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.     

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.     

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 between light and temperature on the physiological ecology ofGracilaria tikvahiae (Gigartinales: Rhodophyta)

Main effects and interactions of light and temperature on rates of growth (), net photosynthesis (P_s), and dark respiration (R) of the red seaweedGracilaria tikvahiae were investigated in outdoor, nutrient-replete continuous-flow seawater culture chambers. Below 15°C,G. tikvahiae did not grow and between 15° and 30°C, both main effects and interactions of light and temperature on and P_s were significant, which explains the occurrence of this alga as a summer annual in its northern range. Temperature interacted with light (I) through its influence on the vs I and P_s vs I curves. The initial slope of the vs I curve, , the light saturation intensity, I_s, and maximum growth rate, _max, were all significantly lowerat 15°C compared to 20°, 25°, or 30°C. Maximum values of _max, the P_s:R ratio and the net photosynthesis:gross photosynthesis ratio (P_s:P_g) all occurred at 25°C, suggesting that this is the best temperature for growth ofG. tikvahiae. Values for P_max increased up to 30°C, indicating that the temperature for maximum growth and net photosynthesis are not the same forG. tikvahiae. Significant photoinhibition of growth and photosynthesis at full incident sunlight (I₀) occurred at 15°C but not at 20°, 25°, or 30°C. Steele's equation fit the 15°C vs I data best, whereas the hyperbolic tangent function fit the 20°, 25°, and 30°C data best. Main effects and interactionof light intensity and temperature on rates of R were also significant (P<0.001). R was highly intercorrelated with and P_s (0.86r0.94), indicating that R inG. tikvahiae is primarily regulated by growth rate and not temperatureper se. Environmental factors that regulate growth, such as light intensity, exert a great influence on R inG. tikvahiae.     

Nutrient enrichment and the ultrastructure of zooxanthellae from the giant clam Tridacna maxima

The separate and combined effects of ammonium (10M) and phosphate (2M) on the ultrastructure of zooxanthellae (Symbiodinium sp.) from giant clams, Tridacna maxima, were examined in the field. Nitrogen addition significantly changed the ultrastructure of the zooxanthellae inhabiting the clams. After 9 mo exposure, the cross-sectional area of zooxanthellae from N-treated clams was significantly lower than that from other treatments [N=39.3 m²; C=47.9 m²; P=43.2m²; N+P=44.5 m²; (P=0.001)]. There was also a significant decrease in the size of starch bodies, especially around the pyrenoid of the zooxanthellae from N and N+P treatments [N=1.2 m²; C=2.0 m²; P=1.8 m²; N+P=1.2 m²; (P=2.08E-11)]. This presumably occurs as a result of the mobilization of organic carbon stores in response to stimulated amino acid synthesis under enriched nutrient conditions. These data strongly suggest that the symbiotic zooxanthellae of clams are limited to some extent by the availability of inorganic nitrogen, and that relatively minor changes to the nutrient loading of the water column can have substantial effects on the biochemistry of symbioses such as that which exists between clams and zooxanthellae.     

Effect of ionizing radiation on haemoglobin of marine lamellibranchs

The effect of ionizing radiation on the iron-linked protein (haemoglobin) of the marine lamellibranchs Anadara granosa (Linn.) and Cardita antiquata (Lam.) from Bombay waters, India is discussed. Purified haemoglobin solutions were exposed to a ⁶⁰Cobalt source delivering a dose of about 4,600 rad/min, at the sample irradiation point. Radiation damage or degradation was measured spectrophometrically by studying changes in the absorption spectra following irradiation in the presence and absence of oxygen. Exposure to ionizing radiation in general caused a decrease in absorption of both haemoglobins, irrespective of location, viz extra-versus intracellular, at Soret (412 m), (540 to 42 m) and (574 to 76 m) peaks, and an increase in absorption at 510 and 630 m. Upon exposure to a higher dose, O₂Hb of C. antiquata showed an increase in absorption at the protein peak (280 m); exposure to a lower dose, however, resulted in decreased absorption. Furthermore, the changes following irradiation were dependent upon the initial state of the pigment. Oxyhaemoglobin, when exposed to radiation, oxidized to hemiglobin, and hemiglobin reduced to oxyhaemoglobin. The extracellular haemoglobin of high molecular weight (3x10⁶) of the false cockle C. antiquata was found to be extremely radio-resistant, whereas intracellular haemoglobin of low molecular weight (74,000) of the arcid clam A. granosa was highly radiosensitive, since it could not be exposed to doses exceeding 18,000 r.     

