Susan L. Eggert

EFFECTS OF RESOURCE LIMITATION ON A DETRITAL-BASED ECOSYSTEM ' ~:'

We examined the importance of terrestrial detrital inputs to secondary productivity of a headwater stream. Following a year of pretreatment studies on two headwater streams, we excluded terrestrial litter inputs (=treatment) to one stream while using the other as a reference. We excluded litter for 3 yr followed by 1 yr of small woody debris (≤10 cm diameter) removal and litter exclusion. Monthly benthic samples were collected from dominant mixed substrate (cobble, pebble, and sand-silt) as well as from moss-covered bedrock outcrop substrates. We used randomized intervention analysis (RIA) to test the null hypotheses that no change in abundance or biomass of functional feeding groups or specific taxa occurred in the treatment stream relative to the reference stream. Benthic organic matter was significantly lower in mixed substrate habitats of the treatment stream; however, small woody debris did not show a significant reduction prior to manual removal during year 4. At the end of the treatment period, tot...

Organic matter flow in stream food Webs with reduced detrital resource base

Food webs based on flows of organic matter were developed for two small streams to examine food web response to a large reduction in detrital inputs. At the study site, Coweeta Hydrologic Laboratory in the southern Appalachians, leaf litter inputs and associated microbial assemblages are the main energy source for food webs in headwater streams. We eliminated leaf litter inputs to one stream using a net placed over the first 180 m of stream from its origin. Food webs based on flow of organic matter were developed for a reference stream and the litter-excluded stream for two months, July and December of year 1 of the litter exclusion, to examine effects of leaf litter exclusion on the trophic base of the food web, size distribution of flows, predator-prey interactions, and trophic structure. Flows (mg AFDM·m 22 ·d 21 ; AFDM 5 ash-free dry mass) were estimated using gut content analyses for detritus and prey items, coupled with secondary production esti- mates. We used a whole-stream 13 C tracer method to estimate assimilation of bacteria by invertebrates. The food webs encompassed most (84-91%) of invertebrate secondary pro- duction, but ,30% of the estimated total links. The primary sources of organic matter for the food web in the reference stream were leaf tissue, bacterial carbon, and animal prey, with ;25-30% of total secondary production derived from each. In-stream primary pro- duction led to ,1% of invertebrate secondary production. A higher fraction of food web production in the litter-excluded stream was derived from wood. Magnitudes of detrital flows were lower in the litter-excluded stream, and some taxa were missing compared to the reference stream. The fraction of predator ingestion approached 100% of total secondary production for both streams, but this predation was distributed diffusely among several taxa. Flows to predators were fewer and smaller in the litter-excluded stream, yet these flows had higher per-biomass consumption coefficients, suggesting stronger interactions among the remaining common taxa. These food webs enabled us to examine interactions among taxa in the streams; hence, we found responses of the stream ecosystem to litter- exclusion that we would not have considered had we only measured changes in invertebrate population sizes or system-level changes in organic matter flow.

WHOLE-SYSTEM NUTRIENT ENRICHMENT INCREASES SECONDARY PRODUCTION IN A DETRITUS-BASED ECOSYSTEM

Although the effects of nutrient enrichment on consumer-resource dynamics are relatively well studied in ecosystems based on living plants, little is known about the manner in which enrichment influences the dynamics and productivity of consumers and resources in detritus-based ecosystems. Because nutrients can stimulate loss of carbon at the base of detrital food webs, effects on higher consumers may be fundamentally different than what is expected for living-plant-based food webs in which nutrients typically increase basal carbon. We experimentally enriched a detritus-based headwater stream for two years to examine the effects of nutrient-induced changes at the base of the food web on higher metazoan (predominantly invertebrate) consumers. Our paired-catchment design was aimed at quantifying organic matter and invertebrate dynamics in the enriched stream and an adjacent reference stream for two years prior to enrichment and two years during enrichment. Enrichment had a strong negative effect on standing crop of leaf litter, but no apparent effect on that of fine benthic organic matter. Despite large nutrient-induced reductions in the quantity of leaf litter, invertebrate secondary production during the enrichment was the highest ever reported for headwater streams at this Long Term Ecological Research site and was 1.2-3.3 times higher than predicted based on 15 years of data from these streams. Abundance, biomass, and secondary production of invertebrate consumers increased significantly in response to enrichment, and the response was greater among taxa with larval life spans < or = 1 yr than among those with larval life spans >1 yr. Production of invertebrate predators closely tracked the increased production of their prey. The response of invertebrates was largely habitat-specific with little effect of enrichment on food webs inhabiting bedrock outcrops. Our results demonstrate that positive nutrient-induced changes to food quality likely override negative changes to food quantity for consumers during the initial years of enrichment of detritus-based stream ecosystems. Longer-term enrichment may impact consumers through eventual reductions in the quantity of detritus.

