Photosynthetic Quotient Research Articles

Molecular and physiological responses of diatoms to variable levels of irradiance and nitrogen availability: Growth of Skeletonema costatum in simulated upwelling conditions

Molecular mechanisms that drive metabolic acclimation to environmental shifts have been poorly characterized in phytoplankton. In this laboratory study, the response of light- and N-limited Skeletonema costatum cells to an increase in light and NO, availability was examined. C assimilation was depressed relative to N assimilation early in enrichment, and the photosynthetic quotient (0, : CO,) increased, consistent with the shunting of reducing equivalents from CO, fixation to NO; reduction. The concomitant increase in dark respiration was consistent with the increased energetic demand associated with macromolecular synthesis. The accelerations of N-specific rates of NO,- uptake and nitrate rcductase activity (NRA) over the first 24 h were comparable to observations for coastal upwelling systems. Increases in cell-specific rates of these proccsscs, however, were confined to the first 8 h of enrichment. The abundance of 18s ribosomal ribonucleic acid (rRNA) increased immediately after the environmental shift, followed by increases in levels of NR-specific mRNA that coincided with the acceleration in NO,- assimilation. NRA, however, exhibited a diurnal rhythm that did not correspond to changes in NR protein abundance, suggesting that enzyme activity was also regulated by direct modulation of existing NR protein by light and NO, - availability.

Photosynthetic quotients in the Gulf of California during autumn 1987 in the central region and spring 1989 in Puerto Don Juan

In order to study the photosynthetic quotient variability (PQ = O2/14CO2) and to infer the relative contribution of new production in the Gulf of California, simultaneous determinations of photosynthesis were made using the 14C and oxygen techniques. Additionally, nutrient concentration was determined in the central region during autumn 1987 and during spring 1989 in a coastal lagoon in the same region. In both areas nutrient distribution suggests consumption by photosynthesis within the euphotic zone. The PQ integrated in the water column for single experiments goes from 1.43 ± 0.29 to 4.01 ± 0.33 in the central region and from 1.32 ± 0.22 to 3.08 ± 0.38 in the coastal lagoon Puerto Don Juan and does not differ from the PQ calculated for each depth. Calculated PQ values indicate the presence of NO3– during photosynthesis. The global PQ for all the experiments in both study areas was calculated by regression analysis for the central region (1.56 ± 0.45) and Puerto Don Juan (1.68 ± 0.56), suggesting NO3– influence during both seasons in both areas. A greater relative contribution of new production than regenerated production in both areas of the Gulf of California is inferred.

Utilization of Inorganic Carbon by Ulva lactuca

Thalli discs of the marine macroalga Ulva lactuca were given inorganic carbon in the form of HCO(3) (-), and the progression of photosynthetic O(2) evolution was followed and compared with predicted O(2) evolution as based on calculated external formation of CO(2) (extracellular carbonic anhydrase was not present in this species) and its carboxylation (according to the K(m)(CO(2)) of ribulose-1,5-bisphosphate carboxylase/oxygenase), at two different pHs, assuming a photosynthetic quotient of 1. The K(m)(inorganic carbon) was some 2.5 times lower at pH 5.6 than at the natural seawater pH of 8.2, whereas V(max) was similar under the two conditions, indicating that the unnaturally low pH per se had no adverse effect on U. lactuca's photosynthetic performance. These results, therefore, could be evaluated with regard to differential CO(2) and HCO(3) (-) utilization. The photosynthetic performance observed at the lower pH largely followed that predicted, with a slight discrepancy probably reflecting a minor diffusion barrier to CO(2) uptake. At pH 8.2, however, dehydration rates were too slow to supply CO(2) for the measured photosynthetic response. Given the absence of external carbonic anhydrase activity, this finding supports the view that HCO(3) (-) transport provides higher than external concentrations of CO(2) at the ribulose-1,5-bisphosphate carboxylase/oxygenase site. Uptake of HCO(3) (-) by U. lactuca was further indicated by the effects of potential inhibitors at pH 8.2. The alleged band 3 membrane anion exchange protein inhibitor 4,4'-diisothiocyanostilbene-2,2'disulphonate reduced photosynthetic rates only when HCO(3) (-) (but not CO(2)) could be the extracellular inorganic carbon form taken up. A similar, but less drastic, HCO(3) (-)-competitive inhibition of photosynthesis was obtained with Kl and KNO(3). It is suggested that, under ambient conditions, HCO(3) (-) is transported into cells at defined sites either via facilitated diffusion or active uptake, and that such transport is the basis for elevated internal [CO(2)] at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation.

