Evidence for a depth refuge effect in artisanal coral reef fisheries

Natural refuges can play a fundamental role in protecting species from overexploitation but have not been adequately quantified in the marine environment. We quantified the effect of a depth refuge on all fish species in an artisanal coral reef fishery in Zanzibar, Tanzania by comparing changes in fish species richness and relative abundance with depth at five fished and three unfished reefs across the region. Commercial species richness was depleted by 15.1% at shallow depths in fished reefs, but there was no difference between the reefs deeper than seven metres. Non-commercial species were not affected by fishing or depth. Evidence for similar patterns in fish communities in other countries and depth-limitations to artisanal fishing methods imply this effect is widespread. The depth refuge effect could be sustaining coral reef fisheries and should be taken in to account before implementing policies or subsidies that encourage or allow fisheries to exploit deeper waters.     

Rooting patterns of four crop legumes in response to seed-placement depths in the dry season

Abstract

On sandy paddy fields, key factors for successful crops in the dry season without irrigation are a shallow water table and practices such as deep seed-placement but only some legume species are adapted to such conditions. To understand the adaptation of legume species to deep seed-placement over shallow water tables, we studied their rooting patterns on two sandy soils. Cowpea (Vigna unguiculata), mungbean (Vigna radiata), peanut (Arachis hypogaea) and soybean (Glycine max) seeds were sown shallow (～5 cm) or deep (～15 cm) in deep sandy soils after harvesting rice in two shallow water table locations in north-east Thailand. The legumes depended mainly on capillary water rising from the water table and none experienced water deficit throughout the growing season. Generally, deeper seed-placement decreased overall root dry weight, but it increased the root surface area to weight ratio. Deep seed-placement promoted a greater fraction of root growth into the subsoil for cowpea (86–99% of total root length), mungbean (61–93% of total root length) and peanut (78–98% of total root length) where the soil contained more water throughout the growing season. Moreover, deep seed-placement at the site with the lower water table promoted deeper penetration of roots of cowpea (～20 cm deeper), mungbean (～20–40 cm deeper) and peanut (～20–40 cm deeper) which improved water access, especially late during the growing season when topsoils dried to close to wilting point. Unlike other species, the soybean rooting pattern did not respond much to seed-placement depths, or soil moisture.     

Manatee metabolism and its influence on distribution in Florida

The metabolic rate of three captive manatees was measured at different water and air temperatures. The animals responded metabolically to cold water (15–20°C), but not to cold air (10–20°C). Average metabolic rates in the thermal neutral zone ($\text{T}{}_1\text{? 24°}$) were 15–22% of predicted weight-specific values, and thermal conductances were 117–229% of predicted levels. Body temperatures averaged 36·4°C. The low heat production and high thermal conductance suggest that manatees are poorly adapted energetically to winter water temperatures in much of Florida. These results support behavioural observations of captive and free-living manatees which suggest that 20°C is the minimum suitable water temperature for manatees. Manatees wintering in colder waters are probably in marginal habitat and are most susceptible to cold-related mortality. Warm water refuges reduce energetic maintenance costs and probably temper the effects of occasional periods of severely cold weather, particularly in northern Florida.     

河蟹养殖自动作业船导航控制系统设计与测试

针对河蟹养殖过程中存在的水草清理难度大、喂料投饵不均匀、人力成本高等问题,该文设计了一种基于ARM(advanced RISC machine)和GPS/INS(global positioning system/inertial navigation system)组合导航的多功能全自动河蟹养殖作业船导航控制系统。该系统由明轮驱动船、ARM主控制器、GPS/INS组合导航装置等组成。为降低传统的基于有限目标点航道位置计算方法的复杂度并减小船体偏离航道的误差,该文提出了一种基于实时插点的航道位置计算方法,实时地解算出当前时刻的目标位置,并设计了相应的转弯及航道切换策略。针对明轮船具有非线性、大时滞、欠阻尼的运动特点,设计了基于模糊PID(proportion integration differentiation)的航向、航速双闭环运动控制算法;基于嵌入式Linux操作系统,设计了船载子系统软件,并编写了上位机监控程序对船载子系统的运行状态进行实时监控。利用河蟹养殖作业船试验平台进行了航速及自动导航试验,并对有限目标点和实时插点的航道位置计算方法的控制效果进行了对比。试验结果表明:船体速度响应较快,超调量不超过5%,稳态误差可控制在3%以内;采用有限点的航道位置计算方法时,船体在转弯与直行时偏离航道的最大误差分别为2.12和1.52m;采用实时插点的航道位置计算方法时,船体在转弯与直行时偏离航道的最大误差分别为0.36和0.09m,分别下降了83.02%和94.08%,船体的控制精度得到了全面的改善。该文可以为多功能河蟹养殖作业船的研究提供重要参考。     

