The darkness caused by the collision would impair photosynthesis and reduce nannoplankton reproduction. While full darkness did not occur, the effects in the north would have lasted for up to six months. However, with ample sunlight and large amounts of nutrients in the oceans, the populations should have bounced back, even in the North, but they did not. The researchers suggest that toxic metals that where part of the asteroid, heavily contaminated the Northern oceans and were the major factor inhibiting recovery.
"Metal loading is a great potential mechanism to delay recovery," said Bralower. "Toxic levels in the parts per billions of copper, nickel, cadmium and iron could have inhibited recovery."
On the one hand, the researchers considered an impact scenario causing perpetual winter and ocean acidification to explain the slow recovery, but neither explains the lag between Southern and Northern Hemispheres. Trace metal poisoning, on the other hand, would have been severe near the impact in the Northern Hemisphere. When the high temperature debris from the impact hit the water, copper, chromium, aluminum, mercury and lead would have dissolved into the seawater at likely lethal levels for plankton. Iron, zinc and manganese -- normally micronutrients -- would reach harmful levels shortly after the impact. Other metal sources might be acid-rain leached soils or the effects of wildfires. Metals like these can inhibit reproduction or shell formation.
The toxic metals probably exceeded the ability of organic compounds to bind them and remove them from the system. Because nannoplankton are the base of the food chain, larger organisms concentrate any metals found in nannoplankton making the metal poisoning more effective. With the toxic metals remaining in the oceans and the lack of sunlight, the length of time for recovery might increase.
Erich Feldmeier on 28 Jun 13"These findings clearly state that there is a relationship between summer suicide rates and biochemical (e.g., plasma L-TRP and melatonin levels, [3H]paroxetine binding to blood platelets), metabolic (serum total cholesterol, calcium and magnesium concentrations), and immune (number of peripheral blood lymphocytes and serum sIL-2R) variables.[18] Another study focused on the association between depression, suicide, and the amount of polyunsaturated fatty acids (PUFA). They state that "depression is accompanied by a depletion of n-3 poly-unsaturated fatty acids".[22] Their methodology involved taking periodic blood samples-every month for one year-of healthy volunteers, allowing them to analyze the "PUFA composition in serum phospholipids and [relating] those data to the annual variation in the mean weekly number of suicides". They used an analysis of variance (ANOVA) to document their results, finding that PUFA like arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid all occurred at significantly lower rates in winter than in summer months. The association between depression, suicide, and PUFA rates is indicative of there being a biological factor in seasonal effects on suicide rates"
