Photo-oxidation of dissolved organic carbon (DOC) to inorganic carbon (DIC) in five oligotrophic south Swedish lakes of different humic content (3.9-19 mg DOC liter-r, O-140 mg Pt liter-*, Secchi depth 7.6-1.5 m) was investigated. Sterile-filtered (0.2 pm) water was incubated in UV-transparent quartz tubes and in tubes covered with aluminum foil. Samples were incubated at different depths (0,0.20,0.65, and 2.00 m) from sunrise to sunset (0400 to 2200 hours) in July. Inorganic C was measured before and after incubation. Total plankton respiration in unfiltered lake-water (production of DIC) was also measured. At the surface, photo-oxidative DIC production was 86410 mg C m-3 d-l, while plankton community respiration was 10 l-274 mg C m-3 d-l. Photo-oxidation integrated over depth was 44-l 7 1 mg C m-2 d-r, while respiration was 201-547 mg C m-2 d-r to a depth of 2 m and 398-860 mg C m-2 d-* over the depth of the epilimnion. Depth-integrated photo-oxidation was independent of water color or DOC concentration in the lakes. Photooxidation was detected deeper than the penetration of UV-B radiation, indicating that longer wavelengths (UV-A and possibly PAR) are also active. DIC production was linearly related to loss of fluorescence (excitation 355 nm, emission 455 nm) in light-incubated samples. Our study shows that photo-oxidation of DOC may be an important process causing the regularly observed supersaturation of DIC in lakes.

We tested the hypothesis that light, especially UV‐B radiation, increases the availability of dissolved organic matter (DOM) to pelagic bacteria in lake water. Filtered (0.2 µm) and autoclaved humic lake water (water color 70 mg Pt liter−1, 12 mg dissolved organic C liter−1) was exposed to simulated sunlight (UV‐B, 1.14 W m−2; UV‐A, 3.87 W m−2; and PAR, 20 W m−2) for various periods of time (0–100 h). Irradiated water was then inoculated with a natural bacterial assemblage (0.6‐µm filtered water) and bacterial yield was measured in the stationary phase of the resulting batch cultures. Both bacterial numbers and cell volumes increased (numbers by 65% and volumes as much as 360%) with increasing UV radiation, resulting in an almost sixfold increase in bacterial biomass. Our experiment shows that light exposure can enhance availability of natural lake DOM to bacteria, possibly through cleavage of macromolecules into smaller units, and may influence both carbon cycling and food webs in lake water.

No abstract

Organic substrates for pelagic bacteria are derived from dissolved organic carbon (DOC) in the water column. DOC is a heterogeneous mixture of molecules, some of which are imported from the watershed (allochthonous DOC) and others that are produced by autotrophs within the system (autochthonous DOC). We examined the importance of autochthonous versus allochthonous DOC in supporting the growth of pelagic bacteria by manipulating the 13 C content of autochthonous sources in a whole-lake experiment. NaH 13 CO 3 was added daily to two small forested lakes for a period of 42 d, thereby strongly labeling autochthonous primary production. To obtain bacterial carbon isotopes, bacteria were regrown in vitro in particle-free lake water and in situ in dialysis tubes; little difference was found between the two methods. The contribution of autochthonous versus allochthonous carbon to the bacterial biomass was estimated by applying a two-member mixing model using a 13 C of Ϫ28‰ as the allochthonous end member. The autochthonous end member, which varied over time, was estimated indirectly by several approaches. The bacterial biomass consisted of 35-70% allochthonous carbon. This result confirms the often-stated hypothesis that autochthonous carbon alone does not support bacterial production. On the other hand, autochthonous DOC was preferentially utilized relative to terrestrial DOC. On the basis of 13 C measurements, only 13% of the DOC standing stock was of recent autochthonous origin, but it supported 30-65% of bacterial production.

1. Human activities have promoted the spread of species worldwide. Several crayfish species have been introduced into new areas, posing a threat to native crayfish and other biota. Invader success may depend on the ability to utilise a wide variety of habitats and resources. Successful invaders are generally expected to have broader niches and to be more plastic than non-invasive species. 2. Using stable isotope ratios of carbon and nitrogen we compared the niche widths of native noble crayfish and introduced signal crayfish, a successful invader of Swedish streams. The calculation of niche width took account of between-site differences in basal resource isotope signature ranges. We also assessed whether population density, prey biomass or prey diversity affected niche width. 3. At the species level, signal crayfish had twice the niche width of noble crayfish. However, individual populations of noble crayfish and signal crayfish in Swedish streams had similar niche widths. This suggests that signal crayfish has greater plasticity with respect to habitat utilisation and feeding than noble crayfish. Niche width in both species correlated positively with benthic invertebrate biomass and diversity, indicating that animal food sources are important for crayfish. 4. We find that assessing niche width in relation to invader success can be a useful tool trying to predict the impact of invasions on different scales. The findings in this study suggest that invaders and natives will have a similar impact on the stream scale whereas the invader will have a larger impact on the regional scale due to the ability to utilise a wider range of streams.