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.     

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.     

Temperature and salinity effect on growth and chemical composition of Gyrodinium aureolum Hulburt in culture

A factorial experiment shows highly significant effects of temperature(12 5–22.5°C) and salinity (17.8–34 S) on thegrowth rate of Gyrodinium aureolum, with a significant temperature-salinityinteraction. The maximum growth rate of G aureolum is measuredto 0.61 div. day^–1 at 20°C and 22.3 S. Gyrodiniumaureolum does not grow at temperatures :10 °C or 25°Cand at salinities 12 S. The cellular content of carbon (C) andnitrogen (N) and the elemental ratios N/C, P/C and N/P are significantlyaffected by the temperature The cellular content of phosphorus(P) and the elemental ratios P/C and N/P vary significantlywith salinity Significant temperature-salinity interactionsare found for the cellular content of carbon, nitrogen and phosphorus.Variations in the N/P ratio indicate that G.aureolum has a largestorage capacity for phosphorus It is suggested that temperatureis one important limiting factor in the initiation of bloomsof G.aureolum in north European waters.     

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.     

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 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.     

Aspects of the feeding ecology of turbid water zooplankton. In situ studies of community filtration rates in silt-laden Lake le Roux, Orange River, South Africa

Diel vertical studies of zooplankton community filtration rates(CFR) were undertaken in situ over two annual cycles in a shelteredbay of Lake le Roux, a large silt-laden oligotrophic reservoirin the arid subtropics of South Africa. The grazer communitywas dominated by a copepod, Meradiaptomus meridianus, whileDaphnia gibba. D. barbara and Moina brachiara accounted forthe balance. Spot estimates of CFR varied from 0.1 to 75% d^–1( = 12.2) in the upper 10 m, while depth-integrated values ranged seasonally from 0.1 to 15% d^–1 ( = 7.4). Most variation in CFR was attributable toseasonal changes in grazer biomass (0–408, = 58.2 µg 1^–1 or 3–1000, 360 mg m^–2 dry wt), and temperature (12–22C). Thesevariables are used to construct multiple linear regression modelsfor the prediction of CFR in this system. Some higher CFR values(up to 260% d^–1 were measured in wanner (up to 26.5C)surface waters with an unusually rich zooplankion (786 rg 1^–1).No significant did vertical changes in CFR or grazer biomasswere observed. Both variables declined sharply with depth. Inorganicturbidity dominated the seston, and algal carbon probably accountedon average for less than about 20% of the total POC available.The feeding responses of this turbid-water community were generallyconsistent with observations made on other assemblages, apartfrom seemingly high specific filtration rates which, atypically,were inversely related to temperature.     

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.     

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.     

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.     

Interspecific variability and environmental influence on particulate organic carbon {delta}13C in cultured marine phytoplankton

The stable carbon isotope composition of particulate organicmatter expressed as ¹³C was measured in cultures of 13 speciesof marine microalgae in different phylogenetic groups. The effectsof salinity variations and changes in photoperiod were alsoassayed for three of them (i.e. Skeletonema costatum, Amphidiniumopercularum and Isochrysis galbana); the effect of nature ofnitrogen supply (nitrate. ammonium) was studied for one (S.costatum).These environmental parameters were chosen because of theirvariability in the ocean and their possible effects on ¹³C valuesof phytoplankton organic carbon. Batch culture conditions andsampling time after inoculum were strongly controlled in orderto provide cells in good physiological state which were comparablefrom one culture to the other. In the same way, sampling waslimited to the first 2 days of exponential growth, in orderto avoid a possible dissolved inorganic carbon (DIC) limitation.Carboxylase activities [of the enzyme ribulose 1,5-bisphosphatecarboxylase oxygenase (Rubisco), and the three ß carboxylases:phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvatecarboxykinase (PEPCK) and pyruvate carboxylase (PC)] and totalchlorophyll a concentrations were assayed simultaneously. The¹³C values observed were between –30.2 and –12.7i.e. comparable to those observed in the world's oceans. Theisotopic composition of phytoplankton organic carbon was shownto be under the influence of the parameters tested but ¹³C variationsare specific to the species considered. The nature of ßcarboxylase found in each species, or systematic position, couldnot be linked to the isotopic composition of organic carbon.No linear or single correlation between ¹³C variations and environmentalmodifications were observed and there is no evidence for a simpleand universal relation between ¹³C of phytoplankters and theirenvironment. In monospecific cultures as in the field, ¹³C fractionationby Rubisco (and eventually by PEPCK) may be counterbalancedby other mechanisms.     