Photosynthesis, dark respiration and light independent carbon fixation of endemic Antarctic macroalgae

The light saturated photosynthesis, dark respiration and light independent carbon fixation of macroalgal species endemic to the Antarctic were measured. Five brown algae. Ascoseira mirabilis, Desmarestia anceps, D. antarctica, Phaeurus antarcticus, Himantothallus grandifolius and the red alga Palmaria decipiens were included. Rates of these three parameters at 0°C were very similar to those measured in other studies on temperate algae at higher temperature. This indicates a high degree of physiological adaptation to the Antarctic environment within these species. A comparison was made of polarographic and chemical means of measuring oxygen flux during photosynthesis and dark respiration at low temperature. There was a good correlation between measurements of oxygen evolution and carbon fixation, although apparent photosynthetic quotient values were in most cases high.

Effect of light on oxygen and carbon dioxide fluxes and on metabolic quotients measured in situ in a zooxanthellate coral

Oxygen and CO2 fluxes were measured in situ as a function of depth on colonies of two species of the zooxanthellate coral Stylophora and were used to compute metabolic quotients. We report for the first time on the photosynthetic quotient of zooxanthellae in hospite (PQz); it ranges from 1.14 to 1.57 and depends on the light regime (daily photosynthetic photon flux density). The comparative effect of light on PQz and on the respiratory quotient of the coral suggests that glycerol is the main substance photosynthesized, translocated, and consumed in shallow‐water colonies, whereas it is replaced by lipid with decreasing light (increasing depth). Such a result is potentially important for the energy budget considerations of any plant‐animal endosymbiosis and provides some evidence that light controls the quality of products photosynthesized by the plant and catabolized by the host.

A novel in situ respirometer for investigating photosynthesis and calcification in crustose coralline algae

This communication describes a novel respirometer for measuring the metabolism in situ of encrusting coralline algae and it provides an example of the data which can be gathered. The respirometer includes a pair of UV-transparent electrode chambers: one accommodates the specimen, the other contains a light sensor. Oxygen, pH and temperature probes monitor changes in the seawater surrounding the specimen while light is recorded concurrently in the second chamber mounted adjacent. A centrifugal pump flushes the sample chamber at regular intervals during deployment. Analog signals from the probes are transmitted via underwater cables to a data-logging device contained within a submersible pressure-resistant housing. The signals are digitized, stored in RAM and later off-loaded to a shore-based microcomputer. Oxygen and pH data are converted into rates of primary production and calcification using equations associated with the alkalinity anomaly technique and experimentally determined values for the photosynthetic and respiratory quotients. On a clear day in May 1986, on the windward reef crest at Lizard Island, Great Barrier Reef, Australia, rates of primary production and calcification were determined for a specimen of Porolithon onkodes (Heydrich) Foslie. The maximum rate of net photosynthesis was 18.9 mmol O 2 · m −2 crust surface · h −1. The average rate of nocturnal respiration was 4.4 mmol O 2· m −2· h −1. The ratio of maximum gross photosynthesis to respiration was 5.25. Using a photosynthetic quotient of 1.21, the data convert to a production rate of 1.5 g C · m −2 · day −1. The maximum rate of calcification during the day was 8.2 mmol CaCO 3· m −2· h −1. At night the specimen dissolved at an average rate of 0.5 mmol CaCO 3· m −2· h −1. Net calcification was estimated to be 8.5 g CaCO 3· m −2· day −1.

