Food Webs I
Presiding: R P Creed, Department of Biology, Appalachian State University; J Benstead, The Ecosystems Center, Marine Biological Laboratory, Woods Hole
NB23D-01 13:30h
Using stable isotopes to track biomagnification of polychlorinated biphenyls (PCBs) through stream food webs
Biomagnification studies of PCBs in streams are rare, even though PCBs are known to biomagnify and persist in aquatic ecosystems. We investigated PCB contamination in Twelvemile Creek (Clemson, SC, USA), a stream that received >400,000 lbs of PCBs from 1955-1978. Our goals were to determine if PCBs biomagnify in streams and to measure the relative importance of heterotrophic and autotrophic pathways for biomagnification. Sites were sampled during spring (n=6) and fall (n=4) 2003-04. Major components of the foodweb were analyzed for total PCBs, Δ13C and Δ15N. Biomagnification was apparent as mean Δ15N strongly predicted PCBs (r2 = 0.58), and PCBs consistently increased with trophic level. We found no consistent patterns related to carbon pathways. Δ13C and PCBs were uncorrelated, suggesting that carbon source was unrelated to biomagnification. However, mean PCB tissue concentration in grazers were double those in shredders (Stenonema modestum, 970 ppb versus Tipula and Pteronarcys spp, 370 ppb) even though concentrations were similar for periphyton and conditioned leaves. Our results show that PCBs biomagnify and persist in streams but the importance of autotrophic and heterotrophic pathways are unclear. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.
NB23D-02 13:45h
Δ15N-Size Relationships in River Invertebrate Communities: An Integrated Measure of Food web Structure?
Anthropogenic perturbations can alter the composition and structure of benthic communities in rivers. Here, we examine the use of estimates of fish trophic position measured with stable nitrogen isotope ratios (Δ15N), and Δ15N-size relationships for the invertebrate community, as indicators of food web structure in rivers affected by various anthropogenic disturbances. Results revealed that Δ15N-size relationships of invertebrate communities showed a variety of responses, some sites (50%) showing significant increases in Δ15N with invertebrate size, whereas others did not. Similarly, the number of trophic levels between invertebrate-eating fish and primary consumers varied greatly among study sites, ranging from 0.6 to 2.2 levels. To investigate whether these estimates of fish trophic position integrated changes in invertebrate community structure, we compared slopes of invertebrate Δ15N versus size with slopes of baseline-corrected fish Δ15N versus size. Both slopes tended to be correlated, although the relationship was not significant (r2 = 0.26, p = 0.07). This trend suggests, however, that invertebrate-feeding fish trophic position measured with Δ15N might reflect overall community structure, and could possibly be used as an easily measured indicator of food chain collapse following anthropogenic perturbations.
NB23D-03 14:00h
Tracing Fluxes Of Aquatic Production And Contaminants Into Terrestrial Food Webs With Nitrogen Stable Isotopes
Biomagnifying contaminants such as mercury can be transported and redistributed across the watershed by streams and rivers. Their fate and effects on consumers depend on food web transfer both within and between aquatic and terrestrial ecosystems. The Truckee River (Ca/Ne) is heavily contaminated by Hg originating from century-old upstream mining operations. We used nitrogen stable isotope analysis to trace the incorporation of Hg transported by the Truckee and transferred by emerging aquatic insects into the riparian food web. N-isotope ratios and Hg of aquatic primary consumers were significantly elevated compared to that of terrestrial arthropods (13.3 vs 5.6 % and 110 vs 17 ngg-1). Estimates of dependence on aquatics in 16 riparian passerine bird species based on blood delta 15N ranged between 0.0 and 0.95 and were significantly related to Hg in blood. Similar correlations between Hg and delta 15N measured in tail tips of western fence lizard (Sceloporus occidentalis) collected at increasing distances from the river were observed. High inter-individual variation in bird Hg was highly correlated with delta 15N. These results show how stable isotopes and contaminant fluxes can reveal important food web linkages across aquatic/terrestrial ecotones.
