Trophic status and lake sedimentation fluxes

In limnological studies the measure of sedimentation fluxes of seston is neglected, in spite of the importance it can have in determining water quality, studying biogeochemical cycles, evaluating the distribution of chemical species, etc. Often sedimentation is obtained only from mass balance models, not taking into account the fact that the uncertainty of determining inputs and outputs makes this evaluation from their difference rather unreliable; other factors of the balance, such as exchanges with the atmosphere, between water and sediments, are equally difficult to define. Though the direct measurement of sedimentation also presents some methodological and logistic difficulties, such as resuspension of material from the bottom, grazing, etc., this does not justify the very scarce attention paid to this kind of determination. This paper reports the sedimentation fluxes of 39 lakes, in different parts of the world, having different limnological and trophic characteristics (0.4 < TP < 369 mu g L-1; 0.5 < chlorophyll a <50 mg m(-3)). The fluxes of PM, C, N and P show a log-log relationship (r approximate to 0.6, p less than or equal to 0.05) with the common trophic variables (Secchi disk, total phosphorus, chlorophyll and primary production), independently of the morphometric characteristic of lakes. Hence sedimentation seems not to be an intrinsic property of the environment but is related to the trophic state of the system. The results achieved tend to confirm that the nature of sedimenting seston is generally autochthonous, even though the poor correlation between PM and the same trophic variables suggests that PM is more influenced by allochthonous material.