Differential effects of Aureococcus anophagefferens isolates ("brown tide") in unialgal and mixed suspensions on bivalve feeding

Picoplanktonic brown tides of Aureococcus anophagefferens have had devastating effects on production of commercially exploited bivalve populations in shallow, mid-Atlantic estuaries in the United States. The toxin produced by this alga has not been chemically characterized. This study develops a bioassay using juvenile mussels, Mytilus edulis, based on the inhibitory effect of brown tide on bivalve suspension-feeding, to compare the cellular toxicity of three Long Island, New York, clonal isolates of A. anophagefferens. Two recent (1995) isolates (CCMP 1707 and 1708) from Peconic Bay proved highly toxic and caused greater than 100-fold reduction in clearance rates (CR) of juvenile mussels in unialgal and mixed suspensions with a nutritious alga, Isochrysis galbana (clone T-iso), relative to controls. A third 1986 isolate from Great South Bay (CCMP 1784) showed no detectable toxicity in 24-h trials, and may have lost its initial potency over more than a decade of laboratory culture. Identification of a non-toxic strain provides a useful tool for future research. Cultures of the toxic isolate CCMP 1708 in late-stationary growth phase were significantly more toxic than those in early-exponential phase. The threshold concentration of toxic A. anophagefferens cells that inhibits clearance on co-occurring phytoplankton species was determined for juvenile (10-mm) hard clams, Mercenaria mercenaria. Relatively low concentrations (⣃᎒³ to 50᎒³ cells ml^-1) of isolate CCMP 1708 were sufficient to sharply reduce clam CR of I. galbana. Calculations based on these results suggest that, at peak historical densities of M. mercenaria in Great South Bay, removal of A. anophagefferens at low cell densities by suspension-feeding benthos could provide an effective top-down grazing control mechanism to prevent the initiation of brown tide in shallow, inner bays.     

Characterization of urease activity in three marine phytoplankton species,<Emphasis Type="Italic"> Aureococcus anophagefferens</Emphasis>,<Emphasis Type="Italic"> Prorocentrum minimum</Emphasis>, and<Emphasis Type="Italic"> Thalassiosira weissflogii</Emphasis>

The availability of different forms of nitrogen in coastal and estuarine waters may be important in determining the abundance and productivity of different phytoplankton species. Although urea has been shown to contribute as much as 50% of the nitrogen for phytoplankton nutrition, relatively little is known of the activity and expression of urease in phytoplankton. Using an in vitro enzyme assay, urease activities were examined in laboratory cultures of three species: Aureococcus anophagefferens Hargraves et Sieburth, Prorocentrum minimum (Pavillard) Schiller, and Thalassiosira weissflogii (Grunow) Fryxell et Hasle. Cultures of P. minimum and T. weissflogii were grown on three nitrogen sources (NO₃^m, NH₄⁺, and urea), while A. anophagefferens was grown only on NO₃^m and urea. Urease was found to be constitutive in all cultures, but activity varied with growth rate and assay temperature for the different cultures. For A. anophagefferens, urease activity varied positively with growth rate regardless of the N source, while for P. minimum, urease activity varied positively with growth rate only for cultures grown on urea and NH₄⁺. In contrast, for T. weissflogii, activity did not vary with growth rate for any of the N sources. For all species, urease activity increased with assay temperature, but with different apparent temperature optima. For A. anophagefferens, in vitro activity increased from near 0-30°C, and remained stable to 50°C, while for P. minimum, increased in vitro activity was noted from near 0-20°C, but constant activity was observed between 20°C and 50°C. For T. weissfloggii, while activity also increased from 0°C to 20°C, subsequent decreases were noted when temperature was elevated above 20°C. Urease activity had a half-saturation constant of 120-165 wg atom N l^у in all three species. On both an hourly and daily basis, urease activity in A. anophagefferens exceeded nitrogen demand for growth. In P. minimum, urease activity on an hourly basis matched the nitrogen demand, but was less than the demand on a daily basis. For T. weissflogii, urease activity was always less than the nitrogen demand. These patterns in urease activity in three different species demonstrate that while apparently constitutive, the regulation of activity was substantially different in the diatom. These differences in the physiological regulation of urease activity, as well as other enzymes, may play a role in their ecological success in different environments.     

