Nitrate and ammonium uptake by plankton in an Amazon River floodplain lake

Uptake of ammonium and nitrate by plankton was measured in tropicalLake Calado, Brazil. Nitrate uptake was strongly influencedby light and was light saturated at {small tilde}340 µEm^–2 s^–1. In contrast, uptake of ammonium was lessinfluenced by light, and saturated at {small tilde}250 µEm^–2 s^–1. Uptake rates of both forms of nitrogenwere inhibited by up to 80% at light intensities higher thanthose required for saturation. Concentrations of ammonium andnitrate also had a strong influence on uptake rates. Half-saturationconstants (0.3–5 µM) were usually greater than ambientconcentrations (0.1–0.6 µM), indicating that uptakerates at ambient concentrations were less than one-half of thesaturated rates. Ammonium is the more important type of inorganicnitrogen for plankton of Lake Calado because nitrate concentrationsremain low to undetectable except during periodic inputs ofnitrate-rich water from the Amazon River. Using the observeddependence of uptake on concentration and light, maximum uptakerates per unit chlorophyll were computed to be in reasonableagreement with rates derived from P^B_m values for carbon uptake. ¹ Present address: Florida Department of Natural Resources,Marine Research Laboratory, St Petersburg, FL 33701, USA     

Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000

Microbial potential uptake and regeneration rates of ammonium(NH₄⁺) were studied along a salinity gradient (salinities 0.2–34.4)in the Mississippi River plume during an extreme drought inspring 2000. Chlorophyll concentrations up to 30 µg L^–1were highest in the low- and mid-salinity regions (salinities8.5–28.2) and comparable to records of other years butextended over smaller areas than during periods of normal riverflow. Bacterial biomass (5.1–28.3 µg C L^–1)was at the low end of the range observed in normal flow years,decreased with distance from the river mouth and did not peakwith chlorophyll. Heterotrophic nanoflagellate abundance (1.4–4.0µg C L^–1) did not reflect phytoplankton and bacterialspatial distribution but peaked at 9.2 µg C L^–1at salinity 8.5. Microbial NH₄⁺ regeneration rates were estimatedby ¹⁵NH₄⁺ isotope dilution experiments for the whole microbialcommunity, under light and dark conditions, and for the <2µm bacterium-dominated size fraction. Microbial NH₄⁺ regenerationrates (0.018–0.124 µmol N L^–1 h^–1) werelow relative to previous reports and peaked at salinity 28.Total NH₄⁺ regeneration rates were higher than those in the<2 µm size fraction at only four stations, suggestingthat bacterial mineralization was a significant component ofNH₄⁺ recycling in some parts of the river plume. Higher NH₄⁺regeneration in whole-water samples versus <2 µm fractionsprovided evidence for microbial grazing in regions where chlorophylland regeneration rates peaked and at two full-salinity stations.     

Respiratory rates in the cladoceran Ceriodaphnia dubia in Lake Rotongaio, a monomictic lake

The experimentally measured oxygen consumption rate by the cladoceran,Ceriodaphnia dubia, showed a linear increase between 5 and 20°C.Oxygen consumption rates of C. dubia were estimated in situfrom respiratory electron transport system (ETS) activity inLake Rotongaio during summer stratification and winter mixing.Oxygen consumption was 0.002 µl O₂ animal^–1 h^–1in the hypolimnion and 0.076 µl O₂ animal^–1 h^–1in the epilimnion during stratification. Implications of respiredoxygen for metabolic carbon requirements are discussed.     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

Ca2+ influx through alpha1S DHPR may play a role in regulating Ca2+ release from RyR1 in skeletal muscle

