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The amount of terrestrial particulate organic matter (t-POM) entering lakes is predicted to increase as a result of climate change. This may especially alter the structure and functioning of ecosystems in small, shallow lakes which can rapidly shift from a clear-water, macrophyte-dominated into a turbid, phytoplankton-dominated state. We used the integrative ecosystem model PCLake to predict how rising t-POM inputs affect the resilience of the clear-water state. PCLake links a pelagic and benthic food chain with abiotic components by a number of direct and indirect effects. We focused on three pathways (zoobenthos, zooplankton, light availability) by which elevated t-POM inputs (with and without additional nutrients) may modify the critical nutrient loading thresholds at which a clear-water lake becomes turbid and vice versa. Our model results show that (1) increased zoobenthos biomass due to the enhanced food availability results in more benthivorous fish which reduce light availability due to bioturbation, (2) zooplankton biomass does not change, but suspended t-POM reduces the consumption of autochthonous particulate organic matter which increases the turbidity, and (3) the suspended t-POM reduces the light availability for submerged macrophytes. Therefore, light availability is the key process that is indirectly or directly changed by t-POM input. This strikingly resembles the deteriorating effect of terrestrial dissolved organic matter on the light climate of lakes. In all scenarios, the resilience of the clear-water state is reduced thus making the turbid state more likely at a given nutrient loading. Therefore, our study suggests that rising t-POM input can add to the effects of climate warming making reductions in nutrient loadings even more urgent.
The density of organisms declines with size, because larger organisms need more energy than smaller ones and energetic losses occur when larger organisms feed on smaller ones. A potential expression of density-size distributions are Normalized Biomass Size Spectra (NBSS), which plot the logarithm of biomass independent of taxonomy within bins of logarithmic organismal size, divided by the bin width. Theoretically, the NBSS slope of multi-trophic communities is exactly - 1.0 if the trophic transfer efficiency (TTE, ratio of production rates between adjacent trophic levels) is 10% and the predator-prey mass ratio (PPMR) is fixed at 10(4). Here we provide evidence from four multi-trophic lake food webs that empirically estimated TTEs correspond to empirically estimated slopes of the respective community NBSS. Each of the NBSS considered pelagic and benthic organisms spanning size ranges from bacteria to fish, all sampled over three seasons in 1 yr. The four NBSS slopes were significantly steeper than -1.0 (range -1.14 to -1.19, with 95% CIs excluding -1). The corresponding average TTEs were substantially lower than 10% in each of the four food webs (range 1.0% to 3.6%, mean 1.85%). The overall slope merging all biomass-size data pairs from the four systems (-1.17) was almost identical to the slope predicted from the arithmetic mean TTE of the four food webs (-1.18) assuming a constant PPMR of 10(4). Accordingly, our empirical data confirm the theoretically predicted quantitative relationship between TTE and the slope of the biomass-size distribution. Furthermore, we show that benthic and pelagic organisms can be merged into a community NBSS, but future studies have yet to explore potential differences in habitat-specific TTEs and PPMRs. We suggest that community NBSS may provide valuable information on the structure of food webs and their energetic pathways, and can result in improved accuracy of TTE-estimates.
The fate of allochthonous dissolved organic carbon (DOC) in aquatic systems is primarily controlled by the turnover of heterotrophic bacteria. However, the roles that abiotic and biotic factors such as light and DOC release by aquatic primary producers play in the microbial decomposition of allochthonous DOC is not well understood. We therefore tested if light and autochthonous DOC additions would increase allochthonous DOC decomposition rates and change bacterial growth efficiencies and community composition (BCC). We established continuous growth cultures with different inocula of natural bacterial communities and alder leaf leachates (DOCleaf) with and without light exposure before amendment. Furthermore, we incubated DOCleaf together with autochthonous DOC from lysed phytoplankton cultures (DOCphyto). Our results revealed that pretreatments of DOCleaf with light resulted in a doubling of bacterial growth efficiency (BGE), whereas additions of DOCphyto or combined additions of DOCphyto and light had no effect on BGE. The change in BGE was not accompanied by shifts in the phylogenetic structure of the BCC, but BCC was influenced by the DOC source. Our results highlight that a doubling of BGE is not necessarily accompanied by a shift in BCC and that BCC is more strongly affected by resource properties.
