Sulphur stable isotopes can distinguish trophic dependence on sediments and plankton in boreal lakes

1. Stable isotopes of carbon are useful for differentiating between freshwater food chains based on planktonic algae or benthic algae, but are reported to be of limited use for identifying food chains based on sedimentary detritus. Because data from marine systems suggest that stable isotopes of sulphur (δ³⁴S values) have potential in this regard, we tested their utility in freshwater lakes.
2. We found that sulphate in the water column of four boreal lakes was enriched in ³⁴S compared to the sulphur in bulk sediments from these lakes. Furthermore, within a given lake, insects known to feed on sediment (directly or via predation) had δ³⁴S values similar to those of sediment, whereas planktonic and benthic invertebrates known to feed on suspended particles had δ³⁴S values similar to those of sulphate in the water column.
3. Using the stable S isotope values of invertebrates that obtain their S from either the sediment or the water column as end members in a two-source mixing model, we show that two fish species obtain their food from both planktonic and sedimentary sources. Furthermore, model results suggest that, as expected, the more benthic-feeding fish species obtains more of its S from the sediment compartment than does the species that feeds in the water-column.
4. Our results suggest that measurements of stable sulphur isotopes provide a means of distinguishing between members of food chains that are based in the water column from those based on sedimentary detritus. As such, they would be a useful complement to stable C isotopes that are used to distinguish between food chains based on planktonic or benthic algae.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

1. The cascading effect of predators on the functioning of adjacent ecosystems can occur when the life cycles of their prey include two ecosystems. However, there has been little consideration of which habitat attributes can modify the strength of these cross‐ecosystem trophic cascades. Habitat size can mediate the strength of predator–prey interactions, and thus affect within‐ecosystem trophic cascades. We hypothesise that similar effects of habitat size might affect cross‐ecosystem trophic cascades.
2. It has been shown that terrestrial predators (e.g. spiders) can capture terrestrial adult insects as they attempt to oviposit in the waters of bromeliads. Such terrestrial predators could therefore alter the trophic structure and functioning of the aquatic food web. If spiders affect an aquatic trophic level that is influenced by bromeliad size, then the strength of the cross‐ecosystem trophic cascade will also depend on bromeliad size.
3. To test this general hypothesis, we manipulated the presence of a funnel‐web spider (Aglaoctenus castaneus, Lycosidae), which builds a single web over water‐filled bromeliads, in bromeliads differing in size and examined effects on the aquatic invertebrate community and on ecosystem functions (decomposition, detrital nitrogen flux).
4. The effects of spiders were largely independent of bromeliad size. Spiders did initiate changes in the trophic structure of aquatic food webs, reducing the biomass of predators, especially damselflies and dytiscid beetles. Spiders also increased decomposition despite having no effect on detritivore biomass or composition.
5. These results are most parsimoniously explained by (i) a behaviourally mediated trophic cascade, whereby damselfly adults avoid bromeliads with spiders, and aquatic detritivores increase rates of detrital processing in the absence of damselfly larvae, and (ii) stimulation of decomposition through nutrients added from spider faeces and prey carcasses. We believe that this is the first study to show that terrestrial predators can affect decomposition by reducing the flux of keystone aquatic predators with complex life cycles.

     

Transformation of the offshore benthic community in Lake Michigan: recent shift from the native amphipod Diporeia spp. to the invasive mussel Dreissena rostriformis bugensis

