Schematic dinoflagellate life cycle. A) Haploid phase where individual cells divide through mitosis. This is the vegetative stage, which leads to the formation of bloom events. Following an internal and/or external stimuli, gametes are formed and will fuse to produce a diploid zygote; B) Planozygote (pelagic motile zygote) phase where the diploid cell will eventually lose motility, and in which the cyst will develop and eventually be released in the water column, then down to the sediment surface; C) Hypnozygote stage where the cyst will undergo a mandatory dormancy period controlled by an endogenous clock before excystment takes place. Modified after Evitt [31]. 

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... are single-celled eukaryotic organisms that inhabit both freshwater and marine environments from equatorial to polar latitudes. In marine environments, they constitute together with diatoms and coccolithophores an important group of primary producers. About half of the extant dinoflagellates are autotrophic, or mixotrophic (combination of phototrophy and heterotrophy), while the other half are heterotrophic, and some are parasitic. Approximately 10-15% of dinoflagellates form a cyst composed of a highly resistant polymer, as part of their life cycle [1]. The cyst stage, or hypnozygote, is believed to be a benthic phase in the life cycle of the dinoflagellates [2] and can remain dormant in the sediments from several days to years. Quaternary dinoflagellate cysts, known as dinocysts, assemblages are widely used by marine palynologists as a proxy indicator of conditions in the upper part of the water column [3, 4, 5], eutrophication [6, 7, 8] and productivity [9, 10]. Over the last few decades, the distribution of dinocyst assemblages in the surface sediments of the ocean basins, especially the Arctic basins, have been the subject of numerous studies in an effort to develop their potential for paleoceanographical reconstructions [11, 12, 13, 14, 15, 16, 17]. So far, approximately 31 dinocyst taxa have been observed in surface sediments of the Canadian Arctic [3, 14, 41, 45]. Several studies reported the distribution of the vegetative motile forms of dinoflagellates in the Arctic [18, 19, 20, 21, 22, 23, 24, 25, 25, 26, 27] and ~192 taxa belonging to 42 genera have been recorded from the water column of the Canadian High Arctic [18, 23, 24, 24, 27, 27, 28, 29]. However, our knowledge of the relationships between the plankton motile forms and their cyst counterpart in the sediments is rather limited. For example, the cyst stage of several motile forms collected from the phytoplankton have not been recorded in surface sediments yet, while several motile forms related to the known Arctic cyst taxa have not been observed in Arctic phytoplankton samples or described yet. The knowledge of the biological affinity or cyst-theca relationships for Arctic dinoflagellates is important because it provides useful information on dinoflagellate life cycle. In addition, the increasing understanding of the motile form will prove extremely useful for paleoceanographical reconstructions and interpretation. A better knowledge of the distribution of the dinoflagellate motile forms will also provide hints on the transport mechanisms affecting the dinocysts after their formation and subsequent deposition in the surface sediments. Dinoflagellates have a relatively complex life cycle that includes a motile plankton or vegetative stage, and some include a resting or cyst stage (Figure 1). The motile forms are characterized by the presence of two flagella, one longitudinal and the other located in a depression or cingulum, around the cell. The cell is comprised within a membrane, the amphiesma, which can include a series of cellulosic plates (referred to as “armored” or thecate), while others lack those cellulosic plates and are referred to as “naked” or “athecate”. Dinocysts are either composed of dinosporin, a complex biomacromolecular substance composed of phenolic, alcoholic and/or carboxylic hydroxides, and fatty acids ranging from C14 to C16, accompanied by tocopherols and sterols (mostly cholesterol and dinosterol) [30], or calcium carbonate. The morphology of most motile and cyst species are often very different, which makes it difficult to understand the complete life cycle. Also, the motile and cyst forms were studied separately by biologists and marine palynologists respectively. This gave rise to two distinct nomenclature systems, which further complicates the understanding of their life cycle. The distribution of dinoflagellates is not only dependent on the physico-chemical variables (currents, temperature, salinity, irradiance, nutrients) of the water column, but also on their feeding strategies, which may include mixotrophy and phagotrophy (ingestion of large food particles or preys) as well as the distribution of their prey. Over the last decade, several research programs aimed at studying the changing Arctic environment were conducted in the northern Baffin Bay (International North Water Polynya Study), the Beaufort Sea (Canadian Arctic Shelf Exchange Study) and the Canadian Arctic Archipelago (ArcticNet). The ecology of plankton and ice diatoms and other protists have been extensively studied over long-term periods as part of these Arctic research programs, but very little has been done on the ecology of dinoflagellate bloom events. The main reasons are that dinoflagellates are usually less abundant in the water column and they are adapted to growth under low turbulence and low nutrients concentrations [56]. Dinoflagellate blooms often occur after the one of diatoms and usually are restricted in time and space, therefore easily missed. For example, in highly productive Arctic basins, such as the North Water Polynya in northern Baffin Bay, diatoms usually dominate the phytoplankton and may account up to 70% of the total phytoplankton carbon biomass, while dinoflagellates and ciliates comprise approximately 24% of the total phytoplankton carbon biomass [24, 24, 32]. However, dinoflagellates may account for the majority of the phytoplankton carbon biomass during certain periods in relation to the depth of the surface mixed layer. 2.1. Cyst-theca relationships of Arctic dinoflagellates Dinoflagellates have been studied by biologists since the mid-late eighteenth century, while dinoflagellate cysts were studied by marine palynologists since the early nineteenth century [31], which lead to a double nomenclature system for the motile and cyst stages. The first cyst incubation experiments in the 1960s [33] paved the way to a number of studies aiming at establishing the cyst- theca relationship of the cyst stage [34, 35, 36, 37, 38, 39]. Several dinoflagellate species closely resemble each other and, in several cases, only detailed examination in scanning electron microscopy resulted in accurate identification. The best example is the Gonyaulax spinifera (Claparède et Lachmann 1859) Diesing 1866 complex, in which ~13 cyst species were supposedly derived from one single motile species [39]. Therefore, species identification can sometimes be erroneous when observing plankton samples because the minute details may have been missed. Six studies provide checklists of motile dinoflagellate species present in the Canadian High Arctic [18, 23, 24, 27, 28, 29] and they accounted for most of the phytoplankton surveys in the area since the 1970s. A total of ~192 taxa belonging to 42 genera have been identified, among which 10 species are known to produce resistant resting cysts as part of their life cycle ( Brigantedinium cariacoense = Protoperidinium avellana (Meunier 1919) Balech 1974; B. simplex = Protoperidinium conicoides (Paulsen 1905) Balech 1973; Pentapharsodinium dalei Indelicato et Loeblich 1986 = theca-based name; Polykrikos kofoidii Chatton 1914 = theca-based name; Polykrikos schwartzii Bütschli 18, 1873 = theca-based name; Protoperidinium americanum (Gran et Braarud 1935) Balech 1974 = theca-based name; Protoperidinium nudum = theca-based name; Spiniferites elongatus = Gonyaulax elongata (Reid 1974, 74) Ellegaard, Daugbjerg, Rochon, J. Lewis et Harding 2003; Trinovantedinium applanatum = Protoperidinium pentagonum (Gran 1902) Balech 1974). In some cases, the cyst has been observed while forming inside the theca within plankton samples, while in other cases, the relationship has been confirmed through laboratory cyst incubation experiments. Several studies have ...