Seasonal changes in the dissolved free amino acid and DOC concentrations in a hypertrophic African reservoir and its inflowing rivers

Dissolved free amino acid (DFAA) concentration and composition and dissolved organic carbon (DOC) concentration were measured over 16 months at three depths in hypertrophic Hartbeespoort Dam, South Africa and in its two perenially inflowing rivers. The range of DFAA concentrations in the reservoir and both rivers were similar with dominant DFAA consisting of serine, glycine, alanine and ornithine in all three systems. The range of DOC concentrations in the rivers was 1.5–11.1 mg l^–1, the major river (Crocodile) having about twice the DOC concentration of the Magalies River. The DFAA/DOC ratios ranged between 0.02–1.1% in the Crocodile River and 0.13–3.7% in the Magalies River. DFAA and DOC concentrations were positively correlated to the Magalies River flow, but for the Crocodile River, which received domestic and industrial effluents, DOC was inversely correlated to flow. The source of DFAA in both rivers was mainly terrestrial, in contrast to the main DOC source in the Crocodile River which was the effluents. The DFAA load of the Crocodile River ranged between 0.22 and 208 kg C d^–1.DOC (5.0–24.8mg l^–1) in Hartbeespoort Dam generally decreased with depth but DFAA (15–4800 nmol l^–1) concentration showed no clear trend. The DFAA/DOC ratios varied between 0.02 and 2.9%. DFAA concentrations were correlated (r = 0.3, n = 30, p = 0.04) with bacterial numbers at 0 and 10 m only while no significant correlations were found with bacterial production, chlorophyll a concentration and phytoplankton primary and EDOC (extracellular DOC) production at any depth. The rate of bacterial utilization of DFAA was low compared with data from other lakes. Diurnal phytoplankton production of DFAA in the euphotic zone of the whole lake was calculated to vary between 268 and 30 780 t C d^–1 indicating autochthonous DFAA sources were dominant to allochthonous DFAA sources. The autochthonous production of DFAA was > 2 × gross bacterial production of the euphotic zone indicating that although DFAA concentrations were frequently < 10 g C l^–1, the rate of DFAA production exceeded bacterial requirements.     

Concentrations and fluxes of organic carbon substrates in the aquatic environment

Data concerning concentrations and fluxes of dissolved organic compounds (DOC) from marine and lacustrine environments are reviewed and discussed. Dissolved free amino acids and carbohydrates comprised the main fraction in the labile organic carbon pool. Dissolved free amino acids in marine waters varied between 3–1400 nM and those of freshwaters between 2.6–4124 nM. Dissolved free carbohydrates varied between 0.4–5000 nM in marine systems and between 14–1111 nM in freshwaters. The turnover times of both substrate pools varied in marine waters between 1.4 hours and 948 days and in freshwaters between 2 hours and 51 days. Measurements of stable^12/13C-ratio and¹⁴C-isotope dating in ocean deep water samples revealed DOC turnover times between 2000–6000 years. Studies on carbon flows within the aquatic food webs revealed that about 50% of photosynthetically fixed carbon was channelled via DOC to the bacterioplankton. Excreted organic carbon varied between 1–70% of photosynthetically fixed carbon in marine waters and between 1–99% in freshwaters. The labile organic carbon pool represented only 10–30% of the DOC. The majority (70–90%) of the DOC was recalcitrant to microbial assimilation. Only 10–20% of the DOC could be easily chemically identified. Most of the large bulk material represented dissolved humic matter and neither the chemical structure nor the ecological function of the DOC is as yet clearly understood.Abbreviations ATP Adenosine Tri-Phosphate - AMS Accelerated Mass Spectrometry - BSA Bovine Serum Albumin - GlAse GlucosidAse activity - DAA Dissolved Amino Acids - DCAA Dissolved Combined Amino Acids - DFAA Dissolved Free Amino Acids - DTAA Dissolved Total Amino Acids - DCHO Dissolved Carbohydrates - DCCHO Dissolved Combined Carbohydrates - DFCHO Dissolved Free Carbohydrates - DTCHO Dissolved Total Carbohydrates - DLCFaAc Dissolved Long Chain Fatty Acids - DSCFaAc Dissolved Short Chain Fatty Acids - DOC Dissolved Organic Carbon - DOM Dissolved Organic Matter - DHM Dissolved Humic Matter - DTPhOH Dissolved Total Phenolic compounds - DCPhOH Dissolved Combined Phenolic compounds - DFPhOH Dissolved Free Phenolic conpounds - EOC Excreted Organic Carbon - HS Humic Substances - HPLC High Performance Liquid Chromatography - HTCO High-Temperature Catalytic Oxidation - (K_t+S_n) Transport Constant + Natural Substrate from Michaelis Menten Kinetics - LOCP Labile Organic Carbon Pool - OM Organic Matter - MEE Microbial Extracellular Enzymes - PER Percent of Extracellular Release - PhDOC Photosynthetically derived Dissolved Organic Carbon - POC Particulate Organic Carbon - ROCP Recalcitrant Organic Carbon Pool - T_t Turnover time - UDOC Utilizable Dissolved Organic Carbon - V_max Maximum Uptake Velocity - WCO Wet Chemical Oxidation Dedicated to Prof. Drs. J. Overbeck on the occasion of his 70th birthday     

