High Spring Flows Research Articles

Synoptic sampling reveals spatial stability in flow and chemistry patterns despite large seasonal variability in a subarctic mountain catchment

AbstractSeasonality plays a critical role in cold mountain regions as variation in air temperature, ground thermal status, and precipitation phase alter the rate, timing and magnitude of hydrological and chemical transport. Additionally, cold mountain catchments can have highly variable topography, geology, permafrost, and landcover, which intrinsically add to this irregularity. Understanding how external and internal variability act to control mass fluxes requires sampling at a high spatial resolution over time, which rarely occurs in cold remote regions. In this work, we conduct five snapshot sampling surveys across 34 subcatchments during the ice‐free period in Wolf Creek Research Basin (a mesoscale montane subarctic catchment) and two additional winter surveys across a subset of sites to assess the drivers of variability in stream chemistry and discharge. We sampled for specific conductance (SpC), major ions, and dissolved organic carbon (DOC) and used statistical metrics and Bayesian mixing analysis to quantify patterns of flow and chemistry across space and time. Our results indicate patterns in both flow and chemistry remain largely consistent across seasons for all solutes. However, there was weaker correlation of chemistry between sites, suggesting asynchronous behaviour within the catchment. There was evidence of increasing production of ions and DOC along the stream network during high spring flows but not during low flows. Although concentrations and flows exhibit high seasonality in subarctic mountains, this seasonal variability does not alter spatial patterns that arise from highly variable catchment characteristics.

Spring peak flows control abundance of Cladophora in a Hydropower‐Impacted River

AbstractThe lower Deschutes River represents the 161‐km reach downstream of three hydroelectric dams flowing to its confluence at the Columbia River. We examined hydrology and nutrient chemistry from 2015–2017 to 2019 to determine factors affecting periphyton community composition and abundance following a change in the outlet structure and depth of release waters from the major upstream impoundment. The periphyton community composition was abundant and diverse in 2015 and 2016 and lower in 2017 and 2019. Filamentous chlorophytes were dominant in the first 2 years but were sparse in the subsequent years. Cyanophytes were dominant in 2017 and 2019. The expectation entering the study was that altered nutrient chemistry associated with the change in operations resulted in conditions more favorable for periphyton growth. Concentrations of total and soluble phosphorous are independent of reservoir operations, being high in all years because of natural weathering of P‐rich volcanic rocks in the basin. Concentrations of TN and DIN were low all study years, resulting in a system that was N‐limited, but with similar chemistry among the study years. The hydrology during 2015 and 2016, with abundant chlorophytes, exhibited low flow in the spring, whereas high spring flows were observed in the latter years. We conclude that high spring flows were far more influential in affecting the periphyton community composition in the lower Deschutes River compared to nutrient chemistry.

Further evidence for lower Columbia River green sturgeon spawning

The Green Sturgeon is a long-lived anadromous fish known to spawn in only three locations. Southern Distinct Population Segment (DPS) Green Sturgeon spawn only in the Sacramento drainage, while the Northern DPS spawns in the Klamath and Rogue rivers. In 2011, a young of year (YOY) Green Sturgeon was captured in the Columbia River below Bonneville Dam, providing preliminary evidence of a fourth spawning location. In 2017, four additional Green Sturgeon YOY of similar size were captured in the Columbia River below Bonneville Dam. Genetic analysis of all YOY samples indicates that they belong to the Northern DPS, confirming that the Northern DPS actually spawns in at least three locations. The detection of Green Sturgeon spawning in the Columbia River only in years with relatively high spring flow (2011, 2017), combined with confirmed correlations between flow and spawning for other White and Green Sturgeon populations, suggests that appropriate spawning conditions may only exist above a specific flow threshold. The Columbia River has the potential to play an important role in Green Sturgeon climate change adaptation due to its location at the northernmost end of the species’ reproductive range. We recommend future research to better quantify Green Sturgeon recruitment magnitude, frequency and dynamics in the Columbia River.

