Natural fluctuations of mercury and lead in Greenland lake sediments

Given the current scenario of increasing global temperatures, it is valuable to assess the potential influence of changing climate on pollution distribution and deposition. In this study we use long-term sediment records from three lakes (spanning ca. 1000, 4800, and 8000 years, respectively) from the Greenland west coast to assess recent and long-term variations in mercury (Hg) and lead (Pb), including stable Pb isotopes (206Pb and 207Pb), in terms of pollution and climate influences. The temporal trends in sediment deposited from about the mid-19th century and forward are in general agreement with the history of industrial emissions at lower latitudes. Therefore, in recent sediment a possible influence from changing climate is difficult to assess. However, by using deeper sediment layers we show that changes in Greenland climate caused changes in the lake influx of material from regional aeolian activity, which resulted in large fluctuations in Hg and Pb concentrations and 206Pb/207Pb ratios. The aeolian material is primarily derived from glacio-fluvial material with low Hg and Pb concentrations and a different isotopic composition. For one of the lakes, the fluctuations in Hg concentrations (10 to 70 ng g(-1)) prior to the 19th century are equal to the anthropogenic increase in the uppermost layers, suggesting that when studying recent concentrations and time trends of pollution in relatively low-contaminated areas such as the Arctic, the early natural fluctuations must be considered.     

Salmon farms as a source of organohalogenated contaminants in wild fish

Organohalogenated contaminants (OHCs), including organochlorines (OCs; PCB, and OC-pesticides), brominated flame retardants (BFRs; polybrominated diphenyl ethers [PBDE], hexabromocyclododecane [HBCD]) and perfluorooctane sulfonate (PFOS), were measured in livers of Atlantic cod (Gadus morhua) and saithe (Pollachius virens) caught in the vicinity of salmon farms (n = 75) and control sites (n = 80) in three regions (59°-70°N) in Norway. Forty-five percent of the farm-associated (FA) fish (60% of the saithe and 30% of the cod) and none of the control fish had salmon feed (aquaculture food pellets) in their digestive tracts. Concentrations of OCs and BFRs were about 50% higher and dominated more by persistent compounds in Atlantic cod compared to saithe. After controlling for a set of confounding variables (location, sex, size, weight, gonads size, hepatosomatic index, and % lipids in the liver), the concentrations of ∑OC and ∑BDE were 50% higher in FA cod compared to control fish, whereas they were 20% higher in FA saithe than control fish. Hence, salmon farms are a source of lipid-soluble OHCs to wild marine fish, but variation in life-history and habitat use seems to affect the levels of OHCs in the different fish species. In contrast to the lipid-soluble OHCs, control fish had 67% higher PFOS levels than FA fish, which suggests that natural food contains higher loads of this compound than the commercial feed used in salmon farms. Some OHCs are known to act as endocrine disruptors, thus further work is required to determine if OHCs negatively affect reproductive processes of wild fish associated with salmon farms.     

Brain potential correlates of supraliminal contrast functions and defocus

In six subjects with normal corrected vision, perceived suprathreshold contrast of a sine‐wave grating pattern was assessed by an intermodal matching technique in combination with the method of ratio estimation. Vertical gratings were produced on an oscilloscope screen at spatial frequencies of 1.0, 2.0, 3.5, 5.0, 9.0 and 15.0 cycles per degree (c/deg) and at contrast levels between 0.03 and 0.55. Steady‐state visual evoked potentials (VEP) were recorded from the occiput to phase‐reversing gratings of the same spatial frequency and contrast range. A linear relation was found in logarithmic coordinates between perceived and physical contrast and between VEP amplitude and physical contrast. The slopes varied with spatial frequency and were steeper at low and high spatial frequencies than in the intermediate range for both subjective contrast and VEP.

In a second group of six subjects, the effect of retinal image defocus (myopia) on the psychophysical power functions was determined by placing spherical plus lenses in front of the eyes. A marked change in slope was obtained with blurred stimuli, and this effect was more pronounced with an increase in spatial frequency. Power transformations can thus be induced by neural and optical factors. Growth rate of supraliminal response functions seems to be inversely related to detection threshold of sensory systems.