Saline seepage and vertical distribution of oxygen in a brackish ditch

Summary Seepage caused salinity gradients in a 120 cm deep ditch adjacent to Lake Veere in the Dutch Delta area. The water was mixed by strong horizontal currents, forced by a pumping engine. The restoration of vertical chloride and oxygen gradients was studied.During summer when the level of Lake Veere was raised by 70 cm, a rapid seepage into the ditch was observed, ranging from 6 to 10 mm.h^–1. This caused an anoxic saline layer. At the anaerobic-aerobic interface 1.4 to 2.4 mg O₂.l^–1.h^–1 of the oxygen present in the upper water column was taken up. Sulfide oxidation required one quarter of this amount of oxygen.During winter the level of Lake Veere was lowered again. Normally seepage was absent then, and chloride entered the water column by diffusion; the mud chlorinity decreased from 15 to 3 Cl^– on its way to the ditch. Only after maximum discharge of polder water a moderate seepage of less than 4.4 mm.h^–1 restored a chloride gradient; about 1.6 mg O₂.l^–1.h^–1, produced by benthic diatoms, had to be supplied to the reducing materials in the seepage water in less than 12 hours.A saline seepage was simulated in the laboratory. These simulation tests showed that seepage below 1 mm.h^–1, as well as benthic respiration only, had a small impact on the vertical oxygen profile in the water column. Bottom oxygen demand increased with seepage velocity. At a strong seepage the water column could become entirely anaerobic.Communication no. 226.     

Function and regulation of the avian caecal bulb: influence of dietary NaCl and aldosterone on water and electrolyte fluxes in the hen (Gallus domesticus) perfused in vivo

Summary The function of the caecal bulb, and its adaptation to chronic high- or low-Na⁺ intake, was investigated by in vivo perfusion of anaesthetised birds. Effects of acute aldosterone injection (125 g·kg^–1 body mass) were also measured.Evidence was found for primary active net absorption of Na⁺, inducing parallel Na-linked absorption of water and Cl^– and secretion of K⁺. Around 20–35% of total Cl^– absorption and K⁺ secretion were independent of Na⁺ fluxes, and these components appear to be driven by passive processes with apparent conductances of 6.3×10^–3 (G _Cl) and 1.1×10^–3 (G _K) S·cm^–2.Acetate (40mM) stimulated Na⁺ fluxes (8.5–9.9 Eq·cm^–2·h^–1) and Na-linked water fluxes (27–44 l·cm^–2·h^–1). Increased coupling ratios (2.9–4.6 l·Eq^–1) and other data indicate that these effects may be due to increased osmotic permeabilities of barriers involved in the Na-linked water transfer pathway.Low-Na⁺ maintenance enhanced EPD (49–69 mV, serosa positive) and all net fluxes:J _Na (6.8–11.6);J _K (–3.2––4.3);J _Cl (4.3–5.6 Eq·cm serosal area^–2·h^–1);J _v (28–43 l·cm^–2·h^–1) (mucosal-serosal fluxes positive).Acute aldosterone enhancedJ _Na (10.8–14.0 Eq·cm^–2·h^–1) and EPD (54–66 mV) by 3 h after injection, but had no effect on the Na-linked components ofJ _K orJ _Cl.Abbreviations ECPD, EPD Electrochemical or electrical potential difference - G _Cl ,G _K ionic conductances (Cl^–, K⁺) - J _v ,J _ion net volume or ion flux rate, mucosa-serosa positive;P _d (Cl) diffusive permeability coefficient (of Cl^–) - SEDM standard error of difference between means     

In situ primary production in young Antarctic sea ice

An in situ incubation technique used successfully to measure the photosynthetic carbon assimilation of internal algal assemblages within thick multiyear Arctic ice was developed and improved to measure the photosynthetic carbon assimilation within young sea ice only 50 cm thick (Eastern Weddell Sea, Antarctica). The light transmission was improved by the construction of a cylindrical frame instead of using a transparent acrylic-glass barrel. The new device enabled some of the first precise measurements of in situ photosynthetic carbon assimilation in newly formed Antarctic sea ice, which is an important component in the sea ice ecosystem of the Antarctic Ocean. The rates of carbon assimilation of the interior algal assemblage (top to 5 cm from bottom) was 0.25 mg C m^–2 d^–1 whereas the bottom algal community (lowest 5 cm) attained only 0.02 mg C m^–2 d^–1. Chl a specific production rates (P^Chl) for bottom algae (0.020 – 0.056 g C g chl a ^–1 h^–1) revealed strong light limitation, whereas the interior algae (P^Chl = 0.7 – 1.2 g C g chl a ^–1 h^–1) were probably more limited by low temperatures (< –5 °C) and high brine salinities.     

Diel Production and Microheterotrophic Utilization of Dissolved Free Amino Acids in Waters Off Southern California

Diel patterns of dissolved free amino acid (DFAA) concentration and microheterotrophic utilization were examined in the spring and fall of 1981 in euphotic waters from the base of the mixed layer off the southern California coast. The average depths of the isotherms sampled were 19.2 m for spring and 9.0 m for fall. Total DFAA levels were generally higher in the spring than in the fall, 18 to 66 nM and 14 to 20 nM, respectively. Two daily concentration maxima and minima were observed for total DFAAs as well as for most individual DFAAs. Maxima were usually measured in the mid-dark period and in the early afternoon; minima were typically observed in early morning and late afternoon. Bacterial cell numbers reached maximal values near midnight in both seasons. These increases coincided with one of the total DFAA maxima. The second total DFAA maximum occurred in early to midafternoon, during the time of maximum photosynthetic carbon production and rapid dissolved amino acid utilization. Microbial metabolism (incorporation plus respiration) of selected ³H-amino acids was 2.7 to 4.1 times greater during the daylight hours. DFAA turnover times, based on these metabolic measurements, ranged between 11 and 36 h for the amino acids tested, and rates were 1.7 to 3.7 times faster in the daylight hours than at night. DFAA distributions were related to primary production and chlorophyll a concentrations. Amino acids were estimated to represent 9 to 45% of the total phytoplankton exudate. Microheterotrophic utilization or production of total protein amino acids was estimated as 3.6 μg of C liter⁻¹ day⁻¹ in spring and 1.9 μg of C liter⁻¹ day⁻¹ in the fall. Assimilation efficiency for dissolved amino acids averaged 65% for marine microheterotrophs.     