Effects of season and waterbody on transparency tube estimates of suspended sediment in large rivers

This paper reports the measurement (N>132) of the effect of river and season on transparency in cm and Total Suspended Solids in mg/L in a navigational pool of the Ohio River and one of its tributaries, the Muskingum River. Both river of origin and season affected water transparency. The transparency-TSS relationship was stronger in spring (R2 = 0.894) than autumn (R2 =0.710), with an overall correlation of R2=0.86 for N=93 observations in both water bodies and seasons. Regression equations for the transparency-TSS relationships for the two rivers under low (autumn) and high (spring) flow conditions were developed. Our study demonstrates that properly trained volunteers can assist with rapid assessment of water turbidity from suspended solids in large rivers, reservoirs and lakes, but seasonal calibration of these measures will improve accuracy of sediment monitoring and management.Keywords: Transparency tube, turbidity, monitoring, sediment transport, citizen science, water quality

Quantifying population-level effects of water temperature, flow velocity and chemical-induced reproduction depression: A simulation study with smallmouth bass

Evaluating stochastic abiotic factors and their combined effects on fish and wildlife populations have been challenging in environmental sciences. Contributing to this challenge is the paucity of data describing how observations made on individuals related to exposure to chemical compounds ultimately effect population vital rates, and how this exposure interacts with other abiotic drivers. Using three smallmouth bass populations in Pennsylvania as a case study, we explored both single-factor and combined effects of water temperature, flow velocity and chemical exposure on populations through a model simulation. Although there are many pathways through which chemicals can affect population vital rates, we focused on one where chemical exposure leads to reduced reproduction. Among the three populations considered, two (the Juniata and Susquehanna populations) have experienced adverse health, including the potential adverse effects of environmental stress and chemical contamination that may cause disease and mortality of young-of-year (YOY), various skin lesions and a high prevalence of intersex or testicular oocytes in adults. The third population (The Alleghany population) has not encountered mortality events of YOY and intersex prevalence is much lower. The simulation involved projecting populations using a length-based model under different environmental conditions. In the simulations, abiotic factors influenced population dynamics through their impacts on growth, survival and recruitment. Response to the same environmental stress varied by population and life-stage of the species. Factors affecting young adult and adult life-stages had great influence on proportional stock density (PSD) and the probability of having PSD within the suggested range (PSD probability). Increases in water temperature had a negative effect and dominant role in the combined effect on population size structure (e.g., PSD and PSD probability) – increases in flow velocity during the spring season also had a negative effect on abundance. Populations with high recruitment rates sustained relatively large population size, even under high water temperature and/or high flow velocity, which suggests that factors and management strategies that benefit recruitment (such as reduced chemical contaminants) may compensate for the negative effects of warming water temperatures and high spring flow velocity on population size.

Is climate change driving recruitment failure in Australian bass Macquaria novemaculeata in southern latitudes of the species range?

Flow regimes have been fundamentally altered in many of the world’s river systems. There is a need to restore components of natural flow variability to protect freshwater biodiversity. The Australian bass is a long-lived, catadromous percichthyid endemic to coastal drainages of south-eastern Australia. Little is known of the timing and magnitude of flows considered important for recruitment and growth of individuals in southern latitudes of the species range. Herein we generate nearly 50 years of otolith-derived recruitment and growth histories for Australian bass and relate these to hydrologic, climatic and demographic variables. We found that younger Australian bass grew fastest in years following a cooler spring, whereas the growth of older fish exhibited little response to temperature. Australian bass year class strength was positively related to high flows in spring and in a negative curvilinear way to spring temperature. A lack of suitable natural flows in spring may be responsible for little to no recruitment of Australian bass in the Genoa River over the past two decades. Based on our findings, the continuing trend of lower rainfall and higher temperatures, coupled with river regulation in some systems in the region, will likely result in depressed juvenile growth and prolonged periods of recruitment failure over the medium to longer term, with significant implications for the viability of populations.

