Limnetica 35

Nutrient fluxes take place in lakes through different flow paths, and the importance of their nutrient flux changes depend on the biogeochemical processes involved. In flow-through lakes, the hydrological control of these flux processes appears to be very important, but the significance of other processes is poorly understood. To test this significance, we analyzed C, N and P fluxes on a weekly scale during mid-stratification in a Spanish calcareous lake fed by agriculturally-drained streams and groundwater. As expected, hydrologic flow paths were more important than those of biogenic fluxes for the nutrient budgets. The organic C (OC hereafter), total N and total P surface inflows and outflows differed. While C and N surface hydrological fluxes covaried, P input or output fluxes were unrelated with those of other nutrients. Net groundwater C and P fluxes covaried, but they were not related with those of N. The accumulations of OC, N and P were also related with each other. All of the biogenic fluxes were highly variable. Primary production by emergent plants (Cladium) were higher than water column photosynthesis, the latter being net autotrophic most of the time. Particulate nutrient settling rates were within the range of those in oligotrophic lakes. Denitrification was not as high as expected in the nitrate-rich water of this lake, which was limited by low levels of soluble reactive phosphorus and dissolved organic carbon. CO₂ degassing from the surface to the atmosphere was always positive, suggesting net heterotrophy, but this was the opposite of previous results. These conflicting results could be reconciled by the high hydrological inputs of inorganic carbon, which yielded more carbon dioxide to be outgassed. Albeit low, the CO₂ production from CaCO₃ formation could sometimes be a source of CO₂. However, the biogenic fluxes were higher than expected in flow-through lakes, mostly arising from the role of littoral emergent plants in nutrient fluxes, which was often neglected in biogeochemical budget studies. Furthermore, no relationship was found between water renewal time and accumulation, thus emphasizing the significance of biogenic fluxes on nutrient retention. At the ecosystem level, our study highlighted both the importance of the hydrogeological setting of the basin on nutrient fluxes and the importance of emergent vegetation on lake nutrient budgets.