Clams used to chart loss of bio-productivity in watersystems

For the first time shellfish have been used as an indicator of past bioproductivity with researchers discovering a 95% drop in Colorado River Delta life due to human interference.


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Researchers from US and Mexican universities have pioneered a new technique of examining the health of waterways globally, by studying old accumulations of shells recording past shellfish populations. Scientists discovered that the biological productivity of the Colorado River Delta is at the most 5% of what it was before dams and agriculture made use of the river’s water, said Michal Kowalewski, a geological scientist at Virginia Polytechnic Institute and State University, who has been involved in the research, begun almost 10 years ago by Karl W. Flessa from the University of Arizona.

“This is probably the first research that uses fossil shells to estimate former bio-productivity in a region severely altered by humans,” Kowalewski told edie. “For the first time we are able to date shells fast and inexpensively and thus estimate rigorously the amount of time recorded in shell-rich beach deposits.”

The research has revealed how, since the 1930’s, an environment that supported billions of clams and other life has disappeared because dams and irrigation projects have reduced the flow of nutrient laden fresh water to the Colorado Delta. “Our estimate that the current shellfish populations represent only 5% of their past levels is highly conservative. This is because at every step of our calculations of past productivity we have assumed the minimal possible values for estimated parameters,” Kowalewski said.

“The research has two important implications. First, we have estimated the productivity of an ecosystem before it was altered by humans, continued Kowalewski. In the case of the Colorado River Delta, we showed how the abundance of shellfish has been dramatically reduced by agriculture and dams. Environmental scientists and conservation biologists cannot produce this type of estimate, only paleontological data can provide such insights. The second implication is methodological. We can apply our approach that combines paleontology, geochemistry, field geology, geochronology, and marine ecology to almost any coastal system, and thus gain important information about their pre-industrial state.”

Islands and beaches lining the Colorado Delta, where the river empties into the Gulf of California, are made up of at least two trillion white clam shells. By combining satellite data with radiocarbon and amino acid dating, the researchers examined specimens of clam shells from the delta coming from the last 1000 years before dams and irrigation projects, started in the 1930’s, diverted Colorado River water from the estuary.

Because shells grow as the animal secretes new layers of material, the scientists could measure the life span of long dead clams by counting the seasonal oxygen isotope cycles in the shell -in the spring, water from snow melt has a much lighter isotope signature. They discovered that the average clam lived three years, thus in 1,000 years, there were 333 generations of clams and so, at any time in the past there were six billion clams living on the delta, with an estimated density of 50 clams per square meter. This number has now been reduced to about three clams per square meter, indicating a 95% decrease in the shellfish productivity in the delta since the river was diverted by humans in around 1950.

Since 1981, co-ordinated efforts have been made to release more water into the delta by both the US and Mexico, resulting in some revival of bio-productivity in the upper delta, but not in the lower delta. The researchers, who also include G.E. Avilo Serrano from the Universidad Autonoma de Baja California, and G.A. Goodfriend from George Washington University, found only 12 live specimens of the mollusc Mulinia coloradoensis, but its dead shells indicate that it was the most prolific species, accounting for 90% of the delta’s shells. The decline in shell fish has a negative impact on the entire area’s ecosystem, as they form a vital part of the local wildlife’s food chain.

The research has important implications for the restoration of delta ecosystems, as it can be used to gain insights into their original conditions and assess the negative consequences of water management, especially important for deltas where no biological survey was previously carried out. The researchers also say that the approach can be used almost anywhere, including rivers, deltas, coastal zones or marine shelves because shellfish are common and their shells survive around the surface for hundreds of years. “We are at the initial stages of conducting research off the coast of Sao Paulo state in Brazil, an immensely populated area,” Kowalewski said. “By looking at old shells we can estimate when the pollution started and by how much the levels of harmful elements have increased since pre-industrial times.” He added that first reports would be available within a year.

The research report has been published in the December issue of the journal Geology.

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