Life on a hot dry beach: behavioural,physiological, and ultrastructural adaptations of the littorinid gastropod <Emphasis Type="Italic">Cenchritis</Emphasis> (<Emphasis Type="Italic">Tectarius</Emphasis>) <Emphasis Type="Italic">muricatus</Emphasis>

The distribution and abundance of the littorinid gastropod Cenchritis (Tectarius) muricatus were determined on an exposed shore at Lee Stocking Island, Bahamas in July/August 1995 and June 1999. Activity patterns were observed on two occasions shortly before a hurricane passed near the island in 1995. C. muricatus extended from the low water mark to 3.6 m vertical height above it. Highest densities (20/m²) were found just above the high tide mark but the animal remained relatively abundant (8/m²) at the highest point. Small animals were only found at high shore levels. With the exception of animals very near to the water's edge C. muricatus was inactive by day and on dry nights. Many individuals of the high shore populations became active at night during periods of high humidity and rainfall, moving distances of up to 4 m overnight. At lower levels, extensive pairing and copulation was associated with these periods of rainfall. Abundance changed dramatically after the passage of the hurricane. At the level of maximum abundance numbers were reduced from 20/m² to 2.8/m². Long-term experimental studies of water loss rate and haemolymph concentration showed that both water loss rate and increase in haemolymph concentration are greatest in the early period of desiccation. The concentration of the blood after 20 weeks is 250% of the original and the water lost is of the order of 22.5%. After an initial increase there appears to be evidence of regulation of haemolymph concentration possibly by exchange with free water in the shell itself. The considerable reserve capacity in tolerance is no doubt in part due to the structure and functioning of the kidney. Adaptive features of the excretory system identified from ultrastructural examination include the replacement of podocyte-containing filtration chambers in the auricle by extracellular tubules permeating the walls of both auricle and ventricle, which suggests a reduction in the rate of primary urine formation. This is also indicated by a reduced surface area of the nephridial gland, which resorbs organic solutes from the urine. Excretory cells contain single large vacuoles in which arise large multi-layered concretions composed of phospholipids and calcium salts typical of prosobranchs in which excretory products may be stored for long periods.     

Estimating the influence of prawn stocking density and seagrass type on the growth of juvenile tiger prawns (Penaeus semisulcatus): results from field experiments in small enclosures

Fine mesh enclosures (0.9 m² in basal area, 1 m high, with 100 µm mesh) and a jet-net retrieval system were developed to test the influence of juvenile prawn stocking density on growth rates in (1) different months (April and October/November) and (2) different types of intertidal seagrass beds in the Embley River estuary of tropical Australia. Small juvenile tiger prawns (3-6 mm in carapace length, CL) were stocked in enclosures at densities of 4-32 prawns per enclosure (4.4-35.5 prawns m^-2) on a high biomass seagrass bed (about 70 g m^-2 of mostly Enhalus acoroides) and one with low biomass (about 10 g m^-2 of mostly Halodule uninervis). After 2-3 weeks in the enclosures, recovery rates, and hence possibly survival, were greater on the high biomass Enhalus than on the low biomass Halodule. However, not all fish and crustaceans could be excluded from the enclosures. Growth rates were twice as fast on the high biomass Enhalus than on the low biomass Halodule. It is likely that the high biomass Enhalus, with its greater surface area, supported more epiphytic flora and fauna and reduced the potential for interference competition between prawns, compared with the low biomass Halodule. Growth rates on Enhalus were significantly faster at a stocking density of 4 prawns per enclosure (1.3 mm CL week^-1) than at a stocking density of 16 and 32 prawns per enclosure (both 0.8 mm CL week^-1), and did not differ significantly between April and October/November (temperatures were about 30°C at both times). The mean growth rate at 8 prawns per enclosure (1.1 mm CL week^-1) did not differ significantly from those at 4, 16 and 32 prawns per enclosure. These results from two seagrass beds suggest that the carrying capacity for juvenile tiger prawns was greater in the high biomass Enhalus than the low biomass Halodule bed.     

