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We calculated average aquatic dissolved organic carbon (DOC) age (the time span from soil discharge to observation) in water from the inlets and outlets of two unproductive Swedish lakes at different times during an annual cycle. Bacterial production (BP) and bacterial growth efficiency (BGE) determined during 7-d bioassays decreased with increasing average aquatic DOC age. Parallel to the declines in BP and BGE there was a rise in specific ultraviolet absorbance at the wavelength of 254 nm (SUVA254), which indicates that decreasing BP and BGE were connected to a shift to a more aromatic and recalcitrant DOC pool. The relationships between bacterial metabolism and DOC age were stronger after a Q10 correction of the DOC age, showing that temperature affected rates of DOC quality changes over time and should be taken into account when relating lake bacterial growth to substrate aging in natural environments. We propose that hydrological variability in combination with lake size (water renewal time) have a large influence on pelagic BP in lakes with high input of terrigenous DOC. [ABSTRACT FROM AUTHOR]

Recruitment ofAnabaenaandAphanizomenonfrom the sediments to the water column was investigated in shallow (1–2 m) and deep (6–7 m) areas of Lake Limmaren, central Sweden. Recruitment traps attached to the bottom were sampled weekly throughout the summer season (June through September). A comparison between the two sites shows that the largest part of the recruited cells originated from the shallow site, although recruitment occurred at all depths in the lake. There were also differences between the species, regarding the site as well as the timing of the recruitment. The contribution of the inoculum to the pelagic population was calculated to vary between 0.003% and 0.05% for the different species. From these results we conclude that shallow sediments are more important than deep ones for the recruitment and that the inoculum in Lake Limmaren is small but may still be an important factor in the population dynamics. [ABSTRACT FROM AUTHOR]

Bacterial grazing was measured from June to August 1995 in Lake Örträsket, a deep brown-water lake in northern Sweden. Mixotrophic chrysophytes were the dominating bacterivores at all times, grazing 3-14% of bacterial standing stock daily. The effects of altered nutrient supply and light availability on grazing activity and growth were studied in two mesocosm experiments. Incubation in the dark did not stimulate phagotrophy, which would otherwise be expected if bacteria were mainly being used as an energy source. Furthermore, clearance rates were not reduced after alleviation of nutrient limitation conditions. Rather, phagotrophy may work as a relatively fixed attribute of the mixotrophic community in this lake. When availability of dissolved nutrients is restricted, phagotrophy permits the mixotrophs to outcompete other phytoplankton, but they become less competitive at high nutrient concentrations. The relative share of mixotrophs in relation to total phytoplankton decreased considerably after enrichment with nitrogen + phosphorus. [ABSTRACT FROM PUBLISHER]