Primary production and environmental factors in an oligotrophic biome in the Aegean Sea

The rate of the primary production of the phytoplankton community in the Petalion Gulf, Aegean Sea, was studied from January 1970 to May 1971, at a station situated at approximately Latitude 37°54N; Longitude 24°11E. A variety of physical and chemical parameters such as chlorophyll, primary nutrients (N,P,Si), temperature, salinity, oxygen and light penetration were also studied simultaneously. The rate of the gross primary production varied from 40 to 200 mg C m^-2 day^-1, with a mean value of 90 mg C m^-2 day^-1. The annual gross primary production was calculated to be 33 g C m^-2, which is the minimum known value in the Aegean and Mediterranean Seas. Maximum production was found at the depth of 20 m on the average, mainly due to high light intensities. Petalion Gulf supports a small photosynthetic biomass, as indicated by the low seasonal values of chlorophyll a (0.01 to 0.18 mg m^-2), the highest values being found in the summer. The low production rate noted may have been due to the low nutrient concentrations found: N, 0.04 to 0.32 g-at/1; P, 0.00 to 0.15 g-at/1; Si, 0.45 to 2.25 g-at/1. It is suggested that inorganic phosphorus and nitrogen may alternate in limiting primary production rates in these oligotrophic waters. The temperate waters of the Petalion Gulf are stratified in summer (15.5° to 24.7°C) and well-mixed in winter (12.9° to 15.0°C); they are oxygen-saturated throughout the year, and of high transparency, with 86 m depth for the euphotic zone on the average yearly. The Petalion Gulf is therefore characterized as a typical oligotrophic biome in the Aegean and Eastern Mediterranean Seas.     

Growth response of Chaetoceros calcitrans (Bacillariophyceae) in batch culture to a range of initial silica concentrations

Batch cultures of the marine unicellular centric diatom Chaetoceros calcitrans (Paulsen) Takano were maintained by serial subculturing every 4 d into nutrient-enriched natural sea-water medium supplemented with 350, 950 and 1 400 g-at Si l^-1. The diatom cultures removed initial silica concentrations of 350 and 950 g-at l^-1 from the medium within 2 and 3 d, respectively. About 30 g-at l^-1 of the highest initial concentration remained in the medium after 4 d. The mean final cell density with an enrichment of 350 g-at Si l^-1 was 3.43±0.26×10⁴ cells l^-1 (median cell volume = 77.5±5.0 m³); with 950 g-at Si l^-1, 8.55±0.55×10⁴ cells l^-1 (50.0±4.5 m³); and with 1 400 g-at Si l^-1, 9.72±0.48×10⁴ cells l^-1 (37.3±5.0 m³). There was no significant difference in the final total organic weight of cells produced, which was in the range of 170 to 190 mg per 250 ml culture. This consisted of proportionately more lipid and carbohydrate and less protein from the treatment with 350 g-at Si l^-1 than from the 1 400 g-at Si l^-1 treatment.     

Interactions of inorganic nitrogen in the uptake and assimilation by marine phytoplankton

The effect of ambient ammonium concentration on the nitrate uptake rate of marine phytoplankton was investigated. These studies consisted of laboratory experiments using unialgal species and field experiments using natural phytoplankton communities. In laboratory experiments, ammonium suppressed the uptake rates of nitrate and nitrite. Approximately 30 min were required for ammonium to exhibit its fully inhibitory effect on nitrate uptake. At high ammonium concentration (>3 g-at/l), a residual nitrate uptake rate of approximately 0.006 h^-1 was observed. When the ambient ammonium concentration was reduced to a value less than 1 g-at/l, the suppressed nitrate uptake rate subsequently attained a value comparable to that observed before the addition of ammonium. A range of 25 to 60% reduction in the nitrate uptake rate of natural phytoplankton communities was observed at ambient ammonium concentrations of 1.0 g-at/l. A mechanism is proposed for the suppression of nitrate uptake rate by ammonium through feedback control of the nitrate permease system and/or the nitrate reductase enzyme system. The feedback control is postulated to be regulated by the level of total amino acids in the cell.Contribution No. 936 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA. This paper represents a portion of a dissertation submitted to the Department of Oceanography, University of Washington, Seattle, in partial fulfillment of the requirements for the Ph.D. degree.     

