EFFECTS OF LIBBY DAM, HABITAT, AND AN INVASIVE DIATOM, DIDYMOSPHENIA GEMINATA, ON BENTHIC MACROINVERTEBRATE ASSEMBLAGES OF THE KOOTENAI RIVER, MONTANA Prepared by Brett D. Marshall, M.Sc. Prepared for: Montana Division of Fish, Wildlife and Parks, FWP Region 1 Headquarters 490 North Meridian Road Kalispell, MT 59901 April 2007 Version 2.1 1 - - - CONTENTS - - - EXECUTIVE SUMMARY ................................................................................................... 3 ACKNOWLEDGEMENTS ................................................................................................. 5 1. INTRODUCTION ........................................................................................................... 6 1.1. Background.............................................................................................................................................................. 6 1.2. Scientific Rationale for Additional Research below Libby Dam......................................................................... 7 2. METHODS................................................................................................................... 10 2.1. Study Sites.............................................................................................................................................................. 10 2.3 Laboratory Analysis............................................................................................................................................... 15 2.4. Data Analysis-Supporting data and covariates................................................................................................... 16 2.5. Data Analysis-Qualitative Macroinvertebrate Abundances.............................................................................. 17 2.6. Data Analysis-Quantitative Macroinvertebrate Abundances............................................................................ 17 2.7. Data Analysis-Quantitative Macroinvertebrate Community Function............................................................18 2.8. Data Analysis-Quantitative Response to D. geminata........................................................................................ 21 3. RESULTS .................................................................................................................... 23 3.1 Supporting field data and Covariates................................................................................................................... 23 3.2. Benthic Macroinvertebrate Sampling Overview ................................................................................................ 26 3.3. Results-Qualitative Macroinvertebrate Abundances ......................................................................................... 27 3.4. Results-Quantitative Macroinvertebrate Abundances....................................................................................... 36 3.5. Results-Quantitative Macroinvertebrate Community Function ....................................................................... 41 3.6. The Role of Habitat and D. geminata .................................................................................................................. 49 3.7. Data Analysis-Quantitative Response to D. geminata........................................................................................ 53 3.7. Results summary ................................................................................................................................................... 58 4.0 IMPLICATIONS OF D. GEMINATA FOR THE KOOTENAI RIVER .......................... 63 5.0 LITERATURE CITED ................................................................................................ 68 2 EXECUTIVE SUMMARY We collected benthic macroinvertebrate and algae samples from five locations below Libby Dam, along with ancillary habitat data. Our goals were: (1) to assess changes in benthic community structure downstream from Libby Dam, (2) to assess the effects of the invasive algae Didymosphenia geminata on benthic assemblages and (3) to make recommendations for natural resource management based upon our findings and those of others. We developed a study design to maximize the potential uses of the data. One of FWP’s priorities was to keep the sites and methods consistent with earlier river surveys (Perry et al. 1987, Hauer and Stanford 1997). We also wanted to quantify downstream gradients in community structure below the dam (something that earlier surveys failed to do with their qualitative analysis). In addition, we wanted to use the samples to quantify the effects of D. geminata—independent of dam-related station effects. We had a limited budget with which to generate a data set to support all these uses. We ultimately used a sampling method that used the same basic field methods as earlier studies (Perry et al. 1987, Hauer and Stanford 1997), but used a flow-standardized stratified sampling design. This allowed us to reduce unwanted variation among samples (increasing statistical power and reducing the required sampling effort), and to use a General Linear Modeling (GLM) to adjust benthic response-variable means for the variation related to physical habitat (flow, substrate, algae). Thus the same range of near-substrate flow velocities were sampled at each of the five study sites used by earlier investigators. We found that Libby Dam continues to exert a major influence on the structure and function of downstream benthic food webs. However, community dissimilarity analysis suggested that the direct influence of Libby Dam was somewhat less intense than indicated by sampling in the 1990’s (Hauer and Stanford 1997). This could be due to (1) landscape or climate changes that are unrelated to the dam, to (2) homogenizing habitat quality by D. geminata coating the substrata and reducing diversity in community composition, to (3) changing river-flow management, or because (4) we sampled the same flow velocity at all sites (earlier investigators did not control this variable nor did they correlate it with the macroinvertebrates). The reality is probably that a combination of all these factors reduced the effect of dam-related changes in benthic community structure. This does not account for the long-term effects of the dam and reservoir as a potential nutrient sink. We ran GLM stepwise procedures with all the habitat variables except for epilithic biomass (which was primarily D. geminata growth). We found that for most response variables, some 3 variation could be significantly explained by habitat variables. When we added the average Ash- free-dry-mass (AFDM) of epilithon as a covariate and re-ran the stepwise analysis, we found this term explained more variation in any other habitat variable. After AFDM variation was explained the treatment “SITE“ no longer significantly explained variation. This indicates that many aspects of community structure were strongly correlated with the thickness of epilithic biofilms. It is noteworthy that this variable explained more variation than flow—which often accounts for the most spatial variation in community structure within a riffle (e.g., Hart and Fonseca 1995). Both shredders and scrapers feed upon biofilm-colonized substrata. Therefore we expected that changes in the quantity or quality of biofilms could affect the success (and thereby abundance) of these two functional feeding groups. These taxa should be less common in areas of very low epilithic growth than in areas with moderate growth. In the case of the Kootenai River and the spread of D. geminata, biofilms are likely to become sufficiently thick as to interfere with feeding and mobility of these taxa (many of which are clingers). Therefore we performed a nonlinear regression to fit a dose-response curve for the abundance of scrapers and shredders relative to the mass of epilithic biofilms. We identified a threshold level of epilithic biofilms which should not be exceeded to maintain natural ecosystem function. Concentrations of organic material exceeding 8mg/cm2, completely excluded shredders and dramatically reduced scrapers. The abundance of these important taxa began declining at organic biofilms concentrations between 3-5 mg/cm2. Ideal production of these important links to higher trophic levels should occur at concentrations less than 5 mg/cm2. The factors contributing to population explosions (blooms) of D. geminata are not understood. The phenomenon began occurring all over the northern hemisphere in the late 1980’s and coincided with dramatic reductions in stratospheric ozone levels and marked increases in the amount of ultra-violet radiation reaching the earth’s surface. Controlled field and laboratory investigations should provide greater insight into the factors contributing to the growth of D. geminata—and help determine which factors can be controlled and which cannot. The abundance of oligochaete worms was strongly correlated with D. geminata concentrations, and could increase the exposure risk of some fishes to the parasite Myxobolus cerebralis—the organism responsible for whirling disease. 4 ACKNOWLEDGEMENTS Many people assisted this project along the way. First I gratefully thank the Montana Department of Fish, Wildlife and Parks (FWP), especially Jim Dunnigan, for funding and initiating this study. The field and laboratory aspects of the study were conducted by EcoAnalysts, Inc. (EAI). Some of the EAI members that specifically helped the project included: Gary Lester, Scott Lindstrom,