Organic Matter Processing and Detrital Food Webs II
Presiding: T Crowl, Ecology Center, Utah State University; A Rosemond, University of Georgia
NB14A-01 15:30h
The Role of Road Corridors on Riparian Vegetation and Stream Ecosystem Dynamics.
Stream ecosystems are dependent on organic material from the riparian zone as a major energy source for the food web. Leaf litter (organic matter) entering streams is processed by a combination of physical and biological mechanisms. In temperate streams, microbial conditioning is important for detrital processing. Much less is known in tropical systems, especially those dominated by large macro-consumers such as decapods. There is also variation among species in terms of processing rates that are explained by nutritional value, chemical defenses and palatability. These traits are a function of plant life history. If riparian species are being significantly altered through invasions by exotic species along road corridors, then we can expect changes in detrital processing rates and ultimately, ecosystem function. As part of a biocomplexity project in Puerto Rico, we are quantifying the changes to species composition and trait-mediated decomposition and foodweb dynamics. Where roads are constructed, exotic invasives include Spathodea, Bambusa, Syzigium and a various grasses. Because of the chemical defenses and their high nutritional value, decomposition rates on these species is much higher than for native riparian species. The increased breakdown rates may `accelerate' ecosystem processes and either enhance or destabilize existing food web linkages.
http://biocomplexity.cnr.colostate.edu
NB14A-02 15:45h
Long-Term Studies of the Effects of Salmon Spawners on Stream Ecosystems
To determine the ecological effects of salmon-derived nutrients (SDN) transported into fresh waters by spawning adult Pacific salmon (Oncorhynchus spp.), we monitored several ecological parameters in Fish Creek, Southeast Alaska from 2000 to 2004. Pink and chum salmon spawn in the lower reach of Fish Creek, but cannot move further upstream because of a waterfall, 4 km from saltwater. We estimated spawner densities and measured dissolved nutrient concentrations and epilithon abundance before, during, and after the salmon run, in reaches above and below the waterfall barrier. Salmon spawners increased streamwater concentrations of ammonium (2.3 - 148x) and soluble reactive phosphorus (0.4 - 17x), and epilithon chlorophyll a (14 - 29x) and ash-free dry mass (1.4 - 4x) in lower reaches. However, the duration and magnitude of these effects varied widely among years, and did not appear to vary solely with spawner densities. Our results suggest that although SDN can stimulate primary production through increased nutrient concentrations, other environmental factors, such as temperature, irradiance, and discharge, can modulate the influence of salmon spawners on stream ecosystems. To better assess the ecological influence of SDN, future studies should consider the influence of key environmental factors and their temporal and spatial dynamics.
NB14A-03 16:00h
Effect of 15 N Labeled Riparian Fertilization And Salmon Carcass Analog Addition On Food Web Dynamics And Productivity In Four Idaho Streams
Nutrient budgets of stream/riparian ecosystems in the Intermountain West have been depleted through declining salmon populations and certain anthropogenic disturbances (e.g. forestry practices). We measured stream food web responses to a riparian fertilization and an in-stream carcass analog addition in 4 Idaho streams with a 15 N tracer. Aerial application of fertilizer pellets to light (224 kg/ha) and heavy (448 kg/ha) treatment sections of 2 streams and carcass analog additions to 2 others were completed in autumn. Periphyton response was measured through chlorophyll a, nutrient diffusing substrata, and stable isotope analyses. Macroinvertebrates were analyzed for abundance, biomass, community structure, and stable isotope composition. Also, willow (Salix) breakdown rates were determined. Pre-treatment chlorophyll a values showed no significant difference between treatment and reference reaches. Post-treatment results showed significantly higher chlorophyll a and Δ 15 N values in treatment reaches compared to reference reaches. Macroinvertebrate abundance, richness, biomass, and Δ 15 N values also increased in treated reaches. No significant differences were detected in leaf breakdown rates between reaches. Riparian fertilization effects were longer lasting than the in-stream treatment. These results suggest that nutrient addition to streams and riparian areas can be used as a management tool to increase stream productivity where nutrients are limiting.
