Photochemical mineralization of synthetic lignin in lake water indicates enhanced turnover of aromatic organic matter under solar radiation

The degradation of ¹⁴C-[ring]-labelled syntheticlignin (¹⁴C-DHP) and dissolved organic carbon(DOC) from lake water were studied simultaneously.¹⁴C-DHP was incubated in humic lake water (colour173 mg Pt l^-1) for 7 d in the dark or under solarradiation. In the dark <0.4% of the introduced¹⁴C-DHP label and 4% of the indigenous DOC weremineralized, indicating that the ¹⁴C-labelledaromatic rings of DHP and the humic DOC weremicrobiologically recalcitrant. Under solar radiation(116 MJ m^-2), 17–21% of the ¹⁴C-labelledcarbons in DHP and 18–23% of the indigenous DOC weremineralized in 7 d. Simultaneously the watersolubility of ¹⁴C-DHP increased. Solar radiationconverted the aromatic cores of synthetic lignin toCO₂ and soluble organic photoproducts. Theresults suggest that solar radiation plays a key rolein the decomposition of natural polyaromatic matter.     

Bacterial Growth in Mixed Cultures on Dissolved Organic Carbon from Humic and Clear Waters

Interactions between bacterial assemblages and dissolved organic carbon (DOC) from different sources were investigated. Mixed batch cultures were set up with water from a humic and a clear-water lake by a 1:20 dilution of the bacterial assemblage (1.0 μm of prefiltered lake water) with natural medium (sterile filtered lake water) in all four possible combinations of the two waters and their bacterial assemblages. Bacterial numbers and biomass, DOC, thymidine incorporation, ATP, and uptake of glucose and phenol were followed in these cultures. Growth curves and exponential growth rates were similar in all cultures, regardless of inoculum or medium. However, bacterial biomass produced was double in cultures based on water from the humic lake. The fraction of DOC consumed by heterotrophic bacteria during growth was in the same range, 15 to 22% of the total DOC pool, in all cultures. Bacterial growth efficiency, calculated from bacterial biomass produced and DOC consumed, was in the order of 20%. Glucose uptake reached a peak during exponential growth in all cultures. Phenol uptake was insignificant in the cultures based on the clear-water medium, but occurred in humic medium cultures after exponential growth. The similarity in the carbon budgets of all cultures indicated that the source of the bacterial assemblage did not have a significant effect on the overall carbon flux. However, fluxes of specific organic compounds differed, as reflected by glucose and phenol uptake, depending on the nature of the DOC and the bacterial assemblage.     

Spatial distribution,size structure,and prey of Craspedacusta sowerbyi Lankester in a shallow New Zealand lake

Solar ultraviolet radiation both degrades and alters the quality of natural organic matter as well as organic pollutants in surface waters. Still, it is only recently that this indirect influence of photochemical processes on aquatic organisms (e.g. bacteria) has received attention. We experimentally studied the photochemical degradation of three PAHs; anthracene, phenanthrene and naphthalene, in water. Anthracene and phenanthrene were rapidly photodegraded (half-lives of 1 and 20.4 hours, respectively), while the photochemical half-life of naphthalene exceeded 100 hours. Hence photodegradation is most likely a less important removal mechanism for the latter compound. The influence of humic substance additions (0–25 mg C l^–1) on degradation rates was also assessed, and while photodegradation of anthracene was not affected by these additions, phenanthrene photodegradation slowed down considerably at the higher humic substance concentrations. These differential responses of anthracene and phenanthrene can at least partially be explained by differences in the spectral absorbance of the two compounds. In contrast, ionic strength did not have any appreciable effect on the estimated photodegradation rates of either compound. The influence of PAHs on growth of aquatic bacteria was assessed in dilution cultures with and without exposure to PAHs and simulated solar UV radiation. Separately, neither PAHs nor simulated solar UV radiation had any effect on bacterial growth. However, when combined, a marked inhibition of bacterial growth could be observed in water obtained from a clearwater lake. This could be due to the formation of toxic photodegradation products such as quinones (detected in our incubations) or other reactive species that affect bacteria negatively. Hence, in addition to influencing the fate and persistence of PAHs in aquatic systems, solar radiation and natural organic matter and regulate the toxicity of these compounds to indigenous micro-organisms.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Effect of humic substance photodegradation on bacterial growth and respiration in lake water

