Organic matter in the Elbe estuary

In the low salinity region of the Elbe estuary in March–April 1992 the turbidity zone was characterized by high loads of suspended matter, 7% of which was organic material (750 μM C) at the surface. Particulate nitrogen, phosphorus and carbohydrates concentrations reached 55 μM N, 10 μM P and more than 15 μM glc. eq., corresponding to 13% of total C, at the surface and increasing threefold near the bottom. In spite of the peaking of particulate organic material levels in the maximum turbidity zone, there were only consistent qualitative changes in total particulate C, N, P, and carbohydrates along the Elbe estuary. Downstream, both the percentage of particulate organic material and the turbidity: organic material ratio decreased, indicating decomposition in the upper estuary and dilution with inorganic suspended matter from the lower estuary. Diatoms, the dominant phytoplankton group, decreased from the upper reaches towards the turbidity zone by 0.3 (surface) and 1.5 mg C l⁻¹ (bottom). This corresponded to 12 and 60% of the decrease in total particulate carbon. Estimated local input of organic carbon by primary production (21 μg Cl⁻¹d⁻¹) was almost compensated by calculated minimum grazing (14 μg C l⁻¹d⁻¹). Considering net primary production and grazing, the dissimilation by zooplankton (5 μg C l⁻¹d⁻¹) and heterotrophic bacterial decomposition (48 μg C l⁻¹d⁻¹), when summed over the estimated flushing time (12 days) represented a loss of suspended organic matter of 0.6 mg Cl⁻¹. Since this was only 20% of the observed decrease in particulate carbon, significant dilution processes must be assumed. Dissolved organic nitrogen decreased from 35 to 10 μM N and dissolvd organic phosphorus from 0.6 to 0.1 μM P towards the sea, mainly due to dilution. The distribution of phosphate, with highest loads in the turbidity maximum of 2.4 μM, suggested an interaction with the accumulated load of particulate material.     

Sources,concentrations and characteristics of organic matter in softwater lakes and streams of the Swedish forest region

18 Swedish forest lakes covering a wide range of dystrophy were studied in order to quantify and characterize the organic matter in the water with respect to origin (allochthonous or autochthonous), physical state (particulate or dissolved) and phosphorus content. Samples were collected repeatedly during a two-year period with unusually variable hydrological conditions. Water from three different depths and from tributaries was analysed with standard monitoring methods, including water colour, Secchi disk transparency, total organic carbon (TOC), COD_Cr, COD_Mn, total phosphorus and molybdate reactive phosphorus. Interrelationships were used to compare different methods and to assess the concentration and composition of organic matter. It is estimated that in remote softwater lakes of the Swedish forest region, autochthonous carbon is typically < 5 g m⁻³. Most lakes in this region receive significant amounts of humic matter originating from coniferous forest soils or peatland in the catchment area. In most humic lakes with a water colour of ≥ 50 g Pt m⁻³, more than half of the organic carbon in the surface water is of allochthonous origin, and in polyhumic lakes (> 200 g Pt m⁻³) the proportion can exceed 90%. Secchi depth readings were related similarly to organic matter from both sources and provided good estimates of TOC with a single optical measurement. Water colour was used to distinguish allochthonous and autochthonous matter. High concentrations of phosphorus were found in humic waters, most of it being molybdate reactive, and probably associated with humic matter rather than as dissolved free inorganic forms. COD_Mn yielded only 25–60% of TOC and appears to include mainly truly dissolved substances of low molecular weight.     

Speciation,phase association and potential bioavailability of phosphorus on a Svalbard glacier

Glacier surfaces are known to harbour abundant and active microbial communities. Phosphorus has been shown to be deficient in glacial environments, and thus is one of the limits on microbial growth and activity. We quantified the phosphorus pool in cryoconite debris and the concentration of dissolved phosphorus in supraglacial water on Werenskioldbreen, a Svalbard glacier. The mean total P content of the cryoconite debris was ~2.2 mg g⁻¹, which is significantly more than would be expected in rock debris from local sources. 57% of this P was present in the fraction defined as organic P. It may account for the P in excess of the rock debris, and could be explained by allochthonous input of organic matter. The concentration of total dissolved P in supraglacial water was very low (5.2–8.5 μg l⁻¹), which was probably caused by efficient flushing and re-adsorption onto mineral surfaces. Dissolved organic P (DOP) was a very important component of the dissolved phosphorus pool on Werenskioldbreen, as concentrations of DOP typically exceeded those of dissolved inorganic P (or SRP) by more than four times in all the glacial water types. It is very difficult to assess whether P was limiting in this environment solely on the basis of the N:P ratios in the debris or biomass. There may be some degree of biological control over the C:N:P ratios in the debris, but the phosphorus cycling in the supraglacial environment on this glacier seems to be mainly controlled by physical and geochemical processes.     

