Nutrient Concentrations are High in the Turbid Waters of the Colorado River Delta

We studied the seasonal change of the spatial distribution of nitrite (NO-2), nitrate (NO-3), reactive phosphate (PO3-4), and silicate (SiO2) in the Colorado River Delta. We also generated 24-h time series at one location to study their short-period variability. The delta is a negative estuary. During summer, salinity may be as high as 40. Amplitude of spring tides is as large as 9 m, and this causes great water turbidity by sediment resuspension. Nutrient concentrations were high throughout the whole year, with lower values towards the oceanic region. Maximum nutrient values in the river delta were 15, 53, 11•5 and 92 μM, for NO-2, NO-3, PO3-4, and SiO2, respectively. Most values were under 2, 40, 5, and 60 μM, for NO-2, NO-3, PO3-4, and SiO2, respectively. Our nutrient data show no clear seasonal pattern. Possibly, high NO-3 values in the delta are due to groundwater input, mostly at the internal extreme, and high NO-2, PO3-4, and SiO2 values are due to resuspension of sediments and mixing of porewaters with the water column, caused mainly during spring tides. In the case of NO-2, oxidation of NH+4 in the water column would be part of the mechanism. This would explain the high negative correlation between NO-3 and sea-level, and the relatively low correlation between the other nutrients and sea-level, for the time series generated at a single location.