Corrigendum

Due to an apparent fault in the telex system, a number of mistakeswere not corrected in this paper. The corrected lines are givenbelow p. 539: line 17In vivo fluorescence action spectra of chl a p. 541: Figure 2 legend, line 92.5 µW cm^–2 at 550nm (0.12 µE m^–2 s^–1 p. 542: Figure 3 legend, line 5( 89 µE m^–2 s^–1).Monochromatic beam intensity was 6 µW cm^–2 at 550nm (–0.28 µE m^–2 s^–1), Figure 3 legend, lines 8 and 9with intensity of 3 mW cm^–2( 179 µE m^–2 s^–1) Monochromatic beam intensitywas 7 5 µW cm^–2 at 550 nm (0.35 µE m^–2s^–1). line 6tivity. The match between the spectra of chl a fluorescenceand PSII O₂ evolution is lines 23–25fluorescence increasingly deviate from thoseof PSII O2 evolution. We attribute this discrepancy to selectivelight scattering by the algae. This scattering increases substantiallywith decreasing wavelength at that region when using a standardspectrofluorometer p. 543: Figure 4 legend, lines 3 and 4as in Chroomonas, withintensity of 2 mW cm^–2 ( 120 µE m^–2 s^–1).Monochromatic beam intensity was 15 µW cm^–2 at 550nm ( 0.7 µE m^–2 s^–1). line 7:ment types in the oceans. While all photoautotrophicorganisms have chl a (the bulk lines 12 and 13:and Barrett (1983). Our spectral data reflectthe great variability in pigment composition and functionalassociation in the major groups of algae. lines 25 and 26:Hiller, 1983). However, blue-violet and redPSII activity is much lower in cryptomonads than might be expectedfrom their absorption spectra (Haxo and Fork, 1958; Haxo, 1960), p. 544: Table I, column 3, entry 6:R-phycocyanin Table 1 legend, lines 1 and 2:^aHaxo and Blinks (1950); ^bFork(1961); ^cHaxo et al. (1955); ^dO'Carra and O'hEocha (1976); ^cresemblesDelesseria decipiens, Haxo and Blinks (1950, Figure 20); ^fHaxoand Fork (1969), like lines 1 and 2:I) indicates that these pigments are affiliatedwith PSII, perhaps exclusively, as in the red algae. In Rhodomonas,the peak of activity at 465 nm may be due to chl c absorp- p. 545: line 2:xanthophyll (fucoxanthin in diatoms, peridininin dinoflagellates and chl c. The ac line 7:shown to be similar to those of the much-studied Chlorella(Vidaver, 1966; Ried, 1972). line 14:tivity of PSII in algae. Spectrofluorometers, with theirsuperior sensitivity and stability, line 34:of natural populations, making these spectra more similarto the PSII photosynthesis lines 36 and 37:Large differences between the values of FIIfor different components within the algal population can distortfluorescence spectra, if they do not correspond with dif lines 41 and 42:different components are not similar to eachother. The necessary handling procedures of natural samples,such as filtration (Yentsch and Yentsch, 1979; Neori et al.,1984), p. 546: lines 20 and 21:DOE contract DE-AT03-82ER60031, anda grant to A.N., O.H.H. and F.T.H. from the Foundation for OceanResearch. Travel to the IInd GAP workshop was facilitated by lines 25 and 26:the culture of Chroomonas, J.Lance for helpwith cultures, J. and E.Yguerabide for the use of their spectrofluorometer,C.R.Booth, Y.Blatt and L.Petrosian for technical line 28:This study was in partial fulfilment for a Ph.D. degreeby A.N. line 42:Dutton.H.J., Manning.W.M. and Duggar.B.M. (1943) Chlorophyllfluorescence and energy transfer in the     

Effects of temperature, salinity and food level on the life history traits of the marine rotifer Synchaera cecilia valentina, n. subsp.