Long-term nutrient enrichment decouples predator and prey production.

Increased nutrient mobilization by human activities represents one of the greatest threats to global ecosystems, but its effects on ecosystem productivity can differ depending on food web structure. When this structure facilitates efficient energy transfers to higher trophic levels, evidence from previous large-scale enrichments suggests that nutrients can stimulate the production of multiple trophic levels. Here we report results from a 5-year continuous nutrient enrichment of a forested stream that increased primary consumer production, but not predator production. Because of strong positive correlations between predator and prey production (evidence of highly efficient trophic transfers) under reference conditions, we originally predicted that nutrient enrichment would stimulate energy flow to higher trophic levels. However, enrichment decoupled this strong positive correlation and produced a nonlinear relationship between predator and prey production. By increasing the dominance of large-bodied predator-resistant prey, nutrient enrichment truncated energy flow to predators and reduced food web efficiency. This unexpected decline in food web efficiency indicates that nutrient enrichment, a ubiquitous threat to aquatic ecosystems, may have unforeseen and unpredictable effects on ecosystem structure and productivity.

Nutrient enrichment alters storage and fluxes of detritus in a headwater stream ecosystem.

Responses of detrital pathways to nutrients may differ fundamentally from pathways involving living plants: basal carbon resources can potentially decrease rather than increase with nutrient enrichment. Despite the potential for nutrients to accelerate heterotrophic processes and fluxes of detritus, few studies have examined detritus-nutrient dynamics at whole-ecosystem scales. We quantified organic matter (OM) budgets over three consecutive years in two detritus-based Appalachian (U.S.A.) streams. After the first year, we began enriching one stream with low-level nitrogen and phosphorus inputs. Subsequent effects of nutrients on outputs of different OM compartments were determined using randomized intervention analysis. Nutrient addition did not affect dissolved or coarse particulate OM export but had dramatic effects on fine particulate OM (FPOM) export at all discharges relative to the reference stream. After two years of enrichment, FPOM export was 340% higher in the treatment stream but had decreased by 36% in the reference stream relative to pretreatment export. Ecosystem respiration, the dominant carbon output in these systems, also increased in the treatment stream relative to the reference, but these changes were smaller in magnitude than those in FPOM export. Nutrient enrichment accelerated rates of OM processing, transformation, and export, potentially altering food-web dynamics and ecosystem stability in the long term. The results of our large-scale manipulation of a detrital ecosystem parallel those from analogous studies of soils, in which net loss of organic carbon has often been shown to result from experimental nutrient addition at the plot scale. Streams are useful model systems in which to test the effects of nutrients on ecosystem-scale detrital dynamics because they allow both the tracking of OM conversion along longitudinal continua and the integrated measurement of fluxes of transformed material through downstream sites.

Effects of litter exclusion and wood removal on phosphorus and nitrogen retention in a forest stream

Many studies in the past have shown indirect evidence of the importance of terrestrial detritus in woodland streams, but recently WALLACE et al. (1997b) eliminated leaf and wood inputs to a small stream and direcdy demonstrated die importance of this material to stream food webs. Additionally, this whole-stream experiment has shown that terrestrial detritus is more than just food for invertebrates. TANK & WEBSTER (1998) found accelerated wood biofilm development and wood decomposition in the litter exclusion stream, and MEYER et al. (1998) used die litter exclusion experiment to estimate that leaves contribute approximately 30% of dissolved organic carbon exports. Previous studies have also suggested that leaf litter in streams is important to nutrient retention (MULHOLLAND et al. 1985, ELWOOD et al. 1988). The purpose of the current study was to examine the effects of litter exclusion and wood removal on retention of dissolved nutrients.

