Variations saisonnières de la production primaire dans les eaux de surface de la rivière du Saguenay

Experiments of primary production were carried out at weekly intervals in the surface waters at one station (maximum depth of 20 m) in the Saguenay River, near Chicoutimi, during May–December 1978. The photic zone was very thin (maximum depth of 2 m). Phosphates are very low during the season sampling (maximum of 0.1 µat-g.^–1). Maximum of production rates and biomass are respectively 3.5 mg C.m^–3.h^–1 and 3.7 mg.m^–3. The river receives both industrial and urban runoff. Trace metals (Mercury, Copper, Lead, and Iron) seemed to be one of the important limiting factors for phytoplankton growth.

     

Denitrification in the water column of an intermittently anoxic fjord

Denitrification was studied in the water column in the Bunnefjord, inner part of the Oslofjord in southern Norway, using a ¹⁵N-technique (the isotope pairing method). The fjord is 150 m deep and during our surveys in September–December 1998 hydrogen sulphide was present in the deep water below 80 m. No significant denitrification was found in water samples from the surface layer (4 m depth), but high rates were observed within a deep density gradient between 62 and 78 m depth. Oxygen concentration within this layer was low (<21 mmol m^–3), and the concentration of NO₃ decreased from ca. 15 mmolm^–3 at 62 m depth to not detectable below 78 m. Pronounced peaks of NO₂ up to 4.4 mmol m^–3 were observed at 70–78 m depth. The maximum denitrification rate of 1.5 mmol N m^–3 d^–1 was observed at 70 m depth. Integrated for the whole layer, the denitrification rate was 13 mmol N m^–2 d^–1. A significant linear correlation was found between the denitrification rate and the ambient nitrate concentration which indicated that the rate was primarily controlled by the availability of nitrate in the O₂-poor water. Compared to rates reported for coastal water, denitrification in the water column in the Bunnefjord was high and the process appears to be a major sink of bioavailable nitrogen in the fjord.     

Effects of floods on epilithon and benthic macroinvertebrate populations in an unstable New Zealand river

Temporal changes in epilithon biomass and benthic macroinvertebrate density were investigated in the Ashley River, a flood-prone river with an unpredictable discharge regime. Biomass, primary production and respiration of the epilithic community were highest in spring when filamentous algae were present and lowest following two large floods that occurred in close succession. Sixty invertebrate species were taken in benthic samples including 24 species of Trichoptera, 15 Diptera and 4 Ephemeroptera. Larvae of the mayfly Deleatidium (Leptophlebiidae) were numerically dominant and comprised up to 83 % of the fauna in any one month. Mean benthic invertebrate density was highest (9170–18 580 m^{–2) following long periods of low stable flow (< 30 m–3} s^–1) and lowest (230 m^–2) after a major flood (454 m^–3 s^–1). Reductions in benthic density occurred when flow exceeded about 30 m^–3 s^–1, the minimum discharge at which small cobbles are moved. Refuge seeking behaviours, flexible life histories and effective recolonization mechanisms enable the benthos of the Ashley River to persist and recover from frequent, temporally unpredictable disturbances.     

Controlled artificial upwelling in a fjord using a submerged fresh water discharge: computer and laboratory simulations

To enhance the conditions for producing shellfish in coastal waters, the possibility of employing artificial upwelling of nutrients is explored. The effectiveness of a submerged discharge of fresh water is studied by means of a numerical buoyant plume model, BJET, and laboratory simulations. An optimisation of the entrainment of deeper, nutrient-rich water to the proper intrusion depth is demonstrated. The studies show that a downward directed jet of fresh water below the euphotic zone can lift significant amounts of nutrients to the primary production near the surface. The outlet must be large enough to lift the deeper water through the pycnocline to the desired depth of primary production. The results are applied to a possible discharge arrangement in the Samnangerfjord, to the east of Bergen, Norway, using field data from 1999. With a discharge of up to 8 m³/s of fresh water at 35 m depth, the entrainment of deeper water into the buoyant plume, up to the 15 m depth, is 12 to 13 times as much. The chosen arrangement could give an expected vertical transport to the euphotic zone of 467 kg d^–1 N, 46 kg d^–1 P and 555 kg d^–1 Si during the summer growth period. This includes periods of coastal downwelling with greatly reduced values of nutrients, but not periods of strong, deep stratification with deeper intrusions. Further optimisation is possible using active controls of the discharge system.     

