Spring benthic macroinvertebrate communities of selected streams in the High Caucasus (Azerbaijan SSR)

Investigations on invertebrate fauna were carried out at fifteen sites in some chosen streams of the Caucasus (Azerbaijan SSR) in March 1970. Seventy-five taxa of invertebrates were found in the investigated streams. Chironomidae constituted the most numerous group at all sites, Ephemeroptera, Plecoptera, and Simuliidae being other important components of the fauna. On the basis of percentage structure of dominance in the examined streams four types of faunistic communities were distinguished. The most common one was the type characteristic of high mountain streams and rivers with Diamesa sp. (gr. latitarsis) predominant. Nevertheless, in springtime this community was characterized by a greater number of taxa and a greater abundance of specimens as compared with the summer period. It may be thus assumed that the spring period in high mountain streams and rivers is more conducive to the development of the invertebrate fauna.     

Relationships between benthic macrofauna and biogeochemical properties of sediments at different spatial scales and among different habitats in mangrove forests

In muddy intertidal sediments, there are reported complex interactions between the biological, physical and chemical properties of the sediment and the benthos that live in and on it. These are expected to be reflected in strong relationships between benthic animals and particular properties of the sediment, although some research has shown these relationships to be relatively weak. This study investigates the relationships between benthic macrofauna and biogeochemical properties of sediments within and among different habitats in multiple mangrove forests in a temperate estuary, in order to address the generality of any such relationships. Matched samples of benthos and sediment were collected from three habitats, which differed with respect to shading, the amounts of algae and leaf litter and the presence of pneumatophores. The sediment was sampled for water content, grain size, organic matter, chlorophylls a and b and colloidal and total carbohydrate.Spatial variation in sediment and benthos were significantly correlated across habitats in two of the three bays, but the sediment properties that contributed to differences between habitats and those that best correlated with the benthos varied among bays. In all bays, the single taxon that best correlated with the sediment was spionid polychaetes, but correlations were generally weak. There was no generality in the patterns of variation among the benthos or sedimentary properties among habitats. The benthos differed significantly among bays in all habitats, with large variation within and among sites. The sediment varied significantly at small scales in all habitats, but significant differences among bays were only found in two habitats. All spatial scales contributed to the total amount of variability in the sediment and there was little predictability from the patterns shown in one habitat to those in other habitats, or from one component of sediment to other components. Such variability suggests that there may be structural redundancy in this fauna, with different components of the benthos contributing to similar functions in different places. Such variability must be considered in experiments designed to understand relationships between ecological structure and function in these complex habitats and in any sampling studies to identify environmental impacts in these habitats.     

Spatial patterns and ecology of benthic communities on a high-latitude South Florida (Broward County,USA) reef system

High-latitude reef communities consisting of typical Caribbean fauna of variable composition and density exist on four parallel ridges at varying depths along the Broward County (Florida, USA) coast. Two of these ridges, at 7–13 and 15–30 m in depth, are drowned early Holocene coral reefs of 5 and 7 ky uncorrected radiocarbon age. In this study, community data were collected on each reef using 50-m, line-intercept transects. Using multivariate non-metric statistics, the data show distinct differences in benthic community structure across several spatial gradients. Diversity indices revealed that while all values were low, species diversity (H) was consistent throughout the county, and species richness (d) and evenness (J) increased along a north-south gradient. Scleractinian coral (27 species) cover was low (<6%) in all areas, and Montastrea cavernosa dominated as the major hermatypic scleractinian. Notably absent was the major Caribbean reef-builder Acropora palmata. A rich alcyonacean fauna was present on all studied reefs, and these were typically the most important faunal group determining community structure. Although during the 2-year period 2000–2002 mean water temperatures never fell below 20 °C, scleractinia were small in size.     

Modification of benthic insect communities in polluted streams: combined effects of sedimentation and nutrient enrichment