Does leaf chemistry differentially affect breakdown in tropical vs temperate streams? Importance of standardized analytical techniques to measure leaf chemistry

Abstract Comparisons of the effects of leaf litter chemistry on leaf breakdown rates in tropical vs temperate streams are hindered by incompatibility among studies and across sites of analytical methods used to measure leaf chemistry. We used standardized analytical techniques to measure chemistry and breakdown rate of leaves from common riparian tree species at 2 sites, 1 tropical and 1 temperate, where a relatively large amount of information is available on litter chemistry and breakdown rates in streams (La Selva Biological Station, Costa Rica, and Coweeta Hydrologic Laboratory, North Carolina, USA). We selected 8 common riparian tree species from La Selva and 7 from Coweeta that spanned the range of chemistries of leaf litter naturally entering streams at each site. We predicted that concentrations of secondary compounds would be higher in the tropical species than in the temperate species and that high concentrations of condensed tannins would decrease breakdown rates in both sites. Contrary to our predictions, mean concentration of condensed tannins was significantly greater (2.6×, p < 0.001) for species at Coweeta than for species at La Selva. Concentration of condensed tannins was negatively correlated with breakdown rate among Coweeta species (r = −0.80), not among La Selva species, and negatively correlated when the 2 sites were combined (r = −0.53). Concentrations of structural compounds were strongly correlated with breakdown rate at both sites (Coweeta species, lignin r = −0.94, cellulose r = −0.77; La Selva species, cellulose r = −0.78, C r = −0.73). The chemistries of 8 riparian species from La Selva and 7 riparian species from Coweeta were not as different as expected. Our results underline the importance of standardized analytical techniques when making cross-site comparisons of leaf chemistry.

Litter breakdown and invertebrate detritivores in a resource-depleted Appalachian stream

We measured breakdown rates of leaves and small wood for the first three years in a stream in which detrital inputs were excluded for 7 years and in a reference stream located in the Appalachian Mountains of North Carolina, USA. Leaf and wood inputs were excluded using a gill-net canopy constructed over a 170-m section of stream. We hypothesized that red maple (Acer rubrum) and rhododendron (Rhodo- dendron mima) leaf breakdown rates would decline in the litter exclusion stream as shredder production decreased with each year of litter exclusion. In contrast, we expected faster wood breakdown rates in the litter exclusion stream as microbes and invertebrates shifted from leaves to wood as their primary organic matter resource. Consistent with our predictions, wood breakdown rates were significantly faster in the litter exclusion stream. We also found sigdicantly slower processing rates of maple leaves in the litter exclusion stream compared to the reference stream during years 2 and 3. Slower breakdown rates for red maple leaves in the litter exclusion stream were associated with lower shredder production and estimated consumption rates in years 2 and 3. Shredder production and consumption rate estimates were also lower in the ex- clusion stream for rhododendron leaves, but leaf breakdown rates were not affected. We also found that shredder production in litterbags was 3-4 x greater than in benthic substrates in the litter exclusion stream. In contrast, shredder production in litterbags was similar to that in benthic substrates in the reference stream. These differences were probably due to the relatively low availability of organic matter in benthic sub- strates in the litter exclusion stream. Our data show shredders track high quality or- ganic matter resources (leaves) and contribute to their loss rate, illustrating the interde- pendence of stream organisms and terrestrial organic matter input.

Reduced detrital resources limit Pycnopsyche gentilis (Trichoptera: Limnephilidae) production and growth

Leaf inputs in temperate forest streams may limit caddisfly production because leaf detritus serves both as a food and case-material resource. We estimated Pycnopsyche gentilis production in a stream experimentally decoupled from its riparian habitat and a reference stream for 8 y in the southern Appalachians. We also examined laboratory survivorship, growth, and case-building activities of P. gentilis in substrate containing various quantities of leaf material. Pycnopsyche gentilis production declined to 0 within 3 y of the start of litter exclusion. Abundance, biomass, and production of P. gentilis were positively related to leaf litter standing crops. Maximum individual length of P. gentilis was reduced when annual leaf standing crops fell below 25 to 50 g AFDM/m2. Observations of case construction for instars removed from their original leaf cases and kept in substrate with low leaf standing crop, showed that P. gentilis was capable of rebuilding a case of available substrate and surviving for 3 to 4 wk before dying of starvation. Survivorship and growth were significantly greater for larvae reared at high and intermediate leaf standing crops, than at low leaf standing crop. Older instars had higher survivorship rates but lower growth rates than younger instars in the low litter substrates. Survivorship and growth rates were lower for some individuals forced to rebuild new cases, indicating an energetic cost associated with case-building activities. Our results demonstrate that the linkage between terrestrially derived organic matter and production of a caddisfly shredder was a consequence of food availability.

Stream Organic Matter Inputs, Storage, and Export for Satellite Branch at Coweeta Hydrologic Laboratory, North Carolina, USA

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