The positive correlation between BOD and DO at 24 of 49 sites is at first sight surprising, only four sites showing a significant negative relationship. While excessive plant growth (macrophytes and algae) may deplete DO at night through respiration, diurnal photosynthesis will increase DO during the day when it is measured. Excessive plant growth will, during its senescence, also increase organic load and BOD [18,4], likely explaining the apparent anomalous positive relationships between DO and BOD based on summer mean values. It should also be noted that high algal populations may also register high BODs (effectively sample respiration) measured over 5 days in the
laboratory, methodological differences being the reason for the positive relationship between DO and BOD. This relationshiprequires further detailed investigation. The
general decline in BOD over time at a number of stretches could be related to the provision of phosphate stripping at some STWs, resulting in a decline in algal
populations.