The influence of temperature and light on the upper limit of Microcystis aeruginosa production in a hypertrophic reservoir

Primary production was measured for 7 years, using the in situ¹⁴C-method in hypertrophic Hartbeespoort Dam, South Africa,to examine the influence of light and water temperature on theupper limit of Microcystis aeruginosa production. Water temperaturesvaried from 11 to >25°C and chlorophyll concentrationsreached 6500 mg m^–3. The maximum volumetric rate of production(A_max) was 12->8800 mg C m^–3 h^–1 with areal productions(A) of 69->3300 mg C m^–2 h^–1 for euphotic zonedepths of <0.5–8.4 m. The intrinsic parameters of phytoplanktonproduction (, A_max/B, I_k) indicated that the phytoplankton populationwas adapted to high light levels. Both A_max/B and I_k were correlatedwith temperature. Under optimal conditions, , the theoreticalupper limit of A, was calculated to be 2.8 g Cm^–2 h^–1,while the measured rate was 2.5 g Cm^–2 h^–1. Measuredareal rates exceeding were overestimated due to methodologicalproblems when working with Microcystis scums. Light and watertemperature interacted to yield high production rates: watertemperature through its direct effect on photosynthetic ratesand indirectly in the formation of diurnal mixed layers; lightindirectly through water temperature and directly through itsattenuation and induction of light-adapted physiology in Microcystis.     

Photosynthetic parameters, pigment composition and respiration rates of the marine diatom Skeletonema costatum grown in continuous light and a 12:12 h light-dark cycle

Nutrient-sufficient cultures of a Trondheimsfjord (Norway) cloneof the marine centric diatom Skeleionema costatum (Grev.) Clevewere grown at 75 µmol m^–2 s^–1 and 15C at24 and 12 h daylength to study diurnal variations and the effectof daylength on pigment and chemical composition, photosyntheticparameters, dark respiration rates and scaled fluorescence excitationspectra (F), the latter used as estimates for the absorptionof energy available to Photosystem II. Specific growth rateswere 1.06 and 0.56 day^– in 24 and 12 h daylength, respectively,while dark respiration rates were generally 85% of the net growthrate. The Chla-normalized photosynthetic coefficients P^B_m anda^B were {small tilde}20–25% higher in continuous lightthan at 12 h daylength, while the Chla:C ratio was {small tilde}15%lower (0.051 versus 0.061 w:w). Thus, the carbon-normalizedcoefficients P^c_m and a^c were <11% lower at 24 h than at 12h daylength. The maximum quantum yield _max, the Chla:C ratioand F differed negligibly, as did the light saturation indexl_k, the N:C ratio and the ratios Chlc:Chla and Fucoxanthin:Chla. P^B_m and l_k did not exhibit diurnal variations at 24 hdaylength, and varied within 23% of the daily mean at 12 h daylength.Predictions of the daily gross photosynthetic rate based ondata for a given time of the day should thus not be >10%in error relative to an integrated value based on several datasets collected through 24 h. _max was 0.084–0.117 mol O₂(mol photons)^– for gross oxygen evolution. However, ifused in mathematical models for predicting the gross and netgrowth rates (i.e. the gross and net carbon turnover rates),‘practical’ values of 0.076 and 0.040 g-at C (molphotons)^–, respectively, should be employed. Correspondingly,values for a^B and P^B_m should be adjusted pro rata. ¹Present address: College of Marine Studies, Sjmannsveien 27,N-6008 lesund, Norway     

Occurrence of mycosporine-like amino acids in the red-tide dinoflagellate Alexandrium excavatum: UV-photoprotective compounds?