Effects of light intensity on nitrate and nitrite uptake and excretion by Chaetoceros curvisetus

Michaelis-Menten uptake kinetics were observed at all light intensities. With constant illumination, the V_max and K₁ in nitrate uptake over the natural light intensity range of 0 to 2000 E were 0.343 g-at NO₃–N(g)^-1 at protein-N h^-1 and 26 E, respectively. Nitrate uptake was inhibited at higher light intensities. The K_s for nitrate uptake did not vary as a function of light intensity remaining relatively constant at 0.62 g-at NO₃–N 1^-1. With intermittent illumination, the V_mzx for light intensity in nitrate uptake over a light intensity range of 0 to 5000 E was 0.341 g-at NO₃–N(g)^-1-at protein-N h^-1. No inhibition of nitrate uptake was observed at higher than natural light intensities. Chaetoceros curvisetus will probably never experience light inhibition of nitrate uptake under natural conditions.     

Nitrogenous nutrition in the euphotic zone of the Central North Pacific Gyre

The uptake rates of the three nitrogen compounds ammonium, nitrate, and urea were measured in the oligotrophic North Central Pacific Gyre in August–September 1985. The measurements were performed by using ¹⁵N-labelled substrates and incubating for short-time periods (3 to 4 h) under simulated in situ conditions. Ambient concentrations of the nitrogenous nutrients were generally below 0.10 mol l^-1. The average total daily nitrogen uptake rate, integrated over the euphotic zone, was 12.5 mmol N m^-2 d^-1. Diel studies in the upper water mass resulted in a calculated phytoplankton growth rate of 1.3 d^-1. Ammonium was the dominating nutrient, accounting for on the average 54% of the total nitrogen uptake, while urea uptake represented 32% and nitrate 14%. Ammonium uptake rates at a coastal station off the Hawaiian Islands were very close to the rates found at the oceanic station. Organisms <3 m dominated the nitrogen assimilation, being responsible for about 75% of the ammonium uptake. The nitrogen uptake rates in this study seem to be higher than those found by earlier investigations in the area, but correlated well with other productivity measurements performed during the same cruise.     

Light-saturated photosynthesis by phytoplankton size fractions in the New York bight,USA

The relationships between netplankton and nanoplankton assimilation numbers, temperature, and major nutrient concentrations were studied and evaluated in the context of seasonal patterns in the biomass of these phytoplankton size fractions. Netplankton and nanoplankton blooms typically occur during late winter (2° to 8°C) and summer (18° to 24°C), respectively. Variations in nanoplankton and netplankton assimilation numbers were not statistically related to the development or collapse of specific blooms based on weekly sampling, but assimilation numbers were higher during the bloom periods than during transition periods of rapid temperature change (8° to 18°C). Differences in the assimilation numbers between size fractions could account for the dominance of the nanoplankton fraction during the summer bloom period but not for the dominance of netplankton during the winter bloom period. Nanoplankton and netplankton assimilation numbers were exponential functions of temperature between 8° and 24°C and 8° and 20°C, respectively. Below 8°C the assimilation numbers of both fractions were higher than expected on the basis of temperature. Above 20°C netplankton assimilation numbers declined with temperature. Netplankton and nanoplankton assimilation numbers were occasionally correlated with dissolved inorganic nitrogen concentrations from less than 1.0 to more than 15 g-at l^-1. Under these conditions, nanoplankton growth rates (calculated from assimilation number and carbon:chlorophyll) were higher and increased more rapidly with dissolved inorganic nitrogen than netplankton growth rates.     

Primary productivity and sizes of pools of organic carbon in the mixed layer of the ocean

We estimated primary productivity and distributions of carbon in the phytoplankton, micro-zooplankton, and suspended and dissolved matter in various areas of the World Ocean to increase our information about the organic carbon cycle in the surface layer of the sea. Primary productivity ranged from about 0.1 gC m^–2 day^–1 in the Gulf of Mexico to 9 gC m^–2 day^–1 in nutrient-rich water off Peru. Phytoplankton carbon ranged from less than 10 g/l in the former to 750 g/l in the latter and in nutrient-rich water off southwest Africa. Micro-zooplankton carbon usually was less than 50 g/l in all waters, and was dominated by ciliates, copepodids, and copepod nauplii in all areas. Concentrations of particulate carbon ranged from 12 g/l off the east coast of South America to 850 g/l off southwest Africa. Concentrations of dissolved organic carbon varied between 0.5 and 1.5 mg/l in all areas except off Peru, where maximum values of 4.5 mg/l were observed. Turnover rates of carbon by small standing crops of micro-flagellates (1 to 5 longest dimension) and dinoflagellates in nutrient-poor waters were lower than those by large standing crops of diatoms and micro-flagellates in nutrient-rich waters. Concentrations of phytoplankton usually accounted for 20 to 55% and micro-zooplankton for 2 to 30% of the particulate carbon in the surface layer of the sea. Concentrations of dissolved organic carbon were not related to concentrations of particulate carbon in most waters except off Peru, where they appear to be directly related.     

