Spring phytoplankton in the Subarctic North Pacific Ocean

The report includes quantitative and qualitative data on the phytoplankton from the First Canadian Transpacific Oceanographic Cruise from March to May 1969. Nanoplanktonic species (<20 m) were numerically dominant and, together with nertic diatoms, made up the larger portion of the biomass at all stations outside the Western Subarctic and Alaskan Gyres, where Denticulopsis seminae predominated. The nanoplankton consisted mainly of haptophycean, dinophycean and cryptophycean flagellates. Tropical oceanic dinoflagellates and diatoms were abundant in the net samples from Station 17 in the warm Kuroshio current but absent from Stations 24–27 in the Western Subarctic Gyre, Stations 2, 37–39 in the Alaskan Gyre, in which cold water oceanic species predominated, and Stations 31 and 33, south of the Aleutians. Subsurface maxima were common on the outward bound (westerly) leg in March.     

Siliceous nanoplankton. II. Newly discovered cysts and abundant choanoflagellates from the Weddell Sea,Antarctica

Siliceous choanoflagellates and previously undescribed, siliceous cyst-like spheroids (in the size range 2.5 to 15 m) were found below sea-ice in the Weddell Sea and studied with scanning electron microscopy. Cell counts from water samples obtained over a 10,000 km area indicated that both spheroids and choanoflagellates were abundant in the upper 100 m of the water column, averaging about 10⁵ cells l^-1. The large numbers of phagotrophic choanoflagellates suggest that bacterioplankton-picoplankton food chains are important and may indicate the presence of longer, more complex trophic networks than previously considered in Antarctic waters. Circumstantial evidence suggests that the spheroids may be the cysts of choanoflagellates; if so, the production of siliceous cysts by members of this enigmatic taxon may be of considerable interest in discussions of the phylogenetic position of the group in relation to the algae and the metazoans. Similar cysts were found simultaneously in the North Pacific Ocean (see preceding paper: Booth et al. 1980), and recently we have discovered the cysts in the upwelling area of the eastern tropical Pacific Ocean. The occurrence of the cyst in such geographically distant localities suggests that they have a worldwide distribution. Since the minute cysts are easily overlooked on the light microscope, their recognition and further study undoubtedly require the electron microscopic examination of plankton samples.     

Grazing of Acartia hudsonica (A. clausi) on Skeletonema costatum in Narragansett Bay (USA): Influence of food concentration and temperature

Grazing experiments were performed with temperatureacclimated Acartia hudsonica fed the diatom Skeletonema costatum in concentrations ranging from 50 to 3×10⁴ cell ml^-1 at 5°, 10° and 15°C. The ingestion data were best fit by an Ivlev equation. Feeding threshold values of 39 and 59 cells ml^-1 were not significantly different from zero; however, filtration rates were depressed at low food concentrations. Maximum filtration rates increased exponentially with temperature, reaching a maximum with copepods collected at 14°–15°C, and then declining. Both the increase in ingestion rate with increasing food concentration and the maximum ingestion rate were significantly greater as experimental temperature was increased. Maximum ingestion rates were reached at concentrations greater than 6×10³ cells ml^-1. Percent of body carbon ingested per day at 5 g C L^-1 increased from 1.5% at 5°C to 6.7% at 15°C. At 500 g C L^-1, the ingestion increased from 84% (5°C) to 660% (15°C). Percent of body nitrogen at 0.5 g N L^-1 increased from 0.6% per day at 5°C to 2.5% per day at 15°C. At 50 g N L^-1, the ingestion was 42% body nitrogen at 5°C and 250% at 15°C. The influence of grazing by A. hudsonica on phytoplankton in Narragansett Bay, USA was estimated for 1972–1977. The percent of standing stock removed by grazing rarely exceeded 5% per day except during the late spring when S. costatum growth becomes nutrient limited and higher temperatures favor the rapid population growth of A. hudsonica.     

