Diel patterns of algae and water quality constituents in the San Joaquin River, California, USA

The San Joaquin River (SJR) is a hypereutrophic river that contributes to hypoxia in the downstream Stockton Deep Water Ship Channel. Oxidizable materials, in the form of algal biomass from upstream sources, contribute to the hypoxic conditions, especially from July to October. Our earlier work demonstrated the existence of strong chlorophyll-a diel cycles which complicated the calculation of algal loads for the watershed-scale monitoring program, necessary to address the total maximum daily load (TMDL) for dissolved oxygen (DO). The purpose of this study was to determine if diel patterns existed for other water quality constituents, and to determine the role of algal growth dynamics in driving these diel changes. Studies conducted between 30 June and 15 October 2004 evaluated temporal changes for several water quality constituents over four, 48 h studies at two sites along the mainstem of the SJR. Strong diel (24 h) patterns were observed for chlorophyll-a and pheophytin-a (algal pigments), temperature, DO, pH, dissolved inorganic nitrogen (DIN, NH4+-N + NO3−-N), soluble reactive phosphorus (SRP), and volatile suspended solids (VSS). Patterns of DIN and SRP were inverse of those observed for algal pigments, temperature, DO, pH, and VSS. The observed diel patterns of algal pigments and temperature were greater in the beginning of the summer (June/July) and diminished by the end of summer (September/October) due to the decreased photoperiod. Within a 24 h period the fluctuations observed in algal pigments and nutrients suggest that growth of algae during daylight hours and depletion at night are largely responsible for the observed diel patterns. Due to the observed diel variability in these water quality constituents, the samples collected for TMDL programs may not be representative unless samples are collected at the daily mean for a system.