Assessing variability in algal primary productivity and photophysiology: Case studies of sea ice algae from Svalbard fjords

Abstract

Salinity is a major factor influencing sea ice algal activity in experimental settings and in nature. Decreases in salinity, occurring during the ice melting procedure, impact measurements of primary productivity and photophysiology of sea ice algae via ¹⁴C-based incubations. Sea ice algae also face low salinity stress in their natural environments, for instance, due to seasonal melting of sea ice and snow cover, or via freshwater inputs from glaciers and rivers. The primary aim of this project was to study the primary productivity and photophysiology of sea ice algal communities in fjord systems of contrasting freshwater influences. In this thesis, a manuscript (to be submitted to peer-reviewed journal) combines research on i) natural bottom-ice algal communities from Svalbard fjords (Tempelfjorden and Van Mijenfjorden), and ii) a cultured ice algal community from Van Mijenfjorden on short-term (4h and 24h) and long-term (168h) acclimation to a decrease in salinity from 33 to 10. Main findings indicate differences in bottom-ice chlorophyll <i>a</i> and photophysiological parameters (α^B, I_k and β^B) between the fjords and sampling sites, which were mainly driven by the various impacts of freshwater inputs, being greater in Tempelfjorden. Under-ice irradiance and sea ice community composition were also important drivers of the photophysiological responses, specifically of I_k and β^B. A reduction in salinity from 33 to 10 led to significantly lower photophysiological parameters (P_s^B, α^B and I_k) for an ice pennate diatom-dominated cultured community from Van Mijenfjorden, suggesting that the algae experienced hypoosmotic stress. Despite significant partial recovery in the photophysiological parameters (P_s^B and α^B) after a 168h exposure to the fresher salinity conditions, the chlorophyll <i>a</i> and cell abundance remained significantly lower in comparison to the control treatment of 33. Adding filtered seawater during the melting of sea ice samples is clearly needed to buffer rapid declines in salinity that would induce similar hypoosmotic stress, and to obtain estimates of primary productivity most representative of true algal productivity. This thesis also includes: (Chapter 1) an introduction; (Chapter 2) a background; (Chapter 4) an additional chapter on optimizing the assessment of photophysiological responses; (Chapter 5) a concluding chapter including a summary, recommendations and author contribution statement; and two supplementary chapters: i) an outreach paper directed towards youth, giving an overview of the acclimation responses of sea ice algae to the extreme seasonal changes in the Arctic Ocean; ii) a collaborative work with Jon Brage Svenning, which aimed to investigate the chromatic acclimation responses of marine diatoms to three different light colorations.