Interferences between root plaque formation and phosphorus availability for isoetids in sediments of oligotrophic lakes

Freshwater isoetids exchanges a high proportion of the photosynthetically produced oxygen over the extensive root system and, therefore, they influence the redox potential (E_h) and phosphorus (P) availability in their sediments. Because isoetids rely on the sediment for P uptake, P may be a key element in controlling the distribution of isoetids. We investigated biomass and P availability to isoetids (Littorella uniflora and Isoetes lacustris) in a transect of five stations across the littoral zone in oligotrophic Lake Kalgaard, Denmark. At the two shallowest stations (0.6 and 1.0 m depth) the redox potential in the low organic rhizosphere sediment was high (>300 mV) and low concentrations of reduced exchangeable iron (Fe) and manganese (Mn) compounds in the sediment and of precipitated Fe and Mn oxides on isoetid roots (plaques) were found. The concentration of sediment P pools was low and so was isoetid P content and isoetid biomass. At intermediate water depth (1.8 m) sediment E_h was high (300 mV) and isoetids showed low root plaque concentrations. However, higher concentration of P pools in the rhizosphere was found at 1.8 m and isoetids showed the highest P content and biomass. At deeper stations (2.8 and 4.6 m depth) E_h was low (<100 mV) in the high organic rhizosphere and high concentrations of plaques were found. The P content in the sediment was high, however, isoetids showed low biomass and low P content. We suggest that the low P content in isoetids growing on P rich organic sediments is partly due to inhibition of the P uptake because of adsorption of P to the oxidized Fe and Mn plaques. However, ratios between oxidized Fe and Fe-bound P, 150 for plaques and 40 for sediment, suggest the isoetids are able to access some of the P that is bound in the plaques. The pools of dissolved P in the porewater were 25–1100 times lower than the estimated annual P requirement for net growth of isoetids while solid fraction P pools were 20–260 times higher than the estimated annual P requirement. Clearly, the oxygen release from isoetid roots decreases the availability of P either by keeping the entire rhizosphere oxidized (low organic sediments) or by the formation of root plaques (high organic sediments).     

Microdistribution of major to trace elements between roots of Halimione portulacoides and host sediments (Tagus estuary marsh,Portugal)

Aim

This study presents a micrometre-scale map of the elemental distribution within roots and surrounding sediment of Halimione portulacoides of a contaminated salt marsh in the Tagus estuary.

Methods

Microprobe particle induced X-ray emission analysis was performed in sediment slices containing roots with tubular rhizoconcretions attached to host sediments.

Results

Strong concentration gradients were found particularly in the inner part of rhizoconcretions adjacent to the root wall. Local enrichment was observed in sediment interstices with Fe precipitates and other associated elements. A maximum of 55 % of Fe was measured near the concretion–root interface, with a decrease to <5 % in the host sediment. Maximum concentrations of P (3 %), As (1,200 μg g^?1) and Zn (3,000 μg g^?1) were registered in concretions, one order of magnitude above the values of the host sediment. The elemental concentration profiles across roots showed that the epidermis was an efficient selective barrier to the entrance of elements. Fe and As were retained in the epidermis. The highest Cu and Zn concentrations were also observed in the epidermis. However, the concentrations of Mn, Cu and Zn increased in the inner root.

Conclusions

As and Fe were mostly retained in the concretion, whereas P, Mn, Cu and Zn were mobilised by the root.     

Chemical composition of modern and pre-acidification sediments in the Tatra Mountain lakes