Development of predictive models to explain the distribution of the West Indian manatee Trichechus manatus in tropical watercourses

Logistic regressions were used to study the relationship between habitat variables and the use of tropical watercourses by the West Indian manatee Trichechus manatus at Northeastern Costa Rica and Southern Nicaragua. Presence of manatees in watercourses was assessed through direct and reported sightings of individuals and feeding signs on aquatic vegetation. Indirect methods provided good approximations to the actual distribution that could not have been achieved through direct observations or aerial surveys. Best multivariate models showed that manatees were most present in watercourses that presented abundant aquatic vegetation, warm, and clear waters, high forest cover, and are wider than those where the species is absent. Although habitat variables that explain habitat use of manatees differed for the two sectors found within the study area, manatees preferred lagoons to other watercourses in both areas. These findings point to forest clearing on the shores as a threat for manatee conservation. Habitat variables are excellent predictors of manatee presence, and predictive models as those developed in this study can help assess potential distribution of manatees in areas where this information is lacking as well as to assist identify potential reintroduction areas.     

An assessment of the behaviors of overwintering manatees as influenced by interactions with tourists at two sites in central Florida

We used a combination of focal animal and scan samples over the course of two winter seasons to assess behavioral patterns of manatees as a function of the presence and activities of recreational swimmers and boats in and around Crystal River National Wildlife Refuge, Florida. The nature and outcome of human-manatee interactions and the frequency of harassment of manatees by swimmers (as defined by the US Fish and Wildlife Service) were also recorded. The use of protected (no-entry) sanctuaries by manatees was significantly greater when both the numbers of swimmers and boats increased, and when water temperatures were lower in surrounding areas. The time manatees spent bottom-resting and nursing decreased while the time spent milling and swimming increased when swimmers were present compared with when they were absent. Over half of the direct interactions recorded between swimmers and manatees constituted some form of harassment as defined by Refuge guidelines, and manatees were far more likely to terminate these interactions than were swimmers. We conclude that the existence of no-entry sanctuaries is very important for the conservation of manatees in the area and thought must be given to expanding the sanctuary network. Furthermore, additional efforts at enforcement and public education are needed. We address, qualitatively, several potential management options to better-secure manatees at this critical time of the year.     

Root growth and nitrogen uptake of carrot, early cabbage, onion and lettuce following a range of green manures

An experiment was performed to study the significance of rooting depth of four vegetable crops on their utilization of green manure nitrogen (N). Rates of rooting depth development were estimated as approximately 0.2, 0.7, 1.2 and 1.2 mm day °C^?1 for onion, carrot, lettuce and cabbage, respectively. At harvest, onion and lettuce were found to be shallow‐rooted with final rooting depths of only 0.3 and 0.6 m, respectively, whereas carrot and cabbage reached rooting depths of at least 1.1 m. The two deep‐rooted vegetables increased their N uptake by 46, 24 and 7 kg N ha^?1 when following winter‐hardy legumes, non‐hardy legumes and rye, respectively; the equivalent responses by the two shallow‐rooted crops were 23, 9 and 15 kg N ha^?1, respectively. Thus the deep‐rooted crops used the legume N more efficiently but the shallow‐rooted crops made better use of N left by the non‐legume rye crop. These interactions between green manure type and vegetable crop N response are the result of the dual effects of the green manures: biological N fixation by the legumes, and the variable ability of the green manure crops to concentrate available N in the topsoil. Before shallow‐rooted crops, the ability of rye to concentrate N in the topsoil may be as important as the N fixing ability of legumes.     