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.     

Grazing and ingestion rates of nauplii,copepodids and adults of the marine planktonic copepod Calanus helgolandicus

The average grazing and ingestion rates of all stages of the marine planktonic copepod Calanus helgolandicus (Calanoida) from nauplius stage IV to adults were measured experimentally at 15°C in agitated cultures. The chain-forming diatom Lauderia borealis and the unarmoured dinoflagellate Gymnodinium splendens were offered as food. The food concentrations were close to natural conditions and ranged from 36 to 101 g of organic carbon per liter. The medium body weights expressed in g of organic carbon of almost all larval stages raised at 49 g C/1 were identical with the weight of the same stages caught in the Pacific Ocean off La Jolla, California, USA. In a log-log system, grazing and ingestion rates increased almost linearly with increasing body weight. Grazing rates ranged from 4 to 21 ml/day/nauplius stage IV to 286 ml to 773 ml/day/female. Ingestion rates increased from 0.2 g to 0.8 g C/day/nauplius stage IV to 18 g to 69 g C/day/female. Grazing and ingestion rates per unit body weight decreased gradually with increasing body weight. The daily ingested amount of food decreased from 292 to 481% of the body weight (g C) of nauplius stage V to 28–85% of the body weight of adult females. Grazing and ingestion performances of all stages increased with increasing particle size. Grazing rates decreased and ingestion rates increased with increasing food concentrations. The published data on food intake of the different age groups of C. helgolandicus show that the young stages of herbivorous planktonic copepods can play a major part in the consumption of phytoplankton in the sea due to their high grazing and ingestion rates.     

Influence of three sediment types on copper toxicity to the polychaete Neanthes arenaceodentata

Adult male Neanthes arenaceodentata were exposed to 0.10±0.015 mg l^-1 copper in the seawater of a continuous-flow bioassay system in the presence of a sand, a mud, a mixture of the sand and mud, and no sediment, to assess the influence of sediment type on Cu-induced mortality. The sediment type did influence mortality. The time to 50% mortality was 7.8 days without sediment, 36.5 days with sand, 54.5 days with the mixture, and 50.0 days with mud. There was no threshold Cu body burden that caused death. The mean Cu concentration per gram of N. arenaceodentata (24 h after death) without sediment was 270 g, in sand 994 g, in the mixture 1047 g, and in mud 1464 g. The differences in the toxic responses are discussed.     

Feeding,growth and bioluminescence of the heterotrophic dinoflagellate Protoperidinium huberi

Feeding, growth and bioluminescence of the thecate heterotrophic dinoflagellate Protoperidinium huberi were measured as a function of food concentration for laboratory cultures grown on the diatom Ditylum brightwellii. Ingestion of food increased with food concentration. Maximum ingestion rates were measured at food concentrations of 600 g C l^-1 and were 0.7 g C individual^-1 h^-1 (1.8 D. brightwelli cells individual^-1 h^-1). Clearance rates decreased asymptotically with increasing food concentration. Maximum clearance rates at low food concentration were ca. 23 l ind^-1 h^-1, which corresponds to a volume-specific clearance rate of 5.9x10⁵ h^-1. Cell size of P huberi was highly variable, with a mean diameter of 42 m, but no clear relationship between cell size and food concentration was evident. Specific growth rates increased with food concentration until maximum growth rates of 0.7 d^-1 were reached at a food concentration of 400 g C l^-1 (1000 cells ml^-1). Food concentrations as low as 10 g C l^-1 of D. brightwellii (25 cells ml^-1) were able to support growth of P. huberi. The bioluminescence of P. huberi varied with its nutritional condition and growth rate. Cells held without food lost their bioluminescence capacity in a matter of days. P. huberi raised at different food concentrations showed increased bioluminescence capacity, up to food concentration that supported maximum growth rates. The bioluminescence of P. huberi varied over a diel cycle, and these rhythmic changes persisted during 48 h of continuous darkness, indicating that the rhythm was under endogenous control.     