Fluxes of free amino acids in three Danish lakes

SUMMARY. 1. Heterotrophic assimilation rates and concentrations of dissolved free amino acids (DFAA) were followed during diel studies in the eutrophic Lake Mossø, Lake Esrom and Lake øm in spring and summer in 1982. In all three lakes, three to four fold diel variations in concentrations and assimilation rates were measured. These fluctuations appeared to be iindependent of phytoplankton and bacteria production. Pools of DFAA varied from 380 nM (Lake Mossø) to 2430 nM (Lake ørn), with serine, glycine, alanine and ornithine as dominant free amino acids.
2. When similar water samples were incubated in a natural light-dark cycle or in total darkness, different pools of DFAA were measured in light and dark.
3. Decomposition of organic matter or zooplankton activity (rather than e.g. phytoplankton exudates) appear to be responsible for the concentration changes.
4. Observed discrepancies between simultaneous concentration changes and assimilation rates are discussed in relation to the applied tracer procedure and the concentration measurements.
5. Assimilation of DFAA sustained from 6% to 25% of the bacterial carbon requirement, corresponding to 2–12% of the phytoplankton production in the lakes.     

Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH₄ ⁺) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing ¹⁵N-NH₄ ⁺ as a tracer showed the bacterial population taking up 0.041–0.128 g-atoms Nl^–1d^–1, which was 8–25% of total NH₄ ⁺ uptake rates. The large bacterial uptake of NH₄ ⁺ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using ³H-leucine and -adenine were l.0gCl^–1 d^–1 before the bloom and 16.2 g Cl^–1 d^–1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH₄ ⁺ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH₄ ⁺. Bacterial production of NH₄ ⁺ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH₄ ⁺ in surface waters. Using a variety of ¹⁵N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.     

Grazing and assimilation rates of natural populations of planktonic rotifersKeratella cochlearis,Keratella quadrata andKellicottia longispina in a eutrophic lake (Aydat,France)

The filtering rates of¹⁴C labelledChlamydomonas sp. by 3 dominant species of rotifers were studied in eutrophic Lake Aydat. They varied from 4 to 53 µl ind^–1 h^–1 forKeratella cochlearis, from 2 to 56 µl.^–1h^–1 forKeratella quadrata and from 3 to 52 µl ind^–1 h^–1 forKellicottia longispina. Their maximum assimilation efficiency was 32%. At the measured grazing rates, these populations could clear the water in less than two days during July. In Lake Aydat, the rotifers community could play an important role in the regulation of seasonal succession of phytoplankton and bacteria.     

Nitrate deposition in northern hardwood forests and the nitrogen metabolism of Acer saccharum marsh

It is generally assumed that plant assimilation constitutes the major sink for anthropogenic Nitrate NO ₃ ^– deposited in temperate forests because plant growth is usually limited by nitrogen (N) availability. Nevertheless, plants are known to vary widely in their capacity for NO ₃ ^– uptake and assimilation, and few studies have directly measured these parameters for overstory trees. Using a combination of field and greenhouse experiments, we studied the N nutrition of Acer saccharum Marsh. in four northern hardwood forests receiving experimental NO ₃ ^– additions equivalent to 30 kg N ha^–1 year^–1. We measured leaf and fine-root nitrate reductase activity (NRA) of overstory trees using an in vivo assay and used ¹⁵N to determine the kinetic parameters of NO ₃ ^– uptake by excised fine roots. In two greenhouse experiments, we measured leaf and root NRA in A. saccharum seedlings fertilized with 0–3.5 g NO ₃ ^– –N m^–2 and determined the kinetic parameters of NO ₃ ^– and NH ₄ ⁺ uptake in excised roots of seedlings. In both overstory trees and seedlings, rates of leaf and fine root NRA were substantially lower than previously reported rates for most woody plants and showed no response to NO ₃ ^– fertilization (range = non-detectable to 33 nmol NO ₂ ^– g^–1 h^–1). Maximal rates of NO ₃ ^– uptake in overstory trees also were low, ranging from 0.2 to 1.0 mol g^–1 h^–1. In seedlings, the mean V _max for NO ₃ ^– uptake in fine roots (1 mol g^–1 h^–1) was approximately 30 times lower than the V _max for NH ₄ ⁺ uptake (33 mol g^–1 h^–1). Our results suggest that A. saccharum satisfies its N demand through rapid NH ₄ ⁺ uptake and may have a limited capacity to serve as a direct sink for atmospheric additions of NO ₃ ^– .     