Use of artificial eggs in studies of washout depth and drift distance for salmonid eggs

Colour-coded artificial trout eggs were used in investigations of washout depth in a natural stream and of drift distance relative to water velocity in an experimental channel and in a section of natural stream.Washout depth was studied in a spawning riffle of a stream whose bankful discharge is 5.6 m³ s^–1. During an experiment when spates never exceeded 6.5 m³ s^–1 egg washout was severe at 5 cm depth within the gravel, variable at 10 cm and negligible at 15 cm. During another experiment when a spate of 9.0 m³ s^–1 (return period 10–20 years) occurred, washout was severe at 5 and 10 cm depth and variable at 15 cm. There was also evidence that some eggs were moved short distances downstream within the gravel without being washed out.Within experimental channels, drift distance could be predicted from multiple regressions relating logarithms of water velocity, percentage of eggs settled and distance from point of release. At a water velocity of 100 cm s^–1 at 0.6 depth, 50% of eggs would settle within 8 m of the point of release. At water velocities of 75 to 100 cm s^–1 drifting eggs would, on average, travel at c. 60% of water velocity and make 1 to 2 bed contacts m^–1 of travel.A similar multiple regression can be applied to data from a natural stream channel. It predicts much larger drift distances (50% settled in 42 m at 100 cm s ^–1 ). However, in the natural channel, settlement appears aggregated and the validity of the concept of permanent settlement is in doubt.     

Primary productivity and related parameters in three different types of inland waters in Sri Lanka

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m^–2 d^–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m^–2 d^–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m^–2 d^–1. Mean maximum photosynthetic rate was 0.30 mg C m^–3 h^–1 but was lower in the reservoirs than in the temporary ponds and lagoons.     

Facteurs contrôlant la production primaire dens deux rivières soumises a une forte réduction de débit

The effects of river diversion on phytoplankton primary production and biomass in the downstream part of two rivers were studied in relation to physical and chemical variables. These rivers, situated north of the 52nd parallel, are characteristic of oligotrophic systems with phytoplankton primary production less than 10.76 mg C m^–2 h^–1, chlorophyll -a lower than 3.0 mg m^–3 and biomass between 118–1007 mg m^–3. The decrease in flow favored the establishment of an algal biomass approximately two times greater then that present before diversion. This increase in biomass was associated in one river with an increase of 2.5 times of the mean primary production. In the other river the primary production per unit of surface area remained stable but increased when expressed by unit volume, due to a great decrease in underwater light penetration, consequence of inorganic particular matter increase.

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Facteurs contrôlant la production primaire dens deux rivières soumises a une forte réduction de débit

     

Physical and chemical post-dam alterations in the Jamari River, a hydroelectric-developed river of the Brazilian Amazon

The regulation of the Jamari River advanced peak floods by 1–2 months and increased dry-season discharges from 60 to 200 m³ s^–1, resulting in water levels approximately 1 m above those recorded before regulation. Daily variation in water level associated with fluctuations in electricity production by the dam propagated to the lower reaches of the tributary Candeias River. Dissolved oxygen (DO), temperature, pH, and conductivity measured over 1.5 years on three locations along the regulated and two free-flowing rivers indicated important alterations in the case of oxygen concentrations. DO levels shifted from saturated (7–8 mg l^–1) during the flood season (when the spillways were open releasing epilimnetic water) to hypoxic (1.5–3 mg l^–1) during the dry season (when the floodgates were closed and only hypolimnetic water used to run the turbines was feeding the river). Fluctuations in water level and variation in dissolved oxygen tended to be greater at the site closest to the dam, gradually attenuating downstream. Mitigation of the downstream effects of river regulation would require modifications in the operation of the dam.     