NB14A-04 16:15h
Top-down Feedback Exacerbates Effects of Nutrient Enrichment in Detritus-based Ecosystems via Increased Loss of Basal Carbon
In primary producer-based food webs faced with chronic nutrient enrichment, increased plant biomass may ultimately be suppressed by consumers. Effects of chronic enrichment of detritus-based ecosystems may be fundamentally different, with consumers exacerbating, rather than suppressing, the effects of enrichment on basal resources. We have continuously enriched a headwater stream at the Coweeta LongTerm Ecological Research site (North Carolina, USA) with N & P (to ca. 400 ug/L dissolved inorganic nitrogen and 50 ug/L soluble reactive phosphorus) for 4.5 yrs and quantified leaf breakdown rates, microbial production, invertebrate production, and leaf litter standing crop in the enriched stream and in an adjacent reference stream. Whereas nutrient enrichment largely increases basal carbon in primary producer-based systems, detrital carbon in the treatment stream was drastically diminished relative to the reference stream. During each successive year of enrichment, rates of leaf breakdown were progressively faster and standing crops of detrital carbon were progressively lower. This `ramping up' of loss rates of carbon appear to be driven by increased consumption by invertebrates, rather than increased microbial activity. Our results illustrate that fundamentally different trophic feedback effects can potentially occur in detrital vs primary producer components of food webs in response to increased nutrient concentrations.
NB14A-05 16:30h
Tadpoles of Early Breeding Amphibians are Negatively Affected by Leaf Litter From Invasive Chinese Tallow Trees
As wetlands are invaded by Chinese tallow trees (Triadica sebifera), native trees are displaced and detrital inputs to amphibian breeding ponds are altered. I used a mesocosm experiment to examine the effect of Chinese tallow leaf litter on the survival to, size at, and time to metamorphosis of amphibian larvae. Fifty 1000-L cattle watering tanks were treated with 1500 g dry weight of one of five leaf litter treatments: Chinese tallow, laurel oak (Quercus laurifolia), water tupelo (Nyssa aquatica), slash pine (Pinus elliottii), or a 3:1:1:1 mixture. Each tank received 45 tadpoles of Pseudacris feriarum, Bufo terrestris, and Hyla cinerea in sequence according to their natural breeding phonologies. Every Pseudacris feriarum and Bufo terrestris tadpole exposed to Chinese tallow died prior to metamorphosis. Hyla cinerea survival in tanks with tallow-only was significantly lower than that observed for all other leaf treatments. Hyla cinerea tadpoles from tallow-only and mixed-leaf treatments were larger at metamorphosis and transformed faster than those in tanks with native leaves only. These results suggest that Chinese tallow leaf litter may negatively affect tadpoles of early breeding frogs and that Chinese tallow invasion may change the structure of amphibian communities in temporary ponds.
NB14A-06 16:45h
Effects of Clearcut Logging on Export of Invertebrates, Fine Particulate Organic Matter and Dissolved Organic Carbon From Headwater Streams.
We compared the export of aquatic invertebrates (drift) and fine particulate organic matter (FPOM) from headwater streams directly affected by logging to that of control streams. Thirty subalpine streams were sampled in the southern interior of BC within two watersheds, Bone and Damfino, in 2003 and 2004. Streams were categorized into three treatment types: new clearcuts logged 1-3 years previous, old clearcuts logged 5-7 years previous, and controls. Discharge rates ranged from 0.0142 to 11.329 L sec-1 and drift densities ranged from 0.0132 to 30.139 individuals L-1. Paired sites were positioned above and below disturbance in clearcuts, and equivalent elevation gradients in control streams. At Damfino, FPOM and drift densities of both clearcut treatments increased in downstream pairs, opposite to differences between control pairs (p=0.004, 0.003). Old clearcuts showed increases of Ephemeroptera (p=0.029), and both clearcut treatments displayed increases of Diptera (p=0.027) compared with controls.