Variations In Hydrological Regime Research Articles

Phytoplankton community structure in the dry zone reservoirs of Sri Lanka at the declining phase of the tropical monsoon

We studied the phytoplankton community structure and their relationship to the physicochemical properties of water by selecting five major freshwater reservoirs; Bandagiriya, Kattakaduwa, Lunugamvehera, Ridiyagama, and Tissa in the Southern dry zone of Sri Lanka during the declining phase of tropical monsoon rainfall (December 2019 to January 2020). Fifty-seven species, dominated by Cyanophyta (63%), followed by Chlorophyta, Bacillariophyta, and Euglenophyta were identified morphologically and through DNA sequencing. The majority of Cyanophyta were filamentous forms (62%) with approximately 88% being cyanotoxin producing species. The species composition of phytoplankton communities is reservoir specific. Microcystis was dominant in Lunugamvehera while the diatom Melosira was dominant in Ridiyagama, Kattakaduwa and Tissa. The lowest phytoplankton density and diversity were observed in Bandagiriya mainly due to high turbidity and total suspended solids which interfere with light penetration through the water column. Physicochemical properties of water were significantly different among reservoirs, leading to reservoir specific correlations between phytoplankton density and physicochemical properties of water which might have been driven by the inter-correlative effects of biotic and abiotic factors at the time of sampling. Therefore, the interactive effects might be responsible for the observed variations in phytoplankton community composition. Thus, the present study provides important information on the phytoplankton community structure at the onset of successional episodes in five tropical freshwater reservoirs in relation to their spatial variations in hydrological regimes and physicochemical properties. Such data would provide essential information for planning and implementation of reliable and efficient strategies for monitoring, sampling, forecasting, and managing algal blooms.

Insights into antibiotic stewardship of lake-rivers-basin complex systems for resistance risk control

Antibiotic stewardship is hindered by a lack of consideration for complicated environmental fate of antibiotics and their role in resistance development, while the current methodology of eco-toxicological risk assessment has not been fully protective against their potential to select for antibiotic resistance. To address this problem, we established a novel methodologic framework to perform comprehensive environmental risk assessment of antibiotics in terms of resistance development, which was based on selection effect, phenotype resistance level, heteroresistance frequency, as well as prevalence and stability of antibiotic resistance genes. We tracked the contribution of antibiotic load reduction to the mitigation of environmental risk of resistance development by fate and transport modeling. The method was instantiated in a lake-river network-basin complex system, taking the Taihu Basin as a case study. Overall, antibiotic load posed no eco-toxicological risk but an average medium-level environmental risk for resistance development in Taihu Lake. The effect of antibiotic load on resistance risk was both seasonal-dependent and category-dependent, while quinolones posed the greatest environmental risk for resistance development. Mass-flow analysis indicated that temporal-spatial variation in hydrological regime and antibiotic fate together exerted a significant effect on antibiotic load in the system. By apportioning antibiotic load to riverine influx, we identified the hotspots for load reduction and predicted the beneficial response of resistance risk under load-reduction scenarios. Our study proposed a risk-oriented strategy of basin-scaled antibiotic load reduction for environmental risk control of resistance development.

Nutrients retention of a series of small dam-impacted urban rivers in northern China

How a series of small rubber dams distributed within urban watersheds impact nutrients transport remains less investigated. To fill this research gap, this study assesses the composition of nitrogen (N) and phosphorus (P) and their retention in five urban river segments across the Qingshui-Yanghe rivers, China from March 2019 to September 2021. The upstream and downstream of the Qingshui river (QR-up and QR-city) were enriched with dissolved N and particulate P, suggesting a greater P contribution of agricultural origin. NH4-N concentrations were lower upstream but increased downstream of the Yanghe river due to the direct discharge of sewage and other human activities. N and P concentrations varied along the urban river and were relatively low at QR-city. A total of 159.2 and 8.4 metric tons/yr, corresponding to 63% and 70% of the TN and TP loading, were trapped within QR-up and QR-city, of which the dissolved N and particulate P load retention accounted for 58% and 78%. N and P retention rates within the river segments sharply decreased from 97 to –320% and were mainly sequestered in the upstream river segments (QR-up- QR-city). The retention rate of nitrate (78%) and dissolved reactive P (67%) were higher than that of TPN (9.75%) and TPP (78%) because of biological assimilation. Further, with retention rates of −320%, the upstream river segments were a net source of dissolved organic P. These results confirm that the small dams have an important nutrient retention function that is highly variable on a spatiotemporal scale similarly to large-scale reservoirs. Due to the cumulative influence of sewage discharge downstream of the Yanghe River, the nutrient load retention is significantly higher than that in the Qingshui river segments. The variations in hydrological regimes due to seasonal events and small dam regulation have a substantial impact on nutrient retention, resulting in higher loads retention in the flood period than in other periods. Overall, the results suggest that management plans must not only focus on decreasing nutrient export from the mountainous area of the Qingshui river basin but also on sewage outfalls and enhancing the hydrodynamic status of the dammed rivers with respect to eutrophication.

A Dynamic, Network Scale Sediment (Dis)Connectivity Model to Reconstruct Historical Sediment Transfer and River Reach Sediment Budgets

AbstractModeling network‐scale sediment (dis)connectivity and its response to anthropic pressures provides a baseline understanding of river processes and sediment dynamics that can be used to forecast future hydro‐morphological changes in river basins. However, this requires a solid understanding of how a system is currently operating, and how it operated in the past. We present the basin‐scale, dynamic sediment connectivity model D‐CASCADE, which combines concepts of network modeling with empirical sediment transport formulas to quantify spatiotemporal sediment (dis)connectivity in river networks. D‐CASCADE accounts for multiple factors affecting sediment transport, such as spatiotemporal variations in hydrological regime, different sediment grain sizes, sediment entrainment and deposition. Add‐ons are included in D‐CASCADE to model local changes in river geomorphology driven by sediment‐induced variations in features such as channel width. We apply D‐CASCADE to the well‐documented Bega River catchment, NSW, Australia, where significant geomorphic changes to rivers have occurred post European colonization (after 1850s), including widespread channel erosion and sediment mobilization. The Bega catchment provides a useful case study to test D‐CASCADE, as original source data on the historical sediment budget are available. By introducing historic drivers of change in the correct chronological sequence, the D‐CASCADE model successfully reproduced the timing and magnitude of major phases of sediment transport and associated channel adjustments over the last two centuries. With this confidence, we then ran the model to test how well it performs at estimating future trajectories of basin‐scale sediment transport and sediment budgets at the river reach scale.

Community Trait Responses of Three Dominant Macrophytes to Variations in Flooding During 2011-2019 in a Yangtze River-Connected Floodplain Wetland (Dongting Lake, China).

In lacustrine wetlands connected to rivers, the changes in flood regimes caused by hydrological projects lead to changes in the community traits of dominant macrophytes and, consequently, influence the structure and function of wetland vegetation. However, community trait responses of macrophytes to the timing and duration of flood disturbance have been rarely quantified. In 2011–2019, we investigated plant species diversity, density, and biomass in three dominant macrophyte communities (Carex brevicuspis C.B. Clarke, Miscanthus sacchariflorus (Maxim.) Hackel, and Polygonum hydropiper L.) through monthly field surveys in Dongting Lake wetlands. Partial least squares regressions were used to analyze how the variations in hydrological regimes affected plant community traits. Apparent inter-annual fluctuations in plant community traits were detected during 2011–2019. The species richness and Shannon index of diversity of Miscanthus and Polygonum communities increased, whereas the Shannon index of diversity of Carex community decreased. Variation in flooding had a greater effect on Polygonum and Carex community traits than on Miscanthus community traits. Flooding disturbed all plant communities, especially when the duration and timing varied. Shorter inundation periods caused the biomass of Miscanthus community to decline, and that of Carex and Polygonum communities to increase. Earlier flood recession caused the species richness and Shannon index of diversity of Polygonum and Miscanthus community to increase, and those of Carex community to decrease. These findings imply that shorter inundation durations and earlier flood recession generated by the operation of the Three Gorges Dam have changed the macrophyte growth pattern.

