Nutrient (N,P, Si) fluxes between marine sediments and water column in coastal and open Adriatic

Nutrient benthic fluxes, as well as sediment phosphorus concentration at the open sea and coastal water stations of the Central and South Adriatic were studied during 1997–98. The fluxes were in the ranges: 0.16–2.67 mmol m^–2 d^–1 (silicate); –0.031–0.164 mmol m^–2 d^–1 (phosphate); –0.51–2.03 mmol m^–2 d^–1 (ammonia); and –1.32–1.62 mmol m^–2 d^–1 (nitrate + nitrite). Silicate flux showed a gradient from the coastal area to the open sea. Ammonia was the main nitrogen species in the flux at the estuary and bay stations, while the sum of nitrate and nitrite was predominant at the open sea stations. Relationships between phosphate and ammonia fluxes (r = 0.699, p<0.01) as well as phosphate and silicate (r = 0.529, p<0.01) were established.     

Nitrification, denitrification, and nitrate ammonification in sediments of two coastal lagoons in Southern France

Summary Seasonal and diurnal variations in sediment-water fluxes of O₂, NO ₃ ^– , and NH ₄ ⁺ as well as rates of nitrification, denitrification, and nitrate ammonification were determined in two different coastal lagoons of southern France: The seagrass (Zostera noltii) dominated tidal Bassin d'Arcachon and the dystrophic Etang du Prévost. Overall, denitrification rates in both Bassin d'Arcachon (<0.4 mmol m^–2 d^–1) and Etang du Prévost (<1 mmol m^–2 d^–1) were low. This was mainly caused by a combination of low NO ₃ ^– concentrations in the water column and a low nitrification activity within the sediment. In both Bassin d'Arcachon and Etang du Prévost, rates of nitrate ammonification were quantitatively as important as denitrification.Denitrification played a minor role as a nitrogen sink in both systems. In the tidal influenced Bassin d'Arcachon, Z. noltii was quantitatively more important than denitrification as a nitrogen sink due to the high assimilation rates of the plants. Throughout the year, Z. noltii stabilized the mudflats of the bay by its well- developed root matrix and controlled the nitrogen cycle due to its high uptake rates. In contrast, the lack of rooted macrophytes, and dominance of floating macroalgae, made nitrogen cycling in Etang du Prévost more unstable and unpredictable. Inhibition of nitrification and denitrification during the dystrophic crisis in the summer time increased the inorganic nitrogen flux from the sediment to the water column and thus increased the degree of benthic-pelagic coupling within this bay. During winter, however, benthic microalgae colonizing the sediment surface changed the sediment in the lagoon from being a nitrogen source to the over lying water to being a sink due to their high assimilation rates. It is likely, however, that this assimilated nitrogen is liberated to the water column at the onset of summer thereby fueling the extensive growth of the floating macroalgae, Ulva sp. The combination of a high nitrogen coupling between sediment and water column, little water exchange and low denitrification rates resulted in an unstable system with fast growing algal species such as phytoplankton and floating algae.     

Nitrous Oxide Formation in the Colne Estuary, England: the Central Role of Nitrite

Nitrate and nitrite concentrations in the water and nitrous oxide and nitrite fluxes across the sediment-water interface were measured monthly in the River Colne estuary, England, from December 1996 to March 1998. Water column concentrations of N₂O in the Colne were supersaturated with respect to air, indicating that the estuary was a source of N₂O for the atmosphere. At the freshwater end of the estuary, nitrous oxide effluxes from the sediment were closely correlated with the nitrite concentrations in the overlying water and with the nitrite influx into the sediment. Increases in N₂O production from sediments were about 10 times greater with the addition of nitrite than with the addition of nitrate. Rates of denitrification were stimulated to a larger extent by enhanced nitrite than by nitrate concentrations. At 550 μM nitrite or nitrate (the highest concentration used), the rates of denitrification were 600 μmol N · m⁻² · h⁻¹ with nitrite but only 180 μmol N · m⁻² · h⁻¹ with nitrate. The ratios of rates of nitrous oxide production and denitrification (N₂O/N₂ × 100) were significantly higher with the addition of nitrite (7 to 13% of denitrification) than with nitrate (2 to 4% of denitrification). The results suggested that in addition to anaerobic bacteria, which possess the complete denitrification pathway for N₂ formation in the estuarine sediments, there may be two other groups of bacteria: nitrite denitrifiers, which reduce nitrite to N₂ via N₂O, and obligate nitrite-denitrifying bacteria, which reduce nitrite to N₂O as the end product. Consideration of free-energy changes during N₂O formation led to the conclusion that N₂O formation using nitrite as the electron acceptor is favored in the Colne estuary and may be a critical factor regulating the formation of N₂O in high-nutrient-load estuaries.     