Concentrations of major nutrients (C, N, P) and acid soluble metals (Ca, Mg, K, Al, Fe, Mn, Pb, and Zn) were determined in modern (0–1 cm) and pre-acidification (5–10 cm) sediment layers collected from 37 alpine and 3 forest lakes in the Tatra Mountains (Slovakia, Poland) in 1996–1998. Sediment composition reflected catchment characteristics and productivity of lakes. In the sediments of alpine lakes, C and N concentrations decreased and Mg increased with a decreasing proportion of vegetation and soil in the catchment. Decreasing Ca:Mg ratios in sediments along the vegetation gradient was inverse to that in water, and could be associated with different ratios of cations in water leachate from catchments and in solids which enter the lake due to soil erosion. Phosphorus concentrations increased with the proportion of moraine areas, with till soils rich in P. Concentrations of C, N, P, and Ca in sediments positively correlated to their concentrations in water. Sediment concentrations of Al and Al:Ca ratios increased with decreasing sediment and water pH. A negative correlation between water pH and concentrations of organic C in water and sediments indicated the important impact of organic acids on the acid status of the lakes exposed to higher terrestrial export of organic matter. Compared to the pre-acidification period, the modern sediments had significantly higher Fe, Mn, Zn, Pb, and K, but lower Mg concentrations. The Zn and Pb enrichment was more evident in oligotrophic alpine lakes than in more productive forest lakes and was independent of lake water or sediment pH. Fe and Mn concentrations in the modern sediments were higher than in ambient soils and bedrock, while those in pre-acidification sediments were similar to contemporary soils and to the rock layer. The enrichment of the modern sediments with Fe and Mn thus probably resulted from both their redox recycling and ecosystem acidification.     

Phosphorus Fractions and Fluxes in the Water Column and Sediments of a Tropical Reservoir (Lobo‐Broa – SP)

Sedimentation rates of organic and inorganic matter, chlorophyll a, P fractions, Ca, Mn, Fe and Al, were determined by sediment traps in a tropical oligo‐mesotrophic reservoir of São Paulo (Brazil). Vertical profiles of the sediments were analyzed for organic content, metals, P and surface P fraction composition. Estimated mean sedimentation rates, corrected for resuspension were: total solids, 1068 g m^—2 y^—1 (OM = 44.7%); chlorophyll a, 2.1 g m^—2 y^—1 and total phosphorus, 2.9 g m^—2 y^—1. The predominant P fraction in the settling flux was associated with aluminum minerals while surface sediments were dominated by organic P. The reservoir exhibited low sediment retention of P (13.0%), Al (9.9%), Fe (9.9%), Mn (1.4%) and Ca (traces), compared to trap sedimentation. This feature was related with a high vertical dynamics (resuspension and bottom release) and with the low retention time of the system.     

Limnology of high mountain tropical lake, in Ecuador: characteristics of sediments and rate of sedimentation

Equatorial high mountain lakes are a special type of lake occurring mainly in the South American Andes as well as in Central Africa and Asia. They occur at altitudes of a few thousand meters above sea level and are cold-water lakes (< 20 degrees C). Relatively little is known about them. A long-term limnological study was therefore undertaken at Lake San Pablo, Ecuador, to analyze the basic limnological processes of the lake, which has a tendency for eutrophication. Sediment quality of San Pablo Lake is given under consideration of horizontal and vertical distribution using sediment cores. Significance of sediments for eutrophication process of lakes is demonstrated using phosphorus concentration of sediments as well as the phosphorus retention capacity of the sediments by ratio Fe/P. Dating of the sediments is done using 137Cs and 210Pb, but the activity of 137Cs in the sediment was very low nearly at the detection level. Sedimentation rate is determined to be 3.5 mm/year and the sediment cores represent about 110 years. P concentration of the sediments is high (approximately 5 g/kg dry substance), and P retention capacity by Fe is insufficient (Fe/P = 4). The sediment quality did not change significantly during the past decades, and the trophic state of San Pablo Lake was already less or more eutrophic 110 years ago. The contamination of the lake sediments by heavy metals is insignificant.     

环境因子对杭州西湖沉积物各形态磷释放的影响

对西湖沉积物的磷形态、粒径组成、化学组成进行了分析, 模拟研究了上覆水磷含量、光照、pH、温度、水动力条件等不同环境因子对西湖沉积物各形态磷释放的影响。结果表明, 上覆水为蒸馏水时的最大释磷量约为底泥-湖水系统的1.15倍, 且释放形态均以IP中的Fe/Al-P为主。在蔽光条件下的最大TP释放量约为光照条件下最大TP释放量的1.35倍。pH 是影响磷释放的重要因素, 在碱性条件下, 促进Fe/Al-P的释放; 在酸性条件下, 促进Ca-P 的释放。在高温条件下沉积物的释磷量会高于低温条件下的释磷量。沉积物各形态磷的释放量在15h后逐渐趋于平衡扰动状态达到平衡时TP释放量是静态释放平衡状态的1.61倍。研究结果旨在探讨不同环境因子对湖泊沉积物磷迁移转化的生态环境效应, 预测西湖内源磷释放的发展趋势, 为控制沉积物内源污染提供理论基础。     

