Intra-annual patterns in biofilm communities and cellulose decomposition in a headwater stream network with spatially variable groundwater inputs

The role of groundwater as a driver of spatial and temporal heterogeneity of ecological conditions in streams is not well understood, particularly at larger spatial scales. To evaluate the association between groundwater and ecological communities and processes, we assessed stream biofilm communities (biomass and diatom assemblage composition) and cellulose decomposition over four seasons in 19 reaches with varying amounts of groundwater input in a headwater stream network in southern Ontario, Canada. Seasonal variation was found to drive diatom assemblage composition and cellulose decomposition among seasons. Moreover, there were clear seasonal patterns in taxa succession in the diatom assemblages where the dominant taxa shifted in accordance with ecological preferences of the taxa. Within season assessments of stream biofilms among individual reaches showed that biomass, diatom assemblage composition, and cellulose decomposition were not associated with amount of groundwater input. The lack of concordance between groundwater influence and stream biofilm condition suggests that the groundwater signal may have been overwhelmed by surface water influences, such as nutrients and thermal effects, at the reach scale. Our findings also demonstrate the importance of assessing stream biofilm communities and cellulose decomposition in multiple seasons to identify and understand discrete shifts in ecosystem conditions of temperate streams.