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.     

Glucose uptake in ocean profiles with special reference to temperature

Glucose uptake was measured at three different stations in the northwestern Pacific Ocean. The measurements were made at various temperatures (5° to 35°C) and glucose concentrations (3 to 100 g glucose/l). The uptake of almost all samples taken from the surface to 1,500 m was affected primarily by temperature. The temperature dependence of samples from near the transition region between the Kuroshio and the Oyashio current waters was twice as large as that of both current samples. Furthermore, the samples from the former, transition, region and the aphotic samples of the latter, current, regions showed an optimal temperature 15° to 30°C higher than the in situ temperature; the optimal temperature was only 5° to 10°C higher in the photic samples of the stable region. The average potential glucose uptake at/near the optimal temperature was 0.026 g glucose/l/h in the photic zone and 0.015 g glucose/l/h in the aphotic zone. However, after correction by the in situ temperature, the uptake was 0.024 and 0.003 g glucose/l/h, respectively.     

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.     

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.     

Capillary supply and mitochondrial volume density in the axial muscles of the mesopelagic teleost Argyropelecus hemigymnus

Quantitative measurements have been made on the ultra-structure and capillary supply to the axial muscles of the mesopelagic hatchet fish Argyropelecus hemigymnus (Cocco, 1829). Fish were collected at Eastern North Atlantic Ocean Station 10244, 32°48N; 31°15W during November 1980, from a depth of 480 to 550 m. Mitochondria with densely packed cristae occupy 44.3% of slow-fibre volume. Each myofibril is in direct contact with a mitochondrion. Compared with other fishes studied, the capillary supply to A. hemigymnus slow fibres is poorly developed. The average number of capillaries per fibre is 0.9, such that each m of capillary contact supplies 0.011 m² of fibre cross-sectional area. The capillary surface area (m²) supplying 1 m³ of slow-fibre mitochondria is 0.17 in anchovy (Engraulis encrasicolus), 0.14 in rat-fish (Chimaera monstrosa), 0.14 in tench (Tinca tinca), 0.16 in catfish (Clarias mossambica), and only 0.025 for A. hemigymnus. It is suggested that, relative to the former species, some modifications in factors determining tissue oxygenation (e.g. myoglobin concentrations, blood flow, perfusion distribution or haemoglobin) and/or mitochondrial respiration rate are required in order to match oxygen supply and demand to the slow muscle in A. hemigymnus.     

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.     

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.     

Growth and survival of Mytilus edulis larvae exposed to low levels of dibutyltin and tributyltin

Two studies were conducted to observe effects of dibutyltin (DBT) and tributyltin (TBT) on larvae of Mytilus edulis for an exposure period of 25 d. Endpoints for evaluation were shell growth and mortality measured at 33 d. Larvae were cultured in a new laboratory assay chamber in a recirculating static test. The control, 2, 20, and 200 g/l DBT-treated populations had mean shell lengths of 527, 523, 417, and 180 m, respectively. Survival was 1% for the 200 g/l DBT-treated population, but ranged from 73 to 83% for controls, 2, and 20 g/l treatments. The no-observed-effect concentration (NOEC) was 2 g/l for DBT, while the lowest-observed-effect concentration (LOEC) was 20 g/l. The chronic toxicity value was 6.3 g/l. In the TBT bioassay, mean shell lengths for the control, 0.006, 0.050, and 0.130 g/l-treated populations were 565, 437, 385, and 292 m, respectively. Control survival was 74%, whereas TBT-treated populations survival ranged from 52 to 58%. The NOEC for TBT was 0.006 g/l TBT and the LOEC was 0.050. A chronic toxicity value of 0.017 g/l was calculated. The results of this study indicated that the toxicity of DBT was less than that of TBT. It was concluded that shell length was inversely related to exposure level in both DBT and TBT bioassays. In this study, we have observed TBT effects at lower exposure levels in the laboratory than previously reported, and also report the first data for DBT effects on mussel larvae.     

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.     

Size composition of particulate organic matter in the lagoon of Tikehau atoll (Tuamotu archipelago)

Suspended particulate matter was comprehensively investigated from 6 to 17 April 1986 in the lagoon of Tikehau atoll (15°00S; 148°10W). Dry weight (DW), particulate organic carbon (POC), adenosine triphosphate (ATP), and chlorophyll a were measured for five size-classes (0.2 to 0.8 m, 0.8 to 3 m, 3 to 35 m, 35 to 200 m, and 200 to 2000 m). Taxa were identified and counted for the whole plankton (both autotrophic and heterotrophic). Particles <3 m accounted for 81% of the total POC (192 mg m^-3), and detritus comprised 82% of the total POM. Phytoplankton (cyanobacteria plus algae) accounted for 35% of the living carbon, 75% of which consisted of heterotrophic bacteria and cyanobacteria. The zooplankton biomass was composed of 31% nano-, 26% micro-, and 43% mesoplankton.     

