Progression of marine phytoplankton blooms and environmental dynamics from sea-ice coverage to open waters in the coastal Arctic: comparing experimental data with continuous cabled observations

In this thesis, I present a unique temporal study of phytoplankton, nutrient and environmental dynamics that focussed on the transitional period between sea-ice cover conditions and open waters in a coastal inlet of the Canadian Arctic during 2016. I also compared the 2016 experimental data with continuous observations made by the Ocean Networks Canada (ONC) underwater observatory. Surface seawater sampling was conducted in Cambridge Bay with high temporal resolution from June 16 to August 3, to measure phytoplankton carbon and nitrate utilisation, silica production, phytoplankton biomass, phytoplankton taxonomy and dissolved nutrients. Throughout the study period, nitrate concentrations averaged 0.67  0.08 µmol L-1, and chlorophyll a and primary production were low at 0.11  0.005 µg L-1 and 0.25  0.02 µmol C L-1 d-1, respectively. The presence of sea-ice reduced physical mixing, which resulted in low surface nitrate concentrations. Phytoplankton assemblages, production rates and biomass were dominated by small flagellated cells (<5 µm) until late July, yet increases in temperature and nitrate later in the season enabled larger Chaetoceros spp. diatoms to bloom. The Chaetoceros bloom coincided with a peak in silica production (0.429 µmol Si L-1 d-1), which was otherwise low, but variable. The time series was divided into three phases based on changes in environmental conditions, these phases were used to evaluate changes in biological dynamics. Phase I was characterised by sea-ice, low nitrate and increasing phytoplankton biomass and primary production. Phase II was a transitional period, with calm water conditions a drop in phytoplankton biomass, however, an increase in the mean nitrate concentration enabled more consistent carbon fixation. PIII had greater environmental variability driven by mixing events. The mixing of the water column in PIII enabled larger Chaetoceors spp. to become prevalent in the surface waters contributing increasingly to the biomass and carbon utilisation. Overall, the nutrient concentrations, levels of biomass and production rates in Cambridge Bay were more reflective of those from oligotrophic regions. When comparing experimental data with observations made by the ONC observatory, a strong relationship between carbon utilisation and apparent oxygen utilisation became evident. This finding suggests that long-term in situ observations can potentially be used to monitor biological rates in the Arctic. The temporal resolution of this field study adds a seasonal perspective to our understanding of Arctic ecosystems, complements studies with greater spatial and interannual coverage, and can contribute to future numerical modelling of Arctic change. Furthermore, this study provides a first-time comparison between experimentally-measured phytoplankton production and cabled observations in the Arctic.

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