Lake ecosystems are strongly linked to their terrestrial surroundings by material and energy fluxes across ecosystem boundaries. However, the contribution of terrestrial particulate organic carbon ...(tPOC) from annual leaf fall to lake food webs has not yet been adequately traced and quantified. In this study, we conducted whole-lake experiments to trace artificially added tPOC through the food webs of two shallow lakes of similar eutrophic status, but featuring alternative stable regimes (macrophyte rich vs. phytoplankton dominated). Lakes were divided with a curtain, and maize (
Zea mays
) leaves were added, as an isotopically distinct tPOC source, into one half of each lake. To estimate the balance between autochthonous carbon fixation and allochthonous carbon input, primary production and tPOC and tDOC (terrestrial dissolved organic carbon) influx were calculated for the treatment sides. We measured the stable isotope ratios of carbon (δ
13
C) of about 800 samples from all trophic consumer levels and compared them between lake sides, lakes, and three seasons. Leaf litter bag experiments showed that added maize leaves were processed at rates similar to those observed for leaves from shoreline plants, supporting the suitability of maize leaves as a tracer. The lake-wide carbon influx estimates confirmed that autochthonous carbon fixation by primary production was the dominant carbon source for consumers in the lakes. Nevertheless, carbon isotope values of benthic macroinvertebrates were significantly higher with maize additions compared to the reference side of each lake. Carbon isotope values of omnivorous and piscivorous fish were significantly affected by maize additions only in the macrophyte-dominated lake and δ
13
C of zooplankton and planktivorous fish remained unaffected in both lakes. In summary, our results experimentally demonstrate that tPOC in form of autumnal litterfall is rapidly processed during the subsequent months in the food web of shallow lakes and is channeled to secondary and tertiary consumers predominantly via the benthic pathways. A more intense processing of tPOC seems to be connected to a higher structural complexity in littoral zones, and hence may differ between shallow lakes of alternative stable states.
Many ecosystems are linked to their adjacent ecosystems by movements of organisms. For instance, aquatic and terrestrial ecosystems are linked via emerging aquatic insects that serve as prey for ...terrestrial consumers. However, the role of these organisms in returning recycled carbon to the ecosystem from which it originated is not well known. This is due to the fact that values of carbon isotope signatures from terrestrial leaves and aquatic resources are usually similar and hence results of isotope mixing models need to be considered with caution. We overcame this problem by adding isotopically distinct terrestrial particulate organic carbon (tPOC) as a tracer to the experimental sides of two lakes that were divided in two equal halves with plastic curtains. We focused on aquatic insect larvae (Chironomidae) that fed on maize Zea mays leaves experimentally added to the lakes, and subsequently became prey for terrestrial predators (spiders) after emergence. The carbon isotope values of Chironomidae and spiders were significantly elevated in the lake treatment sides as compared to reference sides, whereas the values of all autochthonous resources were not affected by maize additions. Estimates from stable isotope mixing models indicated a low but demonstrable contribution of maize leaves to the diet of Chironomidae. Overlap between the isotope values of alder leaves, the major natural tPOC source, and autochthonous resources prevented a reliable quantification of allochthony of Chironomidae. However, we qualitatively demonstrated the flow of terrestrial particulate organic carbon to lakes, as leaf fall, and back to terrestrial surroundings via emerging insects. This 'boomerang' carbon flux between land and lakes blurs the distinction between autochthonous and allochthonous carbon sources.
Microbial communities play a key role in biogeochemical processes by degrading organic material and recycling nutrients, but can also be important food sources for upper trophic levels. Trophic ...cascades might modify microbial communities either directly via grazing or indirectly by inducing changes in other biotic or in abiotic factors (e.g., nutrients). We studied the effects of a tri‐trophic cascade on microbial communities during a whole‐lake manipulation in which European perch (Perca fluviatilis) were added to a naturally fishless lake divided experimentally into two basins. We measured environmental parameters (oxygen, temperature, and nutrients) and zooplankton biomass and studied the changes in the bacterial community using next generation sequencing of 16S rRNA genes and cell counting. Introduction of fish reduced the biomass of zooplankton, mainly Daphnia, which partly altered the bacterial community composition and affected the bacterial cell abundances. However, the microbial community composition was mainly governed by stratification patterns and associated vertical oxygen concentration. Slowly growing green sulfur bacteria (Chlorobium) dominated the anoxic water layers together with bacteria of the candidate division OD1. We conclude that alterations in trophic interactions can affect microbial abundance, but that abiotic factors seem to be more significant controls of microbial community composition in sheltered boreal lakes.