Settlement patterns of newly settled plaice (Pleuronectes platessa) in a non-tidal Swedish fjord in relation to larval supply and benthic predators

In demersal fish species with a pelagic larval stage, settlement patterns may be a consequence of variations in larval supply, habitat selection at settlement, and processes acting between the time of settlement and the time of benthic sampling. This study describes temporal (1994-1998) and spatial variation in plaice (Pleuronectes platessa L.) settlement densities in four semi-isolated nursery areas with similar habitat characteristics, in the non-tidal Gullmarsfjord on the west coast of Sweden. Juvenile abundance varied by a factor of ten, both among years and among nursery grounds. For the 3 years when larval sampling was undertaken (1994-1996) and all nursery areas, there was a significant positive relationship between larval supply and juvenile abundance (linear regression: r²=0.45, n=24, P<0.001). On the southern side of the fjord, a significant positive relationship between larval and juvenile abundance was found in one area (r²=0.62, n=6, P<0.05). The absolute mortality rate of plaice after settlement was related to the initial settlement density (r²=0.95, n=20, P<0.001), and to the abundance of predatory shrimps Crangon crangon (r²=0.44, n=20, P<0.01). Plaice otoliths were found in 6% of the shrimp stomachs analysed from an area with high density (13.3 m^-2) of newly settled plaice. The present study suggests that the density of juvenile plaice was limited by larval supply to the nursery grounds. Consistency in the relative abundance of juveniles among nursery grounds between years also suggested that some nursery areas may be in the settlement shadow of others. The irregular nature of the coastline in combination with larval depletion could thereby cause small-scale (10³-10⁴ m) variation in settlement densities of the same order of magnitude as the inter-annual variability in recruitment to individual nursery grounds.     

Annual population development and production by Calanus finmarchicus, C. glacialis and C. hyperboreus in Disko Bay, western Greenland

The populations of the copepod species Calanus finmarchicus, C. glacialis and C. hyperboreus were investigated in Disko Bay during a 14-month period in 1996-1997. The three species were predominant in the copepod community. The biomass reached a maximum at the beginning of June (127 mg C m^-3). From the end of July until the end of April the following year, the biomass was <1-6 mg C m^-3. All three species showed seasonal ontogenetic migration. The spring ascent for all three species was just prior to or in association with the break-up of sea ice and the development of the spring bloom, whereas descent occurred over a larger time span during summer. The main overwintering stages were CV for C. finmarchicus, CIV and CV for C. glacialis and C. hyperboreus. Peak abundance of juvenile copepodites, representing the new generation, was in August for C. finmarchicus, in July for C. glacialis and in May/June for C. hyperboreus. From the timing of reproduction and the population development, the life cycles were deduced to be 1 year for C. finmarchicus and at least 2 years for C. glacialis and C. hyperboreus. Secondary production and potential grazing impact of the Calanus community were estimated by two methods based on specific egg-production rates and temperature-dependent production. The Calanus community was not able to control the primary producers during the spring bloom but probably did during post-bloom. The estimates also indicated that grazing on ciliates and heterotrophic dinoflagellates contributes as an essential food source in the post-bloom period.     

Distribution of squid paralarvae, <Emphasis Type="Italic">Loligo opalescens</Emphasis> (Cephalopoda: Myopsida), in the Southern California Bight in the three years following the 1997–1998 El Niño

Large numbers of paralarvae of the California market squid, Loligo opalescens (10,560 paralarvae from 422 plankton samples), were collected in the Southern California Bight in 1999, 2000, and 2001 during the spawning season. Paralarval abundance increased dramatically (P<0.0041) from 1.5 squid/1,000m³ in 1999 to 77.9 squid/1,000m³ in 2000, and 73.6 squid/1,000m³ in 2001, following the El Niño of 1997-1998. The effects on the squid fishery of the 1997-1998 El Niño were thus extended for two years, with larval abundance reduced until the 1999-2000 spawning season. Paralarvae were abundant close to shore for up to a month after hatching in 2000 (P<0.003), with tidal surface currents adjacent to shore in the Channel Islands strongly affecting paralarval abundance. Tidally reversing currents within 1-3 km of shore created a boundary layer of "sticky water" within which paralarvae remained entrained inshore immediately after hatching. Neritic currents farther from shore dispersed older paralarvae within the Southern California Bight. The greatest change in paralarval abundance, for all transects, was observed within 1 km of the transition between these two flow regimes. Age of paralarvae (from statolith increments) entrained within the Catalina Island boundary layer averaged 13-16 days, but some individuals remained nearshore for up to a month. Paralarvae in the boundary layer occurred above 80 m depth both day and night, and exhibited a statistically significant pattern of vertical diel migration (P<0.01). Paralarvae at sea were disproportionately abundant adjacent to fronts associated with uplifted isotherms.     