A strain of the marine rotifer Synchaeta cecilia valentina,n. subsp., isolated from the Hondo of Elche Spanish Mediterraneancoastal lagoon at 22 salinity, was cultured in the laboratoryin 20 ml test tubes and fed with the alga Tetrasemis suecica.The effect of two temperatures (20 and 24°C), four salinities(20,25,30 and 37) and two food levels (15 000 and 25000 cellsml^–1) on the life history traits of this rotifer werestudied in life tables performed with replicated individualcultures. Temperature and salinity had a significant negativeeffect (P < 0.001) on the average lifespan (LS) and on thenumber of offspring per female (R₀) The effect of food levelon LS is unclear, whereas R₀ is greater at 20°C with thelower concentration of algae and at 24°C with the higheralgal concentration. The maximum values of LS and R₀, 5.6 daysand 9.2 offspring per female, respectively, were recorded at20°C, 25o salinity and low food concentration. There isalso a clear negative effect on the intrinsic growth rate (r)due to salinity. The effect of temperature depends on the foodlevel and, as occurs with R₀ the maximum values of r occur withthe lower algal concentration at 20°C, whereas at 24°Cthey are obtained with the higher algal concentration. Theser values, from 1.04 to 1.10 day^–1, were reached at 24°C,salinities of 20–25 and with high food concentration.     

A comparison of net and gross rates of oxygen production as a function of light intensity in some natural plankton populations and in a Synechococcus culture

In vitrorates of gross and net oxygen production were measuredas a function of light intensity in some plankton communitiescollected from Bedford Basin, Nova Scotia, and in a monoclonalculture of Synechococcus. The rate of gross oxygen productionwas measured by a technique in which the stable oxygen isotope,¹⁸O, serves as a photosynthetic tracer Net oxygen productionwas measured by automated Winkler technique. The rate of communityrespiration in the light was then determined by the differencebetween gross and net rates of oxygen production. In the naturalpopulations examined, neither gross nor net oxygen productionrates were significantly inhibited at the highest light intensitymeasured (500–800 µE m^–2 s^–1) In a samplein which the dark respiration rate was small relative to themaximal rate of production [P_max;sensu Platt et al (1980) JMar. Res., 38, 687–701] the rates of ‘light’respiration were 3 times greater. In two other communities,with high rates of dark respiration relative to P_maxthe ratesof ‘light’ respiration were closer to rates of darkrespiration. In the Synechococcus clone, both gross and netoxygen production rates were inhibited at high light intensities.Rates of ‘light’ respiration were found to varyas a function of light intensity. The greatest rates of respirationwere measured in samples incubated at light intensities thatwere just saturating (100 µE m^–2 s^–1). Therates of ¹⁴C production were also measured as a function oflight intensity The photosynthetic quotients, based on ¹⁴C productionrates and gross oxygen production rates, average 1 9     

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).     

Daily vertical migration of dense deep scattering layers related to the shelf-break area along the northwest coast of Baja California, Mexico

Echo signals were collected with a Simrad 200 kHz transduceracross the shelf-break features off the northwest coast of BajaCalifornia (30°05'–30°42'N, 115°50'–116°26'W)during two diel cycles (July 1995) with the objective of describingvertical migrations of two dense deep scattering layers (DSL)found near the shelf break. DSL records were made within anarea ±50 m in the neritic zone, ±200 m at theshelf break and ±1000 m depth in the offshore station.Using an Isaacs-Kidd midwater net and Bongo nets, we inferredthat the DSL were composed mainly of juveniles and adults ofthe euphausiids Nyctiphanes simplex Hansen and Euphausia pacificaHansen. These aggregations have a close interaction with theocean bottom during the daytime and display a vertical migration,rising near to the surface at night where a progressive horizontaldispersion then occurs. The DSL measured 3.5–6.0 km horizontallyduring the night. The ascent and descent migrations of the DSLwere significantly fitted to a polynomial function of the secondorder, suggesting different swimming behavior during these twoprocesses. Vertical migrations of the DSL were between 60 and90 m. During the descent migration, maximum instantaneous speedsreached 0.7 cm s¹, and during the ascent, maximum instantaneousspeeds were 0.3 cm s¹. These coastal euphausiid species, alongwith other macrozooplankton and nektonic organisms, have a dailyclose interaction with the ocean bottom and also with the pelagicenvironment, suggesting that they play an important role providingfood for demersal and pelagic organisms on the slope and shelfbreak in the upwelling region off the northwest coast of BajaCalifornia.     

