Biological oceanographers examine 'regime shifts' in complex marine ecosystems

04/29/04

"Regime shifts" are infrequent, large changes in oceanic conditions that spread through the food web. Depending on dynamics of the ecosystem, the response of a biological organism to some external forcing can be smooth, abrupt, or discontinuous.

In a paper published in the current issue of Progress in Oceanography, Dr. Jeremy Collie, University of Rhode Island Graduate School of Oceanography, Dr. Katherine Richardson, University of Arhus, Denmark, and Dr. John Steele, Woods Hole Oceanographic Institution, examine the mechanisms that can cause regime shifts in ecosystems and use oceanographic and ecological theory to shed light on the relationships between forcing and response variables.

The focus of the paper is to establish a theoretical basis for the occurrence of regime shifts in the ocean. Our understanding of regime shifts relies on observations of organisms and their marine environment taken on times scales of years and decades. Given the size, scope and interrelatedness of marine ecosystems, experimental manipulation is infeasible. Ecological theory and mathematical models are therefore essential tools for developing this understanding.

Regime shifts can occur, for example, when changes in ocean conditions, such as warming, affect plankton production, an event that easily propagates up the food chain. Changes in ocean conditions can also affect migration patterns, growth rates, and mortality of fish species.

Overfishing a species can also force a regime shift, changing the population dynamics of an entire ecosystem. A regime shift occurred at several levels of the food web in the English coinciding with lower phosphate levels. Other well-documented regime shifts have occurred in the northeast Pacific Ocean and in the North Sea zooplankton populations.

Using diagnostic criteria for regime shifts developed by Scheffer and Carpenter in 2003, Collie, Richardson, and Steele adapted this protocol for examining marine data for the occurrence of regime shifts. The scientists analyzed data on the commercially important haddock stock on Georges Bank off the coast of New England. This 69-year data set indicates that a regime shift in the haddock population occurred in 1965, most likely triggered by high fishing mortality.

"The difficulty in finding marine data that clearly demonstrate the occurrence of regime shifts in the ocean may be that, until recently, major disturbances in the oceans were of natural rather than anthropogenic (man-made) origin and our data collection following such disturbances has been insufficient to identify possible changes in ecosystems," said Collie.

"The assumption that major disturbances or perturbations of marine ecosystems will not result from human activities appears to no longer be valid," added Collie. "Marine ecosystems are increasingly influenced by human perturbations such as fishing, contamination, and the introduction of non-indigenous species. Fishing, in particular, has been identified as having a pervasive impact on marine ecosystems.

"In addition," said Collie, "with man-made induced climate change, we can no longer treat oceanic variability as being purely natural. Thus, there appears to be a very real possibility that anthropogenic activity may be sufficient to substantially perturb ecosystems. If such disturbance does provide the conditions for the occurrence of regime shifts, then understanding and identifying them becomes critical for developing responsible management strategies for the utilization of marine resources."

Source: Eurekalert & others

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