Skeletal muscle fiber types differ in their contents of total phosphate, which includes inorganic phosphate (P_i) and high-energy organic pools of ATP and phosphocreatine (PCr). At steady state, uptake of P_i into the cell must equal the rate of efflux, which is expected to be a function of intracellular P_i concentration. We measured ³²P-labeled P_i uptake rates in different muscle fiber types to determine whether they are proportional to cellular P_i content. P_i uptake rates in isolated, perfused rat hindlimb muscles were linear over time and highest in soleus (2.42 ± 0.17 µmol·g^–1·h^–1), lower in red gastrocnemius (1.31 ± 0.11 µmol·g^–1·h^–1), and lowest in white gastrocnemius (0.49 ± 0.06 µmol·g^–1·h^–1). Reasonably similar rates were obtained in vivo. P_i uptake rates at plasma P_i concentrations of 0.3–1.7 mM confirm that the P_i uptake process is nearly saturated at normal plasma P_i levels. P_i uptake rate correlated with cellular P_i content (r = 0.99) but varied inversely with total phosphate content. Sodium-phosphate cotransporter (PiT-1) protein expression in soleus and red gastrocnemius were similar to each other and seven- to eightfold greater than PiT-1 expression in white gastrocnemius. That the PiT-1 expression pattern did not match the pattern of P_i uptake across fiber types implies that other factors are involved in regulating P_i uptake in skeletal muscle. Furthermore, fractional turnover of the cellular P_i pool (0.67, 0.57, and 0.33 h^–1 in soleus, red gastrocnemius, and white gastrocnemius, respectively) varies among fiber types, indicating differential management of intracellular P_i, likely due to differences in resistance to P_i efflux from the fiber. inorganic phosphate; sodium-inorganic phosphate transporters; PiT-2; inorganic phosphate efflux     

Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

The isotope ¹⁵N was used to examine nitrogen dynamics in LakesFryxell and Vanda, two permanently ice-covered Antarctic lakes.Half-saturation constants for NH₄⁺. uptake in the shallow watersof both lakes were <10 µg N l^–1; uptake kineticexperiments on populations forming the deep-chlorophyll layersof these lakes showed zero-order kinetics and could not be fittedwith the Michaelis-Menten equation. Elevated uptake within thefirst few minutes following pulses of NH₄⁺. and NO₃^– occurredin both lakes. NH₄⁺ regeneration, determined from isotope dilutionexperiments, exceeded uptake at 4.6 m in Lake Fryxell, was lessthan uptake at 9 m in Lake Fryxell and was equal to uptake at10 m in Lake Vanda under the experimental conditions. NO₃^–uptake was suppressed by NH₄⁺ levels as low as 2 µg NH₄⁺-N l^–1 in Lake Fryxell; the suppression was strongestin the near-surface populations. Substrate-saturated C:N uptakeratios (g:g) in Lake Fryxell decreased from 8.4 near the surfaceto 1.8 at the bottom of the trophogenic zone. Overall, the nitrogendynamics in these lakes are similar to other lakes and the openocean in that biological productivity during the austral summeris supported by regenerated nutrients.     

Estimation of ammonium regeneration efficiencies associated with bacterivory in pelagic food webs via a 15N tracer method

Phagotrophic protists are major components of pelagic food webs,both as consumers of bacterial and phytoplankton cells, andas regenerators of inorganic nutrients. In this study, we estimatedthe efficiency of ammonium regeneration by protists feedingon bacteria within natural plank-tonic assemblages, using a¹⁵N tracer method, in which the excretion of ¹⁵N-labeled ammoniumdue to grazing on ¹⁵N pre-labeled bacteria was followed overtime. We tested this approach in experiments based on the additionof heat-killed ¹⁵N-labeled bacteria to laboratory cultures andto samples of coastal seawater. During two experiments, variationin abundance of bacterivores and bacterioplankton resulted innon-constant grazing rates. Deterministic computer models thatused abundance of bacteria and protists as variables were developedto estimate best-fit values of grazing mortality (g, h^–1)and of ammonium regeneration efficiency (R_E, fraction of theinitial ¹⁵N label in added bacteria which is released as ammonium).Estimated ammonium R_E were 0.30–0.35 for one trophic linksystems with both a monospecific culture and a mixed speciesassemblage of bacterivorous flagellates. R_E was higher for multi-trophicstep food webs: 0.60 for 5 µm pre-screened coastal seawaterand 0.90 for whole coastal seawater.     

