Seasonal photoreactivity of dissolved organic matter from lakes with contrasting humic content

We studied seasonal variability in photodegradation of dissolved organic carbon (DOC) resulting from artificial ultraviolet-A (UV-A) and UV-B irradiation. Water samples were taken approximately monthly from the surface layers of two oligotrophic lakes with contrasting humic content, situated in southern Sweden. Lake water was filter-sterilized (0.2 μm) and exposed to artificial UV radiation in quartz tubes. Potential DOC photodegradation, measured as a photoproduction of dissolved inorganic carbon (DIC) and oxalic, malonic, formic, and acetic acid in irradiated samples, was observed throughout the sampling period. In addition, exposure to UV radiation resulted in a decrease in DOC, absorbance, and humic substance fluorescence. The photoproduction of DIC and the low molecular weight (LMW) organic acids varied seasonally, being generally higher in winter and spring (December-May), while DOC appeared to become less photoreactive after the extensive exposure to solar radiation during summer. Production rates of both DIC and LMW organic acids were approximately eight times higher in the humic lake despite that the DOC concentration was only two times higher than in the clearwater lake. This is most probably due to the high input of allochthonous DOC and the resulting higher absorbance to DOC ratio in the humic system. Furthermore, the longer hydraulic residence time in the clearwater system could have resulted in an accumulation of residual DOC, recalcitrant to further photodegradation

Diurnal and seasonal variations in grazing by bacterivorous mixotrophs in an oligotrophic clearwater lake

The biomass and bacterial ingestion rates of mixotrophic phytoflagellates were examined on diurnal and seasonal scales in oligotrophic Lake Skarlen (S. Sweden). Ingestion rates were measured using fluorescent microspheres in situ on three depths every 6(th) hour during 24 hours in summer in order to quantify diurnal variations and once at midday during autumn, winter and spring. Mixotrophic phytoflagellates composed a large part of the phytoplankton biomass (18-31 %) throughout the year. Nonetheless, calculations suggest that the mixotrophs only consumed between 0.5 and 4.4 % of the bacterial carbon production, with the highest consumption percentage in winter under ice. Two groups of algae, Cryptomonas spp. and Dinobryon spp., were commonly found with ingested prey and both exhibited significant diurnal differences in bacterial ingestion rates with the highest mean rates measured in late afternoon (0.50 and 0.27 bacteria cell(-1) hour(-1), respectively) and very low rates during night (0.09 and 0.06 bacteria cell(-1) hour). In Cryptomonas spp., ingestion rates also varied.somewhat over the year and bacterial ingestion was highest during winter (0.57 bacteria cell(-1) hour), possibly as a result of the lengthy impaired light climate

Nutrient limitation of autotrophic and mixotrophic phytoplankton in a temperate and tropical humic lake gradient

Nutrient enrichment experiments were carried out in three tropical (once) and three temperate (twice) lakes differing in humic content in order to examine whether there was a relationship between the limiting nutrient for algal growth [nitrogen (N) or phosphorus (P)] and humic content, and whether the prevailing limitation was connected to the relative abundance of autotrophic and phagotrophic phytoplankton (mixotrophs). In both climatic regions, there was a stronger tendency for total phytoplankton biomass accumulation to be N limited in lakes with a high humic content. However, in contrast to what we expected, there was no tendency for the mixotrophs to be more favored by the addition of N than of P. In the temperate lakes, the relative abundance of mixotrophs increased in the treatments receiving N or P separately or no nutrients (control) when exposed to a high light availability. In the following year, when the light availability was low, the mixotrophs increased relative to the obligate autotrophs in all treatments, irrespective of nutrient addition. Possibly, this was a result of their ability to supplement photosynthesis with the ingestion of prey. The results indicate that mixotrophy is an advantageous strategy when the availability of light and/or nutrients is low

Effects of fetch and dissolved organic carbon on epilimnion depth and light climate in small forest lakes in southern Sweden

We sampled small (0.03 to 3.3 km(2) surface area) forest lakes in southern Sweden during two summers to investigate how concentration of dissolved organic carbon (DOC) and lake size influence epilimnion depth (z(e)) and light conditions in the epilimnion. z(e) increased with increasing fetch (square root of lake area) but did not decrease with increasing DOC concentration. This suggests greater importance of wind mixing in our study area than in areas with more continental climate, i.e., epilimnion-deepening wind mixing overrides the tendency for epilimnion depth to decrease with increasing DOC concentration. Extinction of photosynthetically active radiation was mainly caused by DOC. The euphotic zone was shallower in the first year than in the second, probably because of higher precipitation and lower solar irradiance in 2007, which together led to higher water color. Mean epilimnetic irradiance was low, especially in 2007, mainly because the increase in DOC did not lead to shallower epilimnia. Browning of lakes in southern Sweden, in combination with a predicted increase in the number of storm events, may lead to more severe light limitation of phytoplankton in small, nutrient-poor lakes, since it may not be accompanied by a compensatory shallowing of the epilimnion. As a consequence, lake ecosystems will become more heterotrophic, CO2-evasion to the atmosphere will increase, and fish production decrease

