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Institute
- Institut für Biochemie und Biologie (91) (remove)
In large and deep Lake Constance, total phosphorus concentrations during winter mixing (TPmix) were reduced by a factor of three (> 80 to ca. 30 ;g/l) from about 1979 to 1993. This resulted in an amplification and lengthening of phosphorus (P) depleted conditions throughout the season and water column. The response of the phytoplankton community depended on the time of the year and the level of aggregation under consideration. Total phytoplankton biomass quantified in terms of algal biovolume or chlorophyll concentrations decreased in summer, i. e. during the period of most severe P depletion, to about half of the original values during the first decade. In subsequent years, summer chlorophyll concentrations remained at this lower level whereas total biovolume increased again despite further decreases of TPmix. Average algal biomass in spring and autumn fluctuated without a distinct relationship to TPmix although P was depleted below the detection level during parts of these time intervals in recent years. This moderate response by community level parameters is attributed to changes in the temporal and internal organization of the algal community. Population dynamics and the relative importance of various taxonomical and functional groups such as mixotrophs and less-edible forms clearly changed in spring and summer. The renewed increase in algal biovolume in summer is mostly caused by species which are able to exploit additional P sources. For example, Dinobryon is an evidently mixotrophic organism which ingests P rich bacteria, its strongest competitors for soluble reactive phosphorus (SRP). Ceratium hirundinella might be migrating between the euphotic zone and deeper, P enriched water layers under suitable hydrodynamical conditions. At the level of genera and higher taxa, consistent trends in respect to TPmix were observed in spring and summer mostly indicating an adaptation to more oligotrophic conditions. In contrast, the functional group of well-edible algae showed little interannual variability and did not change in absolute numbers. This suggests that, in contrast to less-edible algae, well-edible forms are more strongly under top-down than bottom-up control, and that the nutritional basis of most herbivores changed less than it would be expected from the decrease in total algal biomass.
Large (472 km2) and deep (zmean=101 m) Lake Constance is undergoing re-oligotrophication. Total phosphorus during winter mixing (TPmix) decreased from >80 during 1975-1981 to 22 ;g/l in 1996. Average summer values of secchi and euphotic depth increased significantly from 4.5 to 6.5 m and from 10.5 to 13 m, respectively. The algal species composition changed and, during summer, total algal biomass decreased by 50 % and primary production by 25 %. Standing stocks of well-edible algae, rotifers, and herbivorous and carnivorous crustaceans did not exhibit a trend with TPmix, whereas their species compositions or egg-ratios were partially altered. The age-at-capture of planktivorous whitefish increased slightly. I tested the hypotheses that (1) changes should first be observed at the level of individuals or within species (altering e. g. C:P or egg-ratios) prior to changes within communities (affecting e. g. the taxonomic composition) and at the community level (affecting e. g. total biomass or production). This would imply that it is more appropriate to conceptualize step-wise responses along a hierarchical gradient of increasing aggregation as suggested by hierarchy theory, rather than simultaneous changes at all hierarchical levels. (2) Responses become dampened along the food chain and with increasing body size, i. e. bottom-up control is most important for autotrophs. All communities studied (phytoplankton, crustaceans, fish) reacted at the individual level (e. g. by changes of (re)production rates), and/or within the community (e. g. altered taxonomic composition) whereas changes of bulk parameters of the entire community were restricted to phytoplankton. Hence, the first hypothesis is partially supported by the observed reactions and demands further testing. The second hypothesis is clearly supported by our data when comparing autotrophs and consumers, but not when comparing crustaceans and fish. The testing of these hypotheses is complicated by the large differences in size and, consequently, in reaction times of pelagic organisms on the one hand and the rather fixed time scale of limnological research on the other hand. The different time scales imply a selective perception of the various potential responses of the differently sized organisms as the time scales of the responses depend on body size and the level of aggregation. For example, we are more likely to establish physiological or behaviourial changes of fish, and taxonomical or biomass changes of phytoplankton. Acknowledging the scale dependence and level of aggregation is also crucial for cross-system comparisons.
