Seasonality
Water quality in this study showed strong seasonality. BOD and the
concentrations of ammonia-N, nitrate-N, and orthophosphate were highest
in the fall and winter (Figures 4, 6, 7, 8, respectively). These times
coincide with the lowest flows in the river (Figure 3). Low flow can
result in higher concentrations of nutrients as there is less water for
dilution. In addition, biological uptake of nutrients is lower during
the cold months of the year (Olatunde et al., 2015; Rolls et al., 2012).
Not surprisingly, water temperature also showed strong seasonality, with
temperatures reaching their maximum values in July (max 25.9oC) (Figure 9). DO saturation is inversely related to
temperature but is also influenced by hydrology, BOD, and pH. In this
study, we measured the lowest DO values in early fall when temperatures
were still relatively high and flow in the river was low (Figure 5).