NB23D-04 14:15h
Stoichiometric Effects of Phosphorus Enrichment in the Kuparuk River Food web
A 3-km reach of the oligotrophic Kuparuk River (Alaska) has been fertilized with phosphorus (10 Μg/L) for 22 consecutive Arctic summers. In 2004 we added a high-P (30 Μg/L) reach to the experiment. As in previous years, phosphorus addition significantly increased chlorophyll a and decreased epilithon C:P ratio relative to the reference reach. Higher algal biomass or lower epilithon C:P in fertilized reaches caused faster growth of some primary consumers (e.g., Orthocladius rivulorum) but not others (e.g., Acentrella sp.), presumably because threshold C:P ratios varied among taxa. Young-of-year Arctic grayling (Thymallus arcticus) showed a significant step response to phosphorus addition. Some insect primary consumers are known to contain more phosphorus in fertilized reaches, because of storage or higher RNA content. Higher phosphorus content of insect prey may explain higher grayling growth rates in fertilized reaches, because production budgets suggest that carbon limitation of grayling growth is unlikely under natural prey densities, at which models of mass balance constraints indicate phosphorus limitation. Such an interaction would be evidence for increased growth of a nutrient-limited predator in response to modified carbon:nutrient ratios at two lower trophic levels - a fundamentally different effect of fertilization from that classically mediated by increased primary production.
NB23D-05 14:30h
Standing Out From the Crowd: New Fish Recruits Have Unique Δ15N Signatures in a Heavily Polluted Intermittently Open Estuary.
Intermittently open estuaries (IOEs) are the dominant coastal ecosystems of the Australian coastline. Conditions in IOEs range from freshwater when closed, to marine when open to the ocean, during which time marine fish often migrate into these systems. Many IOEs are facing increasing pressure from coastal development and ongoing nutrient inputs from industrial and urban sources have been shown to increase the Δ15N isotope signatures of biota in receiving waters. In this study, we examined the food web structure of Tallows Creek, a relatively small IOE in northern NSW, Australia, which receives ongoing sewage treatment plant inputs. We first sampled Tallows Creek when it was closed to the ocean and found that all components of the food web had enriched Δ15N signatures relative to biota sampled from a relatively undisturbed neighbouring system. However, samples collected immediately following an entrance opening event revealed some individuals of highly mobile taxa with Δ15N signatures more typical of unpolluted estuaries. We suggest that these isotopically light individuals were recent migrants into Tallows Creek and that Δ15N signatures can act as a guide to the importance of heavily polluted systems as nursery grounds for mobile taxa, owing to the distinction between resident and migrant signatures.
NB23D-06 14:45h
a Comparative Study of Techniques to Elucidate the Relative Importance of Autochthonous and Allochthonous Resources in Tropical Streams
To determine whether allochthonous or autochthonous energy resources are more important to aquatic consumers in tropical streams with different degree of riparian shading, stoichiometric analysis (STA of C and N), fatty acid profiling (FAP) and bulk tissue C and N stable isotopic analysis (SIA) were undertaken. Basal food sources (leaf litter, periphyton, cyanobacteria) and common consumer species (snails, shrimps, fishes) were collected in three Hong Kong (China) streams (2 shaded and 1 unshaded) in the dry season of 2004. The results showed that the primary consumers relied primarily on the autochthonous sources, even in the shaded streams. Further investigation focused on laboratory populations of the omnivorous snail Brotia hainanensis (Pachychilidae) that were fed for six months with periphyton or Liquidambar formosana (Hamamelidae) leaf litter. The results of STA, FAP and SIA of these animals with known feeding history were compared with field-collected individuals. SIA and FAP were generally more sensitive than STA in elucidating basal food sources of the consumers in all cases, and the laboratory feeding trials confirmed the autochthonous 'signal' in diets of field-collected animals. SIA and FAP yielded complementary but non-identical results, and their combined use in studies of food webs is recommended.