Long-term trends of epilimnetic and hypolimnetic bacteria and organic carbon in a deep holo-oligomictic lake

We analysed the long-term dynamics (1980–2007) of hypolimnetic and epilimnetic bacterial abundances and organic carbon concentrations, both dissolved (DOC) and particulate (POC), in the deep holo-oligomictic Lake Maggiore, included in the Southern Alpine Lakes Long-Term Ecological Research (LTER) site. During the 28 years of investigation, bacterial abundance and POC concentrations did not decrease with declining phosphorus concentrations, while DOC concentrations showed a pronounced decrease in the epi- and hypolimnion. We used the annual mean total lake heat content and total annual precipitation as climate-related variables, and in-lake total phosphorus as a proxy for trophic state. The model (forward stepwise regression, FSR) showed that reduced anthropogenic pressure was more significant than climate change in driving the trend in DOC concentrations. Bacterial dynamics in the hypolimnion mirrored the fluctuations observed in the epilimnion, but average cell abundance was three times lower. The FSR model indicates that bacterial number variability was dependent on POC in the epilimnion and DOC in the hypolimnion. In the hypolimnion, cell biovolumes for rod and coccal morphotypes were significantly larger than in the epilimnion.     

Effects of re‐oligotrophication and climate change on lake thermal structure

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Volumetric and aerial rates of heterotrophic bacterial production in epi- and hypolimnia: the role of nutrients and system morphometry

Epilimnetic and hypolimnetic bacterial production (BP) were measured once in summer, by the incorporation of [³H] - Leucine in each of 14 Quebec (Canada) lakes varying in nutrient content and morphometry. The epilimnetic and hypolimnetic BP were evaluated at two scales: the common per unit volume and areal (m^–2) scale. The per unit volume scale epilimnetic BP was best predicted by total phosphorus (TP, r ²=0.63), and by water residence time (WRT r ²=0.57), with WRT serving as a surrogate for the nutrient and organic matter supply from the catchments. Total phosphorus and lake mean depth (Z _m) together explained 79% of the variation in epilimnetic BP (l^–1). In contrast, hypolimnetic BP (l^–1) was neither linked to nutrients (TP or TN) or dissolved organic carbon (DOC) but only to measures of lake morphometry and best of all to hypolimnetic thickness (Zh; r ²=0.74). With increased Zh, there is an increased dilution of settling organic particles and their nutrients, resulting in a decrease in BP per litre. Conversely, when BP is expressed in areal units (m^–2), hypolimnetic production increases with increasing hypolimnetic thickness. Water column thickness is a master variable, which together with Chl a (abundance of particles) determines hypolimnetic BP at the whole system scale even though the trophic status is the best single indicator of epilimnetic BP on a volumetric scale. Conclusions drawn invariably change with the scale of investigation. Moreover, it is clear that lake morphometry has a major impact on BP. A comparison of whole water column integrated BP with literature derived estimates of the equivalent sediment production (m^–2) below suggests that if the estimated sediment rates are not complete technique artefacts, they are likely to be an order of magnitude higher than the water column rates (m^–2) at the maximum depth sampling sites. The relative importance of the sediments could be expected to rise with a decline in the maximum depth of lakes, characterized by progressively thinner hypolimnia. The present findings point to both a primarily allocthonous fuelling of sediment production and an uncoupling of water and sediment BP.     

