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Not So Easy EZ

A new study of the EZ - otherwise known as the entrapment zone where ocean tides and river outflows overlap and support copious life at the base of the estuarine food chain - suggests that the zone isn't where or what scientists once thought it was. According to the Romberg Tiburon Center's Wim Kimmerer, who headed up the multi-scientist Interagency Ecological Program research effort, "Tidal flows are more important than we suspected."

For years, scientists have thought "gravitational circulation" - two layer circulation in which heavier, salty tidal water moves upstream along the estuary bottom and lighter, fresh river water moves downstream on the surface - occurred in and enhanced the riches of Suisun Bay. But research by the U.S. Geological Survey's Jon Burau found this type of circulation rare in Suisun Bay. "Some other mechanism must be responsible for the aggregation of organisms in the bay, and it's probably tidal migration," says Kimmerer.

Just how organisms use the tides to move around the EZ and Suisun Bay was a major question for the two-year, high-tech field study which examined the relationships between the positions of organisms - in terms of their height in the water column and their location upstream or downstream of the EZ and physical parameters like tidal flow, river flow and time of day. Reseachers found that most organisms ride the higher velocity flood tides upstream, then drop down to the lower velocity layers on the ebb. In this way, minute specks of life that any casual observer would assume just drift around and go with the flow are actually behaving in a way that maintains their position within range of the EZ.

The study also showed how strong an influence the burgeoning exotic Asian clam Potamocorbula amurensis - which is consuming the EZ food supply - has had on the abundance of life in the zone. "Abundance peaks are not in the same place, nor nearly as pronounced, nor as year round as they was before the clam's arrival," he says. "There's been an upstream shift in abundance peaks from the 2 parts per thousand isohaline in Suisun Bay to 1 ppt or less." (An isohaline refers to a line in the estuary with a specific amount of salt in the water). This shift corresponds with the area outside the clam's reach.

In addition, work by the Romberg Center's Tim Hollibaugh indicated that many bacteria in the EZ are associated with particles of organic matter in the water, rather than free-living. This suggests, says Kimmerer, that organisms such as copepods and filter feeders which are too big to eat free-living bacteria could be feeding on the particle-bound brand, and that the latter may be a larger contributor to the estuarine food chain than once thought. The final report on the study is due out around June.

Contact: Wim Kimmerer (510)525-9073

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