The ecology of chroococcoid picocyanobacteria was studied from 1987 to 1997 in large, deep, mesotrophic Lake Constance in relation to various abiotic and biotic factors that may influence their population dynamics. Picocyanobacteria dominated the autotrophic picoplankton (APP) numerically in this lake at all depths and times. Their abundances did not respond unequivocally to the decline of winterly phosphorus concentrations by a factor of 2.5 during the decade of investigation. They showed a recurrent seasonal pattern with peaks in spring and late summer, interspersed by a pronounced minimum during and after the clear-water phase around June. The magnitude, timing, and number of peaks and troughs which varied interannually, could in part be related to weather conditions or the impact of other plankton groups. Larger phytoplankton and picocyanobacteria exhibited a distinct and predictable response to the vertical mixing intensity during early spring. Except for 1993, picocyanobacteria and larger phytoplankton decreased simultaneously during the mass development of daphnids in late May or June which gave rise to the clear-water phase. As the daphnid development depends more strongly on surface water temperature than on vertical mixing intensity an early onset of stratification may imply a longer spring development which contributed to a higher seasonal average of picocyanobacterial abundances in 1989-1991. The decline in picocyanobacteria around the clear-water phase was often more pronounced and lasted longer than did the decline in larger algae. The rate of decrease may be related to daphnid abundance, however, no such relationship existed in respect to its duration. Summer peaks of picocyanobacteria were recorded despite the presence of relatively high densities of daphnids. We conclude that with the exception of the clear- water phase, grazing control by nano- and microzooplankton may be more important for controlling picocyanobacterial numbers than is grazing by daphnids. Picocyanobacteria declined in autumn prior to or concomitant with larger algae without any obvious relationship to phytoplankton biovolume or the extent of vertical mixing within the uppermost 20 m. The as yet unexplained variation in the population dynamics of picocyanobacteria points to the significance of species- specific protist grazing and to shifts in picocyanobacterial species composition which should be tackled in future studies.
Seasonal changes of the impact of daphnids on the plankton biomass size distribution, the biomass within individual size ranges, the average predator-prey weight ratios, and the efficiency to transfer matter and energy from small to large organisms are analyzed in large and deep Lake Constance based on comprehensive long-term observations. A comparison of daphnid biomass and production with those of other herbivorous groups (i. e. ciliates, rotifers, herbivorous crustaceans) reveals that in early spring daphnids play a minor role in relative and absolute values as compared to small fast growing ciliates. During this time, small algae and ciliates dominate which gives rise to a decreasing Sheldon-type size spectrum, low predator-prey weight ratios, and a low transfer efficiency along the size gradient. Around June, daphnids reach maximum abundances and become keystone species for the shape of the biomass size distribution, the food web structure, and the energy flow. They accumulate biomass in their size range one order of magnitude above the average. The slope of the normalized biomass size spectrum is less negative and positively correlated with daphnid biomass if the latter exceeds about 200 mg C/m2. This indicates a more efficient transfer along the size gradient with high predator-prey weight ratios and high trophic transfer efficiencies. The coefficients of determination of regression lines fitted to size distributions decrease with daphnid abundance, i. e. the size spectra become more irregular when daphnids dominate. In midsummer, daphnids lose their dominance and coexist with other herbivores (especially ciliates) in a highly diverse plankton community. The latter gives rise to a relatively smooth and almost flat Sheldon-type size distribution, lower predator-prey weight ratios, and a slightly reduced transfer efficiency along the size gradient. In late spring/early summer, negative relationships are found between daphnid biomass and the biomasses in the size ranges of autotrophic picoplankton, small phytoplankton, heterotrophic flagellates, and small and medium sized ciliates (0.06 - 32 pg C and 100-30,000 pg C). In mid- and late summer or on annual average, hardly any of these relationships existed. This cannot solely be attributed to lower daphnid abundance but points also to a more diverse control of small plankton organisms including nutrient limitation in summer. Ciliates influence the slope and shape of the size distribution much less than daphnids although they are at least of equal importance as daphnids in respect to herbivory and related fluxes in Lake Constance on annual average. The findings on the impact of daphnids on the energy flow within the plankton food web derived from size distributions are compared to, and are consistent with results obtained by mass-balanced carbon flow diagrams.
In Lake Constance, phytoplankton productivity, together with parameters relevant for the production process, was assessed year-round at about 500 dates between 1980 and 1995/1996. During this period, the concentration of total phosphorus during winter circulation decreased from more than 80 to 22 ;g/l as a consequence of sewage diversion and waste water treatment within the catchment area. By contrast, annual photosynthetic rates remained virtually unchanged for about 10 more years following phosphorus decline (mean value 288 " 21 g C m-2 a-1), and thereafter decreased only by about 25 % until 1996. The aim of this study is to analyze factors responsible for this pronounced resilience.
