Fossil clam shells reveal unintended carbon cycling consequences of Colorado River management
Authors
Water management that alters riverine ecosystem processes has strongly influenced deltas and the people who depend on them, but a full accounting of the trade-offs is still emerging. Using palaeoecological data, we document a surprising biogeochemical consequence of water management in the Colorado River basin. Complete allocation and consumptive use of the river’s flow has altered the downstream estuarine ecosystem, including the abundance and composition of the mollusc community, an important component in estuarine carbon cycling. In particular, population declines in the endemic Colorado delta clam, Mulinia coloradoensis, from 50–125 individuals m−2 in the pre-dam era to three individuals m−2 today, have likely resulted in a reduction, on the order of 5900–15 000 tCyr−1 (4.1–10.6 molCm−2 yr−1), in the net carbon emissions associated with molluscs. Although this reduction is large within the estuarine system, it is small in comparison with annual global carbon emissions. Nonetheless, this finding highlights the need for further research into the effects of dams, diversions and reservoirs on the biogeochemistry of deltas and estuaries worldwide, underscoring a present need for integrated water and carbon planning.
Figure 1. A subset of the processes involved in estuarine carbon cycling. (1) Sequestration of carbon via vegetation (e.g. salt marshes, mangroves) growth, death and burial; (2) emission of carbon due to reworking of carbon-rich sediments; (3) constant gas exchange
between ocean and atmosphere; (4) emission of carbon via respiration bymicrobes and zooplankton; (5) sequestration of carbon via burial of dead plankton; (6) filter feeding by bivalves; (7) carbon emission via bivalve respiration; (8) carbon sequestration via biodeposition and (9) carbon sequestration and emission via biocalcification.