Particulate phosphorus in the Humber estuary

The biogeochemical controls on particulate phosphorus distribution in the Humber estuary have been investigated with regard to salinity and season. Surveys of the Humber were conducted in August 1991 and January, March and June 1992. Samples were collected for particulate phosphorus, dissolved phosphorus, suspended load and salinity analysis. Particulate phosphorus forms 16 to 93% of the total phosphorus present in the water column of the Humber estuary. Total particulate phosphorus concentrations in the Humber ranged from 1 to 45 μmol l⁻¹. This range is comparable to that observed in similar sized industrialized estuaries of the U.S.A. and northern Europe. Flocculation at low salinities was found to be a major influence on particulate phosphorus concentrations. Highest concentrations of particulate phosphorus were found in reaches below a salinity of 5 and may result in phosphorus being trapped in the sediments of the estuary. There is evidence that this phosphorus may be released as oxygen concentrations decrease during the summer. Suspended load, adsorption/desorption equilibrium and external inputs were also found to influence the particulate phosphorus concentrations.     

Phosphorus concentration in barley (Hordeum vulgare L.) seed: Influence on seedling growth and dry matter production

The effect of phosphorus (P) concentration in barley seed on seedling growth has not been much investigated. Consequently, two experiments were conducted in the greenhouse to determine the effect of P concentration in barley seed (Hordeum vulgare L., cv. Empress) on the seedlings grown in sand-filled boxes receiving a culture solution without P. Seeds were selected with three P concentrations: high-P (113.0 mmol P kg⁻¹), medium-P (80.7 mmol P kg⁻¹) and low-P (54.9 mmol P kg⁻¹). At 21 days after sowing, the shoot and root yield or shoot height was the least with seedlings from low-P seed. In the other experiment, high-P and low-P seeds were wetted with distilled water or with a solution of 25.8 cmol L⁻¹ of NaH₂PO₄ for 24 h, and then grown for 31 days. Solution P had been imbibed by seeds whether low or high in native P, but only the imbibed P held by low native P seed benefited seedling dry matter accumulation and shoot elongation. The lack of benefit from seed-imbibed P on seedlings grown from high-P barley seed was associated with low recovery of the imbibed P in those seedlings.     

Evaluation of ammonium and phosphate release from intertidal and subtidal sediments of a shallow coastal lagoon (Ria Formosa – Portugal): a modelling approach

During an annual cycle, overlying water and sediment cores were collected simultaneously at three sites (Tavira, Culatra and Ramalhete) of Ria Formosa’s intertidal muddy and subtidal sandy sediments to determine ammonium, nitrates plus nitrites and phosphate. Organic carbon, nitrogen and phosphorus were also determined in superficial sediments. Ammonium and phosphate dissolved in porewater were positively correlated with temperature (P < 0.01) in muddy and sandy sediments, while the nitrogen-oxidized forms had a negative correlation (P < 0.02) in muddy sediments probably because mineralization and nitrification/denitrification processes vary seasonally. Porewater ammonium profiles evidenced a peak in the top-most muddy sediment (380 μM) suggesting higher mineralization rate when oxygen is more available, while maximum phosphate concentration (113 μM) occurred in the sub-oxic layer probably due to phosphorus desorption under reduced conditions. In organically poor subtidal sandy sediments, nutrient porewater concentrations were always lower than in intertidal muddy sediments, ranging annually from 20 μM to 100 μM for ammonium and from 0.05 μM to 16 μM for phosphate. Nutrient diffusive fluxes predicted by a mathematical model were higher during summer, in both muddy (104 nmol cm⁻² d⁻¹––NH₄⁺; 8 nmol cm⁻² d⁻¹––HPO₄⁻²) and sandy sediments (26 nmol cm⁻² d⁻¹––NH₄⁺; 1 nmol cm⁻² d⁻¹––HPO₄⁻²), while during lower temperature periods these fluxes were 3–4 times lower. Based on simulated nutrient effluxes, the estimated annual amount of ammonium and phosphate exported from intertidal areas was three times higher than that released from subtidal areas (22 ton year⁻¹––NH₄⁺; 2 ton year⁻¹––HPO₄⁻²), emphasizing the importance of tidal flats to maintain the high productivity of the lagoon. Global warming scenarios simulated with the model, revealed that an increase in lagoon water temperature only produces significant variations (P < 0.05) for NH₄⁺ in porewater and consequent diffusive fluxes, what will probably affect the system productivity due to a N/P ratio unbalance.     