Spatial mesoscale patterns of West Pacific picophytoplankton as analyzed by flow cytometry: their contribution to subsurface chlorophyll maxima

Vertical distributions of picophytoplankton (ppp) (<2 m) were studied by ship-board flow cytometry during two cruises in Western Pacific waters to Palau and to Australia in 1990. Weak red-fluorescing small ppp, supposed to be free-living prochlorophytes (Chisholm et al. 1988), were abundant in the area surveyed. These ppp, designated the prochlorophytes, were abundant in the surface waters (>10⁴ cells ml^-1) at the northern region (27°03N; 7°11N) in November, whereas in December at the southern tropical stations (0°23.54S; 9°20.30S; 13°50.6S), they formed subsurface maximum layers (>10⁵ cells ml^-1) on a nitracline at a depth of 3.5 to 5.4% surface irradiation. Their fluorescence intensity increased with depth below 10% surface irradiation. The prochlorophytes at a depth of 1% surface irradiation had ten times higher fluorescence than those at the surface layer. The total fluorescence intensity of the prochlorophytes accounted for 32 to 63% of the sum of the total fluorescence intensity of all fluorescing phytoplankton detected at subsurface chlorophyll maxima in the tropical area. These results suggest that distribution of the prochlorophytes is greatly affected by nitracline and by light intensity and that their chlorophyll is a major contributor to the subsurface chlorophyll maximum in the pelagic West Pacific Ocean.     

Spring and summer growth rates of subarctic Pacific phytoplankton assemblages determined from carbon uptake and cell volumes estimated using epifluorescence microscopy

In order to determine whether phytoplankton growth rates were normal or depressed, total plant carbon (g l^–1) and in situ production rates (g C l^–1 d^–1) were measured for phytoplankton assemblages at Weathership Station P (50°N; 145°W) and at 53°N; 145°W in the subarctic Pacific in May and August 1984. Plant carbon, estimated from cell volumes determined using epifluorescence microscopy, was distributed as follow: 28% in the <2 m fraction, 38% in the 2 to 5 m size fraction, and the remainder in size classes >5 m. Carbon-specific growth rates (k), as doublings d^–1, were calculated for the phytoplankton assemblages as a whole at each sampling depth down to 100 m for three days in May and for four days in August. The populations in the upper part of the euphotic zone showed average doubling rates of 1 d^–1 and thus appeared to be growing at rates normally expected for the prevailing conditions of light and temperature. The low chlorophyll concentrations (0.3 to 0.4 mg chl a m^–3) characteristically found in this oceanic region do not seem to be due to very slow growth of algal populations.Contribution No. 1695 of the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

Nitrogen regeneration by the surf zone penaeid prawn Macropetasma africanus

Nitrogen excretion of individual Macropetasma africanus (Balss) from an exposed beach/surf zone in Algoa Bay, South Africa was monitored under laboratory and field conditions in relation to body mass, temperature and feeding during 1985. Excretion rate experiments were performed on starved prawns at 15°, 18°, 20° and 25°C, as well as on individuals fed on four different diets (mussel, fish, shrimp and natural diet) at 15° and 20°C. The ratios of the excreted compounds to total nitrogen excreted were similar for the four diets despite differences in their nitrogen content and in the amount of food consumed. At 15° and 20°C, ammonia excretion rates of fed individuals were four to seven times higher than in starved prawns. the excretion rates were not correlated with nitrogen content of diets. M. africanus recycles 1 557 g NH₄–N per metre strip per year or 1 832 g total nitrogen m^-1 yr^-1, which constitute 12 and 14%, respectively, of total phytoplankton requirements of the surf zone. This study indicates that large motile crustaceans, when abundant, can play an important role in nutrient recycling in turbulent marine environments.     

