Quantitative Predictions of the Responses of Aquatic Ecosystems to Disturbance I Posters
Presiding: E Marti, Centre d'Estudis Avancats de Blanes (CSIC); C Hoagstrom, South Dakota State University
NB33N-01 1330h
Alpine Lake Phytoplankton Responses to the 2002 Drought in Colorado
Because the hydrologic regime of alpine catchments is dominated by snow melt, their terrestrial and aquatic ecosystems are extremely sensitive to physical and climatic fluctuations in the amount of snow and timing of snowmelt. The drought of 2002 in Colorado was the most extreme drought of the past 100 years of record for the state. Measured at the outflow of Green Lake 4, an alpine lake in Green Lake Valley, located within the Niwot Ridge Long Term Ecological Research (LTER) site in the Front Range of Colorado, the 2002 drought resulted in snowmelt discharge that was just 60% of recent historical averages for stream discharge. We examined how the drought affected the phytoplankton population in this alpine lake. Algal biomass was quantified and samples for community composition analysis were collected during the summers of 2000, 2001 and 2002. The results of the statistical comparison indicate a significant increase in total density of algal cells, possibly caused by drought-induced factors such as earlier ice-out and hence higher irradiance, higher hydraulic residence time and lower wash-outs, plus higher surface water temperatures. Species in three divisions, Bacillariophyta, Chlorophyta and Cyanophyta, showed significant responses. A principal components analysis confirmed a shift in the community composition during the drought. Although based on only three seasons of monitoring data, these results may foreshadow climate change and implicate subsequent biological effects in high altitude watersheds.
NB33N-03 1330h
Wildfire and Headwater Stream Productivity: Effects of Intense Fire on Food Subsidies to Downstream and Riparian Habitats in Eastern Washington
Frequent intense, stand-replacing wildfires have occurred throughout the western United States in recent years. Fish bearing streams and riparian areas receive food subsidies of invertebrates and detritus from headwater streams, however, little is known about how fire impacts these foodwebs. The objectives of this study are to examine the effects of forest fire on the source of these subsidies from headwater streams and quantify the effects of forest fire on the abundance and diversity of macroinvertebrate communities. Benthic, emergent, and drift samples of macroinvertebrates were taken from five streams with burned watersheds and five control streams once per month in summer 2004. Invertebrates are identified to family and measured to the nearest millimeter to determine biomass. Counts of individuals from June 2004 drift samples show that burned streams have ten times more drifting invertebrates than control streams. This research will provide timely information for riparian forest management in fire-prone regions of North America. Wildfires may be beneficial to aquatic habitats both in creating diverse habitat and conditions that support abundance and diversity of invertebrates and other organisms.
NB33N-04 1330h
Stream Characteristics and Aquatic Macroinvertebrates Following Wildland Fire in Western Wyoming
We examined fire effects on aquatic habitat and benthic macroinvertebrates in streams of the Little Granite Creek watershed, Wyoming, where a wildfire burned approximately 75% of one basin, while an adjacent (reference) basin remained largely unburned. Embeddedness (% of channel surface covered by fines) was substantially greater in the burned stream (92%) than in the reference stream (59%). Stream temperatures were consistently higher in summer and lower in fall in the burned drainage, reflecting basin differences in vegetative cover. Although macroinvertebrate taxonomic richness was similar in both streams (45 families), benthic communities in the burned stream were dominated by Dipterans (77% total abundance) with 22% EPT, while communities in the unburned stream were composed of 46% Dipterans and 53% EPT. Benthic communities in both streams had similar proportions of generalist feeders (mostly Chironomidae, 31 to 33%), and collector-gatherers and scrapers (mostly Baetidae, 17%). However, collector-filterers (mostly Simulidae) accounted for over 40% of the total abundance in the burned stream, and only 3% in the unburned stream. Despite differences between the study basins, fire effects in the Little Granite Creek watershed are less dramatic than those reported from other regions.
NB33N-05 1330h
Effects of wildfire on aquatic invertebrates in British Columbia streams
I examined the effects of the 2003 McClure fire on aquatic invertebrates in three forested streams. Two sites were sampled on Fishtrap Creek, which was extensively burned, one site was sampled on Peterson Creek, which was patchily burned, and one site sampled on Jamieson Creek, which served as the unburned control. The intensely burned areas of Fishtrap Creek had all vegetation removed around the creek, and burned the ground to the mineral soil. Random Hess samples were taken in riffles in all creeks in April 2004. Densities of mayflies, stoneflies, caddisflies and diptera varied significantly among the creeks, and were lower in the burned streams compared to the control. The lower site on Fishtrap Creek, which flowed through the most intensely burned area, had a mean of only 67 individuals per square meter, compared to a mean of 1823 in the control. As expected due to the loss of riparian vegetation, shredder densities (Nemouridae+ Leuctridae+Capniidae) were significantly reduced in the burned streams (p=0.008).