Water extracts of the red-tide dinoflagellate Alexandrium excavatumgrown at ‘high’ light intensity (200 µE m^–2s^–1) show a broad absorbance maximum in the UV regionof the spectrum (310–360 nm). Using TLC and reverse-phaseHPLC a series of mycosporine-like amino acids have been characterized:mycosporine-glycine (_max = 310 nm), palythine (_max = 320 nm),asterina-330 (_max = 330 nm), shinorine (_max = 334 nm), porphyra-334(_max= 334 nm), palythenic acid (_max = 337 nm) and the isomericmixture of usujirene and palythene (_max = 359 nm). From theobserved spectral changes during transference from ‘low’(20 µE m^–2 s^–1) to ‘high’ (200µE m^–2 s^–1) light intensities and vice versa,the series of compounds are supposed to be biogenically relatedto one another. The presence of these compounds in A.excavatumis discussed in relation to their possible role in the photoprotectionto deleterious UV radiation.     

Factors controlling primary production in a hypertrophic lake (Hartbeespoort Dam, South Africa)

The physical factors controlling algal primary production weredemonstrated from data collected for a hypertrophic lake. A_maxranged between 12.4 and 5916 mg C m^–3 h^–1. Arealrates (A) varied between 46.9 and 3381 mg C m^–2 h^–1.The factors permitting and controlling production were subjectivelyseparated into two categories. In category 1, nutrients (N +P), which were in overabundance, permitted large standing cropsof Microcystis aeruginosa to develop (>1000 µg chla 1^–1). Wind patterns determined the dramatic spatialand temporal changes in algal standing crop which could dropto 2.7 µg chl a 1^–1. In category 2 were the factorswhich affected the rate processes. The buoyancy mechanism ofMicrocystis usually kept the alga in the euphotic zone. A powerrelationship (r = 0.92, n = 54) between A and A_max/_min showedthat with increasing phytoplankton vertical stratification,A_max was increasingly important in the integral. The saturationparameter I_K and photosynthetic capacity were temperature dependent.Variations of A were significantly related to changes in watercolumn stability (g cm cm^–2) because both axes of thephotosynthesis depth-profile were affected by stability changes.     

Planktonic primary production in the German Wadden Sea

By combining weekly data of irradiance, attenuation and chlorophylla concentrations with photosynthesis (P) versus light intensity(E) curve characteristics, the annual cycle of planktonic primaryproduction in the estuarine part of the Northfrisian WaddenSea was computed for a 2 year period. Daily water column particulategross production ranged from 5 to 2200 mg C m^–2 day^–1and showed a seasonal pattern similar to chlorophyll a. Budgetcalculation yielded annual gross particulate primary productionsof 124 and 176 g C m^–2 year^–1 in 1995 and 1996,respectively. Annual amounts of phytoplankton respiration, calculatedaccording to a two-compartment model of Langdon [in Li,W.K.W.and Maestrini,S.Y. (eds), Measurement of Primary Productionfrom the Molecular to the Global Scale. International Councilfor the Exploration of the Sea, Copenhagen, 1993, pp. 20–36],and dissolved production in 1996, were both in the range of24–39 g C m^–2 year^–1. Annual total net productionwas thus very similar to particulate gross production (127 and177 g C m^–2 year^–1 in 1995 and 1996, respectively).Phytoplankton growth was low or even negative in winter. Inspring and summer, production/biomass (Pr/B) ratios varied from0.2 up to 1.7. Phytoplankton growth during the growth seasonalways surpassed average flushing time in the area, thus underliningthe potential of local phytoplankton bloom development in thispart of the Wadden Sea. The chlorophyll-specific maximum photosyntheticrate (P^B_max) ranged from 0.8 to 9.9 mg C mg^–1 Chl h^–1and was strongly correlated with water temperature (r² = 0.67).By contrast, there was no clear seasonal cycle in ^B, which rangedfrom 0.007 to 0.039 mg C mg^–1 Chl h^–1 (µmolphotons m^–2 s^–1)^–1. Its variability was muchless than P^B_max and independent of temperature. The magnitudeand part of the variability of P^B_max and ^B are presumably causedby changes in species composition, as evidenced from the rangeof these parameters found among 10 predominant diatom speciesisolated from the Wadden Sea. The ratio of average light conditionsin the water column (E_av) to the light saturation parameterE_k indicates that primary production in the Wadden Sea regionunder study is predominantly controlled by light limitationand that nutrient limitation was likely to occur for a few hoursper day only during 5 (dissolved inorganic nitrogen) to 10 (PO₄,Si) weeks in the 2 year period investigated.     