The effect of specific growth rates on the recovery of amino acids

Three specific growth rates, 0.23, 0.45 and 0.51 h^–1, were used to cultivate Corynebacterium glutamicum in a pH-auxostat. The specific formation rates of most amino acids increased by raising the specific growth rates. The highest specific growth rate, 0.51 h^–1, favors the production of LEU; whereas the highest production yield for ALA and GLU were at = 0.23 h^–1. A correlation among specific growth rates, glucose consumption rate, and production yields of amino acids was obtained.     

Heterotrophic utilization and decomposition of extracellular carbon released by the aquatic angiosperm Littorella uniflora (L.) aschers

The heterotrophic assimilation of extracellular organic carbon (EOC) released by Littorella uniflora (L.) Aschers. was assayed. Utilization and decompostion of EOC by heterotrophs in the periphyton community were measured in a two-chamber open flow-system and, by the bacterioplankton, in bottle incubations. Under the experimental conditions 12–30% of the released EOC was metabolized in the periphyton community. This corresponded to a theoretical removal rate of 72–100% h^?1. The bacterioplankton assimilated EOC more slowly (1% h^?1). These results show that the EOC (mainly small molecules < 700 Daltons) was highly labile and accessible to heterotrophic microorganisms. The EOC released by submerged macrophytes in Lake Kalgaard is distributed approximately 20% to the periphyton community, 20% to the bacterioplankton, and 60% to the sediment.     

Are dissolved free amino acids free?

Microbial assimilation of 3 amino acids (glutamic acid, alanine, and ornithine) was characterized in 3 lakes and 2 marine stations using the Michaelis-Menten kinetic approach. The calculated K_t + S_n concentrations were related to chemical concentration measurements of dissolved free amino acids (DFAA) to evaluate the biological and the chemical determinations of the DFAA pools. Concentrations of K_t + S_n always were larger than chemical measurements of the S_n concentrations. K_t + S_n and S_n varied from 11.5 and 9.5 nM (alanine, oligotrophic lake) to 288.7 and 89.9 nM (ornithine, marine harbor station), respectively. Subtracting S_n from the K_t + S_n concentrations, K_t was found to range from 12–897% of the chemically measured S_n concentrations. To test whether the DFAA actually were free, dissolved molecules, dissolved material in the water samples was separated into various molecular size classes by means of gel permeation chromatography. From 47–116% of the DFAA in the untreated water samples was recovered in the low molecular fraction (<700 Daltons). Variation in recoveries mainly appeared to be due to an incomplete chromatographic separation and difficulties in obtaining proper blank levels. The present observations suggest that labeled tracers can be used in the study of DFAA assimilation and that the DFAA are free, dissolved molecules. This partly conflicts with previously published reports.     

