Sources and sinks of nutrients and organic carbon during the 2014 pulse flow of the Colorado River into Mexico

For the first time in history a short pulse of water was intentionally released for environmental purposes in April-May 2014 into the final 160 km stretch of the Colorado River, south of the Mexican border. During this pulse the sources and turnover of dissolved nutrients (nitrate, ammonium, phosphate, silicate and carbon), particulate organic matter, along with stable isotopes of suspended particulate carbon and nitrogen, were studied. The river water, as it flowed along the dry riverbed became rapidly depleted in N-NO3 (from 21 to 0.5 μmol L−1), while dissolved organic carbon of terrestrial C3 plant origin became depleted in 13C and thus more negative δ13CDOC values. At the confluence with the Hardy River, the pulsed water along the Colorado River mixed with perennial return flows from agriculture with high δ15N values (+25‰) due to microbial uptake of NH4 and discrimination against 15N and/or volatilization of ammonia. From nutrient and isotopic evidence, no mixing of the CR water with seawater was evident, which had a typical phytoplankton δ13C composition of about −20‰. However, increases of chlorophyll-a after the pulse ended (up to 32 mg L−1) suggest a possible effect of the pulse on estuary primary productivity.