Modeling potential river management conflicts between frogs and salmonids

Management of regulated rivers for yellow-legged frogs (Rana boylii) and salmonids exemplifies potential conflicts among species adapted to different parts of the natural flow and temperature regimes. Yellow-legged frogs oviposit in rivers in spring and depend on declining flows and warming temperatures for egg and tadpole survival and growth, whereas salmonid management can include high spring flows and low-temperature reservoir releases. We built a model of how flow and temperature affect frog breeding success. Its mechanisms include adults selecting oviposition sites to balance risks of egg dewatering by decreasing flow versus scouring by high flow, temperature effects on development, habitat selection by tadpoles, and mortality via dewatering and scouring. In simulations of a regulated river managed primarily for salmonids, below-natural temperatures delayed tadpole metamorphosis into froglets, which can reduce overwinter survival. However, mitigating this impact via higher temperatures was predicted to cause adults to oviposit before spring flow releases for salmonids, which then scoured the egg masses. The relative timing of frog oviposition and high flow releases appears critical in determining conflicts between salmonid and frog management.

Flow management and fish density regulate salmonid recruitment and adult size in tailwaters across western North America.

Rainbow and brown trout have been intentionally introduced into tailwaters downriver of dams globally and provide billions of dollars in economic benefits. At the same time, recruitment and maximum length of trout populations in tailwaters often fluctuate erratically, which negatively affects the value of fisheries. Large recruitment events may increase dispersal downriver where other fish species may be a priority (e.g., endangered species). There is an urgent need to understand the drivers of trout population dynamics in tailwaters, in particular the role of flow management. Here, we evaluate how flow, fish density, and other physical factors of the river influence recruitment and mean adult length in tailwaters across western North America, using data from 29 dams spanning 1-19 years. Rainbow trout recruitment was negatively correlated with high annual, summer, and spring flow and dam latitude, and positively correlated with high winter flow, subadult brown trout catch, and reservoir storage capacity. Brown trout recruitment was negatively correlated with high water velocity and daily fluctuations in flow (i.e., hydropeaking) and positively correlated with adult rainbow trout catch. Among these many drivers, rainbow trout recruitment was primarily correlated with high winter flow combined with low spring flow, whereas brown trout recruitment was most related to high water velocity. The mean lengths of adult rainbow and brown trout were influenced by similar flow and catch metrics. Length in both species was positively correlated with high annual flow but declined in tailwaters with high daily fluctuations in flow, high catch rates of conspecifics, and when large cohorts recruited to adult size. Whereas brown trout did not respond to the proportion of water allocated between seasons, rainbow trout length increased in rivers that released more water during winter than in spring. Rainbow trout length was primarily related to high catch rates of conspecifics, whereas brown trout length was mainly related to large cohorts recruiting to the adult size class. Species-specific responses to flow management are likely attributable to differences in seasonal timing of key life history events such as spawning, egg hatching, and fry emergence.

Spawning migration of American eel Anguilla rostrata from pristine (1843–1872) to contemporary (1963–1990) periods in the St Lawrence Estuary, Canada

Daily American eel Anguilla rostrata catches and their dates of passage (starting, median and ending dates) were compared between pristine (1843-1872) and contemporary periods (1963-1990) to determine any changes and see whether these were related to environmental variations and water discharge regulation. Timing and duration of A. rostrata migration patterns differed significantly between the two periods. In the contemporary period, migrating A. rostrata were intercepted significantly earlier than in pristine times (18 days earlier on average), and ended at the same average period. Early A. rostrata migration was also significantly related to high spring flow and secondarily to high spring temperature, while migration ended later when high temperature or low water level occurred during the autumn period. A recent slight increase in the water temperature of the St Lawrence River could partially explain the earlier A. rostrata migration observed during the contemporary period. In return, the effect of high spring flow should have been more contrasted if the river discharge would have not been regulated. Recent A. rostrata production now being mainly restricted to the lower part of St Lawrence River mainstream, resulting shorter travelling distance to the estuary may explain why migrating progress was earlier during the contemporary period.

