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

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 volume 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 (l^–1). In contrast, hypolimnetic BP (l^–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 Zh, there is an increased dilution of settling organic particles 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 production (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.