Answer: see below.
Explanation:
gases dissolve more in colder water, so in summer the water can become oxygen depleted. Evaporation in summer can reduce water volume; even in summer rainfall climate evaporation rates may exceed precipitation. Evaporation will also cause higher conentrations of solutes of which some may reach toxic levels. Summer sun may also cause algal or cyanobacterial blooms which can further deplete dissolved oxygen and may release toxins into the water.
