The temperature of a body of water directly affects the amount of dissolved oxygen it can hold.  Following Henry's law, as water becomes warmer, oxygen becomes less soluble in it.  This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on larger scales.  Thus, bodies of water are more vulnerable to anoxic conditions during the warmest period of the day and during summer months.  This problem can be further exacerbated in the vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released.
Daily cycles are also influenced by the activity of photosynthetic organisms.  The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise.