Benthic fluxes of nutrients and some trace metals in the Tamar estuary,SW England

Pore water concentration gradients and fluxes of chemical components have been studied in sediments from six intertidal sites in the Tamar Estuary, SW England over the course of a year. Fluxes of nutrients (ammonia, nitrate, nitrite, phosphate and silicate) and trace metals (iron, manganese, zinc, copper and cadmium) were determined using a laboratory microcosm incorporatingin situ pore water samplers. Nutrients (except nitrate) were transported out of the sediment throughout the year, but nitrate fluxes were directed into the sediment in the summer (denitrification) and out of the sediment in the winter (nitrification). The activities of benthic macrofauna resulted in enhanced fluxes but these differed between sites depending on population structure and density and whether irrigation or sediment reworking predominated. Fluxes of trace metals were seasonally and spatially variable and specific differences were observed that could be attributed to both chemical and biological activity.     

Genetic variation in Atlantic salmon, Salmo salar L., within the Tamar catchment in south-west England

Electrophoretic analysis of enzymes in 383 juvenile Atlantic salmon, Salmo salar L., within the Tamar catchment (south-west England) revealed significant genetic differences at the IDHP-3* locus between the three tributaries studied. Aspects of temporal and spatial variation, and management policy within catchments, are discussed.     

Correlation of the distribution ofMesopodopsis slabberi (Crustacae,Mysidacea) with physico-chemical gradients in a partially-mixed estuary (Tamar,England)

The physical and chemical processes operating in the River Tamar Estuary (south-west England) have been comprehensively described and reported in the literature. There are well-established gradients of salinity, suspended sediment and oxygen which vary both on short-term (tidal) and long-term (seasonal) cycles. Freshwater runoff, the main factor determining salinity distribution, is also the cause of the high variability in suspended sediment concentrations. The biological processes are less well studied and information on the link between the benthic and pelagic systems is particularly lacking. Mysids, through their role as detritivores and as a major component in the diet of some fish, provide this link. Of the four species of mysid distributed longitudinally in the Tamar Estuary, the most abundant isMesopodopsis slabberi which occurs between 5 and 25 km from the estuary head. Observations over an annual cycle have shown marked seasonal changes in both abundance and distribution in the estuary. During winter and spring, densities remained generally low (<50 m⁻³) but, as water temperatures increased, the density increased and reachedca 1200 individuals m⁻³ in July. There was a shift in the longitudinal distribution ofM. slabberi in response to changes in the position of the salinity gradient. Adults comprised the majority of the population in salinities less than 10‰ whereas juveniles and immature animals were distributed over a wider area than the adults and occurred in water of higher salinity than the main adult distribution.M. slabberi appears to utilise the two-layered estuarine circulation to maintain its position in the estuary.     

Bioavailability of arsenic in soil and mine wastes of the Tamar valley,SW England

ABSTRACT

The southern part of the Tamar valley area in SW England is highly mineralised and mines in the region were the world's principal producers of tin, copper and arsenic during the mid nineteenth century. The Devon Great Consols Mine, covering 67.6 ha (167 acres) is situated in this area. Residues from the mining activity resulted in unvegetated spoil tips and local soils highly contaminated with As (range 120–52600 μg/g As). Sequential chemical extraction procedures were conducted on eight surface samples (0–15 cm) taken from a 2.0 km long transect from within the mine site to agricultural grassland. The proportion of water extractable As in agricultural top soils was lower (0.05–0.3%) than the values obtained for mine wastes (0.02–1.2%). Arsenic was found to be concentrated in the Fe-organic and residual fractions, which accounted for up 93 % of the total As in mine spoil and nearby soils.     

An assessment of the potential impact of dredging activity on the Tamar Estuary over the last century: Bathymetric and hydrodynamic changes

The Tamar Estuary, S.W. England, is a commercial and military port and the site of a major naval dockyard. Parts of the estuary require regular maintenance dredging and some capital dredging has been undertaken. The estuary is also assigned Special Area of Conservation (SAC) status under the European Habitats Directive and is an important site for migrating wildfowl and wading birds. As part of an assessment undertaken for a consortium of the port operators and regulatory authorities, we have analysed the ecological, physical, chemical and socio-economic impacts of dredging on the Tamar Estuary ecosystem. This paper focuses on the physical changes through the analysis of historical survey data and the use of hydrodynamic modelling. The objective was to evaluate whether dredging has influenced the estuary morphology or hydrodynamic regime and thus impacted on the ecological habitat. Bathymetric survey data collected between 1895 and 1968 were compared with multi-beam echo sounder data obtained in 2001. The paper survey records were digitised, geo-referenced and adjusted to the modern datum and units. The accuracy and limitations of this approach are discussed. Both sets of data were contoured and analysed using a GIS package. Comparisons between 1895 and 2001 hydraulics were made using a 1-D hydrodynamic and sediment transport model set up for the bathymetry at those times but ‘driven’ using a single, common, year-long data set for tides and river flows. The results suggest that although the deep channel in the region that is mainly south of the Tamar road bridge, is on average 0.5 m deeper in 2001 than it was in 1895, this has had a negligible effect on the area of intertidal mud and, thus, on habitat available for wading birds, and minimal impact on the large-scale hydrodynamics of the estuary.     

Nutrient stoichiometry of linked catchment‐lake systems along a gradient of land use

1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed‐use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed‐use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed‐use lake shifted seasonally from P‐ to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.     

Potential modification of the fluxes of nitrogen from the Humber Estuary catchment (U.K.) to the North Sea in response to changing agricultural inputs and climate patterns

The catchment of the Humber Estuary drains approximately 20% of the land area of England via two main rivers, the Trent and the Ouse, and a number of tributaries. The catchment is home to major metropolitan and industrial centres, as well as to extensive areas of agricultural land; for this reason, the river and estuarine systems have been subject to considerable anthropogenic inputs. The Humber Estuary is one of the largest U.K. estuaries and the major U.K. freshwater input to the North Sea. The U.K. Natural Environment Research Council (NERC) Land Ocean Interaction Study (LOIS), which combined extensive physical and biogeochemical measurements with an integrated modelling programme, was established to examine the transport and fate of nutrients and other constituents through the land-sea boundary. In this paper, a model of nitrogen (nitrate, nitrite, ammonium, particulate nitrogen) transport and cycling in the Humber Estuary, calibrated on the basis of measured constituent concentrations at its riverine and marine boundaries, is linked off-line to a Humber catchment and rivers model of nitrogen transport, which furnished simulated constituent values at the tidal limits, and the resulting estuarine nitrogen profiles compared to those of the standalone estuarine model. The estuarine model is then re-run using simulated concentration values at the tidal limits from catchment-river model simulations incorporating realistic changes in agricultural fertiliser inputs and climate forcing functions. The standalone estuarine model simulation estimated nitrate+nitrite (total nitrogen) export to the North Sea to be ca. 53000 t in 1994 and 44000 t in 1995. Following linkage of the estuarine and catchment-river models, the estimated fluxes for these years increased by 20–30%, {relative to the standalone simulation}. Higher {winter} riverine flows largely accounted for this difference. The altered flows also markedly changed the simulated concentrations and distributions of suspended particulate matter (SPM) within the estuary, indicating strongly that the transport and fluxes of particle-reactive and particle-associated constituents would show measurable differences. Scatter in the measured SPM data precluded identification of the more precise simulation run, however. Subsequent simulations using the linked models estimated that a 50% reduction in artificial fertiliser applications within the catchment gave a 10–15% decrease in nitrogen loads to the North Sea, relative to the 1994–95 input, whilst forcing the catchment model with a climate perhaps appropriate for the mid-21st century yielded nitrogen fluxes that were similar to those of the mid-1990s.     

Anthropogenic nitrogen sources and relationships to riverine nitrogen export in the northeastern U.S.A.

Human activities have greatly altered the nitrogen (N) cycle, accelerating the rate of N fixation in landscapes and delivery of N to water bodies. To examine relationships between anthropogenic N inputs and riverine N export, we constructed budgets describing N inputs and losses for 16 catchments, which encompass a range of climatic variability and are major drainages to the coast of the North Atlantic Ocean along a latitudinal profile from Maine to Virginia. Using data from the early 1990's, we quantified inputs of N to each catchment from atmospheric deposition, application of nitrogenous fertilizers, biological nitrogen fixation, and import of N in agricultural products (food and feed). We compared these inputs with N losses from the system in riverine export.The importance of the relative sources varies widely by catchment and is related to land use. Net atmospheric deposition was the largest N source (>60%) to the forested basins of northern New England (e.g. Penobscot and Kennebec); net import of N in food was the largest source of N to the more populated regions of southern New England (e.g. Charles & Blackstone); and agricultural inputs were the dominant N sources in the Mid-Atlantic region (e.g. Schuylkill & Potomac). Over the combined area of the catchments, net atmospheric deposition was the largest single source input (31%), followed by net imports of N in food and feed (25%), fixation in agricultural lands (24%), fertilizer use (15%), and fixation in forests (5%). The combined effect of fertilizer use, fixation in crop lands, and animal feed imports makes agriculture the largest overall source of N. Riverine export of N is well correlated with N inputs, but it accounts for only a fraction (25%) of the total N inputs. This work provides an understanding of the sources of N in landscapes, and highlights how human activities impact N cycling in the northeast region.     

