Temporal Variation In Density Research Articles

The effect of sampling frequency and spatial and temporal variation in the density of fecal indicator bacteria on the assessment of coastal bathing water quality

The time and location of sampling as well as the number of samples per season can influence a reliable assessment of bathing water quality. In this study, we investigated the spatio-temporal variation of fecal indicator bacteria (FIB) density and the effects of FIB variability and sampling frequency on the assessment of a single sample and the annual and final assessment of coastal bathing water quality. Increasing the number of samples from 10 to 20 per bathing season had a significant impact on bathing water quality assessment at sites where water quality fluctuations had previously been observed, resulting in a change in water quality to a lower category in 36 % of annual and 54 % of final assessments, suggesting that the minimum number of samples per season should be increased at such sites. Increasing the number of samples at sites assessed as excellent over a longer period had no impact on water quality assessment.Spatial and temporal variations in FIB density were significant at a considerable number of sites both in the single sample assessment and in the annual and final assessment. Bathing water quality was spatially unrepresentative at a quarter of the sites analyzed and temporally unrepresentative at a fifth, as there was at least one additional point with a lower bathing water quality than the official sampling point or the bathing water quality was lower in the afternoon than in the morning.When revising the current recreational water quality regulations, the impact of sampling frequency of and of spatio-temporal variation of FIB density on the relevance of bathing water quality assessment at sites subjected to pollution needs to be seriously considered.

Temporal Variations in Density and Distribution of Neutrophils with Associated Morphological Changes in Uterus at All Stages of Estrus Cycle in Mice

Temporal Variations in Density and Distribution of Neutrophils with Associated Morphological Changes in Uterus at All Stages of Estrus Cycle in Mice

Anthropogenic and climatic drivers of population densities in an African savanna ungulate community

AbstractMany ungulate species in Africa range in habitats that vary in type and quality over space and time, but ongoing environmental change is substantially altering their habitats. Identifying key environmental variables that regulate ungulate population densities can guide management actions for effective conservation. We studied the local population density responses of a community of sympatric ungulate species in the Tarangire Ecosystem of northern Tanzania, to a suite of environmental factors that vary over space and time, to quantify population trends, determine the primary environmental correlates of densities, and identify covariation in densities among species. We estimated seasonal densities of five commonly detected species (impala, dik–dik, Grant's gazelle, eland, and waterbuck) based on 7 years of distance‐sampling data from 41 replicate surveys of 237 line transects. We systematically analyzed the effects of spatial, seasonal, and annual environmental covariates on variation in transect species‐specific densities across space and time. Large fluctuations in climatic factors mediated highly synchronous temporal density variation among all species. We documented more spatial than temporal variation in four of the five species, suggesting that spatial heterogeneity may provide some buffer against temporal variation in the environment. Protection of sufficient habitats and water sources should allow ungulates to respond to a temporally changing world by moving across space. Further, among‐species covariation patterns identified two potential ungulate guilds (impala—dik–dik—waterbuck; eland—Grant's gazelle) that should aid in developing efficient and coordinated management actions.

Resolving the variability in habitat use by juvenile small pelagic fish in a major tidal system by continuous echosounder measurements

Shallow coastal areas are important nurseries for larvae and juveniles of many fish species. However, empirical data on small pelagic fish (SPF) in such regions are lacking, and temporal variability in abundance and habitat use by SPF are unknown. Given the critical role of SPF as a trophic link, their commercial value and their sensitivity to climate variability, there is a need for a quantitative and high-resolution monitoring method. We deployed a bottom-mounted echosounder, combined with a water current profiler, to examine the temporal variation in density, vertical distribution and movement behaviour of SPF in the Marsdiep, a dynamic tidal inlet between the North Sea and the Wadden Sea. The acoustic setup provided year-round records of fish density distribution in the water column every 90 min. Monthly fish samples were collected to help interpret acoustic observations. Our analyses indicated that the Marsdiep is a migration gateway for SPF from the North Sea into the Wadden Sea, particularly for juvenile herring and sprat. We observed clear seasonality with high fish densities from mid-spring to early autumn. Fish typically used the upper half of the water column. Swimming behaviour was primarily driven by currents. Yet, during outgoing tide, SPF resisted the outflowing current, suggesting efforts to remain in the Wadden Sea, supporting the hypothesis that the Wadden Sea serves as a preferred nursery area for SPF. Our high frequency, multi-seasonal and vertically resolved observations provide unique insight into the use of the Marsdiep by SPF. Furthermore, potential applications of autonomous echosounder moorings are discussed.