Differential bacterial dynamics promote emergent community robustness to lake mixing: an epilimnion to hypolimnion transplant experiment

Lake mixing disrupts chemical and physical gradients that structure bacterial communities. A transplant experiment was designed to investigate the influence of post‐mixing environmental conditions and biotic interactions on bacterial community composition. The experimental design was 3 × 2 factorial, where water was incubated from three different sources (epilimnion, hypolimnion, and mixed epilimnion and hypolimnion) at two different locations in the water column (epilimnion or hypolimnion). Three replicate mesocosms of each treatment were removed every day for 5 days for bacterial community profiling, assessed by automated ribosomal intergenic spacer analysis. There were significant treatment effects observed, and temperature was the strongest measured driver of community change (r = ?0.66). Epilimnion‐incubated communities changed more than hypolimnion‐incubated. Across all treatments, we classified generalist, layer‐preferential and layer‐specialist populations based on occurrence patterns. Most classified populations were generalists that occurred in both strata, suggesting that communities were robust to mixing. In a network analysis of the mixed‐inocula treatments, there was correlative evidence of inter‐population biotic interactions, where many of these interactions involved generalists. These results reveal differential responses of bacterial populations to lake mixing and highlight the role of generalist taxa in structuring an emergent community‐level response.     

Bacterioplankton Abundance and Activity in a Small Hypertrophic Stratified Lake

Bacterioplankton abundance and production were followed during one decade (1991–2001) in the hypertrophic and steeply stratified small Lake Verevi (Estonia). The lake is generally dimictic. However, a partly meromictic status could be formed in specific meteorological conditions as occurred in springs of 2000 and 2001. The abundance of bacteria in Lake Verevi is highly variable (0.70 to 22 × 10⁶ cells ml⁻¹) and generally the highest in anoxic hypolimnetic water. In 2000–2001, the bacterial abundance in the hypolimnion increased probably due to meromixis. During a productive season, heterotrophic bacteria were able to consume about 10–40% of primary production in the epilimnion. Our study showed that bacterioplankton in the epilimnion was top-down controlled by predators, while in metalimnion bacteria were dependent on energy and carbon sources (bottom-up regulated). Below the thermocline hypolimnetic bacteria mineralized organic matter what led to the depletion of oxygen and created anoxic hypolimnion where rich mineral nutrient and sulphide concentrations coexisted with high bacterial numbers.     

Lake microbial communities are resilient after a whole-ecosystem disturbance

Disturbances act as powerful structuring forces on ecosystems. To ask whether environmental microbial communities have capacity to recover after a large disturbance event, we conducted a whole-ecosystem manipulation, during which we imposed an intense disturbance on freshwater microbial communities by artificially mixing a temperate lake during peak summer thermal stratification. We employed environmental sensors and water chemistry analyses to evaluate the physical and chemical responses of the lake, and bar-coded 16S ribosomal RNA gene pyrosequencing and automated ribosomal intergenic spacer analysis (ARISA) to assess the bacterial community responses. The artificial mixing increased mean lake temperature from 14 to 20 °C for seven weeks after mixing ended, and exposed the microorganisms to very different environmental conditions, including increased hypolimnion oxygen and increased epilimnion carbon dioxide concentrations. Though overall ecosystem conditions remained altered (with hypolimnion temperatures elevated from 6 to 20 °C), bacterial communities returned to their pre-manipulation state as some environmental conditions, such as oxygen concentration, recovered. Recovery to pre-disturbance community composition and diversity was observed within 7 (epilimnion) and 11 (hypolimnion) days after mixing. Our results suggest that some microbial communities have capacity to recover after a major disturbance.     

The hypolimnetic protozoan plankton of a eutrophic lake

The seasonal distribution of benthic species in the water column above and below the thermocline in a small eutrophic lake is described. During summer stratification populations of Spirostomum spp, Loxodes spp., Plagiopyla and Deltopylum become established in the plankton on or below the oxycline/thermocline. At shallow sites no migration occurred and populations of the migratory species in the benthos were sparse, with the exception of Plagiopyla which occurred in high densities in the sediment. Two distinct planktonic populations are established during stratification: an epilimnetic community of obligate planktonic ciliates and a hypolimnetic community of benthic migrants.     