Influence of Primary Producers on Bioavailability of Desorption Resistant Organic Pollutants in the Sediments

The experiments were conducted on three freshwater microalgal strains, which were grown in the top chambers of retrofitted filter assemblies. The bottom receivers contained control sediments and two types of ¹⁴C-labeled, phenanthrene contaminated sediments. The first contaminated sediment had both the reversible and desorption resistant fractions (referred to as partially washed sediment), while the second contaminated sediment had the desorption resistant fraction only (completely washed sediment). A second set of controls was added to isolate the effect of microalgae. Despite minor variations in toxicity, there was no significant difference (P > 0.05) between the growth curves of the three algal strains. From a bioavailability perspective, there was a significant difference (P < 0.05) in desorption rates in chambers containing microalgae. Pooled data from the three cultures indicated that, for assemblies with desorption resistant sediment (completely washed) and microalgae, the top chamber phenanthrene concentrations were approximately 90–100% of the theoretical maximum concentrations. However, filter assemblies without microalgae had only 5–6% of the theoretical maximum. Results also indicated that approximately 91.7–92.4% of the desorbed phenanthrene is directly sequestered by the microalgae in assemblies with the completely washed sediment. These results indicated that the term “desorption-resistant” fraction from a purely physical perspective may not be truly desorption-resistant in presence of microalgae.     

Assessment of Contaminant Retention in Constructed Wetland Sediments

The A‐01 wetland treatment system (WTS) was designed to remove metals from an industrial effluent at the Savannah River Site, Aiken, SC. Sequential extraction data were used to evaluate remobilization and retention of Cu, Pb, Zn, Mn, and Fe in the wetland sediment. Remobilization of metals was determined by the Potentially Mobile Fraction (PMF) and metal retention by the Recalcitrant Factor (RF). The PMF, which includes water soluble, exchangeable, and amorphous oxides fractions, is the contaminant fraction that has the potential to enter into the mobile aqueous phase under fluctuating environmental conditions. PMF values were low for Cu, Zn, and Pb (13–27 %) and high for Fe and Mn (62–70 %). The RF, which includes crystalline oxides, sulfides or silicates and aluminosilicates, is the ratio of strongly bound fractions to the total concentration of elements in sediment. RF values ranged from 73–87 % for Cu, Zn, and Pb, indicating high retention in the sediment and from 30–38 % for Fe and Mn, indicating low retention. Contaminant retention, which is determined by solid phase metal speciation, determines the potential mobility and bioavailability of captured metals in wetland sediments; hence, their likelihood of being released if chemical, physical, or biological conditions within the wetland change.     

Phosphorus mobility in sediments of acid mining lakes,Lusatia, Germany

Changes in benthic retention and mobility of P were studied in a chronosequential approach in acid (pH 2.5–3.5) to moderately acid (pH 5.6–6.8) mining lakes (MLs) with respect to the threat of eutrophication during maturation. Although various MLs exhibit a high pelagic P availability there is currently no distinct threat of an enhanced eutrophication due to a C limitation and a high benthic P immobilization potential. The sequential P extraction applied revealed an increase in sedimentary total P with increasing pH due to the additional uptake of P by Al- and Fe-hydroxides, but no increase in mobile P forms. Currently, the excess in reactive Fe at a high Fe:P ratio in surface sediment at pH ∼3 favours an efficient P precipitation of Fe-oxi-hydroxides with a high specific surface area for P adsorption. However, an enhanced future P mobility of ML sediments and a threat of eutrophication results from: (1) a distinct reduction of the import of Fe and Al from the catchment over time, resulting in a lower supply towards the sediments, and (2) a reduction in the sedimentary Fe mineral surface area due to the future rise in pH.     

