Long-term impacts of sea-ice melt on primary productivity in the Subarctic North Atlantic

Dr Thibodeau, Benoit   (Principal Investigator (PI))

Dr Chan Phoebe   (Co-Investigator)

Dr Halfar Jochen   (Co-Investigator)

Dr Not Christelle Aurelie   (Co-Investigator)

24

2017-04-01

2019-03-31

55400

Long-term impacts of sea-ice melt on primary productivity in the Subarctic North Atlantic

Climate change, North Atlantic, primary productivity, sea-ice, Subarctic

Earth SciencesEnvironmental Studies and Science

Physical Sciences (P)

201611159215

Seed Fund for PI Research – Basic Research

2016

Completed

Arctic sea-ice thickness and concentration have dropped by approximately 9% per decade since 1978 (Comiso, 2012). Concurrent with this sea-ice decline is a marked 30% increase in satellite-based estimates of marine primary productivity (1998 – 2012), driven by shoaling of the mixed layer and enhanced transmittance of solar radiation into the surface ocean (Arrigo and van Dijken, 2015). This has recently been confirmed by phytoplankton studies in Arctic and Subarctic basins that have revealed earlier timing (Harrison et al., 2013), prolonged duration (Arrigo et al., 2008; Drinkwater and Pepin, 2013), and increased primary productivity of the spring phytoplankton bloom (Arrigo et al., 2008; Brown and Arrigo, 2012; Arrigo and van Dijken, 2015). Phytoplankton abundance not only influences marine biodiversity and trophic dynamics, but also plays an important role in carbon sequestration into the deep sea, curtailing the rise of atmospheric carbon dioxide realized to date (Feely et al., 2009). The Subarctic North Atlantic is one of the most seasonally productive marine environments in the world, accounting for roughly 50% of global ocean productivity (Tilstone et al., 2014). However, difficulties of navigating in remote ice-laden waters and harsh polar climates have often resulted in short and incomplete records of in-situ plankton abundance in the Subarctic Northwest Atlantic. Alternatively, information of ocean productivity may be gained through the study of trace nutrient distributions in the surface water column. Biologically available nitrate (NO3-) can be used as an indicator for productivity, such that the enhanced uptake of NO3- (i.e. nitrate assimilation) is associated with higher levels of productivity, and vice versa. Therefore, by monitoring the proportion of assimilated NO3-, we can better elucidate the response of ocean productivity to climatic change. Unfortunately, observational records of dissolved NO3- concentrations are both spatially and temporally limited (since 1960s), and are thus unable to resolve long-term natural baseline trends in productivity prior to the onset of anthropogenic warming. In contrast, stable isotopes of nitrogen (δ15N) preserved in ocean sediments and biological archives can be used as a proxy for the reconstruction of past nitrogen assimilation and thus ocean productivity. While marine sediments have generated the bulk of δ15N records to date extending back tens of thousands of years, the temporal resolution of records are often compromised by low sedimentation rates and diagenetic effects. Alternatively, long-lived biogenic archives such as corals and coralline algae may be used to determine multi-centennial to millennial δ15N variability at annual resolution. However tropical corals have limited distributional ranges, while the complex skeletal morphologies of cold-water deep-sea corals complicate the identification of growth patterns. Crustose coralline algae Clathromorphum compactum are calcareous photoautotrophic marine algae that form hard rock-like encrustations in shallow rocky sublittoral zones, and are exceptionally abundant along the Eastern Canadian coastline. Crustose coralline algae can offer an unprecedented opportunity to study high-latitude surface water paleoceanography because they: 1) are widely distributed in high-latitude oceans where other paleoclimate archives are sparse, 2) are long-lived (up to multiple centuries), and 3) produce clear and even annual growth increments, enabling the precise calendar dating and geochemical sampling of hard tissue. In the Northwest Atlantic, coralline algae can form accretions of up to 10.5cm in thickness (at an average vertical extension rate of 170μm/year), resulting in age spans of several hundred years. In fact, a living specimen of Clathromorphum compactum collected from the northern Labrador, Canada has been dated using annual layer counting methods, backed by radiocarbon dating to an age of 646 years (Halfar et al., 2013). The overarching goal of this research is to examine δ15N in crustose coralline algae to reconstruct changes in nitrogen uptake and productivity associated with sea-ice melt in the Arctic and Subarctic North Atlantic Ocean. In order to address this goal, we will test the following hypotheses: 1) Organic material within coralline algal skeleton records ambient seawater NO3- concentrations; and 2) Increasing phytoplankton productivity driven by the loss of sea-ice cover in the Arctic and Subarctic North Atlantic is accompanied by enhanced nitrogen utilization and thus δ15N recorded in the algal skeleton. Preliminary studies on the algal genus Clathromorphum show that calcification takes place within a dense organic matrix, which can allow for the uptake and preservation of ambient seawater NO3- into the algal skeleton over the lifetime of the organism.