Showing papers in "Marine Biology in 1984"

RNA-DNA ratio: an index of larval fish growth in the sea

Abstract: Data on water temperature, RNA-DNA ratio, and growth of eight species of temperate marine fish larvae reared in the laboratory were fit to the equation: $$G_{pi} = 0.93{\text{ }}\operatorname{T} + 4.75{\text{ RNA - DNA}} - 18.18$$ where Gpi is the protein growth rate in % d-1 and T is the water temperature. Water temperature and larval RNA-DNA ratio explained 92% of the variability in growth rate of laboratory-reared larvae. The model is useful over the entire range of feeding levels (starvation to excess), temperatures (2° to 20°C) and fish species studied. Estimates of recent growth of larval cod, haddock, and sand lance caught at sea based on water temperature and RNA-DNA ratio ranged from negative to 26% d-1. These data demonstrate the importance of food availability in larval fish mortality and suggest that short-term growth under favorable conditions may be considerably higher than expected from long-term indicators. RNA-DNA ratio analysis offers new possibilities for understanding larval growth and mortality, and their relation to environmental variability.

Species differences in accumulation of nitrogen pools in phytoplankton

Abstract: The changes in the intracellular concentrations of nitrate, ammonium, free amino acids, protein, DNA, RNA and total nitrogen were measured in batch cultures of seven species of marine phytoplankton as they progressed from being nitrogen sufficient to being nitrogen starved. After several days of nitrogen starvation, either nitrate or ammonium was added to the cultures, and the measurements were continued for 10 to 36 h. By this means it was possible to assess the long-term and short-term changes in cellular nitrogen compounds and how they relate to phytoplankton nitrogen uptake and growth. Considerable species differences were observed in the amounts and kinds of nitrogen compounds which were stored and the degree to which the utilization of these compounds could support growth if the external nitrogen supply is low or variable. Despite the species variation, the results suggest that the concentrations or ratios of a number of intracellular nitrogen compounds can be used to assess the nitrogen deficiency and/or growth rate of natural phytoplankton populations.

Comparisons of nutrient uptake rates for Baltic macroalgae with different thallus morphologies

Abstract: In-situ experiments were performed during different seasons to determine uptake rates of PO3-4, NH+4 and NO-3 within ecologically representative ranges of nutrient concentrations, of dominant macroalgae in the Baltic Sea. Uptake rates were governed by nutrient concentrations, water temperature and thallus morphology, but not by the phylogenetic affinity of the species. Nitrogen uptake rates were always higher than those of phosphorus at the same concentrations, and NH+4−N uptake rates exceeded those of NO-3−N. The lowest uptake rates occurred among the late successional, long-lived, coarse species with low surface: volume ratios (Fucus vesiculosus, Furcellaria lumbricalis andPhyllophora truncata). The highest uptake rates were measured for short-lived, opportunistic algae, filamentous or with numerous hairs, (Cladophora glomerata, Enteromorpha ahlneriana, Scytosiphon lomentaria, Dictyosiphon foeniculaceus andCeramium tenuicorne). The latter group also had the highest Vmax:kmax ratios, which indicates a more competitive advantage for nutrient uptake at low concentrations.

Genetic heterozygosity and growth rate in Mytilus edulis

Abstract: Individuals of Mytilus edulis of the same age (ca 2 months) were collected as spat from natural populations. Relative growth rates were determined among individuals differing in heterozygosity at five enzyme loci. Growth rate was positively correlated with individual heterozygosity and each of the five loci contributed about equally to the relatinship. More heterozygous individuals also achieved more uniform average growth rates. Although there was a deficiency of heterozygotes at each locus, relative to Hardy-Weinberg expectations, the magnitude of the deficiency, measured as FIS, was less among faster growing mussels. Our results conform closely with those of Zouros et al. (1980) on the American oyster. We conclude that the relationship between multiple locus heterozygosity and growth rate is one that is general to a diversity of outbreeding plant and animal populations. Other studies indicate that this relationship is due to a greater average metabolic efficiency of more heterozygous individuals. This relationship does not emerge from experimental designs in which there has been limited genetic sampling of the natural genetic variation.