In aquatic systems, terrestrial dissolved organic matter (t-DOM) is known to stimulate bacterial activities in the water column, but simultaneous effects of autumnal leaf input on water column and sediment microbial dynamics in littoral zones of lakes remain largely unknown. The study's objective was to determine the effects of leaf litter on bacterial metabolism in the littoral water and sediment, and subsequently, the consequences for carbon cycling and food web dynamics. Therefore, in late fall, we simultaneously measured water and sediment bacterial metabolism in the littoral zone of a temperate shallow lake after adding terrestrial particulate organic matter (t-POM), namely, maize leaves. To better evaluate bacterial production (BP) and community respiration (CR) in sediments, we incubated sediment cores with maize leaves of different quality (nonleached and leached) under controlled laboratory conditions. Additionally, to quantify the incorporated leaf carbon into microbial biomass, we determined carbon isotopic ratios of fatty acids from sediment and leaf-associated microbes from a laboratory experiment using C-13-enriched beech leaves. The concentrations of dissolved organic carbon (DOC) increased significantly in the lake after the addition of maize leaves, accompanied by a significant increase in water BP. In contrast, sediment BP declined after an initial peak, showing no positive response to t-POM addition. Sediment BP and CR were also not stimulated by t-POM in the laboratory experiment, either in short-term or in long-term incubations, except for a short increase in CR after 18 hours. However, this increase might have reflected the metabolism of leaf-associated microorganisms. We conclude that the leached t-DOM is actively incorporated into microbial biomass in the water column but that the settling leached t-POM (t-POML) does not enter the food web via sediment bacteria. Consequently, t-POML is either buried in the sediment or introduced into the aquatic food web via microorganisms (bacteria and fungi) directly associated with t-POML and via benthic macroinvertebrates by shredding of t-POML. The latter pathway represents a benthic shortcut which efficiently transfers t-POML to higher trophic levels.
Most studies of aquatic plankton focus on either macroscopic or microbial communities, and on either eukaryotes or prokaryotes. This separation is primarily for methodological reasons, but can overlook potential interactions among groups. Here we tested whether DNA metabarcoding of unfractionated water samples with universal primers could be used to qualitatively and quantitatively study the temporal dynamics of the total plankton community in a shallow temperate lake. Significant changes in the relative proportions of normalized sequence reads of eukaryotic and prokaryotic plankton communities over a 3-month period in spring were found. Patterns followed the same trend as plankton estimates measured using traditional microscopic methods. The bloom of a conditionally rare bacterial taxon belonging to Arcicella was characterized, which rapidly came to dominate the whole lake ecosystem and would have remained unnoticed without metabarcoding. The data demonstrate the potential of universal DNA metabarcoding applied to unfractionated samples for providing a more holistic view of plankton communities.
Contrasting response of two shallow eutrophic cold temperate lakes to a partial winterkill of fish
(2015)
Food-web effects of winterkill are difficult to predict as the enhanced mortality of planktivorous fish may be counterbalanced by an even higher mortality of piscivores. We hypothesised that a winterkill in a clear and a turbid shallow lake would equalise their fish community composition, but seasonal plankton successions would differ between lakes. After a partial winterkill, we observed a reduction of fish biomass by 16 and 43% in a clear-water and a turbid small temperate lake, respectively. Fish biomass and piscivore shares (5% of fish biomass) were similar in both lakes after this winterkill, but young-of-the-year (YOY) abundances were higher in the turbid lake. Top-down control by crustaceans was only partly responsible for low phytoplankton biomass at the end of May following the winterkill in both lakes. Summer phytoplankton biomass remained low in the clear-water lake despite high abundances of YOY fish (mainly roach). In contrast, the crustacean biomass of the turbid lake was reduced in summer by a high YOY abundance (sunbleak and roach), leading to a strong increase in phytoplankton biomass. The YOY abundance of fish in shallow eutrophic lakes may thus be more important for their summer phytoplankton development after winterkill than the relative abundance of piscivores.
Groundwater influx can significantly contribute to nutrient budgets of lakes and its influence is strongest in shallow littoral areas. In oligo-or mesotrophic systems, additional nutrient supply by groundwater influx may affect benthic primary producers and their interactions. Potential changes can be expected in community composition, biomass, stoichiometry and interactions between submerged macrophytes and epiphyton.