Growth rate and reproductive differences in a widespread estuarine harpacticoid copepod (Scottolana canadensis)

We demonstrate the presence of significant genetically based differentiation in growth rate (g dry weight d^-1) and reproductive traits (percent reproductive females and mean clutch size g dry weight^-1) among females of an harpacticoid copepod (Crustacea),Scottolana canadensis (Willey), taken from a broad range of latitudes and reared in the laboratory under the same conditions. As temperature increases (15°–25° C), the growth rate of southern-derived copepods continues to increase, while that of northern-derived copepods levels off or decreases. Southern-derivedS. canadensis also have a higher percentage of reproducing females at high temperature (25°C) when rations (cells ml^-1) are reduced, while northern-derived females are at an advantage at low temperature (15°C). Both life-history traits indicate local adaptation to maximize scope for growth and reproduction at prevailing temperatures. The data support our hypothesis that evolution has occurred to maximize feeding minus metabolic energy expended, and that this maximization requires changes in feeding efficiency with differing temperatures.     

Effects of suspended sediments on egg production of the calanoid copepod Acartia tonsa

The estuarine copepod Acartia tonsa was collected on several occasions between 4 April and 14 August 1985 from Terrebonne Bay, Louisiana (29°08N; 90°36W) and the effects in its diet of suspended sediments, collected from the same area, were measured at five different concentrations of sediment (100 to 1 000 ppm) and six phytoplankton concentrations (500 to 13 000 cells ml^-1 Thalassiosira weissflogii). Egg production rate was used as an index of diet quality. At low phytoplankton concentrations (500 cells ml^-1), and at intermediate phytoplankton concentrations (2 000 cells ml^-1) for previously starved copepods, egg production was reduced by up to 40% at a sediment concentration of 250 ppm and further reduced at higher sediment concentrations. At higher food concentrations (4 000 to 13 000 cells ml^-1), suspended sediment had no effect on egg production rates at sediment concentrations up to 500 ppm. Rates were reduced only at the highest sediment concentration of 1 000 ppm. Under most natural conditions, suspended sediment would not significantly affect egg production rates in A. tonsa.     

Distribution of the marine littoral Foraminifera in Argentina,Uruguay and Southern Brazil

Benthic foraminifers collected in the marine littoral zone (sensu lato) of Argentina, Uruguay and Southern Brazil have been studied and compared with the Foraminifera collections from the same area studied previously. Two figures have been prepared, which show the geographical range and relative abundance of the most important species (cosmopolitan and non-cosmopolitan). The following biogeographical conclusions were drawn on the basis of an analysis of these figures. The whole area, from a latitude of about 32° S up to the southern coast of Tierra del Fuego, should be considered as one biogeographical unit (a kingdom of Buccella peruviana, sensu lato). This unit has 2 internal boundaries, which are located approximately at latitudes 41° and 52° S. They divide the whole area into 3 sub-areas which can be called (from North to South): North Patagonian (32° to 41° S), South Patagonian (41° to 52° S) and Malvinian (52° S—southern coast of Tierra del Fuego). With the exception of Buccella peruviana, sensu lato, which is typical of all 3 sub-areas, these sub-areas are characterized by the following faunal features. The North Patagonian sub-area contains abundant numbers of Elphidium discoidale. The South Patagonian sub-area is characterized by abundant numbers of Elphidium macellum. The Malvinian sub-area contains abundant numbers of Elphidium macellum, and also several very typical representatives of the Malvin current water, for instance, Cassidulina crassa, sensu stricto, Angulogerina angulosa, Uvigerina bifurcata, Cassidulinoides parkerianus, Ehrenbergina pupa and others. It is a matter of opinion whether the sub-areas cited be considered biogeographical sub-provinces, or even smaller units. They cannot be interpreted as biogeographical provinces.Contribution No. 62 of the Puerto Deseado Marine Biological Station.     

