Sources and fates of dissolved organic carbon in lakes as determined by whole-lake carbon isotope additions

Four whole-lake inorganic ¹³C addition experiments were conducted in lakes of differing trophic status. Inorganic ¹³C addition enriched algal carbon in ¹³C and changed the C-DOC by +1.5‰ to +9.5‰, depending on the specific lake. This change in C-DOC represented a significant input of algal DOC that was not completely consumed by bacteria. We modeled the dynamics in C-DOC to estimate the fluxes of algal and terrestrial carbon to and from the DOC pool, and determine the composition of the standing stock. Two experiments in lightly stained, oligotrophic lakes indicated that algal production was the source of about 20% of the DOC pool. In the following year, the experiment was repeated in one of these lakes under conditions of nutrient enrichment, and in a third, more humic lake. Algal contributions to the DOC pool were 40% in the nutrient enriched lake and 5% in the more humic lake. Spectroscopic and elemental analyses corroborated the presence of increased algal DOC in the nutrient enriched lake. Natural abundance measurements of the C of DOC in 32 lakes also revealed the dual contributions of both terrestrial and algal carbon to DOC. From these results, we suggest an approach for inferring the contribution of algal and terrestrial DOC using easily measurable parameters.     

Distribution of dissolved organic carbon in Lake Baikal and its watershed

 Concentrations of dissolved organic carbon (DOC) in Lake Baikal ranged from 90 to 110 μM C, considerably higher than those in oceanic environments. The DOC concentrations in the epilimnion were higher than those in the hypolimnion. Since particulate organic carbon (POC) concentrations in the pelagic waters of Lake Baikal were <10–40 μM C in the epilimnion and 2–5 μM C in the hypolimnion, DOC constitutes a major component of the organic carbon pool in Lake Baikal, especially in the deep layers. The DOC concentrations downstream of the Barguzin and Selenga Rivers were quite high (400–500 μM C). Probably because of the high concentrations of DOC in these rivers, the DOC levels in Barguzin Bay and offshore at the mouth of the Selenga River were higher than those in the pelagic regions of the central and south basins of Lake Baikal. The relationship between DOC and electric conductivity revealed the transport of DOC from rivers to the pelagic area in Lake Baikal. The spatial distribution of DOC suggested that a major part of DOC in the lake was allochthonous (land-derived). Received: July 26, 2002 / Accepted: September 16, 2002 Present address:Research Institute for Humanity and Nature, 335 Takashima-cho, Marutamachi Kawaramachi, Kamigyo-ku, Kyoto 602-0878, Japan Tel. +81-75-229-6167; Fax +81-75-229-6150 e-mail: yos@chikyu.ac.jp Acknowldgments The authors wish to thank Director Prof. M. Grachev and Dr. O. Timoshkin of the Limnological Institute, Siberian Branch, Academy of Science, Russia, for arranging the cruise on Lake Baikal. We are also indebted to Drs. V. Sinyukovich, I. Khanaev, and A. Zhdanov for their kind assistance during the expeditions. We wish to thank Ms. Y. Ito for measuring the DOC concentrations. This work was supported and financed by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (No. 09041159) and for Scientific Research of Priority Area B (No. 11213101) and by the International Geosphere-Biosphere Programme (IGBP) at Nagoya University. Correspondence to:T. Yoshioka     

Stable isotope analysis of macroinvertebrates and their food sources in a glacier stream

1. Food sources and trophic structure of the macroinvertebrate community along a longitudinal gradient were examined in a glacier stream of the Swiss Alps (Val Roseg). Analysis of multiple stable isotopes (δ¹³C and δ¹⁵N) and measurement of C : N ratios were used to differentiate between allochthonous and autochthonous organic matter.
2. Although isotopic signatures of algae varied widely among sites and dates, it was possible to discriminate between allochthonous and autochthonous food sources using a site-specific approach.
3. Dominant food sources of herbivorous invertebrates in all main channel sites were epilithic diatoms and the filamentous gold alga Hydrurus foetidus . Allochthonous organic matter was of some importance only in a groundwater-fed stream close to the floodplain margin.
4. Seasonal changes in the δ¹³C signature of the macroinvertebrates corresponded with seasonal changes in δ¹³C of the gold alga H. foetidus . This indicated that the energy base remains autochthonous throughout the year.
5. Because of limited food sources, feeding plasticity of the invertebrate community was high. Both grazers and shredders fed predominantly on algae, whereas gatherer-collectors seemed to be omnivorous.
6. The overall enrichment of δ¹⁵N was 2.25‰ ( r ²=0.99) per trophic level. On a gradient from the glacier site to a downstream forested site trophic enrichment was constant but variation in δ¹⁵N within trophic levels decreased.     