NB33N-06 1330h
Does Nutrient Enrichment Alter the Response of Stream Macroinvertebrates to Disturbance?
We examined the influence of nutrient enrichment on the response of benthic macroinvertebrates to disturbance in springbrooks of an alpine floodplain. Individual stones in three different springs were physically disturbed at frequencies of 0, 4, 8 and 16 days, and then collected on day 32. In another experiment, nutrients (slow-release fertilizer pellets) were added to one of two streams, and stones in both treated as in the first experiment. In both experiments, stones were analyzed for periphyton biomass, total invertebrate density, and the density and relative abundance of four common families: Baetidae, Nemouridae, Chironomidae, and Simuliidae. N, P, and C were analyzed for periphyton, benthic sediments and invertebrates in the 2nd experiment to test for stoichiometric changes resulting from nutrient enrichment. In the 1st experiment, disturbance affected the relative abundance of the four families. Fertilization changed the stoichiometry of periphyton, invertebrates and sediments and altered macroinvertebrate assemblages. Here, the effect of disturbance differed between the two streams, and the non-fertilized stream displayed a similar disturbance response pattern by macroinvertebrates as in the 1st experiment. These results suggest that nutrient enrichment alters the effects of physical disturbance on macroinvertebrates by changing food resource stoichiometry and shifting macroinvertebrate assemblage structure.
NB33N-07 1330h
Responses of Meiofauna to Flooding in Stream Mesocosms Along a Gravelbed Stream in the Ozark Highlands
We tested the responses of meiofauna to disturbance by flooding using stream mesocosms on the Illinois River, Arkansas (U. S. A.), a riffle/pool, gravelbed stream in the Ozark Highlands. For 18 days in July and August 2003, one group of stream mesocosms was not disturbed and two were subjected to flooding treatments; moderate/frequent, and severe/infrequent. On day eighteen core samples were taken of the gravel/sand substrate in the mesocosms. Planktonic samples taken entering the mesocosms identified prospective colonizers. Core samples taken from the natural stream bed served as reference meiofauna communities. Statistical differences in densities of meiofauna L-1 of substrate were observed in ten of the 19 taxa and for total meiofauna. In all taxa where there were significant differences, the undisturbed stream mesocosms had significantly higher densities than at least one of the two disturbance treatments. Total meiofauna mean densities in the undisturbed treatments, 1809 ± 258 (2 s. e.), were significantly higher than both the moderate/frequent and the severe/infrequent mesocosms (F = 21.35, P < 0.000). Shannon's diversity index (H) was highest at the moderate/frequent disturbance regime. Four rare taxa were absent and evenness of the remaining meiofauna increased at the moderate/frequent treatment.
NB33N-08 1330h
Meiofauna Responses to Mechanical Disturbances in Stream Mesocosms on the Illinois River, Arkansas (U. S. A.)
Benthic meiofauna responses to mechanical disturbance of the substrate was studied in August, 2003 on the Illinois River, Arkansas, (U. S. A.) in stream mesocosms. The Illinois River is a gravel/bed, riffle/pool stream located in the Ozark Highlands. Three levels of disturbance (undisturbed, moderate/frequent, and severe/infrequent) were assigned randomly to twelve stream mesocosms. After 18 days, substrate samples were taken from each mesocosm using a core sampler. Planktonic and natural stream bed samples were taken every third day during the study to identify potential colonizers and to serve as a reference. Within the mesocosms, four of the 17 taxa found and total density of meiofauna L-1 of substrate were significantly different between treatments with the undisturbed channels having higher densities than the severe/ frequent treatment. Total meiofauna densities L-1 in the undisturbed treatment, 4471.9 ± 1308 (2 s. e.), were significantly higher than either of the other treatment levels (F = 15.43, P = 0.001). Chironomidae and Ephemeroptera were numerically dominant in undisturbed channels but both were significantly reduced by the treatments (F = 12.28, P = 0.003; F = 61.22, P < 0.000 respectively). Shannon's diversity index ('H) was highest in the moderate/frequent treatment reflecting the increase in evenness.