Effects of algal concentration, bacterial size and water chemistry on the ingestion of natural bacteria by cladocerans

Journal of Plankton Research, 11, 1273–1295, 1989. The values of P/U⁰ (Table I) and fluid velocity used to calculatethe energy required for sieving (pp. 1289–1290) and severalequations (footnote b of Table I; p. 1290, lines 3–4)are incorrect. The corrected table appears below: Table I. Filter setule measurements (mean and within specimenstandard deviation) of the gnathobases for the cladocerans studied^aGnathobaseof trunklimb number. ^bP = 8µU₀/(b(1 – 21nt + 1/6(t²) - 1/144(t⁴))), whereP = pressure drop in dyn cm^–2, =3.1416, U₀ = fluid velocityin cm s^–1, b = distance between setule centres in cm,t = ( x setule diameter)/b and µ = 0.0101 dyn s^–1cm^–2. Formula from Jørgensen (1983). The text (p. 1289, line 19 to p. 1290, line 10) should read: organism. Using a similar argument, a 0.5 mm Ceriodaphnia witha filter area of 0.025 mm² (Ganf and Shiel, 1985) and pressuredrop P = 2757 dyn cm^–2 (with fluid velocity of 0.07 cms^–1) allocates only 2171 ergs h^–1 to filtrationof a total energy expenditure of 10⁴ ergs h^–1 [filtrationenergy (ergs h^–1) = area (cm²) x pressure drop (dyn cm^–2)x 3600 (s h^–1) x 1/0.2 (efficiency of conversion of biochemicalinto mechanical work); total energy (ergs h^–1) = respiration(0.05 µl O₂ ind^–1 h^–1 consumed; Gophen, 1976)x conversion factor (2 x 10⁵ ergs µl^–1 O₂). Withan estimated 0.034 mm² in filter area, fluid velocity of 0.041cm s^–1 and respiration of 1.8 x 10⁴ ergs h^–1 (calculatedfrom Porter and McDonough, 1984), a 0.5 mm Bosmina uses <4%of its metabolism to overcome filter resistance. The velocities used in the original examples (0.4 cm s^–1for Ceriodaphnia, 0.2 cm s^–1 for Bosmina) were derivedfrom literature values of appendage beat rate and estimatesof the distance travelled by the appendages during each beatcycle. This approach unnecessarily assumes that all water movedpasses through the filter. In the new calculations, the flowacross the filter needed for food to be collected by sieving(0.07 cm s^–1 for Ceriodaphnia and 0.041 cm s^–1 forBosmina) was determined from the maximum clearance rate/filterarea. The amended energy expenditures, although higher, do notrefute the sieve model of particle collection.     

Resting egg production and oviducal cycling in two sympatric species of alpine diaptomids (Copepoda: Calanoida) in relation to temperature and food availability