Fluxes of dissolved organic carbon,other nutrients and microbial communities in a water-filled treehole ecosystem

Water-filled treeholes are temporally and spatially variable habitats that consist of communities of a limited number of insect orders, namely dipterans and beetles. Since these systems are largely heterotrophic, treeholes are dependent on the surrounding terrestrial ecosystem for their basal energy input. In this study, we observed a cyclic succession of three system states in a water-filled Fagus grandifolia treehole: ‘connected’ during rainfall; ‘isolated’ during periods without rain; and ‘dry’ when no freestanding water was present. During the isolated phase, a rapid, microbially mediated turn-over of nitrogen and sulphate took place, coincident with an accumulation of orthophosphate. Ammonium was the dominant form of nitrogen in the treehole water, and a net decrease in its concentration was observed when the water volume decreased. Normally, nitrate concentration showed only minor fluctuations (0.3–1.3 mg l⁻¹) and concentrations of nitrite were very low (3–18 μg l⁻¹). Concentration of sulphate showed a net decrease, coincident with an increase in sulphide. During the connected phase, the effect of stemflow and throughfall on the nutrient concentrations in the treehole water proved to be variable. Over time, both dilution of, and increase in, treehole water nutrient concentrations were observed. Dissolved organic carbon (DOC) concentration showed a net decrease during the isolated phase. Spring coarse particulate organic matter input into the treehole was variable (0.20–1.74 mg cm⁻² week⁻¹). The observed variability in precipitation inputs as well as fall-in of organic matter underlines the pulsed character of basal energy entering the treehole food-web. DOC concentrations varied mainly with depth (15–57.4 mg l⁻¹) as concentrations were almost three times greater within the detritus than within the water column. Bacterial abundances ranged from 7.3 × 10⁵ to 9.3 × 10⁶ cells ml⁻¹ and did not appear to vary within the water column or in the detrital sediment. Rates of bacterial production increased 24 h after rain events, suggesting that the combined nutrient fluxes due to increased stemflow stimulate the microbial community. Handling editor: J. Cole     

Interaction Effects of Ambient UV Radiation and Nutrient Limitation on the Toxic Cyanobacterium <Emphasis Type="Italic">Nodularia spumigena</Emphasis>

Nodularia spumigena is one of the dominating species during the extensive cyanobacterial blooms in the Baltic Sea. The blooms coincide with strong light, stable stratification, low ratios of dissolved inorganic nitrogen, and dissolved inorganic phosphorus. The ability of nitrogen fixation, a high tolerance to phosphorus starvation, and different photo-protective strategies (production of mycosporine-like amino acids, MAAs) may give N. spumigena a competitive advantage over other phytoplankton during the blooms. To elucidate the interactive effects of ambient UV radiation and nutrient limitation on the performance of N. spumigena, an outdoor experiment was designed. Two radiation treatments photosynthetic active radiation (PAR) and PAR +UV-A + UV-B (PAB) and three nutrient treatments were established: nutrient replete (NP), nitrogen limited (−N), and phosphorus limited (−P). Variables measured were specific growth rate, heterocyst frequency, cell volume, cell concentrations of MAAs, photosynthetic pigments, particulate carbon (POC), particulate nitrogen (PON), and particulate phosphorus (POP). Ratios of particulate organic matter were calculated: POC/PON, POC/POP, and PON/POP. There was no interactive effect between radiation and nutrient limitation on the specific growth rate of N. spumigena, but there was an overall effect of phosphorus limitation on the variables measured. Interaction effects were observed for some variables; cell size (larger cells in −P PAB compared to other treatments) and the carotenoid canthaxanthin (highest concentration in −N PAR). In addition, significantly less POC and PON (mol cell⁻¹) were found in −P PAR compared to −P PAB, and the opposite radiation effect was observed in −N. Our study shows that despite interactive effects on some of the variables studied, N. spumigena tolerate high ambient UVR also under nutrient limiting conditions and maintain positive growth rate even under severe phosphorus limitation.     