Bacteria,Archaea, and Crenarchaeota in the Epilimnion and Hypolimnion of a Deep Holo-Oligomictic Lake

In a deep, subalpine holo-oligomictic lake, the relative abundance of Archaea and Crenarchaeota, but not that of Bacteria, increases significantly with depth and varies seasonally. Cell-specific prokaryotic productivity is homogeneous along the water column. The concept of active Archaea observed in the deep ocean can therefore be extended to a deep oxic lake.The abundance, activity, and community composition of epilimnetic and hypolimnetic prokaryotes have been less thoroughly investigated in deep lakes than in oceans. Strong evidence that the depth gradient plays a role in modulating the balance between the domains of Bacteria and Archaea has been found in various marine systems (8, 12, 13, 20). It has been shown that the percentage of Bacteria in the deep marine hypolimnion decreases (up to 5,000 m) while, conversely, the percentage of Archaea increases. The percentage of Crenarchaeota is also higher in the mesopelagic zone than in surface waters (10).Although Archaea have been found in a variety of freshwater habitats (3), little has thus far been published on differentiating between Bacteria, Archaea, and Crenarchaeota in the hypolimnion of deep lakes. An exception is a study of the high-altitude ultraoligotrophic Crater Lake (21, 22), where group I marine Crenarchaeota were observed in deep-water populations (22). This study and another study of various lakes from three continents (9) are based on summer sampling, making it impossible to ascertain the effects of temporal variability on the vertical distribution of Archaea and Crenarchaeota, as has been done for marine systems and shallow lakes (for examples, see references 8 and 11).Our primary objective was to follow variations in the relative abundance of Bacteria, Archaea, and Crenarchaeota found in the hypolimnetic waters of a deep holo-oligomictic lake with a permanent oxic hypolimnion and compare them with those in the epilimnetic assemblages. We used the catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) technique and compared the data thus obtained with prokaryotic productivity.     

Effects of hypolimnetic oxygenation on water quality: results from five Danish lakes

Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4–20 years. In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited low mean summer levels (<2.2 mg O₂ l⁻¹). Oxygenation generally increased the hypolimnetic water temperature by 0.5–2°C, but in one lake it increased by 4–6°C. In all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification. The accumulation of phosphorus and ammonia typically decreased by 40–88%. In two lakes oxygenation was stopped for 1–2 years and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water quality. Handling editor: Luigi Naselli-Flores     

Interannual variations in atmospheric forcing determine trajectories of hypolimnetic soluble reactive phosphorus supply in a eutrophic lake

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Phosphorus inactivation in a eutrophic lake by the direct addition of ferric aluminium sulphate: impact on iron and phosphorus

SUMMARY.

• 1 Following addition of ferric aluminium sulphate (FAS) in February 1980 to a eutrophic lake, White Lough, Northern Ireland, the cycles of phosphorus (P) and iron (Fe) over 34 months were compared to those in the 18 months preceding the application.
• 2 During the summer of 1980, P release to the hypolimnion was reduced by 92% compared to 1979 but by 1982 had returned to pre-treatment levels. Phosphorus budget calculations for the post- treatment period indicated that P was accumulating in the sediment at a rate sufficient to permit the recovery of P release.
• 3 In the three summers following FAS addition, Fe released into the hypolimnion increased by 56–71% compared to 1979 but this increase represented less than 10% of Fe added in the FAS.
• 4 Prior to FAS treatment, hypolimnetic Fe and P were mixed throughout the lake at overturn, resulting in high winter concentrations, but after treatment, overturn was accompanied by large reductions in the lake Fe and P content due to precipitation. As a result, overwintering P concentrations in 1980/81 and 1981/82 were in the range 31–53μg P1^-1 compared to 111–194μg P1^-1in 1978/79 and 1979/80.
• 5 The reduction of hypolimnetic P release in 1980 and 1981 and lower overwintering P had little impact on summer epilimnetic P concentrations which for 1979–82 were 51, 36, 47 and 48μg P1^-1respectively. Phosphorus concentrations in the epilimnion were raised between April and June in each year by a release of P into the epilimnion. August and September was the only period when reductions in hypolimnetic P were reflected in lower P concentrations in the epilimnion.