Monsoonal differences in primary production in the eastern Banda Sea (Indonesia)

14C incorporation into phytoplankton of the eastern Banda Sea was registered during incubations of samples, both in situ and in an incubator equipped with blue filters. Light conditions throughout the euphotic zone were apparently simulated quite well in the deck incubator: the agreement between the two series of measurements was very good. Primary production was 1.85 g C·m −2..d t-1 in August 1984, after upwelling forced by the southeast monsoon; values ranged from 0.5 g C·m −2..d t-1 in the open Banda Sea up to 7 g C·m −2..d t-1 south of Irian Jaya. In February 1985, when the upper part of the water column was stratified due to persistent monsoonal winds from the northwest, primary production was less: 0.91 g C·m −2..d t-1. In August 1984 production was also measured with the oxygen method. The photosynthetic quotient was 1.05 near the surface and up to 1.86 near the bottom of the euphotic zone. Extrapolation of all measurements to an estimate of annual primary production resulted in a value as high as 5·10 2 g C·m −2..d t-1. The assimilation ratio in surface samples was locally up to 8 mg C·(mg chl. a) −1.h −1 (in the photoperiod) during both cruises, with a mean of 5.85 in August 1984 and 5.95 in February 1985. The surprisingly high values during the oligotrophic conditions of February underline the importance of rapid recycling of nutrients in the euphotic zone.

CO2 and O2 Exchanges in the CAM Plant Ananas comosus (L.) Merr

Photosynthesis and light O(2)-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO(2) and O(2) gas exchange measurements. The amount of CO(2) which was fixed during a complete day-night cycle was equal to the amount of total net O(2) evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O(2)-evolution and of net light atmospheric CO(2)-uptake give the rates of malate-decarboxylation-dependent CO(2) assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O(2) evolved/net CO(2) fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO(2) for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO(2) assimilation deduced from net O(2)-evolution was 3 times higher than the maximum rate of atmospheric CO(2)-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO(2)-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO(2)-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO(2) fixed during the last hour of the light period was stored as malate. Light O(2)-uptake determined with (18)O(2) showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O(2)-uptake increased progressively; during the following part of the light period, the rate of O(2)-uptake was 3.5 times higher than the maximum rate of CO(2)-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO(2)-free atmosphere or in continuous illumination, the rate of O(2)-uptake was higher than in the control. This supports the hypothesis that the low rate of O(2)-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO(2) concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O(2) and CO(2) gas exchange suggest that at the end of the light period the intracellular CO(2) concentration was very low. We propose that the high ratio of O(2)-uptake/CO(2)-fixation is principally caused by the stimulation of photorespiration during this period.

The influence of macrofauna on estuarine benthic community metabolism: a microcosm study

Effects of benthic macrofauna (Corophium volutator, Hydrobia sp., Nereis virens) on benthic community metabolism were studied over a 65-d period in microcosms kept in either light/dark cycle (L/D-system) or in continuous darkness (D-system). Sediment and animals were collected in January 1986 in the shallow mesohaline estuary, Norsminde Fjord, Denmark. The primary production in the L/D-system after 10 d acted as a stabilizing agent on the O2 and CO2 flux rates, whereas the D-system showed decreasing O2 and CO2 flux throughout the period. Mean O2 uptake over the experimental period ranged from 0.38 to 1.24 mmol m−2 h−1 and CO2 release varied from 0.80 to 1.63 mmol m−2 h−1 in both systems. The presence of macrofauna stimulated community respiration rates measured in darknes, 1.4 to 3.0 and 0.9 to 2.0 times for O2 and CO2, respectively. In contrast, macrofauna lowered primary production. Gross primary production varied from 1.06 to 2.26 mmol O2 m−2 h−1 and from 1.26 to 2.62 mmol CO2 m−2 h−1. The community respiratory quotient (CRQ, CO2/O2) was generally higher in the begining of the experiment (0–20 d, mean 1.89) than in the period from Days 20 to 65 (mean 1.38). The L/D-system exhibited lower CRQ (ca. 1) than the D-system. The community photosynthetic quotient varied for both net and gross primary production from 0.64 to 1.03, mean 0.81. The heterotrophic D-system revealed a sharp decrease in the sediment content of chlorophyll a as compared to the initial content. In the autotrophic L/D-system, a significant increase in chlorophyll a concentration was observed in cores lacking animals and cores with C. volutator (The latter species died during the experiment). Due to grazing and other macrofauna activities other cores of the L/D-system exhibited no significant change in chlorophyll a concentration. Community primary production was linearly correlated to the chlorophyll a content in the 0 to 0.5 cm layer. Fluxes of DIN (NH4++NO2−+NO3−) did not reveal significant temporal changes during the experiment. Highest rates were found for the cores containing animals, mainly because of an increased NH4+ flux. The release of DIN decreased significantly due to uptake by benthic microalgae in the L/D-system. No effects of the added macrofauna were found on particulate organic carbon (POC), particulate organic nitrogen (PON), total carbon dioxide (TCO2) and NH4+ in the sediment. The ratio between POC and PON was nearly constant (9.69) in all sediment dephts. The relationship between TCO2 and NH4+ was more complex, with ratios below 2 cm depth similar to those for POC/PON, but with low ratios (3.46) at the sediment surface.