Rates of Photosynthesis in Characean Cells: I. PHOTOSYNTHETIC 14CO2 FIXATION BY NITELLA TRANSLUCENS

A study has been made of photosynthetic ¹⁴CO₂ fixation by isolated‘mature’ internodes of Nitella translucens. Experimentalconditions were similar to those used in studies of the ionicrelations of these cells. Maximum rates of photosynthesis were33–40µµmoles CO₂, fixed per cm² of surfacearea per second (equivalent to 12–15 /xmoles fixed permg chlorophyll per hour). ^l4CO₂ fixation was inhibited to thedark level by 3(3,4,dichlorophenyl)-1, 1-dimethylurea (at 0-6µM or 10µM) and by the uncoupler carbonyl cyanide-m-chlorophenylhydrazone(SµM). The presence of imidazole or ammonium sulphate(both of which uncouple ATP production in vitro) did not resultin an inhibition of 14CO2 fixation. These results are discussedin relation to published work on solute uptake by Nitella translucens.During photosynthesis there was rapid movement of ¹⁴C-labelledorganic compounds out of the chloroplasts. ¹⁴C-labelled sucrose,ammo-acids, and sugar phosphates were found in samples of vacuolarsap.     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

Uptake of 13C and 15N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

The uptake rate of carbon and nitrogen (ammonium, nitrate andurea) by the Microcystis predominating among phytoplankton wasinvestigated in the summer of 1984 in Takahamaira Bay of LakeKasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046h^–1) and ammonium (0.15–0.17 h^–1) were considerablyhigher than other natural phytoplankton. The ammonium, nitrateand urea uptake by Microcystis was light dependent and was notinhibited with nigh light intensity. The K₁ values were farlower than the I_k values. The carbon uptake was not influencedby nitrogen enrichment. Microcystis accelerated the uptake rateby changing V_max/K _s value when nitrogen versus carbon contentin cells declined. Nitrate was scarcely existent in TakahamairiBay during the summer, when Microcystis usually used ammoniumas the nitrogen source. However, the standing stock of ammoniumin the water was far lower than the daily ammonium uptake rates.Therefore, the ammonium in this water had to be supplied becauseof its rapid turn-over time (–0.7–2.6 h).     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Nitrogenous nutrient uptake by phytoplankton and ammonium regeneration by microbial assemblage in Lake Biwa

In situ rates of nitrate, ammoniwn and urea uptake by the phytoplanktonassemblage, and the regeneration rate of ammonium by the microbialassemblage, in Lake Biwa were measured using the nitrogen 15tracer method from 1985 to 1987. The rate of total nitrogen(sum of ammonium, nitrate and urea) uptake was in the rangeof 62–594 ng N^–1 r^–1 h^–1. The percentagecontribution of ammonium uptake was 41–92%, that of urea4–58% and that of nitrate <1–28% of total uptake.The annual mean new production which was supported by nitrateuptake was 18% of the total production in 1986. The phytoplanktonassemblage in Lake Biwa preferentially utilized regeneratednitrogen, such as ammonium and urea, whose concentration wasmuch lower than that of nitrate throughout the observation penodwithout in summer. The in situ nitrogen uptake rate was almostsufficient to meet the nitrogen requirement of the phytoplanktonassemblage, except in midsummer when the nitrate concentrationwas below the detection limit of 0.3 µg N r^–1. Inthe trophogemc layer, the rate of ammonium regeneration was66–272 ng N 1^–1 h^–1 Although the ambient ammoniumconcentration in the trophogenic layer was maintained at aroundthe half-saturation constant for ammonium uptake kinetics, theammomum uptake rates were always highly correlated with ammoniumregeneration rates. From the size fractionation experimentsand estimates from the literature, it was suggested that themicrobial assemblage <1 µm may have been the most importantagent responsible for the ammonium regeneration processes inthe trophogenic layer.     