Modeling the Impact of Soil and Water Conservation on Surface and Ground Water Based on the SCS and Visual Modflow

Soil and water conservation measures can impact hydrological cycle, but quantitative analysis of this impact is still difficult in a watershed scale. To assess the effect quantitatively, a three-dimensional finite-difference groundwater flow model (MODFLOW) with a surface runoff model–the Soil Conservation Service (SCS) were calibrated and applied based on the artificial rainfall experiments. Then, three soil and water conservation scenarios were simulated on the sand-box model to assess the effect of bare slope changing to grass land and straw mulching on water volume, hydraulic head, runoff process of groundwater and surface water. Under the 120 mm rainfall, 60 mm/h rainfall intensity, 5 m² area, 3° slope conditions, the comparative results indicated that the trend was decrease in surface runoff and increase in subsurface runoff coincided with the land-use converted from bare slope to grass land and straw mulching. The simulated mean surface runoff modulus was 3.64×10⁻² m³/m²/h in the bare slope scenario, while the observed values were 1.54×10⁻² m³/m²/h and 0.12×10⁻² m³/m²/h in the lawn and straw mulching scenarios respectively. Compared to the bare slope, the benefits of surface water reduction were 57.8% and 92.4% correspondingly. At the end of simulation period (T = 396 min), the simulated mean groundwater runoff modulus was 2.82×10⁻² m³/m²/h in the bare slope scenario, while the observed volumes were 3.46×10⁻² m³/m²/h and 4.91×10⁻² m³/m²/h in the lawn and straw mulching scenarios respectively. So the benefits of groundwater increase were 22.7% and 60.4% correspondingly. It was concluded that the soil and water conservation played an important role in weakening the surface runoff and strengthening the underground runoff. Meanwhile the quantitative analysis using a modeling approach could provide a thought for the study in a watershed scale to help decision-makers manage water resources.     

Variability of phytoplankton distribution and primary production around Elephant Island,Antarctica, during 1990–1993

The distribution and abundance of phytoplankton within a sampling grid of 50×10³ km² around Elephant Island were determined from early January to mid-March of 4 successive years, 1990–1993. The number of stations where physical-optical-biological data were obtained from the surface to a maximum of 750 m ranged from 74 in 1990 to 206 in 1993. Contour maps of chlorophyll-a (chl-a) concentrations showed marked mesoscale patchiness that varied from month to month and also interannually. The distribution patterns for chl-a were similar when plotting surface concentrations or integrated values to 100 m. Three major zones could be distinguished that differed in both physical and biological characteristics. Stations in the northwest portion of the grid (Drake Passage waters) and in the southeast portion of the grid (Bransfield Strait waters) showed the most pronounced interannual variations, with phytoplankton biomass and rates of primary production being considerably higher in 1990–91 than in 1992–93. The central portion of the sampling grid, which included the major frontal system north of Elephant Island, showed the smallest interannual variations in both biological and physical parameters and the highest rates of primary production. Phytoplankton biomass and rates of primary production were correlated with depth of the upper mixed layer (UML), which in turn was correlated with the measured wind stress. The mean depth of the UML was 50 m, while the mean depth of the euphotic zone was 90 m. Using the measured mean surface solar irradiance (550 Einsteins m^–2 s^–1), the mean irradiance experienced by cells in the UML of 50 m would be around 105 E m^–2 s^–1, which is similar to the measured I_k (light saturation) value for photosynthesis (101 Em^–2 s^–1). The mean value from all cruises for chl-a in surface waters was 0.7 mg m^–3, while the mean rate of primary production was 374 mg Cm^–2 day^–1.     

Ecosystem respiration and organic carbon processing in a large,tidally influenced river: the Hudson River

We estimated whole-ecosystem rates of respiration over a 40-km stretch of the tidally influenced freshwater Hudson River every 2 to 3 weeks from May through November. We measured in situ concentrations of oxygen over depth at dusk and dawn at 10 stations spaced over this interval. The use of multiple stations allowed for the consideration of the influence of tidal advection of water masses. Respiration was estimated from the decrease in oxygen overnight with a correction for diffusive exchange of oxygen with the atmosphere. We estimated this flux of oxygen to or from the atmosphere using the measured oxygen gradient and a transfer velocity model which is a function of wind velocity.Integration of the data for the period of May through November yields an estimate of whole-ecosystem respiration of 591 g C m^–2 (S.E. = 66). That the standard error of this estimate is relatively low (11% of the estimate) indicates that the use of multiple stations adequately deals with error introduced through the advection of water between stations. The logarithm of average daily respiration rate was correlated with average daily temperature (p = 0.007;r ² = 0.62). We used this temperature-respiration relationship to derive an estimate of the annual respiration rate of 755 g C m^–2 yr^–1 (S.E. = 72). This estimate is moderately sensitive to the estimated flux of oxygen between the atmosphere and water; using the lower and upper 95% confidence limits of our model relating the transfer velocity of oxygen to wind speed gives a range of annual respiration estimates from 665 g C m^–2 yr^–1 to 984 g C m^–2 yr^–1.The river is strongly heterotrophic, with most respiration driven by allochthonous inputs of organic matter from terrestrial ecosystems. The majority of the allochthonous inputs to the river (over 60%) are apparently metabolized within the river. Any change in allochthonous inputs due to changes in land use or climate patterns would be expected to alter the oxygen dynamics and energy flow within this tidally influenced river.     