Land use dynamics in the context of variations in hydrological regimes in the upper Senegal River basin

Land use dynamics in the context of variations in hydrological regimes in the upper Senegal River basin

Periphytic algal community in lentic environments of the Upper Paraná River floodplain: seasonal and spatial variation

Abstract Aim: This study aimed to evaluate the influence of seasonal variation in the hydrological regime and of limnological variables on species richness of periphytic algae in lakes of the Upper Paraná River floodplain. Methods The study area is located in the last undammed stretch of the Paraná River in this floodplain, in which three lakes were sampled quarterly (May, August and November 2014, and February and May 2015) for one year. The periphytic material was obtained from petioles of the aquatic macrophyte Eichhornia azurea (Sw.) Kunth. Results We registered 149 taxa of periphytic algae, distributed in 9 classes. Algal richness changed as a function of the seasonal dynamics of the Paraná River’s hydrological regime. Water level fluctuations directly influenced periphytic algae species. During periods of high water level, species richness increased. Limnological variables showed a close relationship with the hydrological regime, and the dissolved nutrients and turbidity were the abiotic factors that most influenced the algal community. The following species were present in all lakes: Fragilaria capucina Desmazières, Gomphonema gracile Ehrenberg and Navicula cryptotenella Lange-Bertalot (Bacillariophyceae) and two species of the genus Oedogonium (Oedogoniophyceae). Conclusions The hydrological regime of the Paraná River played an important role in structuring communities of periphytic algae. Seasonal variation in hydrological regime and in limnological variables together influenced the species richness of these organisms in these floodplain lakes.

Landward Perspective of Coastal Eutrophication Potential Under Future Climate Change: The Seine River Case (France)

Studies quantifying the impact of climate change have so far mostly examined atmospheric variables, and few are evaluating the cascade of aquatic impacts that will occur along the land–ocean continuum until the ultimate impacts on coastal eutrophication potential. In this study, a new hydro-biogeochemical modeling chain has been developed, based on the coupling of the generic pyNuts-Riverstrahler biogeochemical model and the GR4J-CEMANEIGE hydrological model, and applied to the Seine River basin (France). Averaged responses of biogeochemical variables to climate-induced hydrological changes were assessed using climate forcing based on twelve projections of precipitation and temperature (BC-CORDEX) for the stabilization (RCP 4.5) and the increasing (RCP 8.5) CO2 emission scenarios. Beyond the amount of nutrients delivered to the sea, we calculated the indicator of coastal eutrophication potential (ICEP). The models run with the RCP4.5 stabilization scenario show low variations in hydrological regimes and water quality, while five of the six models run with the increasing CO2 emissions scenario (RCP8.5) leads to more intense extreme streamflow (i.e., higher maximum flows, lower and longer minimum flows), resulting in the degradation of water quality. For the driest RCP 8.5 projection, median biogeochemical impacts induced by decreasing discharge (until -270 m3 s-1 in average) are mostly located downstream of major wastewater treatment plants. During spring bloom, e.g., in May, the associated higher residence time leads to an increase of phytoplankton biomass (+31% in average), with a simultaneous -23% decrease of silicic acid, followed downstream by a -9% decrease of oxygen. Later during low flow, major increases in nitrate and phosphate concentrations (until +19% and +32% in average) are expected. For all considered scenarios, high ICEP values (above zero) lasted, indicating that coastal eutrophication is not expected to decrease with changing hydrological conditions in the future. Maximum values are even expected to be higher some years. This study deliberately evaluates the impact of modified hydrology on biogeochemistry without considering the simultaneous alteration of water temperatures, in order to disentangle the causes of climate change-induced impact. It will serve as a first comparative step toward a more complete modeling experiment of climate change impacts on aquatic systems.