Changes in pH as a measure of photosynthesis by marine macroalgae

An automatically operated method for high precision measurements of steady-state photosynthesis by macroalgae was developed. Changes in pH and oxygen content of seawater passing the algae in a flowthrough system, could be measured with extremely high accuracy over very long periods of time. The method is especially suitable for measurements on flowthrough systems with high rates of water exchanges (i.e. short retention time), and can be used to study exchange processes for marine plants, animals and small ecosystems. Since the same measuring unit is used for several flowthrough chambers, the method is very suitable for comparisons between different species, or between differently pretreated specimens of the same species (e.g. in toxicological studies). The method was used to study the ratio: [oxygen production] to [CO2+H+ uptake] at different light intensities for several macroalgae belonging to different systematic groups and from different habitats. At lower photosynthetic rates this ratio was similar for all of the algae studied (1.17±0.02). For brown algae of the fucacean family, the ratio increased by 0.08 units at higher photosynthetic rates. This increase was thought to be related to the crassulacean acid metabolism (CAM)-like strategies connected to these algae. For all other algae studied, the ratio remained constant or decreased slightly (at most by 0.04 units) at higher photosynthetic rates. The relations between the abovementioned ratio and the photosynthetic quotient are discussed on a theoretical basis.

Computation of metabolic quotients in plant-animal symbiotic units

Equations of several metabolic quotients of symbiotic units are derived taking a zooxanthellate scleractinian coral as a case study but are useful in any plant-animal symbiotic association. These quotients are: the photosynthetic quotient of the coral (PQ net ) and zooxanthellae (PQ z ), and the respiratory quotient of the coral (RQ c ).

Phytoplankton biomass and production in the river meuse (Belgium)

The biomass and production of the phytoplankton in a relatively unpolluted reach of the River Meuse (Belgium) were followed through two years (1983 and 1984). Chlorophyll a varied from 0.2 to about 120 mg m −3, and production ranged between 0.05 and 5.78 gC m −2 d −1. The mean photosynthetic quotient (PQ) was 1.25. The parameters of the light-photosynthesis relationship ( P opt and l k ) were calculated and related to the variations of temperature and light in the water column. A simple model allowed calculations of the annual production, which was estimated to be 494 gC m −2 yr −1 in 1983 and 547 gC m −2 yr −1 in 1984. Finally, a simple model is developed, which explains the relationship between phytoplankton development and discharge; this model shows how the effect of discharge can be described by a “dilution rate” of the plankton growing in the river water.

Carbon fixation and oxygen evolution by phytoplankton in the Canadian high arctic

In the Canadian high arctic, in summer, in situ profiles of oxygen evolution and carbon fixation by phytoplankton showed excellent qualitative and quantitative agreement with an apparent photosynthetic quotient in the range from 1.3 to 1.8. Water column photosynthesis was linear in the available light at the surface. Daily rates photosynthesis exceeded 1 mg C m-2 d-1, implying a minimal estimate for annual production in the range from 35 to 70 g C m-2 y-1.

The C4-like characteristics of the intertidal macroalga Ascophyllum nodosum (L.) Le Jolis (Fucales, Phaeophyta)

Numerous methods have been employed to ascertain whether Ascophyllum nodosum exhibits the characteristics of photorespiration. Different oxygen concentrations appear to have little effect on the apparent rate of photosynthesis, the alga displays low CO2 compensation points in air and water, postillumination CO2 bursts (and O2 ‘gulps’) were not observed in either environment, and 14CO2 evolution into a CO2-free atmosphere was inhibited in the light compared with that observed in the dark. Suggestions that photosynthetic quotients can be used as an indicator of the photorespiratory state were found to be unsuitable as determinations of the total inorganic carbon concentration from pH measurements were found to be inconsistent which resulted in variable apparent photosynthetic quotient values. Thus photorespiration is suppressed in Ascophyllum nodosum and it is suggested that the mechanism is based on an inorganic carbon transport pump.