Grazing,defecation and excretion rates of copepods from inter-island channels of the Canadian Arctic archipelago

Zooplankton (>800 m) and water samples were collected at night at eleven stations in Parry Channel and adjacent waters, between 28 August and 14 September 1986. Chlorophyll concentrations varied between 17 g l^-1 at the surface at one station in Wellington Channel, Canada, and 1.5 g l^-1 throughout the top 30 m at one station in Byam Martin Channel, Canada. In tows from 0 to 50 m the zooplankton community at all stations was dominated by varying proportions of three species,Calanus hyperboreus, C. glacialis andMetridia longa. Levels of chlorophyllderived pigments inC. hyperboreus Stages V, IV and III were correlated with the concentration of chlorophyll at the chlorophyll maximum. Defecation rate constants, measured for the different stages where present, were variable but not correlated with ambient chlorophyll concentrations. Ammonia excretion was measured simultaneously for the communities in which defecation was being measured in the invividual species and stages. In these experiments the rate of ammonia accumulation decreased significantly over the period during which the copepods were actively defecating (usually the first 3 h) and then tended to a constant level (over the next 18 to 20 h). The time courses of ammonia accumulation could be described by a model comprised of the sum of a straight line and a saturating curve. For seven experiments the ammonia release given by the asymptote of the saturating component was correlated with the estimate of community defecation, obtained by summing the individual defecations, suggesting that the two processes were closely associated. Weight specific defecation and weight specific defecation-associated ammonia excretion were both correlated with ambient chlorophyll concentration. The ratio of initial to basal ammonia excretion rate varied between 2 and 20, so that defecation-associated ammonia release may be important in the estimation of in vivo nitrogen excretion or regeneration rates.     

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     

Diurnal patterns in photosynthetic capacity and depth-dependent photosynthesis-irradiance relationships inSynechococcus spp. and larger phytoplankton in three water masses in the Northwest Atlantic Ocean

The photosynthetic characteristics of prokaryotic phycoerythrin-rich populations of cyanobacteriaSynechococcus spp. and larger eukaryotic algae were compared at a neritic frontal station (Pl), in a warm-core eddy (P2), and at Wilkinson's Basin (P3) during a cruise in the Northwest Atlantic Ocean in the summer of 1984.Synechococcus spp. numerically dominated the 0.6 to 1 m fraction, and to a lesser extent the 1 to 5 m size fractions, at most depths at all stations. At P2 and P3, all three size categories of phytoplankton (0.6 to 1 m, 1 to 5 m, and >5 m) exhibited similar depth-dependent chages in both the timing and amplitude of diurnal periodicities of chlorophyllbased and cell-based photosynthetic capacity. Midday maxima in photosynthesis were observed in the upper watercolumn which damped-out in all size fractions sampled just below the thermocline. For all size fractions sampled near the bottom of the euphotic zone, the highest photosynthetic capacity was observed at dawn. At all depths, theSynechococcus spp.-dominated size fractions had lower assimilation rates than larger phytoplankton size fractions. This observation takes exception with the view that there is an inverse size-dependency in algal photosynthesis. Results also indicated that the size-specific contribution to potential primary production in surface waters did not vary appreciably over the day. However, estimates of the percent contribution ofSynechococcus spp. to total primary productivity in surface waters at the neritic front were significantly higher when derived from short-term incubator measurements of photosynthetic capacity rather than from dawn-to-duskin situ measurements of carbon fixation. The discrepancy was not due to photoinhibitory effects on photosynthesis, but appeared to reflect increased selective grazing pressure onSynechococcus spp. in dawn-to-dusk samples. Low-light photoadaptation was evident in analyses of the depth-dependency ofP-I parameters (photosynthetic capacity,P _max; light-limited slope, alpha;P _max alpha,I _k ; light-intensity beyond which photoinhibition occurs,I _b ) of the > 0.6 m communities at all three stations and was attributable to stratification of the water column. There was a decrease in assimilation rates andI _k with depth that was associated with increases in light-limited rates of photosynthesis. No midday photoinhibition ofP _max orI _b was observed in any surface station. Marked photoinhibition was detected only in the chlorophyll maximum at the neritic front and below the surface mixed-layer at Wilkinson's Basin, where susceptibility to photoinhibition increased with the depth of the collected sample. The 0.6 to 1 m fraction always had lower light requirements for light-saturated photosynthesis than the > 5 m size fraction within the same sample. Saturation intensities for the 1 to 5 m and 0.6 to 1 m size fractions were more similar whenSynechococcus spp. abundances were high in the 1 to 5 m fraction. The > 5 m fraction appeared to be the prime contributor to photoinhibitory features displayed in mixed samples (> 0.6 m) taken from the chlorophyll maxima. InSynechococcus spp.-dominated 0.6 to 1 and 1 to 5 m size fractions, cellular chlorophylla content increased 50- to 100-fold with depth and could be related to increases in maximum daytime rates of cellularP _max at the base of the euphotic zone. Furthermore, the 0.6 to 1 m and > 5 m fractions sampled at the chlorophyll maximum in the warm-core eddy had lower light requirements for photosynthesis than comparable surface samples from the same station. Results suggest that photoadaptation in natural populations ofSynechococcus spp. is accomplished primarily by changing photosynthetic unit number, occuring in conjuction with other accommodations in the efficiency of photosynthetic light reactions.     