Concentrations and Distribution of Trace Metals in Water and Streambed Sediments of Orogodo River,Southern Nigeria

The distribution of Cd, Pb, Ni, Cr, Cu, Mn, Fe, and Zn in sediment and surface water, and some physico-chemical characteristics of Orogodo river sediments, were evaluated. The sediment pH ranged from 5.1–7.3; conductivity values ranged from 34.5 to 389.0 μScm^?1. Total nitrogen values ranged from 0.06–0.10%, NH₃-N values ranged from 0.25–0.44 mgkg^?1, percent total organic carbon ranged from 0.21–1.68%, and total phosphorus values ranged from 0.004–0.02% for dry and wet seasons. The sand fraction consists of 87–95%, silt fractions ranged from 0–2%, and clay fraction between 4–13%. The mean concentrations of metals (dry weight basis) in the streambed sediments ranged from 1.92–17.37 mgkg^?1 for Cu, 0.98–4.78 mgkg^?1 for Ni, 0.01–32.98 mgkg^?1 for Mn, 353.22–2045.64 mgkg^?1 for Fe, 69.96–100.16 mgkg^?1 for Zn, 0.21–1.32 mgkg^?1 for Cr, and Cd was less than 0.001 mgkg^?1 for wet and dry seasons. The mean concentrations of metals in the surface water ranged between 0.01–0.05–0.05 mg/L for Cu, nd-0.11 mg/L for Ni, 0.001–0.31 mg/L for Pb, 0.001–1.82 mg/L Mn, 0.01–3.52 mg/L for Fe, 0.16–0.61 mg/L for Zn, nd-0.007 mg/L for Cr, and <0.001 mg/L for Cd. Based on principal component analysis, two main sources of metals in the Orogodo River can be identified: (i) Cr, Cu, Pb, and Fe are mainly derived from industrial sources; (ii) Mn, Zn, and Ni associated with traffic activities. No element examined had a contamination/pollution index value greater than unity (pollution ranges). This implies that the multiple pollution indices obtained from the analysis showed that Orogodo River sediments were not polluted with heavy metals.     

Phosphorus dynamics during decomposition of mangrove (Rhizophora apiculata) leaves in sediments

Rhizophora apiculata leaf litter decomposition and the influence of this process on phosphorus (P) dynamics were studied in mangrove and sand flat sediments at the Bangrong mangrove forest, Phuket, Thailand. The remaining P in the mangrove leaf litter increased with time of decomposition to 174% and 220% of the initial amount in the litter in sand flat and mangrove sediment, respectively, although about 50% of the dry weight had been lost. The incorporation of P into the litter was probably associated with humic acids and metal bridging, especially caused by iron (Fe), which also accumulated in considerable amounts in the litter (5-10 times initial concentration). The addition of leaves to the sediment caused increased concentrations of dissolved reactive phosphate (DRP) in the porewater, especially in sand flat sediment. The DRP probably originated from Fe-bound P in the sediment, because decomposition of buried leaf litter caused increased respiration and reduced the redox potential (E_h) in the sediments. Binding of P to refractory organic material and oxidized Fe at the sediment-water interface explains the low release of DRP from the sediment. This mechanism also explains the generally low DRP concentration in the mangrove porewater, the low nutrient content of the R. apiculata leaves, but also the higher total sediment P concentration of the mangrove sediment as compared to sediments outside the mangrove. Both the low release rates for DRP from the sediment and the accumulation of P associated with leaf litter decomposition tend to preserve P in the sediments.     

The budget of dissolved trace metals in Lake Biwa,Japan

A comprehensive study on the dynamics of dissolved elements (Mg, Al, Si, P, Ca, V, Cr, Mn, Fe, Ni, Zn, As, Sr, Y, W, and U) in Lake Biwa was carried out using a clean technique. Lake water samples (n = 523) were collected from six stations in the North Basin and three stations in the South Basin. River water samples (n = 178) were collected from 14 major rivers flowing into the North Basin. Rainwater samples (n = 89) were collected at Otsu. The river water was enriched with Mn, Al, Fe, P, and Zn and the rainwater was enriched with Zn, Al, Fe, and Mn compared to North Basin water during winter mixing. The residence times of dissolved species were estimated on the basis of input through the rivers and rain. The residence times for Ca, Mg, and Sr were about 8 years, the same as that for water. Mn, Al, Fe, and Zn showed the shortest residence times (0.05–0.19 year). A budget calculation suggested that more than 60% of the input of dissolved Si, P, V, Cr, Mn, Fe, Ni, and Zn was scavenged and retained in the lake sediments and/or discharged as suspended particles.     