Growth patterns of Codium fragile ssp. tomentosoides in response to temperature,irradiance, salinity,and nitrogen source

Seasonal patterns of growth, reproduction, and productivity of Codium fragile spp. tomentosoides (van Goor) Silva were monitored at 3 locations in Rhode Island. Maximal growth occurred during the summer and was more significantly correlated with temperature than any other factor measured in this study. Multiple correlation models suggested an interaction between temperature, irradiance, and available nitrogen. Maximal reproduction occurred in late summer and early fall. The maximal productivity, based on harvested quadrats, was 2. 10 g dry weight m^-2 day^-1. A large amount (up to 87.3%) of the annual production entered the detrital food chain during the winter by fragmentation of the thallus. Culture studies examined the effects of temperature (6° to 30°C), irradiance (7 to 140 E m^-2 sec^-1), daylength (8 h light: 16 h dark to 24 h light: O h dark) and salinity (6 to 48) on growth. Differentiated thalli grew over a broad range of experimental conditions, with maximal growth at 24°C, 24 to 30 S, a minimal irradiance of 28 E m^-2 sec^-1, and 16 h daylength. The effect of increasing daylength was due to increased total daily irradiance rather than to a true photoperiodic effect. Undifferentiated sporelings survived and grew in a narrower range of environmental conditions than thalli. c. fragile spp. tomentosoides grew equally well with nitrate, nitrite, ammonium, and urea as a nitrogen source. The addition of NaHCO₃ stimulated growth at levels of 2.4 to 4.8 mM, suggesting an inorganic carbon limitation in static cultures. This study supports the hypothesis that the in situ seasonal growth pattern of c. fragile spp. tomentosoides is primarily due to the interaction of temperature and irradiance.     

Chemical composition,metabolic rates and feeding behavior of the midwater ctenophore Bathocyroe fosteri

Individuals of the midwater ctenophore Bathocyroe fosteri (0.01 to 1.6 g dry weight, DW) were collected from Bahamian waters by the submersible Johnson-Sea-Link during May and September/October 1983 and October/November 1984 from 530 to 700 m depth. Metabolic rates were measured and showed oxygen consumption to be in the range of 0.01 to 0.18 mg O₂ g^-1 DW h^-1 at temperatures ranging from 9° to 12°C. Ammonium excretion (0.01 to 0.14 g-at N g^-1 DW h^-1) was typically low. Energy expenditures estimated from respiration data (ca. 7% body C d^-1) indicated that one to three midwater crustacean prey (ca. 150 g C d^-1) could provide the daily maintenance ration required by a 40 mm ctenophore. These metabolic characteristics complemented in situ observations of poor locomotor ability and passive feeding behavior.     

Photosynthesis-irradiance responses and photosynthetic periodicity in the sea anemone Aiptasia pulchella and its zooxanthellae

Sea anemones (Aiptasia pulchella) containing zooxanthellae (Symbiodinium microadriaticum) were maintained in a long-term laboratory culture on a 12 h light (100 E m^-2 s^-1):12 h dark cycle. Photosynthetic oxygen production was measured for the symbiotic association and for freshlyisolated zooxanthellae. Light utilization efficiencies () were similar for both sets of zooxanthellae, suggesting negligible shading of zooxanthellae by animal tissue in this association. Whereas freshly-isolated zooxanthellae were photoinhibited at high irradiances (800 to 1 800 E m^-2 s^-1), zooxanthellae in the host continued to function at photosynthetic capacity. Time of day may influence photosynthetic measurements in symbiotic organisms, as it was found that photosynthesis in A. pulchella followed a diel periodicity at both light-saturating (1 200 E m^-2 s^-1) and subsaturating (150 E m^-2 s^-1) irradiances. There was a peak period of photosynthesis between 12.00 and 14.00 hrs. Light stimulated dark respiration rates of A. pulchella. Dark respiration of sea anemones increased somewhat towards the end of the light cycle and was always greater after exposure to high irradiances.     

