Seasonal changes in primary production and respiration in a subtropical lake undergoing eutrophication

The balance between gross primary production (GPP) and respiration (R) is frequently used to estimate the role of lakes in the carbon cycle. Seasonal changes in the carbon cycle of subtropical lakes are often underestimated, but changes in meteorological and limnological characteristics often follow the well-defined climatic seasons. Based on 1 year's free-water dissolved oxygen and temperature measurements, we investigated the seasonal changes in primary production and respiration in subtropical Peri Lake in Southern Brazil, which is currently undergoing eutrophication. We expected that periods of high light availability and temperature would lead to a net autotrophic condition. Furthermore, we explored the seasonal coupling between GPP and R, expecting that different sources of organic matter would have different effects on the metabolic rates. We found that Peri Lake was predominately net heterotrophic (GPP  < R). GPP was high during summer and autumn and low in winter, as was R, coinciding with the seasonal changes occurring in light and temperature. Light conditions were of essential importance for the variations in GPP, while respiration was fueled by both autochthonous and allochthonous organic matter. Constant external input of organic matter resulted in a generally low coupling between GPP and R. A tighter coupling between GPP and R was observed in spring as a result of higher productivity, while a decoupling in autumn was due to intensified allochthonous organic matter runoff caused by rainfall and wind. We found that higher productivity rates in summer did not shift the system to an autotrophic condition and that Peri Lake functioned as a carbon source, light and organic matter being the prime drivers for the metabolic rates.