The Colorado River Delta: A high primary productivity ecosystem

Abstract

The construction of major dams in the Colorado River has produced a drastic effect on the amount of fresh water entering the Colorado River Delta. After 1935 there was a dramatic decrease in fresh-water input, causing ecological impacts and a decrease of organisms in the natural habitat. We carried out six spatial samplings during a period of no fresh-water inflow to the delta, in June, August and November 1989, and February, April and June 1990; during each sampling, a 24-hour time series was conducted at one station and seawater samples were taken every two hours. Also, in May 1991 we carried out an eight-day time series, taking samples of seawater at 08:00, 12:00 and 16:00 hours. The objective was to estimate phytoplankton abundance, chlorophyll concentration and primary productivity during a period of no fresh-water input. In the spatial distribution, the chlorophyll a concentrations and phytoplankton abundance were higher on the Baja California side than on the Sonora side, with concentrations of 2.6–18.2 mg m^–3 and 1.8–12.7 mg m^–3 and of 274 and 166 cells mL^–1, respectively. In the time series, the dinoflagellates were more abundant than the diatoms. The primary production showed a tendency to decrease from August 1989 to June 1990, with values of 15.0 to 0.5 mgC m^–1. The primary productivity values in the time series presented a tendency to increase during the tide transition, reaching values of 76 mgC m^–1. The variations of chlorophyll concentration, phytoplankton abundance and primary productivity were strongly influenced by the effects caused by the tidal currents.