Responses of the benthic insect community of a southern Appalachian trout stream to inorganic sedimentation and nutrient enrichment were monitored over a period of eight months. Entry of pollutants from point sources established differentially polluted zones, allowing an assessment of impacts due to sedimentation alone and in association with elevated nutrient levels. Input of sediment resulted in a significant increase in bed load and decrease of pH at the substrate-water interface (P < 0.05). The zone receiving nutrient runoff from livestock pasture exhibited elevated levels of nitrate and phosphate, but available data indicated such concentrations to be quite low. Species richness, diversity, and total biomass of filter feeding Trichoptera and Diptera, predaceous Plecoptera, and certain Ephemeroptera were significantly reduced in the polluted zones. Inorganic sedimentation, operating indirectly through disruption of feeding and filling of interstitial spaces, was considered to be the primary factor affecting filter feeding taxa. Decomposition of compounds associated with materials in the bed load may depress pH and eliminate acid sensitive species of Plecoptera and Ephemeroptera. Such processes of acidification may be particularly important to Appalachian streams since the pH of regional surface waters is characteristically acidic prior to sedimentation. Accumulation of particles on body surfaces and respiratory structures, perhaps as a function of wax and mucous secretion or surface electrical properties, appears to be the major direct effect of inorganic sedimentation on stream insects. Growths of the filamentous bacterium Sphaerotilus natans were also frequently associated with silted individuals in the zone receiving nutrient addition. Distribution of the bacterium suggested that silted substrates, perhaps as related to the presence of iron compounds, are required for colonization in dilute nutrient solutions. The primary effect of Sphaerotilus colonies appears to be augmentation of particle accumulation through net formation by bacterial filaments. Data indicate that inorganic sedimentation and nutrient addition operate synergistically, eliminating a significantly greater number of taxa than exposure to one pollutant alone.     

Atyid shrimps (Decapoda: Atyidae) influence the spatial heterogeneity of algal communities over different scales in tropical montane streams, Puerto Rico

1. Atyid (Decapoda: Atyidae) shrimps influence the distribution of algal communities over different scales in tropical montane streams of Puerto Rico. Within pools of an atyid-dominated stream, atyid shrimps enhanced patchiness in algal communities along the depth gradient. Algal bands occurred in shallow pool margins where atyids did not forage (< 3 cm below water surface), with significantly greater standing crop, taxon richness, and structural complexity than deeper areas. In deeper water, atyids reduced small-scale patchiness in algal community composition and maintained a low-growing understorey turf dominated by sessile diatoms (Bacillariophyta) and, sometimes, closely cropped, filamentous blue-greens (Cyanophyta).
2. Among pools of the atyid-dominated stream, atyids interacted with light to determine algal patchiness between stream margins and deeper areas. In sunny pools, algal standing crop was 140-fold greater in pool margins than in deeper areas where atyids foraged. In shaded pools, however, standing crop in pool margins was only 5-fold greater than in deeper areas. Effects of light on algal standing crop were greater outside atyid foraging areas than within, indicating that shrimp grazing overrides the positive effects of light.
3. In contrast to the atyid-dominated stream, algal communities in an atyid-poor stream were characterized by a high biomass of loosely attached epipelic diatoms and no depth zonation. Interstream rock and shrimp transplant experiments indicated that atyids significantly reduced algal standing crop and altered community composition on rocks from atyid-poor streams within 24 h. Results support the hypothesis that atyid shrimps play a major role in determining observed interstream differences in algal communities.     

Atyid shrimps (Decapoda: Atyidae) influence the spatial heterogeneity of algal communities over different scales in tropical montane streams, Puerto Rico

1. Atyid (Decapoda: Atyidae) shrimps influence the distribution of algal communities over different scales in tropical montane streams of Puerto Rico. Within pools of an atyid-dominated stream, atyid shrimps enhanced patchiness in algal communities along the depth gradient. Algal bands occurred in shallow pool margins where atyids did not forage (< 3 cm below water surface), with significantly greater standing crop, taxon richness, and structural complexity than deeper areas. In deeper water, atyids reduced small-scale patchiness in algal community composition and maintained a low-growing understorey turf dominated by sessile diatoms (Bacillariophyta) and, sometimes, closely cropped, filamentous blue-greens (Cyanophyta).
2. Among pools of the atyid-dominated stream, atyids interacted with light to determine algal patchiness between stream margins and deeper areas. In sunny pools, algal standing crop was 140-fold greater in pool margins than in deeper areas where atyids foraged. In shaded pools, however, standing crop in pool margins was only 5-fold greater than in deeper areas. Effects of light on algal standing crop were greater outside atyid foraging areas than within, indicating that shrimp grazing overrides the positive effects of light.
3. In contrast to the atyid-dominated stream, algal communities in an atyid-poor stream were characterized by a high biomass of loosely attached epipelic diatoms and no depth zonation. Interstream rock and shrimp transplant experiments indicated that atyids significantly reduced algal standing crop and altered community composition on rocks from atyid-poor streams within 24 h. Results support the hypothesis that atyid shrimps play a major role in determining observed interstream differences in algal communities.     