     

Epilimnetic and hypolimnetic symptoms of eutrophication in Great Mazurian Lakes, Poland

SUMMARY. An increase in the trophic state and productivity of a lake is not necessarily accompanied by a decrease in epilimnetic Secchi disc transparency. In smaller lakes with sharp thermoclines the symptoms of eutrophication may be more evident as a decrease in transparency in the epilimnion but, in larger lakes with poorly defined thermoclines, symptoms of eutrophication may be more evident in the hypolimnion as shown by an increase in the rate of hypolimnetic oxygen consumption.     

Planktonic food chains of a highly humic lake

The development and metabolism of epilimnetic plankton from a highly humic lake was followed in late summer, when the predominant zooplankton species, Daphnia longispina, was very abundant (ca. 200 ind. l^?1). The experiment was made in two tanks: one with an unaltered plankton assemblage and one with larger zooplankton removed. The scarce phytoplankton community was also simple, consisting mainly of one Cryptomonas and two Mallomonas species. The abundance and species composition of smaller plankton was heavily influenced by grazing of Daphnia. In particular, the biomass, of heterotrophic flagellates increased after the removal of Daphnia. The biomass and production of bacterioplankton were not affected, and remained several times higher than that of phytoplankton. Bacterial production and grazing on bacteria were balanced, and when Daphnia was removed its grazing activity was compensated by flagellates. The removal of Daphnia did not affect the respiration or community net production of plankton. Among organisms smaller than zooplankton, bacteria seemed to be responsible for most of the respiration. The community net production was consistently negative even at the water surface, indicating an allochthonous carbon source. The results suggest that phytoplankton primary production was insufficient for the secondary production in the epilimnetic water of the study lake. The food requirements of bacteria and zooplankton, as well as of flagellates, each exceeded that supplied by phytoplankton primary production. The simple food chains in this experiment made it possible to reveal the functioning of the community so completely that dissolved organic matter is certainly comparable to or exceeds the importance of phytoplankton primary production as an energy and carbon source for food webs in this humic lake.     

Aeromonas Distribution and Survival in a Thermally Altered Lake

Par Pond is a thermally enriched monomictic southeastern lake which receives heated effluent from a production nuclear reactor. Fish populations in the lake have lesions of epizooty from which Aeromonas spp. are readily isolated. Distribution and population densities of Aeromonas in the water column were measured along an oxygen and temperature gradient as well as seasonally. Greater population densities of Aeromonas occurred below the oxygen chemocline when the lake was stratified. Survival of Aeromonas hydrophila under in situ conditions in both epilimnetic and hypolimnetic waters was determined through the use of polycarbonate membrane diffusion chambers during two separate reactor operating conditions. Survival levels of pure cultures of A. hydrophila corresponded to the distribution patterns of the naturally occurring Aeromonas-like populations. The greater survival of A. hydrophila during full reactor operation suggests that the fish populations may be exposed to Aeromonas for a longer period of time than when the reactor is not operating.     

Thymidine Incorporation by Free-Living and Particle-Bound Bacteria in a Eutrophic Dimictic Lake

The percentage of [methyl-³H]thymidine incorporated into samples from a dimictic eutrophic lake and retained on polycarbonate membranes of 3.0-, 1.0-, and 0.2-μm pore size was studied in a lake with filamentous cyanobacteria as the dominant phytoplankton type throughout the period of thermal stratification. Water samples were also examined by epifluorescence microscopy for evidence of algal senescence and bacterial colonization of intact and damaged cyanobacterial filaments. A small percentage (2 to 20%) of bacterial activity was retained by filters with pore sizes ≥ 1 μm in epilimnetic samples. Epilimnetic samples also had a small percentage of cyanobacterial filaments, either intact or damaged, which were visibly colonized by bacteria in summer and fall samples. A significant proportion (20 to 35%) of bacterial activity was retained by filters with pore sizes ≥ 1 μm in samples collected from the metalimnion and hypolimnion during late summer and fall. The proportion of damaged cyanobacterial filaments was higher in these samples than in those from the epilimnion or from those obtained early in the summer. Furthermore, the filaments in these samples were more heavily colonized by bacteria. Overall, particle-bound production accounted for only 2 to 19% of total bacterial production from April to August in all water layers. It appears that the supply of colonizable particles (damaged cyanobacterial filaments) is an important factor affecting the level of particle-bound bacterial activity in this lake.     