Distribution, abundance and sighting patterns of multiple stocks of bottlenose dolphins (Tursiops truncatus) in coastal Virginia waters

Three stocks of bottlenose dolphins (Tursiops truncatus) are currently known to co‐occur in the coastal waters off Virginia. A combination of visual line‐transect survey and photo‐identification (photo‐ID) methods was used to gather information on the occurrence, distribution and density of bottlenose dolphins in the coastal areas of Norfolk and Virginia Beach. Sixty‐one line‐transect surveys were completed in two zones (inshore and offshore) between August 2012 and August 2015, with on‐effort coverage of 6,550 kilometres (km) of trackline over 349.6 hours. Conventional line‐transect analysis of bottlenose dolphin sightings showed both spatial and temporal variation in density (D) and abundance (N), with highest density in the inshore zone during warm‐water months (June–November). Inshore densities were calculated as 3.51 individuals per square kilometre (km2) (N = 1,090) in the warm‐water season and 0.73 individuals/km2 (N = 225) in cool‐water months (December–May). Densities in the offshore zone were 1.24 individuals/km2 (N = 741) in the warm‐water season and 0.61 individuals/km2 (N = 366) in the cool‐water season. Warm‐water density peaks support previous research; however, this study documents a cold‐season presence of dolphins in both zones not previously reported. Twenty‐seven photo‐ID surveys were completed and a photo‐ID catalogue was created using photos taken during both dedicated photo‐ID and line‐transect surveys, containing 1,335 unique individuals. Re‐sighting numbers were low, with less than 17% of individuals documented on more than one occasion, and only 4% sighted on three or more occasions, with a maximum of 4 re‐sightings. Days between re‐sightings ranged from 5 to 970, though distance between re‐sightings spanning greater time was minimal (less than 25km). Sighting locations of photo‐IDed dolphins were broken into three regions to investigate seasonal and spatial patterns of individual distribution; dolphins sighted only in the Cape Henry (CH) or Inner Bay (IB) region were observed mostly during summer and fall seasons (93% CH, 100% IB) with none occurring during winter. For the Outer Coastline (OC) region, the majority of sightings (88%) occurred during fall and winter. Additionally, there were two individuals sighted during winter months also sighted during summer more than 130km to the north inside Chesapeake Bay (matched to a different catalogue). These sightings support the possibility that the current stock structure does not account for all dolphins in the region. Stock structure as currently understood is complicated, with overlapping spatial distribution of three stocks that separate seasonally. Our study provides information that may be useful for management, but also additional findings that could impact future stock designations

Monitoring partially marked populations using camera and telemetry data.