Relationships among biological elements (macrophytes, macroinvertebrates and ichthyofauna) for different core river types across Europe at two different spatial scales

The objective of this study was to evaluate differences in correlations among Biological Elements and environmental parameters for different river types, analysed at two different spatial scales. A total of 82 sites, with at least good ecological status, were sampled across Europe, representing three core river types: Mountain rivers (26 sites); Lowland rivers (29 sites) and Mediterranean rivers (17 sites). At each site samples of macrophytes, macroinvertebrates and fishes were taken during spring, following the methodological procedures established by the European STAR project. Environmental parameters were also recorded, based on a site protocol developed by the European projects AQEM and STAR. Environmental parameters were divided into three categories: aquatic habitats (mesohabitat scale), global features (reach scale) and obligatory typology parameters of Water Framework Directive (WFD) (geographical scale). Data were analysed to evaluate at the two scales, first, relationships among biological elements, and second, relationships between biological elements and environmental parameters. Within each river type, correlation matrices (Bray–Curtis distance) were calculated separately for each biological element and for each category of environmental parameters. All biological elements were correlated (p<0.01) to the larger spatial scale: macrophytes and macroinvertebrates are more correlated in lowland and mountain rivers, while in Mediterranean rivers, fish and macrophytes presented higher correlations. These links tend to be consistent for different spatial scales, except if they are weak on a larger regional scale, obligatory parameters of WFD were, in most cases, significantly correlated with the three biological communities (p<0.05). Results at different spatial scales supported the hierarchical theory of river formation. Reach and mesohabitat environmental parameters tend to explain aquatic communities at a lower spatial scale, while geographical parameters tend to explain the communities at a major spatial scale.     

Community structure of benthic invertebrates in interior Alaskan (USA) streams and rivers

Taxonomic composition of benthic invertebrates in interior Alaskan streams and rivers is summarized from published and unpublished data. Diptera dominate the Alaskan stream fauna and constitute a larger proportion of the benthos in Alaskan streams than in streams of temperate North America. Plecoptera and Ephemeroptera are the next most abundant in Alaskan streams with Trichoptera generally very scarce. Several orders that occur regularly in streams of temperate North America are absent (or in very low abundance) in interior Alaskan streams: Hemiptera, Odonata, Megaloptera, Coleoptera. Net-spinning caddisflies, burrowing mayflies, and several families of stoneflies (Pteronarcyidae, Peltoperlidae and Perlidae) are conspicuous by their absence or extreme scarcity. Taxonomic composition varies significantly among hydrologic regions (major watersheds) and among stream types (springs, headwater streams, small rivers, and large rivers). Only two taxa (Chironomidae and Nemouridae) significantly increase in proportional contribution from south to north while many taxa decrease.     

Trophic models of four benthic communities in Tongoy Bay (Chile): comparative analysis and preliminary assessment of management strategies

Steady-state trophic flow models of four benthic communities (seagrass, sand-gravel, sand and mud habitats) were constructed for a subtidal area in Tongoy Bay (Chile). Information of biomass, catches, food spectrum and dynamics of the commercial and non-commercial populations was used and the ECOPATH II software of Christensen and Pauly [Ecol. Modell. 61 (1992a) 169] was applied. The sea star Meyenaster gelatinosus and the crabs Cancer polyodon, C. porteri and Paraxanthus barbiger were found to be the most prominent predators in the benthic system. The scallop Argopecten purpuratus as well as other bivalves represented the principal secondary producers in the seagrass, sand-gravel and sand habitats, while the Infauna dominated the mud habitat. The highest total biomass and system throughput (33579.3 t/km²/year) was calculated for the sand-gravel habitat. The sand habitat had a negative net system production due to the amount of primary production imported from deeper waters to satisfy the food requirements of the large beach clam (Mulinia sp.) populations. The mean trophic level of the fishery varied between 2.06 (sand-gravel) and 3.92 (sand) reflecting the fact that the fishery concentrates on primary producers (i.e. algae and filter feeding), and on top predators (i.e. snails and crabs). Fishery is strongest in sand-gravel habitat, where annual catches amount to 122.05 g/m². Low values of the relative Ascendency (A/C) (from 27.4 to 32.7%) suggest that the systems analysed are immature and highly resistant to external perturbations. Manipulations of the input data for the exploited species suggest that seagrass and sand-gravel habitats have a potential for a ∼3 times higher than the present production of scallops and the red algae Chondrocanthus chamissoi. Preliminary results of Mixed Trophic Impacts (MTI) analysis suggest that any management policy aimed at a man-made increase in the standing stocks of A. purpuratus and Ch. chamissoi in seagrass and sand-gravel habitats, and a removal of the seastar M. gelatinosus in the seagrass habitat appears justified.     

