Morten Holtegaard Nielsen

Removal of snow cover inhibits spring growth of Arctic ice algae through physiological and behavioral effects

The snow cover of Arctic sea ice has recently decreased, and climate models forecast that this will continue and even increase in future. We therefore tested the effect of snow cover on the optical properties of sea ice and the biomass, photobiology, and species composition of sea ice algae at Kangerlussuaq, West Greenland, during March 2011, using a snow-clearance experiment. Sea ice algae in areas cleared of snow was compared with control areas, using imaging variable fluorescence of photosystem II in intact, unthawed ice sections. The study coincided with the onset of spring growth of ice algae, mainly an increase in two pennate diatoms (Achnanthes taeniata and Navicula directa), as temperature increased and ice thickness and brine volume stabilized. The increase in biomass was accompanied by an increase in minimum variable fluorescence (Fo) and the maximum quantum yield of PSII (Fv/Fm) and filling of brine channels with fluorescing cells. In contrast, in thexa0minus snow area, PAR transmittance increased sixfold and there was an exponential decrease in chl-a and no increase in Fo, and the area of fluorescing biomass declined to become undetectable. This study suggests that the onset of the spring bloom is predominantly due to temperature effects on brine channel volume, and that the algal decline after snow removal was primarily due to emigration rather than photodamage.

Photobiology of sea ice algae during initial spring growth in Kangerlussuaq, West Greenland: insights from imaging variable chlorophyll fluorescence of ice cores

We undertook a series of measurements of photophysiological parameters of sea ice algae over 12xa0days of early spring growth in a West Greenland Fjord, by variable chlorophyll fluorescence imaging. Imaging of the ice–water interface showed the development of ice algae in 0.3–0.4xa0mm wide brine channels between laminar ice crystals in the lower 4–6xa0mm of the ice, with a several-fold spatial variation in inferred biomass on cm scales. The maximum quantum yield of photosynthesis, Fv/Fm, was initially low (~0.1), though this increased rapidly to ~0.5 by day 6. Day 6 also saw the onset of biomass increase, the cessation of ice growth and the time at which brine had reached <50xa0psu and >−2xa0°C. We interpret this as indicating that the establishment of stable brine channels at close to ambient salinity was required to trigger photosynthetically active populations. Maximum relative electron transport rate (rETRmax), saturation irradiance (Ek) and photosynthetic efficiency (α) had also stabilised by day 6 at 5–6 relative units, ~30xa0μmol photons m−2xa0s−1 and 0.4–0.5xa0μmol photons m−2xa0s−1, respectively. Ek was consistent with under-ice irradiance, which peaked at a similar value, confirming that daytime irradiance was adequate to facilitate photosynthetic activity throughout the study period. Photosynthetic parameters showed no substantial differences with depth within the ice, nor variation between cores or brine channels suggesting that during this early phase of ice algal growth cells were unaffected by gradients of environmental conditions within the ice. Variable chlorophyll fluorescence imaging offers a tool to determine how this situation may change over time and as brine channels and algal populations evolve.

Dipole vortices in the Great Australian Bight

Shipboard measurements from late 2006 made by the Danish Galathea 3 Expedition and satellite sea surface temperature images revealed a chain of cool and warm ‘mushroom’ dipole vortices that mixed warm, salty, oxygen-poor waters on and near the continental shelf of the Great Australian Bight (GAB) with cooler, fresher, oxygen-rich waters offshore. The alternating ‘jets’ flowing into the mushrooms were directed mainly northwards and southwards and differed in temperature by only 1.5°C; however, the salinity difference was as much as 0.5, and therefore quite large. The GAB waters were slightly denser than the cooler offshore waters. The field of dipoles evolved and distorted, but appeared to drift westwards at 5km day–1 over two weeks, and one new mushroom carried GAB water southwards at 7km day–1. Other features encountered between Cape Leeuwin and Tasmania included the Leeuwin Current, the South Australian Current, the Flinders Current and the waters of Bass Strait.

Environmental Impact of Submarine Rock Blasting and Dredging Operations in an Arctic Harbor Area: Dispersal and Bioavailability of Sediment-Associated Heavy Metals

AbstractIn order to determine the possible impact on the marine environment, we present a study on the dispersal and bioavailability of sediment-associated heavy metals related to underwater blasting and dredging of bedrock operations during a quay construction. The environmental impact was primarily assessed by deploying a buoy setup including sediment traps, blue mussels, and passive samplers (diffusive gradient in thin films, DGTs) in a gradient from the construction site during the operations. Samplings were made during five separate periods covering a total span of about 2.5xa0months. Analyses included sedimentation rates, organic content, and metal concentrations of the material collected in the sediment traps and metal concentrations of the mussels and passive samplers. The construction work was associated with a considerable dispersion of sediments, organic material, and associated heavy metals. The major fraction of the sediment settled in the vicinity of the construction site. While the mussels were found to accumulate some metals in a distance-related manner to the construction site and no such accumulation in the DGTs occurred, we conclude that most of the dispersed metals were particle associated. It was found that while a large part of the material settled in the vicinity of the construction site, most of the fine-grained and/or organic sediment that was brought into suspension was transported further away from the construction site (beyond the 350xa0m) most likely carrying contaminants including heavy metals. For future studies of risks and monitoring of underwater blasting and dredging, we recommend to include a larger monitoring area and more importantly water samples of the suspended plumes.n Graphical AbstractThe dispersal of sediments and bioavailability of heavy metals were assessed using a buoy setup during underwater blasting and dredging.ᅟ