Start of crucial marine event creeping earlier, with unknown impacts on food chain, carbon cycling
Farmers know. So do gardeners. From temperate regions to chilly northern latitudes, ask plant growers and they will tell you that the start of the annual growing season is arriving earlier and earlier, in some areas by a couple of weeks or more.
But is the same thing happening in the ocean? Are marine plants mimicking their early-arriving counterparts on land? New research at Scripps Institution of Oceanography at UC San Diego says "yes."
Warming temperatures and melting ice in the Arctic could be behind a progressively earlier arrival of the yearly spring bloom of phytoplankton — tiny plants at the base of the ocean food chain — and the shift could hold consequences for the entire food chain and carbon cycling in the region.
Scientists at Scripps Oceanography, along with colleagues in Portugal and Mexico, plotted the annual Arctic phytoplankton bloom and found the peak timing of the event has been progressing earlier each year for more than a decade. The researchers analyzed satellite data depicting ocean color and phytoplankton production to determine that the spring bloom has come up to 50 days earlier in some areas in that time span.
The earlier Arctic blooms have roughly occurred in areas where ice concentrations have dwindled and created gaps that make early blooms possible, say the researchers, who published their findings in the journal Global Change Biology.
During the one- to two-week spring bloom, a major influx of new organic carbon enters the marine ecosystem through a massive peak in phytoplankton photosynthesis, which converts carbon dioxide into organic matter as part of the global carbon cycle. Phytoplankton blooms stimulate production of zooplankton, microscopic marine animals, which become a food source for fish.
Mati Kahru, lead author of the study and a research oceanographer in the Integrative Oceanography Division at Scripps, said it’s not clear if the consumers of phytoplankton are able to match the earlier blooms and avoid disruptions of their critical life-cycle stages such as egg hatching and larvae development.
"The spring bloom provides a major source of food for zooplankton, fish, and bottom-dwelling animals," said Kahru, who has been studying satellite data for more than 20 years. "The advancement of the bloom time may have consequences for the Arctic ecosystem."
Such a match or mismatch in timing could explain much of the annual variability of fish stocks in the region.
"The trend towards earlier phytoplankton blooms can expand into other areas of the Arctic Ocean and impact the whole food chain," said the authors, who used satellite data from 1997-2010 to create their bloom maps.
The NASA Ocean Biology and Biogeochemistry Program and the National Science Foundation provided financial support for the research. The satellite data were provided by the NASA Ocean Biology Processing Group, ESA GlobColour group, the National Snow and Ice Data Center, and the Japan Aerospace Exploration Agency.
Kahru’s coauthors include Greg Mitchell, a Scripps Oceanography research biologist, Vanda Brotas of the University of Lisbon in Portugal, and Marlenne Manzano-Sarabia of Universidad Autónoma de Sinaloa in Mexico.
—Mario C. Aguilera