Stable isotope analysis reveals variation in trophic niche depending on altitude in an endemic alpine gecko

Interspecific competition is considered a major determinant of ecological niche. It is hypothesized that increased competition should reduce niche breadth. However, there are scarce field tests on this hypothesis. Here, we test this central hypothesis in ecology by using the Atlas day gecko Quedenfeldtia trachyblepharus. This alpine gecko faces fewer competitors as altitude increases, and thereby, we predict that this species should increase niche breadth and relevant fitness parameters with altitude. We tested this prediction by analysing the isotopic signature of carbon (δ¹³C) and nitrogen (δ¹⁵N). Our results reveal that specimens from higher altitudes showed higher values for both carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes, had better body condition and a greater isotopic breadth when compared to specimens from lower altitudes. Altitudinal variation in carbon values was not explained by variation in isotopic concentration in the baseline of the trophic chain. Therefore, our findings support the prediction that relaxed interspecific competition favours increased trophic niche breadth. These results also suggest that global warming may represent an important threat for this species, as it may provoke the ascent in altitude of competitors, with negative consequences for the conservation of this endemism.     

Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1.  The δ¹³C and δ¹⁵N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa ( Daphnia , Holopedium , large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.
2.  δ¹³C signatures of Daphnia , small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ¹³C became enriched with increasing DOC concentration.
3.  The variability of δ¹³C and δ¹⁵N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for δ¹³C. Differences in δ¹³C and POM-corrected δ¹⁵N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.
4.  The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.     

Distribution of dissolved organic carbon in lakes of different trophic types

The distributions of dissolved organic carbon (DOC) in the warm season were elucidated in ten lakes of different trophic types in Japan, Russia, and China. DOC showed similar vertical distributions in all the lakes in summer when thermal stratification occurred. DOC in the epilimnion was higher than the value of 0.8mgCl^–1 found in the hypolimnion. In three Japanese lakes, hypolimnion DOC was negatively correlated with apparent oxygen utilization (AOU), reflecting the net oxidation of DOC using the dissolved oxygen in lake water. The DOC:O₂ ratios (0.115–0.179), calculated by the slopes of the regression lines of DOC versus AOU in hypolimnion water, were as low as those of deep-sea water, which indicates low bioavailability of lake water DOC for heterotrophic bacteria. DOC and conductivity did not correlate well except in two Japanese lakes: one showed a positive correlation and the other a negative correlation, indicating DOC loading from the inflowing rivers. Eutrophic lakes tended to have higher DOC values than meso- and oligotrophic lakes, and DOC values in the surface water negatively correlated with Secchi depths.     

A 200-year coral stable oxygen isotope record from a high-latitude reef off Western Australia

 A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition^*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral δ¹⁸O. At 29′S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral δ¹⁸O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral δ¹⁸O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral δ¹⁸O demonstrate a high variability in spectral composition through time. Oscillations in the 5–7-y, 14–15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral δ¹⁸O suggests a rise in SST by 0.6 ′C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4 ′C since AD 1795 is inferred from the coral record. Accepted: 3 July 1998     

Empirical relationships between cladoceran fauna and trophic state in thirteen northern German lakes: analysis of surficial sediments

The cladoceran faunas from 13 northern German lakes (the Plön district) of different trophic levels were analysed using the bosminid and chydorid remains in the surficial sediment as an integrated sample of the total lake fauna. In the dimictic lakes, a shift occurred in the composition of planktonic Cladocera from Bosmina coregoni, including morphs with a mucro, to predominance of Chydorus sphaericus and Bosmina c. thersites as trophic state increased. In the benthic Cladocera (Chydoridae), there was a decrease in species diversity, in the percentages of phytophilous species and in the portion of species that are typical of clear water lakes, but there was an increase both in the number of mud dwellers and species that are typical of polluted lakes. No positive correlation was found between mean depth of the lakes and the planktonic/littoral ratio.     

