Julie Veillette

Effects of loss of perennial lake ice on mixing and phytoplankton dynamics: insights from High Arctic Canada

Abstract Perennially ice-covered lakes are well known from Antarctica and also occur in the extreme High Arctic. Climate change has many implications for these lakes, including the thinning and disappearance of their perennial ice cover. The goal of this study was to consider the effects of transition to seasonal ice cover by way of limnological observations on a series of meromictic lakes along the northern coastline of Ellesmere Island, Nunavut, Canada. Conductivity-temperature profiles during a rare period of ice-free conditions (August 2008) in these lakes suggested effects of wind-induced mixing of their surface freshwater layers and the onset of entrainment of water at the halocline. Sampling of the mixed layer of one of these meromictic lakes in May and August 2008 revealed a pronounced vertical structure in phytoplankton pigments and species composition, with dominance by cyanobacteria, green algae, chrysophytes, cryptophytes and dinoflagellates, and a conspicuous absence of diatoms. The loss of ice cover resulted in an 80-fold increase in water column irradiance and apparent mixing of the upper water column during a period of higher wind speeds. Zeaxanthin, a pigment found in cyanobacteria, was entirely restricted to the <3μm cell fraction at all depths and increased by a factor of 2–17, with the greatest increases in the upper halocline region subject to mixing. Consistent with the pigment data, picocyanobacterial populations increased by a factor of 3, with the highest concentration (1.65 × 108 cells L−1) in the upper halocline. Chlorophyll a concentrations and the relative importance of phytoplankton groups differed among the four lakes during the open-water period, implying lake-specific differences in phytoplankton community structure under ice-free conditions.

Milne Fiord Epishelf Lake: A Coastal Arctic Ecosystem Vulnerable to Climate Change

Abstract: Milne Fiord in the Canadian High Arctic contains the last known ice-dammed fiord lake (epishelf lake) in the Northern Hemisphere. This freshwater ecosystem is retained by the Milne Ice Shelf and is underlain by sea water that is connected to the Arctic Ocean. Using microscopy, photosynthetic pigment analyses, and molecular techniques we examined the planktonic communities present in Milne Fiord to determine the biotic characteristics of the epishelf lake and the sea water below. Net sampling of the water column of Milne Fiord revealed a mixture of marine, freshwater, and brackish Zooplankton taxa, and high performance liquid chromatography (HPLC) pigment analysis showed pronounced differences in phytoplankton composition through the highly stratified water column. Chlorophytes dominated in the epishelf lake, prasinophytes prevailed in the halocline, and the bottom layer harboured mainly fucoxanthin-containing groups. Clone libraries of a dark-incubated, concentrated sample from below the halocline (30 m depth) yielded marine Archaea (mainly Crenarchaeota) and known bacterial taxa from the Pacific and Arctic oceans (e.g., Roseobacter, Oleispira, Colwellia). An equivalent sample from the epishelf lake (5 m depth) yielded many bacterial taxa that are characteristic of cold, freshwater habitats (e.g., Polynucleobacter, Variovorax, Flavobacterium), the euryhaline genus Polaromonas, and freshwater eukaryotes, notably ciliates. Similarly, denaturing gradient gel electrophoresis (DGGE) analyses of T4-like bacteriophages showed different viral assemblages in the upper and lower water column. This diverse, stratified ecosystem is dependent on the integrity of the bounding ice shelf and is therefore vulnerable to the ongoing effects of climate change in this region.