Nitrogen Behaviour and Nitrous Oxide Emission in the Tidal Seine River Estuary (France) as Influenced by Human Activities in the Upstream Watershed

The Seine River estuary (France) is the receptacle of a drainage basin characterised by high population density, heavy industrial activity and intensive agriculture. Whereas nitrate concentrations are high due to diffuse sources in the upstream drainage basin, ammonium mainly originates from the effluents of the Achères wastewater treatment plant (WWTP) downstream from Paris and its suburbs (6.5 million equivalent-inhabitants). Ammonium is mostly nitrified in the tidal freshwater estuary and nitrification causes a strong summer oxygen deficit. Average longitudinal summer profiles of oxygen and nitrogen concentrations for two periods, between 1993–1997 and 1998–2003 in dry hydrological conditions (excluding the wet years 2000 and 2001) clearly reflect the changes due to the improved treatment of wastewater from Paris and its suburbs. On the basis of daily water flux data and twice monthly nitrogen measurements at the boundaries of the upstream freshwater estuarine section (108 km), we calculated nitrification and denitrification fluxes, whose annual averages were 43 and 71 × 10³ kg N d⁻¹ respectively from 1993 to 2003, with summer values (July–September) representing 73 and 57% of the annual fluxes, respectively. The degree of denitrification in the upper estuary appears to be closely related to the nitrification, itself more loosely related to the amount of reduced nitrogen (Kjeldahl) brought by the treated effluents from the Achères WWTP. We estimated the total N₂O emissions to about 40 kg N d⁻¹ (25–60 kg N d⁻¹) in the same sector.     

Measurement of in situ rates of nitrification in sediment

A method has been developed for the measurement of nitrification rates in intact sediment cores without disturbing the concentration gradients of oxygen and ammonium. N-serve (2-chloro-6-trichloromethyl-pyridine), a specific inhibitor of the autotrophic ammonium oxidation, was injected into a 0–2 cm surface layer of the sediment (20 ppm) and added to the water column of sediment cores (5 ppm). N-serve in these concentrations was sufficient to inhibit nitrification, but did not change the rate of ammonium production or incorporation in sediment suspensions, which were incubated aerobically and anaerobically. The ammonium accumulation in cores injected with N-serve was thus equal to the amount of ammonium which was oxidized to nitrate in the control cores. Nitrification rates were in the range of 0–3 mmol N m^{–2 –1}     

Methane oxidation by methanotrophs and its effects on the phosphate flux over the sediment-water interface in a eutrophic lake

The effect of methane oxidation in aerobic sediment on oxygen consumption and phosphate flux was investigated in diffusion chambers. The diffusion chambers consisted of two compartments separated by a Teflon membrane. In the upper chamber a thin sediment layer was present and the lower chamber was continuously flushed with gas. The hydrophobic membrane allowed for diffusion of gases from the lower chamber through the sediment layer toward the headspace of the upper chamber. In experiments with a methane oxidation rate of 9.8 mmol m^–2 day^–1, the oxygen consumption rate increased by a factor of two compared with controls without methane oxidation (8.6 vs 17.7 mmol m^–2 day^–1). Methane oxidation significantly decreased oxygen penetration depth (2.5–4.0 vs 1.0–2.0 mm). However, despite the shrinkage of the oxidized microlayer, no differences were found in phosphate flux across the sediment water interface. Batch experiments with standard additions of methane revealed that the growth of methanotrophic bacteria contributes to the phosphate uptake of aerobic sediment. From the batch experiments a molar ratio of carbon to phosphate of 45 mol:mol was calculated for the growth of methanotrophs. Results suggest that a decrease in chemical phosphate adsorption caused by a decrease in the oxygen penetration depth could be compensated for entirely by the growth of methanotrophic bacteria. Send offprint requests to: A.J.C. Sinke     