Resting egg production and oviducal cycling were investigatedfor the calanoid copepods Arctodiaptomus alpinus and Acanthodiaptomusdenticornis both in the laboratory and in a small karstic alpinelake by making a census of the number of eggs produced and theproportion of females in each of four morphologically distinguishedreproductive conditions each day in the laboratory or duringa 2–3 week period in lake enclosures. In the laboratory,individuals were maintained on a mixed diet of natural phytoplanktonat constant temperatures of 4, 10, 15 and 20C, respectively.Both species differed considerably in their temperature requirementsfor reproduction. Lifetime fecundity was highest at 10C in A.alpinusand at 20C in A.denticornis, with up to 327 eggs ^–1 spawnedin the former and up to 582 eggs ^–1 in the latter species.Unfavorable temperatures were further reflected in an increasein egg mortality and the allocation of time spent in a post-reproductivephase, as well as in a decrease of longevity. Increasing temperaturesenhanced egg production rates due to decreasing clutch productionperiods, although clutch size was negatively correlated withtemperature. Maximum rates reached 5.88 and 7.98 eggs ^–1day^–1 in the laboratory, and 0.73 and 0.55 eggs ^–1day^–1 in enclosures in A.alpinus and A.denticornis, respectively.Egg production rates and clutch size were clearly governed bynutritional conditions in the lake, but were less affected byfood supply in the laboratory. Here, rates of egg productionwere adapted to improving food supply by increasing the frequencyof spawning events, rather than the number of eggs per clutch.No correlation was found between female body size and reproductiveparameters in the laboratory. A very low proportion of totalclutch production resulted in clutches composed of subitaneouseggs, i.e. 0.14% in A.denticornis and 1.20% in A.alpinus. Oviducalphase duration allocations indicate that there exists a temperatureoptimum for gamete maturation.     

A comparison of nitrogen and carbon metabolism in the shallow and deep-water phytoplankton populations of a subalpine lake: response to photosynthetic photon flux density

Rates of , and CO₂ assimilationby the organisms in the shallow and deepchlorophyll layers ofCastle Lake were measured over a gradient of photosyntheticphoton flux densities (PPFD) during the 1979–1980 ice-freeseasons. The results of these experiments could be fitted witha hyperbolic function in the manner of the Michaelis-Mentenequation (excluding rates of dark assimilation) up to –40%of the surface PPFD after which photon inhibition occurred.The half saturation constants relative to incidence PPFD (K_LT)for assimilation ( = 1.1 E m^–2d^–2) were about twice those for ( = 0.5 E m^–2d^–1).All of the K_LT values correspond to depths in thelakerangingfrom 17–29 m(–1% of surface PPFD). Dark assimilationof both and was –50% of the assimilation at saturating PPFD implying that part ofthe immediate energy required for inorganic nitrogen assimilationmay come from intermediary metabolism. This contention was supportedfor assimilation by the results of experiments performed with specific inhibitors of non-cyclic photophosphorylationand oxidative phosphorylation. The K_LT values for the assimilationof CO₂ were from 2–10 times higher than those for inorganicnitrogen. These values for CO₂ assimilation were not significantlyaltered by the addition of either or during 12 h incubations.     

The maximum quantum yield of phylopiankton photosynthesis in situ

Light-limited photosynthetic carbon incorportion is expectedto be directly proportional to the scalar quantum irradiance.The proportionality constant is , where _mis the maximum quantum yield (mol C Einstein^–1 absorbed)and $$\stackrel{\&macr;}{{\hbox{ k }}_{\hbox{ c }}}$$ isthe mean spectral absorption coefficient (m² mg^–1 chla). Recent efforts to evaluate of in situphytoplankton photosynthesis are variously flawed. Lack of evidenceof proportionality and lack of correction of cosine to scalarirradiance are common deficiencies. Most data, as we interpretthem, indicate values in the range 0.0003 – 0.0006 mol C m² Einstein1 abs mg1 chl a. New determinationsin lrondequoit Bay, New York, lie in this range. Most estimatesof at depth have been about 0.010 m² mg^–1chl a. Similar values are being obtained for total particulatesfrom lrondequoit Bay; whether detritus contributes significantlyis not yet known. Published data, in our view, all point tovalues of m in situ in the range 0.03–0.07 mol C Einstein^–1abs. Published values >0.10 are almost certainly due to imprecisionor systematic error. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Temporal and spatial variations in phytoplankton carbon isotopes in a polymictic subtropical lake