Cell swelling activates separate taurine and chloride channels in Ehrlich mouse ascites tumor cells

The taurine efflux from Ehrlich ascites tumor cells is stimulated by hypotonic cell swelling. The swelling-activated taurine efflux is unaffected by substitution of gluconate for extracellular Cl^– but inhibited by addition of MK196 (anion channel blocker) and 4,4-diisothiocyanostilbene-2,2-disulfonic acid (DIDS; anion channel and anion exchange blocker) and by depolarization of the cell membrane. This is taken to indicate that taurine does not leave the osmotically swollen Ehrlich cells in exchange for extracellular Cl^–, i.e., via the anion exchanger but via a MK196- and DIDS-sensitive channel that is potential dependent. An additional stimulation of the swelling-activated taurine efflux is seen after addition of arachidonic acid and oleic acid. Cell swelling also activates a Mini Cl^– channel. The Cl^– efflux via this Cl^– channel, in contrast to the swelling-activated taurine efflux, is unaffected by DIDS and inhibited by arachidonic acid and oleic acid. It is suggested that the swelling-activated Mini Cl^– channel and the swelling-activated taurine channel in the Ehrlich cell represent two distinct types of channels.This work was supported by the Danish Natural Research Council and by the NOVO foundation. Dr. Birte Kramhøft is acknowledged for critical reading of this paper.     

The release of dissolved organic carbon as a result of diatom fragmentation during feeding byMysis relicta

The release of dissolved organic carbon (DOC) byMysis relicta that occurs while feeding may be an important pathway in the recycling of nutrients in Lake Michigan. Laboratory experiments were conducted to determine the levels of DOC released byM. relicta from Lake Michigan while grazing on laboratory-cultured diatoms. Increasing filtering rates were observed at progressively lower chlorophyll concentrations. Higher filtering rates were accompanied by higher fragmentation of diatoms and a higher concomitant release of DOC from broken cells. While the amount of DOC release is small (0.02–3.30 g C · 1^–1· h^–1), it nonetheless is of importance to bacteria and possibly some algae. After assimilation by bacteria and algae, the organic carbon is then available to consumers.Contribution No. 239, Center for Great Lakes Studies, The University of Wisconsin-Milwaukee, Milwaukee, WI 53201, U.S.A.Contribution No. 239, Center for Great Lakes Studies, The University of Wisconsin-Milwaukee, Milwaukee, WI 53201, U.S.A.     

Vertical planktonic structure in the central basin of Lake Baikal in summer 1999, with special reference to the microbial food web

Planktonic microbial interactions in the central basin of Lake Baikal were examined on a summer day in 1999. The subsurface maxima of bacterial abundance and chlorophyll concentration were recorded at the same depth, whereas the vertical distribution of heterotrophic nanoflagellates was the inverse of those of bacteria and picophytoplankton. Release of extracellular organic car-bon (EOC) from phytoplankton was estimated by the NaH¹⁴CO₃ method as 2.4µgCl^–1day^–1. Bacterial production (4.3µgCl^–1day^–1), estimated in a bottle incubation experiment using size-fractionated water samples, exceeded the EOC released. Thus, other supplying sources of organic matter are needed for the bacterial production. Grazing (2.6µgCl^–1day^–1) was also estimated in the experiment and accounted for 60% of the bacterial production. This is the first report on the microbial food web in the central basin of Lake Baikal.     

n-3 PUFA productivity in chemostat cultures of microalgae

Eicosapentaenoic (EPA) and docosahexaenoic (DHA) acid productivities from chemostat cultures of an isolate of Isochrysis galbana have been studied. The productivities reached in the interval of dilution rates between 0.0295 h^–1 and 0.0355 h^–1 were 1.5mg·1^–1·h^–1 for lipids, 300 g·1^–1·h^–1 for EPA and 130g1·1^–1·h^–1 for DHA. Furthermore, light attenuation by mutual shading, and agitation speed influences on growth and fatty acid composition were analysed. A model relating steady-state dilution rates to internal average light intensity has been proposed, the parameter values of which obtained by non-linear regression were: maximum specific growth rate (_max)=0.0426 h^–1; the affinity of cells to light (I_k) = 10.92 W·m^–2; the exponent (n) = 5.13; regression coefficient (r ²)=0.9999. Correspondence to: E. Molina Grima     