Shifts in distribution of herbivorous geese relative to hydrological variation in East Dongting Lake wetland, China

Studies on distribution dynamics of waterbirds and the relation with hydrological changes are essential components of ecological researches. East Dongting Lake is a Ramsar site and especially important wintering ground for herbivorous geese along the East Asian-Australasian Flyway. In this paper, based on annual (2008/09–2016/17) waterbird census data, we investigated the spatial-temporal distributions of three herbivorous goose species (Lesser White-fronted Goose Anser erythropus, Bean Goose Anser fabalis, and Greater White-fronted Goose Anser albifrons) within East Dongting Lake, and analyzed their distribution dynamics (denoted by percentage similarity index, PSI) relative to variations in hydrological regime. The results demonstrated that the distribution of the globally vulnerable Lesser White-fronted Geese changed obviously between years, whereas that of Bean Geese was more stable. Greater White-fronted Geese suffered drastic distribution variation during the study period. The PSI of Lesser White-fronted Geese was negatively correlated with between-year difference in water recession time and mean water level in October, whereas no obvious trend was found in Bean Geese. The Normalized Difference Vegetation Index (NDVI) was applied to detect changes in food resources of the geese, and significant correlations were also found between NDVI and hydrological factors. It was inferred that the variations in hydrological regime affected the annual distribution dynamics of Lesser White-fronted Geese by changing food conditions; whereas the effect on Bean Geese were not reflected in this study. Species traits may explain the differences in distribution dynamics among the three goose species. It was speculated that Lesser White-fronted Geese might be more sensitive to habitat change, whereas Bean Geese were more resilient. We suggested that regulating hydrological regime was crucial in management works. Our study could offer scientific information for species conservation in the context of habitat changes in East Dongting Lake wetland and provide potential insights into habitat management in this area.

Temporal variability in composition and fluxes of Yellow River particulate organic matter

AbstractThis study examines temporal variations of the abundance and carbon isotopic characteristics of particulate organic carbon (POC) and specific‐source compounds in the context of hydrological variability in the Yellow River. The content and bulk carbon isotopic characteristics (13C and 14C) of POC were relatively uniform over the hydrologic (seasonal) cycle. We attribute these temporally invariant geochemical characteristics to the dominant contribution of loess material to the suspended particulate matter (SPM). In contrast, molecular‐level signals revealed that hydrologic conditions exert a significant influence on the proportional contributions of petrogenic and especially fresh plant‐derived OC, while pre‐aged soil OC is mobilized via deeper erosion processes (e.g., gully erosion, mudslides) and is independent of hydrodynamics and surface runoff. A coupled biomarker‐isotope mixing model was applied to estimate the time‐varying supply of contemporary/modern biomass, pre‐aged soil, and fossil OC components to Chinese marginal seas from the Yellow River. We found that natural (e.g., precipitation) and human‐induced (e.g., water and sediment regulation) variations in hydrological regime strongly influence the flux with the magnitude of the corresponding annual fluxes of POC ranging between 0.343 ± 0.122 Mt yr−1 and 0.581 ± 0.213 Mt yr−1, but less strongly infleunce proportions of the different OC constituents. Inter‐annual differences in pre‐aged soil and fossil OC fluxes imply that extreme climate events (e.g., floods) modulate the exhumation and export of old carbon to the ocean, but the OC homogeneity in the pre‐aged mineral soil‐dominated watersheds facilitates robust predictions in terms of OC transport dynamics in the past (sediment cores) and in the future.

Relationships between reservoir water quality and catchment habitat type

Numerous catchment characteristics including topography, geology, soil and vegetation are reported to exert a strong influence on mean surface water properties. The present study employs a geographical information system (GIS) approach to examine, for the first time, the relationship between reservoir water quality [dissolved organic carbon (DOC) concentration, colour, nitrate concentration and pH] and catchment Phase 1 Habitat coverage. Analysis was conducted on two occasions and at two different spatial scales. Numerous statistically significant correlations were identified, suggesting the use of Phase 1 Habitat data could help improve predictive models of surface water quality. The occurrence and strength of correlations varied seasonally in response, we argue, to temporal variations in hydrological regime and anthropogenic activity. The data also suggest that the proximity of habitat types to the reservoir is significant in affecting reservoir water quality. The findings are used to recommend suitable measures for drinking water companies to mitigate against water quality issues.