This study addresses how humic substance (HS) chemical composition and photoreactivity affect bacterial growth, respiration, and growth efficiency (BGE) in lake water. Aqueous solutions of HSs from diverse aquatic environments representing different dissolved organic matter sources (autochthonous and allochthonous) were exposed to artificial solar UV radiation. These solutions were added to lake water passed through a 0.7-microm-pore-size filter (containing grazer-free lake bacteria) followed by dark incubation for 5, 43, and 65 h. For the 5-h incubation, several irradiated HSs inhibited bacterial carbon production (BCP) and this inhibition was highly correlated with H2O2 photoproduction. The H2O2 decayed in the dark, and after 43 h, nearly all irradiated HSs enhanced BCP (average 39% increase relative to nonirradiated controls, standard error = 7.5%, n = 16). UV exposure of HSs also increased bacterial respiration (by approximately 18%, standard error = 5%, n = 4), but less than BCP, resulting in an average increase in BGE of 32% (standard error = 10%, n = 4). Photoenhancement of BCP did not correlate to HS bulk properties (i.e., elemental and chemical composition). However, when the photoenhancement of BCP was normalized to absorbance, several trends with HS origin and extraction method emerged. Absorbance-normalized hydrophilic acid and humic acid samples showed greater enhancement of BCP than hydrophobic acid and fulvic acid samples. Furthermore, absorbance-normalized autochthonous samples showed approximately 10-fold greater enhancement of BCP than allochthonous-dominated samples, indicating that the former are more efficient photoproducers of biological substrates.     

ALGAE AS COMPETITORS FOR GLUCOSE WITH HETEROTROPHIC BACTERIA1

Dissolved organic carbon (DOC) constitutes the bulk of organic carbon in aquatic environments. The importance of DOC utilization by mixotrophic algae is unclear since heterotrophic bacteria are regarded as more efficient users. We tested the hypothesis that algae decrease the DOC concentration in the light to lower levels than in darkness resulting in competitive exclusion of heterotrophic bacteria according to the mechanistic competition theory. We investigated (a) the uptake kinetics of glucose as a model substrate by two cultured algae and mixed bacteria populations, (b) the competition for glucose between algae and bacteria in chemostats, (c) the effect of discontinuous glucose supply in chemostats, and (d) the minimum glucose concentrations achieved in cultures of algae and bacteria. Bacteria showed higher specific‐glucose‐uptake rates than algae. In chemostats, algae became extinct in the dark and coexisted in the light where they decreased bacteria to lower densities. Discontinuous glucose supply promoted the algae compared to continuous substrate addition. Several algae consumed glucose to lower concentrations in the dark than in the light and showed lower or equal residual glucose concentrations than bacteria. Residual concentrations were not related to allometric traits (cell volume) and photosynthetic potential (chl content). Overall, the hypothesis was not supported, and mechanisms of competition for DOC obviously differed from those for particulate prey. However, since some algae showed lower or equal residual glucose concentrations than bacteria, algal dark uptake of DOC may be important in deep layers of many waters.     

Effect of exposure to sunlight and phosphorus-limitation on bacterial degradation of coloured dissolved organic matter (CDOM) in freshwater

This study reports on the interacting effect of photochemical conditioning of dissolved organic matter and inorganic phosphorus on the metabolic activity of bacteria in freshwater. Batch cultures with lake-water bacteria and dissolved organic carbon (DOC) extracted from a humic boreal river were arranged in an experimental matrix of three levels of exposure to simulated sunlight and three levels of phosphorus concentration. We measured an increase in bacterial biomass, a decrease in DOC and bacterial respiration as CO(2) production and O(2) consumption over 450 h. These measurements were used to calculate bacterial growth efficiency (BGE). Bacterial degradation of DOC increased with increasing exposure to simulated sunlight and availability of phosphorus and no detectable growth occurred on DOC that was not pre-exposed to simulated sunlight. The outcome of photochemical degradation of DOC changed with increasing availability of phosphorus, resulting in an increase in BGE from about 5% to 30%. Thus, the availability of phosphorus has major implications for the quantitative transfer of carbon in microbial food webs.     

