Detritus-based Stream Research Articles

Bottom-up limitation of a stream salamander in a detritus-based food web

The indirect effects that resources can have on higher trophic levels remain poorly understood for detritus-based ecosystems. Our objective was to examine effects of long-term terrestrial litter exclusion on a larval salamander, Eurycea wilderae, in a detritus-based stream. After 4 years of exclusion treatment, we conducted a mark–recapture study and analyzed gut contents of E. wilderae larvae in the litter exclusion reach, a reach downstream of treatment, and in a reference stream. Eurycea wilderae growth rate (per day), density (individuals per square metre), biomass (milligrams ash-free dry mass per square metre), and production (milligrams ash-free dry mass per square metre per year) were all significantly reduced in the litter exclusion reach. Reduced density in the treatment reach was likely due to elevated hatchling drift driven by reduced prey availability. Larvae from the treatment reach had fewer prey items per gut than larvae in the reference stream and their diet consisted of fewer copepods but more midge larvae, nematodes, and terrestrial insects. The reach downstream of treatment was intermediate between reference and litter exclusion reaches for most measured parameters, indicating residual effects of upstream treatment. Our results provide the first comprehensive evidence of bottom-up limitation of a vertebrate predator in a detritus-based ecosystem and further demonstrate the importance of the terrestrial–aquatic linkage.

Responses of organic matter and macroinvertebrates to placements of boulder clusters in a small stream of southwestern British Columbia, Canada

Diversity and productivity of stream food webs are related to habitat heterogeneity and efficiency of energy retention. We tested the hypothesis that experimental boulder placements in a second-order stream would increase diversity and abundance of macroinvertebrates by restoring detrital retention and habitat heterogeneity. Two relatively natural, upstream, reference reaches and a downstream treatment reach with a relatively straight channel and less woody debris were studied for 3 months before and 1.2 years after the placement of six boulder clusters in the treatment reach. Mean velocity and its coefficient of variation increased in the treatment reach (140 and 115%, respectively), whereas the reference reaches remained relatively unchanged after the placements. Enhanced particulate organic matter storage (550%) was accompanied by increased total macroinvertebrate abundance (280%) in the treatment reach, converging with those of the reference reaches almost 1 year after the treatment. Detritivorous taxa numerically dominated the macroinvertebrate community, the total densities of which were best predicted by the fine fraction of organic matter biomass at microhabitat scale. However, the effect of boulder clusters on taxonomic richness was negligible. Our findings suggest that boulder clusters can be used at least as a short-term means to restore macroinvertebrate productivity in detritus-based stream systems.

The Trophic Significance of Bacteria in a Detritus-Based Stream Food Web

We compared relative use of streamwater dissolved organic carbon (DOC) by bacteria and the trophic significance of bacteria to invertebrates in two headwater streams at Coweeta Hydrologic Laboratory in North Carolina: a stream with all leaf litter inputs excluded for 1 yr, and a reference stream. Leaf litter standing crop in the treatment stream was <1% that of the reference stream, and fine benthic organic matter (FBOM) was 50% lower than the reference. We used a whole-stream tracer addition of 13C-1 sodium acetate for 3 wk to label bacteria and hence their consumers during both July and December. Bacterial δ13C was measured by collecting respired bacterial carbon. We estimated the contribution of bacterial carbon to consumers using a mixing model for invertebrates and bacteria. The acetate label declined exponentially downstream with a 10-m uptake length in each stream and season. FBOM and biofilm were the only detrital samples to show a strong label; both were more labeled in the litter-excluded stream. Bacteria in the litter-excluded stream had 7–10 times more label than those in the reference stream during both seasons, showing their higher relative use of streamwater DOC. The percentage of invertebrate carbon derived from bacteria was >20% for many taxa. This was significantly related to the percentage of amorphous detritus in invertebrate guts, suggesting that bacterial carbon supporting higher trophic levels was associated with amorphous detrital particles. Predatory invertebrates were labeled, showing that bacterial carbon was important for higher trophic levels. Some invertebrates were more highly labeled than the bacteria. Stenonema in the treatment stream contained eight times more label than measured bacteria. This suggests that they were using an unmeasured bacterial source such as bacteria in exposed epilithic biofilms, which had higher δ13C than all other detrital components. Invertebrates in the treatment stream did not appear to use more bacterial carbon than in the reference stream despite a lower standing crop of detritus. Tallaperla, a shredding stonefly, derived 20–40% of its carbon from bacteria in both streams, even though it was more labeled in the treatment stream. Our estimates of the percentage of invertebrate carbon derived from bacteria were higher than those found in laboratory-based studies. To investigate reasons for this difference, we examined the possibility that bacterial carbon was principally found in exopolymers, as our labeling method would have labeled exopolymers. We found 6 g/m2 of colloidal carbohydrates in the reference stream, which was five times greater than bacterial biomass; thus the high use of bacterial carbon by invertebrates may be a consequence of the availability of these polymers.

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

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

Dietary overlap and coexistence of chironomid larvae in a detritus-based stream

Of the seven orders of macroinvertebrates that make up the DuffinCreek riffle community, the Chironomidae were the dominant taxonmaking up, numerically, 25.8–53.7% of the total fauna. Cricotopus sp., ?Rheosmittia sp. and a species from the Eukiefferiella brehmi group predominated whereas Parametriocnemus sp., Thienemanniella sp., Tanytarsussp., and Rheotanytarsus sp., were common. The three leastabundant chironomids were Ablabesmyia sp., Microtendipes sp. and a species from the Stempellinamontivaga group. The coexistence of a variety of life stages ofeach species throughout the year indicates a high degree oftemporal overlap. The major food type both in the environment andin the larval guts was detritus. Dietary overlap indices indicatedalmost complete overlap (0.89–0.99) and a convergence of diets –the highest values were generated from hyporheic feeding datacompared with rock surface grazing data. Niche breadth valuesindicated a predominance of generalist feeding throughout most ofthe year (0.62–0.99).