The panorama of artisanal fisheries of the Araguaia River, Brazil

Seasonal changes in composition and yield of artisanal fishery catches may occur due to natural fluctuations in hydrological regime, fishers, and gear used. In this context, the present study aimed to test the hypothesis that the hydrological cycle of the Araguaia River influences the activity of small-scale commercial fishing by examining the following predictions: fishery yield is influenced by the hydrological regime; the main species exploited through artisanal fishing can change with variations in hydrological regime; and there are differences between male and female fishers in both yield and main exploited species due to differences in the sites that they explore and the gear that they use. Results from the study demonstrated a significantly higher fishery yield during the low-water season, when the catch per unit effort (CPUE) was 24.8 kg/fisher/day, compared to an ebb season CPUE of 19.9 kg/fisher/day. The main species caught by artisanal fisheries was indeed influenced by the hydrological regime of the Araguaia River and the fisher’s gender, which was associated with differences in fishing locations and gear used. These findings suggest that both Araguaia River hydrology and fisher gender should be taken into consideration in developing management strategies for achieving sustainable yields. Of note, our results for fishery yields were 305 % higher than official Brazilian estimates for the entire Araguaia basin over the same period, which suggests underestimation in the official statistics.

Plant landscape and environmental changes recorded in marginal marine environments: The ancient Roman harbour of Portus (Rome, Italy)

The interpretation of past environment changes in coastal marshes, river mouths and drainage channels connecting fresh to salt water bodies, is a difficult task since several factors must be considered such as the variable influence of climate, sea level changes, coastline variations and changes in the river discharge. It is even more difficult if this attempt is carried out in basins and channels of an ancient harbour, used for centuries, connected to the sea and to a river. This is the case of the ancient harbour of Rome, located in the Tiber delta area, which had a prominent role during the imperial age in the Mediterranean. A town named Portus arose in the vicinity and, although sacked many times by Barbarians, was still very important during medieval times. The Romans managed the harbour, the city and the surroundings and the impact on the environment must have been very strong.A multi-proxy approach was used to characterize and date environment and landscape changes in the ancient port of Rome area, from the 1st century to the 15th century AD. Plant macrofossil results have been integrated not only with pollen and microcharcoal, but also with the Tiber River bi-millennial record available in literature. This integration, used to improve the chronological framing of the cores, was carried out using ostracod analyses, micro-crustaceans commonly occurring in marginal marine environments and widely used for palaeoenvironmental reconstructions. In particular, the ability of some species to live within specific salinity ranges has been a precious tool to identify the hydrological setting of the depositional environment.The cores recovered allowed recognition of two different sedimentary sequences; the older shows the silting of the harbour during the first centuries, and the younger records the sedimentation that occurred between the 4th and the 15th century AD. This multi-proxy approach applied to the study of plant remains indicates that 1) the human impact was lower during the first centuries AD than during late-Antiquity and Middle Ages or that 2) clues of human presence were easier to detect in a closed marshy area than in an open harbour. The correlations between macroremains, pollen percentage and concentration, freshwater, brackish water and salt water ostracods and the frequencies of Tiber River floods during a 1500 year time span link some environmental changes and the silting of the harbour to the variations in hydrological regime of the Tiber River.

The influence of environmental conditions on early life stages of flounder (Platichthys flesus) in the central Baltic Sea

Flounder (Platichthys flesus) is a temperate marine fish that is well adapted to the brackish waters of the Baltic Sea. There are two sympatric flounder populations in the Baltic Sea, pelagic and demersal spawners, which differ in their spawning habitat and egg characteristics. In the present study, pelagic spawning flounder of the central Baltic Sea was studied. We examined whether variations in hydrological regime can explain fluctuations in flounder early life stages that have occurred over the past 30years (1970–2005). Using generalized additive modeling to explain the abundance of flounder eggs and larvae in a Latvian ichthyoplankton dataset, we evaluate the hypothesis that the available reproductive volume, defined as the water column with dissolved oxygen larger than 1ml/l and salinity between 10.6 and 12PSU, affects the survival of flounder ichthyoplankton and determines recruitment success.Both reproductive volume and spawning stock biomass were significant factors determining flounder ichthyoplankton abundance. Different measures of water temperature did not contribute significantly to the variability of eggs or larvae. However, recruitment did not correlate to the supply of larvae. The findings presented in this study on the relationship between flounder reproduction, spawning stock biomass and reproductive volume, as well as the lack of correlation to recruitment, are valuable for the understanding of flounder ecology in the Baltic Sea, and for developing the management of the species.