Estimating production in plankton food webs from biomass size spectra and allometric relationships
(2000)
Planktische Räuber-Beute-Systeme : experimentelle Untersuchungen von ökologischen Synchronisationen
(2000)
Phytoplankton dynamics in a shallow eutrophic lake were investigated over a 3-year period with respect to environmental forces which drive species composition and diversity. Diversity was calculated on the basis of species as well as on the basis of their functional properties (the C-R-S-concept). Stratification and water column mixing had a strong impact on phytoplankton composition. Application of a similarity-diversity model revealed that a high diversity was a transient non-stable state, whereas drastic changes or long-lasting stable environmental conditions are characterized by low diversity. This effect was more pronounced when the diversity was calculated on the basis of the phytoplankton species functional properties. Thus, this functional approach supports the intermediate disturbance hypothesis from field data.
Natural selection for grazer resistance to toxic cyanobacteria: Evolution of phenotypic plasticity?
(2001)
Interplay between energy limitation and nutritional deficiency: Empirical data and food web models
(2002)
Due to differences in the biochemical composition of autotrophs and their grazers, food quality can strongly influence herbivore population dynamics. Under nutrient depleted conditions the carbon to nutrient ratios of autotrophs can increase to such an extent that consumers become nutrient rather than energy limited. Estimating the importance of this effect in situ in pelagic food webs is complicated by the omnivory of many consumers and rapid nutrient recycling. Isolated predator-prey studies inadequately represent this interaction, instead an ecosystem perspective is required. We used seven years of data from large, deep Lake Constance to develop seasonally resolved flux models of the pelagic food web and analyze the balance between energetic and nutrient constraints. The carbon (C) and phosphorus (P) flows were simultaneously quantified and balanced. C represented food quantity/energy. P was taken as a surrogate of food quality, because algal C:P ratios exceeded the threshold above which P limitation of herbivores is predicted by stoichiometric theory throughout summer and autumn. Primary production exceeded bacterial C production by a factor of 3 but autotrophs and bacteria took up approximately equal amounts of P during summer and autumn. As a consequence the C and P supply of suspension-feeding zooplankton was decoupled: Consumer C demands were largely met by phytoplankton whereas P was mostly obtained from bacteria and their protist predators. The degree of consumer P deficiency varied according to supplementation of their algal diet with P-enriched bacteria or bacterivores. This favored the occurrence of omnivores, i.e. organisms that minimized P deficiencies at the cost of enhanced energy limitation. In contrast with previous perceptions, P remineralization during P depleted summer conditions was dominated by bacterivorous flagellates, carnivorous crustaceans and fish, which fed on prey with an elemental composition similar to their own, whereas herbivores contributed only 30% of P cycling despite their large biomass and C production. Our results suggested a co- limitation of predominantly herbivorous consumers by C and P and a mutual dependence of the two types of deficiency at the individual and system level. This pattern is not specific to pelagic systems but appears to be applicable across ecosystem types.
Mixotrophs combine resource use to outcompete specialists: Implications for aquatic food webs
(2003)
The majority of species can be grouped into those relying solely on photosynthesis (phototrophy) or those relying solely on the assimilation of organic substances (heterotrophy) to meet their requirements for energy and carbon. However, a special life history trait exists in which organisms combine both phototrophy and heterotrophy. Such 'mixotrophy' is a widespread phenomenon in aquatic habitats and is observed in many protozoan and metazoan organisms. The strategy requires investment in both photosynthetic and heterotrophic cellular apparatus, but the benefits must outweigh these costs. In accordance with the mechanistic resource competition theory, laboratory experiments revealed that pigmented mixotrophs combined light and prey as substitutable resources. Thereby, they reduced prey abundance below the critical food concentration of competing specialist grazers [Rothhaupt, K. O. (1996) Ecology 77, 716-724]. Here, we demonstrate for the first time the important consequences of this strategy for an aquatic community. In the illuminated surface strata of a lake, mixotrophs reduced prey abundance so steeply that grazers from higher trophic levels, consuming both the mixotrophs and their prey, could not persist. Thus, the mixotrophs escaped from both competition and grazing, and remained dominant. Furthermore, the mixotrophs structured the prey abundance along the vertical light gradient creating low densities near the surface and a pronounced maximum of their algal prey at depth. Such deep algal accumulations are typical features of nutrient poor aquatic habitats, previously explained by resource availability. We hypothesize instead that the mixotrophic grazing strategy is responsible for deep algal accumulations in many aquatic environments.