Sources and fates of dissolved organic carbon in lakes as determined by whole-lake carbon isotope additions

Four whole-lake inorganic ¹³C addition experiments were conducted in lakes of differing trophic status. Inorganic ¹³C addition enriched algal carbon in ¹³C and changed the C-DOC by +1.5‰ to +9.5‰, depending on the specific lake. This change in C-DOC represented a significant input of algal DOC that was not completely consumed by bacteria. We modeled the dynamics in C-DOC to estimate the fluxes of algal and terrestrial carbon to and from the DOC pool, and determine the composition of the standing stock. Two experiments in lightly stained, oligotrophic lakes indicated that algal production was the source of about 20% of the DOC pool. In the following year, the experiment was repeated in one of these lakes under conditions of nutrient enrichment, and in a third, more humic lake. Algal contributions to the DOC pool were 40% in the nutrient enriched lake and 5% in the more humic lake. Spectroscopic and elemental analyses corroborated the presence of increased algal DOC in the nutrient enriched lake. Natural abundance measurements of the C of DOC in 32 lakes also revealed the dual contributions of both terrestrial and algal carbon to DOC. From these results, we suggest an approach for inferring the contribution of algal and terrestrial DOC using easily measurable parameters.     

Bacterioplankton Growth on Fractions of Dissolved Organic Carbon of Different Molecular Weights from Humic and Clear Waters

The ability of fractions of dissolved organic carbon (DOC) of different molecular weights (MW) to support bacterial growth was studied in batch culture experiments. Natural pelagic bacteria were inoculated into particle-free (0.2-μm filtered) water, taken from 10 oligotrophic lakes of differing humic content, and either used without further treatments or ultrafiltered to remove DOC of >10,000 MW or >1,000 MW. Stationaryphase abundance of bacteria in the cultures was used as an estimate of bacterial carrying capacity. High-MW DOC (>10,000) comprised an increasing fraction of total DOC with increasing total DOC and humic content of the lakes. High-MW DOC was generally more available to bacteria (i.e., more bacteria were produced per unit of organic carbon initially present) than low-MW (<10,000) DOC. The availability to bacteria of this high-MW DOC decreased with increasing humic content. However, although less available in humic lakes than in clearwater lakes, the higher abundance of high-MW DOC made it quantitatively more important as a bacterial substrate; i.e., a larger fraction of the total bacterial yield of the cultures was due to high-MW DOC compounds in humic lakes than in clearwater lakes. On the average, 48% of bacterial growth occurred at the expense of DOC of <10,000 MW. DOC of <1,000 MW was responsible for an average of 22% of bacterial growth, with no significant correlation to humic content and DOC concentration of lakes. The DOC which supports bacterial growth, as well as the total DOC, is of different quality in humic and clearwater lakes.     

Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Water draining from the Everglades marshes of southern Florida containshigh concentrations of dissolved organic C (DOC), N (DON), and in somelocations, P (DOP). These dissolved organic nutrients carry over 90% of the Nand organic C, and about 25% of the P exported downstream in the Everglades.Ourobjectives were to describe the most important aspects of the origin and fateofdissolved organic matter (DOM) in the Everglades, and to describe the processescontrolling its concentration and export. Concentrations of dissolved organicnutrients are influenced by local plant production, decomposition, and sorptionequilibrium with peat. The drained peat soils of the Everglades AgriculturalArea and the more productive marshes of the northern Everglades produce some ofthe highest concentrations of DOC and DON in the Everglades watershed. Inportions of the marshes of the northern Everglades, P enrichment was correlatedwith higher local DOC and DON concentrations and greater production of solubleplant matter. Microbial degradation of Everglades DOM was very slow; less than10% of the DOC was lost after 6 months of incubation in the laboratory andsupplements of inorganic nutrients failed to speed the decomposition. Exposureto solar radiation increased the subsequent decay rate of the remaining DOC(25%in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7%of the DON, and degraded about 50% of the humic substances over 21 days insterile porewater samples and thus degraded DOM faster than microbialdegradation. The humic substances appeared to inhibit biodegradation of theother fractions of the DOC since hydrophilic organic acids decomposed fasterwhen isolated from the humic substances.The fate of DOC and DON is closely linked as indicated by a generally narrowrange of C/N ratios. In contrast, high concentrations of DOP were associatedwith P enrichment (at least in pore water). The DOC was composed of about 50%humic substances, 33% hydrophilic acids, and 15% hydrophilic neutralsubstances,typical of DOC from other environments, despite the fact that it originatesfroma neutral to slightly alkaline peatland. Despite high exports of DON (3.9g m^–2 y^–1 from one area), themarshes of the northern Everglades are a sink for DON on a landscape scale. Theagricultural fields of the Everglades Agricultural Area, however, exported netquantities of DON. High concentrations of DOC desorbed from the agriculturalsoils when water with no DOC was added. Sorption experiments indicated thathighconcentrations of dissolved organic matter flowing into the marshes from theEverglades Agricultural Area could suppress the further desorption ofadditionalsoluble organic matter through physicochemical mechanisms. While biologicalfactors, plant production and microbial decomposition are important inproducingpotentially soluble organic nutrients, physicochemical sorption equilibria,hydrology, and degradation by solar radiation are also likely to control theexport of this material on the landscape scale.     

Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake

Bacterial growth and grazing mortality were estimated from Mayto October in two south Swedish oligotrophic lakes, one beinga clearwater lake (water colour 5–10 mg Pt l^–1 DOC2.9–3.4 mg l^–1, Secchi disk depth 5.0–9.4m) and the other a humic, brownwater lake (water colour 105–165mg Pt l^–1, DOC 13.7–22.7mg l^–1, Secchi diskdepth 1.3–2.1 m). Specific rates of growth and grazingmortality were generally similar for both lakes. However, theabundance of bacteria was consistently 2–3 times higherin the water of the humic lake, suggesting that the total productionand consumption of bacterial cells were also higher than inthe dearwater lake. The ratio of bacterial secondary productionto primary production was higher in the humic lake than in theclearwater lake, indicating that the bacterioplankton of thehumic lake utilize allochthonous substrates, in addition tosubstrates originating from autochthonous primary production.Most of the bacterial loss in both lakes could be attributedto small protozoan grazers. This implies that allochthonousand autochthonous organic carbon fixed by bacterioplankton isless important in terms of carbon flow to higher trophic levelsthan would be expected if macrozooplankton were the dominantbacterivores, providing a more direct and efficient transferof carbon to larger organisms.     

Direct and Indirect Effects of Solar Ultraviolet Radiation on Attached Bacteria and Algae in Lotic Systems

I examined the effects of solar ultraviolet radiation (UVR) on attached bacteria and algal densities in lotic systems in outdoor artificial stream apparatus. Flumes were covered with four types of film for UVR screening treatments, and attached bacterial cell densities and their temporal variations were compared between conditions excluding and including solar UVR. Attached bacterial cell densities were depressed by solar UVR, and both accrual rate and saturated density were significantly lower in the +UVR (full solar radiation) condition than in −UVR and dark conditions. Solar UVR also indirectly affected the rate of algal accrual. Microscopic direct observations of attached bacterial cell density and algae on substrata showed that solar UVR depressed the accrual of attached bacteria and consequently the frequency of sites with high bacterial cell density that can trap suspended algae in the water. The final amount of algal accrual in the +UVR condition was one-fourth of that in the −UVR condition. Therefore, the effects of solar UVR may be more serious in systems where periphyton are frequently removed by floods.     

Effects of different molecular weight fractions of dissolved organic matter on the growth of bacteria,algae and protozoa from a highly humic lake

Effects of different molecular size fractions (< 1000 MW, < 10 000 MW, < 100 000 MW and <0.1 μm) of dissolved organic matter (DOM) on the growth of bacteria, algae and protozoa from a highly humic lake were investigated. DOM from catchment drainage water as well as from the lake consisted mostly (59–63%) of high molecular weight (HMW) compounds (> 10 000 MW). With excess inorganic nutrients, the growth rate and yield of bacteria were almost identical in all size fractions. However, in < 1000 MW fractions and with glucose added, a longer lag phase occurred. Without added nutrients both the growth rates and biomasses of bacteria decreased towards the smaller size fractions and the percentage of dissolved organic carbon (DOC) used during the experiment and the growth efficiency of bacteria were lower than with excess nutrients. The growth efficiency of bacteria was estimated to vary between 3–66% in different MW fractions, largely depending on the nutrient concentrations, but the highest growth efficiencies were observed in HMW fractions and with glucose. The growth of algae was clearly lowest in the < 1000 MW fraction. In dim light no net growth of algae could be found. In contrast, added nutrients substantially enhanced algal growth and in deionized water with glucose, algae achieved almost the same growth rate and biomass as in higher MW fractions of DOM. The results suggested that bacteria and some algae were favoured by DOM, but protozoans seemed to benefit only indirectly, through bacterial grazing. The utilization of DOM by bacteria and algae was strongly affected by the availability of phosphorus and nitrogen.     

