Effects of sediment deposition on periphytic biomass, photosynthetic activity and algal community structure.

Suspended solids and siltation are among the most prevalent problems in streams and rivers of the world; however, because they are often associated with other stresses such as increased nutrient concentrations or changes in channel form, their impacts on the biota and on ecosystem functioning are not fully understood. To assess the effects of pulse sediment deposition on periphyton, we applied an exponential gradient of clay concentration (from 0 to 54.7 g L(-1)) for three days to eleven artificial indoor channels precolonized by algae (three controls+eight treatments). This resulted in a gradient of inorganic particulate matter in the bottom from two to over 200 gm(-2). Periphytic biomass, photosynthetic activity and algal communities were studied during the following four weeks. High sediment loads (>6 g L(-1)) initially reduced algal growth but by the end of the experiment periphytic biomass was similar in all channels. Under high sediment load, algal photosynthetic efficiency showed a quick decrease after three days of exposure, followed by a delayed increase in chlorophyll a contents. After two weeks signs of adaptation were observed, first as an increase in photosynthetic efficiency and then as an increase in pigment concentration. Siltation led to changes in community structure; diatoms increased in high silt treatments although green algae still dominated. Overall, the accumulation of fine sediment affected periphytic biomass, photosynthetic activity and community composition. Periphyton adaptation reduced the initial impact, reaching almost full compensation in terms of chlorophyll a and photosynthetic activity; however, algal community composition did not recover within the time frame of this study. Thus, the frequent siltation pulses observed in many streams throughout the world may have an important impact on the periphyton, which would in turn affect stream ecosystem structure and functioning.