Low Water Level Period Research Articles

Optimizing Ecosystem Services in China

The pool behind the controversial Three Gorges Dam ([1][1], [2][2]) on the Changjiang (Yangtze) River in Hubei Province, China, will top off at 175 m above sea level next winter (2008 to 2009) (“Three Gorges Dam: Into the unknown,” R. Stone, News Focus, 1 August, p. [628][3]). With this flood level, former cities, homes, and farm fields of about 1.5 million people will be seasonally under water, and a set of new unique ecosystems will develop. The extent of the impact of this unprecedented amount of wetland underwater, the potential ecological systems that will result on the borders of this reservoir, and possible approaches to minimize the impacts or enhance ecological services are mostly unknown. Flooding in the pool behind the Three Gorges Dam will extend up to 300 km upstream, almost to the city of Chongqing. The affected area with the most impact on human settlements is on the Pengxi River; it is in this region that a city of 300,000, Kaixian, has actually been relocated to higher grounds. The Pengxi River valley includes 5500 ha of land that will now be seasonally flooded, some for 6 months. Approximately 5% of this newly flooded land is former urban area. Opportunities exist for optimizing ecosystem services through application of ecological engineering ([3][4]). Algal blooms, mainly with Peridiniopsis sp ., were widespread in the pools during flooding in 2008, so nutrient management will need consideration. Conventional agriculture can be practiced during periods of low water level on the riparian slopes, with one major exception—no fertilizers should be used, as they will exacerbate the pool eutrophication the next spring. Nor will fertilizers be needed. Sedimentation of nutrients, especially phosphorus, will be significant during flooding, and an agriculture more harmonious with the new conditions might flourish. Cascading terraced ponds and wetlands such as those at the Honghe River ([4][5]) are another approach for retaining the water as it recedes, while reducing nutrient loss to the river system. Mudflats will be abundant near or at the river in summer, providing ideal habitat for shorebirds and other wading birds. Commercial enterprises for food production can be designed to utilize the pulsing water. Fish-net systems could be used to capture fish as the flood pulse recedes. We agree with Stone that there will be a new, though perhaps uneasy, equilibrium between the Three Gorges Dam and its reservoir area in the next several decades as nature adapts with new emerging ecosystems. 1. 1.[↵][6] 1. J. Wu 2. et al. , Science 300, 1239 (2003). [OpenUrl][7][Abstract/FREE Full Text][8] 2. 2.[↵][9] 1. G. Z. Shen, 2. Z. Q. Xie , Science 304, 681 (2004). [OpenUrl][10] 3. 3.[↵][11] 1. W. J. Mitsch, 2. S. E. Jorgensen , Ecological Engineering and Ecosystem Restoration (Wiley & Sons, New York, 2004). 4. 4.[↵][12] 1. J. Shi , J. Yunnan Nationalities Uni. 21, 77 (2004) (in Chinese). [OpenUrl][13] [1]: #ref-1 [2]: #ref-2 [3]: /lookup/doi/10.1126/science.321.5889.628 [4]: #ref-3 [5]: #ref-4 [6]: #xref-ref-1-1 View reference 1. in text [7]: {openurl}?query=rft.jtitle%253DScience%26rft.stitle%253DScience%26rft.issn%253D0036-8075%26rft.aulast%253DWu%26rft.auinit1%253DJ.%26rft.volume%253D300%26rft.issue%253D5623%26rft.spage%253D1239%26rft.epage%253D1240%26rft.atitle%253DECOLOGY%253A%2BThree-Gorges%2BDam--Experiment%2Bin%2BHabitat%2BFragmentation%253F%26rft_id%253Dinfo%253Adoi%252F10.1126%252Fscience.1083312%26rft_id%253Dinfo%253Apmid%252F12764179%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [8]: /lookup/ijlink/YTozOntzOjQ6InBhdGgiO3M6MTQ6Ii9sb29rdXAvaWpsaW5rIjtzOjU6InF1ZXJ5IjthOjQ6e3M6ODoibGlua1R5cGUiO3M6NDoiQUJTVCI7czoxMToiam91cm5hbENvZGUiO3M6Mzoic2NpIjtzOjU6InJlc2lkIjtzOjEzOiIzMDAvNTYyMy8xMjM5IjtzOjQ6ImF0b20iO3M6MjQ6Ii9zY2kvMzIyLzU5MDEvNTI4LjIuYXRvbSI7fXM6ODoiZnJhZ21lbnQiO3M6MDoiIjt9 [9]: #xref-ref-2-1 View reference 2. in text [10]: {openurl}?query=rft.jtitle%253DScience%26rft.issn%253D0036-8075%26rft.volume%253D304%26rft.spage%253D681%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [11]: #xref-ref-3-1 View reference 3. in text [12]: #xref-ref-4-1 View reference 4. in text [13]: {openurl}?query=rft.jtitle%253DJ.%2BYunnan%2BNationalities%2BUni.%26rft.volume%253D21%26rft.spage%253D77%26rft.atitle%253DJ%2BYUNNAN%2BNATIONALITIES%2BUNI%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx

Recent reduction in the water level of Lake Victoria has created more habitats for Anopheles funestus

BackgroundThe water level of Lake Victoria has fallen more than 1.5 m since 1998, revealing a narrow strip of land along the shore. This study determined whether the recent drop in the water level has created additional breeding grounds for malaria vectors.MethodsThe recent and past shorelines were estimated using landmarks and a satellite image. The locations of breeding habitats were recorded using a GPS unit during the high and low lake water periods. GIS was used to determine whether the breeding habitats were located on newly emerged land between the new and old shorelines.ResultsOver half of the breeding habitats existed on newly emerged land. Fewer habitats for the Anopheles gambiae complex were found during the low water level period compared to the high water period. However, more habitats for Anopheles funestus were found during the high water level period, and they were all located on the newly emerged land.ConclusionThe recent reduction in water level of Lake Victoria has increased the amount of available habitat for A. funestus. The results suggest that the water drop has substantially affected the population of this malaria vector in the Lake Victoria basin, particularly because the lake has a long shoreline that may harbour many new breeding habitats.

Using Affine Jump Diffusion Models for Modelling and Pricing Electricity Derivatives

A seasonal affine jump diffusion spike model with regime switching in the long‐run equilibrium level is applied to model spot and forward prices in the Scandinavian power market. The spike part of the model incorporates different coefficients of mean reversion in the spike and normal variables and thus improves the spot–forward relationship, particularly at time periods when spikes occur. The regime switching part of the model contains two separate regimes to distinguish between periods of high and low water levels in the reservoirs, reflecting the availability of hydropower in the market. The performance of the models is compared with that of other models proposed in the literature in terms of fitting the observed term structure, as well as by generating simulated price paths that have the same statistical properties as the actual prices observed in the market. In particular, the model performs well in terms of capturing the spikes and explaining their fast mean reversion as well as in terms of reflecting the seasonal volatility observed in the market. These issues are very important for the pricing and hedging of derivative instruments.

Home ranges and movements of the Chinese stripe-necked turtle (Ocadia sinensis) in the Keelung River, northern Taiwan

AbstractWe investigated home ranges and movements of 13 adult Ocadia sinensis by radio-tracking in a narrow and disturbed riverine habitat of the Keelung River, northern Taiwan from January 2001 to April 2002. Our results indicated that individuals of this turtle were sedentary, with home range length averaging 703 m (range 170-1460 m). Home range length did not differ between the sexes. Movement patterns of radio-tracked turtles were highly variable, with no apparent seasonal patterns. The daily movement distances ranged from 5-245 m, equalling 0.3 to 47.1% of their home range length. Usually, O. sinensis moved into a new pool during flooding or after a longer period of low water level. This turtle was seldom relocated terrestrially, showing its highly aquatic nature. Ocadia sinensis used the riverine habitat unevenly, preferring the deep and slow-current pools and avoiding highly modified river sections. Our results raise conservation concern about loss of deeper, slow-current pools in many river management projects, which was important habitat for O. sinensis in their home ranges.