Stable carbon isotopes (¹³C) were determined for phytoplanktonand dissolved inorganic carbon (DIC) from Lake Apopka, a shallow,polymictic and hypereutrophic lake in Florida, USA. Bulk planktondominated by pico- and nanqanobacteria were enriched in ¹³(–13.1± 1.1%) as a result of assimilation of extremely ¹³C-richDIC (¹³C = 9.6 ± 3.0%). Diatoms (Aulacoseira spp.) hada ¹³C of –14.3 ± 0.6% that was slightly more negativethan that of small cyanobacteria. Meroplanktonic diatoms hada ¹³C (–13.6 ± 1.8%), similar to their planktoniccounterparts. The ¹³C of a colonial cyanobacterium (Microcystisincerta) was exceptionally heavy (–3.0 ± 1.0%)and attributed to localized carbon limitation. Seasonal variationin ¹³C of bulk plankton was small (4%) relative to reports forother lacustrine systems No difference in the ¹³C of bulk planktonhorn surface water between stratified and non-stratified periodswas found. No measurable changes in ¹³C of bulk plankton wereindicated in light and dark incubation experiments Frequentwind mixing of the water column, high DIC concentration, andconsistently high lake productivity were used to explain thetemporal and spatial isotope consistency of phytoplankton inthis lake.     

The afternoon depression in primary productivity in a high rate oxidation pond (HROP)

Algae (mainly Euglena sp.) from a high rate oxidation pond (HROP)were used for studying the afternoon depression in primary productivity.The phenomenon was observed on the same date by laboratory measurementsof photosynthesis and respiration (oxygen evolution method)as well as by in situ determinations of ¹⁴C incorporation. Thefollowing values of were calculated: morning, 0.014 µmolO₂/mg Chi a/min//mol quanta.m².s; afternoon, 0.008. Assuminga constant k_c of 0.006 m²/mg Chi a we found the quantum requirement(^–1) in the morning sample to be considerably lower thanin the afternoon sample (surface: morning 44 mol quanta/molO₂; afternoon, 71). Besides this reduction in photosyntheticefficiency the afternoon sample also exhibited reduced lightsaturated photosynthetic rates (P_max) and enhanced dark respirationrates. The combination of these three effects led to considerablylower areal primary productivity in the HROP in the afternoon.We suggest that this phenomenon is brought about by carbon limitationand cell overloading by photosynthetic products.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. II. A general review

The causes of interspecific differences in the µ-l relationshipare examined in the context of a mechanistic model which relatesµ to irradiance in terms of six factors:, k_c photosyntheticquotient (PQ), Chl a:C, respiration and excretion. The effectof cell size on the light saturated growth rate is also considered.It is shown that photosynthetic efficiency and PQ exhibit remarkablylittle interspecific variability, and average 0.024 ±0.005 µg C(µg Chl a)^–1 h^–1 (µEm^–2 s^–1)^–1 and 1.5 ± 0.2 mol 02 molC^–1 (when NO₃^– is the nitrogen source) respectively.Two useful relationships were derived: (i) between growth efficiency,_g and Chl a:C at µ. = 0; (ii) between the compensationintensity, I_c and the Chl a-specific maintenance respirationrate. Both relationships were independent of temperature anddaylength. Species best adapted to growth at low light werefound to exhibit high Chl a:C ratios and low maintenance respirationrates. As a group, diatoms were consistently the best adaptedfor growth at low irradiance. Chiorophytes, haptophytes, chrysophytesand cryptophytes were intermediate in their performance at lowirradiance. Dinoflagellates exhibited extreme diversity, withspecies spanning the spectrum from very good performance atlow irradiance to very poor. A new µ_max-cell carbon relationshipis given based on growth rates normalized to 15°C. Evidenceis presented to show that noise in this relationship can besignificantly reduced by using only carbon-specific growth ratesand using only data for species grown at the same daylength.     