Microbial control of dissolved organic carbon in lakes: research for the future

The dissolved organic carbon (DOC) of lakes dominates any budget of organic carbon in these systems. Limnologists are still limited by techniques and particularly by the lack of measurements of rates of microbial transformation and use of this DOC. There are now four different approaches to the study of the microbial control of DOC in lakes. The first is through measurements of the total DOC. Recent advances in measurement with high temperature combustion will likely lead to higher and more consistent measurements in freshwaters than previously. It is possible that a biologically active fraction may be identified. The second approach is through measurements of microbial incorporation and respiration of ¹⁴C-labeled organic matter. The kinetics of this process are well known but advances in measurement of the size of the substrate pool are still being made. The third approach is to use bacterial growth in batch or continuous flow experiments in order to understand how much of the total DOC can be decomposed by microbes. The assay in this approach may be microbial growth (thymidine incorporation, biomass changes) or change in the DOC (total concentrations, specific compounds, or fractions of the DOC by molecular weight). These methods are promising but are not developed enough for routine use. For example, growth measurements in the laboratory are all subject to experimental artifacts caused by changes in the DOC and in the bacterial populations. Finally, the fourth approach is through the use of isotopes of the natural DOC. In the sea this approach has given the age of the bulk DOC (¹⁴C data). In freshwaters it has great potential for differentiating between bacterial use of terrestrial DOC vs. use of algal-derived DOC (13C data). Stable isotopes are also useful for experimentally labeling DOC produced by algae and following the use of this material by bacteria.     

Stable carbon and nitrogen isotope signatures of decomposing tropical macrophytes

The Pantanal of Mato Grosso, Brazil, is a large, seasonal wetland, which exhibits high macrophyte productivity at the beginning of the rainy season, when the floodplain becomes flooded. During inundation, from December through May, there is rapid turnover of decomposing macrophyte litter, which is subsequently colonized and consumed by various organisms. In this paper, the variation in the carbon and nitrogen isotope signatures of decomposing macrophytes and detritus was determined to provide an isotopic baseline for the elucidation of higher trophic levels. Seven abundant macrophyte species, Cyperaceae sp., Pontederia lanceolata, Cabomba furcata, Salvinia auriculata, Eichhornia crassipes, Nymphaea amazonum and Paspalum repens, were exposed in mesocosm decomposition experiments lasting 21 or 100 days. Stable isotope ratios of carbon and nitrogen and the atomic C/N ratios were determined for decomposing plant material, particulate organic matter (POM), the microbial film, and aquatic invertebrate larvae. The ¹³C values for the macrophytes did not change during decomposition. However, the variability of ¹⁵N was high (range of ± 6 ) due to microbial activity. There was no consistent difference in the isotopic signatures of macrophytes and POM. C/N ratios decreased from 17 to 50 in macrophytes, to 7 to 12 in POM. The isotopic signatures and C/N ratios of the microbial film were the same as those of POM. We concluded that heterotrophic processes did not fractionate stable carbon isotopes but caused an increase in the variability of stable nitrogen ratios and a change in the C/N ratios in our experimental system. Therefore, it was not possible to distinguish fresh and senescent material or even POM when used as a food source. The ¹³C values of the aquatic larvae were closely coupled to those of the carbon source provided.     

Carbon, nitrogen, and sulfur isotope changes and hydro-geological processes in a saline lake chain

The ionic concentrations,conductivity and pH of water in the Lake Chany complex in West Siberian Russia change from the mouth to the interior of the lake. This difference is indicative of marked evaporation of lake water from the closed water body system in the dry climate of Western Siberia. The carbon isotope composition of particulate organic matter (POM, composed mainly of phytoplankton) clearly changes, along with the pH of the water, reflecting the concentration of dissolved CO₂. Carbon and nitrogen isotope signatures of Chironomus plumosus larvae, a benthic invertebrate that may feed on bulk lake sediment, systematically increase, along with those of POM and sediment organic matter (SOM), through the lake chain. Both sulfate-sulfur and nitrogen isotope compositions of the POM and SOM increase with distance from the estuary into the Lake Chany complex. Heavier sulfur and nitrogen isotope recycling from the sediment, caused by microbial sulfate reduction and denitrification, respectively, may have led to the increased sulfate-sulfur and nitrogen isotope compositions of the POM and SOM.     