In situ phosphorus release experiments in the Warnow River (Mecklenburg,northern Germany)

Results are presented of in situ benthic phosphorus release experiments in an undercut bank of an impounded river. Due to high sedimentation of phytoplankton biomass high oxygen consumption rates between 259.4 and 947.0 mg O₂ m^–2 d^–1 developed, leading to almost anaerobic conditions and phosphorus releases between 175.2 and 236.3 mgP m^–2 d^–1 over a period of 18 days.In a second series of experiments the water column overlying the sediment was aerated, resulting in much lower P release rates (1.1 to 32.9 mgP m^–2 d^–1) over a period of 30 days. The influence of pH and nitrate was studied by adjusting pH and adding NO₃ ^– to the overlying water. Increasing pH positively affected P release rates and enhanced NO₃ ^– levels led to an increase of benthic P release, too.     

Factors controlling seasonal variability of benthic ammonium release and oxygen uptake in Alfacs Bay (Ebro Delta,NW Mediterranean)

The seasonal variability of sediment–water ammonium flux andoxygen uptake was studied in an estuarine bay (Alfacs Bay, Ebro Delta, NWMediterranean) influenced by temporal freshwater discharges. Three stationswith different organic loading were sampled. The relationships of benthicfluxes to bottom water (temperature, dissolved oxygen, ammonium, nitrateplus nitrite) and to sediment (porosity, chlorophyll a derivative pigments,organic carbon and nitrogen) variables were examined. Oxygen uptake rangedfrom 0.3 to 2.5 mmol m^–2 h^–1 and ammoniumrelease ranged from 6 to 230 µmol m^–2 h^–1.The lowest value was recorded at the station furthest from the freshwaterinputs, and the highest was at the littoral station nearest the freshwaterdischarge channels (for oxygen uptake) and at the deep station at the saltwedge front (for ammonium flux). Water temperature and the concentration ofchlorophyll a derivative pigments on the surface sediment were revealed asthe main variables to be taken into account to explain the variabilityfound. Changes in fluxes reflecting temperature changes were found at thestation furthest from the freshwater inputs, while at the other, fluxvariability was found to be related to the cycle of functioning offreshwater discharge channels. The different patterns of variability arediscussed in relation to the dynamics of the estuary and to the mainfeatures of benthic nitrogen cycling.     

Denitrification potential of a river floodplain during flooding with nitrate-rich water: grasslands versus reedbeds

Denitrification is a major mechanism for nitrogen removal from nitrogen-rich waters, but it requires oxygen-poor conditions. We assessed denitrification rates in nitrate-rich but also oxygen-rich river water during its stay in a floodplain. We measured diurnal oxygen fluctuations in floodwater along the river Rhine, and carried out an experiment to assess denitrification rates during day, evening and night. Denitrification in floodwater and flooded sediment were measured, comparing activity of periphyton and sediment from agricultural grasslands and reedbeds. Floodwater along the river Rhine was oxygen-saturated (> 10 mg O₂/L) during the day, but oxygen largely disappeared during the night (0.4–0.8 mg O₂/L). Independent of oxygen concentrations, denitrification in surface water alone hardly occurred. In flooded sediments, however, denitrification rates were much higher (1.1–1.5 mg N m^–2 h^–1), particularly at dark and oxygen-poor conditions (nighttime). In the experimental jars, reedbed-periphyton bacteria achieved similar denitrification rates as bacteria in sediment, but overall periphyton denitrification was of minor importance when calculated per square meter. Apart from oxygen levels, maximum denitrification appeared to be regulated by nitrate diffusion from water into the sediment, as the maximum quantity of N denitrified in the sediment equalled the quantity of N lossed from the surface water. Assessed 24-hr denitrification rates in the flooded floodplains (c. 15 mg N m^–2 d^–1) were similar in grasslands and reedbeds, and were rather low compared to rates in other floodplains.     

