The nature of zooplankton spatial heterogeneity in a nonriverine impoundment

Knowledge of spatial heterogeneity characteristic of reservoir plankton communities is fundamental to a variety of ecological studies. Degree of spatial heterogeneity in the zooplankton community of Center Hill Reservoir, with water residence times of 50–250 days, was positively correlated with rate of water influx. Important spatial differences resulted from the contrast between zooplankton associated with new and longer-impounded water. The nature of spatial heterogeneity differed fundamentally from the more riverine impoundments where spatial differences are often persistent and characterized by gradual change (as opposed to contrast) in plankton assemblages with respect to location. Magnitude of plankton spatial heterogeneity in nonriverine impoundments may be predictable from inflow rates. Areas, between which major differences in plankton communities exist, may also be definable from knowledge of inflow dispersal patterns in these impoundments.     

Fish parasitism in a new impoundment and differences upstream and downstream

The parasitic fish fauna in Guelph Lake, a four-year old impoundment, were compared with those in upstream and downstream localities. The lacustrine assemblage of the young mesotrophic to eutrophic impoundment was dominated by the cestodes, Ligula intestinalis and Proteocephalus ambloplites, and the nematode, Philometra sp. The acanthocephalan, Neoechinorhynchus saginatus, and the nematode, Rhabdacona cascadilla dominated the rheophilous assemblage of fish parasites, but the prevalence rate and intensity of R. cascadilla was significantly reduced downstream.     

Ecological Effects of Serial Impoundment on the Cotter River, Australia

This study examines the ecological effects of serial impoundments (three dams) on a rocky upland stream in southeastern Australia. Physical, chemical and biological changes were quantified and interpreted within a three-level hierarchy of effects model developed previously by Petts [1984, Impounded Rivers. John Wiley and Sons, New York] and the Australian Rivers Assessment System (AUSRIVAS) to predict pre-dam biota. First-order effects were decreased median monthly discharges and floods of lesser magnitude following construction of the dams. No effect on water characteristics (pH, electrical conductivity and major ions) was evident. The second-order effect on channel morphology was a decrease in bank-full cross-sectional area by up to 75% because of reduced flows. At all sites, the predominantly cobble streambed was armoured and generally highly stable. The discharge required to initiate movement of the streambed surface sediments (38.9 m³ s⁻¹) was 40% less frequent since construction of the dams, implying alteration to the natural disturbance regime for benthic biota. Benthic algal growth appeared more prolific at sites directly below dams. Fewer macroinvertebrate taxa than expected and modified assemblages within 1 km of all three dams were third-order effects. Compared to reference conditions, macroinvertebrate samples from the sites directly below the dams had relatively more Chironomidae larvae, Oligochaeta and Acarina, and fewer of the more sensitive taxa, Plecoptera, Ephemeroptera, Trichoptera and Coleoptera. Biological recovery to the macroinvertebrate assemblage was evident within 4 km downstream of the second dam.     

三峡水库蓄水后香溪河库湾底栖动物群落结构的变化

2003年8月到2004年8月对长江三峡水库香溪河库湾的底栖动物进行了定量研究。共采到底栖动物26种,隶属4门6科,总平均密度为276ind./m2,总平均生物量为0.301g/m2,优势种为霍甫水丝蚓和前突摇蚊幼虫。与其他同类型的水库比较香溪河库湾的现存量相对较低。研究结果同时显示随着蓄水时间的延长,底栖动物的现存量在逐渐增加。霍甫水丝蚓和前突摇蚊幼虫等耐污种发展为优势种,表明水质有恶化的趋势,寡毛类的平缓发展说明目前香溪河库湾的沉积情况尚不严重。     

四种温带森林土壤氧化亚氮通量及其影响因子

以中国东北东部4种典型森林生态系统(人工红松林、落叶松林、天然次生蒙古栎林和硬阔叶林)为研究对象,采用静态暗箱-气相色谱法,比较其土壤N₂O通量的季节动态及其影响因子.结果表明：在生长季, 4种森林生态系统土壤总体上表现为大气N₂O的排放源, 其N₂O通量大小顺序为：硬阔叶林(21.0±4.9 μg·m^-2·h^-1)> 红松林(17.6±4.6 μg·m^-2·h^-1)>落叶松林(9.8±5.9 μg·m^-2·h^-1)>蒙古栎林(1.6±12.6 μg·m^-2·h^-1).各生态系统的N₂O通量没有明显的季节动态,只在夏初出现排放峰值(蒙古栎林为吸收峰).4种生态系统N₂O通量均与10 cm深土壤含水量呈极显著正相关,与NO₃^--N呈显著负相关;N₂O通量对土壤温度和NH₄⁺-N的响应出现分异：针叶林N₂O 通量与NH₄⁺-N呈显著正相关,而与5 cm深土壤温度呈不相关;阔叶林与针叶林正相反.在较为干旱的2007年,土壤水分是影响4种林型土壤N₂O通量的关键因子.植被类型与环境因子及氮素有效性对N₂O通量的相互作用将是未来研究的重点.     

Dams have varying impacts on fish communities across latitudes: a quantitative synthesis

Dams are recognised to impact aquatic biodiversity, but the effects and conclusions diverge across studies and locations. By using a meta‐analytical approach, we quantified the effects of impoundment on fish communities distributed across three large biomes. The impacts of dams on richness and diversity differed across biomes, with significant declines in the tropics, lower amplitude but similar directional changes in temperate regions, and no changes in boreal regions. Our analyses showed that non‐native species increased significantly in tropical and temperate regulated rivers, but not in boreal rivers. In contrast, temporal trajectories in fish assemblage metrics were common across regions, with all biomes showing an increase in mean trophic level position and in the proportion of generalist species after impoundment. Such changes in fish assemblages may affect food web stability and merit closer study. Across the literature examined, predominant mechanisms that render fish assemblages susceptible to impacts from dams were: (1) the transformation of the lotic environment into a lentic environment; (2) habitat fragmentation and (3) the introduction of non‐native species. Collectively, our results highlight that an understanding of the regional context and a suite of community metrics are needed to make robust predictions about how fish will respond to river impoundments.     

The Effect of Sea Level Rise on Dabbling Duck Energetic Carrying Capacity

Waterfowl migrating and overwintering in the Atlantic Flyway depend on adequate availability of wetland plant communities to survive winter and fuel reproduction in the subsequent breeding season. Energetics models are the primary tool employed by conservation planners to estimate energetic carrying capacity based on energy supply and demand in different wetlands to assist with effective habitat conservation. Coastal impoundments have been used to provide a consistent, annual source of energy for migrating and wintering waterfowl. But few studies have attempted to comprehensively assess the relative value of managed coastal impoundments compared with unmanaged tidal salt marshes to wintering waterfowl in the Mid-Atlantic region with further consideration to the effect of sea level rise changing availability. We estimated biomass and energy of preferred foods for 5 dabbling duck species in 7 impoundments and 3 tidal salt marshes over winter by collecting soil core (n = 1,364), nekton (n = 426), and salt marsh snail (Melampus spp.; n = 87) samples in October, January, and April 2011–2013. Food-energy density was greater in freshwater impoundments for nearly all dabbling ducks (range = 183,344–562,089 kcal/ha), and typically greater in brackish impoundments (range = 169,665–357,160 kcal/ha) than most tidal salt marsh communities (range = 55,693–361,429 kcal/ha), whereas mudflat (range = 96,223–137,473 kcal/ha) and subtidal (range = 55,693–136,326 kcal/ha) communities typically contained the least energy. Extrapolating to the state level, we estimated 7.60 × 10⁹–1.14 × 10¹⁰ kcal available within a 16-km buffer from the Delaware Bayshore, depending on species. Combining estimates for daily energy expenditure and food energy, we estimated 2.86 × 10⁷–7.06 × 10⁷ duck energy days currently available to dabbling ducks over winter. We estimated that in the next century, dabbling duck carrying capacities are likely to decrease under all but the most conservative sea level rise scenarios because of the gradual replacement of land-cover types that provide high energy density (i.e., low marsh, high marsh communities) with those that provide low energy density (i.e., subtidal, mudflat communities). Coastal impoundments in Delaware, USA, will provide increasingly important habitat for wintering dabbling ducks in the coming decades provided they are properly maintained and retain their current energetic density because they will contain a growing proportion of the available duck energy days on the landscape. Our research will assist managers in meeting target population goals for dabbling ducks in Delaware and the Mid-Atlantic region by highlighting key differences in the function and value of various wetlands. © 2021 The Wildlife Society.     

WATER TRANSPARENCY CHARACTERISTICS OF SOUTH AFRICAN IMPOUNDMENTS

SUMMARY

Some water transparency characteristics (Secchi disc transparency, mean diffuse light attenuation coefficient and turbidity) of a number of South African impoundments are compared. In general most impoundments may be considered to be highly turbid; for 92 impoundments considered in this paper the modal mean Secchi disc transparency was in the class 0 to 0,5 m. The geographical distribution of water transparency within South Africa shows good agreement with the geographical distribution of sediment production, indicating that suspended inorganic material is the dominant factor governing water transparency in most impoundments. There is evidence that eutrophication and mineralization are processes which lead to increased water transparency. High turbidity and its role in influencing the limnology and management of impoundments is discussed. More in-depth investigations are needed to evaluate the contributions by coloured substances, suspended silt and phytoplankton to light attenuation within individual impoundments.     

Laboratory investigation on community composition, emergence patterns and biomass of wood-inhabiting Chironomidae (Diptera) from a sandy lowland stream in Central Europe (Germany)

Wood-inhabiting chironomid communities were investigated from June 1998 to July 1999 by laboratory rearing of randomly collected submerged branches from a headwater and mid-reach site of a sandy lowland stream, separated by an impoundment used for fishery purposes. Total annual emergence (males and females) from headwater samples was higher (2551 ind m^–2 y^–1) compared to the mid-reach (1576 ind m^–2 y^–1), which could be due to disturbances caused by frequent impoundment openings resulting in high discharge events. Chironomid community from branches comprised three subfamilies, with Orthocladiinae (18 species, 2189 ind m^–2 y^–1) clearly predominating at the headwater (total of 36 species). Mid-reach samples (total of 48 species) showed similar emergent numbers of Orthocladiinae (19 species, 786 ind m^–2 y^–1) and Chironominae (26 species, 764 ind m^–2 y^–1). Tanypodinae were caught very rarely in the laboratory emergence (12 ind m^–2 y^–1 at both sites). Shannon-Wiener diversity index for the mid-reach chironomid community was higher (2.52) than for the headwater community (1.68). Chironomid species composition on woody debris was similar between stream sites, with a Sørensen index of 0.75, but showed different dominance structures indicated by a Wainstein index of 0.26. Total adult biomass (in the manner of dry mass) during the study period was higher for wood-dwelling chironomids from the headwater (158.2 mg m^–2 y^–1) compared to the mid-reach (123.8 mg m^–2 y^–1), but individual biomass was higher for mid-reach chironomids (0.079 mg ind^–1 vs. 0.062 mg ind^–1 at the headwater), indicating the predominance of larger species. Total biomass of wood-inhabiting chironomids in the investigated lowland stream was low compared to other field emergence studies, which could be attributed to the laboratory approach for investigating the emergence from a single substrate type (submerged wood) instead of the integrative field surveys where chironomids from all habitats were caught. Main reason for the lack of chironomid species closely associated to wood in this sandy lowland stream could be infrequent but episodic disurbances caused by the anthropogenic induced highly fluctuating discharge regime of the downstream study site.