Using stable-isotope analysis of feathers to distinguish moulting and breeding origins of seabirds

To determine whether stable isotope measurements of bird feathers can be used to identify moulting (interbreeding) foraging areas of adult seabirds, we examined the stable-carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic composition of feathers of chicks and adults of black-browed albatrosses (Diomedea melanophrys) from Kerguelen Islands, southern Indian Ocean. Albatross chicks are fed primarily fish (75% by mass), the diet being dominated by various species of the family Nototheniidae and Channichthyidae which commonly occur in the shelf waters in the vicinity of the colony. δ¹³C and δ¹⁵N values in chick feathers, which are grown in summer in the breeding area, were lower than values in adult feathers, which are grown in winter (δ¹³C: –19.6‰ versus –17.6‰ and δ¹⁵N: 12.4‰ versus 15.7‰, respectively). No differences in δ¹³C and δ¹⁵N values were found in adult wing feathers moulted in 1993 and 1994 and in adult feathers formed at the beginning, middle and end of the 1994 moulting period. These data are consistent with adults moulting in the same area and feeding at the same trophic level from one year to the next and with no major changes in foraging ecology within a given moulting season; they suggest that foraging grounds were different in summer and winter and that these differed in their stable-isotope signature. Changes in both feather δ¹³C and δ¹⁵N values indicated feeding south of the Subtropical Front (STF) during chick rearing, which is in agreement with the known foraging ecology at this time and feeding north of the STF during moult. This, together with band recoveries from adult birds, indicates that black-browed albatrosses from Kerguelen Islands wintered in subtropical waters off southern Australia. The stable-isotope markers in feathers, therefore, have the potential for locating moulting areas of migratory seabird species moving between isotopically distinct regions and for investigating seabirds’ foraging ecology during the poorly known interbreeding period. Such information is needed for studies of year-round ecology of seabirds as well as for their conservation and the long-term monitoring of the pelagic environment. Received: 28 June 1999 / Accepted: 14 September 1999     

Leaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects

 Leaf carbon isotope discrimination (Δ), seasonal estimates of the leaf-to-air water vapor gradient on a molar basis (ω), and leaf nitrogen contents were examined in three riparian tree species (Populus fremontii, P. angustifolia, and Salix exigua) along elevational transects in northern and southern Utah USA (1500–2670 m and 600–1820 m elevational gradients, respectively). The ω values decreased with elevation for all species along transects. Plants growing at higher elevations exhibited lower Δ values than plants at lower elevations (P. fremontii, 22.9‰ and 19.5‰, respectively; P. angustifolia, 23.2‰ and 19.2‰, respectively; and S.␣exigua, 21.1‰ and 19.1‰, respectively). Leaf nitrogen content increased with elevation for all species, suggesting that photosynthetic capacity at a given intercellular carbon dioxide concentration was greater at higher elevations. Leaf Δ and nitrogen content values were highly correlated, implying that leaves with higher photosynthetic capacities also had lower intercellular carbon dioxide concentrations. No significant interannual differences were detected in carbon isotope discrimination. Received: 25 February 1996 / Accepted: 8 September 1996     

Variation in carbon isotope discrimination within and among Sphagnum species in a temperate wetland

Field samples of bryophytes are highly variable in carbon isotope discrimination values (Δ, a measure of ¹³CO₂ uptake relative to ¹²CO₂), but it is unknown what affects Δ under field conditions, or how variation in Δ relates to bryophyte performance. This study employed field and greenhouse common garden studies to evaluate the influence of microsite, seasonal, and genetic variation on Δ in peatmosses. Three species of Sphagnum that occupy hollow (S. recurvum), carpet (S. palustre), and hummock (S. tenerum) habitats were sampled for relative growth rates (RGR), C:N ratio, and Δ throughout a growing season. Values of Δ ranged from 19.0 to 27.1‰. This variation was unrelated to species (P=0.61). However, Δ varied seasonally (P<0.001), with lower discrimination in the spring (mean 22.5‰), followed by summer (23.8‰) and winter (24.7‰). There was also significant microsite variation (P=0.015) which disappeared when plants were grown in a common garden. In both spring and summer, microsite variation in Δ was inversely related to RGR (P<0.001), but unrelated to C:N ratios (P>0.08). These results suggest that environmental, not genetic, variation at microsites affects Δ in non-vascular plants. However, environmental control of Δ is unlike that in vascular plants where water limitation lowers chloroplastic demand and increases resistance to carbon uptake. In non-vascular plants, water limitation lowers chloroplastic demand and decreases resistance to carbon uptake. These processes have additive effects and generate high spatial and seasonal variability in Δ. Received: 29 April 1999 / Accepted: 8 November 1999     

A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake

The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,¹²C and¹³C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in ¹³C_DIC), due to preferential uptake of¹²C by phytoplankton and a change in the dominant CO₂ source from inflow andin situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in ¹³C_DIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion.Mass balance calculations of DI¹²C and DI¹³C in the epilimnion for the summer (June 23–September 25) yield a mean rate of net conversion of DIC to organic carbon (C_org) of 430 ± 150 moles d^-1 (6.5 ± 1.8 m moles m^-2 d^-1. Net CO₂ invasion from the atmosphere was 420 ± 120 moles d^-1 (6.2 ± 1.8 m moles m^-2 d^-1) with an exchange coefficient of 0.6 ± 0.3m d^-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO₂. About 50% of CO₂ invasion and conversion to C_org for the summer occurred during a two week interval in mid-summer.DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d^-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to C_org in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The ¹³C of DIC added to the deep water (-22.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO₂ (-7) was produced by a combination of POC oxidation and as a by-product of methanogenesis.     