Changes in N and P limitation induced by water level fluctuations in Nature Park Kopački Rit (Croatia): nutrient enrichment bioassay

The importance of nutrients as limiting factors might vary in environments with different limnological characteristics. In this article we evaluate the effect of frequency and duration of flooding on nutrient limitation in a riparian floodplain. Variations in N and P limitations were studied in the period of low (2003) and high (2004) water level in two different floodplain habitat types in the Nature Park Kopacki Rit (Croatia), a floodplain area of the Danube River. In 2003 and 2004, the limnological characteristics of floodplain lake (Lake Sakadas) and the channel (Stara Drava) differed due to their hydrological regimes. Potential for nutrient limitation was determined by DIN:TP and TN:TP ratios, while the actual nutrient limitation was assessed by nutrient enrichment bioassay. A change from non-limited to N-limited conditions in the channel, and consistent actual N limitation in the lake was determined by the nutrient enrichment experiment. Of the two ratios, DIN:TP matched better with the bioassay data. Phosphorus limitation was only occasionally evident. Changes in trophic conditions from hypereutrophy to eutrophy (between low and high water levels) reflected the importance of the hydrologic regime as a factor which can modify the trophic state of Lake Sakadas. Compared with 2003, the increase of total nitrogen concentrations in 2004 calls attention to the importance of nitrogen inputs from the Danube to the system with excessive phosphorus concentrations.

Phytoplankton productivity in a highly colored shallow lake of a South American floodplain

ABSTRACT We described phytoplankton productivity in a floodplain wetland of the Lower Parana River, Argentina. Four samplings encompassing periods of high and low water levels were conducted in a highly colored shallow lake. Photosynthesis-irradiance (P-E) curves and the areal photosynthetic rate (PA) were estimated following the 14C assimilation technique. Likewise, physical and chemical variables and phytoplankton composition, density, and chlorophyll a concentration were measured. Phytoplankton assemblages shifted from cyanobacteria blooms in summer to cryptophycean dominance in winter, and co-dominance of cryptophytes and chlorophytes in autumn. Assimilation number Pmax ranged from 3–7.8 µg C (µg Chl a h)−1 and peaked in early summer (low water level) when water color was highest, suggesting that phytoplankton productivity was not depressed by the high content of colored humic acids. Photosynthetic efficiency (α) ranged from 0.021–1 µg C (µg Chl a h)−1 µmol photons−1 m2 s and reached its maximum value...

Habitat Use of the Shoal Bass, Micropterus cataractae, in an Alabama Stream

ABSTRACT Habitat use of larval, juvenile and adult shoal bass (Micropterus cataractae) was monitored in Little Uchee Creek (Chattahoochee River drainage), Alabama in 2005 and 2006. Larvae and juveniles used different habitats than adults and each other. All groups used habitat that differed from what was generally available. Adult shoal bass used different habitat in 2005 when water levels were relatively greater than in 2006. During low water level periods adults moved into pool mesohabitat, emphasizing the importance of habitat connectivity for the persistence of this species.

Condition-dependent dispersal of a patchily distributed riparian ground beetle in response to disturbance

In common with many habitat elements of riverine landscapes, exposed riverine sediments (ERS) are highly disturbed, naturally patchy and regularly distributed, whose specialists are strongly adapted to flood disturbance and loss of habitat due to succession. Investigations of dispersal in ERS habitats therefore provide an important contrast to the unnaturally fragmented, stable systems usually studied. The present investigation analysed the three interdependent stages of dispersal: (1) emigration, (2) inter-patch movement and (3) immigration of a common ERS specialised beetle, Bembidion atrocaeruleum (Stephens 1828) (Coleoptera, Carabidae), in a relatively unmodified section of river, using mark-resight methods. Dispersal was correlated with estimates of local population size and density, water level and patch quality in order to test for condition-dependent dispersal cues. Flood inundation of habitat was found to increase strongly the overall rate of dispersal, and the rate of emigration was significantly higher from patches that were heavily trampled by cattle. Strongly declining numbers of dispersers with distance suggested low dispersal rates during periods of low water level. Dispersal in response to habitat degradation by cattle trampling would likely lead to a higher overall population fitness than a random dispersal strategy. Dispersal distances were probably adapted to the underlying habitat landscape distribution, high-flow dispersal cues and ready means of long-distance dispersal through hydrochory. Species whose dispersal is adapted to the natural habitat distribution of riverine landscapes are likely to be strongly negatively affected by reduced flood frequency and intensity and habitat fragmentation through flow regulation or channelisation.