黄土地区降雨的优势入渗深度

[目的]分析黄土地区降雨入渗深度,为研究黄土滑坡破坏机理提供依据。[方法]通过现场观测实验监测不同深度土体的含水量变化趋势,并基于颗粒离散元方法对降雨入渗过程进行数值计算。[结果]监测结果表明黄土地区降雨入渗深度不超过1.5m。进一步通过数值计算发现:优势通道入渗的深度和裂缝几何尺寸有关,裂缝越深、越宽,降雨入渗的深度越大,影响范围越广,且裂缝的几何形态随着降雨入渗过程的发展而发生变化。[结论]黄土降雨入渗分为非饱和入渗和优势通道入渗2种,非饱和入渗深度有限,优势通道入渗深度更大,易诱发大规模滑坡灾害。     

Hydrogeochemistry and Water Balance in the Coastal Wetland Area of “Biviere di Gela,” Sicily, Italy

In the study area physical and chemical factors control the composition of surface and groundwaters, which in turn determine the water quality of the “Biviere di Gela” lake. These factors combine to create diverse water types which change their compositional character spatially as rainfall infiltrates the soil zone, moves down a topographically defined flow path, and interacts with bedrock minerals. Low-salinity waters, which represent the initial stage of underground circulation, start dissolving calcium carbonate from the local rocks. The progressive increase in salinity, characterized by substantially higher Ca, SO₄, Na and Cl concentrations, suggests that dissolution of CaSO₄ and NaCl is an important process during water–rock interactions. The “Biviere di Gela” lake is often separated into two units (Lago Grande and Lago Piccolo). “Lago Grande” water is generally of Na-SO₄-Cl-type, whereas “Lago Piccolo” water is of Na-Cl-SO₄-type. Their total content of dissolved salts varies with season, the amount of rainfall, and inflow of ground and drainage water. Over time, an increasing trend towards greater salinity and also sudden changes in the relative abundances of Cl and SO₄ have been recorded for the “Lago Grande.” The isotope composition of the lake water appears to be affected by inflow of ground and surface waters and also by evaporative loss. The nitrate content of waters from the recharge basin is of particular concern because it contributes to lake eutrophication. The trace element contents do not evidence the presence of any significant metal contamination of lake waters, although a future potential hazard of metals bioaccumulation by the aquatic biota must be taken into consideration. Finally, a water balance for the basin shows that a drop in precipitation of about 20% might be critical for lake survival.     

Soil water content,absorption of nutrient elements,and responses to fertilization of sunflower: A case study

Sunflower is considered to be a drought tolerant crop due to its root system. In Argentina, water availability and mineral nutrition are the main factors determining yield. We evaluated the behavior of a sunflower hybrid subjected to fertilization in a period of drought. A trial with the hybrid Paraiso 5 was carried out on a Typic Hapludoll located in western Buenos Aires province. The six treatments were: control; 150 kg or 300 kg available nitrogen (N) ha^‐1 (Native N supplemented with fertilizer); 150 kg or 300 kg available N ha^‐1 (Native N supplemented with fertilizer) plus other macro‐ and micronutrients, and macro‐ and micronutrients without N. Leaf area, plant biomass, root biomass, and yield components were determined. Plant tops and grain were analyzed for N, phosphorus (P), sulfur (S), potassium (K), calcium (Ca), magnesium (Mg), boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn). Soil water content to a 1.2 m depth was measured periodically. At sowing, soil moisture was adequate, but during the first two months, the soil water content dropped to near the wilting point. Roots were largely located in the first 10 cm and dropped abruptly at deeper depths. The highest grain yield resulted from the highest N rates, but there were no effects from other nutrient elements. The oil content was not affected by these treatments. The chemical composition of the straw showed significant differences in N, P, S, and Mn concentration. In grain, only N increased as a response to N fertilization. This hybrid absorbed most nutrient elements during the first half of the life cycle and this corresponded to a time of drought conditions. Apparently roots are able to absorb water from the deep soil depths and still absorb nutrient elements from top soil. These traits might be crop strategies to respond to fertilization under drought.     

Mechanism and Degradation Pathways of Bisphenol A in Aqueous Solution by Strong Ionization Discharge

The effects of anthropogenic noise have garnered significant attention in marine ecosystems, but comparatively less is known about its impacts on freshwater ecosystems. For fish that provide parental care, the effects of acoustic disturbance could have fitness-level consequences if nest tending behavior is altered. This study explored the effects of motorboat noise on the parental behavior of nesting male smallmouth bass (Micropterus dolomieu; Lacépède, 1802), an important freshwater game fish in North America that provides sole paternal care to offspring. Specifically, we evaluated how boat noise proximity to a bass nest (ranging from 4.5 to 90 m) influenced paternal care behaviors. A total of 73 fish were exposed to a 3-min motorboat playback designed to simulate a boat sound that typically occurs in areas near littoral nesting sites. The fish were video recorded, and their behaviors were analyzed before, during, and after exposure to the playback. Residency time was the only behavioral metric to be adversely affected by noise playbacks but only when in close proximity to the speaker. Our results suggest that boat noise may have an impact on bass reproductive fitness in specific contexts where combustion motors are used near shore during the nesting period. The largely null findings may indicate a resilience to boat noise and/or habituation to the noise. In addition, boats displace water and create waves that represent another form of disturbance that could be experienced by fish but was not simulated here. Future research should integrate behavioral and physiological responses to boat noise and other aspects of boat disturbance to better understand the fitness impacts of boating activity on freshwater fish.