Succession of bacterial community composition over two consecutive years in two aquatic systems: a natural lake and a lake-reservoir

The succession in bacterial community composition was studied over two years in the epilimnion and hypolimnion of two freshwater systems: a natural lake (Pavin Lake) and a lake-reservoir (Sep Reservoir). The bacterial community composition was determined by cloning-sequencing of 16S rRNA and by terminal restriction fragment length polymorphism. Despite large hydrogeological differences, in the Sep Reservoir and Pavin Lake the dominant bacteria were from the same taxonomic divisions, particularly Actinobacteria and Betaproteobacteria. In both ecosystems, these major bacterial divisions showed temporal fluctuations that were much less marked than those occurring at a finer phylogenetic scale. Nutrient availability and mortality factors, the nature of which differed from one lake to another, covaried with the temporal variations in the bacterial community composition at all sampling depths, whereas factors related to seasonal forces (temperature and outflow for Sep Reservoir) seemed to account only for the variation of the hypolimnion bacterial community composition. No seasonal reproducibility in temporal evolution of bacterial community from one year to the next was observed.     

Lake Kinneret dissolved oxygen regime reflects long term changes in ecosystem functioning

Lake Kinneret (Israel) has undergone several prominent chemical and biological changes since 1970. Between 1970 and 1991 significant, long-term gradual increase were recorded in epilimnetic dissolved oxygen (DO) concentrations (about 20%), and in pH levels (0.2 units). Concomitantly there was a significant increase in hypolimnetic H₂S concentrations (about 75%) and a long-term gradual drop in zooplankton biomass (50%). Since 1994 these trends were reversed and the levels of the three chemical parameters have returned to those found in the 1970's and that of zooplankton to mid 1980's levels. The present study is an attempt to relate some of these long term changes by means of yearly oxygen budgets, based on fluxes of oxygen producing and consuming processes. This analysis raises the possibility that part of the long-term increase in epilimnetic DO and pH between 1970 to 1990 may be attributed to reduced inputs of organic matter from alochthonous sources and possibly to enhanced burial of organic matter in the bed sediments. However, the major cause for the observed increase in epilimnetic DO and pH is increased sedimentation of organic matter to the hypolimnion during stratification. As indicated by the amount of H₂S formed in the hypolimnion during stratification added to the amount of oxygen entrapped in this layer at the onset of thermal stratification, between 1970 to 1991 the sedimentation flux of organic matter increased by approximately 40%. It is estimated that during these two decades hypolimnetic respiration increased from ca. 8% of the annual amount of oxygen evolved due to photosynthesis during the early 1970's to ca. 12.5% during the 1980's. The shift in the layer of oxidative processes is suggested to be the result of a multi-annual decline in zooplankton grazing pressure, which led to increased sedimentation of organic matter. The reversed trends for DO, pH and H₂S since 1994 may have partially been due to the increase in zooplankton activity and partially due to changes in phytoplankton community structure.     