Postglacial Vegetation, Climate History and Land-Sea Interaction at Island Lake, Baie des Chaleurs, New Brunswick, as Documented by Palynological Analysis*

This study demonstrates that lakes located near the coast, close to large bodies of water, can document sea-level fluctuations and the subtle sea-land interaction that governs coastal areas. At Island Lake, located close to the head of Baie des Chaleurs, northern New Brunswick, the postglacial marine transgression corresponds with a reversal from a cold and dry herb and shrub tundra environment to open pioneer forest that was warmer and wetter. Successive incursions of poplar/aspen and spruce were succeeded by boreal forest dominated by spruce, alder and birch. A forest dominated by alder and fir, indicators of wetter and even colder local conditions followed. Paleoclimatic reconstruction inferred from pollen stratigraphy indicates that an early warming period culminating ca. 9450 BP, was followed by a period when temperatures remained cool. At the same time, the annual precipitation rose sharply, suggesting increased availability of moisture in this area. This is interpreted as a cooling effect due to the proximity to a large body of water to the study site and is attributed to the marine transgression into Baie des Chaleurs following the discharge of glacial lakes Agassiz and Barlow-Ojibway into the Great Lakes and Goldthwait Sea and the isostatic adjustment of the landmass. The cooling effect was recorded by a vegetation change from ca. 9450 to 8100 BP. Within that period, from ca. 8500 to ca. 8400 BP, the vegetation did not record the effect of the proximity to the sea. This period is postulated to be a period of low water levels in the Baie. Recovery to the regional climate norm occurred after 8100 BP when the climate was warmer than today. Since then the climate gradually cooled.

Effects of summer drawdown on cyprinid fish larvae in Lake Biwa, Japan

The effects of artificial water-level manipulations in Lake Biwa, initiated in 1992 to prevent flooding, on cyprinid fish larvae were assessed by measuring and estimating the topographic change in the reed zones. The shallow, litter-accumulated water in those zones was inhabited by larvae of two species, Carassius spp. and Cyprinus carpio. More than 70% (by volume) of these shallow areas in Lake Biwa was estimated to be lost when the water level was artificially lowered by 30cm from June to October 1997. During the low water level period, significantly fewer eggs of these cyprinid fishes were collected in a reed zone than in April and May of the same year, a comparison with a previous study suggesting that spawning of the fishes was inhibited during the period of low water level. The artificial reductions in water level probably resulted in a significant decrease in the volume of shallow water in the lake and may be linked to the drastic decline in these cyprinid fishes.

Inter-annual variability of Potamogeton perfoliatus stands

Yearly differences in the composition and nutrient content of submerged macrophyte stands, mainly of the long-term dominant Potamogeton perfoliatus L. were observed at 10 stations along 83 km route of the Estonian shore of L. Peipsi (total area 3555 km 2) in the period of 1999–2002. Changes consisted in the differences in the presence of plants (three stations), replacement of dominant (one station), removal of mud sediment (one station), mass propagation of Lemna trisulca (all stations) in 1999 and of Cladophora glomerata on P. perfoliatus (six stations) in 2000. The amount of C. glomerata on pondweed, similar to the biomass of host plant, decreased in 2001–2002, whereas markedly thick layer of epiphytic microalgae covered P. perfoliatus in 2002. In the samples of P. perfoliatus collected at the stations of the mass propagation of C. glomerata in the next year, 2001, inflorescences were absent. The appearance of an increase in P. perfoliatus stands was probably associated with improving sediment conditions in the period of low water level in the last observation years. A trend of constant increase in the mean molar N/P ratio in the tissues of P. perfoliatus in the study period (from 17.8 to 25.2) was statistically significant ( p = 0.003). Classical canonical analysis, used to study two complexes correlated between themselves characteristics of P. perfoliatus and environmental conditions, revealed that plant growth factor (including biomass, tissue N%, P% and N/P) was in strong correlation ( p < 0.0001) with water condition factor (including mean temperature in May, sum of temperatures between ice break and sampling, mean water level, N tot and P tot of water in June). More important for water condition factor were mean temperature in May and P tot of water in June. Also the independent significance of water N tot and P tot for pondweeds was tested and confirmed by ANOVA analysis.