Vertical differences in food web structure were examined in an extremely acidic, iron-rich mining lake in Germany (Lake 111; pH 2.6, total Fe 150mg L-1) during the period of stratification. We tested whether or not the seasonal variation of the plankton composition is less pronounced than the differences observed over depth. The lake was strongly stratified in summer, and concentrations of dissolved organic carbon and inorganic carbon were consistently low in the epilimnion but high in the hypolimnion. Oxygen concentrations declined in the hypolimnion but were always above 2mg L-1. Light attenuation did not change over depth and time and was governed by dissolved ferric iron. The plankton consisted mainly of single-celled and filamentous bacteria, the two mixotrophic flagellates Chlamydomonas sp. and Ochromonas sp., the two rotifer species Elosa worallii and Cephalodella hoodi, and Heliozoa as top predators. We observed very few ciliates and rhizopods, and no heterotrophic flagellates, crustaceans or fish. Ochromonas sp., bacterial filaments, Elosa and Heliozoa dominated in the epilimnion whereas Chlamydomonas sp., single-celled bacteria and Cephalodella dominated in the hypolimnion. Single-celled bacteria were controlled by Ochromonas sp. whereas the lack of large consumers favoured a high proportion of bacterial filaments. The primarily phototrophic Chlamydomas sp. was limited by light and CO2 and may have been reduced due to grazing by Ochromonas sp. in the epilimnion. The distribution of the primarily phagotrophic Ochromonas sp. and of the animals seemed to be controlled by prey availability. Differences in the plankton composition were much higher between the epilimnion and hypolimnion than within a particular stratum over time. The food web in Lake 111 was extremely species-poor enabling no functional redundancy. This was attributed to the direct exclusion of species by the harsh environmental conditions and presumably enforced by competitive exclusion. The latter was promoted by the low diversity at the first trophic level which, in turn, was attributed to relatively stable growth conditions and the independence of resource availability (inorganic carbon and light) from algal density. Ecological theory suggests that low functional redundancy promotes low stability in ecosystem processes which was not supported by our data.
Functional groups with diverse responses to environmental factors sum to produce communities with less temporal variability in their biomass than those lacking this diversity. The detection of these compensatory dynamics can be complicated by a spatio-temporal alternation in the environmental factors limiting growth (both abiotic and biotic), which restricts the occurrence of compensatory dynamics to certain periods or locations. Hence, resolving the spatio- temporal scale may uncover important spatial and/or temporal components in community variability. Using long-term data from Lake Constance (Bodensee), we find that a reduction in grazing pressure and relaxed competition for nutrients during winter and spring generates coherent dynamics among edible and less edible phytoplankton. During summer and fall, when both grazing pressure and nutrient limitation are present, edible and less edible phytoplankton exhibit compensatory dynamics. This study supports recent work suggesting that both abiotic and biotic interactions promote compensatory dynamics and to our knowledge, this is the first example of a system where compensatory and coherent dynamics seasonally alternate.
