Volumetric and aerial rates of heterotrophic bacterial production in epi-and hypolimnia:the role of nutrients and system morphometry

摘要

Epilimnetic and hypolimnetic bacterial production (BP) were measured once in summer,by the incorporation of [~3H]-Leucine in each of 14 Quebec (Canada) lakes varying in nutrient content and morphometry.The epilimnetic and hypolimnetic BP were evaluated at two scales:the common per unit volume and areal (m~(-2) scale.The per unit volumes scale epilimnetic BP was best predicted by total phosphorus (TP,r~2=0.63),and by water residence time (WRT r~2=0.57),with WRT serving as a surrogate for the nutrient and organic matter supply from the catchments.Total phosphorus and lake mean depth (Z_m) together explained 79% of the variation in epilimnetic BP (1~(-1)).In contrast,hypolimnetic BP (1~(-1)) was neither linked to nutrients (TP or TN) or dissolved organic carbon (DOC) but only to measures of lake morphometry and best of all to hypolimnetic thickness (Zh;r~2=0.74).With increased Zn,there is an increased dilution of settling organic partices and their nutrients,resulting in a decrease in BP per litre.Conversely,when BP is expressed in areal units (m~(-2)),hypolimnetic production increases with increasing hypolimnetic thickness.Water column thickness is a master variable,which together with Chl a (abundance of particles) determines hypolimnetic BP at the whole system scale even though the trophic status is the best single indicator of epilimnetic BP on a volumetric scale.Conclusions drawn invariably change with the scale of investigation.Moreover,it is clear that lake morphometry has a major impact on BP.A comparison of whole water column integrated BP with literature derived estimates of the equivalent sediment prodiction (m~(-2)) below suggests that if the estimated sediment rates are not complete technique artefacts,they are likely to be an order of magnitude higher than the water column rates (m~(-2)) at the maximum depth sampling sites.The relative importance of the sediments could be expected to rise with a decline in the maximum depth of lakes,characterized by progressively thinner hypolimnia.The present findings point to both a primarily allocthonous fuelling of sediment production and an uncoupling of water and sediment BP.