Effects of nitrogen and phosphorus enrichement on in vivo symbiotic zooxanthellae of Pocillopora damicornis

The reef coral Pocillopora damicornis (Linnaeus) was grown for 8 wk in four nutrient treatments: control, consisting of ambient, unfiltered Kaneohe Bay seawater [dissolved inorganic nitrogen (DIN, 1.0 M) and dissolved inorganic phosphate (DIP, 0.3 M)]; nitrogen enrichment (15 M DIN as ammonium); phosphorus enrichment (1.2 M DIP as inorganic phosphate); and 15 M DIN+1.2 M DIP. Analyses of zooxanthellae for C, N, P and chlorophyll a after the 8 wk experiment indicated that DIN enrichment increased the cellular chlorophyll a and excess nitrogen fraction of the algae, but did not affect C cell^-1. DIP enrichment decreased both C and P cell^-1, but the decrease was proportionally less for C cell^-1. the response of cellular P to both DIN and DIP enrichment appeared to be in the same direction and could not be explained as a primary effect of external nutrient enrichment. The observed response of cellular P might be a consequence of in situ CO₂ limitation. DIN enrichment could increase the CO₂ (aq) demand by increasing the net production per unit area. DIP enrichment could slow down calcification, thus decreasing the availability of CO₂ (aq) in the coral tissue.Hawaii Institute of Marine Biology Contribution No. 920     

Application of a 15N tracer method to the study of dissolved organic nitrogen uptake during spring and summer in Chesapeake Bay

The dissolved organic nitrogen (DON) pool in marine waters contains a diverse mixture of compounds. It is therefore difficult to accurately estimate planktonic uptake of DON using the limited number of radiolabeled compounds commercially available. We describe a method to estimate DON uptake rates using ¹⁵N-labeled DON recently released from phytoplankton. To make ¹⁵N-labeled DON, we incubated surface water with ¹⁵NH ₄ ⁺ and then isolated the DON, including any recently released DO¹⁵N, with ion retardation resin. This DON was then added to a freshly collected water sample from the same environment to quantify the rate of DON uptake. The technique was applied to investigate rates of DON uptake relative to inorganic nitrogen in the mesohaline Chesapeake Bay during May 1990 and August 1991. The May experiment took place after the spring bloom, and rates of DON uptake [ranging from 0.31 to 0.53 g-atom (g-at) Nl^-1 h^-1] often exceeded rates of NH ₄ ⁺ and NO ₃ ^- uptake combined. The rates of DON uptake at this time were higher than estimated bacterial productivity and were not correlated with bacterial abundance or bacterial productivity. They were, however, correlated with rates of NO ₃ ^- uptake. In May, we estimate that only 7 to 32% of DON uptake was a result of urea utilization. In contrast, in August, when regenerated nutrients predominate in Chesapeake Bay, rates of DON uptake (ranging from 0.14 to 0.51 g-atom Nl^-1 h^-1) were an average of 50% of the observed rates of NH ₄ ⁺ uptake. Consistent with the May experiment, rates of DON uptake were not correlated with bacterial production. A sizable fraction of DON uptake, however, appeared to be due to urea utilization; rates of urea uptake, measured independently, were equivalent to an average of 74% of the measured rates of DON uptake. These findings suggest that, during both periods of study, at least a fraction of the measured DON uptake may have been due to utilization by phytoplankton.     

Growth of plankton algae as a function of N-concentration,measured by means of batch technique

Employing a special batch technique which involves extremely low concentrations of the alga Selenastrum capricornutum, it is shown that a growth rate of ca. 75% of optimum growth occurs at a concentration of 0.02 g-at N(NH₄ ⁺) l^-1. The low concentrations of this N-source in the oligotrophic parts of the oceans (ca. 0.06 g-at l^-1) thus give rise to a borderline deficiency. As the half-saturation constant (K _S) for growth was recorded at 0.05 g-at N(NO₃ ^-), this ion can hardly be of much importance at the low concentrations present in these parts of the oceans. It is wrong to assert that N is the primary limiting nutrient. Phosphate also is found at concentrations giving rise to a borderline deficiency.     

