Warmer Oceans Lead To Phytoplankton Declines, SeaWeb, 04/04/07
Source: SeaWeb Ocean Update
The productivity of phytoplankton is geared to fluctuations in the global climate, and recent warmer temperatures correspond to lower oceanic biomass and productivity. This is the conclusion of a study published in the journal Nature.
The study, by Michael J. Behrenfeld of Oregon State University and colleagues, analyzed a decade of data from satellites which measured surface chlorophyll. Behrenfeld and his co-authors combined these readings with empirical data to estimate phytoplankton growth rates and net primary production. Using this method, the study’s authors found that global chlorophyll and productivity increased sharply from 1997-98, and then declined steadily until 2005.
According to Behrenfeld and colleagues, the sharp increase occurred during a negative, or cold, phase of the El Niño- Southern Oscillation (ENSO), and the subsequent slow drop occurred as the planet moved into a warm phase. In a companion article in Nature, Scott C. Doney of the Woods Hole Oceanographic Institution explains that the “climate–plankton link is found primarily in the tropics and mid-latitudes, where there is limited vertical mixing because the water column is stabilized by thermal stratification (that is, when light, warm waters overlie dense, cold waters). In these areas, the typically low levels of surface nutrients limit phytoplankton growth. Climate warming further inhibits mixing, reducing the upward nutrient supply and lowering productivity.”
Extrapolating these observations into the future, continues Doney, “suggests that marine biological productivity in the tropics and mid-latitudes will decline substantially.” However, ecosystem dynamics are complex and nonlinear, “and unexpected phenomena may arise as we push the planet into this unknown climate state.”
Sources: Behrenfeld, M.J., et al. 2006. Climate-driven trends in con- temporary ocean productivity. Nature 444: 752-755; Doney, S.C., 2006. Plankton in a warmer world. Nature 444: 695-696.
Contact: Michael J. Behrenfeld, Oregon State University. E-mail. Scott C. Doney, Woods Hole Oceanographic Institution. E-mail.