Long‐Term Dynamics of Native and Nonnative Fishes in the San Juan River, New Mexico and Utah, under a Partially Managed Flow Regime

AbstractNonnative fishes and flow alteration are primary threats to native fish persistence in lotic systems. We used several flow regime attributes and fish sampling data obtained from the San Juan River, New Mexico and Utah, during 1993–2010 to evaluate the potential use of flow manipulations to increase recruitment of native fishes that must cope with nonnative species. During this period, discharge in the river was partially manipulated by reservoir releases that augmented naturally high spring flows in this snowmelt‐driven system. An information theoretic approach was used to rank candidate models that predicted species densities based upon selected combinations of flow attributes and abundances of nonnative species. Autumn density of age‐0 fishes in secondary channels was the main response variable. The main predictor variables included flow attributes associated with interannual variation in daily discharge and water temperature; densities of nonnative competitors; and catch rates of a numerically dominant nonnative predator (the channel catfish Ictalurus punctatus). Top‐ranked models for native species included positive associations with small‐bodied nonnative fishes and negative associations with the abundance of channel catfish adults. Densities of native speckled dace Rhinichthys osculus and flannelmouth suckers Catostomus latipinnis increased with mean spring discharge, but the density of native bluehead suckers C. discobolus did not. With the exception of juvenile channel catfish, the top candidate models predicting densities of nonnative fishes all included the duration of low summer flows. These results confirmed findings from a previous study that demonstrated different responses of native and nonnative fishes to seasonal flows; the present study also revealed that densities of all fishes were generally lower in years with greater abundance of adult channel catfish. Regression analysis indicated that seasonal flow manipulations and suppression of nonnative predator populations could be effective management tools to restore and maintain the native fish community.

Tributary discharge, lake circulation and lake biology as drivers of water quality in the Canadian Nearshore of Lake Ontario

A combination of data from shipboard and deployed sensors, nutrient and major ion concentration data, and tributary discharge and current data was used to assess water quality patterns on the Canadian nearshore of Lake Ontario in 2008 and the factors that impact these patterns. Mixing areas at tributary mouths were characterized by high spatial and temporal variability in water quality. Conductivity at sensors offshore from tributaries was variably correlated with discharge, with the correspondence most evident during high spring flow. Tracers of land runoff were positively correlated with tributary discharge at three of five study locations. The spatial distribution of water quality was strongly influenced by the predominating alongshore circulations, with gradients frequently observed parallel to the shoreline away from tributary mouths. Upwelling, evident from temperature bands along the coastline, periodically created onshore–offshore gradients in chlorophyll a and dissolved inorganic nitrogen. Lake whiting, starting in July, further enhanced the effect of upwelling by accentuating differences in water clarity, suspended solids, and ionic composition between surface and bottom strata. In general, turbidity decreased and water clarity increased with lake depth. Gradients in water clarity and chlorophyll a suggestive of dreissenid mussel feeding were uncommon. Interacting land- and lake-based factors contributed to heterogeneous water quality over the coastline of Lake Ontario highlighting needs for water resource assessment methods sensitive to the temporal and spatial scales of problems as well as the connectivity between the land and the lake.

Understanding the dynamics of high and low spring flow: a key to managing the water resources in a small urbanized hillslope of Lesser Himalaya, India

Water scarcity is becoming a major problem in High Mountain Basins of Indian Himalaya. Access to fresh water becomes a core concern for every dweller in the summer season all across the Indian Himalayan Region. This paper presents an assessment of two low yielding perennial springs in one of the urban center located in the very steep mountain slopes of Lesser Himalaya. Spring hydrographs were analyzed to investigate the variability and availability of spring flow in three seasons. Dynamic spring flow volume in storage and time at which the flow may cease were also assessed, as they play a crucial role for planning short-term measures. The linear model using the exponential decay equation is found to be sufficiently accurate to model the recession characteristics of spring outflow from the hardrock aquifers for winter season, as the main contribution comes from baseflow with delayed interflow, which is slow diffuse flow in nature. The present study examines the findings from 1 year of spring outflow and rainfall when deficient monsoonal rainfall (moderate drought) is recorded in major parts of the country. The data can play a significant role in estimating dependable minimum water availability from the spring outflow and planning for appropriate capacity of low to medium storage structure for conserving the excess water which goes unutilized even during the high scarcity period. The analysis of duration curve suggests, more focus should be given for conserving the surplus water available during the monsoon through appropriate storage structures, which can be used during the leanest month.

