Growth as a function of sea ice cover, light and temperature in the arctic/subarctic coralline C. compactum: A year-long in situ experiment in the high arctic

Abstract

Long-term, high-resolution measurements of environmental variability are sparse in the High Arctic. In the absence of such data, we turn to proxies recorded in the layered skeletons of the long-lived crustose coralline algae Clathromorphum compactum. Annual growth banding in this alga is dependent on several factors that include temperature, light availability, nutrients, salinity, and calcium carbonate saturation state. It has been observed that growth slows during winter as sunlight reaching the seafloor diminishes due to decreased insolation and the build-up of sea-ice, such that the relationship between sea-ice cover extent and algal growth has allowed for reconstructions of relative sea-ice variability through time. However, recent laboratory work has shown that C. compactum continue growing in complete darkness (sea-ice cover). Therefore, a more complete understanding of algal growth is necessary for the refinement of the sea-ice proxy. Here, we present the results of a ~year-long in-situ growth and environmental monitoring experiment in Arctic Bay, Nunavut, Canada (~73°N) which addresses, for the first time in situ, the gaps in our understanding of growth over an annual cycle in the High Arctic. Algal growth was assessed on a quasi-monthly basis, where specimens were subsampled to quantify monthly extension in the context of ocean temperature and light availability. By measuring extension rate through time, we observed that the algae grew on average 72 µm yr-1, with ~54% of annual growth occurring during the sea-ice free summer months (June-September), ~25% during the winter months (November-April), and ~21% occurring during the transition months of May and October. Although winter growth slowed, we did not observe a consistent cessation of linear extension during low-or no-light months. We posit that substantial growth during the winter months at this latitude is most likely a consequence of the mobilization of stored energy (photosynthate) produced during the photosynthetically active summer months. However, we also discuss the possibility of low light-photosynthetic activity and/or dark carbon fixation, which could also facilitate extension through time. Overall, the novel growth model presented here has implications for the use of C. compactum growth for reconstructing the environment as well as for trace-element-based (typically Mg/Ca) algal chronologies.

Structural and Geochemical Assessment of the Coralline Alga Tethysphytum antarcticum from Terra Nova Bay, Ross Sea, Antarctica

“…These possible layering does not show any difference in Mg content which is relatively constant along the thallus. Seasonal layering/bands with different Mg content are quite common in subpolar species such as Clathomorphum nereostratum and Clathomorphum compactum [7,52] or in free living coralline algae (rhodoliths) such as Lithothamnnion glaciale [6].…”

Linear extension and calcification rates in a cold‐water, crustose coralline alga are modulated by temperature, light, and salinity

“…Although temperature appears to be a consistent and major determinant of growth rate in crustose coralline algal species (e.g., Adey 1970; Halfar et al 2011; Williams et al 2018; Williams et al 2021 b ; Westfield et al 2022; Gould et al 2022), the influence of other important environmental factors, such as light and salinity, are far less understood. For example, Westfield et al (2022) investigated the isolated and interactive effects of increased temperature and p CO 2 on algal growth and calcification in C. compactum sampled from the Gulf of Maine and found normal parabolic responses to warming (i.e., highest growth at intermediate conditions), with varied response to increased p CO 2 .…”