Seasonal and spatial hydrological variability drives aquatic biodiversity in a flood‐pulsed, sub‐tropical wetland

Summary1. Flood‐pulsed wetlands make vital contributions to local and global biodiversity. However, the patterns and controls of spatial and temporal variation in aquatic biodiversity in flood‐pulsed wetlands are not well understood. We analysed the relationship between variation in hydrological regime and the patterns of aquatic biodiversity in a large pristine flood‐pulsed wetland, the Okavango Delta, Botswana.2. Surveys of water chemistry, diatoms and macroinvertebrates were conducted over the seasonal phases of the flood pulse. Hydrological variables of flood frequency and hydroperiod class were collated from 16 years of satellite images. Multivariate regression trees and generalised least squares regression were used to determine the chief controls of community composition and taxon richness.3. Hydroperiod class, phase of the flood and conductivity explained 32% and 43% of the variation in diatom and invertebrate taxon richness, respectively. There was a negative relationship between hydroperiod class and invertebrate taxon richness on the rising, peak and receding flood, whereas at low flood there was no significant relationship. Multivariate regression tree analysis revealed hydroperiod class, phase of the flood and conductivity as the dominant forces shaping invertebrate and diatom community composition.4. Seasonal and spatial variation in hydrological conditions are the principal drivers of variation in aquatic biodiversity in flood‐pulsed wetlands. In pristine flood‐pulsed wetlands, increased productivity caused by the arrival of the flood waters appears to override disturbance and connectivity in shaping taxon richness and community composition. Thus, these data suggest that the maintenance of a rich mosaic of habitats covering a broad range of hydroperiod is the key to preserving aquatic biodiversity and natural ecosystem function in flood‐pulsed wetlands.

Development of a method for assessing flood vulnerability

Over the past few decades, a growing number of studies have been conducted on the mechanisms responsible for climate change and the elaboration of future climate scenarios. More recently, studies have emerged examining the potential effects of climate change on human societies, including how variations in hydrological regimes impact water resources management. According to the Intergovernmental Panel on Climate Change's third assessment report, climate change will lead to an intensification of the hydrological cycle, resulting in greater variability in precipitation patterns and an increase in the intensity and frequency of severe storms and other extreme events. In other words, climate change will likely increase the risks of flooding in many areas. Structural and non-structural countermeasures are available to reduce flood vulnerability, but implementing new measures can be a lengthy process requiring political and financial support. In order to help guide such policy decisions, a method for assessing flood vulnerability due to climate change is proposed. In this preliminary study, multivariate analysis has been used to develop a Flood Vulnerability Index (FVI), which allows for a comparative analysis of flood vulnerability between different basins. Once fully developed, the FVI will also allow users to identify the main factors responsible for a basin's vulnerability, making it a valuable tool to assist in priority setting within decision-making processes.

Spatiotemporal variations in hydrological regimes within Central Africa during the XXth century

Using several statistical tests, we divided into homogeneous segments the XXth century long discharges data series and the rainfall data series of the Congo River and of several of its right bank sub-catchments. These tests revealed that the River Congo at Brazzaville experienced a phase of so called stable discharge from the beginning of the XXth century until 1960, a phase of surplus discharge during the 1960s, testimony to its centennial flooding, and then from 1971 onwards, two successive phases of lower discharge. The second one, stretching from 1980 up to now, being the most accentuated with a 10% drop in the total series' interannual discharge (40 600m3s−1). It has only been very recently, in 1990 that the river dropped to its lowest level of the century. The dates at which hydrological discontinuities appear, are observed to be globally similar within practically all of the tributaries studied, the most significant being that of 1970, already well known in West Africa. During the second half of the century, these discontinuities occur roughly every ten years at the beginning of the 1960s, 1970s, and 1980s. Rainfall discontinuities are not as numerous as discharges ones. They separate homogeneous phases whose variations between them are less pronounced than that of those separated by discharges discontinuities. This underlines the major importance of the geographical location and of the physiographical characteristics of river basins on hydrological spatiotemporal variations. Lastly, we show that the effects of the recent drought are more visible and pronounced in the northern areas of this vast basin than in the southern ones.