Ecological studies on the phytoplankton of Boknafjorden, western Norway. II. Environmental control of photosynthesis

Both predicted (incubator) and measured (in situ) ¹⁴C-assimilationrates were studied from February to November 1981 at three stationsin Boknafjorden, a deep silled fjord of western Norway. Sampleswere taken from different light depths within the euphotic zone.A high degree of conformity was found between the two approaches.Daily values of carbon assimilation integrated over the euphoticzone varied between 0.05 and 1.4 g C m^–2. Yearly primaryproduction varied between stations from 82 to 112 g C m^–2(120–148 g C m^–2 when based on average light conditions).The light-saturation curve parameters ^B and P^B_max ranged from0.0056 to 0.0537 mg C mg Chla^–1 h^–1 µE^–1m² and from 0.7 to 8.5 mg C mg Chla^–1 h^–2 (in situassimilation numbers ranged from 0.9 to 9.3 mg C mg Chla^–1h^–1) respectively, which compare well with those publishedfrom the northwestern side of the Atlantic. The overall importanceof light in controlling photosynthesis throughout the year wasrevealed by the light utilization index , estimated to be 0.43mg C mg Chla^–1 E^–1 m². The maximum quantum yieldwas encountered on August 17, with 0.089 mol CE^–1. Chla/Cratios above and below 0.010 were found to be typical for shade-and light-adapted cells respectively. Assimilation numbers andgrowth rates were linearly related only when considering light-adaptedcells. Consistent with the findings of this study, the applicabilityof I_K, ^B and P^B_max as indicators of light-shade adaptation propertiesshould be questioned. Maximum growth rates were encounteredduring an autumn bloom of the dinoflagellate Gyrodinium aureolum(1.0 doublings day^–1), while 0.7–0.8 doublings day^–1were found for a winter bloom (water temperature of 2°C)of the diatom Skeletonema costatum. No unambiguous temperatureeffect on assimilation number and growth of phytoplankton couldbe recognized in Boknafjorden. A tendency towards increasedassimilation numbers coinciding with increased water columnstability was revealed. The highest P^B_max values were oftenencountered at almost undetectable nutrient concentrations.At least during summer this could be attributed to recyclingof nutrients by macro- and/or microzooplankton, responsiblefor a greater part of the primary production now being grazeddown. This study supports the convention that the depth of theeuphotic zone may extend considerably below the 1% light depth.     

Phytoplankton uptake of amumonium and urea during growth on oxidized forms of nitrogen

p. 383, Figure 2. The legend to Figure 2 should read: Fig. 2. Cumulative urea-N taken up as % total cellular N vs.time of incubation for T. pseudonana. Closed symbols = ureauptake; open symbols =urea uptake in the presence of NH₄⁺. = pre-depletion ([NO₂^– ] in culture medium = 5.0 µg-atomNO₂-N 1^–1 ), • = at depletion, = post-depletion(16 h after nitrogenous nutrient could no longer be detectedin culture medium).     

Joint field experiments for comparisons of measuring methods of photosynthetic production

During the 1st GAP Workshop at Konstanz in April 1982 comparativemeasurements of phytoplankton primary production by severaltechniques were conducted simultaneously at an offshore stationin Lake Konstanz and an experimental algal pond. Suspended glassbottle exposure techniques using ¹⁴C and ¹³C uptake gave P_z(mg C m^–3 h^–1) values which varied considerablynear-surface, but estimates of areal rates for the euphoticzone P_cu(mg C m^–3 h^–1) which were reasonably close.In the lake, P_z, from a vertical tube exposure (with ¹⁴C uptake)was greater than rates derived for integrated bottle samples.The oxygen bottle method permitted a good estimate of compensationdepth, corresponding to in situ growth studies. There were difficultiesin direct comparison between O₂ and carbon methods. Correlationbetween them for P_z was good in the lake but poor in the pond,both for suspended bottle and vertical tube methods. This seriesdemonstrates that despite reasonable overall estimates, comparativelyminor methodological differences in experimental technique cancause large variation. ⁺ Coordinator of the group for comparative measurements of photosyntheticproduction at the GAP Workshop, Konstanz, April 1982. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