Predator‐induced changes in dissolved organic carbon dynamics

The fate of dissolved organic carbon (DOC) is partly determined by its availability to microbial degradation. Organisms at upper trophic levels could influence the bioavailability of DOC via cascading effects on primary producers and bacteria. Here we experimentally tested whether the presence of fish in aquatic food webs can indirectly affect the composition of the DOC pool. We found that fish had strong positive effects on phytoplankton biomass that affected the dynamics of DOC composition. Specifically, fish increased protein‐like, algae‐derived DOC mid‐experiment, concurrent with the strongest fish‐induced increase in phytoplankton biomass. Fish also increased bacterial abundance, altered the community composition and diversity of bacteria, and temporarily increased DOC compounds with fluorescence properties indicative of microbially‐reprocessed organic matter. Overall, our experiment revealed that fish can positively influence the substrate (algae‐produced DOC) and the key players (bacteria) of the microbial carbon pump. Consequently, fish could contribute to carbon sequestration by stimulating both the production of bioavailable DOC and the microbial degradation of bioavailable to persistent DOC. We propose this as a novel mechanism whereby the loss of predators from global ecosystems could alter carbon cycling.     

Effective isolation of bacterioplankton genus Polynucleobacter from freshwater environments grown on photochemically degraded dissolved organic matter

Effective isolation of freshwater bacterioplankton belonging to genus Polynucleobacter from a shallow eutrophic lake and its tributary was achieved by size-selective filtration with a 0.7-μm pore filter and cultivation on R2A agar medium. Partial 16S rRNA gene analysis showed that over 80% of all the strains were highly similar to the Polynucleobacter cluster. Essential medium components for effective cultivation are pyruvate, yeast extract and peptone, whereas soluble starch and glucose are not necessary. Isolate KF001 (affiliated with Polynucleobacter subcluster D) has a strict requirement for organic acids as carbon sources, and we hypothesize that the Polynucleobacter cluster of bacteria could utilize compounds formed via photochemically dissolved organic matter (DOM) degradation for growth. Because organic acids form from solar radiation of DOM in aquatic environments, carbon sources that are typical products of DOM photochemical degradation were added to the medium. These compounds were readily utilized by KF001 in this study. Finally, we observed the stimulation of strain KF001 activity by photochemical degradation of natural lake water. Our findings suggest a carbon flow of DOM photoproducts to Polynucleobacter in the freshwater microbial loop.     

Bacterioplankton Production in Humic Lake Örträsket in Relation to Input of Bacterial Cells and Input of Allochthonous Organic Carbon

In order to compare riverine bacteria input with lake water bacterial production and grazing loss with output loss, a bacterial cell budget was constructed for humic Lake ?rtr?sket in northern Sweden. The riverine input of bacterial cells in 1997 represented 29% of the number of bacterial cells produced within the layer of the lake affected by inlet water. A large share of the in situ lake bacterial production was consumed by grazers, mainly flagellates, which stresses the importance of bacteria as energy mobilizers for the pelagic food web in the lake. The bacterial production in Lake ?rtr?sket, which is almost entirely dependent on humic material as an energy source, was clearly stimulated by high flow episodes which brought high amounts of little degraded material into the lake. During base flow condition the bacterial production in the inlet rivers was high, which led to an input of more degraded material to the lake. This material did not stimulate the lake bacterial production. Internal factors that determined the utilization of the allochthonous DOC in the lake were the retention time and the exposure to light and high temperatures. Thus, the potential for in situ production of bacteria in Lake ?rtr?sket was to a large extent a function of how precipitation and runoff conditions affected terrestrial losses and river transport of humic material.     

Littoral-pelagial interchange and the decomposition of dissolved organic matter in a polyhumic lake

The small, polyhumic lake, Mekkojärvi (southern Finland), is bordered by a moss vegetation zone (Warnstorfia and Sphagnum species) which provides a habitat-rich and productive environment for many planktonic and periphytic animals. Impacts of moss on the metabolism of bacterioplankton, phytoplankton and zooplankton in polyhumic water were investigated in laboratory throughflow systems. Growing Warnstorfia (together with epiphytic algae and bacteria) suppressed the production of planktonic algae but had no clear effect on leucine uptake, and hence bacterial production, or on the decomposition of humic substances. Phenol uptake and mineralization rates, however, were lower in the littoral water than in the pelagial water. Excretion of organic carbon by Warnstorfia algae or Daphnia longispina (the predominant crustacean in the pelagial water) provided only a minor contribution to bacterial production; therefore, a major contribution had to be from humic substances. A bacterial production efficiency of 31–38% could account for the microbial respiration in the water. The results indicated that bacterial, or detrital matter (originating largely from the littoral zone), could not obviate the need for algal food, and that a great deal of particulate matter in the water was poor or useless food for Daphnia. In all, the bulk of dissolved organic matter in Lake Mekkojärvi was biochemically highly recalcitrant. Our results indicate that humic substances (from watershed or littoral area) which, through bacterial degradation, enter the planktonic food web of the lake are mainly lost through respiration by microorganisms.     