Numerical indices applied to the results of a survey of the macro-invertebrate fauna ofthe Tamar catchment (southwest England)

A survey ofthe macro-invertebrate fauna in the River Tamar revealed that a division ofthe catchment can be made which separates an organically enriched upper region from a more stable environment in the lower tributaries and reaches. Invertebrate species normally associated with organic pollution were found immediately downstream from farm waste and sewage effluent outfalls. Invertebrate species normally associated with an unpolluted condition were widely distributed throughout the catchment, and revealed that the Tamar is essentially free from gross pollution causing the continuous elimination of stream fauna. Systems used by pollution control organizations to codify biological results are reviewed and applied comparatively to the results. The Diversity index was found to be the most consistent method in assessing the biological state of the river. The Trent, Lothian, Chandler and Carpenter indices were found to give either anomalous figures or proved insensitive to conditions beyond a certain quality.     

Diet overlap and resource partitioning among three forage fish species in Coorong,the largest inverse estuary in Australia

Protracted drought in southern Australia has degraded the system function of the Coorong estuary and intensified resource competition among forage fish species. The present study investigates the gut content, prey composition and dietary overlap of three forage fishes: smallmouth hardyhead (Atherinosoma microstoma), Tamar River goby (Afurcagobius tamarensis) and sandy sprat (Hyperlophus vittatus) influenced by environmental variation in the Murray Estuary and Coorong. The prey species identified in fish stomachs were dominated by crustaceans (amphipods, ostracods and harpacticoids), but nematodes and acanthocephalans were also common in all forage fishes. The diet of the sandy sprat and Tamar River goby highly overlapped (α = 0.8) in the Murray Estuary and all three forage fishes showed potential diet overlap (α ≥ 0.6) in the North Lagoon. Spatiotemporal variation of prey diversity was observed in smallmouth hardyhead while temporal variation of prey diversity was observed in sandy sprat and Tamar River goby. Overall, the prey abundance was temporally variable and predominantly regulated by salinity, pH, dissolved oxygen, water transparency and chlorophyll a in the Murray Estuary and Coorong. This study adds to our knowledge on dietary overlap and resource partitioning among small-bodied forage fishes mediated by environmental factors in the Murray Estuary and Coorong.     

The determination of total nitrogen and total phosphorus concentrations in freshwaters from land use, stock headage and population data: testing of a model for use in conservation and water quality management

1. Nutrient concentrations (particularly N and P) determine the extent to which water bodies are or may become eutrophic. Direct determination of nutrient content on a wide scale is labour intensive but the main sources of N and P are well known. This paper describes and tests an export coefficient model for prediction of total N and total P from: (i) land use, stock headage and human population; (ii) the export rates of N and P from these sources; and (iii) the river discharge. Such a model might be used to forecast the effects of changes in land use in the future and to hindcast past water quality to establish comparative or baseline states for the monitoring of change. 2. The model has been calibrated against observed data for 1988 and validated against sets of observed data for a sequence of earlier years in ten British catchments varying from uplands through rolling, fertile lowlands to the flat topography of East Anglia. 3. The model predicted total N and total P concentrations with high precision (> 95% of the variance in observed data explained). It has been used in two forms: the first on a specific catchment basis; the second for a larger natural region which contains the catchment with the assumption that all catchments within that region will be similar. Both models gave similar results with little loss of precision in the latter case. This implies that it will be possible to describe the overall pattern of nutrient export in the UK with only a fraction of the effort needed to carry out the calculations for each individual water body. 4. Comparison between land use, stock headage, population numbers and nutrient export for the ten catchments in the pre-war year of 1931, and for 1970 and 1988 show that there has been a substantial loss of rough grazing to fertilized temporary and permanent grasslands, an increase in the hectarage devoted to arable, consistent increases in the stocking of cattle and sheep and a marked movement of humans to these rural catchments. 5. All of these trends have increased the flows of nutrients with more than a doubling of both total N and total P loads during the period. On average in these rural catchments, stock wastes have been the greatest contributors to both N and P exports, with cultivation the next most important source of N and people of P. Ratios of N to P were high in 1931 and remain little changed so that, in these catchments, phosphorus continues to be the nutrient most likely to control algal crops in standing waters supplied by the rivers studied.     