Size classes and major taxonomic groups of phytoplankton at two locations in the subarctic pacific ocean in May and August, 1984

In order to assess the relative importance of the pico- and nanoplankton fractions, the composition of entire phytoplankton communities at Weathership Station P (50°N; 145°W) and at 53°N; 145°W were studied in May and August, 1984, using epifluorescence, scanning electron, and inverted light microscopy. The biomass of major taxa within five size classes was estimated from cell volume and cell concentration. For both months, approximately twothirds of the total phytoplankton carbon were contributed by cells<5 m. In May, 16% of plant biomass was contributed by cells<2 m, and in August 39%. (In both months 90% of plant carbon<2 m was contributed by the bluegreen coccoid Synechococcus spp.) Cells 2 to 5 m contributed about 39% to total plant carbon; they were mostly flagellates in May and nonmotile coccoids in August. The remaining one-third of algal carbon was composed of dinoflagellates, cryptomonads, other flagellates and diatoms, all >5 m. Very little difference between taxa was observed with respect to vertical stratification. Small taxonomic changes were observed in the community between May and August, and within each month.Contribution No. 1694 of the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

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.     

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 heavy metals (Zn,Hg, Cu,Cd, Pb,Ni) on the length growth of Mytilus edulis

The shortterm (10–22 d) effect of Zn, Hg, Cu, Cd, Pb, and Ni on the length growth of Mytilus edulis is studied. Significant reductions of growth rate was found at 0.3 g Hgl^-1, 3 g Cul^-1, 10 g Znl^-1, and 10 g Cdl^-1 added to the local sea water, while concentrations of up to 200 gl^-1 of Pb and Ni had no effect on the growth. With exposure to Cu and Zn, there was a linear reduction in growth rate with increasing metal concentration up to about 6 g Cul^-1 and 100 g Znl^-1. Above these levels, growth stopped with Cu, while with Zn it was stabilized at about 20% of control growth. When Hg and Cd were added, a curvilinear relationship between growth and metal concentration is indicated. With Hg, growth rate is nearly zero above 3–4 g Hgl^-1, while the growth rate was 50% of control after 10 d of exposure to 100 g Cdl^-1. At 2 g Cdl^-1 there was a significant stimulation of length increase. Observed EC₅₀-values for growth were 0.3–0.4 g Hgl^-1, 3–4 g Cul^-1, 60 g Znl^-1, and 100 g Cdl^-1.     

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.     

Calcification in the maerl coralline alga Phymatolithon calcareum: Effects of salinity and temperature

The coralline alga Phymatolithon calcareum was dredged from 13 m in the Kattegatt, Baltic Sea, in December, 1980, and its rate of calcification was measured by ⁴⁵Ca⁺⁺-uptake methods. Light-saturated calcification rates at 5°C ranged from 15.8 g CaCO₃ g^-1 dry wt h^-1 for the basal parts of the plants to 38.7 g CaCO₃ g^-1 dry wt h^-1 for the tips. These age gradients were not apparent when calcification rates were expressed on the basis of surface area. Experiments with salinity (10, 20, 30) and temperature (0°, 5°, 10°, 20°C) indicated that optimum conditions for calcification were at 30 S and at temperatures above 10°C. Salinity had a greater influence on calcification rate than did temperature, and there was a positive relationship between salinity and calcification rate at all temperatures. In 6 mo old cultures, salinity was again the important factor, with all plants remaining healthy at 30 except those at the highest temperature (20°C). These trends, and the low calcification rates at 10S (4.6 g CaCO₃ g^-1 dry wt h^-1 at 5°C to 8.6 g CaCO₃g^-1 dry wt h^-1 at 20°C) suggest that low salinity may be the explanation for the general absence of P. calcareum from the brackish waters of the Baltic Sea. Short-term experiments in which salinity was kept constant while Ca⁺⁺ concentration was altered, and experiments in which salinity was varied and Ca⁺⁺ concentration kept constant, suggest that it is the calcium ion concentration and not salinity per se which affects calcification rates.     

Biological production of nitrite in seawater

The relative importance of 3 different sources for biological production of nitrite in seawater was studied. Decomposition of fecal pellets of the copepod Calanus helgolandicus (at a concentration of approximately 12 g-at N/l), in seawater medium, released small amounts of ammonia over a 6 week period. It nitrifying bacteria were added to the fecal pellets nitrite was barely detectable over the same period. Decomposition of phytoplankton (present at a concentration of about 8 g-at particulate plant N/l) with added heterotrophic bacteria, released moderate amounts of ammonia over a 12 week period. If the ammonia-oxidizing bacterium Nitrosocystis oceanus was added to the decomposing algae, nitrite was produced at a rate of 0.2 g-at N/l/week. Heterotrophic nitrification was not observed when 7 open-ocean bacteria were tested for their ability to oxidize ammonia. The diatom Skeletonema costatum, either non-starved or starved of nitrogen, produced nitrite when growing with 150 or 50 g-at NO ₂ ^- -N/l at a light intensity of about 0.01 ly/min. When nitrate in the medium was exhausted, S. costatum assimilated nitrite. If starved of vitamin B₁₂, both non-N-starved and N-starved cells of S. costatum produced nitrite in the medium with 150 g-at NO ₃ ^- -N/l. Nitrate was not exhausted and cell densities reached 2x10⁵/ml due to vitamin B₁₂ deficiency. If light intensity was reduced to 0.003 ly/min under otherwise similar conditions, cells did not grow due to insufficient light, and nitrite was not produced. In the sea, it appears that, in certain micro-environments, decomposition of particulate matter releases ammonia with its subsequent oxidation to nitrite. The amounts of these nutrients and the rate at which they are produced are dependent upon the nature of the materials undergoing decomposition and the associated bacteria. In certain other areas of the sea, where phytoplankton standing stock is high and nitrate is non-limiting, excretion by these organisms is a major source of nitrite.     

