar oceans, phytoplankton inevitably experience extremely low to zero light availability during winter and accumulation of phytoplankton biomass only begins as light returns (Venables and Moore 2010; Venables et al. 2013). The mixed layer is still deep and average light levels are low (~ 1 µmol quanta m-2 s-1; Venables et al. 2013). As day length rises after winter (and wind speeds decline), the annual onset of vertical stratification resulting from seasonal warming and fresh water input (ice melt), further improves light conditions and generally initiates the (highly productive) phytoplankton Spring bloom (Thomalla et al. 2011; Venables et al. 2013; Llort et al. 2015; Eveleth et al. 2017). Phytoplankton can experience low light intensities again during high biomass phytoplankton blooms as a result of self-shading (Vernet et al. 2008). In response to seasonal and weather-induced light variability, phytoplankton have to adapt their photosynthetic efficiency and light saturation point (MacIntyre et al. 2002), a process of photoacclimation. Overall, the phytoplankton community response to light could be due to physiological changes within cells and populations, a shift in community or size composition (Timmermans et al. 2001; Moore et al. 2006; Arrigo et al. 2010; Alderkamp et al. 2012). Still, the mechanisms that regulate peak phytoplankton biomass and seasonal carbon flow, especially after periods of low light and in the more productive coastal regions of the Southern Ocean, are not fully understood and require further investigation. As it is the sum of production and losses over an annual cycle that determines the ecosystem carrying capacity (Sarker and Wiltshire 2017), studies that span entire productive seasons have the potential to vastly improve our understanding of phytoplankton dynamics. 11 Chapter 1 Polar phytoplankton are strongly linked to the dynamics of sea ice (Garibotti et al. 2003; Arrigo et al. 2008b; Leu et al. 2015; Moreau et al. 2015) as it not only influences light availability but also acts as a vector for seeding phytoplankton cells into the water column (Tison et al. 2010). Rapid warming in recent decades has increased ice melt (a faster retreat and melting of glaciers and ice sheets) and freshwater input as well as a decline in sea ice (Cook 2005; Vaughan 2006; Stammerjohn et al. 2012; Depoorter et al. 2013; Rignot et al. 2013) that has resulted in an earlier spring retreat and a later autumn advance, increasing the ice-free period (with high light availability) and consequently the length of the productive season (Arrigo et al. 2008b; Stammerjohn et al. 2008). Furthermore, the period of solar heating is extended (due to less sea ice) and leads to an increase in polar surface water temperatures (Meredith and King 2005; Perovich et al. 2007). Temperature imposes a fundamental control on phytoplankton metabolic processes (Raven and Geider 1988; Moisan et al. 2002) and can impose biogeographic boundaries for major taxonomic groups e.g. the absence of cyanobacteria from polar waters could be explained by a higher optimum temperature for growth than other groups of algae (Thomas 2013). The oceans serve as one of the largest natural CO2 reservoirs on earth and especially important are the cold polar oceans that take up relatively large amounts of anthropogenic CO2 (Bates et al. 2006; Arrigo et al. 2008a). Carbon dioxide generated by anthropogenic activities is one of the main drivers of global warming (Lam et al. 2012) and the global ocean heat content has been rising since the early 90s (Cheng et al. 2017, 2018), with the most rapid warming observed in polar regions (Vaughan et al. 2003; Screen and Simmonds 2010; Bromwich et al. 2013). Ocean climate models predict that warming of ocean surface waters (due to global warming) combined with increased fresh water input at high latitudes (due to increased precipitation and ice melt) will strengthen vertical stratification (Sarmiento et al. 2004; Toggweiler and Russell 2008). In the Arctic this may reduce the input of dissolved inorganic nutrients from the deep to the surface ocean, increasing the share of small-sized phytoplankton (Li et al. 2009; Tremblay and Gagnon 2009; Ardyna 12 General Introduction et al. 2011). Predictions for coastal waters of the Antarctic Peninsula are that the phytoplankton community will also change with fewer micro-sized diatoms in favour of smaller-sized cells, impacting overall phytoplankton abundance, size class 1 and taxonomic structure (Garibotti et al. 2005; Hilligsøe et al. 2011; Doney et al. 2012; Mendes et al. 2017; Rozema et al. 2017). These changes will alter the structure and functioning of the polar marine pelagic food webs, carbon cycling and ultimately carbon sequestration to the deep ocean (biological pump) (Hoegh-Guldberg and Bruno 2010). Cell size is an important functional trait that influences almost every aspect of phytoplankton biology such as physiological rates (growth, photosynthesis, respiration) and ecological function such as grazing (Finkel et al. 2010; Key et al. 2010; Marañón 2015). Smaller cells have a larger surface to volume ratio which typically gives them a competitive advantage in oligotrophic waters with relatively low nutrient availability, whilst larger cells dominate under eutrophic conditions where nutrient concentrations are higher (Marañón et al. 2001; Hirata et al. 2011). The smaller
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