Treatment wetlands are solar powered ecosystems. Solar radiation varies diurnally, as well as on an annual basis. Wetland processes are therefore driven to respond on these same two cyclic periods. The first and most obvious effect is on the temperature of the water and soils in the wetland. Intense summer radiation results in warmer conditions and higher evapotranspiration. Winter radiation is smaller, and results in cooler temperatures and less evapotranspiration. Other meteorological variables, such as air temperature, humidity and precipitation, also have annual cycles, but with considerable stochastic variability. The water and soil temperature variations cause changes in microbial activity, which in turn creates changes in microbially-mediated water quality improvement. The cyclic changes in rain and evapotranspiration may create significant effects on the water budget for the wetland, and thus influence treatment efficiency. In addition, there are seasonal cycles in the vegetation and litter in the system, which occur in response to solar inputs and meteorological factors. This causes seasonal changes in nutrient and chemical uptake and release. This combination of cyclic influences is reflected in the treatment performance of the constructed wetland. It is shown that wetland water temperature alone is not a sufficient descriptor of wetland biogeochemical cycles. Mass balances demonstrate cyclic interactions in treatment wetlands. The effects of vegetative cycles are quantified for an example system.