Evidence is provided from stable isotope analysis that aggregations of small ocean sunfish Mola mola (total length <1 m) feed broadly within coastal food webs and their classification as obligate ...predators of gelatinous zooplankton requires revision.
We evaluated the potential utility of stable isotope analysis of tissues commonly archived by aquatic biologists. Previous studies with chemically preserved samples have shown contradictory results, ...which present an obstacle for the use of archived sample materials. We tested the effects of ethanol and formalin preservation on zooplankton and of ethanol on benthic macroinvertebrate δ¹³C and δ¹⁵N values. We found that neither formalin nor ethanol had a significant effect on δ¹³C and δ¹⁵N values of preserved zooplankton. Nor did ethanol significantly affect δ¹³C or δ¹⁵N values of macroinvertebrates. However, ethanol preservation slightly, but significantly decreased C:N ratios of both zooplankton and macroinvertebrates, probably reflecting some extraction of lipids. Overall, the effects of preservatives on δ¹³C and δ¹⁵N values that we observed were minor. We also compared δ¹³C and δ¹⁵N values analysed from roach scales and perch operculum bones with those analysed from muscle tissue. Decalcification of scales and operculum bones only slightly improved our comparison to muscle tissue δ¹³C and δ¹⁵N values. Decalcified scales had slightly higher δ¹³C and lower δ¹⁵N values. Similarly, decalcified operculum bones showed slightly increased δ¹³C and decreased δ¹⁵N values to those for fish muscle. Our results confirm that scales and operculum bones can provide a suitable proxy for fish muscle in isotope studies with minor correction. We conclude that various archived sample materials can indeed be used with confidence for historical reconstructions of freshwater food webs by stable isotope analysis.
Although being a widely introduced and successfully established species, the European catfish Silurus glanis L. (the world's third largest freshwater fish) remains poorly studied in its introduced ...areas. Here we studied the trophic ecology of non-native European catfish in a large river system in south-western France using stomach content and stable isotope analyses (SIA). We used fin samples for SIA of catfish and hence tested the validity of using fin tissue as a proxy for muscle in SIA. The mean d super(15)N and d super(13)C values analysed from fin tissues did not differ from those analysed from muscle tissue and reflected strong and consistent relationships (r super(2) = 0.95 for carbon and r super(2) = 0.98 for nitrogen). The d super(15)N values varied almost 5ppt among the analysed catfish individuals, while d super(13)C values varied >5ppt. Total length of these catfish ranged from 200 to 2240 mm and was correlated with d super(15)N and especially with d super(13)C values. Although catfish length and d super(15)N values were positively correlated, the mean trophic positions of catfish increased only slightly from smaller individuals to larger ones (4.3 to 4.7). However, larger catfish were considerably super(13)C-enriched in their d super(13)C values compared to smaller individuals and had up to 4ppt higher d super(13)C values than their expected aquatic prey. This might indicate frequent consumption of mammals and/or non-aquatic birds by the larger sized individuals, which were found in the catfish stomachs.