Perturbations of a water column of Lake Onogawa by local heavy rainfall

The perturbations of a water column at the deepest part of Lake Onogawa by a local heavy rainfall were analyzed. Mixing throughout the water column (18.8-m deep) was indicated by changes in the distribution of water temperature. This mixing perturbed the hypolimnetic seasonal distributions of dissolved components. All partic-ulate components peaked at 10 m, suggesting a center of inflowing water. Compared with background levels, particulate nitrogen (PN), particulate carbon (PC), particulate phosphorus (PP), and suspended solids (SS) increased from 16-fold for PN to 100-fold for SS. Soluble reactive phosphorus was the only dissolved component that formed a clear maximum at 10 m, as did the particulate components. Assuming that SS consists mainly of mineral particles, SS can be classified into three categories: (1) A rapidly sinking fraction, the main body of the loaded SS, with a sinking rate exceeding 1 m day⁻¹ and radii exceeding 2–3 μm. (2) A slowly sinking fraction with a sinking speed of about 10 cm day⁻¹ and radii of 0.7–0.9 μm; this fraction is calculated to be about 4% of the total loaded SS at most. (3) A fraction that was essentially retained in the water column. The maximum estimate of this fraction was 0.5% of the total. Received: January 1, 2000 / Accepted: August 10, 2000     

Reversibility of phosphorus sorption by ferruginous nodules

Ferruginous nodules sorb significant amounts of available soil and fertiliser phosphate. The effect of this sorption on phosphorus availability of an agricultural soil was tested by sequential extraction and by exhaustive cropping with millet (Pennisetum typhoides) in a greenhouse trial following fertilisation of the original soil containing 70% nodules and of prepared samples containing various mixes of separated soil fines and nodules. Phosphorus sorption maxima by the soil fines and nodules were 190 mg kg⁻¹ and 380 mg kg⁻¹ respectively. Samples of fines and nodules which had sorbed 110 and 194 mg kg⁻¹ were submitted to 8 successive extractions with 0.01 M KCl, after which P desorption amounted to 117 mg kg⁻¹ and 103 mg kg⁻¹ respectively. Hysteresis between sorption and desorption was negligible for the soil fines and increased with increasing nodule content of the samples. In the greenhouse experiment, P uptake at the first cropping was highest in the soil fines at all levels of phosphorus applied. Subsequent croppings, however, showed higher P uptake in the concretionary soils. These results indicate a higher initial P release from the soil fines with cropping followed by an earlier exhaustion of phosphorus. At the end of the greenhouse experiment, yields were low in spite of the large quantities of P still remaining in the soils. Phosphorus fractionation showed that, of the P left in the soil after cropping 20% was in labile, 29% in Fe or Al-associated, and 51% in low-availability forms.     

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m⁻² lake surface area d⁻¹ with EPC0 values ranging between 180 and 270 μg P L⁻¹. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.     

Nitrogen and phosphor compounds in bottom sediments: mechanisms of accumulation,transformation and release