Population dynamics and production of euphausiids. I. Thysanoessa longicaudata in the North Atlantic Ocean

Adults and furciliae of Thysanoessa longicaudata (Krøyer) taken in continuous plankton records (CPR) in 1966 and 1967 were examined and measured. The measurements were converted to biomass using a length/weight equation, and net production values were calculated from these data. Two generations were spawned annually in the eastern North Atlantic Ocean and only one in the colder western waters and in the Norwegian Sea. The timing of reproduction in the spring was correlated with the mean sea-surface temperature. The highest annual production values (up to 12.24 mg dry weight m^-3 year^-1) were recorded in the eastern areas, where the ratio between production and biomass was usually between 4:1 and 8:1 compared with ratios usually between 2:1 and 3:1 in the western waters. The values for daily production per individual ranged from 3.34 g dry weight day^-1 to 17.54 g day^-1. Mortality in the early stages of development was calculated as about 98.2% of the eggs produced.     

Ecological growth strategies in the seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae): Soluble nitrogen and reserve carbohydrates

The seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae) were grown in an outdoor continuous-flow system at both ambient incident light (I₀) and 0.13 I₀. During the winter, both species accumulated substantial soluble nitrogen reserves (up to 1020 g-at N·g dry wt^-1 in G. foliifera and 630 g-at N·g dry wt^-1 in Ulva sp.). The rate at which these N reserves were depleted was proportional to the growth rate. Seaweeds grown at 0.13 I₀ had lower growth rates and higher levels of soluble tissue N than plants grown at I₀. During the spring-summer growing season, peaks in tissue N followed nutrient peaks in the ambient seawater. Ulva sp. had higher nutrient uptake and growth rates than G. foliifera and showed greater fluctuations in soluble tissue N. This may characterize opportunistic seaweed species with high biomass turnover rates. At I₀, the levels of starch (up to 340 mg·g dry wt^-1 in G. foliifera and 170 mg·g dry wt^-1 in Ulva sp.) were highest during the spring and summer. During this period, fluctuations in starch content were inversely related to growth rate and soluble tissue N. Seaweeds grown at 0.13 I₀ did not accumulate starch. Neither species was found to overwinter with starch reserves.     

Respiration and excretion by the ctenophore Mnepiopsis leidyi

Respiration (dissolved oxygen and carbon dioxide) and excretion (dissolved organic carbon, inorganic and organic nitrogen and phosphorus) rates were measured for a variety of sizes of Mnemiopsis leidyi over a temperature range of 10.3° to 24.5°C. Both respiration and excretion rates were a direct linear function of animal weight and very temperature sensitive (Q₁₀4). Oxygen uptake ranged from 155 to 489 g at O/(g dry weight) day^-1 and carbon dioxide release from 43 to 166 M. Organic carbon made up about 38% of the total carbon released. Inorganic nitrogen excretion, exclusively in the form of ammonium, comprised 54% of the total nitrogen release and ranged from 10 to 36 M NH₄/(g dry weight) day^-1. Average release of dissolved primary amines (expressed as glycine equivalents) equaled 43% of the organic nitrogen fraction. Inorganic phosphorus release ranged from 2.0 to 4.9 M/(g dry weight) day^-1 and made up about 72% of the total phosphorus loss. The turnover of elements in the body was calculated as 5 to 19% per day for carbon and nitrogen, depending on the temperature, and an even higher 20 to 48% per day for phosphorus. These values are comparable to rates observed for small, active zooplankton.     

The ammonium/methylammonium uptake system of Symbiodinium microadriaticum

The substrate analogue [¹⁴C]-methylammonium was used to study ammonium/methylammonium uptake by Symbiodinium microadriaticum (zooxanthellae). The value of the Michaelis constant (K _m) for the uptake system was approximately 35 M with methylammonium as substrate; ammonium was a competitive inhibitor of methylammonium uptake, and the K _m for ammonium uptake (determined as the inhibition constant, K _i, for methylammonium) was 6.6 M. Methylammonium uptake by zooxanthellae was light-dependent. Methylammonium uptake rates of zooxanthellae which had been freshly isolated from the hermatypic coral Acropora formosa (0.85±0.05x10^-10 mol min^-1 cell^-1) were lower than those of axenic cultures of the zooxanthellae from Montipora verrucosa (Acroporidae) grown under various nitrogen regimes (1.6 to 12x10^-10 mol min^-1 cell^-1). Maximum uptake rates were found for ammonium-starved cultured M. verrucosa zooxanthellae (10.2 to 12x10^-10 mol min^-1 cell^-1); M. verrucosa zooxanthellae growing with ammonium as nitrogen source and zooxanthellae which had been freshly isolated from A. formosa gave similar and considerably lower uptake rates (0.85 to 1.6x10^-1 mol min^-1 cell^-1). These results suggest that either coral tissue contains sufficient ammonium to repress synthesis of the uptake system of the algal symbionts or, alternatively, there are additional barriers to ammonium transport for zooxanthellae in vivo.     