Mathematical model to identify nitrogen variability in large rivers

AbstractThe river Swale in Yorkshire, northern England has been the subject of many studies concerning water quality. This paper builds on existing data resources and previous 1D river water quality modelling applications at daily resolution (using QUESTOR) to provide a different perspective on understanding pollution, through simulation of the short‐term dynamics of nutrient transport along the river. The two main objectives are (1) building, calibration and evaluation of a detailed mathematical model (Advection‐Dispersion Model: ADModel), for nutrient transport under unsteady flow conditions and (2) the development of methods for estimating key parameters characterizing pollutant transport (velocity, dispersion coefficient and transformation rates) as functions of hydrological parameters and/or seasonality.The study of ammonium and nitrate has highlighted temporal variability in processes, with maximum nitrification and denitrification rates during autumn. Results show that ADModel is able to predict the main trend of measured concentration with reasonable accuracy and accounts for temporal changes in water flow and pollutant load along the river. Prediction accuracy could be improved through more detailed modelling of transformation processes by taking into account the variability of factors for which existing data were insufficient to allow representation. For example, modelling indicates that interactions with bed sediment may provide an additional source of nutrients during high spring flows. Copyright © 2010 John Wiley & Sons, Ltd.

Seasonal movements of non‐native lake trout in a connected lake and river system

Abstract Non‐native lake trout, Salvelinus namaycush (Walbaum), threaten native salmonid populations in the western United States. Effective management of lake trout requires understanding movements within connected lake and river systems. This study determined the seasonal movements of subadult lake trout in the Flathead River upstream of Flathead Lake, Montana, USA using radio telemetry. The spatiotemporal distribution of lake trout in the river was related to water temperature. Lake trout were detected in the river primarily during autumn, winter and spring, when water temperatures were cool. By contrast, fewer were detected when temperatures were warmest during summer and during high spring flows. Downriver movements to Flathead Lake occurred throughout autumn and winter when water temperature decreased below 5 °C, and in late spring as water temperature rose towards 15 °C and river discharge declined following spring runoff. Upriver movements occurred primarily in October, which coincided with migrations of prey fishes. These results suggest that lake trout are capable of moving throughout connected river and lake systems (up to 230 km) and that warm water temperatures function as an impediment to occupancy of the river during summer. Controlling source populations and maintaining natural water temperatures may be effective management strategies for reducing the spread of non‐native lake trout.

GROUNDWATER INFLUENCES ON THE DISTRIBUTION AND ABUNDANCE OF RIVERINE SMALLMOUTH BASS, MICROPTERUS DOLOMIEU, IN PASTURE LANDSCAPES OF THE MIDWESTERN USA

ABSTRACTThis study examined how spring‐flow (SF) contributions to streams related to the distribution and abundance of smallmouth bass Micropterus dolomieu in a predominately pasture landscape in Missouri, USA. Stream segments (N = 13) with similar landscape characters were classified by SF volume into high SF (HSF) or low SF (LSF) groups. The densities of smallmouth bass, channel unit (CU) use and temperature‐selection patterns were assessed for several life stages and frequency distributions for age 0 fish. More smallmouth bass were present in stream segments with HSF influence. Age 0 fish were twice as likely to be present in HSF stream segments. Older age classes were present in stream reaches independent of SF contribution. For all age classes, the use of particular CUs did not depend on SF influence. All age classes were more likely to be present in pools than other CUs. Microhabitat temperature selection differed among age classes. Age 0 fish selected warmer temperatures with a gradual shift towards cooler temperatures for older age classes. The length frequency of age 0 fish was skewed towards larger individuals in streams with limited SF influence, whereas the length frequency in HSF stream segments was skewed towards smaller individuals. The benefits of significant groundwater via SF influence seem to be related to increased hatch or survival of age 0 fish and the availability of optimal temperatures for adult smallmouth bass growth. Thermal refugia and stable flows provided by springs should be recognised for their biological potential to provide suitable habitat as climate change and other land‐use alterations increase temperature regimes and alter flow patterns. Published in 2011 by John Wiley & Sons, Ltd.