A comparison of oxygen,nitrate, and sulfate respiration in coastal marine sediments

Aerobic respiration with oxygen and anaerobic respiration with nitrate (denitrification) and sulfate (sulfate reduction) were measured during winter and summer in two coastal marine sediments (Denmark). Both aerobic respiration and denitrification took place in the oxidized surface layer, whereas sulfate reduction was most significant in the deeper, reduced sediment. The low availability of nitrate apparently limited the activity of denitrification during summer to less than 0.2 mmoles NO ₃ ^– m^–2 day^–1, whereas activities of 1.0–3.0 mmoles NO ₃ ^– m^–2 day^–1 were measured during winter. Sulfate reduction, on the contrary, increased from 2.6–7.6 mmoles SO ₄ ^2– m^–2 day^–1 during winter to 9.8–15.1 mmoles SO ₄ ^2– m^–2 day^–1 during summer. The aerobic respiration was high during summer, 135–140 mmoles O₂ m^–2 day^–1, as compared to estimated winter activities of about 30 mmoles O₂ m^–2 day^–1. The little importance of denitrification relative to aerobic respiration and sulfate reduction is discussed in relation to the availability and distribution of oxygen, nitrate, and sulfate in the sediments and to the detritus mineralization.     

Solubility of cadmium and cobalt in a post-oxic or sub-oxic sediment suspension

A sediment sample with high organic matter and trace metal content was suspended in synthetic river water for four weeks under an inert gas atmosphere. Subsequently, the anaerobic suspension was reoxidized by bubbling air through it. The concentrations of dissolved oxygen, sulfide, ferrous iron, manganese, cadmium, cobalt and the pH-value were measured at close time intervals during the anaerobic incubation. The anaerobic suspension was a post-oxic or sub-oxic environment with oxygen and total sulfide concentrations less than 1 µmole 1^–1. Concentrations of dissolved ferrous iron and manganese were 50–150 µmole 1^–1 and 5–30 µmole 1^–1, respectively. The total sulfide concentration was measured using a sensitive voltammetric technique, with a detection limit of 1 nmole 1^–1. A sequential extraction procedure was applied to two sediment samples taken at the end of the anaerobic incubation and after one week of reoxidation. The extractions indicated that cadmium was bound in sulfide minerals under post-oxic conditions. Thermodynamic equilibrium calculations revealed that the concentrations of dissolved cobalt in the post-oxic suspension were limited by the precipitation of cobalt sulfide minerals.     

Biogeochemical processes in intensive zero-effluent marine fish culture with recirculating aerobic and anaerobic biofilters

The biogeochemical processes that drive nutrient transformations and recycling in organic marine sediment-water environments were studied for 17 months in a zero-effluent intensive recirculating culture system. The system consisted of a 10 m³ gilthead seabream (Sparus aurata) tank coupled to aerobic and anaerobic water treatment elements. Nutrients and alkalinity were measured in the system to quantify the main biogeochemical processes. Fractions of the carbon fed in feed were found in fish (18.3%) and in sludge (11%); the missing carbon was respired by fish (45%) and by aerobic (8.4%) and anaerobic (7.7%) microorganisms. Fractions of the nitrogen fed in feed were found in fish (15.4%) and in sludge (14.3%); the missing nitrogen was eliminated by nitrification-denitrification. Most of the phosphorus and ash fed in feed and not found in fish accumulated within the sludge in the system. The rates of nitrification, denitrification and sulphate reduction increased with time, reaching 0.3 g N m^− 2 d^− 1, 53 g N m^− 2 d^− 1 and 145 g S m^− 2 d^− 1, respectively. Nitrification developed more rapidly than denitrification, leading at first to nitrate accumulation (to 20 mmol NO₃ l^− 1 by day 200) and a decrease in alkalinity. Once denitrification surpassed nitrification, nitrate concentrations decreased, eventually being reduced to < 0.3 mmol NO₃ l^− 1 by day 510, and alkalinity stabilized. Toxic hydrogen sulphide, generated within the anaerobic sludge, was oxidized by oxygen and nitrate as it diffused through the anaerobic-aerobic sediment-water interface. When nitrate levels in the water above the sludge dropped below 2 mmol l^− 1, sulphide was also oxidized in the fluidized bed reactor. Denitrification reduced nitrate in the water, respired (jointly with sulphate reduction) carbon in the sludge, oxidized the hydrogen sulphide, and contributed to stabilization of alkalinity and accumulation of polyphosphate in bacteria as a major sink of labile P.     