This paper is an overview of Russian literature dealing with the accumulation, the transformations and the release of phosphate and nitrogen compounds in a great number of Russian lakes and reservoirs. A considerable data bank has been analysed. Special attention is given to the relations of N- and P-accumulation with the input and transformation of organic carbon, as well as to the release mechanisms, often in relation to eutrophication of the lakes and reservoirs. It is shown that the major input of organic matter into the sediments comes from autochthonous material, and is usually > 70 %. The relative importance of phytoplankton and macrophytes as sources of organic matter is discussed; it appears that trophic state, depth and other factors may have a large influence on this ratio. In shallow eutrophic lakes macrophytes may be the source of organic matter, which source can amount to 1.5–2.5 times that of phytoplankton. It is also shown that the C/N ratio is not a good indicator of the source of the organic matter, because their C/N ratios often are not very different. The decomposition rate of organic matter was analysed; it depends on trophic state and other factors. Sediment N accumulation is mostly (> 90%) in organic form, and depends on nitrogen and organic matter inputs coming from phytoplankton or macrophytes. A correlation coefficient of 0.9–0.95 was found in 176 lakes. In 113 lakes the N accumulation was 0.11 x C accumulation, with C/N ratios between 7.4 and 12.9. Ammonification was rather constant in different groups of lakes; values were often about 20–25 mg m⁻² d⁻¹. The presence of the different forms of nitrogen in interstitial water and in adsorbed forms is discussed. The N in interstitial water is usually in the form of NH₃. Sediment P-accumulation is usually in inorganic form and is related to primary production. Three different groups of sediments could be distinguished with C/P ratios of 31–100, of 101–350 and > 350. In hard water lakes P sedimentation was found to be 0.3–0.5 times that in soft water lakes with comparable primary production. The relative occurrence of apatite, non-apatite and residual P in sediments was calculated. In the interstitial water the P concentration appeared to be controlled by the input and decomposition of organic matter. The concentration of phosphate dissolved in the interstitial water of the top 2 cm layer is often 10–100 times lower than that of the dissolved N. The concentrations of interstitial phosphate are from a few μgl⁻¹ up to 15 mgl⁻¹, but the higher concentrations occur only rarely. Different types of vertical profiles of P compounds in the sediments were shown to be related with the presence of an oxidised zone, the presence of clay etc. Autochthonous apatite and non-apatite phosphates are more mobile than the allochthonous ones and are in equilibrium with interstitial phosphate. Accumulation of autochthonous apatite in sediments is controlled by decomposition of organic matter and accumulation of carbonates.     

Aluminum in lake water and organs of a fish Tribolodon hakonensis in strongly acidic lakes with a high aluminum concentration

Aluminum in lake water and in the organs of the fish Tribolodon hakonensis was investigated in Lake Usoriko (pH 3.6), Lake Inawashiroko (pH 5.0), and the Tenryu River (pH 7.7). The concentration of total soluble aluminum in the water was 0.51 mg l⁻¹ in Usoriko, 0.05 mg l⁻¹ in Inawashiroko, and less than 0.01 mg l⁻¹ in the Tenryu. The chemical forms of soluble aluminum in the acid water were characterized as Al³⁺, AlL²⁺, and AlL^≦1+. More than 90% of soluble aluminum in the water of Usoriko was Al³⁺, whereas AlL²⁺ was dominant in the water of Inawashiroko. The aluminum concentration in the organs of T. hakonensis in Usoriko was 42 μg g⁻¹ wet weight in gills, 4.2 μg g⁻¹ in muscle, 6.9 μg g⁻¹ in bone, 12.7 μg g⁻¹ in liver, 6.0 μg g⁻¹ in kidney, and 6.0 μg g⁻¹ in intestine, indicating accumulation of aluminum in the gills. The aluminum concentration in the organs of T. hakonensis living in Inawashiroko was approximately the same, in spite of the difference in water chemistry of the two acid lakes, especially for pH and aluminum. This suggests that aluminum accumulation might be controlled in the fish living in the acid lakes. In contrast, the aluminum concentration in the gills of T. hakonensis from the Tenryu was 2 μg g⁻¹. Received: May 20, 1999 / Accepted: December 10, 1999     

Characterization of phosphorus fractions in the sediments of a tropical intertidal mangrove ecosystem

