As warming starts to shake up marine food webs, ecologists say it may give an unexpected boost to some fisheries – but also make them more precarious.
This is one of the implications of a new experimental study in PloS Biology that takes a panoramic point of view. Rather than tackling complex food webs species by species, the authors look at how warming affects growth and metabolism across the board within the broad groups of organisms at the base of the web.
Mary O’Connor of the University of North Carolina, Chapel Hill, and colleagues turned up temperatures in outdoor containers holding phytoplankton, the ocean’s primary producers, and bacteria and zooplankton, the smallest consumers. Warming of two to six degrees drove up the productivity of phytoplankton, as expected. But consumers increased their growth even more. Zooplankton retain only about 10% of the biomass they eat, so total biomass declined as the hungry hordes munched on the phytoplankton.
Zooplankton are fish food. O’Connor tells New Scientist, “The effect could be translated up the food chain” to a gain in fisheries, but “that top-heavy food web structure could be less stable, and crash all together." The group found that the consumer boom was much greater when nutrients were added, so they suggest that food webs in nutrient-poor waters – such as the ocean surface – may be more resilient to climate change.
The study is timely: NOAA’s National Climatic Data Center recently announced that world ocean temperatures were the hottest on record last month. The temperatures beat the 20th-century average by nearly 0.6 degrees Celsius. According to AP, meteorologists are attributing the record high to a combination of global warming trends, an El Nino phase just getting started, and other natural variation. Apparently, an unusual and unexplained weather pattern this summer is concentrating warmth over the ocean while land surfaces stay cooler.
And this won’t be a brief blip in sea temperatures:
Breaking heat records in water is more ominous as a sign of global warming than breaking temperature marks on land, because water takes longer to heat up and does not cool off as easily as land.
“This warm water we’re seeing doesn’t just disappear next year; it’ll be around for a long time,” said climate scientist Andrew Weaver of the University of Victoria in British Columbia.