Binding of phosphate in sediment accumulation areas of the eastern Gulf of Finland,Baltic Sea

The relationships between P and components binding P were studied by analysing the concentrations of N, P, Fe, Mn, Ca and Al in sediments and pore water along the estuarine transect of the River Neva in August 1995. The high sediment organic matter concentration resulted in low surface redox potential and high pore-water o-P concentration, whereas the abundance of amphipods resulted in high surface redox potentials and low pore-water o-P concentration. However, despite the variation in sediment organic matter and the abundance of amphipods, very reduced conditions and slightly variable concentrations of Tot-P (0.7–1.1 mg g^–1 DW) were observed in the 10–15 cm sediment depth along the estuarine gradient, indicating that the pools of mobile P were largely depleted within the depth of 0–15 cm. Multiple regression analysis demonstrated that organic matter and Tot-Fe concentration of the sediment were closely related to the variation in Tot-P concentration of the sediments (r ² = 0.817, n=32). In addition, the high total Fe:P ratio suggested that there is enough Fe to bind P in sediments along the estuarine gradient. However, low Fe_diss concentrations in the pore water of reduced sediment (redox-potential <–50 mV) indicated efficient precipitation of FeS (FeS and FeS₂), incapable to efficiently bind P. Consequently, the low Fe_diss:o-P ratio (< 1) recorded in pore water in late summer implied that Fe³⁺ oxides formed by diffusing Fe_diss in the oxic zone of the sediments were insufficient to bind the diffusing o-P completely. The measured high o-P concentrations in the near-bottom water are consistent with this conclusion. However, there was enough Fe_diss in pore water to form Fe³⁺ oxides to bind upwards diffusing P in the oxic sediment layer of the innermost Neva estuary and the areas bioturbated by abundant amphipods.     

Distinguishable root plaque on root surface of Potamogeton crispus grown in two sediments with different nutrient status

The properties of plaques were different on the root surface of Potamogeton crispus planted in sediments from two different shallow lakes. Lake Tangxunhu sediment, with low pH, contained low organic matter, whereas Lake Yuehu sediment, with high pH, had high calcium deposits mixed with high organic matter. The contents of mineral elements in sediment of Lake Tangxunhu was lower than that of Lake Yuehu, except for iron (Fe) content, but the contents of mineral elements extracted by sodium dithionite–sodium citrate–sodium bicarbonate (DCB) from root plaques were higher in Lake Tangxunhu than those in Lake Yuehu, except for Fe. These element distributions on P. crispus root plaques were characterized by scanning electron microscope combined with energy-dispersive X-ray spectrometer and were consistent with the contents of mineral elements in sediment. The root plaque of P. crispus planted in Lake Tangxunhu sediment mainly contained silicon (Si) and Fe, and the content of Si was greater than Fe, which may be contributed to the formation of poly-silicic-ferric in the natural conditions. However, the root plaque of P. crispus planted in the sediment with higher calcium content of Lake Yuehu was rich in Fe, Si, phosphorus (P), and calcium (Ca). Due to oxygen secretion by plant roots, the root plaque has more Fe₃(PO₄)₂ and a certain amount of Ca₃(PO₄)₂. The ratio of magnesium (Mn) to Fe extracted by DCB from root plaque in Lake Tangxunhu sediment was 0.031 and 0.010 in Lake Yuehu sediment. In Lake Tangxunhu sediment, lower content of organic matter results in weak reducibility. Enhanced oxidation ability by oxygen secretion of P. crispus root could oxidize low-valent Fe and Mn into iron–manganese oxide, which leads to formation of iron–manganese plaque on the root surface. However, this case is different in Lake Yuehu sediment, where Fe and Mn can be reduced in high organic sediment and low-valent Mn can precipitate in the sediment in which pH is >8. Thus, low-valent Fe in Lake Yuehu sediment moves to the root surface of P. crispus, where it oxidizes into Fe oxide, i.e., Fe plaque.     