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 influence of salinity fluctuation on the ammonium metabolism of the marine diatom Skeletonema costatum grown in continuous culture

Ammonium-limited cultures of Skeletonema costatum were grownat dilution rates from 0.019 to 0.038 h^–1 at an averagesalinity of 22.4 For a few days cultures were exposed to a freshwaterpulse. When salinity was decreased to 8.6 (average minimum)photosynthesis and cell division were inhibited. Both in vivoand DCMU-enhanced fluorescence per cell were statistically constant:photosystems I and II were not inhibited by a gradual salinitydecrease. Ammonium assimilation was affected via an inhibitionof carbon fixation. Ammonium concentrations increased in thecontinuous cultures, whereas the overcapacity of ammonium uptakedeclined: the nitrogen limitation was relieved. When salinitywas increased again, photosynthesis and cell division were stimulated.Salinity fluctuations were accompanied by a fluctuation in thepools of aspartic acid (0.5–1.0 mM), glutamine acid (0.9–4.1mM) and glutamine (0.5–2.0 mM). The pool of glutamic acidfollowed the salinity pattern (r=0.67, P<0.05). The correlationbetween the amino acid pool and the osmotic value of the mediumwas significant (r=0.72, P<0.05). Cellular glutamic acidand glutamine levels increased until the nitrogen limitationwas restored.     

On the trophic fate of Phaeocystis pouchetii (Harlot). III. Functional responses in grazing demonstrated on juvenile stages of Calanus finmarchicus (Copepoda) fed diatoms and Phaeocystis

The objective of this study was to quantify the functional responsein feeding rate in the various developmental stages of Calanusfinmarchicus to different concentrations of the diatoms Thalassiosiranordenskioeldii and Porosira glacialis, and the haptophyseanPhaeocystis pouchetii. Grazing of copepodite stage I–VC.finmarchicus was measured using two different approaches.Feeding rates were obtained from either incubation experiments,estimating the rate of removal of particles from suspension,or by quantifying the turnover rate of the plant pigments inthe gut. Clearance as a function of algal concentration (1–30µg plant pigment 1^–1) was described in juvenilestages of C.finmarchicus fed the diatoms T.nordenskioeldii [20µm equivalent spherical diameter (ESD)], P.glacialis (40µm ESD), and two size categories (30–100 µmand >100 µm ESD) of the gelatinous alga P.pouchetii.When the copepodite stages were fed T.nordenskioeldii, the gutcontent of plant pigments was in general higher than when fedP.glacialis. Rates obtained were variable when the same copepoditestages were offered the two size categories of P.pouchetii,but within the same order of magnitude as those obtained forthe larger diatom. At unialgal diets, diatoms were more readilyconsumed than the larger size fraction among colonies of P.pouchetiiby copepodite stage I–III C.finmarchicus. But given anappropriate prey size, C.finmarchicus grazed both diatoms andcolonies of gelatinous algae at equal rates. A linear relationshipbetween gut content and food concentrations <10 µgchlorophyll 1^–1 was found. This indicates that the ingestionrate in C.finmarchicus is directly proportional to the ambientfood concentration during the most productive period in Mayand June in high latitudes irrespective of algal species present. ¹Present address: Marine Biological Laboratory, University ofCopenhagen, Strandpromenaden 5, DK-3000 Helsingør, Denmark ²Present address: Greater Copenhagen Council, Gl. KøgeLandevej 1–3, DK-2550 Valby, Denmark     

Trophic diversity within the planktonic food web of the Elbe Estuary determined on isolated individual species by 13C analysis

The uptake of organic substrates by heterotrophic planktonicorganisms was studied along the freshwater Elbe Estuary in May,July and October 2000 using ¹³C analysis of individually isolateddominant species of copepoda, cladocera, rotifera and ciliata.Non-sedimenting suspended particulate matter (SPM_ns) was separatedfrom sedimenting matter and further analysed for the chemicalcomposition of its different size fractions in order to estimatesubstrate availability. Particles <5 µm accounted for15% of total SPM_ns [40 mg dry weight (DW) L^–1] and containedC:N ratios indicating a predominance of living matter (i.e.mass C:N of phytoplankton). All species under study exhibiteda high capacity for selective feeding with little variationin the diet along the whole freshwater profile. Picoplanktonof 0.2–1.2 µm formed mainly by bacteria had a ¹³Cvalue of –26 in May and July and –29 in October,similar to the ¹³C of the dissolved organic carbon (DOC). Bacteriadid not fractionate isotopically and did not preferentiallyconsume specific subunits of their substrate, i.e. they exhibitedno trophic shift. It appears that phytoplankton exudates werea minor component to total DOC in this estuary. Phytoplanktonwas the exclusive food for all phagotrophic organisms understudy in July, thus only one trophic level was exhibited. DuringMay and October the grazers under study used different substrates,resulting in a planktonic food web of three trophic levels formedby different species of the taxonomic groups under study. Theresults indicate a conditioned behaviour with regard to substrateselection allowing the grazers to produce high abundances evenwhen particles were abundant and competition for phytoplanktonwas high.