Bernard LeBlanc

Springtime sensible heat, nutrients and phytoplankton in the Northwater Polynya, Canadian Arctic

Abstract Sampling was conducted in the Northwater Polynya (between 70°20′ and 77°20′N), on 17 and 19 May 1991. At each of the 14 sampling stations, CTD profiles were recorded from surface to bottom and nutrients and phytoplankton were determined at four depths down to 30 m. The presence, between 220 and 400 m, of water temperatures >0°C is an indication that, in winter, the West Greenland Current enters the Northwater along the Greenland coast. The warm water is progressively mixed as it moves northward and eastward. It was thus hypothesized that sensible heat is as an important factor in keeping the Northwater open. Measured chemical and biological variables were quite homogeneous on the vertical down to 30 m and they showed longitudinal gradients. From east to west, the average concentrations of nutrients increased (phosphate from 0.5 to 1.4, nitrate from 3.7 to 10.8, and silicate from 6.8 to 34.2 mmol m −3 ), whereas the areal concentrations of phytoplankton decreased (from 47 to 9 x 10 9 cells m −2 and from 506 to 50 mg Chl a m −2 ). Nutrient ratios indicated possible silicon deficiency in the easternmost part of the polynya. Diatoms dominated cell numbers (≥87% at all stations). Concentrations of the three nutrients were inversely correlated with both Chl a and cell numbers. The Y-intercepts of regressions of Chl a on nutrients provided an estimate of potential maximum biomass in the upper 30 m, which was ca. 600 mg Chl a m −2 , or lower if there was silicon limitation. The overall picture was that of a diatom bloom, moving westward and progressively exhausting the nutrients. Initiation of the bloom appeared to have been linked to the absence of sea ice. A source of heat for this would have been the above sensible-heat process.

Temporal and spatial patterns in the surface-water biomass of phytoplankton in the North Water

Temperature, salinity, and in vivo fluorescence of surface seawater in the North Water were recorded continuously, using a CTD+fluorometer, in August 1997, April–July 1998 and August–October 1999. The phytoplankton bloom started in the polynya on the Greenland side in April. In April and May, high phytoplankton biomass coincided with saline water on the Greenland side, while biomass was low on the Ellesmere Island (Canada) side where a deep mixed layer prevailed. High phytoplankton biomass extended over the whole polynya in June, when surface temperature increased due to solar heating and salinity decreased due to freshwater input. The initiation of the bloom was about 2 months earlier on the Greenland than the Canadian side. In July and August, phytoplankton biomass became low in the southern survey area, indicating that the phytoplankton bloom had ended. In September, relatively saline and warm water occurred in the southeastern part of the study area where, consistent with the change in water properties, high concentrations of chlorophyll a were observed again. These results imply that both the earlier start of the algal bloom in spring and the eventual increase in phytoplankton biomass in summer contribute to the high annual primary production along the Greenland side, thus influencing the structure and biological productivity of the entire North Water ecosystem.

Sustained disruption of narwhal habitat use and behavior in the presence of Arctic killer whales

Significance Predators are widely understood to impact the structure and stability of ecosystems. In the Arctic, summer sea ice is rapidly declining, degrading habitat for Arctic species, such as polar bears and ringed seals, but also providing more access to important predators, such as killer whales. Using data from concurrently tracked predator (killer whales) and prey (narwhal), we show that the presence of killer whales significantly changes the behavior and distribution of narwhal. Because killer whales are effective predators of many marine mammals, similar predator-induced changes would be expected in the behavior of tracked animals in marine ecosystems worldwide. However, these effects are rarely considered and may frequently go unrecognized. Although predators influence behavior of prey, analyses of electronic tracking data in marine environments rarely consider how predators affect the behavior of tracked animals. We collected an unprecedented dataset by synchronously tracking predator (killer whales, N = 1; representing a family group) and prey (narwhal, N = 7) via satellite telemetry in Admiralty Inlet, a large fjord in the Eastern Canadian Arctic. Analyzing the movement data with a switching-state space model and a series of mixed effects models, we show that the presence of killer whales strongly alters the behavior and distribution of narwhal. When killer whales were present (within about 100 km), narwhal moved closer to shore, where they were presumably less vulnerable. Under predation threat, narwhal movement patterns were more likely to be transiting, whereas in the absence of threat, more likely resident. Effects extended beyond discrete predatory events and persisted steadily for 10 d, the duration that killer whales remained in Admiralty Inlet. Our findings have two key consequences. First, given current reductions in sea ice and increases in Arctic killer whale sightings, killer whales have the potential to reshape Arctic marine mammal distributions and behavior. Second and of more general importance, predators have the potential to strongly affect movement behavior of tracked marine animals. Understanding predator effects may be as or more important than relating movement behavior to resource distribution or bottom-up drivers traditionally included in analyses of marine animal tracking data.