Temporal variation and distribution of trace metals in freshwater and fish from Calabar River, S.E. Nigeria

An investigation on the abundance and distribution of trace metals (Fe, Cu, Zn, Mn, Cr, Cd and Pb) in water, and nine species of fish samples from Calabar river was carried out in 1992. The concentrations of iron (6000–7240gl^–1), zinc (4910–7230gl^–1), and cadmium (3–7gl^–1) showed moderate pollution while those of copper (420–630gl^–1), manganese (23–48gl^–1), chromium (<10–20gl^–1) and lead (<1–10gl^–1) in water were well below WHO permissible levels. Significant seasonal changes (0.001p0.25) were obtained for iron, copper, zinc, manganese and cadmium in water. Furthermore, iron, zinc and cadmium showed statistically significant spatial changes (0.005p0.10). Of the nine fish species studied, no statistically significant relationship between body weight and the concentrations of the metals was observed. The concentrations of the metals per mean total body weight apparently decreases in the order Fe>Zn>Cu>Mn>Pb>Cd=Cr and were within the limits that were safe for consumption.     

Effect of pentachlorophenol (PCP) on meiobenthic communities established in an experimental system

Aquaria containing clean sand received a continuous supply of seawater from Santa Rosa Sound, Florida, USA, mixed with known concentrations (7, 76 and 622 g l^-1) of pentachlorophenol (PCP). After 9 weeks, nematodes accounted for 87% of the total meiofauna. Nematode biomass and densities were greatest in aquaria exposed to 76 g PCP l^-1 and were least in aquaria exposed to 622 g PCP l^-1. Epistrate feeders were abundant in control aquaria and aquaria exposed to 7 and 76 g PCP l^-1, but not in aquaria exposed to 622 g PCP l^-1. Selective deposit feeders were not abundant in the control aquaria and aquaria exposed to 7 g PCP l^-1, but comprised 19% of the nematodes in aquaria exposed to 76 g PCP l^-1 and 61% in aquaria exposed to 622 g PCP l^-1.     