Natural and anthropogenic rapid changes in the Kara-Bogaz Gol over the last two centuries reconstructed from palynological analyses and a comparison to instrumental records

Palynological analyses (pollen and dinocysts) of a sediment core taken in the Kara-Bogaz Gol have been used to reconstruct rapid and catastrophic environmental changes over the last two centuries (chronology based on 210Pb). A natural cyclicity (65 years) of water level changes in the Caspian Sea (CS) and in the Kara-Bogaz Gol (KBG) and anthropogenic factors (building of a dam separating the CS and the KBG waters) combine to induce rapid changes in water levels of the KBG, in the salinity of its waters and in vegetation cover of its surroundings. The impact of low water levels on the dinocysts is marked by a lower diversity and the survival of two species that are typical of the KBG, the Caspian Sea species present in the KBG having disappeared. During periods of higher water levels (AD 1871–1878), the lake is surrounded by a steppe-like vegetation dominated by Artemisia; whereas during periods of low water levels (AD 1878–1913 and AD 1955–1998), the emerged shore are colonised by Chenopodiaceae. The period of AD 1913–1955 corresponding to decreasing water levels has an extremely low pollen concentration and a maximum of reworking of arboreal taxa. During the last low-level period, humans responded by abandoning the shores of the bay. What happened to the KBG can be used as an example of what may happen in the future for the Aral Sea. A problem of reworking of Tertiary dinocysts into modern deposits has been detected owing to the knowledge of the modern dinoflagellate assemblages recently made available through a water survey. A comparison to modern surface pollen samples from Central Asia (Anzali, Caspian Sea south and central basins, Aral Sea, Lake Balkhash, Lake Issyk-Kul and the Chinese Tien-Shan range) allows us to establish the potential reworking of at least five arboreal pollen taxa possibly by run-off and dust storms.

Seasonal Movements of Podocnemis sextuberculata (Testudines: Podocnemididae) in the Mamirauá Sustainable Development Reserve, Amazonas, Brazil

We studied the linear home range and seasonal movements of Podocnemis sextuberculata from September 1996 to December 1998 in the Mamirauá Sustainable Development Reserve located in the middle Solimões River, near Tefé, Amazonas, Brazil. We collected data on turtle movements and home range through mark and recapture and radiotelemetry, focusing on seasonal differences between periods of high and low water levels. Based on radiotelemetry of 6 females in the Jarauá River system, linear home range varied from 16.52 to 44.5 km. Nesting migrations from the Jarauá to the beaches of the Japurá River averaged 18.04 km. Males did not move significantly between captures. For conservation purposes, it is of critical importance to protect both the canals that the turtles use to travel between the lakes and the river as well as the deep holes in the river where mature turtles congregate during the dry season.

Hydrochemistry of reservoirs of Damodar River basin, India: weathering processes and water quality assessment

Water samples collected from the six reservoirs of Damodar River basin in pre- and post-monsoon, have been analysed, to study the major ion chemistry and the weathering and geochemical processes controlling the water composition. Ca, Na and HCO3 dominate the chemical composition of the reservoir water. The seasonal data shows a minimum concentration of most of the ions in post-monsoon and a maximum concentration in pre-monsoon seasons, reflecting the concentrating effects due to elevated temperature and increased evaporation during the low water level period of the pre-monsoon season. Water chemistry of the reservoirs strongly reflects the dominance of continental weathering aided by atmospheric and anthropogenic activities in the catchment area. Higher concentration of SO4 and TDS in Panchet, Durgapur and Tenughat reservoirs indicate mining and anthropogenic impact on water quality. The high contribution of (Ca+Mg) to the total cations, high concentration of dissolved silica, relatively high (Na+K)/TZ+ ratio (0.3) and low equivalent ratio of (Ca+Mg)/(Na+K) suggests combined influence of carbonate and silicate weathering. Kaolinite is the possible mineral that is in equilibrium with the water, implying that the chemistry of reservoir water favours kaolinite formation. The calculated values of SAR, RSC and sodium percentage indicate the ‘excellent to good quality’ of water for irrigation uses.

