THE POWER OF CLAMS: BIVALVE MOLLUSKS AS ECOSYSTEM ENGINEERS IN THE COLORADO RIVER ESTUARY

In the era before upstream dams and diversions, abundant filter-feeding bivalves may have cleared the water of suspended sediment and made the Colorado River estuary a nutrient-limited system. Today, despite the lack of fluvial sediment, productivity in the estuary is limited by light – light can penetrate only a few centimeters into waters made turbid by the resuspension of delta mud. In the “predambrian”, an estimated six billion clams inhabited the shallow subtidal and they may have profoundly affected their habitat through filter-feeding and the production of feces and pseudo-feces. Literature-based estimates of clearance rates of filter-feeding bivalves range from 0.0036 to 22 liters per hour per gram of dry tissue weight. Using the median rate of 2.0 L/hr/gram, a dry tissue weight of one gram and continuous year-round filtering, the collective efforts of six billion clams could have cleared 105 x 10⁹ m³ per year – a volume approximately five times greater than the 18.0 x 10⁹ m³ annual flow of the river. Even if the clams were working only halftime, clearance rates would have exceeded flow rates even during the annual spring flood. Production of feces and pseudo-feces during filtering would have deposited sediment as organic-rich pellets close to the river's mouth. Clearing the water of suspended sediment would have allowed greater light penetration and could have made productivity limited by the supply of river-borne nutrients rather than by light. These estimates, crude as they are, help explain very high predambrian productivity in a system currently limited by light. Paradoxically, the estuary's water may have been clearer when the river still delivered sediment. The bivalves of the Colorado River estuary may have been superb ecosystem engineers, affecting the sedimentary regime of their habitat as well as their own food supply.