The Permeability of Ammonia, Methylamine and Ethylamine in the Charophyte Chara corallina (C. australis)

Ritchie, R. J. 1987. The permeability of ammonia, methylamineand ethylamine in the charophyte Chara corallina (C. australis).—J.exp. Bot. 38: 67–76 The permeabilities of the amines, ammonia (NH₃), methylamine(CH₃NH₂) and ethylamine (CH₃CH₂NH₂) in the giant-celled charophyteChara corallina (C. australis) R.Br. have been measured andcompared. The permeabilities were corrected for uptake fluxesof the amine cations. Based on net uptake rates, the permeabilityof ammonia was 6?4?0?93 µm s^–1 (n = 38). The permeabilitiesof methylamine and ethylamine were measured in net and exchangeflux experiments. The permeabilities of methylamine were notsignificantly different in net and exchange experiments, norto that of ammonia (P_methylamine = 6?0?0?49 µm s^–1(n = 44)). In net flux experiments the apparent permeabilityof ethylamine was slightly greater than that of ammonia andmethylamine (P_{ethylamine, net} = 8?4?1?2 µm s^–1 (n= 40)) but the permeability of ethylamine based on exchangeflux data was significantly higher (P_{ethylamine, exchange} =14?1?2 µm s^–1 (n = 20)). Methylamine can be validlyused as an ammonium analogue in permeability studies in Chara. The plasmalemma of Chara has acid and alkaline bands; littlediffusion of uncharged amines would occur across the acid bands.The actual permeability of amines across the alkaline bandsis probably about twice the values quoted above on a whole cellbasis i.e. the permeability of ammonia across the permeablepart of the plasmalemma is probably about 12 µm s^–1. Key words: Chara, permeability, ammonia, methylamine     

Seasonal uptake and regeneration of inorganic nitrogen and phosphorus in a large oligotrophic lake: size-fractionation and antibiotic treatment

Uptake and regeneration of inorganic N and P in oligotrophicFlathead Lake (Montana) were measured with ¹⁵N and ³²P incorporationand dilution experiments, six times over a seasonal cycle. Theannual mean molar N P uptake ratio at ambient concentrationswas 13 9 (range = 4 8–34.2); uptake of nitrate, ammoniumand phosphate were always below saturation indicating both Nand P deficiency Organisms >280 µm were responsiblefor 0–60% of ammonium and 0–40% of phosphate regeneration,40–100% of the ammonium and 34–98% of phosphateregeneration occurred in the <3 µm fraction The <3µm fraction accounted for 7–70% of the ammoniumand 6–64% of the phosphate uptake. Results from antibiotictreatments indicated that both prokaryotic and eukaryotic ammoniumuptake was important, and that eukaryotes accounted for 53–98%of the ammonium regeneration. During thermal stratification,heterotrophic ammonium and phosphate regeneration by organisms>3 µm supplied much of the inorganic N and P in theepilimnion. Estimated rates of allochthonous and diffusive (i.e‘new’) ammonium, nitrate and phosphate input were<5% of biotic regeneration. These results suggests that (i)both N and P dynamics should be considered when examining nutrientregulation of primary productivity of oligotrophic lakes, (ii)bacteria probably compete with phytoplankton for both ammoniumand phosphate, (iii) biotic regeneration is the main sourceof nutrients to the epilimnion during stratification, and (iv)crustacean zooplankton were relatively unimportant sources ofregenerated ammonium and phosphate.     

Defoliation in White Clover: Nodule Metabolism, Nodule Growth and Maintenance, and Nitrogenase Functioning During Growth and Regrowth