Long-term monitoring is an important component of effective wildlife conservation. However, many methods for estimating density are too costly or difficult to implement over large spatial and temporal extents. Recently developed spatial mark-resight (SMR) models are increasingly being applied as a cost-effective method to estimate density when data include detections of both marked and unmarked individuals. We developed a generalized SMR model that can accommodate long-term camera data and auxiliary telemetry data for improved spatiotemporal inference in monitoring efforts. The model can be applied in two stages, with detection parameters estimated in the first stage using telemetry data and camera detections of instrumented individuals. Density is estimated in the second stage using camera data, with all individuals treated as unmarked. Serial correlation in detection and density parameters is accounted for using time-series models. The two-stage approach reduces computational demands and facilitates the application to large data sets from long-term monitoring initiatives. We applied the model to 3 years (2015-2017) of white-tailed deer (Odocoileus virginianus) data collected in three study areas of the Big Cypress Basin, Florida, USA. In total, 59 females marked with ear tags and fitted with GPS-telemetry collars were detected along with unmarked females on 180 remote cameras. Most of the temporal variation in density was driven by seasonal fluctuations, but one study area exhibited a slight population decline during the monitoring period. Modern technologies such as camera traps provide novel possibilities for long-term monitoring, but the resulting massive data sets, which are subject to unique sources of observation error, have posed analytical challenges. The two-stage spatial mark-resight framework provides a solution with lower computational demands than joint SMR models, allowing for easier implementation in practice. In addition, after detection parameters have been estimated, the model may be used to estimate density even if no synchronous auxiliary information on marked individuals is available, which is often the case in long-term monitoring.

An open spatial capture-recapture model for estimating density, movement, and population dynamics from line-transect surveys.

The purpose of many wildlife population studies is to estimate density, movement, or demographic parameters. Linking these parameters to covariates, such as habitat features, provides additional ecological insight and can be used to make predictions for management purposes. Line‐transect surveys, combined with distance sampling methods, are often used to estimate density at discrete points in time, whereas capture–recapture methods are used to estimate movement and other demographic parameters. Recently, open population spatial capture–recapture models have been developed, which simultaneously estimate density and demographic parameters, but have been made available only for data collected from a fixed array of detectors and have not incorporated the effects of habitat covariates. We developed a spatial capture–recapture model that can be applied to line‐transect survey data by modeling detection probability in a manner analogous to distance sampling. We extend this model to a) estimate demographic parameters using an open population framework and b) model variation in density and space use as a function of habitat covariates. The model is illustrated using simulated data and aerial line‐transect survey data for North Atlantic right whales in the southeastern United States, which also demonstrates the ability to integrate data from multiple survey platforms and accommodate differences between strata or demographic groups. When individuals detected from line‐transect surveys can be uniquely identified, our model can be used to simultaneously make inference on factors that influence spatial and temporal variation in density, movement, and population dynamics.

How does urban population density decline over time? An exponential model for Chinese cities with international comparisons

It is widely observed urban land expands faster than population grows, which results in declining urban population densities over time. However, it is not known how densities decline. By taking 35 major Chinese cities as examples, we propose an exponential model to quantify the temporal decline in urban population densities from 2001 to 2015. The average adjusted R2 of the exponential model for 35 cities is 0.68, indicating a moderate but powerful correlation with temporal variations in densities. The exponential model implies that the temporal decline in densities in cities is occurring at a constant but differentiated rate. Apart from Beijing and Shanghai, the annual decline rates vary from 1% to 9%, with an average of 4.65%, which means urban population densities declined by 4.65% per annum from 2001 to 2015. The bivariate correlation analysis and multiple regression model reveal that the initial urban population density and the growth rates of urban land, population, and GDP in a city are significantly correlated with the decline rates and that these four factors can explain 72.3% of the variance in the decline rates among cities. We further verified the transferability and applicability of the exponential model by applying it to 20 U.S. cities. Using a global dataset, we found the temporal decline rate in densities in Chinese cities was greater than those in India and other countries or regions, which highlights the necessity of efficient urban development strategies to slow down the temporal decline in urban population densities in China.