Solid phases of phosphorus fractions in the surface and core sediments were studied to understand the biogeochemical cycling and bioavailability of phosphorus in the Pichavaram intertidal mangrove sediments of India. Total P in surface and core sediments ranged between 451–552 and 459–736 μg g⁻¹ respectively and Fe bound P was the dominant fraction. Low levels of Fe bound P in the mangrove zone than the two estuarine zones may be because of high salinity inhibition of phosphate adsorption onto the Fe-oxides/hydroxides. Post-depositional reorganization of P was observed in surface sediments, converting organic P and Fe bound P into the authigenic P. High levels of organic P in the mangrove zone is primarily due to intensive cycling and degradation of organic matter and adsorption of phosphate on the organic molecules. The burial rates and regeneration efficiency of P in the intertidal mangrove ecosystem ranged from 5.41 to 7.27 μmol P cm⁻² year⁻¹ and 0.122 to 0.233 μmol P cm⁻² year⁻¹, respectively. High burial efficiency (≈99%) of P proves the earlier observation of limiting nature of P for the biological productivity. Further, bioavailable P (exchangeable P + Fe bound P + organic P) constituted a considerable proportion of sedimentary P pool of which an average accounted for 55 and 50% in surface and core sediments respectively. The results indicate that significant amount of P is locked in sediments in the form of authigenic P and detrital P which makes P as a limiting nutrient for the biological productivity.     

Nutrient limitation of the primary production of phytoplankton in Lake Baikal

Nutrient limitation of the primary production of phytoplankton at some stations in southern and central Lake Baikal was studied by nutrient enrichment experiments in August 2002. Chlorophyll (Chl.) a concentrations ranged from 0.7 to 5.8μgl⁻¹. Inorganic nutrient concentrations were low: soluble reactive phosphorus ranged from 0.05 to 0.20μmoll⁻¹, ammonia from 0.21 to 0.41μmoll⁻¹, and nitrite plus nitrate from 0.33 to 0.37μmoll⁻¹. In the five enrichment experiments, phosphate spikes and phosphate plus nitrate spikes always stimulated primary production. Nitrate spikes also stimulated primary production in four of the experiments. Significant differences were detected between the controls and phosphate spikes and between the controls and phosphate plus nitrate spikes. Thus, the first limiting nutrient is thought to be phosphorus, but once phosphorus is supplied to the surface water, the limiting nutrient will quickly shift from phosphorus to nitrogen.     

Biomass of pelagic fungi in Baltic rivers

Concentrations of pelagic fungal biomass, determined as the content of ergosterol in particulate matter, were measured in 49 Baltic rivers during summer 1999. The ergosterol concentration varied 12-fold, from 12.6 to 152.5 ng l⁻¹ (average of 56.4 ng l⁻¹) and correlated positively with concentrations of dissolved organic matter and inorganic nutrients as well as with spectral DOM properties indicative of terrestrial sources. The fungal biomass was 12- to 100-fold lower than the biomass of pelagic bacteria, suggesting that fungi in the water column of the rivers probably were of minor importance in the riverine ecosystems at the sampling time. Handling editor: J. Padisak     

Export of Nitrogen,Phosphorus, and Suspended Solids from a Southern New England Watershed to Little Narragansett Bay

The Pawcatuck River watershed (764 km²) is a mainly forested drainage basin with a low population density (80 people km⁻²) that discharges to a shallow estuary, Little Narragansett Bay (RI and CT, USA). In order to quantify the nitrogen (N) and phosphorus (P) flux to the estuary, we measured all forms of nitrogen and phosphorus, as well as suspended solids at the mouth of the river above tidal influence, on more than 80 occasions over an annual cycle. The annual export of total nitrogen, total phosphorus, and total suspended solids amounted to 16.0×10⁶ mol y⁻¹, 0.97×10⁶ mol y⁻¹, and 1.4×10⁶ kg y⁻¹, respectively. Nitrogen export was equally divided between dissolved inorganic (83% NO₃⁻) and organic forms, with particulate nitrogen comprising 17% of the total flux. Phosphorus export was dominated by particulate forms (67%), with dissolved inorganic phosphate contributing 30% and dissolved organic phosphorus contributing 8% of the annual flux. Preliminary nutrient budgets for the Pawcatuck watershed suggest that only about 10% of the nitrogen and phosphorus inputs are exported from the system. Strong regressions between water discharge and TN enabled us to extrapolate the data collected during the relatively dry study period to a long term average discharge year. Under normal river discharge conditions, the N flux would be approximately 26.0×10⁶ mol y⁻¹ or about 20% of the nitrogen inputs to the watershed. This value is very close to the N flux predicted by a regression developed by others from a wide range of larger watersheds. The relatively large size of the Pawcatuck watershed relative to the estuary (9.6 km²), makes Little Narragansett Bay one of the most intensively nitrogen loaded estuaries on the Atlantic coast in spite of the dominant forest cover of the watershed.     