In two experiments, the functioning and metabolism of nodulesof white clover, following a defoliation which removed abouthalf the shoot tissue, were compared with those of undefoliatedplants. In one experiment, the specific respiration rates of nodulesfrom undefoliated plants varied between 1160 and 1830 µmolCO₂ g^–1h^–1, of which nodule ‘growth and maintenance’accounted for 22 ± 2 per cent, or 27 ± 3.6 percent, according to method of calculation. Defoliation reducedspecific nodule respiration and nodule ‘growth and maintenance’respiration by 60–70 per cent, and rate of N₂ fixationby a similar proportion. The original rate of nodule metabolismwas re-established after about 5 d of regrowth; during regrowthnodule respiration was quantitatively related to rate of N₂,fixation: 9.1 µmol CO₂ µmol^–1N₂. With the possible exception of nodules examined 24 h after defoliation,the efficiency of energy utilization in nitrogenase functioningin both experiments was the same in defoliated and undefoliatedplants: 2.0±0.1 µmol CO₂ µmol^–1 C₂H₄;similarly, there was no change in the efficiency of nitrogenasefunctioning as rate of N₂ fixation increased with plant growthfrom 1 to 22 µmol N₂ per plant h^–1. Exposure of nodulated white clover root systems to a 10 percent acetylene gas mixture resulted in a sharp peak in rateof ethylene production after 1.5–2.5 min; subsequently,rate of ethylene production declined rapidly before stabilisingafter 0.5–1 h at a rate about 50 per cent of that initiallyobserved. Regression of ‘peak’ rate of ethyleneproduction on rate of N₂ fixation indicated a value of 2.9 µmolC₂H₄ µmol^–1 N₂, for rates of N₂ fixation between1 and 22 µmol N₂ per plant h^–1. The relationshipsbetween nitrogenase respiration, acetylene reduction rates andN₂ fixation rates are discussed. Trifolium repens, white clover, defoliation, nodule respiration, N₂, fixation, nitrogenase     

Carbon Fixation, Ammonium Uptake and Regeneration in an Equatorial Lake: Biological Versus Physical Control

Ammonium uptake and regeneration were measured in the euphoticzone of Petit Saut Lake, French Guyana, to examine nitrogencycling in this recently flooded equatorial forest environment.Am-monium regeneration rates were extremely high (mostly inthe range 1–6 µmol N l^–1 h^–1), and aredue to the very high grazing rates of the microzooplankton,which consumed between 56 and 95% of the phytoplankton productionin any given incubation. These regeneration rates were aboutan order of magnitude higher than the net ammonium uptake rates.This imbalance is probably due to dissolved organic nitrogenrelease during grazing. At the bottom of the euphotic zone (4–5m), photosynthetic bacteria are responsible for ammonium uptake.Diffusion-driven ammonium fluxes are an order of magnitude lowerthan biologically driven fluxes. Therefore, ammonium fluxesare dominated by biology rather than by physics in this lake.     

DCEBIO stimulates Cl- secretion in the mouse jejunum

We investigated the effects of 5,6-dichloro-1-ethyl-1,3-dihydro-2H-benzimidazol-2-one(DCEBIO) on the Cl^– secretory response of the mouse jejunum using the Ussing short-circuit current (I_sc) technique. DCEBIO stimulated a concentration-dependent, sustained increase in I_sc (EC₅₀ 41 ± 1 µM). Pretreating tissues with 0.25 µM forskolin reduced the concentration-dependent increase in I_sc by DCEBIO and increased the EC₅₀ (53 ± 5 µM). Bumetanide blocked (82 ± 5%) the DCEBIO-stimulated I_sc consistent with Cl^– secretion. DCEBIO was a more potent stimulator of Cl^– secretion than its parent molecule, 1-ethyl-2-benzimidazolinone. Glibenclamide or NPPB reduced the DCEBIO-stimulated I_sc by >80% indicating the participation of CFTR in the DCEBIO-stimulated I_sc response. Clotrimazole reduced DCEBIO-stimulated I_sc by 67 ± 15%, suggesting the participation of the intermediate conductance Ca²⁺-activated K⁺ channel (IK_Ca) in the DCEBIO-activated I_sc response. In the presence of maximum forskolin (10 µM), the DCEBIO response was reduced and biphasic, reaching a peak response of the change in I_sc of 43 ± 5 µA/cm² and then falling to a steady-state response of 17 ± 10 µA/cm² compared with DCEBIO control tissues (61 ± 6 µA/cm²). The forskolin-stimulated I_sc in the presence of DCEBIO was reduced compared with forskolin control tissues. Similar results were observed with DCEBIO and 8-BrcAMP where adenylate cyclase was bypassed. H89, a PKA inhibitor, reduced the DCEBIO-activated I_sc, providing evidence that DCEBIO increased Cl^– secretion via a cAMP/PKA-dependent manner. These data suggest that DCEBIO stimulates Cl^– secretion of the mouse jejunum and that DCEBIO targets components of the Cl^– secretory mechanism. 1-ethyl-2-benzimidazolinone; forskolin; glibenclamide; clotrimazole; H89     