Response of Gonyaulax tamarensis to the presence of a pycnocline in an artificial water column

In nature, large concentrations of the toxic bloom-forming dinoflagellate,Gonyaulax tamarensis, are frequently observed in the vicinityof the pycnocline. In the absence of a pycnocline the organismis usually recorded near the surface, where light levels aremore advantageous for photosynthesis. In this paper we examinethe swimming behaviour of G.tamarensis when exposed to varyingdegrees of stratification and investigate whether the maintenanceof a subsurface (pycnocline) population is the result of retentionof the algae by a physical barrier or active accumulation ofthe organisms at a density interface. The study indicates thatG.tamarensis cells presented with a halocline of S<{smalltilde}6–7 (occurring over a few centimeters) cross thissalinity barrier and accumulate at the highest available photonflux density ({small tilde}100 µmol m^–2 s^–1).Cells exposed to a gradient of S>{small tilde}7remain atthe halocline (pfd={small tilde}40 µmol m^–2 s^–1).However, when light above the pycnocline is attenuated by theaddition of food colour to the medium, the cells cross a haloclineof S=10 and accumulate at the highest available photon fluxdensity. In the absence of added nutrients (inorganic N andP) the organism fails to exhibit a phototactic response. Thus,the presence of a strong halocline does not represent an inpenetrablephysical barrier for G.tamarensis and the development of pycnoclinepopulations of this organism is a function of density, lightand nutrient climate.     

The optical properties of a turbid reservoir and its phytoplankton in relation to photosynthesis and growth (Mount Bold Reservoir, South Australia)

In situ light measurements were used to obtain information oninherent and apparent optical properties. The average verticalattenuation coefficient K_d(ave) varied from 1.1 to 4.6 In unitsm^–1 During three periods the variation in K_d(ave) correlatedwith changes in chlorophyll a concentration and specific attenuationcoefficients K_s, of 0.013, 0.014 and 0.022 m² mg Chl a^–1were calculated. Chlorophyll-specific diffuse absorption coefficients(A,) for these periods were 0.012. 0.013 and 0.017 m² mg Chla^–1 and only varied significantly from estimates of K_sin the period when scattering was intense. Absorption coefficientsa(z^mid) and scattering coefficients b(z^mid) calculated for themid-point of the euphotic zone ranged between 0.45 and 2.9 mand 3.5–52.0 m respectively. Chlorophyll-specific absorptioncoefficients K_a, of 0.005, 0.006 and 0.007 m² mg Chl a^–1and scattering coefficients K_b of 0.05. 0.09 and 0.191 m² mgChl a^–1 were measured during the three periods. The highK_b value occurred when gas-vacuolate cyanobactena were dominant.Algal photosynthesis and light absorption were related throughthe maximum quantum yield _m which varied between 0.019 and 0.11mol C Einstein^–1 while average quantum yields _a, variedbetween 0.006 and 0.024 with a mean of 0.013 mol C Einstein^–1A comparison of changes in the mean irradiance of the mixedzone and chlorophyll concentration indicated that growth waslight limited below 0.04–0.05 Einsteins absorbed mg Chla^–1 day^–1.     

Photoinhibition and the diurnal variation of phytoplankton photosynthesis--I. Development of a photosynthesis--irradiance model from studies of in situ responses