Nitrogen fluxes and budget seasonality in the Ria Vigo [2pt] (NW Iberian Peninsula)

Inorganic and organic nitrogen fluxes in the Ria Vigo have been quantified in order to recognise the contrasting nitrogen budget scenarios and understand the biogeochemical response to eutrophication events. According to the nitrogen biogeochemical pathways of the ria reservoir (photosynthesis, remineralization, denitrification, PON rain rate and sedimentation), three main seasonal behavioural trends are emphasised: (1) low inorganic nitrogen inputs and low organic nitrogen fluxes, (2) high inorganic nitrogen input and output, (3) high inorganic nitrogen input and high organic nitrogen output. The first scenario occurs in late spring and in summer during non-upwelling situations. The consumption of inorganic nitrogen by net photosynthesis is approximately 2 mol N s^–1 and the ria is oligotrophic (12 mgC m^–2 h^–1). The outgoing estuarine residual current transports phytoplanktonic material towards the mouth of the ria whereupon it sediments and is remineralized as it falls to the lower water layers and the incoming residual current. The regenerated nitrogen is reintroduced to the photic ria layer which leads to the greatest reduction in dissolved oxygen concentration (50% of saturation). Recycled nutrients play an important role in primary production during this oligotrophic state of the ria. Thus, approximately half of the inorganic nitrogen utilised by photosynthesis is ammonium. The majority of PON is deposited inside the ria (0.8 mmol N m^–2 d^–1) and the denitrification rate is 0.3 mmol N₂ m^–2 d^–1. The other two cases occur in winter and spring–summer with upwelling. In winter, estuarine circulation and freshwater contributions control the nitrogen cycle. The ria mainly exports nitrate (up to 14 mol N s^–1) and so there is fertilisation but no eutrophication. In spring and summer, the nitrogen cycle is controlled by upwelling circulation. The inorganic nitrogen consumption by net photosynthesis is high, 7–14 mmol N m^–2 d^–1, and the ria is a natural eutrophic system (70 mgC m^–2 h^–1). Accordingly, 90% of organic nitrogen is synthesised from nitrate and the upwelling-increased circulation exports 6.5 mol N s^–1 of organic nitrogen.     

Ground water flow paths in relation to nitrogen chemistry in the near-stream zone

Interactions between ground water flow paths and water chemistry were studied in the riparian zone of a small headwater catchment near Toronto, Ontario. Significant variations in oxygen — 18 and chloride indicated the presence of distinct sources of water in the ground water flow system entering the near-stream zone. Shallow ground water at the upland perimeter of the riparian zone had nitrate-N, chloride and dissolved oxygen concentrations which ranged between 100–180 µg L^–1, 1.2–1.8 mg L^–1 and 4.6–9.1 mg L^–1 respectively. Concentrations of nitrate — N in deep ground water flowing upward beneath the riparian wetland were < 10 µg L^–1, whereas chloride and dissolved oxygen ranged between 0.6–0.9 mg L^–1 and 0.4–2.2 mg L^–1 respectively. Ammonium — N concentrations (20–60 µg L^–1) were similar in shallow and deep ground water. Ground water was transported through the wetland to the stream by three hydrologic pathways. 1) Shallow ground water emerged as springs near the base of the hillslope producing surface rivulets which crossed the riparian zone to the stream. 2) Deep ground water flowed upward through organic soils and entered the rivulets within the wetland. 3) Deep ground water reached the stream as bed and bank seepage. Springs were higher in nitrate and chloride than rivulets entering the stream, whereas bank seeps had lower concentrations of nitrate and chloride and considerably higher ammonium concentrations than the rivulets. These contrasts in nitrate and chloride concentrations were related to initial differences in the ion chemistry of shallow and deep ground water rather than to element transformations within the riparian wetland. Differences in ammonium concentration between seeps and rivulets were caused by immobilization of ammonium in the substrates of aerobic rivulets, whereas little ammonium depletion probably occurred in deep ground water flowing upward through reduced subsurface organic soils around the stream perimeter.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Close and delayed benthic–pelagic coupling in coastal ecosystems: the role of physical constraints