THE THERMODYNAMIC EFFICIENCY (QUANTUM DEMAND) AND DYNAMICS OF PHOTOSYNTHETIC GROWTH.

The commonly quoted values of maximum photosynthetic efficiency have been those obtained by determining the oxygen yield from suspensions of resting algal cells in which growth was disregarded. The unpredictability of the metabolism of resting cells severely vitiates the reliability of measurements made on their energy metabolism. Also the validity of the measurements with resting cells is made doubtful by anomalous values for the photosynthetic quotient (-δCO2 /δO2 ). The measurements on resting cells fall into two categories: one in which the cells were suspended in acid media (pH 5) with a CO2 partial pressure (pCO2 ) of 5% atmospheric, and one in which the cells were suspended in alkaline media (about pH 9) with a pco2 of 0.25% atmospheric. In acid media with 5% CO2 , the most probable value of the minimum quantum demand is 5 to 6 hv/O2 . With pH 9 media, equilibrated with 0.25% CO2 , the minimum quantum demand found is about 10 hv/O2 . This low efficiency seems to be caused by a sub-optimal CO2 partial presure, since it has been observed that the value at alkaline pH agrees with that at acid pH provided the pCO2 is maintained at 2 % atmospheric. This pCQ2 effect has been neglected by many workers. To avoid the controversial methods using resting cells, it is essential to determine the photosynthetic efficiency of cells in a steady state of growth. The environmental conditions during growth of the cells have a strong influence on the efficiency of photosynthesis; for instance, the efficiency appears to be strongly dependent on the temperature during growth. Under light-limited conditions when the photosynthetic efficiency of growth is optimized the minimum quantum demand of algal cells is found to be 5 to 6 hv/O2 . The minimum quantum demand of CO2 fixation varies from 1.1 to 1.4 times the value for O2 depending on the nature of the nitrogen source for growth. Significant doubt must be attached to measurements of the maximum photosynthetic efficiency with isolated chloroplasts, on the grounds that in vitro conditions may impair their efficiency and that the efficiency may be affected by the growth conditions of the parent plant. Thus, a unified view of the experimental data indicates that the most probable value of the minimum quantum demand is 5 to 6 hv/O2 . The preference for the apparently sub-optimal value of about 10 hv/O2 found with alkaline media and a pco2 of 0.25 %, which is the prevailing view, is necessitated by the requirement of the Z-scheme paradigm of the mechanism of the electron transfer. Thus it appears that hypothesis rather than a unified view of the experimental data on the efficiency is dictating the view of the mechanism involved. A cell of Chlorella (strain 211/8k) fully charged with reducing equivalents and energy can continue to assimilate CO2 and grow at the maximum rate (doubling time 3 h) for 9 s. It is calculated that exposure to 20 W m-2 (daylight PAR) for 0.5 s is sufficient fully to charge such a cell with energy and reducing equivalents. This calculation predicts that, in the steady state of growth, cycles of exposure of each cell to 20 W m-2 for 0.5 s followed by 9 s in the dark will support growth at the maximum rate. The theoretical expressions used to express the maximum thermodynamic efficiency of conversion of radiation to chemical work (ηT ) are shown to be inconsistent. The correct value is taken to be given by Spanner's equation ηT = 1 -(T/Tr ), where T is the ambient temperature and Tr is the radiation temperature. Hence, the maximum value of ηT for conversion of the PAR in sunlight to chemical work varies from 0.93 for unscattered sunlight to 0.70 if it is isotropically scattered. It is deduced that under the usual ambient conditions the value of ηT for photosynthesis will decrease by 0.043 for each log decrease in the irradiance. Contents Summary 3 I. Introduction 4 II. The stoichiometry of photosynthesis 6 III. Thermodynamic limits to photosynthetic efficiency 7 IV. The theoretical quantum demands for production of NADPH and ATP 8 V. The dynamics and energetics of photosynthetic growth 10 VI. The physiology of cells at or near zero growth rate 12 VII. Manometric measurements of quantum demands of resting cells 13 VIII. Non-manometric measurements of quantum demands of resting cells 16 IX. Quantum demands of vascular plants and isolated chloroplasts 18 X. The quantum demands of growing cells 19 XI. The influence of wavelength of radiation on photosynthetic efficiency 22 XII. Conclusion 23 XIII. Appendices 24 References 34.