Automated procedure for determination of dissolved organic nitrogen and phosphorus in aquatic environments

A method is described for estimating dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) in aquatic environments. The method is based on ultraviolet oxidation under successive acid and alkaline conditions, and uses continuous-flow techniques. A number of pure organic nitrogen compounds were tested in the concentration range 2 to 40 g-at Nl^-1. At the 10 g-at Nl^-1 level, recovery ranged from 60 to 100% in deionized water, and from 40 to 80% in seawater (30 S). In general, recovery decreased with increasing nitrogen concentration. For pure organic phosphorus compounds, recovery ranged from 73 to 100% at the 5 g-at Pl^-1 level. An application of the method to the surface water of a clay pond along the Atlantic coast of France for a period of 72 h under natural irradiance revealed very strong diel variations of dissolved organic nitrogen, but no significant trends for dissolved organic phosphorus.     

ATP concentration and adenylate energy charge of tropical zooplankton from waters inside the great barrier reef

The adenylate energy charge ratio (EC) of 8 species of zooplankton ranged from 0.70 to 0.89. Eucalanus subcrassus had significantly (P<0.05) lower EC when captured during a 30-min tow than during a 3-min tow, indicating stress at capture. The concentration of ATP in the 8 species ranged from 9.1 to 36.0 g/mg protein and from 0.17 to 15.7 g/mg dry weight. Excluding a low value for the ctenophore Pleurobrachia sp., the average concentration was 8.1 g ATP/mg dry weight, which is equivalent to an approximate C/ATP ratio of 50.Contribution from the Australian Institute of Marine Science, Australia, and the Institute of Marine Biology, University of Bergen, Norway     

The methane mussel: roles of symbiont and host in the metabolic utilization of methane

Methane mussels (Bathymodiolus sp., undescribed; personal communication by R. Turner to CRF) were collected in September 1989 and April 1990 from offshore Louisiana in the Gulf of Mexico. These mussels contain endosymbiotic methane-oxidizing bacteria and are capable of utilizing environmental methane as a source of energy and carbon. Oxygen consumption, methane consumption, and carbon dioxide production were measured in mussels with intact symbionts, functionally aposymbiotic mussels, and separated symbiont preparations under controlled oxygen and methane conditions, in order to study the roles of the symbionts and the hosts in methane utilization. The association was found to be very efficient in fixing methane carbon (only 30% of CH₄ consumed is released as CO₂), and to be capable of maximal rates of net carbon uptake of nearly 5 mol g^-1 h^-1. Rates of oxygen and methane consumption were dependent upon oxygen and methane concentrations. Maximal consumption rates were measured at 250 to 300 M O₂ and 200 to 300 M CH₄, under which conditions, oxygen consumption by the gill tissues (containing symbionts) had increased more than 50-fold over rates measured in the absence of methane. A model is proposed for the functioning of the intact association in situ, which shows the symbiosis to be capable of achieving growth rates (net carbon assimilation) in the range of 0.003 to 0.50% per day depending upon oxygen and methane concentrations. Under the conditions measured in the seep environment (200 M O₂, 60 M CH₄), a mussel consuming methane at rates found to be typical (4 to 5 mol g^-1 h^-1) should have a net carbon assimilation rate of about 0.1% per day. We suggest that the effectiveness of this symbiosis arises through integration of the morphological and physiological characteristics inherent to each of the symbiotic partners, rather than from extensive specialization exhibited by other deep-sea chemotrophic associations.     

Lead and cadmium in urban allotment and garden soils and vegetables in the United Kingdom

In order to assess the intake of lead and cadmium by consumers of home grown vegetables in urban areas, replicated experimental plots of uniform size, comprising summer and winter crops, were established in 94 gardens and allotments in nine towns and cities in England.The geometric mean lead and cadmium concentrations for the soils (n = 94) were 217 g g^–1 (ranging from 27 to 1,676 g g^–1) and 0.53 g g^–1 (<0.2–5.9 g g^–1), respectively. Compared with agricultural soils, the garden and allotment soils contained elevated levels of lead but not cadmium.Lead concentrations in the vegetables ranged from <0.25 g g^–1 to 16.7 g g^–1 dry weight and cadmium concentrations ranged from <0.025 g g^–1 to 10.4 g g^–1 dry weight. Lead concentrations were higher than reported background levels, although <1% exceeded the statutory limit for saleable food in the UK (1 g g^–1 fresh weight). Cadmium concentrations were generally similar to background levels.     

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.