Quantitative and qualitative observations on Laminaria dentigera and other subtidal kelps of southern Kodiak Island,Alaska

Observations were made and quantitative samples of non-canopy kelps were collected, using SCUBA, at 9 subtidal sites off southern Kodiak Island in May, 1976. At a 10th site, only observations were made. The most abundant species in the quadrats were Laminaria dentigera, L. yezoensis, Pleurophycus gardneri, Agarum cribrosum, and Alaria marginata. We found fertile plants of all these species. L. dentigera dominated at all sampled sites except one within a bed of Nereocystis luetkeana, where P. gardneri was dominant. L. yezoensis was dominant at the site which was visited but not sampled. The average wet weight of non-canopy kelps in the 55 samples was 12 kg m^-2, and the quadrat range was 2 to 35 kg m^-2. The reported ranges of P. gardneri and Alaria marginata are extended westward from Montague Island (147°22W) to Bumble Bay, Kodiak Island (154°43W), where A. marginata was found at a depth of 8.5 m.     

A response-surface approach to the combined effects of temperature and salinity on the larval development of Adula californiensis (Pelecypoda: Mytilidae). II. Long-term Larval survival and growth in relation to respiration

Larvae of the bivalve molluso Adula californiensis (Phillippi, 1847) were reared for 3 days, from fertilization to veliger stage, at optimum conditions (15°C, 32.2 S), and then transferred to experimental temperatures and salinities for 22 more days to determine the effects of these factors on survival and growth. For larvae surviving to 25 days, maximum survival was estimated, by response-surface techniques, to occur at temperatures below 10°C and at salinities above 25. A comparison of 60% survival response contours for 3, 15 and 25-day old larvae indicated a progressive shift in temperature and salinity tolerance with age of larvae. The older larvae became more tolerant to reduced salinity, but less tolerant to high temperatures. Growth of the larvae over 25 days of culture was slight, and relatively independent of temperature and salinity conditions found in the environment. Oxygen consumption of 3-day old veliger larvae measured at various combinations of temperature and salinity generally increased from 7° to 18°C, and then sharply decreased from 18° to 21°C. A plateau of oxygen consumption from 9° to 15°C at 32.9 S indicated that the larvae are adapted to oceanic rather than estuarine conditions. A comparison of 25-day larval survival, mean length, and growth, with oxygen consumption of 3-day old veliger larvae indicated that high temperatures (15°C, and above) coupled with reduced salinities (26.1, and below) were unfavorable for prolonged larval life. Because of the lack of larval adaptations to estuarine conditions, larva survival and, hence, successful recruitment of this species within Yaquina Bay (Oregon, USA) depends upon the essentially oceanic conditions found only during the summer in the lower part of the Bay.     

Influence of food concentration,temperature and salinity on the larval development ofBalanus amphitrite

Influence of food concentration (0.5, 1 and 2 x 10⁵ cell ml^–1 ofSkeletonema costatum), temperature (20 and 30°C) and salinity (15, 25 and 35) on the larval development ofBalanus amphitrite (Cirripedia: Thoracica) was examined. The mortality rate at 20°C was lower than at 30°C in general. Increase in food concentration from 0.5 to 1 x 105 cells ml^–1 improved the survival rate, but this was not evident when food concentration was increased to 2 x 105 cells ml^–1. The results indicate that food availability and temperature jointly determine the energy allocation for metamorphic progress. It was observed that the influence of the tested variables varied with instar. At 20 °C the mean duration of the second instar exceeded 3 d and was much longer than other instar durations. The fourth, fifth and sixth instars and the total naupliar period showed that the effect of different salinities at given food concentrations was negligible at 20°C, while at 30°C there was a marked decrease in duration with increasing salinity.     

Effect of temperature on nitrite excretion by three marine diatoms during nitrate uptake

Nitrite excretion during nitrate uptake by three nitrate-limited diatoms was measured at different temperatures. Phaeodactylum tricornutum Bohlin and Chaetoceros affinis Lauder Hustedt excreted nitrite over the whole range of physiological temperatures, whereas Thalassiosira pseudonana Hasle and Heimdal (Clone 13-1) excreted nitrite only at 25°C. Parallel to growth and nitrate-uptake rates, excretion rates increased exponentially with temperature, attaining a maximum between 20° and 25°C (optimum temperature range). At the end of nitrate uptake, when the nitrate concentration in the culture medium had decreased below 1 M, nitrite was reabsorbed at all temperatures, except when cells were in the dark or at very low light intensity. Nitrite uptake was also inhibited by the presence of nitrate in the medium. These results are discussed in relation to the formation and position of the maximum layer of primary nitrite in the thermocline, below the maximum layer of chlorophyll in stratified oceanic areas.     

