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Phytoplankton in Coastal Waters and Global Climate Change?

Photo of San Francisco Bay, California
Phytoplankton populations in estuaries such as San Francisco Bay, California, are influenced by a host of local stresses that mask plankton responses to global climate change.

Human disturbance of coastal ecosystems is intense and can mask ecological responses to global climate change. This is the major conclusion of a large effort by U.S. Geological Survey (USGS) scientist James E. Cloern and University of California-Davis scientist Alan D. Jassby, who analyzed phytoplankton data from 154 of the world’s coastal water bodies. Phytoplankton (suspended, microscopic algae) are sensitive indicators of long-term climate variability in the open ocean, but apparently not in the nearshore coastal zone where human landscape transformations, fishing, aquaculture, river damming and diversions, introduced species, and contaminants are the dominant causes of biological change. Cloern and Jassby published a recent paper in Ecology Letters to document their analyses of phytoplankton variability, measured as chlorophyll concentration, from coastal sites around the world. Their analyses revealed a wide range of seasonal patterns, with large variability across and within coastal water bodies. Most of the observed variability was attributed to human-induced and natural changes to the local environment of each coastal water body that were unrelated to broad-scale changes in climate.

Knowing how phytoplankton populations respond to environmental stresses is important to scientists and managers because phytoplankton:

  • form the base of food webs that support commercial fisheries,
  • exert a large influence on the variability of atmospheric carbon dioxide,
  • play key roles in the cycling of nutrients and contaminants, and
  • are key indicators and transformers of water quality.
Graph of peak chlorophyll concentrations San Francisco Bay, California
The above bar graph shows the months that peak chlorophyll concentrations occurred in 116 coastal water bodies in the northern temperate zone. The distribution is surprisingly even from March through September, although, peaks occurred throughout the year. The distribution shows no characteristic single seasonal pattern, a large departure from the regular seasonal pattern of plants on land that is tightly tied to the annual climate cycle (The graph is a modified version of figure 4 from Cloern and Jassby, 2008)
(Larger Version)

Terrestrial plants and oceanic phytoplankton have fairly predictable yearly cycles. However, local processes at the land/sea interface—where influences from watersheds, the atmosphere, and coastal ocean intersect—make seasonal phytoplankton changes highly variable and complex. Ecosystems at the land/sea interface are unique in the large number of processes affecting their biological communities. Therefore, plankton monitoring in the coastal zone is most useful for detecting the integrated effects of many stressors rather than as an indicator of a single influence, such as global climate change.


Cloern, J.E., and Jassby, A.D., 2008, Complex seasonal patterns of primary producers at the land-sea interface: Ecology Letters, v. 11, no. 12, p. 1,294-1,303, doi:10.1111/j.1461-0248.2008.01244.x.

USGS Information on Estuaries

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