Cross-shelf variation in calanoid copepod production during summer 1996 off the Oregon coast, USA

The fecundity of nine species of adult female calanoid copepods, and molting rates for copepodite stages of Calanus marshallae were measured in 24 h shipboard incubations from samples taken during the upwelling season off the Oregon coast. Hydrographic and chlorophyll measurements were made at approximately 300 stations, and living zooplankton were collected at 36 stations on the continental shelf (<150 m depth) and 37 stations offshore of the shelf (>150 m depth) for experimental work. In our experiments, maximum egg production rates (EPR) were observed only for Calanus pacificus and Pseudocalanus mimus, 65.7 and 3.9 eggs fem^-1 day^-1 respectively, about 95% of the maximum rates known from published laboratory observations. EPR of all other copepod species (e.g., C. marshallae, Acartia longiremis and Eucalanus californicus) ranged from 3% to 65% of maximum published rates. Fecundity was not significantly related to body weight or temperature, but was significantly correlated with chlorophyll a concentration for all species except Paracalanus parvus and A. longiremis. Copepod biomass and production in on-shelf waters was dominated by female P. mimus and C. marshallae, accounting for 93% of the adult biomass (3.1 mg C m^-3) and 81% of the adult production (0.19 mg C m^-3 day^-1). Biomass in the off-shelf environment was dominated by female E. californicus, P. mimus, and C. pacificus, accounting for 95% of the adult biomass (2.2 mg C m^-3) and 95% of the adult production (0.08 mg C m^-3 day^-1). Copepodite (C1-C5) production was estimated to be 2.1 mg C m^-3 day^-1 (on-shelf waters) and 1.2 mg C m^-3 day^-1 (off-shelf water). Total adult + juvenile production averaged 2.3 mg C m^-3 day^-1 (on-shelf waters) and 1.3 mg C m^-3 day^-1 (off-shelf waters). We compared our measured female weight-specific growth rates to those predicted from the empirical models of copepod growth rates of Huntley and Lopez [Am Nat (1992) 140:201-242] and Hirst and Lampitt [Mar Biol (1998) 132:247-257]. Most of our measured values were lower than those predicted from the equation of Huntley and Lopez. We found good agreement with Hirst and Lampitt for growth rates <0.10 day^-1 but found that their empirical equations underestimated growth at rates >0.10 day^-1. The mismatch with Hirst and Lampitt resulted because some of our species were growing at maximum rates whereas their composite empirical equations predict "global" averages that do not represent maximum growth rates.     

Pools of chlorophyll and live planktonic diatoms in aphotic marine sediments

Chlorophyll a and numbers of live pelagic diatoms were recorded from sediment depth profiles at 11 stations in the oligotrophic Øresund, Denmark, in late-June. Extraction efficiency of chlorophyll a analysed fluorometrically did not differ significantly between paired samples of frozen-thawed and fresh sediment. The depth profiles of chlorophyll a could be explained by a diagenetic model involving two different chlorophyll pools: one reactive pool declining exponentially with core depth, and one non-reactive pool, of about 1 µg Chl ml^-1 wet sediment, being constant with depth. The number of live diatoms, quantified by the dilution-extinction method, and expressed in terms of most probable number (MPN), declined from an average of about 300,000 g^-1 in the surface sediment to zero values at a depth of 13 cm. The number of live cells was significantly correlated with the sediment chlorophyll a, and the profiles of live cells as well as reactive chlorophyll followed the same exponential decline with core depth, suggesting that the main source of chlorophyll in the sediment was live pelagic diatoms. Taxonomic composition of diatoms in the sediment, dominated by the pelagic genera Chaetoceros, Thalassiosira and Skeletonema, matched the species composition in the water column 3 months earlier during the spring bloom. Regular recordings of the phytoplankton community in the water column showed that only these specific bloom species could be the source of the sediment content of diatoms and chlorophyll a. Further, the ratios between live cells and chlorophyll a were similar in the sediment and in the spring bloom. A conservative estimate of depth-integrated pools of diatoms in the sediment suggested that about 44% of the total phytoplankton biomass during the spring bloom was still present as live cells in the sediment after 3 months. This indicates that the spring bloom input to the sediment is not degraded immediately by the benthic fauna.     

