Improved understanding of high-latitude crustose coralline algal growth and application as high-resolution environmental archives

Records of high latitude oceanic and climate dynamics obtained from high-resolution marine archives like the high arctic crustose coralline alga Clathromorphum compactum are invaluable for placing current environmental change in the context of historical variability. In addition to serving as an excellent marine archive, these algae also perform important ecosystem services by providing substrate and habitat for many benthic fauna, as well as contributing to regional primary productivity and carbon sequestration. This dissertation aims to advance our knowledge of the subannual-scale growth and calcification in the long-lived crustose coralline alga Clathromorphum compactum in the context of environmental change and historical reconstructions, and to explore novel proxies for building long-term high-resolution records of surface ocean temperature in the Canadian Arctic and northwest Atlantic Oceans. Chapter 1, "Experimental investigation of the effects of temperature, light, and salinity on rates of linear extension and calcification in the Arctic/Subarctic crustose coralline alga Clathromorphum compactum" examines the effects of environmental variability on algal calcification rates and growth. A 3-month controlled laboratory culturing experiment, isolating the effect of light level, salinity and temperature on net calcification and growth in algal specimens collected from Arctic Bay, Nunavut, Canada is presented. The experiment tests the specific hypotheses that (1) calcification is positively impacted by increased environmental temperature, (2) calcification is negatively impacted by reductions in salinity and (3) calcification rates are increased with increased light availability. The work elucidates C. compactum's potential response to expected warming, declining salinity, and lengthening of the growing season. Algal calcification rates were found to be best described by a combination of light exposure, salinity, and temperature, where temperature and salinity were positively correlated, and light level was negatively correlated with calcification rate. The results have implications for both the future fate of C. compactum in a changing Arctic, and for improved understanding of paleoceanographic records obtained from these algae. Chapter 2, "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" expands knowledge of sub-annually resolved calcification and growth rates in C. compactum in the high Arctic. A yearlong in situ growth monitoring experiment was deployed in Arctic Bay, Nunavut, Canada (72°59.747', 85°04.589') to address gaps in our understanding of annual algal growth in regions with extensive sea-ice cover. Monthly algal growth rates were quantified in the context of variability in sea-ice cover (light) and seawater temperature. A new growth model is presented describing subannual growth in C. compactum in the high Arctic. On average 72 µm yr-1 of growth occurred in the algal community, with 54% of annual growth occurring during the sea-ice free, warm, summer months of June through September, 25% of the growth occurring during the dark, winter months of November through April, and ~21% of the growth occurring in the shoulder months of May and October. Importantly, although growth rates decline in cold, sea-ice covered, winter months, growth does not stop (i.e., we do not observe a consistent cessation of linear extension (calcification)) during the dark of winter. The findings presented inform both the proxy calibration and age-modeling of these important marine archives and reveal insights about the ecology of C. compactum in this changing high-latitude ecosystem. Chapter 3, "Investigation of subannually-resolved elemental paleothermometers in the high-latitude coralline alga Clathromorphum Compactum" aims to improve the accuracy and precision of high-resolution paleo-reconstructions of sea surface temperatures using C. compactum samples collected across a ca. 30° latitudinal range in the northwest Atlantic and Arctic Oceans. A new growth-adjusted age modelling technique was developed and tested to improve high-resolution age assignment of geochemical data in the algal skeleton. Several trace and minor elements were measured via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) techniques and investigated as candidate paleothermometers (Mg/Ca, Sr/Ca, Na/Ca, Li/Ca, Mn/Ca, and Ba/Ca). Using nonlinear statistical techniques improved the calibration methods used for the Mg/Ca- and other paleothermometers in C. compactum when compared to linear calibration of the single Mg/Ca - SST proxy. Multivariate modeling was also explored to improve temperature prediction and resulted in similar uncertainty in temperature prediction as nonlinear models in one of the algal specimens examined here. As an example, the precision of the C. compactum paleothermometer for predicting summer temperature was increased in one algal sample from 1.3°C using a linear calibration model for Mg/Ca - SST to 0.5°C using the non-linear Mg/Ca - SST calibration model. Methods involved are discussed, with recommendations for further improvements.--Author's abstract

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