     

Status of the West Indian Manatee in Honduras

Aerial surveys and interviews with fisherman in Honduras indicate that manatee numbers are low. There is an abundance of suitable habitat, and evidence on the proportion of calves indicates that manatees are reproducing. Although natural mortality occurs when the animals are trapped in small landlocked coastal lagoons, the greatest threat to manatees is from subsistence hunting for their meat. Harpooning from a small motorised canoe is the most common means of capture; however, entanglement in gill nets may also be employed. Enforcement of existing protective laws and enlistment of public support is required to preserve the remaining manatees in Honduras.     

Straw incorporation and tillage for winter barley: Soil structural effects

A large-scale field experiment was conducted over four seasons on a gleysol (24% clay in topsoil) in Scotland. Conventional ploughing, shallow ploughing and shallow rotary or tine cultivation were investigated for the incorporation of straw in winter barley. Straw after harvest was either chopped or removed. Thus the residue treatments were either straw plus stubble or stubble only. Incorporation depths ranged from 0–100 to 0–300 mm.The presence of straw changed soil physical conditions after several seasons in which straw was incorporated. In the ploughed treatments, the presence of straw plus stubble decreased the average soil water content and matric potential in the topsoil in comparison to stubble only, indicating more rapid drainage. This was associated with the presence of buried straw in zones of loose soil just above plough depth and with a long-term increase in soil pore continuity related to the presence of straw residues. The presence of straw plus stubble under conventional ploughing decreased thermal diffusivities in the top 150 mm of soil in comparison with stubble only. Shallow incorporation of straw plus stubble gave higher overall strengths, bulk densities, lower water infiltration rates and poorer drainage in the topsoil than deeper incorporation. However, shallow incorporation caused an accumulation of organic matter over the first three seasons. This accumulation probably contributed to the increased aggregate stability and resistance to compaction of the top 50-mm soil layer, indicating improved resistance to erosion and to further compaction.     

兰州市农村饮用水中硝态氮分布特征及评价

水体硝态氮污染是水污染研究的焦点之一。通过对兰州市7个县区农村98个生活饮用水水样的测定分析,结合国家相关标准（GB5749—2006）,对兰州市农村生活饮用水硝态氮含量进行评价,并通过回归分析推断了不同水源类型水体硝酸根与电导率的相关性。结果显示：①兰州农村生活饮用水水体硝态氮平均含量为（5.42±6.63）mg·L-1,合格率为86.73%;②皋兰县、西固区和安宁区农村生活饮用水硝态氮含量无超标现象,榆中县、七里河区、永登县、红古区农村生活饮用水硝态氮含量较高,有不同程度超标现象,合格率分别为77.27%、77.78%、80.00%和83.33%;③不同水源类型水体硝态氮含量存在显著差异：自来水、河水和水库水合格率均为100%;窖水、深井水（〉30m）和泉水硝态氮含量较高,合格率分别为92.00%、91.66%和55.56%;浅井水（〈30m）硝态氮平均含量最高,为（22.27±10.54）mg·L-1,合格率仅为25.00%;④除水库水和浅井水,其他类型水体硝酸根与电导率呈显著或极显著的正相关性;⑤浅井水中水体硝态氮含量与pH呈显著负相关;⑥浅井水和泉水水体硝态氮含量与农田氮投入量存在显著正相关性。     

Prescribed burning effects on soil physical properties and soil water repellency in a steep chaparral watershed, southern California, USA

Chaparral watersheds associated with Mediterranean-type climate are distributed over five regions of the world. Because brushland soils are often shallow with low water holding capacities, and are on slopes prone to erosion, disturbances such as fire can adversely affect their physical properties. Fire can also increase the spatial coverage of soil water repellency, reducing infiltration, and, in turn, increasing overland flow and subsequent erosion. We studied the impacts of fire on soil properties by collecting data before and after a prescribed burn conducted during Spring 2001 on the San Dimas Experimental Forest, southern California. The fire removed the litter layer and destroyed the weak surface soil structure; leaving a thin band of ash and char on top of, and mixed in with, an unstable, granular soil of loose consistency. Median litter thickness and clay content were significantly decreased after fire while soil bulk density increased. At 7 d post-fire, soil surface repellency in the watershed was significantly higher than prior to the burn. At 76 d post-fire, surface soil water repellency was returning to near pre-fire values. At the 2 and 4 cm depths, 7 d post-fire soil repellency was also significantly higher than pre-fire, however, conditions at 76 d post-fire were similar to pre-fire values. Variability in soil water repellency between replicates within a given 15 × 15 cm site was as large as the variability seen between sites over the 1.28 ha watershed. The increase in post-fire persistence of water repellency was largest beneath ceanothus (Ceanothus crassifolius) as compared to a small increase beneath chamise (Adenostoma fasciculatum). However, pre-fire persistence was higher under chamise than for ceanothus. Post-fire changes to soil properties may increase the watershed hydrologic response, however the mosaic distribution of water repellency may lead to a less severe increase in hydrologic response than might be expected for a spatially more homogenous increase in repellency.     

不同灌溉制度对玉米根系生长及水分利用效率的影响

为了探讨不同灌溉制度对玉米根系生长和水分利用效率的影响及基因型间差异,在大型活动防雨棚和棚外田间条件下,利用一组玉米遗传材料杂交种户单四号、父本803和母本天四进行了研究。结果发现玉米杂交种在根系生长、分布和水分利用效率上表现出显著的杂种优势。在充分灌溉条件下,玉米杂交种在浅层的根长密度大于亲本,但在水分亏缺条件下,玉米杂交种根长密度在整个剖面上都显著大于亲本;同一玉米基因型在不同的灌溉制度下根长密度在土壤剖面的分布也不同,拔节期不灌溉条件下玉米根系在深层土壤中的分布较充分灌溉条件下大,保证了玉米对深层土壤水分的充分吸收,而后期灌水延缓了表层根系生长的衰退,产生明显的补偿效应;拔节期干旱而抽雄期和灌浆期灌水显著提高了3种基因型玉米的水分利用效率。通过合理灌溉优化玉米根系分布特性以提高玉米吸水能力和水分利用效率,是节水栽培上的可行途径。     

基于诊断函数的薄层流对数律研究

薄层流是一种特殊形态的明渠流,其特点是水深浅薄。为探讨薄层流流速分布是否满足对数律,该研究利用高分辨率粒子图像测速(Particle Image Velocimetry,PIV)技术,分析8组薄层流(水深0.49～1.1 cm,雷诺数835～2 877)及1组深水明渠紊流(对照)床面至水面的流速分布、紊动强度及雷诺应力。并基于诊断函数,研究薄层流流速是否满足对数律、对数区的范围及卡门常数变化规律。结果表明,薄层流的无量纲流速从过渡区开始偏离深水明渠水流中的理论曲线;薄层流的流向紊动强度大于深水明渠紊流,但垂向紊动强度小于深水明渠紊流,随着水深的增加,两者的紊动强度逐渐重合;雷诺应力的特征表明,随着水深的增加,受黏性力影响的范围越来越小。薄层流诊断函数曲线的特征说明薄层流中不存在严格意义的对数区,但当水深极浅时(水深≤0.53 cm),流速基本满足对数律,且卡门常数在0.2～0.3范围内。当水深和雷诺数增加,薄层流诊断函数曲线出现波动而不再近似水平。为方便实际计算,若允许诊断函数有一定的倾斜,对数区在极大值与极小值之间的范围,薄层流的卡门常数随着雷诺数的增加而增加。此外,薄层流对数区的范围并非稳定,随着雷诺数的增加,对数区影响的范围变大。该研究可为薄层流的理论研究和流速计算提供参考。     

Quantitative relationships between morphology and current soil hydrology in some Alfisols in semiarid tropical Australia

The shallow groundwater hydrology of a sequence of red, yellow, and grey earths (Paleustalfs, Plinthustalfs and Ochraqualfs of “Soil Taxonomy”) was monitored over a 9-year period (1973–1982) along a drilled transect of 30 boreholes near Torrens Creek, in northeastern Australia. Climatic analysis shows an aridic soil moisture regime under which soil water surpluses should not occur, and thus significant hydromorphism would not be expected in the soils. The present study has demonstrated, however, that profile saturation and groundwater tables may be expected to develop at various depths in the soils as often as 1 year in 3. Besides, strong linear relationships were found between a number of morphological features of the soils and various hydrological regimes currently active in the profiles: the depths to the upper limits of brown, brownish red, or red mottles, to the upper limit of ironstone gravels, and to the maximum content of clay are all most closely related to the mean position of the shallowest water tables observed; the depths to grey mottles and to mottles with chromas of two or less are most directly related to the depths at which the soils are saturated for 5 weeks; the depth to the maximum content of ironstone gravels and to the base of the solum are most closely associated with the depth at which the soils are saturated for 10 to 15 weeks; and dominantly grey colours in the strongly mottled subsols are best related to profile saturations for at least 21 weeks. The results indicate that the gley features present in the soil profiles could have developed, at least in part, in response to the current soil hydrological regime.     

外来调水对华北低平原区地表水和地下水 水化学特征的影响* ——以河北省南皮县为例

华北低平原区有着巨大的粮食增产潜力,同时也是粮食生产和农业水资源矛盾突出的地区。外来调水与浅层微咸水的联合利用是解决区域水资源问题的有效途径之一,同时也将引起区域水循环和水环境的改变。为明确外来调水对华北低平原区地表水和地下水水化学特征的影响,本研究在华北低平原区河北省南皮县域内对调水后不同季节地表水和地下水进行调查和采样,利用水文地球化学和氢氧(d~2H、d~(18)O)稳定同位素相结合的方法,研究外来调水对地表水和地下水转化及其水化学特征的影响。研究结果表明,11月至翌年7月,受蒸发作用的影响,地表水电导率(EC)和钠吸附比(SAR)增加,d2H、d18O同位素不断富集;由于地表水和周围土壤的交换吸附作用使其水化学类型向Na~+、Cl~-和SO_4~(2-)增加、HCO_3~-减少的咸水转变。调水改变了地表水和浅层地下水之间的补给关系,11月至翌年3月,沟渠附近浅层地下水受外来调水直接或者灌溉补给,使得3月浅层地下水EC降低,埋深变浅,部分采样点分布在外来调水的SAR-EC区域。受调水影响,3月沟渠附近浅层地下水水化学类型为Na·Mg·Ca-Cl·SO_4、Na·Mg-Cl·SO_4·HCO_3、Na·Mg-SO_4·Cl·HCO_3等,是11月调水(Na·Mg·CaSO_4·HCO_3·Cl)和浅层地下水(Na·Mg-Cl·SO_4)的过渡类型。3月至7月浅层地下水补给沟渠水,地下水埋深变深,7月浅层地下水水化学类型与3月相似。调水可以季节性地改善区域内沟渠水及其附近的浅层地下水水质,而对深层地下水和坑塘水的水质无改善作用。调水对沟渠水水质的改善体现在调水季节,对浅层地下水水质的改善存在滞后性,2014年11月调水之后,2015年3月浅层地下水的水质得到改善。因此,采用调水和浅层地下水、坑塘水混合灌溉,对合理开发利用区域咸淡水资源以及深层地下水压采,恢复地下水位意义重大。     

Nitrate Depletion During Within-Stream Transport: Effects of Exchange Processes Between Streamwater,the Hyporheic and Riparian Zones

In regions with intensive agriculture and shallow hydrological systems, headstreams are often polluted with nitrate even at the springs. In North-West France, nitrate concentration was seen to decrease downstream during baseflow conditions when the stream flows on granite, but this does not occur on schist. In order to explain this difference in behaviour, we analysed the groundwaters and surveyed the redox conditions (using a field test for ferrous iron) in near-bank wet meadows as well as in the hyporheic zone. We show that the wet meadow groundwater was denitrified and that oxygen and nitrate were presentaround the stream channel in a wide zone on granite,compared with a very restricted zone on schist. Ongranite, exchanges between the stream and the hyporheic zone are favoured by sandy or peaty material having high hydraulic conductivity. This gives rise to two processes (1) lateral inflow of denitrified water from wet meadows, (2) in the opposite direction, supply of stream nitrate to denitrification sites in the hyporheic zone. In the second case, a high hydraulic conductivity also reduces the water residence time and limits denitrification, resulting in high levels of oxygen and nitrate. On schist, the low hydraulic conductivity prevents an efficientconnection between surface and subsurface waters.