Mechanisms underlying export of N from high-elevation catchments during seasonal transitions

Mechanisms underlying catchment export of nitrogen (N) during seasonal transitions (i.e., winter to spring and summer to autumn) were investigated in high-elevation catchments of the Sierra Nevada using stable isotopes of nitrate and water, intensive monitoring of stream chemistry and detailed catchment N-budgets. We had four objectives: (1) determine the relative contribution of snowpack and soil nitrate to the spring nitrate pulse, (2) look for evidence of biotic control of N losses at the catchment scale, (3) examine dissolved organic nitrogen ( DON) export patterns to gain a better understanding of the biological and hydrological controls on DON loss, and (4) examine the relationship between soil physico-chemical conditions and N export. At the Emerald Lake watershed, nitrogen budgets and isotopic analyses of the spring nitrate pulse indicate that 50 to 70% of the total nitrate exported during snowmelt (ca. April to July) is derived from catchment soils and talus; the remainder is snowpack nitrate. The spring nitrate pulse occurred several weeks after the start of snowmelt and was different from export patterns of less biologically labile compounds such as silica and DON suggesting that: (1) nitrate is produced and released from soils only after intense flushing has occurred and (2) a microbial N-sink is operating in catchment soils during the early stages of snowmelt. DON concentrations varied less than 20–30% during snowmelt, indicating that soil processes tightly controlled DON losses.     

The influence of impoundments on nutrient budgets in two catchments of Southeastern Michigan

N and P budgets quantify inputs and outputs of nutrients at the catchment scale to allow evaluation of inputs and outputs as well as inferences about transport and processing based on unaccounted-for nutrients. N and P budgets were constructed for two catchments in southeastern Michigan with markedly different numbers of impoundments, over two years, to evaluate the influence of impoundments on nutrient fluxes from each catchment. The Huron, with 88 impoundments >10 ha, stored 156 kg P km⁻² y⁻¹, while the Raisin (with 14 impoundments) had a net export of 102 kg P km⁻² y⁻¹. The Huron catchment also stored and denitrified more N than the Raisin catchment – 2,418 kg N km⁻² y⁻¹ compared to 1,538 kg N km⁻² y⁻¹. Riverine export of N and P also varied markedly between the catchments, with the Huron River exporting 288 kg N and 7 kg P km⁻² y⁻¹ and the Raisin River exporting 1,268 kg N and 34 kg P km⁻² y⁻¹. We then re-calculated budget results from previous studies using the approach of the present study, altering input and outputs fluxes as well as system boundaries to obtain comparable budgets. For these comparable budgets, annual P outputs on average accounted for 77% of inputs whereas N outputs accounted for only 39% of N inputs. Across catchments, the percent of inputs exported by the river averaged 16% for N and 5% for P, indicating more effective retention of P than N.     

Impact of DOC trends resulting from changing climatic extremes and atmospheric deposition chemistry on periphyton community of a freshwater tropical lake of India

Recent measurements have demonstrated unprecedented increase in atmospheric deposition of nutrients in many parts of India. To determine whether atmospheric nutrient inputs would increase phytoplankton growth and catchment dissolved organic carbon (DOC) flushing to constrain benthic algae, we analyzed NO₃ ^? and PO ₄ ^?3 in atmospheric deposits; nutrients and DOC in runoff and lake water and standing crop biomass of phytoplankton and periphyton at Jaisamand Lake of Rajasthan, India. Atmospheric deposition of NO₃ ^? (7.18–29.95 kg ha^?1 year^?1) and PO ₄ ^?3 (0.56–2.15 kg ha^?1 year^?1) showed a consistently rising trend across the year. Microbial biomass and activity in catchment increased in response to atmospheric deposition. Lake DOC and nutrients showed strong coherence with their terrestrial and atmospheric fluxes. Phytoplankton development showed significant linearity with atmospheric input of nutrients. Air-driven input appeared to have compensated the nutrient constraints to phytoplankton during drought. The N:P stoichiometry of deposition and that of lake water indicated that, although there was a seasonal switchover to N- or P-limitation, phytoplankton were mainly co-limited by N and P due probably to the synergistic effects of combined N + P enrichment in the pelagic zone of the lake. Periphyton standing crop showed inverse relationship with phytoplankton and lake DOC. The study indicated that enhanced phytoplankton development and terrestrial DOC flushing in response to atmospheric nutrient input attenuated light penetration to constrain algal periphyton. We suggests that data on these issues may be considered in developing aquatic ecosystem models to establish future links between changing air–water–land interactions and associated shifts in lake ecosystem functioning for more accurately predicting climate change drivers and designing integrated lake basin management strategies.     