Diurnal series of fluorescence and photosynthesis assays wereconducted in high altitude (3803 m), tropical (16°), LakeTiticaca (Peru/Bolivia). Near-surface diurnal thermoclines formedon typical days of high photon flux density (PFD, {small tilde}2000 µE m^–2 s^–1). In the depth range of diurnalstratification profiles of in vivo fluorescence, both without(F_a and with (F_b DCMU, exhibited a mean decrease of 64% frommorning to mid-day, but little change (mean increase of 1.5%)through the afternoon. Three times during the day surface, mid-depth(3–5 m) and deep (15–20 m) phytoplankton sampleswere incubated with H¹⁴CO₃^– under short (<2 h) exposuresto a range of in situ PFDs. Comparison of phytoplankton in differentsamples (ANOVA) showed identical photosynthetic response insunrise (isothermal) samples but a significant drop in surfaceand mid-depth photosynthesis at all PFDs during times of diurnalstratification. Similarly, both low-light () and light-saturated(P² _max photosynthetic parameters were lower in mid-day surfacesamples compared to deep samples. In addition, previously photoinhibitedsamples had a higher threshold intensity for photoinhibition,I_T. These results, together with diurnal time series of fluorescencefrom in situ incubations, demonstrate that recovery from extendedepisodes of photoinhibition during diurnal stratification isslower than suggested by previous observations in vitro. Photosynthesisby near-surface phytoplankton is different in light increasingup to I_T than light decreasing from I_T. This effect can be modeledby reducing and P_max as a function of the maximum photoinhibitingPFD in the diurnal light history. ¹Present address: Division of Molecular Plant Biology, Universityof California, Berkeley, Berkeley, CA 94720, USA     

Phytoplankton photosynthesis parameters in central Canadian lakes

Values for two biological parameters — (i) P^B_m, the lightsaturated rate of photosynthesis per unit of chlorophyll and(ii) , the slope of the light limited part of the photosynthesisversus light curve per unit of chlorophyll — must be knownin order to be able to estimate rates of phytoplankton primaryproduction from chlorophyll data. These parameters were measuredfor periods of up to 9 years in central Canadian lakes locatedin temperate, subarctic and arctic climatic zones. Regardlessof their geographic locations, lakes where integral photosynthesiswas nutrient limited had lower values of these parameters thandid lakes where integral photosynthesis was light limited. Temperatureset an upper limit to the variability of P^B_m but was not a goodpredictor of its actual value. Year-to-year variability of photosynthesisparameters in the most intensively studied group of lakes waslarge: annual means varied by a factor of three over a 9-yearperiod. Until the sources and extent of this variability areknown, accurate production estimates can be obtained from chlorophylldata only if P^B_m and are measured in each water body and inevery year. Implications for estimating primary production fromremotely-sensed chlorophyll data are discussed.     

Phytoplankton primary productivity characteristics in response to photosynthetically active radiation in three Kenyan Rift Valley saline alkaline lakes

Phytoplankton photosynthetic characteristics in the Kenyan RiftValley lakes Bogoria, Nakuru and Elmentaita were studied betweenNovember 2003 and February 2005. In these world-famous saline–alkalinelake systems, long-term continuous monitoring and photoautotrophicprimary productivity modelling have been done for the firsttime. High light attenuation coefficients were observed withlakes means around 13 m^–1 reflecting the huge phytoplanktonbiomass. No photoinhibition was observed in the primary productivityfield measurements. High values of the photosynthesis–irradiancecurve initial slope () up to 0.85 (mg O₂ mg Chl a^–1 h^–1)(µmolphotons m^–2 s^–1)^–1 and a low onset of productivitysaturation (E_k) down to 11.4 µmol photons m^–2 s^–1as an acclimation to poor light supply were found. For the trophogeniczone, high mean net primary production (NPP) rates of 6.8, 10.7and 8.5 g O₂ m^–2 day^–1 were recorded for Bogoria,Nakuru and Elmentaita. For the whole water column, NPP decreasedto –1.4, 1.6 and 7.2 g O₂ m^–2 day^–1 becauseof high community respiration. Modelling of the gross primaryproduction (GPP, Chlorophyll a, light supply, initial slope, maximum production rates considered) gave annual values of4.9, 6.8 and 4.2 kg O₂ m^–2 year^–1, respectively,for Bogoria, Nakuru and Elmentaita, annual NPP values down tothe compensation depth were 70, 65 and 55% of the GPP.