We aimed to analyse the temporal scales of the variability of benthic ammonium flux using data from an estuarine bay (Alfacs Bay, N. W. Mediterranean). Several nitrogen (N) pools in the sediment, their reactivities and their associated fluxes were estimated. Decomposition of labile buried N (4.5 mol N m^-2) was found to cause an ammonium flux of 0.1 mmol N m^-2 d ^-1, referred to as background flux. The fluxes measured from bell-jar incubations were usually higher, between 2 and 6 mmol N m^-2 d ^-1, which reflects mineralization of recent sedimentation. A typical sedimentation event was found to account for 25 – 75 mmol m^-2 of freshly settled N, which should bring on an ammonium flux of about 1.7–5.0 mmol N m^-2 d ^-1, referred to as fast flux. The concordance between measured and computed benthic fluxes is associated with the coupling of benthic fluxes to production and sedimentation. Close benthic–pelagic coupling was observed in winter and early spring, while a delayed flux response to sedimentation, with transient variations of pore water ammonium profiles, showing surface peaks and decreasing concentrations with depth, was found in autumn. Structures, such as viscous layers, which develop over the sediment–water interface, were found to be essential to the regulation of benthic processes and to lead to transient variations of pore water nutrient concentrations and associated fluxes. The temporal scales of the benthic flux response to sedimentation were discussed in terms of the physical structures involved in decomposition (the bulk sediment, the viscous layers or the fresh settled layer), the processes controlling kinetics and diffusion laws. Several scenarios for the benthic- pelagic coupling in Alfacs Bay, in which local (estuarine circulation) and climatic components combine to yield the variability observed, were examined.     

Denitrification in the periphyton associated with plant shoots and in the sediment of a wetland system supplied with sewage treatment plant effluent

Seasonal variation in denitrification and major factors controlling this process were determined in sediment, microbial communities attached to plant shoots (periphyton) and in the water of a Phragmites and an Elodea-dominated stand of a constructed wetland system between May 1997 and February 1998. The wetland was supplied with effluent from a sewage treatment plant. The denitrification rate in periphyton on plants shoots (expressed per shoot area) was always considerably higher than in the sediment and varied with the chlorophyll-a content of the periphyton in the course of the year. The algae in the periphyton provided attachment surfaces and probably also organic compounds to the denitrifying bacteria. Decreases in periphyton biomass and denitrification rate in the Phragmites and Elodea-dominated stands during the growing season were associated with enhanced shading by Phragmites shoots or a floating layer of macro-algae and Lemna spp., respectively. Light availability and the denitrification rate of periphyton increased again after the Phragmites shoots were cut in October. Nitrate appeared to limit the denitrification rate in the sediment. Periphyton denitrification rates were mostly lower on Elodea shoots than on Phragmites shoots, in spite of the higher living algal biomass on Elodea shoots. This difference was associated with lower nitrate concentrations in the Elodea-dominated stand. In the two stands, the daily denitrification rates in periphyton on shoots of Phragmites australis (44.4–121 mg N m^–2 stand area d^–1) and Elodea nuttallii (14.8–33.1 mg N m^–2 d^–1) were clearly more important than rates in the sediment (0.5–25.5 mg N m^–2 d^–1) or the water (0.4–3.9 mg N m^–2 d^–1). The presence of few bacteria attachment sites or low organic carbon availability possibly resulted in low denitrification rates in the water. Denitrification appeared to be a major process in nitrate removal from the through-flowing water in this wetland system.     

Sediment-water fluxes of nutrients in an Antarctic coastal environment: influence of bioturbation