3‐D flow and scour near a submerged wing dike: ADCP measurements on the Missouri River

Detailed mapping of bathymetry and three‐dimensional water velocities using a boat‐mounted single‐beam sonar and acoustic Doppler current profiler (ADCP) was carried out in the vicinity of two submerged wing dikes located in the Lower Missouri River near Columbia, Missouri. During high spring flows the wing dikes become submerged, creating a unique combination of vertical flow separation and overtopping (plunging) flow conditions, causing large‐scale three‐dimensional turbulent flow structures to form. On three different days and for a range of discharges, sampling transects at 5 and 20 m spacing were completed, covering the area adjacent to and upstream and downstream from two different wing dikes. The objectives of this research are to evaluate whether an ADCP can identify and measure large‐scale flow features such as recirculating flow and vortex shedding that develop in the vicinity of a submerged wing dike; and whether or not moving‐boat (single‐transect) data are sufficient for resolving complex three‐dimensional flow fields. Results indicate that spatial averaging from multiple nearby single transects may be more representative of an inherently complex (temporally and spatially variable) three‐dimensional flow field than repeated single transects. Results also indicate a correspondence between the location of calculated vortex cores (resolved from the interpolated three‐dimensional flow field) and the nearby scour holes, providing new insight into the connections between vertically oriented coherent structures and local scour, with the unique perspective of flow and morphology in a large river.

River regimes and recent hydrological changes in the Duero basin (Spain)

Summary River regimes are key tools in water management as they enable prediction of minimum and maximum flows throughout the year. In the Mediterranean region, flow regimes are very sensitive to climatic variations, and are likely to change with the declining precipitation and higher temperatures predicted for the future. This study explores variability in river regimes of the Duero basin (northern Spain), and changes therein over the period 1961–2006. Principal component analyses were used to classify the main types of flow regimes across the basin, and to identify the most common patterns in the monthly streamflow trends. Three types of river regime were identified: (1) a rainfall-based regime with a peak flow in winter, spatially corresponding to the middle and lower reaches of the rivers; (2) a snow–rainfall regime in the headwaters, with high flows in spring; and (3) a human-induced regime, with peak flows in summer because of release of water stored in spring. There was a marked reduction in river discharge in winter and spring over the course of the study period, leading to significant changes in both the magnitude and timing of flows. These results indicate that river regimes have changed in the direction of reduced flow volumes and shifts in peak flows. The causes appear to be decreasing winter precipitation and a reduction in the snowmelt component, presumably due because of increasing temperatures in winter and spring.

The Role of Environmental Factors in Determining Early Survival and Invasion Success of Exotic Brown Trout

AbstractDue to significant threats to native species posed by nonnative fishes, it is important to understand how species life history strategies interact with environmental conditions to explain the outcome of nonnative fish invasions. Brown troutSalmo truttaare prolific invaders but often exhibit upstream distributional limits in streams of the intermountain western United States. We used redd counts, embryo survival experiments, and temperature modeling to identify limits to brown trout invasion. Brown trout spawned later than previously reported and established spawning areas in high‐elevation stream reaches (1,983‐m elevation), where adult recruitment is typically very low. While embryo survival was lower in high‐elevation, cooler‐water areas, these harsh overwinter conditions did not necessarily preclude hatching success (≥36%). However, model predictions based on winter temperature data indicate that during most years, brown trout fry probably would fail to emerge from the gravel before the onset of peak spring flooding in these high‐elevation reaches, suggesting that high spring flows could limit invasion success. A better understanding of mechanistic limits to invasion success across multiple life stages is crucial to predicting the future expansion of exotic fish species.

