Impact of freshwater runoff from the southwest Greenland Ice Sheet on fjord productivity since the late 19th century

Abstract

Abstract. Climate warming and the resulting acceleration of freshwater discharge from the Greenland Ice Sheet are impacting Arctic marine coastal ecosystems, with implications for their biological productivity. To accurately project the future of coastal ecosystems and place recent trends into perspective, palaeo-records are essential. Here, we show runoff estimates from the late 19th century to the present day for a large sub-Arctic fjord system (Nuup Kangerlua, southwest Greenland) influenced by both marine- and land-terminating glaciers. We followed a multiproxy approach to reconstruct spatial and temporal trends in primary production from four sediment core records, including diatom fluxes and assemblage composition changes and biogeochemical and sedimentological proxies (total organic carbon, nitrogen, C/N ratio, biogenic silica, δ13C, δ15N, and grain-size distribution). We show that an abrupt increase in freshwater runoff in the mid-1990s was reflected by a 3-fold increase in biogenic silica fluxes in the glacier-proximal area of the fjord. In addition to increased productivity, freshwater runoff modulates the diatom assemblages and drives the dynamics and magnitude of the diatom spring bloom. Our records indicate that marine productivity is higher today than it has been at any point since the late 19th century and suggest that increased mass loss of the Greenland Ice Sheet may continue promoting high productivity levels at sites proximal to marine-terminating glaciers. We highlight the importance of palaeo-records in offering a unique temporal perspective on ice–ocean–ecosystem responses to climate forcing beyond existing remote sensing or monitoring time series.

Climate Variability and Glacier Dynamics Linked to Fjord Productivity Changes Over the Last ca. 3300 Years in Nuup Kangerlua, Southwest Greenland

“…Freshwater discharge has influenced the fjord productivity on a decadal scale during the last ca. 100 years (Oksman et al., 2022).…”

“…Sediment core 36R is also referred to by its site number, as “site 6” in Oksman et al. (2022). Age‐depth models were created using the OxCal 4.4 program (Bronk Ramsey, 2009; Bronk Ramsey & Lee, 2013) and by calibrating ages using the Marine20 calibration curve (Heaton et al., 2020) and a ΔR of −70 ± 54 (Pieńkowski et al., 2022).…”

“…The impact of glacial meltwater on fjord productivity has been the subject of several recent studies that have contributed to process-level understanding and decadal-scale insights (e.g., Holding et al, 2019;Hopwood et al, 2020;Meire et al, 2016Meire et al, , 2017Sejr et al, 2022). However, only longer-term records that reach beyond the modern warming (Oksman et al, 2022) can help placing productivity trends into a wider temporal context and, more generally, evaluating the long-term effects of cryosphere changes on high latitude ecosystems. Fjord productivity in Nuup Kangerlua is affected by freshwater discharge entering the fjord, and recent work covering the past century has shown that modern high productivity levels have prevailed only since the late 1990s (Oksman et al, 2022).…”

“…The magnitude and effects of upwelling meltwater plumes on phytoplankton theoretically depend on fjord topography and the grounding line depth of the marine‐terminating glaciers (Hopwood et al., 2018). Recent fjord monitoring efforts have shown a positive impact of subglacial discharge on primary production reaching as far as 30–80 km down‐fjord from the marine‐terminating glaciers (Meire et al., 2017), however, freshwater has also been found to influence marine productivity in glacier‐distal fjord waters (Oksman et al., 2022). In contrast, fjords that receive meltwater only as surface runoff from land‐terminating glaciers have typically a lower primary productivity, because of the lacking upwelling mechanism and because surface runoff increases stratification and water column turbidity (Hopwood et al., 2020; Meire et al., 2017; Sejr et al., 2022).…”

An Interdisciplinary Perspective on Greenland’s Changing Coastal Margins

“…One of the challenges to understanding the evolution of these systems will be identification of the factors governing productivity over longer lead times. Globally, biomass is predicted to decline under all emission scenarios due to increasing temperatures and decreasing global primary production (Lotze et al, 2019); Greenland and other high-latitude regions, however, are expected to respond differently (Wassmann and Registad, 2011;Oksman et al, 2022).…”