Ecosystems are generally linked via fluxes of nutrients and energy across their boundaries. For example, freshwater ecosystems in temperate regions may receive significant inputs of terrestrially ...derived carbon via autumnal leaf litter. This terrestrial particulate organic carbon (POC) is hypothesized to subsidize animal production in lakes, but direct evidence is still lacking. We divided two small eutrophic lakes each into two sections and added isotopically distinct maize litter to the treatment sections to simulate increased terrestrial POC inputs via leaf litter in autumn. We quantified the reliance of aquatic consumers on terrestrial resources (allochthony) in the year subsequent to POC additions by applying mixing models of stable isotopes. We also estimated lake-wide carbon (C) balances to calculate the C flow to the production of the major aquatic consumer groups: benthic macroinvertebrates, crustacean zooplankton, and fish. The sum of secondary production of crustaceans and benthic macroinvertebrates supported by terrestrial POC was higher in the treatment sections of both lakes. In contrast, total secondary and tertiary production (supported by both autochthonous and allochthonous C) was higher in the reference than in the treatment sections of both lakes. Average aquatic consumer allochthony per lake section was 27–40%, although terrestrial POC contributed less than about 10% to total organic C supply to the lakes. The production of aquatic consumers incorporated less than 5% of the total organic C supply in both lakes, indicating a low ecological efficiency. We suggest that the consumption of terrestrial POC by aquatic consumers facilitates a strong coupling with the terrestrial environment. However, the high autochthonous production and the large pool of autochthonous detritus in these nutrient-rich lakes make terrestrial POC quantitatively unimportant for the C flows within food webs.
For many applications and ecological studies in which wild individuals are brought to laboratory it would be essential to know accurately how fast novel diet is reflected in composition of different ...tissues. To study the effects of two different diets on the stable isotope composition of freshwater crayfish muscle and hemolymph, we conducted a three month experiment on noble crayfish (Astacus astacus) and signal crayfish (Pacifastacus leniusculus) by feeding them sweet corn (Zea mays) or Baltic herring (Clupea harengus membras) as novel food. During the experiment, the crayfish were given 0.4 g of selected food daily and the amount consumed was recorded. The samples for the stable isotope analyses were taken at the commencement of the experiment (initial control) and three times (hemolymph) or twice (muscle tissue) during the experiment. We found that stable isotope changes can be similarly, and rather slowly, detected from muscle tissue and hemolymph under studied conditions. Hemolymph sampling, being non-lethal, can be recommended as a practical sampling method. Our results confirm earlier reports according to which diet changes reflect to crayfish isotope ratios slowly implying that isotope ratios indicate long-term diet.
Pour de nombreuses applications et des études écologiques dans lesquelles des individus sauvages sont apportés au laboratoire, il serait essentiel de connaître avec précision à quelle vitesse l’alimentation nouvelle se reflète dans la composition des différents tissus. Pour étudier les effets de deux régimes différents sur la composition en isotopes stables du muscle et de l’hémolymphe d’écrevisses, nous avons mené une expérience de trois mois sur l’écrevisse noble (Astacus astacus) et l’écrevisse signal (Pacifastacus leniusculus) en les nourrissant de maïs doux (Zea mays) ou de hareng de la Baltique (Clupea harengus membras) comme nouveaux aliments. Pendant l’expérience, les écrevisses ont reçu 0,4 g de nourriture sélectionnée par jour et la quantité consommée a été enregistrée. Les échantillons pour les analyses d’isotopes stables ont été pris au début de l’expérience (contrôle initial) et trois fois (hémolymphe) ou deux fois (tissus musculaires) pendant l’expérience. Nous avons trouvé que les changements d’isotopes stables peuvent être similaires, et plutôt lents, détectés à partir de tissu musculaire et de l’hémolymphe dans les conditions étudiées. L’échantillonnage de l’hémolymphe, étant non létal, peut être recommandé comme une méthode d’échantillonnage pratique. Nos résultats confirment des données antérieures selon lesquelles les changements de régime se reflètent lentement dans les rapports isotopiques des écrevisses, impliquant que les rapports isotopiques indiquent le régime alimentaire sur un long terme.
Information on the present distribution and status of the threatened freshwater pearl mussel
Margaritifera margaritifera
populations in Finland was updated. The status of the populations was ...classified into seven categories which are
viable
,
maybe viable
,
non-viable/partly viable
,
dying-out
,
almost extinct
,
probably extinct
, and
extinct
. The main criteria for judging the viability class were the population size and proportion of juvenile mussels. According to calculated estimate 1.7% populations were viable, 8.5%
maybe viable
, 40.2%
non-viable/partly viable
, 14.5%
dying-out
, 30.8%
almost extinct
, and 4.3%
probably extinct
. The present number of rivers with
M. margaritifera
in Finland is 117.
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