Longitudinal Spatial Patterns of Bacterial Production and Respiration in a Large River–Estuary: Implications for Ecosystem Carbon Consumption

Rivers and estuaries transport organic carbon (C) from terrestrial and freshwater ecosystems to the marine environment. During this transit, bacteria actively utilize and transform organic C, but few studies have measured detailed spatial variation in rates of bacterial respiration (BR) and production (BP). We measured BP at 39 stations and BR at 12 stations at monthly intervals along a 200-km reach of the tidal Hudson River. We observed strong repeatable spatial patterns for both BP and BR, with rates declining in the downstream direction. Bacterial Production had much greater dynamic range of spatial variation than BR. We used the detailed seasonal and spatial data on BP and BR to measure the total C demand of bacteria at several scales. We calculated volumetric and areal rates for 12 sections of the Hudson, as well as the total C utilization. Volumetric BR averaged 20 g-C-m^–3 y^–1, but it was highest in the most upstream section at 30 g C m^–3 y^–1. Areal rates averaged over the entire river were 174 g C m^–2 y^–1, but they were 318 g C m^–2 y^–1 in the deepest section of the river, indicating the importance of morphometric variation. Total bacterial C demand increased downriver with increasing total volume. Overall, bacteria in the freshwater section of the river consumed approximately 18–25.5 × 10⁹ g C y^–1, about 20% of the total organic C load.     

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.     

Phytoplankton primary production in a shallow,well- mixed,hypertrophic South African lake

This paper reports on a two-year analysis of the wind climateand its effect on phytoplankton primary production in ashallow (mean depth = 1.9 m), hypertrophic South Africancoastal lake, Zeekoevlei. The lake is subject to continuousmixing of the euphotic zone (Z _eu = 0.8 m), andcomplete mixing of the water column to the mean depth on adaily basis. Median annual wind speeds, prevailing fromeither the north or the south, were 6.4 m s^–1. There wasan almost total absence of calms, measured as hourly meanwind speeds of <1 m s^–1. Notwithstanding the highfrequency of mixing, the lake supports a dense population ofphytoplankton, dominated by Cyanophyte and Chlorophytespecies. Mean concentrations of chlorophyll-a were240 g l^–1. The attenuation of photosyntheticallyavailable radiation, PAR, was high, with mean K _dvalues of 6.4 m^–1 and water transparencies of <0.5 m.Levels of primary productivity, determined using the lightand dark bottle oxygen method, were very high, comparable toor exceeding that of the most productive systems yet studied.Maximum volumetric productivity ranged from 525 to 1524 mg Cm^–3 h^–1, and was confined to the upper 0.5 m of thewater column. Daily areal productivity, P _d,varied between 1.2 and 4.3 g C m^–2 d^–1, and that ofthe maximum chlorophyll-a specific photosynthetic rate,P _{B max}, between 1.6 and 7.9 mg C (mgChl-a)^–1 h^–1. Primary production was limited bywater temperature and the attenuation of PAR. The highfrequency of wind-induced mixing resulted in regular mixingof the phytoplankton through the euphotic zone, and reducedthe overall importance of P _max at a single layer inthe depth profile. Similarly, the regularity of mixing wasrecognized as a limitation of the incubation of bottle chainsto determine primary production levels.     

