1. To help improve our understanding of the nitrogen cycle in lakes, particularly in the context of
climate change, we analysed total nitrogen (TN) and nitrate (NO
3 -N) data from six mesocosm experiments
(in Denmark, U.K., China and Turkey) covering different climatic regions. We assessed the effects
of nitrogen (N) and phosphorus (P) loading, temperature, salinity and water level on N processing.
2. Water column N loss (defined as the nitrogen processed in and lost from the water column in units
of net amount processed per unit area and per unit of time, or in relative terms as the percentage loss
of the total pool in 2 weeks) was particularly sensitive to external nutrient loading to the mesocosms.
Mean water column TN loss at high N loading varied from 111 to 250 mg m
�1 and increased
with N loading. High P loading resulted in increased water column N loss, possibly because of
increased uptake into plants and attached algae and sedimentation of the increased algal crop. High
salinity generally decreased water column TN loss; on average, 10% more TN was in the water column
at 12& salinity than at 2& salinity, while no significant effect of water level was found.
3. Only weak relationships were observed between N processing and temperature, and mesocosms
limited by P accumulated more nitrogen in their water columns than those with high P loadings. Our
results suggest that N processing in lakes appears to be more sensitive to features of the catchment,
such as hydrology and loading, than to climatic effects related to temperature, salinity and water level.