A Record of Late-Quaternary Moisture-Balance Change and Vegetation Response from the White Mountains, New Hampshire

Changes in regional moisture availability over time affect hydrologic systems, such as lakes, and ecological processes, such as plant growth and reproduction. Long-term changes in moisture balance may help to explain broadscale changes in forest composition during the Quaternary period. In this article, Holocene lake-level changes and fossil pollen from Echo Lake, New Hampshire, are documented and the relationship between moisture-balance fluctuations and vegetation change in northern New England over the past 12,000 years is examined. Ground-penetrating radar profiles and five sediment cores provide evidence for two periods of low water levels at ca. 11,000–8,000 cal yr BP and at ca. 5,000–2,000 cal yr BP, respectively. During the first interval of low water level, dry-tolerant white pine (Pinus strobus) became common in the surrounding watershed. By ca. 8,000 cal yr BP, when the water level rose, hemlock (Tsuga), beech (Fagus), and birch (Betula), which prefer high soil-moisture levels, had increased in abundance. The second period of low water levels coincided with a decline in hemlock populations that has been widely attributed to a pathogen outbreak. The correlation between lake-level and vegetation change suggests that moisture availability, in addition to temperature, helps to shape vegetation dynamics by directly affecting plant populations and by influencing plant-pathogen and other ecological interactions.

Composition and dynamics of hyporheic and surface fauna in a semi-arid stream in relation to the management of a polluted reservoir

The mid Tafna wadi (N-W Algeria) is exposed to several man induced disturbances including urban, industrial, and agricultural effluents, and alteration of the natural flow regime. The recent installation of a reservoir upstream of the study area has strongly modified the flow regime by extending the period of low water level, and this stream section is only occasionally fed by water releases. Investigations on the hyporheos and benthos of this stream have allowed the comparison of densities, taxa richness and diversity, during three hydrological periods (natural high water, low flow and reservoir water releases), with the aim to highlight effects of the reservoir management on downstream hydrosystems. Species richness and abundance of stygobite crustaceans were low in the hyporheos. Surface water Crustaceans represented 53% of the overall fauna, with Cyclopoidae composing 50% of the Crustaceans. The dominance of the Cyclopoidae was also observed in the hyporheos. Daphniidae were more abundant (37%) in the benthic habitat than in the hyporheic zone. The alternation of high and low discharges appeared to affect the hyporheos more than the benthic communities. The stability concept of the hyporheic habitat does not fit with our results showing a great range of spatial (mosaic habitat) and temporal changes according to the flow regime.

Suspended sediment transport on a temperate, microtidal mudflat, the Danish Wadden Sea

Time series measurements of suspended sediment transport and bed level change on a microtidal mudflat of the Danish Wadden Sea were carried out between February 1997 and September 2000. The data show that the mudflat is generally accreting except for periods with onshore winds when wave-induced erosion takes place during periods of low water levels at the site. In general, a continuous net input of mud was recorded during periods of weak or offshore winds, i.e. the import is not event controlled, and this was interrupted by episodic loss of mud during a few tidal periods with onshore winds. The net landward transport of suspended sediment during calm weather or periods of offshore wind is caused by settling and scour lag and tidal asymmetry. A very large input of suspended material was observed in the period following a strong storm, the landward directed net transport during the five tidal periods following the storm being equivalent to approximately 40% of the net annual accumulation in the area. The typical depth of sediment reworking at the intertidal study site is 2–5 cm. Some seasonal variation can be deduced from the data set with a tendency for deposition during spring and summer and erosion in the winter period. The seasonality was especially clear at the most landward station where accretion following periods of ice-formation in the area is due to grounding of sediment transported by ice-floes but otherwise deposition is mainly taking place in the warmer seasons. Net accretion over the three-year measuring period along the transect varies between 0.6 and 1.9 cm a−1. Deposition is largest at the most landward station and the accretion found on the basis of repeated bed level measurements compares well with 210Pb and 137Cs dating.