Salvinia auriculata in post-treatment of dairy industry wastewater

Abstract

The present study aimed to evaluate the aquatic macrophyte Savinia auriculata in post-treatment of wastewater from a dairy industry. The experiment was carried out in a greenhouse between February and March 2015. A batch system was used, each reactor was composed of polyethylene and had capacity of 250 liters of post-treated effluent. Every seven days, pH, turbidity, temperature, dissolved oxygen, chemical oxygen demand and series of solids (total, fixed and volatile) were determined in the wastewater. Besides that, the concentration of macro and micronutrients (P, N, K, Ca, Mg, Cu, Zn, Mn, and Fe) were determined in the wastewater and in plant tissue before and after the experiment. The results showed efficiency in the decrease of N, P, turbidity, pH, solids (ST, SF) and inefficiency in the reduction of volatile solids in the wastewater. The concentration of COD, Ca, Mn, Mg, and Fe increased at the end of the experiment. Due to the absorption of some nutrients such as N and P it is possible to conclude that Salvinia auriculata is a good option for the post-treatment of the wastewater from dairy industry. However, plant senescence promotes the elevation of some elements in the effluent because in this process, nutrients that were previously retained in the plant tissue are inserted into the wastewater again. Because of this it is necessary to remove plants of the reactor in the beginning of the process of senescence.     

The turbidity maximum in the Tamar estuary

Many estuaries of medium to high tidal range exhibit an accumulation of fine cohesive material in their upper reaches in the region of the limit of saline intrusion. Much, or all, of this material is suspended each tidal cycle and the entire region undergoes a seasonal variation which appears to depend on fluvial input. Two factors which are throught to influence the formation and maintenance of turbidity maxima are the differing magnitudes of the bed shear stress (τ₀) on flood and ebb tides and the large vertical density gradient which developes on the ebb tide. Crucial to the importance of the first factor is that τ₀ exceeds a critical value, at which erosion occurs, for a greater period on the flood than on the ebb. The effect of the density gradient is that upward propagation of bed generated eddies is inhibited and the sediment is not transported into the upper part of the flow where it will be most effectively transported. It is not clear which, if either, of these mechanisms is dominant. Data consisting of vertical profiles of velocity, salinity and suspended solids were collected at four stations in the Tamar estuary during a high range tidal cycle. One station, at which the depth mean salinity (S _d ) varied from 0.0 to ∼ 12.0‰, was occupied permanently. The other stations were occupied such that data were collected asS _d varied in the range 0.0 to ∼ 4.0‰. In this way each station was occupied for a period of time on the ebb and flood tide. Observations show that during the early ebb, when the flow is relatively deep and slow, stratification persist untilS _d ∼ 0.0‰ and that no significant transport occurs while the flow is saline but that there is a rapid increase in suspended solids concentrations after this time. During the later ebb the shallower faster flow allowed the density gradient to be erode and significant transport was observed atS _d ∼ 5.0‰. On the flood tide the flow in the low salinity region is well-mixed troughout. Computation of the fluxes and total transport per unit breath of estuary show that on the ebb tide the quantity of solid material being transported by the low salinity (0–3‰) region remains nearly constant as this region of the flow is advebted seaward. On the flood tide, however, as the same region is advected landward the quantity of material being transported increases. It is concluded that in the Tamar estuary the early ebb tide stratification contributed to the formation and maintenance of a turbidity maximum which is strongly associated with the low salinity region of the flow. It is also speculated that the differences in the ebb and flood tide transport are caused by differences in the availability of mobile material on the bed at different stages of the tidal cycle.     