Evaluation of fatty acids as biomarkers for a natural plankton community. A field study of a spring bloom and a post-bloom period off West Greenland

An investigation of the fatty acid composition of a natural arctic plankton community was carried out over two fishing banks located between 63°N and 65°N off the West Greenland coast. Samples for fatty acid analyses, species determination and biomass assessments of the plankton community were taken at the depth of fluorescence maximum. High biomass and diatom dominance during the spring bloom and low biomass and flagellate dominance in the post-bloom period were reflected by the fatty acid profiles. The total amount of fatty acid ranged from 55 to 132 µg l^-1 during the spring bloom and from 1 to 5 µg l^-1 during the post bloom. Analysis of the fatty acids showed that when the plankton was dominated by diatoms of the genera Thalassiosira and Chaetoceros, the proportions of C16:1(n-7) and C20:5(n-3) were correspondingly high. C18s, and particularly C18:1(n-9), were more abundant when the plankton was dominated by small autotrophic flagellates, primarily haptophytes. We found a good positive correlation between the common diatom marker, C16:1(n-7)/C16:0, and the biomass percentage of diatoms (r=0.742, P<0.001), as well as between the biomass percentage of flagellates and total C18 fatty acids (r=0.739, P<0.001). This supports the use of these specific fatty acids and fatty acid ratios as general biomarkers of the plankton community. However, the fatty acids are not specific enough to sufficiently characterise the composition of the plankton community, and microscopical support is needed to verify observed trends.     

The importance of micro and macro morphological variation in the adaptation of a sublittoral demosponge to current extremes

Sponges are known to show morphological acclimation in response to habitat variation. However, previous studies have concentrated on only one aspect of morphological variation, either gross morphology or spicule morphology. Cliona celata (Grant) is common in a variety of different habitats on the south-west coast of Ireland and has been investigated with respect to morphological variability on both scales. C. celata exhibited six different gross morphological body forms (ridged, burrowing, massive, massive/chimneys, encrusting, encrusting/chimneys). The body form exhibited was correlated to local environment, showing the extent of morphological adaptation in C. celata. Sponge size varied (from 548ᇟ to 2,345녹 cm²) between sites, with the largest (2,345녹 cm²) being found at the most stable site where flow rates were <5 cm^-1 (F>23.24, P<0.05). This may seem paradoxical as growth conditions were considered poor, but mortality and damage from material in suspension was reduced at low energy sites. At the spicule level, morphological variation was also present. Spicules at high energy sites were significantly longer, narrower and less numerous than at low energy sites (F>15.36, P<0.05). Previously, spicule variation has been associated with increased stiffness in hostile environments. However, longer, thinner spicules, as found in C. celata, may result in a more flexible sponge. This is the first study to show both gross morphological (macro) and spicule (micro) variation in a single species of sponge. However, this study only reinforces some of the previously produced information on both of these adaptations of sponges to varying environments. This study also illustrates how the results of single studies should not be used to draw conclusions for group level adaptation.     