The effects of manipulation of sedimentary iron and organic matter on sediment biogeochemistry and seagrasses in a subtropical carbonate environment

The microbial metabolism of organic matter (OM) in seagrass beds can create sulfidic conditions detrimental to seagrass growth; iron (Fe) potentially has ameliorating effects through titration of the sulfides and the precipitation of iron-sulfide minerals into the sediment. In this study, the biogeochemical effects of Fe availability and its interplay with sulfur and OM on sulfide toxicity, phosphorous (P) availability, seagrass growth and community structure were tested. The availability of Fe and OM was manipulated in a 2 × 2 factorial experiment arranged in a Latin square, with four replicates per treatment. The treatments included the addition of Fe, the addition of OM, the addition of both Fe and OM as well as no addition. The experiment was conducted in an oligotrophic, iron-deficient seagrass bed. Fe had an 84.5% retention efficiency in the sediments with the concentration of Fe increasing in the seagrass leaves over the course of the experiment. Porewater chemistry was significantly altered with a dramatic decrease in sulfide levels in Fe addition plots while sulfide levels increased in the OM addition treatments. Phosphorus increased in seagrass leaves collected in the Fe addition plots. Decreased sulfide stress was evidenced by heavier δ³⁴S in leaves and rhizomes from plots to which Fe was added. The OM addition negatively affected seagrass growth but increased P availability; the reduced sulfide stress in Fe added plots resulted in elevated productivity. Fe availability may be an important determinant of the impact that OM has on seagrass vitality in carbonate sediments vegetated with seagrasses.     

Enrichment and Chemical Fraction of Copper and Zinc in the Sediments of the Daliao River System,China

Copper and zinc contents and their chemical forms in the surface sediments of the Daliao River system (DRS) were investigated to quantitatively assess their contamination, mobility, and potential bioavailability. Twenty-seven surface sediment samples were collected and analyzed for total contents of Cu, Zn, Al, Fe and Mn and for chemical forms of Cu and Zn. These results reveal that the total contents of Cu and Zn in the sediment of DRS mainly ranged from 4.5 to 86.1 and 17.7 to 1340.9 mg kg^?1, respectively. The enrichment factors (EFs) were generally lower than 1.0 at most sampling sites, while at T3, B5 and B7 sites of the Taizi River near Benxi, Liaoyang and Anshan cities, respectively, they were more than 2.0. The primary sources of Zn and Cu contamination at these three sites are considered to be iron mining and smelting and some chemical industries. Hence, the total contents of Cu and Zn in the sediments of the Daliao River system mainly ranged around background levels. In general, less than 2% and more than 60% of total Cu and Zn contents in the sediment at most sampling sites were associated to the exchangeable fraction and residual fraction, respectively, showing their low mobility and bioavailability. The major sink for anthropogenic Cu might be organic matter and Fe oxides, while for anthropogenic Zn it was carbonate and the easily reducible phase dominated by Mn and amorphous Fe oxides. Therefore, anthropogenic Cu and Zn have relatively high potential mobility and bioavailability. In comparison with the sediments of other rivers in China and in the world, the sediment of the DRS contained relatively low contents of Cu and Zn.     

Benthic sediment influence on dissolved phosphorus concentrations in a headwater stream

Phosphate interacts with inorganic sediment particles through sorption reactions in streams. Collectively, this phosphorus (P) buffering mechanism can be an important determinant of soluble reactive P (SRP) concentrations. If sorption reactions control SRP concentrations in a stream, then differences in sediment characteristics may cause spatial differences in SRP concentrations. This prediction was tested by examining sediment-buffering characteristics and spatial variation in SRP among reaches with distinct sediment composition (i.e., fine versus coarse particles) in two tributaries of Boulder Creek, a headwater stream in central Wisconsin. SRP concentrations were significantly lower and algal available P and P sorption capacity were significantly higher in the reach dominated by fine sediments. Although fine particles such as sand had the greatest P sorption capacity, no retention could be attributed to biotic processes, whereas over 50% of P retention in coarse particles such as gravel could be linked to biotic uptake. Equilibrium P concentration (EPC₀) assays from different sediment fractions also indicate that biotic uptake is relatively unimportant in sand particles (EPC_live 10 μg/L: EPC_killed 10 μg/L) but very important in gravel or larger particles (EPC_live 10 μg/L: EPC_killed 80 μg/L). Thus, sediment influence on stream water P concentrations can shift predictably from abiotic sorption in reaches with fine particles to biotic retention in areas dominated by coarse sediments. Consequently, changes in sediment composition due to natural or anthropogenic disturbance have the potential to alter the type and strength of sediment-associated processes determining ambient stream P concentrations.     

