Ocean life in the ice age: time to party.

Given a choice between an ice age climate and today's balmy conditions, most residents of North America and Europe would opt for the warmer weather. But a fish or a whale might pick differently. Emerging evidence suggests that oceanic life enjoyed an extended bash during the last ice age, complete with plenty of nutritious refreshments and unbridled procreation, at least on a microscopic level.

In the Aug. 31 Nature, Raja S. Ganeshram of the University of British Columbia in Vancouver and his colleagues report that the ice age ocean of around 20,000 years ago appeared to hold higher concentrations of nitrate, a key nutrient for the single celled planktonic plants that anchor the marine food chain. The availability of the extra nitrate during the glacial epoch may have stimulated the proliferation of this basic marine staple in waters now considered nutrient deprived.

Ganeshram's team studied nitrate levels by measuring the concentrations of two nitrogen isotopes--N-15 and N-14--in sediments collected off the coast of Mazatlan, Mexico. This area is one of three principal "nitrate sinks" in the modern ocean--places where bacteria, in a process called denitrification, convert nitrate into a form unusable by most organisms. Because the bacteria more readily consume nitrate containing the lighter nitrogen isotope, researchers can gauge the extent of past denitrification by analyzing the ratio of N-15 to N-14 in the sediments.

Off Mexico, sediments formed during the ice age showed evidence of lower rates of denitrification, suggesting that oceans at that time contained more nitrates for organisms to feed on, the scientists found. Researchers from the Woods Hole (Mass.) Oceanographic Institution, reporting in the Feb. 9 Nature, had previously obtained evidence of a similar ice age drop in denitrification rates in the Arabian Sea, another principal nitrate sink in today's ocean.

"This tells you something was different in a major way because denitrification was not occurring in areas where it is now," notes J.R. Toggweiler of the National Oceanic and Atmospheric Administration in Princeton, N.J. But he cautions that scientists lack even basic data about the source of nitrate in today's ocean. Without more information, they cannot definitively deduce the global nitrate content of the ice-age oceans from the measured denitrification rates at just two sites.

Higher nutrient concentrations during the ice age could help explain a persistent puzzle about the glacial world, according to Ganeshram and his coworkers. From studies of ice layers in Antarctica and Greenland, researchers know that atmospheric carbon dioxide concentrations dropped during the glacial period, weakening Earth's natural greenhouse effect. If a surfeit of nutrients fertilized marine plankton during the ice age, the plants would have sopped up carbon dioxide as they grew, thereby reducing atmospheric concentrations of this gas.