Application of N-15 dilution for simultaneous estimation of nitrification and nitrate reduction in soil-water columns

Nitrification and denitrification rates were estimated simultaneously in soil-floodwater columns of a Crowley silt loam (Typic Albaqualfs) rice soil by an¹⁵N isotopic dilution technique. Labeled NO ₃ ^– was added to the floodwater of soil-water columns, half were treated with urea fertilizer. The (NO ₃ ^– +NO ₂ ^– )–N and (NO ₃ ^– +NO ₂ ^– )–N concentrations in the floodwater were measured over time and production and reduction rates for NO ₃ ^– calculated. Nitrate reduction in the urea amended columns averaged 515 mol N m^–2h^–1 and nitrification averaged 395 mol N m^–2h^–1 over the 35–153 d incubation. The nitrification rate for 4–19 d sampling period (1,560 mol N m^–2h^–1) in the urea amended columns was almost 9 times greater than the reduction rate (175 mol N m^–2h^–1) over the same period. Without the addition of urea the NO ₃ ^– production rate averaged 32 mol N m^–2h^–1 and reduction 101 mol N m^–2h^–1.     

The influence of road salts on the salinity and the meromictic stability of Lake Svinsjøen,southeastern Norway

Primarily as a result of road salting, the water masses ofLake Svinsjøen, a small meromictic lake in southeasternNorway, have been subject to great changes in salinity duringthe period 1947–1995. The greatest change in saltconcentration has occurred in the upper part of themonimolimnion (depth 10–15 m) where mean conductivityincreased 104.2 per cent, from 143 to 292 S cm^–1. Inthe upper mixolimnion (depth 0-5 m), mean conductivity rosefrom 130 to 238 S cm^–1 during the same period. Theions responsible for the salinity changes were Na⁺ andCl^– from de-icing salts, and Ca²⁺ and Cl^– fromsalts used to keep down dust from roads. Further sources ofCa²⁺ are the road asphalt and increased weathering andleaching of the lime-rich rocks caused by acid precipitation,the main source of the additional inputs of SO tothe lake. The salinity changes caused major changes inmeromictic stability, S . In the period1947-1966, S increased by 24 g cm cm^–2,and the maximum level of meromictic stability, 125 gcm^–2, was found in 1966. As a result of higher rate ofsalt accumulation in the upper part of the monimolimnion andin the mixolimnion, S decreased by 30 g cmcm^–2 during the period 1966-1991, and a simultaneousrise in the chemocline took place. In the period 1991-1995 anadditional decrease of 26 g cm cm^–2 occurred. Continuedectogenic inputs of salts through processes typical of thetime period investigated will in future further weaken thelake's meromictic stability, and may cause the demise ofmeromixis in Lake Svinsjøen, a development which may haveimportant implications for primary productivity of thelake.     

Carbon assimilation in contrasting streamside and inland Spartina alterniflora salt marsh

Carbon assimilation and standing crop biomass of Spartina alterniflora were studied in a contrasting streamside and inland salt marsh in Louisiana Gulf coast, USA. A substantially lower leaf dry weight, leaf area index, and standing crop biomass were recorded for inland plants as compared to streamside plants. Net assimilation rates ranged between 8 to 25 mol m^–2 s^–1 for streamside and between 4 to 19 mol m^–2 s^–1 for inland plants. The average photosynthetic rates were significantly lower for inland plants which were growing in an apparently more stressed environment. In addition, the differences were more profound with progression of the growing season. The reduced photosynthetic activity in the inland marsh was attributed to greater soil waterlogging, increased anaerobic root respiration, plant toxins (sulfide), restricted nutrient uptake or a combination of these factors.Abbreviations E_h = redox potential - g_w = stomatal conductance - LAI = leaf area index - P_n = net photosynthesis - PPFD = photosynthetic photon flux density - T₁ = leaf temperature