1. The native amphipod Diporeia spp. was once the dominant benthic organism in Lake Michigan and served as an important pathway of energy flow from lower to upper trophic levels. Lake‐wide surveys were conducted in 1994/1995, 2000 and 2005, and abundances of Diporeia and the invasive bivalves Dreissena polymorpha (zebra mussel) and Dreissena rostriformis bugensis (quagga mussel) were assessed. In addition, more frequent surveys were conducted in the southern region of the lake between 1980 and 2007 to augment trend interpretation. 2. Between 1994/1995 and 2005, lake‐wide density of Diporeia declined from 5365 to 329 m⁻², and biomass (dry weight, DW) declined from 3.9 to 0.4 g DW m⁻². The percentage of all sites with no Diporeia increased over time: 1.1% in 1994/1995, 21.7% in 2000 and 66.9% in 2005. On the other hand, total dreissenid density increased from 173 to 8816 m⁻², and total biomass increased from 0.4 to 28.6 g DW m⁻². Over this 10‐year time period, D. r. bugensis displaced D. polymorpha as the dominant dreissenid, comprising 97.7% of the total population in 2005. In 2007, Diporeia was rarely found at depths shallower than 90 m and continued to decline at greater depths, whereas densities of D. r. bugensis continued to increase at depths greater than 50 m. 3. The decline in Diporeia occurred progressively from shallow to deep regions, and was temporally coincident with the expansion of D. polymorpha in nearshore waters followed by the expansion of D. r. bugensis in offshore waters. In addition, Diporeia density was negatively related to dreissenid density within and across depth intervals; the latter result indicated that dreissenids in shallow waters remotely influenced Diporeia in deep waters. 4. With the loss of Diporeia and increase in D. r. bugensis, the benthic community has become a major energy sink rather that a pathway to upper trophic levels. With this replacement of dominant taxa, we estimate that the relative benthic energy pool increased from 17 to 109 kcal m⁻² between 1994/1995 and 2005, and to 342 kcal m⁻² by 2007. We project that previously observed impacts on fish populations will continue and become more pronounced as the D. r. bugensis population continues to expand in deeper waters.     

Understorey environments influence functional diversity in tank‐bromeliad ecosystems

1. A substantial fraction of the freshwater available in neotropical forests is impounded within the rosettes of bromeliads that form aquatic islands in a terrestrial matrix. The ecosystem functioning of bromeliads is known to be influenced by the composition of the contained community but it is not clear whether bromeliad food webs remain functionally similar against a background of variation in the understorey environment. 2. We considered a broad range of environmental conditions, including incident light and incoming litter, and quantified the distribution of a very wide range of freshwater organisms (from viruses to macroinvertebrates) to determine the factors that influence the functional structure of bromeliad food webs in samples taken from 171 tank‐bromeliads. 3. We observed a gradient of detritus‐based to algal‐based food webs from the understorey to the overstorey. Algae, rotifers and collector and predatory invertebrates dominated bromeliad food webs in exposed areas, whereas filter‐feeding insects had their highest densities in shaded forest areas. Viruses, bacteria and fungi showed no clear density patterns. Detritus decomposition is mainly due to microbial activity in understorey bromeliads where filter feeders are the main consumers of microbial and particulate organic matter (POM). Algal biomass may exceed bacterial biomass in sun‐exposed bromeliads where amounts of detritus were lower but functional diversity was highest. 4. Our results provide evidence that tank‐bromeliads, which grow in a broad range of ecological conditions, promote aquatic food web diversity in neotropical forests. Moreover, although bromeliad ecosystems have been categorised as detritus‐based systems in the literature, we show that algal production can support a non‐detrital food web in these systems.     

Effects of fathead minnows and restoration on prairie wetland ecosystems

SUMMARY 1. Research has shown that fish influence the structure and processes of aquatic ecosystems, but replicated studies at the ecosystem level are rare as are those involving wetlands. Some wetlands of the Prairie Pothole Region (PPR) of North America support fish communities dominated by fathead minnows ( Pimephales promelas ) while others are fishless, providing an opportunity to assess the influence of these fish on wetland ecosystems. Additionally, many wetlands have previously been drained and subsequently restored, but the success of these efforts is poorly known and restoration may be impeded by the presence of fish.
2. We assessed the effects of fathead minnows and drainage by studying 20 semipermanent, prairie wetlands in Minnesota from 1996 to 1999. We used a 2 × 2 factorial design to examine the effects of presence and absence of minnows and drainage history (restored/never drained) on the abundance of aquatic invertebrates and amphibians, as well as on the concentrations of chlorophyll a , total phosphorus, total nitrogen and turbidity in the water column.
3. Results showed that fathead minnows are an important determinant of many biotic and abiotic characteristics of wetlands in the eastern PPR. Wetlands with fathead minnows had fewer aquatic insects, large- and small-bodied cladocerans, calanoid copepods, ostracods and larval tiger salamanders, as well as a higher abundance of corixids and greater turbidity and chlorophyll a . A higher concentration of phosphorus in restored basins was the only consistent effect of past management.
4. Fathead minnows usually dominate fish communities in eastern PPR wetlands where fish are present, and can have several strong ecosystem effects. While abiotic variables are important determinants of ecosystem structure in prairie wetlands, they can be strongly influenced by biotic factors.     