Growth dynamics and mortality of the intertidal encrusting sponge Halichondria okadai (Demospongiae, Halichondrida)

A periodical survey of the growth of an intertidal encrusting sponge Halichondria okadai was conducted on a rocky intertidal shore on the Izu Peninsula, southern Japan. The area covered by each of 16 marked sponges was monitored, and the fusion and fission of specimens were recorded from June 1995 to June 1998. During the investigation, 15 of the original specimens managed to survive accompanying fusions or fissions and only one colony completely disappeared. Ten fusions and 23 fissions were observed in total. The maximum death rate during a semilunar period was 16.7% and cumulative mortality throughout the study period was 46.1%, indicating that the mortality of H. okadai is lower than that of other species previously reported. There was no particular seasonal pattern in the occurrences of fusion, fission and death of sponges. However, fissions were frequently observed in the latter half of the study period and were sometimes followed by fusions or death of specimens. Positive growth rates were often observed during warmer months, and specimens showed little growth or regression in winter. Seasonal patterns in surface area were thought to be related to seasonality in reproductive activity.     

Population biology of the annual grassTriplasis purpurea in relation to distance from shore on Staten Island, New York

Four populations of the native annual grassTriplasis purpurea were surveyed on coastal beaches along the south shore of Staten Island, NY, to determine the potential of this species to colonize shoreline habitats mostly devoid of other vegetation. If the species can establish and maintain dense populations, it may have conservation value for urban beaches disturbed by human activities. For two populations, survivorship, growth, and reproduction were monitored at different distances from shore to determine the ability of this species to maintain viable populations. At three sites,T. purpurea occurred in >75% of all quadrats and the highest density was 1195 plants/m² at 74 m from shore in one recently disturbed site. Density generally increased with increasing distances from shore at low tide (from ca. 40 – 90 m). Plants showed the greatest growth and reproduction at close distances to shore (30 – 40 m); part of this effect was due to density in one population, but when density effects were removed statistically, there still remained a decline in growth and reproduction with increasing distance from shore. Improved vigor nearest to shore may be due to continual sand deposition. Survivorship showed a Type I pattern, with low mortality throughout the growing season. By colonizing newly-deposited and continually shifting sands,T. purpurea can contribute to the earliest stages of ecological succession along disturbed beaches in eastern North America and may be valuable to the development and management of urban coastal plant communities.     

Population biology of the annual grass Triplasis purpurea in relation to distance from shore on Staten Island, New York

Four populations of the native annual grassTriplasis purpurea were surveyed on coastal beaches along the south shore of Staten Island, NY, to determine the potential of this species to colonize shoreline habitats mostly devoid of other vegetation. If the species can establish and maintain dense populations, it may have conservation value for urban beaches disturbed by human activities. For two populations, survivorship, growth, and reproduction were monitored at different distances from shore to determine the ability of this species to maintain viable populations. At three sites,T. purpurea occurred in >75% of all quadrats and the highest density was 1195 plants/m² at 74 m from shore in one recently disturbed site. Density generally increased with increasing distances from shore at low tide (from ca. 40–90 m). Plants showed the greatest growth and reproduction at close distances to shore (30–40 m); part of this effect was due to density in one population, but when density effects were removed statistically, there still remained a decline in growth and reproduction with increasing distance from shore. Improved vigor nearest to shore may be due to continual sand deposition. Survivorship showed a Type I pattern, with low mortality throughout the growing season. By colonizing newly-deposited and continually shifting sands,T. purpurea can contribute to the earliest stages of ecological succession along disturbed beaches in eastern North America and may be valuable to the development and management of urban coastal plant communities.     

Large-scale oceanic distribution and population structure of Calanus finmarchicus, in relation to physical environment, food and predators

The investigation was carried out from 62°N to 73°N and from 14°E to 11°W in the Norwegian Sea during 19 June-12 July 1997. Regional differences in the phase of the seasonal development of the plankton community were evident, most pronounced across the Arctic front. In the Coastal and eastern Atlantic domains, post-bloom conditions prevailed, characterised by low chlorophyll a (chl a) levels and a phytoplankton assemblage dominated by coccolithophorids and small flagellates. During the study period, egg production rates of Calanus finmarchicus were low (<10 eggs female^-1 day^-1), older copepodite stages dominated, and the seasonal descent to deeper waters had started. In the Arctic domain, bloom conditions were evident by high chl a levels and a high abundance of large diatoms. Egg production rates were higher (a maximum of 29 eggs female^-1 day^-1), but the dominance of stages CI-CIII indicated that considerable spawning had already occurred prior to the spring bloom. The seasonal descent had barely started. Both invertebrate and fish predators were most abundant in the Coastal and eastern Atlantic domains, with abundance strongly decreasing north-westwards. No tight relationship between total abundance of invertebrate or fish predators and that of C. finmarchicus was apparent. However, a weak, but significant, relationship between abundance of young stages of chaetognaths and Euchaeta spp. versus young stages of C. finmarchicus was found, indicating that these invertebrate predators develop parallel to the development of the new cohort of C. finmarchicus. In early summer, C. finmarchicus had reached overwintering stages, and had started to accumulate in deeper waters in areas with the highest abundance of horizontally migratory planktivorous fish.     