放牧对全球草地生态系统碳氮磷化学计量特征影响的Meta分析

为明确全球尺度下放牧管理措施对草地生态系统碳(C)、氮(N)、磷(P)化学计量特征的影响,提高草地生态系统管理水平,本研究选取国内外83篇中英文文献进行Meta分析,并通过亚组分析探讨了放牧家畜组合(羊单牧、牛单牧和牛羊混牧)和放牧强度(轻度、中度、重度)对草地生态系统叶片、凋落物、根系,以及土壤C、N、P化学计量特征的影响。结果表明: 放牧会显著降低叶片和凋落物C含量、C/N、C/P,增加N、P含量及N/P;显著降低根系和土壤C、N含量,C/P和N/P,增加P含量和C/N。叶片、凋落物化学计量特征变化对牛、羊单独放牧响应更为明显,而根系、土壤化学计量特征变化则对混牧响应更为明显,重度放牧会对草地生态系统化学计量特征产生更大的影响。放牧会降低土壤N含量,增加P含量,表明放牧对草地N、P含量的影响路径不同。进一步研究N、P含量变化对放牧活动不平衡响应机制,将放牧方式、强度的影响纳入草地生态系统预测、管理模型,能够有效提高草地生态系统管理水平。     

The Status and Characteristics of Eutrophication in the Yangtze River (Changjiang) Estuary and the Adjacent East China Sea, China

Eutrophication has become increasingly serious and noxious algal blooms have been of more frequent occurrence in the Yangtze River Estuary and in the adjacent East China Sea. In 2003 and 2004, four cruises were undertaken in three zones in the estuary and in the adjacent sea to investigate nitrate (NO₃–N), ammonium (NH₄–N), nitrite (NO₂–N), soluble reactive phosphorus (SRP), dissolved reactive silica (DRSi), dissolved oxygen (DO), phytoplankton chlorophyll a (Chl a) and suspended particulate matter (SPM). The highest concentrations of DIN (NO₃–N+NH₄–N+NO₂–N), SRP and DRSi were 131.6, 1.2 and 155.6 μM, respectively. The maximum Chl a concentration was 19.5 mg m⁻³ in spring. An analysis of historical and recent data revealed that in the last 40 years, nitrate and SRP concentrations increased from 11 to 97 μM and from 0.4 to 0.95 μM, respectively. From 1963 to 2004, N:P ratios also increased from 30–40 up to 150. In parallel with the N and P enrichment, a significant increase of Chl a was detected, Chl a maximum being 20 mg m⁻³, nearly four times higher than in the 1980s. In 2004, the mean DO concentration in bottom waters was 4.35 mg l⁻¹, much lower than in the 1980s. In comparison with other estuaries, the Yangtze River Estuary was characterized by high DIN and DRSi concentrations, with low SRP concentrations. Despite the higher nutrient concentrations, Chl a concentrations were lower in the inner estuary (Zones 1 and 2) than in the adjacent sea (Zone 3). Based on nutrient availability, SPM and hydrodynamics, we assumed that in Zones 1 and 2 phytoplankton growth was suppressed by high turbidity, large tidal amplitude and short residence time. Furthermore, in Zone 3 water stratification was also an important factor that resulted in a greater phytoplankton biomass and lower DO concentrations. Due to hydrodynamics and turbidity, the open sea was unexpectedly more sensitive to nutrient enrichment and related eutrophication processes.     

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

珠江口及毗邻海域营养盐对浮游植物生长的影响

基于2006年7月(夏季),10月(秋季)和2007年3月(春季)的现场调查数据,对珠江口及毗邻海域中的营养盐和叶绿素a等环境生态因子的时空分布特性进行了对比分析,研究了氮磷比与叶绿素a含量和种群多样性之间的联系,探讨了该海域营养盐对于浮游植物生长的影响。结果表明:(1)研究海域营养盐表现出较强的季节和空间差异性,总氮(TN)和总磷(TP)浓度均值春季(1.545 mg/L、0.056 mg/L)和夏季(1.570 mg/L、0.058 mg/L)均大于秋季(1.442 mg/L、0.034 mg/L),且春夏季浓度空间差异更明显。(2)调查期间海域营养盐含量超标现象突出,夏季尤为明显。无机氮(DIN)总体均值0.99 mg/L,超四类海水标准限值1倍,活性磷酸盐(PO4-P)总体均值0.021 mg/L,DIN∶PO4-P平均值为130;叶绿素a浓度与营养盐、p H、温度有较显著的相关性。(3)叶绿素a浓度较高的站位,具有较高的DIN∶PO4-P值,但浮游植物多样性指数偏低,优势种明显,主要为中肋骨条藻。氮磷比的改变会影响不同生长特性的浮游植物间的竞争和种群结构的改变;今后海洋污染治理中,在控制氮、磷污染时要注意氮磷比的改变可能造成的浮游生态影响。