How aquatic insects live in cold climates

AbstractIn cold climates most aquatic habitats are frozen for many months. Nevertheless, even in such regions the conditions in different types of habitat, in different parts of one habitat, and from one year to the next can vary considerably; some water bodies even allow winter growth. Winter cold and ice provide challenges for aquatic insects, but so do high spring flows, short, cool summers, and unpredictable conditions. General adaptations to cope with these constraints, depending on species and habitat, include the use of widely available foods, increased food range, prolonged development (including development lasting more than one year per generation), programmed life cycles with diapause and other responses to environmental cues (often enforcing strict univoltinism), and staggered development. Winter conditions may be anticipated not only by diapause and related responses but also by movement for the winter to terrestrial habitats, to less severe aquatic habitats, or to different parts of the same habitat, and by construction of shelters. Winter itself is met by various types of cold hardiness, including tolerance of freezing in at least some species, especially chironomid midges, and supercooling even when surrounded by ice in others. Special cocoons provide protection in some species. A few species move during winter or resist anoxia beneath ice. Spring challenges of high flows and ice scour may be withstood or avoided by wintering in less severe habitats, penetrating the substrate, or delaying activity until after peak flow. However, where possible species emerge early in the spring to compensate for the shortness of the summer season, a trait enhanced (at least in some lentic habitats) by choosing overwintering sites that warm up first in spring. Relatively low summer temperatures are offset by development at low temperatures, by selection of warm habitats and microhabitats, and in adults by thermoregulation and modified mating activity. Notwithstanding the many abiotic constraints in cold climates, aquatic communities are relatively diverse, though dominated by taxa that combine traits such as cold adaptation with use of the habitats and foods that are most widely available and most favourable. Consequently, except in the most severe habitats, food chains and community structure are complex even at high latitudes and elevations, including many links between aquatic and terrestrial habitats. Despite the complex involvement of aquatic insects in these cold-climate ecosystems, we know relatively little about the physiological and biochemical basis of their cold hardiness and its relationship to habitat conditions, especially compared with information about terrestrial species from the same regions.

Annual and spatial variation for macroinvertebrates in the Upper Mississippi River near Cape Girardeau, Missouri

We sampled macroinvertebrates at four sites in the Open River section of the Upper Mississippi River near Cape Girardeau, Missouri (Rkm 106-114). In 1999, 2000, and 2002 - 2004, macroinvertebrates were sampled in the fine sediments downstream of four wing dikes with a Ponar dredge. In 1999 and 2001, macroinver- tebrates on rocks were sampled on the upstream face of those same wing dikes with rock baskets. We identified 68 taxa in the fine sediments and 50 taxa on the rocks, with only 27 taxa being found in both. In both habitats, annual variability in population densities and community structure exceeded spatial variability. Macroinvertebrate densities in the fine sediments ranged from approximately 3700 to 11,700 individuals/m 2 with oligochaetes com- prising 77- 95 % of total density. Non-metric Multidimensional Scaling ordination suggests that annual differ- ences in the fine sediment assemblage reflect variation in hydrological conditions (high spring flow and low an- nual flow). Macroinvertebrate density on the rocks ranged from approximately 57,800 to 163,000 individuals/m 2 with hydropsychid caddisflies (Hydropsyche bidens/orris and Potamyia flava) comprising 82 - 97 % of total den- sity. This dominance of oligochaetes in fine bottom substrates and caddisflies on hard substrates is consistent with many other studies on big rivers in the Mississippi River Basin (e. g., Upper and Lower Mississippi, Ohio, Il- linois, and Missouri Rivers), and may be the current reference condition for these big rivers. This makes tradi- tional water quality monitoring and assessment efforts using macroinvertebrates more difficult because increased dominance by more pollution tolerant taxa is perceived as evidence of water or habitat degradation. However, macroinvertebrate assemblages in our study reach seem less stressed than other big rivers known to be impaired. Specifically, densities of the pollution-sensitive mayfly Hexagenia indicate that conditions in our study reach have improved markedly since the 1960's.