Abstract
Meltwater discharge from tidewater glaciers impacts the adjacent marine environment. Due to global warming tidewater glaciers are retreating and will eventually terminate on land. Yet, the mechanisms through which meltwater runoff and subglacial discharge from tidewater glaciers influence marine primary production remain poorly understood, as data in close proximity to glacier fronts are scarce. Here, we show that subglacial meltwater discharge and bedrock characteristics of the catchments control the phytoplankton growth environment inside the fjord, based on data collected in close proximity to tidewater glacier fronts in Kongsfjorden, Svalbard from 26 to 31 July 2017. In the southern part of the inner fjord, glacial meltwater from subglacial discharge was rich in fine sediments derived from erosion of Devonian Old Red Sandstone and carbonate rock deposits, limiting light availability for phytoplankton (0.6 mg m–3 Chl a on average, range 0.2-1.9 mg m-3). In contrast, coarser sediments derived from gneiss and granite bedrock and lower subglacial discharge rates were associated with more favourable light conditions facilitating a local phytoplankton bloom in the northern part of the inner fjord with mean Chl a concentration of 2.8 mg m-3 (range 1.3-7.4 mg m-3). In the northern part, glacier meltwater was a direct source of silicic acid through weathering of the silica-rich gneiss and granite bedrock. Upwelling of the subglacial freshwater discharge plume at the Kronebreen glacier front in the southern part entrained large volumes of ambient, nutrient-rich bottom waters which led to elevated surface concentrations of ammonium, nitrate and partly silicic acid. Total dissolved inorganic nitrogen transported to the surface with the upwelling of the subglacial discharge plume has a significant potential to enhance summer primary production in Kongsfjorden, with ammonium released from the seafloor being of particular importance. The transition from tidewater to land-terminating glaciers may, thus reduce the input of nutrients to the surface layer with negative consequences for summer productivity.
Highlights
High latitude regions are undergoing rapid and considerable alterations due to climate change (IPCC, 2014)
Important differences between physical and biological parameters among zones are summarised in Figure 2 and a more detailed overview of the main parameters used in this study is given in Table 1
Mixing of Surface Water (SW) and Intermediate water (IW) in the Southern Glacier Zone (SGZ) showed a tendency to follow the gradient of the subglacial discharge mixing line, indicating that subglacial discharge may be the dominant source of freshwater in this zone
Summary
High latitude regions are undergoing rapid and considerable alterations due to climate change (IPCC, 2014). At the west coast of Spitsbergen, the largest island of the archipelago, an increased prevalence of warm Atlantic Water (AW) displacing cold coastal water has caused an expedited melting of tidewater glaciers, which terminate directly into the sea (Schauer et al, 2004; Blaszczyk et al, 2009; Walczowski et al, 2017) These glaciers modify the hydrography and biogeochemistry of fjords through meltwater runoff (Straneo et al, 2011; Bartholomaus et al, 2013; Meire et al, 2016b; Kanna et al, 2018; Cape et al, 2019) and offer important foraging areas for seabirds, seals, and white whales (Lydersen et al, 2014; Urbanski et al, 2017; Everett et al, 2018). This meltwater-induced gradient influences the growth environment of primary producers (Piquet et al, 2014; van de Poll et al, 2016; Calleja et al, 2017; Hegseth et al, 2019)
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