Individual effects and interactions of salinity,temperature, and zinc on larval development of the grass shrimp Palaemonetes pugio. I. Survival and developmental duration through metamorphosis

Larvae of the estuarine grass shrimp Palaemonetes pugio (Holthuis) were reared from hatch through successful completion of metamorphosis in 80 combinations of salinity (3 to 31%), temperature (20° to 35°C), and zinc (0.00 to 1.00 ppm Zn⁺⁺). Response-surface methodology was employed to depict the individual effects and interactions of the three factors on survival and developmental duration through total larval development. Outside the optimal salinity-temperature conditions of 17 to 27 S and 20° to 27°C, viability of larvae was reduced by both the individual effects of salinity and temperature and interactions between the two factors. Survival capacity of larvae and resistance adaptations to salinity and temperature were progresively reduced by zinc concentrations from 0.25 to 1.00 ppm Zn⁺⁺. Response-surface analysis of the data suggested that the duration of total larval development of P. pugio was least at salinities from 18 to 23 and at temperatures from 30° to 32°C. At both higher and lower salinity-temperature conditions and in increasing zinc concentrations from 0.25 to 1.00 ppm Zn⁺⁺, developmental rates were retarded. A significant zinc-temperature interaction existed, whereby increasing zinc concentrations reduced both survival and developmental rates of larvae more at suboptimal temperatures. Larval resistance to zinc toxicity was least at supraoptimal salinities, indicative of a significant zinc-salinity interaction. The reduced viability, restricted euryplasticity, and retarded developmental rates of P. pugio larvae developing in media with low-level zinc contamination would limit the distributive properties of the pelagic phase in the life cycle of the species and reduce recruitment both into and out of the parent estuarine population.     

Larval viability and heterozygote deficiency in populations of marine bivalves: evidence from pair matings of mussels

Using a pressurized temperature gradient, which rapidly equilibrates bacteria to a wide range of temperature and pressures, temperature ranges for growth and survival of Vibrio harveyi (strain B 392) were found to narrow with increasing pressures. Both temperature and pressure ranges were greater in an enriched seawater medium than in unenriched seawater. Using a kinetics apparatus, V. harveyi was also exposed to gradual changes in temperature and pressure, simulating a particle sinking at approximately 200 m d^-1 from surface waters (17 °C; 1 bar pressure) to 3 400 m (4°C; 340 bars pressure). With gradual temperature changes, cells were able to grow and survive at lower temperatures than they had in rapid equilibration experiments. Gradual pressurization, however, did not increase pressure tolerance. Under all experimental conditions, the combined effects of low temperatures and high pressures were lethal for V. harveyi. It is hypothesized that temperature and pressure stratify the ocean into regions of growth, survival, and death for V. harveyi, and perhaps other bacteria.     

Life in a warm deep sea: routine activity and burst swimming performance of the shrimp<Emphasis Type="Italic"> Acanthephyra eximia</Emphasis> in the abyssal Mediterranean

Measurements of routine swimming speed, tail-flip escape responses, and oxygen consumptions were made of the deep-sea shrimp Acanthephyra eximia using autonomous landers in the Rhodos Basin at depths of up to 4,400 m and temperatures of 13–14.5°C. Routine swimming speeds at 4,200 m averaged 0.18 m s^–1 or 3.09 body lengths s^–1, approximately double those of functionally similar oceanic scavengers. During escape responses peak accelerations of 23 m s^–2 or 630.6 body lengths s^–2 were recorded, with animals reaching speeds of 1.61 m s^–1 or 34.8 body lengths s^–2. When compared to shallow-water decapods at similar temperatures these values are low for a lightly calcified shrimp such as A. eximia despite a maximum muscle mass specific power output of 90.0 W kg^–1. A preliminary oxygen consumption measurement indicated similar rates to those of oceanic crustacean scavengers and shallower-living Mediterranean crustaceans once size and temperature had been taken into account. These animals appear to have high routine swimming speeds but low burst muscle performances. This suite of traits can be accounted for by high competition for limited resources in the eastern Mediterranean, but low selective pressure for burst swimming due to reductions in predator pressure.Communicated by J.P. Thorpe, Port Erin     

