Shading effects of free-floating plants on drainage-ditch invertebrates

In small, lentic ecosystems in agricultural areas, eutrophication often results in excessive growth of small, free-floating plants. A dense layer of plants on the water surface changes the underwater light climate drastically and in turn leads to hypoxic or even anoxic conditions. Knowledge of the effects on macroinvertebrates of reduced light conditions and oxygen stress as result of eutrophication is limited. We thus examined in a field situation the influence of an unpredictable, 10-day period of low oxygen availability as a result of poor underwater light conditions. In a before-after control-impact design, the underwater light climate and dissolved oxygen concentration of ditch sections were manipulated, and the macroinvertebrate assemblage composition was recorded during the 4 weeks before and after treatment. A poor underwater light climate in combination with normoxic conditions did not affect the invertebrate assemblage composition, but the combination of low-light intensity and anoxic conditions did alter it. Interestingly, these changes were not apparent directly after treatment but developed in the weeks following, indicating that although the invertebrates could cope with a shading-induced period of hypoxia, costs were associated with the event over a longer time period.