Storage,redistribution and net export of dissolved and sediment-bound nutrients,Vejle Fjord,Denmark

Sixty three percent of the nitrogen (total transported 2041 × 10³ kg y^–1) and 17% of the phosphorus (total 159 × 10³ kg y^–1) supplied from terrestrial sources to Vejle Fjord during the period September 1988 to October 1989 is exported to the Kattegat. The sediment nutrient concentrations in the estuary are mainly governed by hydrography and resuspension. The general wind-induced circulation consists of outgoing currents along the southern side and ingoing currents along the northern side of the estuary. The sediments in shallow water on the southern side had higher concentrations of nutrients.Resuspension resulted in large differences between gross sedimentation and net sedimentation, especially in shallow water. Gross sedimentation of total-N in shallow water was 819 × 10³ kg y^–1 compared to a net sedimentation of 19 × 103 kg y^–1. The shallow water areas in the estuary (10% of the area), had a net sedimentation of total-N which was less than 1% of the supply.Wave-induced resuspension only occurs in exposed parts of the deep water area, when wind velocities exceed 10 m s^–1. The concentration of nutrients in the sediments was found to increase with distance from the river (the source) and with increasing depth, as a result of resuspension near the river mouth in the inner part of the estuary. In sheltered parts of the estuary there was no wave-induced resuspension and the net sedimentation equals gross sedimentation. The rate of sedimentation in deep water areas was 12.2 g m^–2y^–1 for total-N and 2.1 gm^–2y^–1 for total-P.     

Seasonal changes in composition and biomass of epilithic algal floras of a chalk stream and a soft water stream with estimates of production

Microscopic epilithic algae in the River Itchen at Otterbourne near Southampton and in the Ober Water in the New Forest were studied during 1984 and 1985. The River Itchen rises from chalk springs and has a steady pH near 8.2 and a mean alkalinity of 236 mg HCO₃ 1^–1; at the study site the river is about 16 m wide and 20 cm deep, with a mean flow rate of 0.33 m s^–1 and a discharge ranging through the year between 0.34 and 2.46 m³ s^–1. The Ober Water, which drains sands and gravels, has a pH between 6.9 and 7.2 and a mean alkalinity of about 50 mg HCO₃ 1^–1; at the study site it is about 6 m wide, with a mean flow rate of 0.27 m s^–1 and a discharge ranging through the year between 0.08 and 1.0 m³ s^–1.Epilithic algae removed from the pebbles that form the major part of the beds of both streams show seasonal changes in abundance and composition. Diatoms peaked in April/May and dominate the epilithic flora in both streams, comprising 70–95% of all algal cells; highest numbers of chlorophytes occurred in summer and cyanophytes increased in autumn. The species composition of the epilithic flora in the two streams was different, as was the population density; algal cell numbers ranged between 500 and 7000 cells mm^–2 of stream floor in the River Itchen and between 8 and 320 cells mm^–2 of stream floor in the Ober Water. The chlorophyll a content of epilithic algae in the River Itchen ranged between 115 and 415 mg m^–2 of stream floor, representing an annual mean biomass of about 8 g m^–2, whereas in the Ober Water a chlorophyll a content of 2.2 to 44 mg m^–2 of stream floor was found, representing an annual mean biomass of about 1 g m^–2. Cautious estimates of the annual production of epilithic algae in these streams suggest a value of about 600 g organic dry weight m^–2 in the River Itchen and about 75 g m^–2 in the Ober Water.     

Seasonal flooding,soil salinity and primary production in northern prairie marshes

Hydrologic regime is an important control of primary production in wetland ecosystems. I investigated the coupling of flooding, soil salinity and plant production in northern prairie marshes that experience shallow spring flooding. Field experiments compared whitetop (Scolochloa festucacea) marsh that was: (1) nonflooded, (2) flooded during spring with 25 cm water and (3) nonflooded but irrigated with 1 cm water · day^–1. Pot culture experiments examined whitetop growth response to salinity treatments. The electrical conductivity of soil interstitial water (EC_e) at 15 cm depth was 4 to 8 dS· m^–1 lower in flooded marsh compared with nonflooded marsh during 2 years. Whitetop aboveground biomass in flooded marsh (937 g · m^–2, year 1; 969 g · m^–2, year 2) exceeded that of nonflooded marsh (117 g · m^–2 year 1; 475 g · m^–2, year 2). Irrigated plots had lower EC_e and higher aboveground biomass than nonflooded marsh. In pot culture, EC_e of 4.3 dS · m^–1 (3 g · L^–1 NaCl) reduced total whitetop biomass by 29 to 44% and EC_e of 21.6 dS · m^–1 (15 g · L^–1 NaCl) reduced biomass by more than 75%. Large reductions of EC_e and increases of whitetop growth with irrigation indicated that plants responded to changes in soil salinity and not other potential environmental changes caused by inundation. The results suggest that spring flooding controls whitetop production by decreasing soil salinity during spring and by buffering surface soils against large increases of soil salinity after mid-summer water level declines. This mechanism can explain higher marsh plant production under more reducing flooded soil conditions and may be an important link between intermittent flooding and primary production in other wetland ecosystems.     

