Because of these daily considerations, it is often helpful to select consistent sampling times for many water quality parameters. However, ideal sampling times may also depend on tides, which could expose nothing but mud where there had been water only a few hours earlier. Tidal stages vary from day to day. Time of Year Environmental conditions change with the seasons, and monitoring results can reflect those variations. For example, nutrient and pesticide concentrations in estuaries vary considerably from season to season.
More runoff enters the estuary during wet weather periods, delivering pollutants and fresh water.
Consequently, pollution concentrations can be higher and salinity lower at these times. On the other hand, dry weather periods mean less runoff to the estuary. Higher salinity and lower pollutant levels may mark such dry periods. The same observation may be made in colder climates during the winter, when snow remains on land and the spring thaw is months away. The seasons have a profound influence on several other water quality measures, particularly dissolved oxygen.