The Carbon Economy of Rubus chamaemorus L. II. Respiration

Respiratory activity and seasonal changes in carbohydrate contentof the storage organs of Rubus chamaemorus L. have been investigated.Leaf dark respiration rate increases in a non-linear mannerfrom 0·7 mg CO₂ evolved dm^–2 h^–1 at 0 °Cto 4·6 rng CO₂ evolved dm^–2 hh^–1 at 30 °C.Root and rhizome respiration rates increase from 1 µ1O₂ uptake g^–1 fresh weight h^–1 at 0.7 ° C to10 µ10, uptake g^–1 f. wt h^–1 at 20 °C.Rhizome carbohydrate reserves decline from a September peakof 33 per cent alcohol insoluble d. wt to 16 per cent in May. The circumpolar distribution of R. chamaemorus is discussedin relation to the evidence presented here and in the precedingpaper of the series.     

Dynamics of inorganic phosphorus in pelagic communities of the Sea of Okhotsk

Inorganic phosphorus uptake and regeneration in the OkhotskSea waters were investigated in July–August 1994 withthe use of radioisotopic techniques. The rates of PO₄-P uptakeby microplankton in the upper mixed layer were between 1.5 and6.6 µg P l^-1 day^-1 (average 2.75) in areas of diatom dominance,and between 0.68 and 1.68 µg P l^-1 day^-1 (average 1.16)in areas of intense warming and summer phytoplankton minimum.The residence time of PO₄-P standing stock in water at differentstations varied between 1.5 and 24 days (mean 9 days). The shareof bacterioplankton contributing to total PO₄-P uptake was 50%in areas of the summer phytoplankton minimum and 20–30%in areas of diatom dominance. The PO₄-P regeneration rate wasmeasured first time experimentally in the temperate sea. Itsrates varied from 0.30 to 1.65 µg P l^-1 day^-1. In areasof diatom dominance, it compensated with 30–60% of PO₄-Puptake. In zones of summer phytoplankton minimum and in thelayers of deep chlorophyll maxima at 10–25 m depths, thePO₄-P regeneration rate often exceeded its uptake. Primary phytoplanktonproduction correlated well with PO₄-P uptake values in the uppermixed layer, while no correlation was found between primaryproduction and the ambient PO₄-P content in water.     

WATER EXCHANGE, TEMPERATURE TOLERANCE, OXYGEN CONSUMPTION AND ACTIVITY OF THE NAMIB DESERT SNAIL, TRIGONEPHRUS SP.

Water exchange, temperature tolerance and oxygen consumptionof the snail, Trigonephrus sp., from the southern Namib desertof Namibia were examined and related to activity. At 25°Cand 15% R.H. mean water loss and food and water uptake were5.95 mg. day^–1 and 630 mg.day^–1, respectively. Bodytemperature tracked sand temperature. Snails tolerated sandtemperatures as high as 45°C. Mean ± S.D. oxygenconsumption rates were 32.0 ± 2.94 µlO₂.g totalbody mass^–1.h^–1 at 15°C, when the snails wereactive, and 11.27 µlO₂.g total body mass^–1.h^–1at 25°C, when the snails were inactive. These values are2-6 times lower than those recorded for the similarly sizedmesic snail, Helix aspersa. Activity experiments indicated thatlow ambient temperatures and high humidities were favoured bythe snails. This, together with the burying behaviour of thesesnails during high temperatures, suggests that they limit stressby restricting activity to physiologically-favourable periods,even though more-extreme conditions may be tolerated. (Received 7 June 1990; accepted 20 November 1990)