Limnetica 41

Dissolved and particulate organic matter are the energy source for secondary production in forested streams. Cycling of or­ganic matter and stream ecosystem functioning are linked to organic matter input and storage capacity and timing. This study assessed the seasonal variation (dry and rainy seasons) of environmental parameters, organic matter stock and input, and stream metabolism in two first-order tropical streams in the Selva Lacandona, Mexico. We also aimed to identify the drivers of organic matter and stream metabolism seasonality. We found seasonal variation in organic matter stock and input correlated with trop­ical seasonality. Dissolved organic matter and seston increased in the rainy season, while benthic primary producers and leaf litter stock and input increased in the dry season correlated with lower water discharge. Gross primary production increased in the dry season, while ecosystem respiration did not differ between seasons. Seasonality defined by the rainfall pattern and its effect on stream hydrology is the main driver of organic matter dynamics in tropical streams. However, environmental parameters and organic matter stock and input were not good predictors of stream metabolism.