In situ measurement of the urea decomposition rate and its turnover rate in the Pacific Ocean

The in situ decomposition rate of urea was measured using ¹⁴C-labelled urea at 3 areas in the North Pacific Ocean: Sagami Bay on the southern coast of central Japan, the northwestern Pacific central waters and the subarctic Pacific waters. The mean values of the decomposition rates of urea in surface waters of these areas were 44.5, 1.51 and 1.32 mol urea m^-3 d^-1, respectively. These rates decreased with depth. High rates of urea carbon incorporation into particulate matter and the CO₂ liberation from urea carbon into seawater were obtained in light bottles in the euphotic zone, while low rates were found in dark bottles. The turnover rates of urea in the 3 areas were calculated respectively as 12, 113 and 110 d at the surface, and the values increased with depth.     

Fractionated phytoplankton primary production,exudate release and bacterial production in a Baltic eutrophication gradient

The distribution of phytoplankton primary production into four size fractions (>10 m, 10-3 m, 3-0.2 m and <0.2 m), the utilization of algal exudates by bacteria and the bacterial production were studied in a eutrophication gradient in the northern Baltic proper. The polluted area exhibits substantially increased nutrient, especially nitrogen, levels while only minor differences occur in salinity and temperature regimes. Total primary production was 160 g C · m^-2 · yr^-1 at the control station and about 275 g C · m^-2 · yr^-1 at the eutrophicated stations. The estimated total exudate release was 16% of the totally fixed ¹⁴CO₂ in the control area and 12% in the eutrophicated area (including the estimated bacterial uptake of exudates). The difference in¹⁴CO₂ uptake rates between incubation of previously filtered water (<3, <2, <1 m) and unfiltered water was used to estimate bacterial uptake of phytoplankton exudates which were found to contribute about half of the estimated bacterial carbon requirement in both areas. Bacterial production was estimated by the frequency of dividing cells (FDC) method as being 38 g C · m^-2 · yr^-1 at the control station and 50 g C · m^-2 · yr^-1 at the eutrophicated stations. To estimate the mean in situ bacterial cell volume a correlation between FDC and cell volume was used. The increased annual primary production in the eutrophicated area was due mainly to higher production during spring and autumn, largely by phytoplankton cells (mainly diatoms) retained by a 10 m filter. Primary production duringsummer was similarin the two areas, as was the distribution on different size fractions. This could possibly explain the similar bacterial production in the trophic layers at all stations since the bulk of bacterial production occurs during summer. It was demonstrated that selective filtration does not quantitatively separate photoautotrophs and bacteria. A substantial fraction of the primary production occurs in the size fraction <3 m. The primary production encountered in the 3-0.2 m fraction was due to abundant picoplankton (0.5 to 8 · 10⁷ ind · l^-1), easily passing a 3 m filter. The picoplankton was estimated to constitute up to 25% of the total phytoplankton biomass in the control area and up to 10% in the eutrophicated area.     

Silicon and the ecology of marine plankton diatoms. II. Silicate-uptake kinetics in five diatom species

The variation of the rate of silicate uptake with varying silicate concentration in the medium was investigated in short-term experiments with the following marine diatom species:Skeletonema costatum, Thalassiosira pseudonana, T. decipiens, Ditylum brightwellii, andLicmophora sp. The uptake conformed to Michaelis-Menten kinetics only after a correction had been made for reactive silicate that apparently could not be utilized by the diatoms. The magnitude of this correction was in the range of 0.3 to 1.3 g-at Si/l. Mean values of the half-saturation constant of silicate uptake were calculated for the different species. The lowest value was found inS. costatum (0.80 g-at Si/l) and the highest inT. decipiens (3.37 g-at Si/l). Growth limitation by low silicate concentrations could be a cause of species succession in marine plankton-diatom blooms.