Image by eelke dekker via Flickr CC
The warming temperatures of the ocean are problematic for many species, but especially worrisome is the impact hotter water has on cornerstone species upon which many other marine animals rely. Usually we hear about changes in ocean temperature impacting coral reefs, but now scientists are finding that across the globe, phytoplankton -- the food for zooplankton which is food for many other ocean species -- is in decline, and that will have massive impacts for not just the marine food chain but ocean systems on the whole. Scientists report in the journal Nature that phytoplankton levels have decreased by about 1% per year as the oceans have warmed. While 1% doesn't sound like much, consider that the algae species represents about half of all our planet's photosynthetic biomass (which means it plays a vital role in processing the carbon dioxide out of the atmosphere, acting as microscopic carbon capture and storage units while also producing about half of the planet's oxygen) and is the basis of the entire oceanic food chain.
By combining satellite-derived observations of phytoplankton activity from 1997 to 2006 with historical shipboard measurements dating back to the beginning of oceanography, the researchers discovered the downward trend. In the past 60 years, algal biomass has decreased by about 40%, with the rate speeding up in recent years, and the scientists are pointing at ocean warming as the culprit.
"Clearly, 40% is a huge number," says Paul Falkowski, an oceanographer at Rutgers University in New Brunswick, New Jersey. "This implies that the entire ocean system is out of steady state, slowing down."
"This is severely disquieting," adds Victor Smetacek, a marine biologist at the Alfred Wegener Institute of Polar and Marine Research in Bremerhaven, Germany. "One must really digest the very magnitude of this decline and its possible implications."
"The study adds to a growing body of global ocean research, all evidencing a fundamentally common result: the net effect of a warming ocean surface is a reduction in phytoplankton surface chlorophyll concentration," says Michael Behrenfeld, a marine ecologist at Oregon State University in Corvallis.
The warming of the ocean causes "stratification" -- or emphasized differences in the ocean's layers. Phytoplankton need sunlight from the upper layers and nutrients from the lower layers, and as the ocean stratifies, the nutrients are harder to obtain. However, decreases in arctic areas aren't necessarily explained by this, and scientists are looking at other factors such as changes in wind and ocean circulation (which also link back to warming oceanic temperatures).
Whatever the specific reasons, there seems to be no doubt that the levels of phytoplankton are diminishing. Adding the dwindling supply of phytoplankton to the dwindling stocks of fish, changing temperatures, acidification and pollution of the ocean, and we're looking at a serious problem that may already be impossible to rectify.
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