Multivariate analysis of microbial communities in the River Elbe (Germany) on different phylogenetic and spatial levels of resolution

The microbial communities of three different habitat types and from two sediment depths in the River Elbe were investigated by fluorescence in situ hybridization at various levels of complexity. Differences in the microbial community composition of free-flowing river water, water within the hyporheic interstitial and sediment-associated bacteria were quantitatively analyzed using domain- and group-specific oligonucleotide probes. Qualitative data on the presence/absence of specific bacterial taxa were gathered using genus- and species-specific probes. The complete data set was statistically processed by univariate statistical approaches, and two-dimensional ordinations of nonmetric multidimensional scaling. The analysis showed: (1) that the resolution of microbial community structures at microenvironments, habitats and locations can be regulated by targeted application of oligonucleotides on phylogenetic levels ranging from domains to species, and (2) that an extensive qualitative presence/absence analysis of multiparallel hybridization assays enables a fine-scale apportionment of spatial differences in microbial community structures that is robust against apparent limitations of fluorescence in situ hybridization such as false positive hybridization signals or inaccessibility of in situ oligonucleotide probes. A general model for the correlation of the phylogenetic depth of focus and the relative spatial resolution of microbial communities by fluorescence in situ hybridization is presented.     

The effects of between-habitat dispersal rate on protist communities and metacommunities in microcosms at two spatial scales

The effect of manipulation of between-habitat dispersal rates in multiple patch systems was examined experimentally using protist communities in laboratory microcosms. Replicate landscapes of eight microcosms (patches) at two spatial scales (patch sizes) were inoculated with 13 species of protists. Dispersal was carried out by transferring a small random sample of medium and protists from one randomly selected microcosm to another within a landscape. Four dispersal rates (24, 6, 2 and 0 transfers very 3 days) were used, and the microcosms were sampled after 6 and 12 weeks. Patch size had a consistent effect on within-path (community) and within-landscape (metacommunity) diversity, both being lower in small patch systems. Higher dispersal rates had a slight effect on community and metacommunity diversity after 12 weeks, with a tendency for higher dispersal to slightly offset the rate of loss of species. Both dispersal and patch size had effects on the abundance of many individual species, though in a variety of ways. The individual species results suggest that extinction is selective with respect to both patch size and dispersal rate treatments, and may be influenced by species interactions. It seems likely that in metacommunity systems of this sort, rather than mainland-island systems, the potential effect of between-patch dispersal rate in rescuing and recolonizing where local extinctions occur may be much reduced by the effect of selective extinction, relative to that expected under the assumption of random extinction.     

Ecological relationships between phytoplankton communities and different spatial scales in European reservoirs: implications at catchment level monitoring programmes

Phytoplankton communities are structured by factors acting over temporal and spatial scales. Identifying which factors are driving spatial patterns in aquatic communities is the central aim of ecology. In this study, data sets of phytoplankton communities and environmental data of two Portuguese reservoirs types (lowland “riverine reservoirs” and higher altitude “artificial lake reservoirs”) were used to determine the importance of environmental variables at different spatial (geographical, regional and local) and time scales (seasons, years) on the community structure. In all the data sets, the multivariate ordination technique Canonical Correspondence Analysis (CCA) showed that regional and local scales explained the majority (9–18% and 13–19%, respectively) of the taxa variance. However, for “riverine reservoirs”, time variables were more important, explaining 27% of the variability in phytoplankton assemblages. Variance partitioning was used to assess the individual importance of the three spatial scales and time for the community structure of the two reservoir types. The majority of among-site variability (5.9–21.4%) was accounted for by time variables, with local, regional, and geographical scale variables accounting for 3.3–5.6%, 3.7–4.5% and 2.6–2.9%, respectively. The effects of different spatial scales on phytoplankton communities were clearly interrelated; thus, implying that phytoplankton assemblages are capable of detecting stress from catchment to site scales. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. Padisak     

Structure and spatial variability of mayfly (Ephemeroptera) communities in the upper Hron River basin

Mayflies were sampled from eleven sites in six tributaries of the upper Hron River. Mayfly communities were ordinated and related to environmental factors using multiple regression and principal component analysis. Communities of headwater sites were dominated by Rhithrogena iridina which was gradually replaced by Baetis alpinus, at the lower reaches. Total mayfly species richness was inversely related to stream slope and overall abundance was inversely related to altitude. The prominent factor influencing mayfly communities was a complex longitudinal gradient as proposed by the river continuum concept. There were no significant differences in community composition and structure between the left-and right-hand tributaries of the Hron River.     