Depth-selection behavior and longevity in Daphnia: an evolutionary test for the predation-avoidance hypothesis

The classic evolutionary theory of senescence predicts that long lifespan evolves under low risk of extrinsic mortality. As lakes present planktonic animals with vertical gradients of mortality risk associated with fish predation, we expected the individual lifespan of Daphnia of the “hypolymnetic” clones to be longer than that of “epilimnetic” Daphnia. In order to test this prediction, we performed a laboratory study on 14 clones from the D. longispina species complex, taken during the daytime either from epilimnion or hypolimnion of three mesotrophic lakes. “Epilimnetic” Daphnia started reproduction earlier, aged faster, and lived shorter than their “hypolimnetic” conspecifics. Our results indirectly support the predation-avoidance hypothesis as the ultimate explanation for depth-selection behavior in Daphnia.     

The Role of Allochthonous Inputs of Dissolved Organic Carbon on the Hypolimnetic Oxygen Content of Reservoirs

Hypolimnetic oxygen content in lentic ecosystems has traditionally been modeled as a function of variables measured at the epilimnion, or that are supposed to drive epilimnetic processes, like total phosphorus load. However, in man-made reservoirs the river inflow can plunge into deep layers, directly linking the hypolimnion with the surrounding watershed. In these circumstances, organic matter carried by the river can influence the hypolimnetic oxygen content without important intervention of epilimnetic processes. Taking long-term data from two reservoirs in Spain, we applied an empirical regression approach to show that the dissolved organic matter carried by the river is the main driver shaping the hypolimnetic oxygen content. By contrast, typical variables commonly included in the modeling of the oxygen content in the hypolimnion (nutrient concentrations, chlorophyll a, and dissolved organic carbon measured in the water column) did not show any significant correlation. Interpretations from this regression approach were supported by a comparison between the monthly oxygen consumption in the hypolimnion and the monthly dissolved organic carbon load from the river inflow. We also revisited the prediction of the year-to-year variability of the Nürnberg’s anoxic factor in four reservoirs from Spain and the USA, explicitly including the allochthonous sources in the equations. These sources were significant predictors of the anoxic factor, especially in those systems subject to relatively high human impact. Thus, effects of allochthonous dissolved organic carbon should always be considered in empirical modeling and management of reservoir hypolimnetic processes related to oxygen content (for example, anoxia, nutrient internal loading, or phosphorus cycle resilience).     

Respiratory Succession and Community Succession of Bacterioplankton in Seasonally Anoxic Estuarine Waters

Anoxia occurs in bottom waters of stratified estuaries when respiratory consumption of oxygen, primarily by bacteria, outpaces atmospheric and photosynthetic reoxygenation. Once water becomes anoxic, bacterioplankton must change their metabolism to some form of anaerobic respiration. Analysis of redox chemistry in water samples spanning the oxycline of Chesapeake Bay during the summer of 2004 suggested that there was a succession of respiratory metabolism following the loss of oxygen. Bacterial community doubling time, calculated from bacterial abundance (direct counts) and production (anaerobic leucine incorporation), ranged from 0.36 to 0.75 day and was always much shorter than estimates of the time that the bottom water was anoxic (18 to 44 days), indicating that there was adequate time for bacterial community composition to shift in response to changing redox conditions. However, community composition (as determined by PCR-denaturing gradient gel electrophoresis analysis of 16S rRNA genes) in anoxic waters was very similar to that in surface waters in June when nitrate respiration was apparent in the water column and only partially shifted away from the composition of the surface community after nitrate was depleted. Anoxic water communities did not change dramatically until August, when sulfate respiration appeared to dominate. Surface water populations that remained dominant in anoxic waters were Synechococcus sp., Gammaproteobacteria in the SAR86 clade, and Alphaproteobacteria relatives of Pelagibacter ubique, including a putative estuarine-specific Pelagibacter cluster. Populations that developed in anoxic water were most similar (<92% similarity) to uncultivated Firmicutes, uncultivated Bacteroidetes, Gammaproteobacteria in the genus Thioalcalovibrio, and the uncultivated SAR406 cluster. These results indicate that typical estuarine bacterioplankton switch to anaerobic metabolism under anoxic conditions but are ultimately replaced by different organisms under sulfidic conditions.