Rates of exchanges of nitrate and ammonium across the sediment-water interface were measured in an inshore marine environment at Signy Island, South Orkney Islands, Antarctica, over 6 months from August 1991 to February 1992. The sediment was a source of ammonium to the water column but a sink of nitrate, although nitrate exchange rates were very variable. Concentration profiles of nitrate and ammonium in the sediment porewater corroborated the measured vertical exchanges. Bioturbation, by a largely amphipod benthic infauna which was confined to the top 2 cm of sediment, was investigated experimentally. Removal of bioturbation depressed sedimentary O₂ uptake by 33% and sedimentary release of NH₄ ⁺ by 50%. In contrast, in the absence of bioturbation, the removal of NO₃ ^– from the water column by the sediment increased in rate. The measured fluxes of ammonium and nitrate from the sediment did not match with the amount of nitrogen mineralised within the sediment, and urea may account for the difference. It is suggested that the export of nitrogen from the bottom sediment may be significant in sustaining primary production in the Antarctic inshore environment. Ammonium and urea are preferred to nitrate as a nitrogen source by phytoplankton. The nitrate concentrations in the sediment porewater were low, but a large pool of nitrate was identified in the top 0–2 cm layer, which was released by KCl extraction or by freezing of the sediment. This extractable pool of nitrate did not equilibrate with the soluble nitrate pool in the sediment, but seemed to be released from components of the benthic infauna, which were also largely confined to the top 0–2 cm. The physiological role of this nitrate is unknown.     

Seasonal variation of methane emissions from a temperate swamp

Methane flux measurements were made at four sites in a freshwater temperate swamp over the 13 month period of April 1985 through May 1986. Emissions were highly variable both between sites and over time at any one site. Ebullition from sediments was an important component of methane release. Although release of methane through bubbling occurred in only 19% of the measurements made between April and June 1985, when instrumentation allowed us to separate diffusive and bubble fluxes, ebullition accounted for 34% of the total flux during this period. Methane release rates showed a strong seasonal variation, with highest emission rates observed in early spring and again in late summer, which was associated with changes in plant growth and physiology. Emission rates were partially correlated with sediment temperature, but the relationship was not straightforward, and resembled a step function. Emissions responded strongly to temperature change through the range of 10–16°C. At winter sediment temperatures between 4–9°C, CH₄ flux continued at low rates (0–28 mg CH₄ m^–2d^–1; average = 7.9 mg CH₄m^–2d^–1) and appeared insensitive to changes in sediment temperature. Annual methane emission from three constantly flooded sites (mean water depth = 35 cm) was 43.7 +/- 7.8 gm^–2 (standard error); annual flux from a bank site was 41.4 +/- 20.5 gm^–2. A comparison of flux measurements from fresh and saline wetlands in the immediate area of Newport News Swamp emphasizes the importance of edaphic factors in controlling flux.     

Internal loading of nutrients and certain metals in the shallow eutrophic Lake Myvatn, Iceland

Sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice cover of 190 days per year. In situ, transparent and dark flux chambers were used for direct measurements of benthic fluxes of dissolved oxygen, nutrients, silica and certain metals, taking into account primary production and mineral precipitation. The range of benthic flux observed for dissolved oxygen (DO), dissolved inorganic carbon (DIC), ammonium, ortho-P, silica, calcium, and magnesium was –45.89 to 187.03, –99.32 to 50.96, –1.30 to 1.27, –0.51 to 0.39, –62.3 to 9.3, –33.82 to 16.83, and –23.93 to 7.52 mmol m^–2 d^–1, respectively (negative value indicating flux towards the lake bottom). Low benthic NH₄ ⁺ and ortho-P fluxes were likely related to benthic algal production, and aerobic bottom water. Ortho-P fluxes could also be controlled by the dissolution/precipitation of ferrihydrite, calcite, and perhaps hydroxyapatite. The negative silica fluxes were caused by diatom frustule synthesis. Benthic calcium and magnesium fluxes could be related to algal production and dissolution/precipitation of calcium and/or Ca,Mg-carbonates. Fluxes of DO, DIC, pH and alkalinity were related to benthic biological processes. It is likely that some of the carbon precipitates as calcite at the high pH in the summer and dissolves at neutral pH in the winter. Mean of the ratio of gross benthic DIC consumption and gross benthic DO production was 0.94 ± 0.18, consistent with algal production using NH₄ ⁺ as N source. During the summer weeks the water column pH remains above 10. This high pH is caused by direct and indirect utilisation of CO₂, HCO₃ ^–, CO₃ ^–2, H₄SiO₄ ^° and H₃SiO₄ by primary producers. This study shows that in shallow lakes at high latitudes, where summer days are long and the primary production is mostly by diatoms, the pH is forced to very high values. The high pH could lead to a positive feedback for the Si flux, but negative feedback for the NH₄ ⁺ flux.