Spatial pattern of parasitism in Diatraea saccharalis Fab. (Lep., Crambidae) populations at two different spatial scales in sugarcane fields in Brazil

Abstract:  The biological control of Diatraea saccharalis is regarded as one of the best examples of successful classical biological control in Brazil. Since the introduction of the exotic parasitoid, Cotesia flavipes , the decrease in D. saccharalis infestation in sugarcane fields has been attributed to the effectiveness of this agent. Native Tachinidae fly parasitoids ( Lydella minense and Paratheresia claripalpis ) have also been implicated in the success. Quantitative data confirming the actual contribution of these agents to the control of D. saccharalis are, however, rather scant. The purpose of this study was to investigate the spatial pattern of parasitism of these parasitoids in D. saccharalis populations at two large spatial scales (fields and zones). To investigate this subject, a large data set comprising information collected from a sugarcane mill located in the state of São Paulo, Brazil (São João sugarcane mill) was analysed. When regressions between the proportion parasitism against host density were computed, the percentage of significant regressions with either a positive or a negative slope was very small at both spatial scales for both parasitoid species. Regressing the densities of tachinid-parasitized hosts against host densities per field showed that these parasitoids presented a 'moderate aggregative' response to host densities, as 53.33% of the regressions were positively significant. Cotesia flavipes was 'weakly aggregated' on host densities at the field level, because only 33.33% of the regressions were positively significant. At the zone level, neither aggregative nor spatial proportion parasitism responses were evident for either parasitoid species due to the small percentage of significant regressions computed.     

Effects of commercial trawling activities in the diet of the flat fish Citharus linguatula (Osteichthyes: Pleuronectiformes) and the starfish Astropecten irregularis (Echinodermata: Asteroidea)

This study assesses the effects of commercial trawling on benthic fauna, by analysing differences in the population structure and diet of benthic organisms inhabiting fishing grounds. The study is focused on two epifaunal species collected from a fishing ground in the north-western Mediterranean, which included a portion of un-trawled seabed used as a reference of undisturbed community. The selected species were the flatfish Citharus linguatula, more abundant at the undisturbed site and previously characterised as a surface predator and vulnerable to trawling, and the starfish Astropecten irregularis, which dominated the epibenthic community and had been characterised as an opportunistic species with low vulnerability to trawling. Both species were selective feeders and ingested similar prey species at each of the two sites, although differences in the relative prey abundance found in the stomachs occurred between sites. A. irregularis at the fished site mainly ingested gastropods, whereas higher proportion of bivalves was ingested at the undisturbed site. C. linguatula diet mainly composed of crustaceans at both sites, but there was an increase in the ingestion of small fish at the fished site. Moreover, prey ingestion of these two species generally increased with fishing activity. Trawling appears to modify the diet of these two species in terms of relative abundance of ingested prey, nevertheless, increased opportunism due to trawling disturbance was not detected and the density of these predators over fishing grounds was more closely related to their vulnerability to trawling.     

Microhabitat and morphometric variation of two Chaetophoracean (Chaetophorales, Chlorophyta) species in tropical streams of southeastern Brazil

Two populations of Chaetophora elegans (Roth) C. Agardh and two of Stigeoclonium helveticum Vischer were investigated for microhabitat characteristics and morphological variation in streams of São Paulo State, southeastern Brazil. Different patterns of microhabitat distribution were found between species investigated. Populations of C, elegans were distributed under relatively narrow microhabitat conditions (high irradiance, low depth, moderate to high current velocity, rocky substrata and lower values of niche width) and showing little morphometric variation (colony diameter, main axis cell size, and apical branch number), Stigeoclonium helveticum occurred under more diverse microhabitat conditions, revealed by lack of significant difference between sampling units with and without the alga and wider niche width, but also exhibited relatively narrow morphometric variation (plant length, main axis cell and ateral branch cell sizes). The narrow microhabitat conditions and smaller niche width of C. elegans can explain its low abundance (percentage cover) in streams from the area studied as well as in other regions of São Paulo State, In contrast, the wider variation of microhabitat conditions and the higher niche widths of S. helveticum suggest that this green alga is able to grow in a high number of stream ecosystems in the region investigated, ranging from undisturbed to highly disturbed habitats. Thus. the results suggest that S. helveticum is a generalist species.     