Growth of juvenile Mercenaria mercenaria and the effect of resuspended bottom sediments

The influence of silt on growth of juvenile hard clams Mercenaria mercenaria (L.) (9 mm in mean shell length) was investigated in the laboratory using mixed suspensions of algae (50x10⁶ Pseudoisochrysis paradoxa cells l^-1) and fine-grained bottom sediments (0 to 44 mg l^-1). Growth rates, expressed as percent increase in ash-free dry tissue weight, were not significantly affected by sediment concentrations up to 25 mg l^-1. Significant reduction in growth (by 16% relative to controls fed only algae), and condition of clams, occurred at 44 mg silt l^-1. The results of the 3-week growth experiment agree well with predictions made in an earlier study by integrating results of shortterm physiological measurements. Growth rates obtained with experimental algal-silt diets at 21°C (2.6 to 3.3% increase in dry tissue weight d^-1) were comparable to those determined at ambient concentrations of Great South Bay particulates at 20°C (0.9 to 4.0% d^-1). Levels of particulate inorganic matter in seawater from Great South Bay, New York, exhibited pronounced daily changes, and ranged from 6 to 126 mg dry weight l^-1. Growth enhancement by the addition of silt to an algal diet, reported in mussels, surf clams and oysters, was not found in M. mercenaria. It is suggested that these three species are better suited than hard clams for culturing efforts in inshore turbid waters above uncompacted, muddy bottoms.Contribution No. 452 from the Marine Sciences Research Center, State University of New York at Stony Brook, USA     

Blue sprat <Emphasis Type="Italic">Spratelloides robustus</Emphasis> (Clupeidae: Dussumieriinae): a temperate clupeoid with a tropical life history strategy?

This article describes the life-history strategy of the blue sprat Spratelloides robustus in South Australia and compares the demographic traits observed with those of other clupeoids. Validation studies that involved marking the sagittae of captive fish with oxy-tetracycline suggested that growth increments are deposited daily. The oldest fish examined was 82 mm caudal fork length and 241 days old, which suggests S. robustus may live for less than 1 year. Growth rates were high during larval stages (0.34 mm day^у) and remained high throughout juvenile (0.33 mm day^у) and adult stages (0.19 mm day^у). S. robustus reached 50% maturity at approximately 60 mm caudal fork length after approximately 135 days. Spawning occurred from October to February (spring to late summer) and larvae were found mainly in Spencer Gulf, Gulf St Vincent, and Investigator Strait. Females spawned multiple batches of demersal eggs every 1-2 days. Batch fecundities were low (mean=756, SD=341) and increased linearly with length and weight. The life history of S. robustus is dissimilar to other small to medium-sized temperate clupeoids, but similar to those of many small sub-tropical and tropical clupeoids, including other Spratelloides species. Gulf St Vincent and Spencer Gulf may be considered to be "seasonally subtropical systems" in an otherwise temperate region that support a suite of species, including S. robustus, that have life-history strategies similar to those of sub-topical and tropical taxa.     

