Food Webs II
Presiding: R P Creed, Department of Biology, Appalachian State University; B Koch, University of Wyoming
NB24C-01 15:30h
Function and Food Webs of Springs Near Treeline in the Swiss National Park
We investigated ecosystem function and food web structure of high elevation springs in the Swiss National Park. Functional characteristics were derived from measures of ecosystem metabolism, sediment respiration, and nutrient uptake experiments in 4 springs. Food webs were assessed using stable isotope analysis (C, N) of different ecosystem compartments from 20 springs encompassing 6 different spring types. Gross primary production (GPP) ranged from 2.4 to 4.3 g O2 m-2 d-1 and ecosystem respiration (ER) from 3.6 to 6.7 g O2 m-2 d-1, suggesting springs were net-heterotrophic (P/R<0.75). One iron-oxide rich spring had GPP = 65 and ER = 96 g O2 m-2 d-1. Bacterial abundances (DAPI) ranged from 2-3×108 cells/ml, and benthic sediments were mostly anaerobic. Uptake lengths of both N and P were <50 m, with uptake rates of P at 0.3-3.1 mg m-2 h-1 and N at 57-178 mg m-2 h-1, suggesting springs were important nutrient sinks. Food webs were simple (<8 taxa) and primarily detritus based. The dominate stonefly predator relied on instream production of invertebrates. Terrestrial predators (lycosid spiders) near springs fed on a terrestrial diet. These data suggest these springs derive most of their energy from allochthonous sources and are net-heterotrophic ecosystems.
NB24C-02 15:45h
Seasonal differences in riparian consumer diet and insect communities in an Oregon Coast Range watershed food web.
In riparian areas, terrestrial and aquatic habitats overlap creating zones where they interact as an aquatic-terrestrial interface. This coupling allows energy to move between systems and generates intertwining food webs. Thus, vertebrate riparian consumers, such as fish or birds, potentially have alternative prey from sources external to their habitats. The purpose of our study was to explore this reciprocal exchange in an alder dominated riparian forest of the Oregon Coast Range. Diet samples were collected from birds and fish in summer and fall 2003 with a suite of insect samples at Honeygrove Creek and two of its small tributaries. In a comparison of emerging aquatic insects and flying terrestrial insects during June and again in September, we detected seasonal differences in terrestrial and aquatic insects available to riparian consumers. Despite this availability of externally derived prey, fish depended more on resources derived from within their respective habitats during summer. Cutthroat trout (Oncorhynchus clarkii) and Coho salmon (Oncorhynchus kitsutch) ate more juvenile aquatic than adult aquatic or terrestrial insects. During fall, the same pattern was exhibited by Coho but Cutthroat trout appeared to consume a slightly greater number of terrestrial insects. The preliminary analysis of bird diet samples from commonly encountered species such as, Swainson's thrush, Song Sparrow, and Pacific-slope Flycatcher, showed more terrestrial derived prey in their diets than aquatic during the summer sampling season.
NB24C-03 16:00h
Importance of Resource Subsidies to Animals in Freshwater Ecosystems
Many streams receive large resource subsidies from adjacent ecosystems. These subsidies take a variety of forms including terrestrial organic matter, dissolved nutrients, or animals. Some subsidies can have important consequences for in-stream consumers in the recipient system, while in other systems they are unimportant. We present a framework for establishing the conditions under which food subsidies to streams are likely to be important to animal consumers. Importance of cross-system subsidies will vary as a function of the spatial/temporal scales of interest. Attributes of the subsidy (e.g., stoichiometric quality) or of the donor/recipient systems (e.g., productivity gradients) can be used to predict conditions under which subsidies are important to in-stream consumers. We propose a hypothesis that classifies the importance of subsidies to streams as high, intermediate, or low based on the subsidy's nutritive quality and its flux relative to autochthonous primary production. Thus, importance will be high when the flux of a highly nutritious subsidy is a large proportion of in-stream productivity. Importance will be low in the opposite case. We test this hypothesis using data from the literature. This framework is intended to facilitate cross-system comparisons that test the ecological conditions governing the importance of resource subsidies to freshwater ecosystems.
NB24C-04 16:15h
The Importance of Insects in Energy Transfers Across Riparian Ecotones Along Hong Kong Streams
Energy and materials in the form of insects transfer reciprocally between land and water through stream riparian ecotones, and may provide important energy subsidies to aquatic and terrestrial consumers. Variation in the magnitude and extent of this transfer was investigated in 2004-05 in six Hong Kong streams: four shaded and two unshaded. A combination of pan traps and light traps were used to investigate seasonal activity of aquatic and terrestrial insects. Both were more abundant during the wet season (April to September). Over 80% of emerging aquatic insects stayed within 20 m of the stream bank at all sites, suggesting that the water to land subsidy was spatially restricted. Inputs of terrestrial insects into shaded streams were 30% greater than at open sites, and drift-feeding Parazacco spilurus (Cyprinidae) ate more terrestrial insects in shaded (>40% of prey) than unshaded streams (25% of prey). Stable isotope analysis (SIA; C & N) of potential prey and fish tissues confirmed the dietary importance of terrestrial insects. The spider Leucauge celebesiana (Tetragnathidae) builds orb web parallel to the water surface during the main period of aquatic insect emergence, and SIA indicated that aquatic insects were the primary prey of this terrestrial consumer.
NB24C-05 16:30h
Ecological Role of the New Zealand Mudsnail in the Provo River, Utah
The invasive New Zealand mudsnail Potamopyrgus antipodarum has been found in the Provo River in central Utah. It does not yet dominate the benthos, but the ecological implications of its invasion are being monitored. Seven of the ecosystem's major benthic taxa, including the New Zealand mudsnail, were collected and analyzed for 15N and 13C levels. Measured nitrogen levels indicate similar trophic status among all taxa under study. The 13C levels, however, reflect differences in carbon sources among taxa. Samples from two taxa, the isopod Asellus and the gastropod Physella, suggest that some individuals are obtaining terrestrially derived carbon. This could be associated with individuals of the same taxon occupying different stream microhabitats. All P. antipodarum samples, however, indicate an aquatic carbon source.
NB24C-06 16:45h
Effect of Spawning Salmon on Seasonal Changes in Structure and Function of the Macroinvertebrate Community of Kennedy Creek
This study investigated the hypothesis that spawning salmon affect macroinvertebrates negatively and positively-the former due to the disturbance of redd excavation and the latter due to nutrients released during spawning and salmon carcass decomposition. To test this hypothesis, I monitored changes for five seasons in density, biomass, and salmon-derived carbon (C) and nitrogen (N) in the benthic macroinvertebrates of a small stream with a large run of chum salmon (Oncorhyncus keta). Stable isotope analysis showed that the macroinvertebrates contained salmon-derived C and N year around, as indicated by the results of pre-spawning samples: 20-41% salmon-derived C and 25-50% salmon-derived N, representing 22% of total macroinvertebrate biomass. Near the end of the spawning run, all macroinvertebrates sampled showed increases in salmon-derived C (41-68%) and N (51-87%) incorporated into their tissues; however, the total macroinvertebrate biomass decreased due to redd excavation. The percentage of salmon-derived C and N in macroinvertebrates remained high 3 months after spawning (49-88% and 60-97%, respectively). Because total macroinvertebrate biomass also increased, this period had the highest salmon-derived macroinvertebrate biomass (2.71 g m-2). Six months after spawning, there were no detectable differences in total macroinvertebrate biomass between reaches with and without salmon.