Spatial and temporal variations in cyanobacteria and microcystins in Aha Reservoir, Southwest China

Toxic cyanobacteria (TCB) are well-known worldwide for their adverse impacts on humans. Species compositions and seasonal variations of TCB in reservoirs depend on interactions between physical and chemical factors. This study was conducted to evaluate the water quality in the Aha Reservoir, Southwest China, focusing on cyanobacteria and cyanotoxins. Water samples were collected weekly or biweekly from May to September of 2015 and used to delineate temporal variations in density and distribution of toxic cyanobacteria and cyanotoxins in the reservoir. Toxic cyanobacteria identified consisted of Aphanizomenon flos-aquae, Pseudanabaena limnetica, Cylindrospermopsis sp., and Microcystis sp., with Aphanizomenon flos-aquae and Pseudanabaena limnetica being the most common and significant toxic genera. The total biomass of cyanobacteria was 17.0 mg/L. Identification and quantification of microcystin variants were conducted by high performance liquid chromatography (HPLC) using a system equipped with a photodiode array detector. Microcystin levels were between 0-3.0 μg/L, MC-RR was around 0-3.0 μg/L and MC-LR was approximately 0-0.9 μg/L. Overall, the results of this study indicate that the investigated reservoirs should be monitored regularly to minimize potential health risks to the human population.

Spatial and temporal distribution of ichthyoplankton during an unusual period of low flow in a tributary of the São Francisco River, Brazil

AbstractFlow‐dependent fish specialists require specific conditions for reproduction, so the success and reproductive intensity of these animals are determined by the flood regime. Thus, this study investigated the spatial and temporal reproductive patterns of fish, especially migratory Prochilodus species (flow‐dependent specialists) in an unusual period of low flow in the Pará River sub‐basin, one of the main tributaries of the upper São Francisco River. For this, ichthyoplankton collections were carried out between November 2013 and February 2014. Data were analysed considering the spatial and temporal variations in density of eggs and larvae for the upper, middle, and lower portions of the Pará River sub‐basin, and correlating this to some environmental variables. The results showed that the small headwater stretch of the Pará River is one relevant spawning area for migratory fish species. However, this area is isolated by the Cajuru reservoir, which makes it uncertain the recruitment of these embryos, due to interruption of natural drift between spawning/nursery areas caused for reservoirs. Larvae of newly hatched migratory species found in tributaries of the lower Pará River sub‐basin also indicate these species use these tributaries as spawning grounds and migratory routes. The period in which the research was conducted represented the most atypical low flow, one in the last 75 years, resulting in the low variability in the environmental parameters. Although few parameters increased briefly in this low flow period influenced by greater rainfall in December, this precipitation was possibly responsible for the final gonadal maturation and spawning of migratory species.

The response of the H geocorona between 3 and 8 <i>R</i><sub>e</sub> to geomagnetic disturbances studied using TWINS stereo Lyman-<i>α</i> data

Abstract. Circumterrestrial Lyman-α column brightness observations from 3–8 Earth radii (Re) have been used to study temporal density variations in the exospheric neutral hydrogen as response to geomagnetic disturbances of different strength, i.e., Dst peak values between −26 and −147 nT. The data used were measured by the two Lyman-α detectors (LAD1/2) onboard both TWINS satellites between the solar minimum of 2008 and near the solar maximum of 2013. The solar Lyman-α flux at 121.6 nm is resonantly scattered near line center by exospheric H atoms and measured by the TWINS LADs. Along a line of sight (LOS), the scattered LOS-column intensity is proportional to the LOS H column density, assuming optically thin conditions above 3 Re. In the case of the eight analyzed geomagnetic storms we found a significant increase in the exospheric Lyman-α flux between 9 and 23 % (equal to the same increase in H column density ΔnH) compared to the undisturbed case short before the storm event. Even weak geomagnetic storms (e.g., Dst peak values ≥ −41 nT) under solar minimum conditions show increases up to 23 % of the exospheric H densities. The strong H density increase in the observed outer exosphere is also a sign of an enhanced H escape flux during storms. For the majority of the storms we found an average time shift of about 11 h between the time when the first significant dynamic solar wind pressure peak (pSW) hits the Earth and the time when the exospheric Lyman-α flux variation reaches its maximum. The results show that the (relative) exospheric density reaction of ΔnH have a tendency to decrease with increasing peak values of Dst index or the Kp index daily sum. Nevertheless, a simple linear correlation between ΔnH and these two geomagnetic indices does not seem to exist. In contrast, when recovering from the peak back to the undisturbed case, the Kp index daily sum and the ΔnH essentially show the same temporal recovery.