Sedimentary lipid biomarkers have become widely used tools for reconstructing past climatic and ecological changes due to their ubiquitous occurrence in lake sediments. In particular, the hydrogen isotopic composition (expressed as delta D values) of leaf wax lipids derived from terrestrial plants has been a focus of research during the last two decades and the understanding of competing environmental and plant physiological factors influencing the delta D values has greatly improved. Comparatively less attention has been paid to lipid biomarkers derived from aquatic plants, although these compounds are abundant in many lacustrine sediments. We therefore conducted a field and laboratory experiment to study the effect of salinity and groundwater discharge on the isotopic composition of aquatic plant biomarkers. We analyzed samples of the common submerged plant species, Potamogeton pectinatus (sago pondweed), which has a wide geographic distribution and can tolerate high salinity. We tested the effect of groundwater discharge (characterized by more negative delta D values relative to lake water) and salinity on the delta D values of n-alkanes from P. pectinatus by comparing plants (i) collected from the oligotrophic freshwater Lake Stechlin (Germany) at shallow littoral depth from locations with and without groundwater discharge, and (ii) plants grown from tubers collected from the eutrophic Lake Muggelsee in nutrient solution at four salinity levels. Isotopically depleted groundwater did not have a significant influence on the delta D values of n-alkanes in Lake Stechlin P. pectinatus and calculated isotopic fractionation factors epsilon(l/w) between lake water and n-alkanes averaged -137 +/- 9%(n-C-23), -136 +/- 7%(n-C-25) and -131 +/- 6%(n-C-27), respectively. Similar epsilon values were calculated for plants from Lake Muggelsee grown in freshwater nutrient solution (-134 +/- 11% for n-C-23), while greater fractionation was observed at increased salinity values of 10 (163 +/- 12%) and 15(-172 +/- 15%). We therefore suggest an average e value of -136 +/- 9% between source water and the major n-alkanes in P. pectinatus grown under freshwater conditions. Our results demonstrate that isotopic fractionation can increase by 30-40% at salinity values 10 and 15. These results could be explained either by inhibited plant growth at higher salinity, or by metabolic adaptation to salt stress that remain to be elucidated. A potential salinity effect on dD values of aquatic lipids requires further examination, since this would impact on the interpretation of downcore isotopic data in paleohydrologic studies. (C) 2017 Elsevier Ltd. All rights reserved.
The sum of benthic autotrophic and bacterial production often exceeds the sum of pelagic autotrophic and bacterial production, and hence may contribute substantially to whole-lake carbon fluxes, especially in shallow lakes. Furthermore, both benthic and pelagic autotrophic and bacterial production are highly edible and of sufficient nutritional quality for animal consumers. We thus hypothesised that pelagic and benthic transfer efficiencies (ratios of production at adjacent trophic levels) in shallow lakes should be similar. We performed whole ecosystem studies in two shallow lakes (3.5ha, mean depth 2m), one with and one without submerged macrophytes, and quantified pelagic and benthic biomass, production and transfer efficiencies for bacteria, phytoplankton, epipelon, epiphyton, macrophytes, zooplankton, macrozoobenthos and fish. We expected higher transfer efficiencies in the lake with macrophytes, because these provide shelter and food for macrozoobenthos and may thus enable a more efficient conversion of basal production to consumer production. In both lakes, the majority of the whole-lake autotrophic and bacterial production was provided by benthic organisms, but whole-lake primary consumer production mostly relied on pelagic autotrophic and bacterial production. Consequently, transfer efficiency of benthic autotrophic and bacterial production to macrozoobenthos production was an order of magnitude lower than the transfer efficiency of pelagic autotrophic and bacterial production to rotifer and crustacean production. Between-lake differences in transfer efficiencies were minor. We discuss several aspects potentially causing the unexpectedly low benthic transfer efficiencies, such as the food quality of producers, pelagic-benthic links, oxygen concentrations in the deeper lake areas and additional unaccounted consumer production by pelagic and benthic protozoa and meiobenthos at intermediate or top trophic levels. None of these processes convincingly explain the large differences between benthic and pelagic transfer efficiencies. Our data indicate that shallow eutrophic lakes, even with a major share of autotrophic and bacterial production in the benthic zone, can function as pelagic systems with respect to primary consumer production. We suggest that the benthic autotrophic production was mostly transferred to benthic bacterial production, which remained in the sediments, potentially cycling internally in a similar way to what has previously been described for the microbial loop in pelagic habitats. Understanding the energetics of whole-lake food webs, including the fate of the substantial benthic bacterial production, which is either mineralised at the sediment surface or permanently buried, has important implications for regional and global carbon cycling.
We studied bacterial associations with the green alga Desmodesmus armatus and the diatom Stephanodiscus minutulus under changing environmental conditions and bacterial source communities, to evaluate whether bacteriaalgae associations are species-specific or more generalized and determined by external factors. Axenic and xenic algae were incubated in situ with and without allelopathically active macrophytes, and in the laboratory with sterile and nonsterile lake water and an allelochemical, tannic acid (TA). Bacterial community composition (BCC) of algae-associated bacteria was analyzed by denaturing gradient gel electrophoresis (DGGE), nonmetric multidimensional scaling, cluster analyses, and sequencing of DGGE bands. BCC of xenic algal cultures of both species were not significantly affected by changes in their environment or bacterial source community, except in the case of TA additions. Species-specific interactions therefore appear to overrule the effects of environmental conditions and source communities. The BCC of xenic and axenic D.armatus cultures subjected to in situ bacterial colonization, however, had lower similarities (ca.55%), indicating that bacterial precolonization is a strong factor for bacteriaalgae associations irrespective of environmental conditions and source community. Our findings emphasize the ecological importance of species-specific bacteriaalgae associations with important repercussions for other processes, such as the remineralization of nutrients, and organic matter dynamics.