Orbital signatures and long-term vegetation patterns in the Mediterranean

Abstract An attempt is made to explore the relation between vegetation phases and orbital configurations by comparing long pollen sequences from the Mediterranean region and astronomical curves on the basis of their independent chronologies. Three recurring patterns emerge from this comparison: (i) all intervals with perihelion passage occurring during northern winter were associated with significant contractions of tree populations; (ii) all temperate-stage expansions of certain Mediterranean vegetation elements occurred when perihelion passage was in summer; and (iii) temperate-stage forests reached maximum extent at times of autumn perihelion. Of these patterns, the first corresponds to extreme dry/cold episodes culminating at times of March perihelion, and the second, to maximum summer temperature and evaporation regimes following June perihelion. The third, associated with the occurrence of September perihelion, appears to be more diffuse both in time and space, possibly reflecting local variations in hydrological regimes. The consistency of at least the first two cases provides some support for the notion that certain recurring vegetation patterns appear to be a result of climate changes linked to specific orbital signatures. This could eventually lead to the development of improved terrestrial chronologies through astronomical tuning.

Leuciscus (Pisces, Cyprinidae) karyotypes: Transect of Portuguese populations

The presently described Iberian chubs - Leuciscus carolitertii and L. pyrenaicus - sampled throughout their distribution ranges in Portugal were cytogenetically analyzed. Their chromosome numbers were consistently 2n = 50, except for two specimens of L. carolitertii, which exhibited a supernumerary chromosome in some of the metaphases. The karyotypes were found to be highly typical for other Leuciscus taxa, as well as for European leuciscine cyprinids: the chromosome sets are dominated by metacentric and submetacentric elements with a reduced number of acrocentric pairs (three to four); the largest pair of the complements belongs typically to this latter category. The chubs from northern drainages, assignable to L. carolitertii, have apparently a more stable karyotype structure (12M:30S:8A) than the chubs from L. pyrenaicus, which have 12M:32S:6A, but may exhibit in the most southern river basins (Guadiana, Mira, Aljezur, Bordeira and Arade) more variable karyotypes. Besides, these data support the very recent discovery of two genetically distinct Leuciscus taxa in this region of the Iberian Peninsula, suggesting the stochastic fixation of structural chromosome rearrangements in these small and isolated drainages, which may be affected by bottlenecks due to significant variations in hydrological regimes. The NORs were apparently located in one small submetacentric pair of chromosomes and the presence of a heteromorphic sex chromosome system of the ZW/ZZ type was also evidenced for the Iberian endemic chubs.

STREAM NITRATE‐N LOADS IN RELATION TO VARIATIONS IN ANNUAL AND SEASONAL RUNOFF REGIMES1

ABSTRACT: The calculation of stream nutrient loads from a sampling period of one year or, at most, a few years may provide an inaccurate estimate of average seasonal or annual loads due to considerable year‐to‐year variations in hydrological regime. The number of years of record required to give a reliable estimate of long‐term average NO3‐N loads was analyzed for E. Duffin Creek and the Nottawasaga River in Ontario, Canada. Nitrate load rating relationships were used in combination with a continuous stream discharge record for 22 years (E. Duffin Creek) and 34 years (Nottawasaga River) to simulate long‐term seasonal and annual variation in NO3.N loads. The errors involved in calculating average loads were examined by comparing the loads derived from sampling periods of one or more consecutive years duration with the estimated long‐term average load for the two rivers. Annual NO3‐N loads for a single year deviated from the long‐term average load by ± 20 to 53 percent in 8 out of 22 years in E. Duffin Creek and in 13 of 34 years in the Nottawasaga River. Six consecutive years of record would be required for both rivers to ensure that an error of > ± 20 percent would occur in only 5 percent of these observation periods. February‐April NO3‐N loads for a single year could deviate by up to +90 percent or ‐61 percent from the long‐term average spring period load for the two rivers. A sampling period of at least 6–7 years would be needed to estimate average NO3‐N loads for the spring runoff season with an error <± 20 percent.