Spatio‐temporal variation in density and size structure of the endangered sea cucumber Isostichopus fuscus in Huatulco National Park, Mexico

AbstractThe brown sea cucumber Isostichopus fuscus (Ludwig, 1875) has been subjected to strong fishing pressure and is currently considered an endangered species by the IUCN Red List of Threatened Species. Spatial and temporal variations in density were analysed and population parameters of I. fuscus were estimated in three localities of the Southern Mexican Pacific. Density was estimated using circular transects covering a 300 m2 area in each locality. Body length and weight were also measured. The mean density was 0.018 ± 0.013 individuals·m−2, and was statistically significantly lower during summer. The mean length and weight were 22.4 ± 3.8 cm and 396 ± 130 g, respectively, and statistically significant differences were found among localities. The weight–length relationship was W = 0.28·L1.32 and the mean relative condition index (Kn) = 1.04 ± 0.31. The von Bertalanffy growth equation was Lt = 38.7·(1–exp(−0.4·t)). The lowest values of length, weight and age were found at Dos Hermanas, suggesting that this locality could be favorable for the settlement of larvae and recruitment and growth of juveniles. The present study is the second to estimate densities of I. fuscus and the first one to publish data on size structure and growth for the population at Bahias de Huatulco. The densities were so low in the three sampled localities that none of them would be able to support a fishery, even under a sustainable management scheme. The information collected in the present study could be helpful for management programs of the Huatulco National Park, as all the data used for these currently come mainly from the populations of I. fuscus in the Gulf of California.

Constraining variable density of ice shelves using wide-angle radar measurements

Abstract. The thickness of ice shelves, a basic parameter for mass balance estimates, is typically inferred using hydrostatic equilibrium, for which knowledge of the depth-averaged density is essential. The densification from snow to ice depends on a number of local factors (e.g., temperature and surface mass balance) causing spatial and temporal variations in density–depth profiles. However, direct measurements of firn density are sparse, requiring substantial logistical effort. Here, we infer density from radio-wave propagation speed using ground-based wide-angle radar data sets (10 MHz) collected at five sites on Roi Baudouin Ice Shelf (RBIS), Dronning Maud Land, Antarctica. We reconstruct depth to internal reflectors, local ice thickness, and firn-air content using a novel algorithm that includes traveltime inversion and ray tracing with a prescribed shape of the depth–density relationship. For the particular case of an ice-shelf channel, where ice thickness and surface slope change substantially over a few kilometers, the radar data suggest that firn inside the channel is about 5 % denser than outside the channel. Although this density difference is at the detection limit of the radar, it is consistent with a similar density anomaly reconstructed from optical televiewing, which reveals that the firn inside the channel is 4.7 % denser than that outside the channel. Hydrostatic ice thickness calculations used for determining basal melt rates should account for the denser firn in ice-shelf channels. The radar method presented here is robust and can easily be adapted to different radar frequencies and data-acquisition geometries.

Muography sensitivity to hydrogeological rock density perturbation: roles of the absorption and scattering on the muon flux measurement reliability

ABSTRACTTomography using muons is a method to investigate the subsurface density. Muons are particles produced in the atmosphere, and their absorption depends on the quantity of matter they pass through, hence on the rock density and thickness. The Temporal Tomography of rock mass Density using Muons flux Measurement (T2DM2) project aims to characterise the spatial and temporal density variations of the first hundreds of metres of the subsurface. These variations can be induced by water transfer within the critical zone or other thermo‐hydro‐mechanical processes like fracturing. The imaging potential of muography and its application to hydrogeological processes are introduced. Numerical simulations are performed to estimate the significance of the muon flux fluctuations associated to rock density and composition variations for depths ranging from 60 to 1000 m. Particular attention is paid to muon scattering in rocks, which should be taken into account in order to achieve a good spatial and angular resolution.