Increased photoreactivity of DOC by acidification: implications for the carbon cycle in humic lakes

Effects of ultraviolet (UV)-B radiation and acidification on pelagic carbon flux in a humic lake (dissolved organic carbon [DOC] �15 mg C L�1) were studied in a mesocosm experiment during the summer of 2000. Triplicate tanks (107 liters volume, 1 m high) were exposed to natural solar radiation, a daily extra dose of UV-B radiation, or kept dark. One set of tanks was submitted to a decrease in pH (from 5.7 to 4.7), and one set was kept at the natural pH level. During 70 d, water samples were taken regularly from the mesocosms for measurements of DOC, absorbance, dissolved inorganic carbon (DIC), and pH. Additionally, we regularly incubated samples to measure photooxidation rates, primary production, and community respiration. We found an increase in the photooxidation rates in the acidified mesocosms relative to ambient pH. The greater abiotic production of DIC (i.e., photooxidation) in acidified conditions could explain �27 % of the decline in DOC in the same conditions. Laboratory experiments were done to test the effects of pH on the dissolved organic matter (DOM) photoreactivity. At lower pH values, we found both higher abiotic DIC production and specific absorbance fading, relative to neutral pH values in water from a humic lake. In a separate experiment, samples were exposed to prolonged irradiation under laboratory conditions, resulting in complete loss of absorptivity in the wavelengths between 290 and 400 nm. Decreases in DOC in the long-term exposure caused by photochemical mineralization were �45 and 55 % of the initial pool for natural p

Photochemical mineralization of dissolved organic carbon in lakes of differing pH and humic content

Photochemical mineralization of dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) was studied in surface water from 10 oligotrophic lakes in Southern Sweden differing in DOC, pH, and iron content. In addition, we studied the effects of acidification on the DOC photoreactivity. Samples from each lake were filtered through 0.2 mum-filters, acidified by one pH unit or kept under ambient pH conditions, and incubated in quartz tubes, in situ and in the laboratory. We found a positive relationship between photochemical DIC production at the surface and DOC concentration or absorbed dose of UV radiation. On average, lake water treated with acid showed higher rates of photooxidation than lake water with ambient pH. However, the difference in rates of photooxidation between acidified and non-acidified treatments was only evident for lake water with high DOC and iron content (i.e. lakes with DOC concentration > 10 mg C/l, and iron concentration > 400 mug/l). Results from this study indicate that acidification may increase photochemical mineralization, especially in humic lakes. Finally, we showed that a substantial increase in UV-B radiation (e. g. a doubling) would influence the rates of photochemical mineralization of DOC in humic lakes (DOC > 10 mg C/l) substantially less than a rather moderate decrease in pH of one unit. In clear lakes an increased UV-B radiation would have a stronger impact on areal DIC-production than a decrease in pH

Influence of solar radiation on the availability of dissolved organic matter to bacteria in the Southern Ocean

The influence of solar radiation on the ability of dissolved organic matter (DOM) to support bacterial growth, was studied in the eastern Atlantic sector of the Southern Ocean during the SWEDARP 1997/1998 cruise with SA Agulhas December 1997-February 1998. Vertical profiles of water samples (2-3000m) were obtained from the Spring Ice Edge (SIE, 60degreesS, high chlorophyll-a) and from the Winter Ice Edge (WIE, 56degreesS, low chlorophyll-a) areas. Filter-sterilized water from each area and depth was incubated under natural solar radiation and in the dark for comparison. Photobleaching of humic substance fluorescence occurred in all studied water samples. The bleaching was typically larger in the initially more fluorescent deep waters, compared to the low-fluorescent surface waters. Both the irradiated water and the dark controls were re-inoculated with a mixed bacterial inoculum from the initially sampled water. Bacterial growth (accumulating cells) and bacterial production (protein synthesis) were monitored during a 16-19 day incubation of these cultures at near in situ temperature (2 degreesC). Bacterial growth in cultures prepared from SIE water was largest at the surface (2-25 m), while the growth in corresponding cultures from the WIE did not vary much over depth. In contrast to the observed photobleaching, no clear effects of the irradiation on the ability of the DOM to support bacterial growth could be observed in either of the experiments. Hence, the degradation of fluorescent structures and other photochemical alterations of the organic matter did not have a major effect on the total pool of biodegradable organic substrates. The lack of effects of photoreactions on bacterial growth potential in the present study disagrees with the short-term bacterial growth response observed in other oceanic environments. This could be due to the different experimental approaches employed (short-vs. long-term incubations) or may indicate that the impact of photoreactions on bacterial growth in the Southern Ocean differ substantially from aquatic systems that are more influenced by terrestrial environments, as well as the warmer oligotrophic oceanic environments. (C) 2004 Elsevier Ltd. All rights reserved

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso- to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed.2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes.3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non-algal and detrital C to the POC pool was high.4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition