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排序方式: 共有475条查询结果,搜索用时 15 毫秒
61.
Carolyn W. Burns 《新西兰海洋与淡水研究杂志》2013,47(3):477-482
Few workers have used enclosures specifically to study interactions between zoo‐plankton and cyanobacteria. Differences among studies in enclosure size, nutrient level, plankton abundance and species composition, presence or absence of fish, and length of experiments make generalisations difficult. Zooplankton had no direct effect on the growth of ungrazed cyanobacteria (Anabaena flos‐aquae, Aphanizomenon flos‐aquae, large Microcystis colonies) in short‐term (<5 days) and long‐term (> 1 month) enclosure studies. When large cyanobacteria were abundant, some Daphnia spp. snowed reduced reproduction and development. When large grazers were abundant they suppressed the growth of edible, colonial cyanobacteria (Aphanizomenon elenkinii, small Microcystis colonies). By altering the ambient light and nutrient environment, large zooplankton may suppress cyanobacteria; evidence for the importance of grazers in promoting cyanobacterial dominance by removing competing phytoplankton species is equivocal. Zooplankton may suppress nitrogen fixation by cyanobacteria through ammonia excretion and may promote a change in dominance from diatoms to cyanobacteria through recycling phosphorus but not silicon. 相似文献
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Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large‐scale nitrate transformation in a groundwater‐fed river
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P. Byrne H. Zhang S. Ullah A. Binley A.L. Heathwaite C.M. Heppell K. Lansdown M. Trimmer 《水文研究》2015,29(6):1385-1396
The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse‐scale (5–10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine‐scale (<1 cm) biogeochemical patterns, especially in near‐surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO3?. In this study, we utilised diffusive equilibrium in thin‐films samplers to capture high resolution (cm‐scale) vertical concentration profiles of NO3?, SO42?, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub‐reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin‐films samplers indicate considerable cm‐scale variability in NO3? (4.4 ± 2.9 mg N/L), SO42? (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface‐subsurface water mixing in the shallow sediments of our sub‐reach under baseflow conditions. The significance of this is that upwelling NO3?‐rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub‐reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper‐seated groundwater fluxes and hydro‐stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub‐reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd. 相似文献
65.
The Mondego estuary (Portugal) suffered major changes in environmental quality due to eutrophication, however, in the late 1990s a restoration project was implemented in order to return the system to its original condition. The main goal of this paper is to evaluate the ecosystem response to the restoration measures applied at three different levels: water quality, primary producers and primary consumers.In post-restoration period a clear decline was observed in dissolved inorganic nitrogen which was reflected in the gradual recovery of Zostera noltii and a concomitant decline in green macroalgae.Macrobenthic assemblages responded variably to the recovery process. In the seagrass bed and intermediate area, there was a large increase in total biomass, but in the eutrophic area species diversity increased. Despite improvement in the ecological status of the system, full recovery has not been achieved yet, possible due to hysteresis in the dynamics of this system. 相似文献
66.
Hyeon Ho Shin Koichiro Mizushima Jong Sick Park Mitsunori Iwataki Yang Ho Yoon 《Marine pollution bulletin》2010,60(8):1243-1258
Dinoflagellate cysts acquired from sediment cores were analyzed in order to reconstruct historical nutrient levels in Gamak Bay, Korea and Ariake Bay, Japan. Dinoflagellate cyst assemblages in Gamak Bay were characterized by high proportions of heterotrophic cysts such as Brigantedinium spp., Protoperidinium americanum and Polykrikos cysts, which suggested that nutrients levels may have already been high before 1970s, and then increased further to the hypertrophic conditions of the 1990s. In contrast, dinoflagellate cyst assemblages in Ariake Bay were characterized by high relative abundances of Lingulodinium machaerophorum and Spiniferites spp., which suggested that nutrient levels in Ariake Bay had increased gradually since the mid 1960s, and may have been significantly enhanced by the mid 1980s. Dinoflagellate cyst assemblages reflecting environmental changes in the two bays are contrasting, perhaps due to different nutrient enrichment mechanisms. This suggests that the indicators of nutrient levels encoded in dinoflagellate cyst assemblages may exhibit site-specific information. 相似文献
67.
Peter Herzsprung Martin Schultze Bertram Boehrer Anja Duffek Andrea Van der Veen Kurt Friese 《Limnologica》2010,40(2):182-190
Between 1999 and 2002, a former open-cast mine was filled with river water forming the recent Lake Goitsche. During filling initially acid water was neutralised. Phosphorus (P) imported from Mulde River was nearly completely removed from the water column by co-precipitation with iron (Fe) and aluminium (Al) and deposited in the sediment.During extremely high waters of the Mulde River in 2002, a dike breach facilitated a second high import of P into Lake Goitsche with suspended and dissolved matter. The analysis of total phosphorus (TP), however, showed that P again had been eliminated from the water body a few months after the flood event. Sediment investigations before filling with river water, during filling, and after the flood event were used to analyse the process of P immobilisation in a lake with acid mine drainage history.The ratios of Fe to soluble reactive P (SRP) of sediment pore water were up to three orders of magnitudes higher than in natural lakes and can serve as an indicator for potential internal P loading from sediments. The SRP concentrations at the oxic/anoxic boundary were near or below the limit of quantification (< 0.2 μmol/L). Fe and manganese (Mn) redox cycling were responsible for hindering P dissolution from sediment to lake water.Finally it can be stated, that the risk of eutrophication for such a lake seems to be low. 相似文献
68.
María M. Castillo 《Limnologica》2010,40(4):322-329
The influence of landscape on nutrient concentration and yield was analyzed in a tropical catchment, the Guare River in northern Venezuela. Spatial and temporal variation in nitrate, SRP and total P were determined in 15 sites located along the river mainstem and tributaries. Higher nitrate concentrations and yields were reported from upper sites and both decreased in the downstream direction along the river mainstem. These trends appear to be related to more pronounced slopes and larger proportions of agricultural and forest lands in subcatchments located in the upper part of the basin, and dense algal mats in the lower reaches. Nitrate values were higher during periods of high discharge, suggesting that nitrate is primarily transported by shallow subsurface flow. SRP represented between 60 and 80% of total P. Phosphorus concentrations were homogeneous along the river mainstem and showed little seasonal variation, while yields were higher in the upper basin. Multiple regression identified slope, runoff and agriculture as primary predictors of nitrate and phosphorus across the watershed, which suggests that both natural and anthropogenic landscape characteristics have a strong influence on nutrient levels in the Guare catchment. 相似文献
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70.
《Marine pollution bulletin》2013,76(1-2):98-104
A field study to check parameter stratification during high density growth of four opportunistic macroalgae was carried out in Orbetello lagoon (Italy). The effects of macroalgal masses were compared with a seagrass meadow and two lagoon areas with bare bottoms as controls for pH, temperature, dissolved oxygen, salinity, nitrite, nitrate ammonium and orthophosphate. The nutrient content of thalli and sediment redox were measured. Macroalgae showed differences in stratification of thalli nutrient content. Mat with low density and high volume produced stratifications in the water column, but it did not produce nutrient release by sediment. In contrast, high density and low and high volume mats led to sharp falls in dissolved oxygen, with negative values of sediment Eh and anoxic trigger processes that presumably led to release of sediment nutrients. This depended on thallus type: heavy thalli compacted the mat and light ones distributed more widely in the water column. 相似文献