Individual-area relationship best explains goose species density in wetlands.

Explaining and predicting animal distributions is one of the fundamental objectives in ecology and conservation biology. Animal habitat selection can be regulated by top-down and bottom-up processes, and is mediated by species interactions. Species varying in body size respond differently to top-down and bottom-up determinants, and hence understanding these allometric responses to those determinants is important for conservation. In this study, using two differently sized goose species wintering in the Yangtze floodplain, we tested the predictions derived from three different hypotheses (individual-area relationship, food resource and disturbance hypothesis) to explain the spatial and temporal variation in densities of two goose species. Using Generalized Linear Mixed Models with a Markov Chain Monte Carlo technique, we demonstrated that goose density was positive correlated with patch area size, suggesting that the individual area-relationship best predicts differences in goose densities. Moreover, the other predictions, related to food availability and disturbance, were not significant. Buffalo grazing probably facilitated greater white-fronted geese, as the number of buffalos was positively correlated to the density of this species. We concluded that patch area size is the most important factor determining the density of goose species in our study area. Patch area size is directly determined by water levels in the Yangtze floodplain, and hence modifying the hydrological regimes can enlarge the capacity of these wetlands for migratory birds.

Simulations of the muon flux sensitivity to rock perturbation associated to hydrogeological processes

Muon tomography is a method to investigate the in-situ rock density. It is based on the absorption of cosmic-ray muons according to the quantity of matter (thickness and density). Numerical simulations are performed in order to estimate the expected muon flux in LSBB Underground Research Laboratory (URL) (Rustrel, France). The aim of the muon measurements in the underground galleries of this laboratory is to characterize the spatial and temporal density variations caused by water transfer in the unsaturated zone of the Fontaine-de-Vaucluse karstic aquifer.

The contribution of abiotic and biotic factors to spatial and temporal variation in population density of the least killifish, Heterandria formosa

We report the results of a long-term examination of variation in adult density in the least killifish, Heterandria formosa, and the associations among adult density and a variety of ecological factors. We used data from 11 populations of H. formosa in Northwest Florida, collected between 2000 and 2010, to examine the relationships among temporal and spatial variation in adult density and (1) the composition of the aquatic fauna community among habitats (springs, ponds, and tidal marshes) where H. formosa are found, (2) an index of predation pressure and (3) the sampling season, distance from shore (cm), water depth (cm), and aquatic vegetation cover (%) in throw traps at each sampling event. We found clear evidence that adult densities of H. formosa vary widely but consistently among populations, with greater spatial variation than temporal variation in density estimates. Site identity explained most of the variation in density among populations, and there was no long-term consistent association of variation in density with different habitats; sites of the same habitat type were as likely to have characteristically different densities as were sites of different habitat types. Average adult densities of H. formosa increased as aquatic vegetation cover increased and as the index of predation pressure decreased, however assemblage structure was not a strong predictor of density. These results show that despite marked community, abiotic and biotic distinctions between habitat types, location-specific variation was the predominant signal in these data.

Spatial and temporal variations in densities of small fishes across different temporary floodplain types of the lower Okavango Delta, Botswana

Small-sized and juvenile fishes as well as physicochemical water parameters in various primary, secondary and rarely flooded temporary floodplains of the Okavango Delta were assessed during the different hydrological phases of the 2009–2010 flooding season. Small fishes were sampled in the marginal zone of the floodplains using a throw-trap net and a suite of physicochemical properties were measured. Both physicochemical water quality parameters and densities of small fishes showed spatiotemporal variations across the temporary floodplain types. Turbidity, dissolved oxygen, conductivity, woody debris and chlorophyll a were all highest in rarely flooded floodplains compared to both primary and secondary floodplains (ANOVA, p < 0.05). Fish from 38 fish taxa belonging to 11 families were identified and classified during the study. The fish families Cichlidae (dominated by tilapias), Poeciliidae (Aplocheilichthys spp.) and Cyprinidae (Barbus spp.), were the most abundant across the study floodplains. The poeciliids were most abundant in frequently flooded primary and secondary floodplains, whereas juvenile cichlids dominated in rarely flooded floodplains. During high floods the rarely flooded portions of the delta function as important nursery habitats for juvenile cichlids, implying that a significant reduction in flooding may have negative effects on the delta's tilapia stocks due to reduced recruitment success.