Comparison of the oxygen and 14C methods to measure phytoplankton production rates: Evaluation of the photosynthetic quotient

(1985). Comparison of the oxygen and 14C methods to measure phytoplankton production rates: Evaluation of the photosynthetic quotient. SIL Proceedings, 1922-2010: Vol. 22, No. 4, pp. 2208-2213.

Assessing Phytoplankton and Bacterioplankton Production During Early Spring in Lake Erken, Sweden

The spring development of both phytoplankton and bacterioplankton was investigated between 18 April and 7 May 1983 in mesotrophic Lake Erken, Sweden. By using the lake as a batch culture, our aim was to estimate, via different methods, the production of phytoplankton and bacterioplankton in the lake and to compare these production estimates with the actual increase in phytoplankton and bacterioplankton biomass. The average water temperature was 3.5 degrees C. Of the phytoplankton biomass, >90% was the diatom Stephanodiscus hantzchii var. pusillus, by the peak of the bloom. The C and O(2) methods of estimating primary production gave equivalent results (r = 0.999) with a photosynthetic quotient of 1.63. The theoretical photosynthetic quotient predicted from the C/NO(3) N assimilation ratio was 1.57. The total integrated incorporation of [C]bicarbonate into particulate material (>1 mum) was similar to the increase in phytoplankton carbon determined from cell counts. Bacterioplankton increased from 0.5 x 10 to 1.52 x 10 cells liter ( approximately 0.5 mug of C liter day). Estimates of bacterioplankton production from rates of [H]thymidine incorporation were ca. 1.2 to 1.7 mug of C liter day. Bacterial respiration, measured by a high-precision Winkler technique, was estimated as 4.8 mug of C liter day, indicating a bacterial growth yield of 25%. The bulk of the bacterioplankton production was accounted for by algal extracellular products. Gross bacterioplankton production (production plus respiration) was 20% of gross primary production, per square meter of surface area. We found no indication that bacterioplankton production was underestimated by the [H]thymidine incorporation method.

Light and the annual variation of oxygen‐ and carbon‐based measurements of productivity in a macrophyte‐dominated river1

Daily integrals of primary productivity were measured for 369 days in the macrophyte‐dominated Gryde River (NW Jutland, Denmark) between July 1979 and September 1980. All seasons were distinct, with high primary productivity during summer, a gradual transition in autumn, low primary productivity in winter, and a rapid transition to summer conditions during April. Those seasons corresponded with the condition of the macrophytes, with high biomass and surface area during summer, a gradual decline in autumn, senescence during winter, and development of new shoots during April and May. The remarkably constant relationship between daily integrals of primary productivity and light (in comparison with that in rivers without macrophytes) could be described during both summer and winter by a hyperbolic equation, with more shade adaptation but less maximum photosynthesis during winter than during summer. The hyperbolic relationship held for both oxygen‐ and carbon‐based measurements. The mean value for the photosynthetic quotient (P.Q.) was close to one, suggesting little transport of oxygen and inorganic carbon to or from roots. We could not discern effects on primary productivity of other variables such as temperature, which seemed unimportant in comparison to light.

The Effect of Nitrogen Supply on the Photosynthetic Quotient of Natural Phytoplankton Assemblages

The Effect of Nitrogen Supply on the Photosynthetic Quotient of Natural Phytoplankton Assemblages

The ecology of Lake Nakuru (Kenya) : III. Abiotic factors and primary production.

Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000-160,000 μS). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450-550 kerg·cm-2·s-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 μg P-PO4·l-1). Oxygen concentrations near sediments were <1 gO2·m-3 for at least 6 h·d-1 in 1972/73, resulting in a release of ∼45 mg P-PO4·m-2·d-1. Attenuation coefficients vary from 3.6-16.5 according to algal densities and mean depth from 0-400 cm. Algal biomass was ∼200 g·m-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2-35 gO2·m-3 do not influence primary production rates. Net photosynthetic rates (mgO2·m-3·h-1; photosynthetic quotient=1.18) reached 12-17.7 in 1972/73 and 2-3 in 1974, but vertically integrated rates were only 1-1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO2·m-3·24 h-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4-2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1-3 gO2·m-2·h-1). The relation of O2 produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.