Population genetic structure of three freshwater mussel (Unionidae) species within a small stream system: significant variation at local spatial scales

1. Unionid mussels are highly threatened, but little is known about genetic structure in populations of these organisms. We used allozyme electrophoresis to examine partitioning of genetic variation in three locally abundant and widely distributed species of mussels from a catchment in Ohio. 2. Within‐population variation was similar to that previously reported for freshwater mussels, but genotype frequencies exhibited heterozygote deficiencies in many instances. All three species exhibited significant among‐population variation. Evidence of isolation‐by‐distance was found in Elliptio dilatata and Ptychobranchus fasciolaris, while Lampsilis siliquoidea showed no geographical pattern of among‐population variation. 3. Our results suggest that the isolating effects of genetic drift were greater in L. siliquoidea than in the other species. Differentiation of populations occurred at a much smaller spatial scale than has previously been found in freshwater mussels. Differences among species may reflect differences in the dispersal abilities of fishes that serve as hosts for the glochidia larvae of mussels. 4. Based on our results, we hypothesise that species of mussels that are common to large rivers exhibit relatively large amounts of within‐population genetic variation and little differentiation over large geographical distances. Conversely, species typical of small streams show lower within‐population genetic variation and populations will be more isolated. If this hypothesis can be supported, it may prove useful in the design of conservation strategies that maintain the genetic structure of target species.     

Macrofauna communities of the Dogger Bank (central North Sea) in the late 1990s: spatial distribution,species composition and trophic structure

Macrofauna samples were taken in May 1996–1998 at 28 stations on the Dogger Bank (central North Sea) to describe species composition, spatial distribution and trophic structure of the macrofaunal communities. In general, five communities could be distinguished depending on the influence of different water masses, depth, type of sediment and food availability. Except for those species which feed in the sediment, macrofauna communities did not show any correlation to the measured sediment variables. In fact, macrofauna communities of the Dogger Bank were mainly structured by lateral food supply via frontal systems. This was indicated by the dominance of interface feeders foraging in the benthic boundary layer, which are able to switch from surface deposit to suspension feeding depending on flow conditions. The importance of sandlickers in the shallowest parts of the Dogger Bank hints at an important contribution of benthic primary production to the nutritional and energetic needs of the benthic community. Electronic Publication     

Variation in the density of questing Ixodes pacificus (Acari:Ixodidae) nymphs infected with Borrelia burgdorferi at different spatial scales in California

The density of, and prevalence of infection with Borrelia burgdorferi in, Ixodes pacificus nymphs as well as the density of infected nymphs were compared at 12 properties at a small rural community at high risk for Lyme disease (CHR) and at 12 areas at the University of California Hopland Research and Extension Center (HREC), Mendocino County, California. The mean infection prevalence and density of infected nymphs were 1.7% (range, 0-4.2%) and 0.10 infected nymphs per 100 m2 (range, 0-0.23 per 100 m2) at the HREC, and 12.4% (range, 3.9-41.3%) and 1.83 infected nymphs per 100 m2 (range, 0.29-22.17 per 100 m2) at the CHR. Thus, the mean density of infected nymphs differed 18-fold between CHR and HREC and 76-fold between properties at the CHR. Also, there was up to 10-fold variation in infection prevalence and 16-fold variation in density of infected nymphs between discrete areas within properties at the CHR. The high densities of infected nymphs recorded at the CHR suggest that, despite the low statewide incidence of Lyme disease, the medical community should be alerted that Lyme disease can be highly endemic in rural areas of northwestern California. The prevalence of spirochetal infection was higher for nymphs collected in southern/western, as compared to northern/eastern, exposures at both HREC and CHR. Infection prevalence and nymphal density were negatively associated at the HREC, whereas they tended to be associated positively at the CHR. A positive association was observed between nymphal density and density of infected nymphs when data from CHR and HREC were combined, and when data from the CHR were considered alone, but not for data from the HREC alone.