Untangling the complex issue of dissolved organic carbon uptake: a stable isotope approach

1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole-stream addition of a ¹³C-DOC tracer coupled with laboratory measurements of bioavailability of the tracer and stream water DOC.
2. The tracer, a leachate of ¹³C-labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2-h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer.
3. From the longitudinal ¹³C uptake curve, we resolved labile and semi-labile DOC classes within the ¹³C-DOC tracer comprising 82% and 18% of the tracer respectively.
4. Plug-flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi-labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients.
5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi-labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi-labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans.     

Stable isotopes analysis to assess the trophic role of ants in a Mediterranean agroecosystem

1 Stable isotopes signatures (δ¹³C and δ¹⁵N) of the most important tree‐dwelling ants in an olive orchard were examined, together with the signatures of the most common herbivores, predators and sap‐sucking insects. The olive orchard consists of separate subunits (trees) surrounded by a matrix of grasses or bare ground, and the role of ants in such a system is not fully understood. 2 None of the selected ant species was exclusive to the olive trees because they were also observed foraging on vegetation (mainly thistle) under the tree crowns. Hence, the relative contributions of these two sources of energy (olive trees versus herbs/grasses) were assessed by comparing the δ¹³C of ants with the signatures of plants and those of other arthropods collected on the trees and on nearby thistles. Next, the trophic level occupied by the ants and their ecological role within the olive food web were determined by examining the δ¹⁵N values and their relationship with indices of ecological performance measuring the potential pressure exerted by each species on the ecosystem. 3 The analysis of ¹³C signatures revealed a different contribution of the two energy sources, olive trees versus herbs and grasses, with the former being more important for ants. The analysis of ¹⁵N signatures suggested separate roles for different ant species: some (Crematogaster scutellaris, Lasius lasioides) occupied a higher trophic level, mostly involved in predation, whereas others (Camponotus piceus, Camponotus lateralis) occupied a lower level, probably involved more in homopteran tending. A fifth species (Camponotus aethiops) was in an intermediate position. Finally, the δ¹⁵N levels of the species were significantly correlated with indices of ecological performance.     

Trophic ecology of the Lake Superior wave zone: a stable isotope approach

Stable carbon and nitrogen isotope ratio analyses were used to characterize the primary energy sources and trophic positions of 16 common Lake Superior wave zone invertebrate species. Isotope data from six tributary species that were taxonomically and ecologically matched with common wave zone species revealed broad energetic separation between these similarly structured benthic food webs. Previously published stable isotope data for Lake Superior wetland and pelagic food webs were used to assess the relative importance of inter-habitat energy flow within the Lake Superior ecosystem. The results of these comparisons indicate that the Lake Superior wave zone is energetically distinct from its tributaries, wetlands, and to a lesser extent from its vast pelagic realm. This information and approach should prove useful in future studies on the bioenergetics of inter-zonal migrants and other species that forage in multiple habitats within the lake and also in revealing energetic connections among terrestrial, riverine, littoral, and pelagic food webs in the coastal ecosystems of Lake Superior.     

Alteration of trophic interactions between periphyton and invertebrates in an acidified stream: a stable carbon isotope study

The hypothesis according to which proliferation of periphytic algae under acid conditions results from a release of grazing pressure is tested. Stable carbon isotope analysis is used to investigate the autochthonous/allochthonous balance of invertebrate feeding in streamside artificial channels that were experimentally acidified. We find that the relative contribution of autochthonous food sources (epilithon) to total invertebrate biomass was slightly lower (after 1 mo of acidification) or not altered (after 2 mo) under acidified conditions when compared with a control. Feeding shifts were exhibited by some invertebrate taxa and provided evidence that acidification modifies trophic interactions between attached algae and primary consumers. Cross-treatment calculations showed that reduction of grazing pressure after the first month of acidification was an effect rather than the cause of periphyton proliferation. Our approach using stable carbon isotope analysis and biomass measurements of macroinvertebrates allows the quantification of the trophic base of lotic secondary producer communities under both experimental and natural conditions.