Daily energy requirements and trophic positioning of the sand shrimp<Emphasis Type="Italic"> Crangon septemspinosa</Emphasis>

Sand shrimp, Crangon septemspinosa Say, are important to the trophic dynamics of coastal systems in the northwestern Atlantic. To evaluate predatory impacts of sand shrimp, daily energy requirements (J ind.^–1 day^–1) were calculated for this species from laboratory estimates of energy losses due to routine (R_R), active (R_A), and feeding (R_SDA) oxygen consumption rates (J ind.^–1 h^–1), coupled with measurements of diel motile activity. Shrimp used in this study were collected biweekly from the Niantic River, Connecticut (41°33N; 72°19W) during late spring and summer of 2000 and 2001. The rates of shrimp energy loss due to R_R and R_A increased exponentially with increasing temperature, with the magnitude of increase greater between 6°C and 10°C (Q₁₀=3.01) than between 10°C and 14°C (Q₁₀=2.85). Rates of R_R doubled with a twofold increase in shrimp mass, and R_SDA was 0.130 J h^–1+R_R, irrespective of shrimp body size. Shrimp motile activity was significantly greater during dark periods relative to light periods, indicating nocturnal behavior. Nocturnal activity also increased significantly at higher temperatures, and at 20°C shifted from a unimodal to a bimodal pattern. Laboratory estimates of daily metabolic expenditures (1.7–307.4 J ind.^–1 day^–1 for 0.05 and 1.5 g wet weight shrimp, respectively, between 0°C and 20°C) were combined with results from previous investigations to construct a bioenergetic model and make inferences regarding the trophic positioning of C. septemspinosa. Bioenergetic model estimates indicated that juvenile and adult shrimp could meet daily energy demands via opportunistic omnivory, selectively preying upon items of high energy content (e.g. invertebrate and fish tissue) and compensating for limited prey availability by ingesting readily accessible lower energy food (e.g. detritus and plant material).Electronic Supplementary Material Supplementary material is available in the online version of this article at Communicated by J.P. Grassle, New Brunswick     

Effect of temperature on the egg and yolk-sac larval development of common pandora,<Emphasis Type="Italic"> Pagellus erythrinus</Emphasis>

Temperature is one of the most critical environmental factors for fish ontogeny, affecting the developmental rate, survival and phenotypic plasticity in both a species- and stage-specific way. In the present paper we studied the egg and yolk-sac larval development of Pagellus erythrinus under different water temperature conditions, 15°C, 18°C and 21°C for the egg stage and 16°C, 18°C and 21°C for the yolk-sac larval stage. The temperature-independent thermal sum of development was estimated as 555.6 degree-hours above the threshold temperature (the temperature below which development is arrested), i.e. 7°C for the egg and 12.1°C for the yolk-sac larval stage. Higher hatching and survival rates occurred at 18–21°C. At the end of the yolk-sac larval stage, body morphometry differed significantly (p<0.05) between the temperatures tested. The growth rate of the total length increased as temperature rose from 16°C to 18°C, while in the range of 18–21°C it stabilized and was independent of water temperature. The estimated Gompertz growth curve for the yolk-sac larvae of P. erythrinus was (r²=0.992) for the 16°C, (r²=0.991) for the 18°C and (r²=0.981) for the 21°C treatment. The efficiency of vitelline utilization during the yolk-sac larval stage was higher at 18°C.Communicated by O. Kinne, Oldendorf/Luhe     

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.     

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.