Availability of dissolved organic carbon for planktonic bacteria in oligotrophic lakes of differing humic content

Bacterioplankton from 10 oligotrophic lakes, representing a gradient from clearwater to polyhumic, were grown in dilution cultures of sterile filtered lake water. The bacterial biomass achieved in the stationary phase of the dilution cultures was positively correlated with the amount of both humic matter and dissolved organic carbon (DOC) in the lakes. About the same fraction of the total DOC pool was consumed in the dilution cultures of all lakes (average 9.5%, coefficient of variation (CV) 24%), with approximately the same growth efficiency (average 26%, CV 28%). Thus, humic lakes could support a higher bacterial biomass than clearwater lakes due to their larger DOC pools. The relevance of the results to planktonic food webs of humic and clearwater lakes is discussed.     

Effect of Humic Substance Photodegradation on Bacterial Growth and Respiration in Lake Water

This study addresses how humic substance (HS) chemical composition and photoreactivity affect bacterial growth, respiration, and growth efficiency (BGE) in lake water. Aqueous solutions of HSs from diverse aquatic environments representing different dissolved organic matter sources (autochthonous and allochthonous) were exposed to artificial solar UV radiation. These solutions were added to lake water passed through a 0.7-μm-pore-size filter (containing grazer-free lake bacteria) followed by dark incubation for 5, 43, and 65 h. For the 5-h incubation, several irradiated HSs inhibited bacterial carbon production (BCP) and this inhibition was highly correlated with H₂O₂ photoproduction. The H₂O₂ decayed in the dark, and after 43 h, nearly all irradiated HSs enhanced BCP (average 39% increase relative to nonirradiated controls, standard error = 7.5%, n = 16). UV exposure of HSs also increased bacterial respiration (by ~18%, standard error = 5%, n = 4), but less than BCP, resulting in an average increase in BGE of 32% (standard error = 10%, n = 4). Photoenhancement of BCP did not correlate to HS bulk properties (i.e., elemental and chemical composition). However, when the photoenhancement of BCP was normalized to absorbance, several trends with HS origin and extraction method emerged. Absorbance-normalized hydrophilic acid and humic acid samples showed greater enhancement of BCP than hydrophobic acid and fulvic acid samples. Furthermore, absorbance-normalized autochthonous samples showed ~10-fold greater enhancement of BCP than allochthonous-dominated samples, indicating that the former are more efficient photoproducers of biological substrates.     

Relationships between picophytoplankton and environmental variables in lakes along a gradient of water colour and nutrient content

SUMMARY 1. Biomass and production of picophytoplankton, phytoplankton and heterotrophic bacterioplankton were measured in seven lakes, exhibiting a broad range in water colour because of humic substances. The aim of the study was to identify environmental variables explaining the absolute and relative importance of picophytoplankton. In addition, two dystrophic lakes were fertilised with inorganic phosphorus and nitrogen, to test eventual nutrient limitation of picophytoplankton in these systems.
2. Picophytoplankton biomass and production were highest in lakes with low concentrations of dissolved organic carbon (DOC), and DOC proved the factor explaining most variation in picophytoplankton biomass and production. The relationship between picophytoplankton and lake trophy was negative, most likely because much P was bound in humic complexes. Picophytoplankton biomass decreased after the additions of P and N.
3. Compared with heterotrophic bacterioplankton, picophytoplankton were most successful at the clearwater end of the lake water colour gradient. Phytoplankton dominated over heterotrophic bacteria in the clearwater systems possibly because heterotrophic bacteria in such lakes are dependent on organic carbon produced by phytoplankton.
4. Compared with other phytoplankton, picophytoplankton did best at intermediate DOC concentrations; flagellates dominated in the humic lakes and large autotrophic phytoplankton in the clearwater lakes.
5. Picophytoplankton were not better competitors than large phytoplankton in situations when heterotrophic bacteria had access to a non-algal carbon source. Neither did their small size lead to picophytoplankton dominance over large phytoplankton in the clearwater lakes. Possible reasons include the ability of larger phytoplankton to float or swim to reduce sedimentation losses and to acquire nutrients by phagotrophy.