Influence of <Emphasis Type="Italic">Potamogeton crispus</Emphasis> growth on nutrients in the sediment and water of Lake Tangxunhu

An incubation experiment was performed on Potamogeton crispus (P. crispus) using sediment collected from Lake Tangxunhu in the center of China, in order to determine the effects of plant growth on Fe, Si, Cu, Zn, Mn, Mg, P, and Ca concentrations in the sediments and overlying waters. After 3 months of incubation, Ca, Mg, and Si concentrations in the water column were significantly lower, and P and Cu concentrations were significantly higher than in unplanted controls. The effect of P. crispus growth on sediment pore waters and water-extractable elements varied. Concentrations of Ca, Mg, Si, Fe, Cu, and Zn were significantly higher, and P was significantly lower, than in pore waters of the control. Water-extracted concentrations of Fe, Mg, and Si in the sediments were lower, and P was higher, than in the control. Presence of P. crispus generally enhanced concentration gradients of elements between pore waters and overlying waters but not for P. The growth of P. crispus was associated with an increase in water pH and formation of root plaques, resulting in complex effects on the sediment nutritional status. Handling editor: S. M. Thomaz     

Nitrogen mass balances and denitrification rates in central Ontario Lakes

Nitrogen mass balances for seven unproductive lakes and 20 forested catchments in central Ontario were measured between 1977 and 1989. Average annual lake denitrification rates calculated with the N/P ratio method were strongly correlated with summer anoxic factor (extent of surficial sediment anoxia) whereas denitrification rates calculated with a²¹⁰Pb sediment N accumulation method were poorly correlated with the anoxic factor suggesting that the N/P method is superior. Substantial denitrification occurred in all lakes — an average of 36% of TN inputs or 75% of the net gain. On a regional area-weighted basis, 67% of bulk atmospheric TN deposition was stored or denitrified terrestrially, 12% was denitrified in lakes, 4% was stored in lake sediments, and 17% was exported from lakes. N/P ratios were generally less in streams than in precipitation suggesting preferential N retention in catchments, whereas the N/P ratios in lake outputs were slightly higher than lake input ratios, suggesting preferential P retention in lakes. This is consistent with the notion that P-limited lakes can exist adjacent to N-limited forests.     

Comparative in situ study of the intestinal absorption of aluminum, manganese, nickel, and lead in rats

This comparative study of the intestinal absorption of four toxic metals (aluminum, manganese, nickel, and lead) carried out in rats using the in situ intestinal perfusion technique was able to measure the partition of each metal between the intestine (intestinal retention), the blood circulation, and target tissues after 1 h. The perfused metal solutions were at concentrations likely to occur during oral intoxication. It was found that aluminum (48 and 64 mM), even as a citrate complex, crossed the brush border with difficulty (0.4% of the perfused amount); about 60% of this was retained in the intestine and the remainder was found in target tissues (about 36%). Conversely, lead (4.8–48 μM) penetrated the intestine more easily (about 35% of the perfused amount), was slightly retained (about 12% of the input), and was soon found in the tissues (about 58% of the input) and to a lesser degree in circulation (about 29%). Within the same concentration range, nickel and manganese showed certain similarities, such as a reduced crossing of the brush border proportional to the increase in the concentration perfused (0.17–9.5 mM). There was similar intestinal retention and absorption (about 80% and 20% of the input, respectively). Manganese crossed the brush border more easily and was diffused more rapidly into tissues. Finally, the addition of equimolar amounts of iron (4.7 mM) produced opposite effects on the absorption of the two elements, inhibiting manganese and showing a trend to increase in nickel absorption. This could be the result of competition between Fe²⁺ and Mn²⁺ for the same transcellular transporters and the slight predominance of paracellular mechanism in the event of “Fe²⁺-Ni²⁺” association.