Biomass, photosynthesis and growth of Laminaria saccharina in a high-arctic fjord, NE Greenland

Biomass, photosynthesis and growth of the large, perennial brown alga Laminaria saccharina (L.) Lamour. were examined along a depth gradient in a high-arctic fjord, Young Sound, NE Greenland (74°18'N; 20°14'W), in order to evaluate how well the species is adapted to the extreme climatic conditions. The area is covered by up to 1.6-m-thick ice during 10 months of the year, and bottom water temperature is <0°C all year round. L. saccharina occurred from 2.5 m depth to a lower depth limit of about 20 m receiving 0.7% of surface irradiance. Specimen density and biomass were low, likely, because of heavy ice scouring in shallow water and intensive feeding activity from walruses in deeper areas. The largest specimens were >4 m long and older than 4 years. In contrast to temperate stands of L. saccharina, old leaf blades (2-3 years old) remained attached to the new blades. The old tissues maintained their photosynthetic capacity thereby contributing importantly to algal carbon balance. The photosynthetic characteristics of new tissues reflected a high capacity for adaptation to different light regimes. At low light under ice, or in deep water, the chlorophyll a content and photosynthetic efficiency (!) were high, while light compensation (E_c) and saturation (E_k) points were low. An E_c of 2.0 µmol photons m^-2 s^-1 under ice allowed photosynthesis to almost balance, and sometimes exceed, respiratory costs during the period with thick ice cover but high surface irradiance, from April through July. Rates of respiration were lower than usually found for macroalgae. Annual elongation rates of leaf blades (70-90 cm) were only slightly lower than for temperate L. saccharina, but specific growth rates (0.48-0.58 year^-1) were substantially lower, because the old blades remained attached. L. saccharina comprised between 5% and 10% of total macroalgal biomass in the area, and the annual contribution to primary production was only between 0.1 and 1.6 g C m^-2 year^-1.     

Large viruses and infected microeukaryotes in Ross Sea summer pack ice habitats

A variety of Ross Sea summer pack ice habitats between 66 and 75°S were examined for viruses 𔒦 nm capsid diameter. Maximum abundances of these viruses likely to infect eukaryotes were 10⁶-10⁷ ml^-1 brine in surface, interior, and bottom habitats and constituted up to 18% of the total (all sizes) viruses. There is abundant ultrastructural evidence for infection of a variety of microheterotrophs and some autotrophs. One station exhibited the classical characteristics of a lower latitude algal bloom with potential viral control. The blooming alga, Pyramimonas tychotreta Daugbjerg 2000, was infected, as were two abundant heterotrophs, Cryothecomonas spp. and an unidentified flagellate, that fed on P. tychotreta. Infections were observed in only one life history stage (multiflagellate cells) of P. tychotreta, suggesting a relationship among virus-induced lysis, life-history stages, physiology, and environmental factors regulating the life cycle. There is good evidence that diatoms are not a likely source of the large viruses, and viruses in general are not a major food source for ice microheterotrophs in summer.     

Significance of food type for growth of ephyrae Aurelia aurita (Scyphozoa)

We studied growth of newly released Aurelia aurita ephyra larvae fed five different food types, including a large-sized copepod, a phytoflagellate, and suspended POM (particulate organic matter) made from bivalve meat. Experiments were run at saturated food concentration in two different temperatures over 10 days. The effect of small differences in temperature was inconsistent and interacted with the effect of food type, which, in turn, was highly significant. A low average growth rate (4-9% day^-1) was shown when feeding on the large-sized copepod Calanus finmarchicus (80 µg AFDW individual^-1), in spite of an extremely high daily ration of up to 1500% of body AFDW. When feeding on the cryptophyte Rhodomonas baltica (ca. 8 µm cell diameter), the ephyrae showed an average growth rate over the 10 day experiment of 7-11%, but with a considerably higher growth rate during the first days. Suspended POM generated an average growth rate of 7-9% day^-1, whereas fresh bivalve meat, manually placed into the stomach of the ephyra, gave an average growth rate of 12-14% day^-1. Artemia nauplii (ca. 3 µg AFDW individual^-1), used as a general reference, resulted in higher growth rates than any of the other food types (17-31% day^-1). We conclude that A. aurita ephyrae can capture and feed on phytoplankton, large copepods, and POM; that phytoplankton might be of nutritive significance early in development; and that the high quantity of large-sized copepods ingested is inefficiently converted to growth during early development. POM is a potential food source because of the ability of the ephyrae to encounter and ingest it, although concentration, size distribution, and nutritional composition of natural POM probably constrain its effect on growth.