Carbon turnover in peatland mesocosms exposed to different water table levels

Changes of water table position influence carbon cycling in peatlands, but effects on the sources and sinks of carbon are difficult to isolate and quantify in field investigations due to seasonal dynamics and covariance of variables. We thus investigated carbon fluxes and dissolved carbon production in peatland mesocosms from two acidic and oligotrophic peatlands under steady state conditions at two different water table positions. Exchange rates and CO₂, CH₄ and DOC production rates were simultaneously determined in the peat from diffusive-advective mass-balances of dissolved CO₂, CH₄ and DOC in the pore water. Incubation experiments were used to quantify potential CO₂, CH₄, and DOC production rates. The carbon turnover in the saturated peat was dominated by the production of DOC (10–15 mmol m^–2 d^–1) with lower rates of DIC (6.1–8.5 mmol m^–2 d^–1) and CH₄ (2.2–4.2 mmol m^–2 d^–1) production. All production rates strongly decreased with depth indicating the importance of fresh plant tissue for dissolved C release. A lower water table decreased area based rates of photosynthesis (24–42%), CH₄ production (factor 2.5–3.5) and emission, increased rates of soil respiration and microbial biomass C, and did not change DOC release. Due to the changes in process rates the C net balance of the mesocosms shifted by 36 mmol m^–2 d^–1. According to our estimates the change in C mineralization contributed most to this change. Anaerobic rates of CO₂ production rates deeper in the peat increased significantly by a factor of 2–3.5 (DOC), 2.9–3.9 (CO₂), and 3–14 (CH₄) when the water table was lowered by 30 cm. This phenomenon might have been caused by easing an inhibiting effect by the accumulation of CO₂ and CH₄ when the water table was at the moss surface.     

The impact of effluents of Pitkäranta pulp mill on the water quality of Lake Ladoga: a model study

The aim of this study is to show the effects of the Pitkäranta pulp mill on the water quality of Northern Ladoga by using water quality models. The effluent loading of the pulp mill with its full production capacity is as follows: Water flow 85 000 m³ d^–1 BOD₅ 2.4 t d^–1 Suspended solids 4.1 t d^–1 tot-N 330 kg d^–1 tot-P 68 kg d^–1 COD(Cr) 14.4 t d^–1 Org. C 6 t d^–1 Lignosulphonates 9.4 t d^–1 Loadings of lignosulphonates and organic C are estimations. Lignosulphonate concentrations of 10.5 mg 1^–1 have been reported in the region of Pitkäranta. The study area of northern Ladoga near the pulp mill was divided into three zones (I, II and III). The mean depth of each of them is considered as 10 m, their respective areas 5, 10 and 50 km², and volumes 50, 100 and 500 million m³. The estimated discharges of the zones are 10, 20 and 50 m³ s^–1, respectively. With the aid of simple water quality models the effects of the pulp mill effluents on the concentrations of oxygen, total phosphorus, lignin, COD(Mn) and Secchi disk depths in each of the zones were estimated. Estimations were made during full production capacity and half production capacity of the pulp mill. The modelling results were compared with the preliminary water analysis results of the Finnish-Russian joint research expedition into Lake Ladoga in August 1993. The results show that near the pulp mill (zone I) phosphorus concentrations are high even with half production capacity. Also lignin and COD(Mn) contents have increased, and oxygen concentrations are low both in the summer and during wintertime. Farther away in the open water (zone III) the pollution effects are low. Eutrophication, indicated by high total phosphorus concentrations, is the main effect of the pulp mill effluents.     

Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavågen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m^–3 day^–1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 g l^–1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m^–2 day^–1 in 1997 to 1200 mg carbon m^–2 day^–1 in 1999. Scallops placed at 10 m depth in Hopavågen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day^–1 in 1997 to 0.53% day^–1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavågen had a significantly lower growth rate than those grown in the fertilised water of Hopavågen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavågen was 2–4 times higher than in the control scallops.