Environmental mechanism of magnetic susceptibility changes of lacustrine sediments from Lake Hulun, China

The changes of magnetic susceptibility (κ) are correlated with those of corresponding sedimentological, geochemical, mineralogical and biological results, which verifies that κ can be taken as one of the environmental proxies. However, usually the exact origin of magnetic signal is poorly understood, and is difficult to relate with the environmental evolution. Magnetic properties of material derived from the catchment and sedimentary environment may affect the accumulation, preservation, or authigenesis and diagenesis of magnetic minerals. In the Lake Hulun region in Inner Mongolia, it is found that muddy sediments, deposited during high water level period (corresponding to humid climate), have comparatively high κ values. In contrast, the sandy sediments, deposited during low water level period (corresponding to arid climate), have low κ values. Detailed rock magnetic investigation confirms that detrital magnetite derived from volcanic rocks in the catchment exists in both muddy and sandy sediments. During high water level period, secondary ferrimagnetic iron sulphide was produced in muddy sediments under relatively reductive conditions. Ferrimagnetic iron sulphide, coexisting with detrital magnetite, predominates the magnetic properties of muddy sediments, resulting in increasing κ. This paper reveals the significance of authigenic ferrimagnetic iron sulphide produced after sediment deposition.

How juvenile Atlantic salmon,Salmo salarL., respond to falling water levels: experiments in an artificial stream

The responses of juvenile Atlantic salmon,Salmo salarL., to an experimentally imposed drop in water level were studied in an artificial stream. In a pilot experiment, 20% of fish which had a feeding station in an area of riffle remained there throughout the period of low water levels. The remaining riffle‐dwelling fish moved into areas of deeper water, although not necessarily to the pool nearest their feeding station. Out of the fish which left the area, 89% moved in an upstream direction and 11% moved downstream. In a second experiment, which was designed to look in more detail at this response to de‐watering, 95% of riffle‐dwelling fish left when the water level dropped and moved into a pool, mostly within the 4‐h period after water levels started to fall. Seventy‐four per cent of fish which left moved upstream and the remainder moved downstream. There was a strong tendency to leave in an upstream direction as shallow areas began to dry out, and this tendency persisted irrespective of variability in fish size, prior social experience and the size of the home range prior to de‐watering. It is suggested that the prior opportunity to explore alternative habitat may be an important determinant of response of riffle‐dwelling salmon to a sudden drop in water level.

Tracing Ground Water Recharge in an Agricultural Watershed with Isotopes

AbstractNitrate and pesticide use in the 800 km2 Raisin River agricultural watershed in eastern Ontario, Canada, threatens the quality of ground water in the highly exploited regional carbonate aquifer overlain by sandy till. To assess local recharge contributions through the cultivated fields, monthly monitoring of ground water levels, geochemistry, and environmental isotopes (δ2H H2O, (δ13CDIC) was carried out for 12 wells over a 14‐month period.Seasonal water level variations suggest that recharge is constrained to spring and late fall when transpiration is minimized and the ground is not frozen. However,2H monitoring shows that early summer precipitation also contributes to recharge. Variations in (δ2H values between monitoring sites suggest a local component to recharge.δ13CDIC was used to distinguish between dissolved inorganic carbon (DIC) originating from natural (C3) vegetation and DIC from the cultivated corridors where corn is grown (C 4 vegetation). Seasonal variations in δ13CDIC are remarkably coherent for all wells, with uniform trends to positive values during periods of low water table elevation. During periods of high water table (spring and late fall), δ13CDIC values are between ‐13 and ‐ 16%c VPDB (Vienna Peedee Belemnite), reflecting DIC originating from a dominantly natural (C3) vegetation. When ground water levels are low (summer and mid‐winter), δ13CDIC values shift to between ‐11 and ‐l%c. The seasonal enrichments in δ13CDIC are clear evidence for a local contribution to recharge by direct infiltration though the fields. This contribution is enhanced during periods of low water level, likely due to drainage from the phreatic aquifer. High DOC (dissolved organic carbon) concentrations (&gt; 10 to 30 mg‐C/L) correlate with periods of high water levels indicating infiltration of labile organics to the carbonate aquifer.The work carried out for this paper shows that the conjunctive use of environmental isotope geochemistry and physical parameters are fundamental to assessing the risk of ground water contamination in agricultural watersheds.