Occurrence of bacterivory in Cryptomonas,a common freshwater phytoplankter

Summary Bacterivory was detected by incorporation of 0.57 m diameter, fluorescent polystyrene beads and fluorescently labeled bacteria (FLB) in two cultured species of Cryptomonas (C. ovata and C. erosa), and a population of Cryptomonas sp in a humic, mesotrophic lake. Rates of ingestion and clearance were very low, and similar for the cultures and the in situ population. The in situ population incorporated 0.7–1.7 bacteria cell^-1 h^-1, thereby ingesting 0.3%–2.0% of the total bacterial numbers present in the water per day, and receiving less than 2% of its carbon content per day through bacterivory. Thus, bacterivory by Cryptomonas was quantitatively important neither as a sink for bacterial biomass, nor as a carbon source for the algal cells. Possibly, it served in the uptake of essential nutrients.     

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.     

Food webs in sub-Antarctic lakes: a stable isotope approach

In order to improve the understanding of food-web interactions in sub-Antarctic freshwater systems, we complemented earlier experimental studies with analyses of differences in stable isotopes (¹⁵N and ¹³C) among organisms in two lakes with contrasting productivity. The distribution of the stable isotopes showed that the small copepod Boeckella michaelseni feeds mainly on pelagic POM (particulate organic material), whereas the larger copepod species B. poppei also feeds on benthic algae. Furthermore, the predatory copepod Parabroteas sarsi seems to mainly feed on B. michaelseni, but also on B. poppei and the benthic cladoceran, Alona weineckii. Moreover, stable-isotope data suggest that the diving beetle, Lancetes angusticollis, is not only feeding on B. poppei as indicated from experimental studies, but also on the benthic cladoceran A. weineckii. Although the food webs of the two lakes are very similar, they show considerable differences in the distribution of stable isotopes. We conclude that monitoring, experiments and stable-isotope analysis in combination give a reasonably clear picture of sub-Antarctic freshwater food webs.     

Multifunctionality and diversity in bacterial biofilms

Bacteria are highly diverse and drive a bulk of ecosystem processes. Analysis of relationships between diversity and single specific ecosystem processes neglects the possibility that different species perform multiple functions at the same time. The degradation of dissolved organic carbon (DOC) followed by respiration is a key bacterial function that is modulated by the availability of DOC and the capability to produce extracellular enzymes. In freshwater ecosystems, biofilms are metabolic hotspots and major sites of DOC degradation. We manipulated the diversity of biofilm forming communities which were fed with DOC differing in availability. We characterized community composition using molecular fingerprinting (T-RFLP) and measured functioning as oxygen consumption rates, the conversion of DOC in the medium, bacterial abundance and the activities of five specific enzymes. Based on assays of the extracellular enzyme activity, we calculated how the likelihood of sustaining multiple functions was affected by reduced diversity. Carbon source and biofilm age were strong drivers of community functioning, and we demonstrate how the likelihood of sustaining multifunctionality decreases with decreasing diversity.     

Contribution of Sediment Respiration to Summer CO2 Emission from Low Productive Boreal and Subarctic Lakes

We measured sediment production of carbon dioxide (CO₂) and methane (CH₄) and the net flux of CO₂ across the surfaces of 15 boreal and subarctic lakes of different humic contents. Sediment respiration measurements were made in situ under ambient light conditions. The flux of CO₂ between sediment and water varied between an uptake of 53 and an efflux of 182 mg C m⁻² day⁻¹ from the sediments. The mean respiration rate for sediments in contact with the upper mixed layer (SedR) was positively correlated to dissolved organic carbon (DOC) concentration in the water (r² = 0.61). The net flux of CO₂ across the lake surface [net ecosystem exchange (NEE)] was also closely correlated to DOC concentration in the upper mixed layer (r² = 0.73). The respiration in the water column was generally 10-fold higher per unit lake area compared to sediment respiration. Lakes with DOC concentrations <5.6 mg L⁻¹ had net consumption of CO₂ in the sediments, which we ascribe to benthic primary production. Only lakes with very low DOC concentrations were net autotrophic (<2.6 mg L⁻¹) due to the dominance of dissolved allochthonous organic carbon in the water as an energy source for aquatic organisms. In addition to previous findings of allochthonous organic matter as an important driver of heterotrophic metabolism in the water column of lakes, this study suggests that sediment metabolism is also highly dependent on allochthonous carbon sources.     