Diurnal patterns of size-fractioned primary productivity across a coastal front

In July 1985, diurnal patterns of photosynthesis and pigmentation were characterized for whole water (>0.4 m) and size-fractioned (>5 m and 0.4 to 5 m) communities from three light depths sampled across a coastal thermal front in the Southern California Bight. Samples were collected predawn and held for 20 h in deck incubators. Variations in chlorophyll a and accessory pigment-to-chlorophyll a ratios showed no obvious diurnal trends. Timing of peak photosynthetic potential (P _max) and its coincidence with variations in light-limited rates of photosynthesis (alpha), as well as diurnal amplitudes in P _max and alpha, often differed between size fractions sampled within the same community. The same was true for identical size fractions collected from different depths and stations transecting the front. Primary productivity was 20-fold greater on the cold water side, where >5 m diatoms dominated the mixed layer and accounted for 80% of daytime productivity. Diatoms collected from the top and bottom of the upper mixed layer displayed nearly identical diurnal patterns in P _max and alpha, with midday peaks exceeding predawn values by four-fold and two-fold respectively. Above the pycnocline, the 0.4 to 5 m fraction had lower assimilation rates than the >5 m fraction and smaller diurnal amplitudes in P _max and/or alpha, with daytime patterns often characterized by two peaks interspersed by a short period of photoinhibition. Within the front, the 0.4 to 5 m fraction accounted for two-thirds of plant biomass and >90% of primary production. Pigment analyses by high-performance liquid chromatography revealed enrichment in 19-hexanoyloxyfucoxanthin, indicative of enhanced numbers of prymnesiophtes. Photosynthetic activity in confined surface communities was susceptible to daytime photoinhibition, but subsurface communities exhibited midday P _max peaks that were three-to seven-fold predawn values. In the warm-water mass, both algal size fractions contributed equally to photosynthesis and chlorophyll a in surface waters, with the 0.4 to 5 m fraction becoming dominant at the base of the euphotic zone. At all depths, peak P _max of the 0.4 to 5 m fraction occurred before noon, while P _max of the >5 m fraction was clearly evident in the afternoon. Elevated chlorophyll b-, 19hexanoyloxyfucoxanthin- and zeaxanthin-to-chlorophyll a ratios indicated a mixture of algal groups, including chlorophytes, cyanobacteria and prymnesiophytes.     

Hydrobiological parameters during an annual cycle in the Arcachon Basin

Temperature, salinity, nutrients and phytoplankton biomass were monitored on a weekly to bimonthly frequency at six stations in the Bay of Arcachon from July 1984 to July 1985. This particular period appears to have differed from the last ten years in displaying higher amplitudes of both temperature and salinity (1.5° to 24.5°C and 20 to 34.5) at high tide. However, although in spring both temperature and salinity were normal (14°C, 28), an important phytoplankton spring bloom occurred, with maximum chlorophyll levels reaching 15 g l^-1 in April. Utilization of nutrients was high, particularly for nitrate and silicate, the concentrations of which decreased, respectively, from 10–15 to 0.1–2 M and 10–20 to 0.25–3 M from February to May. Exhaustion of nitrate was observed in May, except in areas subjected to river input. In contrast, silicate increased throughout the study area from May to July.     

Photoacclimation of Ulva rotundata (Chlorophyta) under natural irradiance

Two vegetative clones (designated 11/85 and 7/86 in accordance with month/year of collection) of the green macroalga Ulva rotundata were collected in the vicinity of Beaufort, North Carolina, USA. Each was grown in an outdoor continuous-flow system in summer (20°C) of 1986 and late winter (10° to 17°C) of 1987, in irradiances ranging from 9 to 100% of full sunlight, with and without NH ₄ ⁺ enrichment. Continuous enrichment of influent estuarine water (dissolved inorganic nitrogen 2 M, N:P5) to 8–12 M NH ₄ ⁺ had only a slight effect on growth rate. Temperature changes of 2 to 3°C had a much greater effect. Prolonged exposure to a given daily irradiance resulted in acclimation, exposure to a given daily irradiance resulted in acclimation, indicated by faster growth of conditioned plants relative to those transferred from a different irradiance. Most of the difference in growth rates between transferred and control plants was attributed to differences in thallus absorptance. Growth was photoinhibited above 40% sunlight at temperatures below 15°C, but not above 20°C. Following interday irradiance transfers, thallus percent dry weight changed in a manner that suggests different response times for photosynthesis and cell division.