Diversity, structure and function of fish assemblages associated with Posidonia oceanica beds in an area of the eastern Mediterranean Sea and the role of non‐indigenous species

Temporal and spatial variation in density, biomass and body size of littoral fish species associated with nearshore Posidonia oceanica meadows was studied over an annual cycle in an area of the eastern Mediterranean Sea. A total of 109,350 littoral fishes were collected, belonging to 34 families and 88 species. Density of fishes peaked during the summer due to high numbers of juveniles. Season was a significant factor determining density, although number of species and biomass did not show any obvious seasonal pattern. Throughout the study, schooling planktivorous fish species such as the picarel Spicara smaris, the bogue Boops boops and the damselfish Chromis chromis were dominant, both in terms of density (80%) and biomass (70%). Temporal variation in density and body size of fishes was used to assess the seasonal and ontogenetic habitat use of each species, with their affinity to seagrass assessed by comparing their respective distribution on sand. Four functional guilds were created (juvenile migrants, seagrass residents, seasonal migrants and occasional visitors) to describe the habitat use of P. oceanica meadows by each species. Several species associated with P. oceanica meadows used this habitat mainly as juveniles during summer, although many others were present concurrently as adults and as juveniles. Among the species encountered, 11 were non-indigenous of Indo-Pacific origin, of which three used seagrasses mainly as juveniles and four as residents. The non-indigenous silverstripe blaasop Lagocephalus sceleratus ranked among the 10 most dominant species in terms of biomass (2%) and was classified as a seagrass resident.

Geographic and Temporal Variations in Population Dynamics ofIxodes ricinusand AssociatedBorreliaInfections in The Netherlands

In a countrywide investigation of the ecological factors that contribute to Lyme borreliosis risk, a longitudinal study on population dynamics of the sheep tick Ixodes ricinus and their infections with Borrelia burgdorferi sensu lato (s.l.) was undertaken at 24 sites in The Netherlands from July 2006 to December 2007. Study sites were mature forests, dune vegetations, or new forests on land reclaimed from the sea. Ticks were sampled monthly and nymphal ticks were investigated for the presence of Borrelia spp. I. ricinus was the only tick species found. Ticks were found in all sites, but with significant spatial and temporal variations in density between sites. Peak densities were found in July and August, with lowest tick numbers collected in December and January. In some sites, questing activities of I. ricinus nymphs and adults were observed in the winter months. Mean monthly Borrelia infections in nymphs varied from 0% to 29.0% (range: 0%-60%), and several sites had significantly higher mean nymphal Borrelia infections than others. Four genospecies of Borrelia burgdorferi s.l. were found, with B. afzelii being dominant at most sites. Borrelia infection rates in nymphal ticks collected in July, September, and November 2006 were significantly higher (23.7%, p<0.01) than those in the corresponding months of 2007 (9.9%). The diversity in Borrelia genospecies between sites was significantly different (p<0.001). Habitat structure (tree cover) was an effective discriminant parameter in the determination of Borrelia infection risk, as measured by the proportion of nymphal ticks infected with B. burgdorferi s.l. Thickness of the litter layer and moss cover were positively related to nymphal and adult tick densities. The study shows that Borrelia-infected ticks are present in many forest and dune areas in The Netherlands and suggests that in such biotopes, which are used for a wide variety of recreational activities, the infection risk is high.