Trophic relationships: integrating meiofauna into a realistic benthic food web

• 1 This paper summarises the most important contributions on trophic relationships of lotic meiofauna. In contrast to marine research, the few quantitative studies of the freshwater meiobenthos have shown that these invertebrates not only take up particulate/fine organic matter, but also dissolved organic substances attached to organic particles. In lotic ecosystems, further estimates of grazing rate and bacterial/algal ingestion rate are needed, particularly in situ measurements.
• 2 The effects of macroinvertebrate predators upon meiofauna are still under debate. Depending on the type of experiments (laboratory vs. field) it seems that macrofauna may or may not affect meiofauna. Field samples and analyses of gut contents of larval tanypod chironomids have shown that the impact upon meiofauna was low and larvae were nonselective predators. Predation amounted to 2.2% of the combined prey density and prey consumption averaged 1.3 individuals per predator individual per year.
• 3 Adding taxonomic resolution by including the meiofaunal component within lotic food webs distinctly increases the number of total species and, as a consequence, changes food web statistics. Webs that included meiofauna revealed that these metazoans contributed substantially to the percentage of intermediate species (species with predators and prey). The resolution of dietary analyses of major consumers of macro‐ and meiobenthos showed that many stream invertebrates feed on meiofauna.

     

A stable carbon and nitrogen isotope study of lake food webs in Canada's Boreal Plain

1.  Stable carbon and nitrogen isotope and fish stomach content analyses were used to investigate food webs in five relatively undisturbed lakes on the Boreal Plain of Canada. Stable isotope analysis was also used to determine the importance of external and internal carbon sources.
2.  Overlap in the carbon and nitrogen signatures of primary producers made it difficult to determine unambiguously the feeding habits of many invertebrates. However, isotope analysis suggested that external carbon inputs were detectable in the aquatic food chains of the one lake with a short water residence time («1 year). In the other four lakes, with water residence times ≥1 year, autochthonous carbon was the only detectable carbon source in the food webs.
3.  Food webs in these lakes spanned a range of four to five trophic levels. Both invertebrates and fish appeared to eat a variety of food, often feeding at more than one trophic level.
4.  With the exception of one lake (SPH20), top predators in these lakes, northern pike ( Esox lucius ) and fathead minnows ( Pimephales promelas ), occupied similar trophic positions despite large differences in body size and trophic morphology. In SPH20, where there were two additional fish species, pike occupied a higher trophic position. However, all the top predators in each lake appeared to be omnivores and generalists.
5.  The prevalence of omnivory and the apparent generalist feeding habits of fish in these lakes suggest that organisms are flexible in their feeding habits and that these food webs will be resilient to disturbance.     

Serological tracers of meiofaunal food webs

Serological methods utilizing taxon-specific antibodies were used to identify trophic connections in a salt marsh of South Carolina (U. S. A.). The incorporation of meiofauna within the benthic invertebrate food web was detected with these methods when microscopial examinations of predator stomachs revealed nothing but amorphous material and detritus. Measurements of soluble prey proteins in both predator guts and surficial sediments provided data to quantify the trophic connections. Difficulties with data interpretation limit the utility of serological methods for quantifying predation in the field.