Integrated tank cultivation of salmonids and Gracilaria chilensis (Gracilariales,Rhodophyta)

In order to study the N and P balances in the Driss I reservoir, we measured concentrations in the water of these elements, their sedimentation rates, and their input by the river Inaouen and output through the dam. Supply and loss of N and P were calculated from samples collected every 48 hours and were for N 15 and 27. 5 mg m⁻² d⁻¹ and for P 33.6 and 1.1 mg m⁻² d⁻¹ respectively. The sedimentation rates, determined using sediment traps, were high, about 75 mg m⁻² d⁻¹ for N and 34 mg m⁻² d⁻¹ for P. Phosphate input came from point sources and was mainly in particulate form. A large fraction of the phosphate sedimented in the form of calcium bound phosphate, with some iron bound phosphate. Bioavailability was, however, low, as the particulate phosphate disappeared rapidly from the epilimnion because of the high sedimentation rate. The ratio Tot-N/Tot-P of the sedimented particulate matter varied between 0.05 and 7.74 and depended on primary production and watergate management.     

Inhibition of mycorrhizal symbiosis in Leucaena leucocephala by chlorothalonil

The effect of the fungicide, chlorothalonil, on vesicular-arbuscular mycorrhizal (VAM) symbiosis was studied in a greenhouse using Leucaena leucocephala as test plant. Chlorothalonil was applied to soil at 0, 50, 100 and 200 μg g⁻¹. The initial soil solution P levels were 0.003 μg mL⁻¹ (sub-optimal) and 0.026 μg mL⁻¹ (optimal). After 4 weeks, the sub-optimal P level was raised to 0.6 μg mL⁻¹ (high). The soil was either uninoculated or inoculated with the VAM fungus, Glomus aggregatum. The fungicide reduced mycorrhizal colonization of roots, development of mycorrhizal effectiveness, shoot P concentration and uptake and dry matter yields at all concentrations tested, although the highest inhibitory effect was noted as the concentration of the fungicide was increased from 50 to 100 μg g⁻¹. Phosphorus applied after four weeks tended to partially offset the deleterious effects of chlorothalonil in plants grown in the inoculated and uninoculated soil which suggests that the fungicide was interfering with plant P uptake. The results suggest that the use of chlorothalonil should be restricted to levels below 50 μg g⁻¹ if the benefits of mycorrhizal symbiosis are to be expected. Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3464. Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3464.     

Shifts in water quality in a drinking water reservoir during and after the removal of cyprinids

Lake ülemiste, the drinking water reservoir of Estonia’s capital city Tallinn, was biomanipulated by manual removal of cyprinids in 2004–2006 and its impact on water quality in the vegetation period was studied. A total biomass of 156 tonnes corresponding to 160 kg ha⁻¹ of fish, predominantly cyprinids, were removed. A decline in the unit catches of fishing was observed. The removed fish biomass versus phosphorus concentration of the lake was considered sufficient to reduce the impact of cyprinids on water quality. The phosphorus removed within fish biomass corresponded to 38 μg l⁻¹ and 21% of the external phosphorus load of the fishing period. The mean total phosphorus concentration dropped from >50 to ≤36 μg l⁻¹. However, the densities of planktivorous young-of-the-year percids remained high and the role of zooplankton grazing in improving water quality was found non-significant or transient. The cladocerans biomass decreased and the small-sized Daphnia cucullata remained almost the only daphnid in Lake ülemiste during and after the manipulation. Predomination of filamentous cyanobacteria was replaced by a more diverse phytoplankton composition and co-domination of micro- and pico-sized colonial cyanobacteria during summer. Mean phytoplankton biomass decreased from 15 to 6 mg l⁻¹ primarily as a result of decreased in-lake TP availability. The Secchi disc transparency increased only in May 2005–2007. The effects of coincidental events, a decline of external loading of phosphorus and a simultaneous flushing induced by heavy rainfall, on lake water quality are discussed with some implications to the future management of the reservoir.