Fossil clam shells reveal unintended carbon cycling consequences of Colorado River management

Abstract

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 t C yr−1 (4.1–10.6 mol C m−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.

A downcore increase in time averaging is the null expectation from the transit of death assemblages through a mixed layer

“…13A). This transience does not reduce the value of SML assemblages to conservation and management, that is, this youngest, newly dead part of the fossil record is the most relevant for detecting recent ecological and environmental change from live–dead discordance, for reconstructing ecological dynamics by age unmixing of these assemblages, and for analytically estimating rates of carbonate shell recycling and sequestration (e.g., Cramer et al 2012; Martinelli et al 2016; Smith et al 2016; Kusnerik et al 2020; Albano et al 2021; Dillon et al 2021; Scarponi et al 2022; Kokesh and Stemann 2023; Tomašových et al 2023). Rather, we simply need to recognize that the permanent fossil record will in many cases be dominated by lower-resolution, more time-averaged IML assemblages.…”

Characteristics and Process Interactions in Natural Fluvial–Riparian Ecosystems: A Synopsis of the Watershed-Continuum Model

“…Colloquially known as dry washes, arroyos, or wadis, ephemeral channels are extremely abundant, comprising far more than half of the global stream channel network [170], and are becoming increasingly abundant as humans and climate change dewater rivers (e.g., [197]). Flooding releases CO 2 sequestered by seasonal or erratic burial of organic matter and invertebrates, such as leaf litter or clams [198]. Benthic invertebrates that shred, graze, or collect organic debris often are generally absent or rare in ephemeral FREs, reducing decomposition rates and transferring those functions to microbial and physical molar actions when the stream floods.…”

Benthic communities under anthropogenic pressure show resilience across the Quaternary

“…Previous studies show that populations of the congeneric Mulinia coloradoensis from the Northern Hemisphere were decimated by a decrease in freshwater and nutrient input [15]. These changes caused by the damming of the Colorado river significantly decreased the productivity of the associated estuary and brought about detrimental ecosystem-level consequences [15][16][17]80]. Therefore, it is likely that Mulinia from Chile and Peru could have also thrived in estuary conditions during more humid weather.…”

“…In addition to overfishing, coastal areas are subject to habitat modification, deviation of watercourses, runoff of pollutants, aquaculture, and among others [ 7 , 12 , 14 , 15 ]. For example, the damming of the Colorado river caused the collapse of once very abundant populations of the clam Mulinia coloradoensis, leading to a dramatic drop in productivity [ 15 ], changes to trophic structure [ 16 ] and a reduction in carbon emission in the river basin [ 17 ]. Ever-growing demands for more food have led to poor aquaculture practices that bring about changes in community structure, ecosystem function, eutrophication and outbreaks of disease [ 5 , 18 ].…”