Inorganic carbon limitation and mixotrophic growth in Chlamydomonas from an acidic mining lake
(2005)
Plankton communities in acidic mining lakes (pH 2.5-3.3) are species-poor because they face extreme environmental conditions, e.g. 150 mg l(-1) Fe2++Fe3+. We investigated the growth characteristics of the dominant pigmented species, the flagellate Chlamydomonas acidophila, in semi-continuous culture experiments under in situ conditions. The following hypotheses were tested: (1) Low inorganic carbon (IC) concentrations in the epilimnion (e.g. 0.3 mg l(-1)) arising from the low pH limit phototrophic growth (H-1); (2) the additional use of dissolved organic carbon (mixotrophy) leads to higher growth rates under IC-limitation (H-2), and (3) phagotrophy is not relevant (H-3). H- 1 was supported as the culture experiments, in situ PAR and IC concentrations indicated that IC potentially limited phototrophic growth in the mixed surface layers. H-2 was also supported: mixotrophic growth always exceeded pure phototrophic growth even when photosynthesis was saturated. Dark growth in filtered lake water illuminated prior to inoculation provided evidence that Chlamydomonas was able to use the natural DOC. The alga did not grow on bacteria, thus confirming H-3. Chlamydomonas exhibited a remarkable resistance to starvation in the dark. The compensation light intensity (ca. 20 mu mol photons m(-2) s(-1)) and the maximum phototrophic growth (1.50 d(-1)) fell within the range of algae from non-acidic waters. Overall, Chlamydomonas, a typical r-strategist in circum-neutral systems, showed characteristics of a K-strategist in the stable, acidic lake environment in achieving moderate growth rates and minimizing metabolic losses. (c) 2005 Elsevier GmbH. All rights reserved
Investigating the mechanisms which underlie the biomass fluctuations of populations and communities is important to better understand the processes which buffer community biomass in a variable environment. Based on long- term data of plankton biomass in Lake Constance (Bodensee), this study aims at explaining the different degree of synchrony among populations observed within two freshwater plankton groups, phytoplankton and ciliates. Established measures of temporal variability such as the variance ratio and cross-correlation coefficients were combined with first- order autoregressive models that allow estimating species interactions from time-series data. We found that predation was an important driver of the observed seasonal variability patterns in phytoplankton and ciliates, and that competitive interactions only played a subordinate role. In Lake Constance copepods and cladocerans, two major invertebrate predator groups, focus their grazing pressure at different times of the season. Model results suggested that compensatory dynamics detected in phytoplankton originate from the differential vulnerability of species to either one of these two predator groups. For ciliates model results advocated that synchrony among species occurs because ciliates tend to be vulnerable to both predator groups. Our findings underline the necessity of extending studies of community variability to multiple trophic levels because accounting for predator-prey interactions may often be more important than accounting for competitive interactions at one trophic level
1. The in situ abundance, biomass and mean cell volume of Actinophrys sol (Sarcodina: Heliozoa), the top predator in an extremely acidic German mining lake (Lake 111; pH 2.65), were determined over three consecutive years (spring to autumn, 2001-03). 2. Actinophrys sol exhibited pronounced temporal and vertical patterns in abundance, biomass and mean cell volume. Increasing from very low spring densities, maxima in abundance and biomass were observed in late June/early July and September. The highest mean abundance recorded during the study was 7 x 10(3) Heliozoa L-1. Heliozoan abundance and biomass were higher in the epilimnion than in the hypolimnion. Actinophrys sol cells from this acidic lake were smaller than individuals of the same species found in other aquatic systems. 3. We determined the growth rate of A. sol using all potential prey items available in, and isolated and cultured from, Lake 111. Prey items included: single-celled and filamentous bacteria of unknown taxonomic affinity, the mixotrophic flagellates Chlamydomonas acidophila and Ochromonas sp., the ciliate Oxytricha sp. and the rotifers Elosa worallii and Cephalodella hoodi. Actinophrys sol fed over a wide-size spectrum from bacteria to metazoans. Positive growth was not supported by all naturally available prey. Actinophrys sol neither increased in cell number (k) nor biomass (k(b)) when starved, with low concentrations of single-celled bacteria or with the alga Ochromonas sp. Positive growth was achieved with single- celled bacteria (k = 0.22 +/- 0.02 d(-1); k(b) = -0.06 +/- 0.02 d(-1)) and filamentous bacteria (k = 0.52 +/- < 0.01 d(- 1); k(b) = 0.66 d(-1)) at concentrations greater than observed in situ, and the alga C. acidophila (up to k = 0.43 +/- 0.03 d(-1); k(b) = 0.44 +/- 0.04 d(-1)), the ciliate Oxytricha sp. (k = 0.34 +/- 0.01 d(-1)) and in mixed cultures containing rotifers and C. acidophila (k = 0.23 +/- 0.02-0.32 +/- 0.02 d(-1); maximum k(b) = 0.42 +/- 0.05 d(-1)). The individual- and biomass-based growth of A. sol was highest when filamentous bacteria were provided. 4. Existing quantitative carbon flux models for the Lake 111 food web can be updated in light of our results. Actinophrys sol are omnivorous predators supported by a mixed diet of filamentous bacteria and C. acidophila in the epilimnion. Heliozoa are important components in the planktonic food webs of 'extreme' environments