Structure and Function of Bacterial Communities Emerging from Different Sources under Identical Conditions

The aim of this study was to compare two major hypotheses concerning the formation of bacterial community composition (BCC) at the local scale, i.e., whether BCC is determined by the prevailing local environmental conditions or by “metacommunity processes.” A batch culture experiment where bacteria from eight distinctly different aquatic habitats were regrown under identical conditions was performed to test to what extent similar communities develop under similar selective pressure. Differently composed communities emerged from different inoculum communities, as determined by terminal restriction fragment length polymorphism analysis of the 16S rRNA gene. There was no indication that similarity increased between communities upon growth under identical conditions compared to that for growth at the ambient sampling sites. This suggests that the history and distribution of taxa within the source communities were stronger regulating factors of BCC than the environmental conditions. Moreover, differently composed communities were different with regard to specific functions, such as enzyme activities, but maintained similar broad-scale functions, such as biomass production and respiration.     

Using Satellite Remote Sensing to Estimate the Colored Dissolved Organic Matter Absorption Coefficient in Lakes

Given the importance of colored dissolved organic matter (CDOM) for the structure and function of lake ecosystems, a method that could estimate the amount of CDOM in lake waters over large geographic areas would be highly desirable. Satellite remote sensing has the potential to resolve this problem. We carried out model simulations to evaluate the suitability of different satellite sensors (Landsat, IKONOS, and the Advanced land Imager [ALI]) to map the amount of CDOM in concentration ranges that occur in boreal lakes of the Nordic countries. The results showed that the 8-bit radiometric resolution of Landsat 7 is not adequate when absorption by CDOM at 420 nm is higher than 3 m⁻¹. On the other hand, the 16-bit radiometric resolution of ALI, a prototype of the next generation of Landsat, is suitable for mapping CDOM in a wider range of concentrations. An ALI image of southern Finland was acquired on 14, July 2002 and in situ measurements were carried out in 15 lakes (18 stations). The results showed that there is a high correlation (R² = 0.84) between the 565 nm/660 nm ALI band ratio and the CDOM absorption coefficient in lakes. Analysis of 245 lakes in the acquired satellite image showed a normal distribution of CDOM concentration among the lakes. However, the size distribution of lakes was highly skewed toward small lakes, resulting in the CDOM concentration per unit lake area being skewed toward high values. We showed that remote sensing enables synoptic monitoring of the CDOM concentration in a large number of lakes and thus enables scaling up to the level of large ecosystems and biomes.     

The effects of shredding invertebrates on the transfer of organic carbon from littoral leaf litter to water-column bacteria

Leaf litter can be of great importance for the productivity of small oligotrophic lakes surrounded by deciduous forests. Feeding invertebrate shredders produce particulate organic leftovers, but their feeding also enhances the release of dissolved organic carbon (DOC). We tested whether invertebrate-mediated DOC release affects the production of heterotrophic water-column bacteria. Submersed leaves were incubated in microcosms with and without shredders; and DOC, absorbance, bacterial abundance and bacterial production in the water column were monitored. We also measured dry weight of the organic particles (FPOC, fine particulate organic carbon, leaf residues and shredders). Total leaf-litter carbon decreased by nearly 80% in the presence of shredders, and on average 56% of the initial leaf carbon ended up as FPOC after 126 days of incubation. Without shredders FPOC production was almost zero, and 72% of the added leaf carbon could be retrieved as leaves when the experiment ended. Both these figures include the rapid release of DOC during the first week of leaf incubation in the lake water (equivalent to 16–19% of total added leaf carbon). Although bacterial production in the water was several times higher in treatments with shredders, bacterial consumption of leaf-derived DOC from shredding was obviously of minor importance in the total carbon budget. This result suggests, although shredders have a strong impact on transformation of leaves to FPOC, they do not greatly enhance the initial rate of mineralization of the leaf-derived detritus.     

Sedimentation in Boreal Lakes—The Role of Flocculation of Allochthonous Dissolved Organic Matter in the Water Column

We quantified sedimentation of organic carbon in 12 Swedish small boreal lakes (<0.48 km²), which ranged in dissolved organic carbon (DOC) from 4.4 to 21.4 mg C l⁻¹. Stable isotope analysis suggests that most of the settling organic matter is of allochthonous origin. Annual sedimentation of allochthonous matter per m² lake area was correlated to DOC concentration in the water (R ² = 0.41), and the relationship was improved when sedimentation data were normalized to water depth (R ² = 0.58). The net efflux of C as CO₂ from the water to the atmosphere was likewise correlated to DOC concentration (R ² = 0.52). The losses of organic carbon from the water column via mineralization to CO₂ and via sedimentation were approximately of equal importance throughout the year. Our results imply that DOC is a precursor of the settling matter, resulting in an important pathway in the carbon cycle of boreal lakes. Thus, flocculation of DOC of terrestrial origin and subsequent sedimentation could lead to carbon sequestration by burial in lake sediments.