Influence of algal and suspended sediment concentrations on the feeding physiology of the hard clam Mercenaria mercenaria

Abstract: Short-term laboratory feeding experiments were conducted to determine the response of the hard clam Mercenaria mercenaria (L.) (32 mm in mean shell length) to increasing sediment concentrations. Clams were fed mixed suspensions of Pseudoisochrysis paradoxa (50 and 150 cells μl-1) and bottom sediments (0 to 44 mg l-1). Algal ingestion rate deelined with increasing sediment loads. This resulted primarily from a reduction in clearance rate, which declined by 0.08 l h-1 g-1 (1.3%) for every 1 mg l-1 increase in sediment load. This reduction was of similar magnitude for juvenile (13 mm) clams. At the algal concentrations tested, pseudofaeces production was intermittent and inconspicuous below about 10 mg silt l-1. Loss of algae in pseudofaeces increased with increasing sediment loads; however, even at the highest silt and algal concentrations, clams lost a maximum of only 18% of the algae cleared from suspension. Thus, pseudofaeces production is not expected to cause significant loss of algal food at the sediment concentrations normally encountered in the natural environment (≦ ca 40 mg silt l-1). Absorption rate of total organic matter remained constant, at least up to silt concentrations of 20 mg l-1. Experiments using dual 51Cr:14C-formaldehyde-labelled sediment indicated that clams were able to counteract the dilution of algae by absorbing a considerable fraction (21 to 22%) of detrital sedimentary organics. Absorption efficiency of pure P. paradoxa ranged from 82% at 50 cells μl-1 to 58% at 150 cells μl-1. Integration of physiological rate measurements suggests that at moderate to high algal concentrations (≧300 μg Cl-1), growth improvement by the addition of silt, documented in mussels, surf clams and oysters, is unlikely to occur in M. mercenaria. It is suggested that a suspension-feeding bivalve's success in maximizing its energy gain in a turbid environment depends on the combination of two features: a high selection efficiency and a high rate of pseudofaeces production. It is proposed that species which regulate ingestion primarily by producing pseudofaeces are better adapted to cope with high suspended sediment loads than species such as M. mercenaria, which control ingestion mainly by reducing clearance rate.

Mass Mortality of Strongylocentrotus-Droebachiensis (Echinodermata, Echinoidea) Off Nova-Scotia,Canada

Abstract: A mass mortality of Strongylocentrotus droebachiensis, attributed to disease, was monitored in an echinoiddominated barren ground at Eagle Head on the south-western coast of Nova Scotia, Canada, in 1982. Mortality was 70% in a shallow (3 m) nearshore area, resulting in a loss of echinoid biomass of 2 042 g fresh weight m-2, and 6% in deeper (7 m, 10 m) offshore areas. Echinoid density, size and nutritional condition (gonad index) were highest in the nearshore area. Survivorship was higher in juveniles (<15 mm diameter) than in adults resulting in the formation of a bimodal size distribution in the nearshore area. Mortality began around early October, near the peak of the annual cycle of seawater temperature (∼15°C), and was arrested by early December (seawater temperature ∼7°C) when morbid echinoids appeared to recover. In laboratory experiments, time to morbidity of S. droebachiensis exposed to morbid conspecifics increased exponentially with decreasing temperature (20° to 8°C). There was no survival at 20° and 16°C, 20% survival at 12°C and 100% survival at 8°C after 60 d; suggesting a lower temperature limit (between 12° and 8°C) for possible transmission of a pathogenic agent. Morbid laboratory echinoids from experiments at 16°C, and recovering echinoids collected in the nearshore area in early December, showed 100 and 85% survival respectively at <=8°C, and 0 and 15% survival respectively at 16°C, after 30 d. Time to morbidity was not affected significantly by nutritional condition and was similar for juvenile and adult echinoids. Time to morbidity was greater in echinoids exposed to one or three morbid individuals continuously, or seven morbid individuals for 1 h, relative to higher levels of exposure (up to seven morbid individuals continuously). Recent mass mortalities in S. droebachiensis have occurred in years of record high sea surface temperatures. The extent of mortality is correlated with the magnitude and duration of temperatures above a lower limit.

Effects of grazing by two species of sea urchins (Strongylocentrotus franciscanus and Lytechinus anamesus) on recruitment and survival of two species of kelp (Macrocystis pyrifera and Pterygophora californica)

Abstract: We studied the effects of grazing by two species of sea urchins on two species of kelp (Macrocystis pyrifera and Pterygophora californica) in the San Onofre kelp bed in southern California from 1978 through 1981. Both red sea urchins, Strongylocentrotus franciscanus, and white sea urchins, Lytechinus anamesus, were abundant and lived in aggregations. The purple sea urchin (S. purpuratus) was rare at the study site and was not studied. The aggregations of red urchins were either relatively small and stationary (for over 3 yr) or relatively large and motile (advancing at about 2 m mo−1). Both stationary and moving aggregations were observed at the same time, and within 100 m of one another. Stationary aggregations of red urchins probably subsisted mainly on drift kelp and had no effect on kelp recruitment or on adult kelp abundance. In contrast, red sea urchins in large, motile aggregations or “fronts” ate almost all the macroalgae in their path. The condition of their gonalds indicated that red urchins in fronts were starved relative to red urchins in the small, stationary aggregations. Large, motile aggregations developed after 2 yr of declining kelp abundance (probably due largely to storms). We propose that a scarcity of drift algae for food results in a change in the behavior pattern of the red urchins and thus leads to the formation of large, motile aggregations. The aggregations of white urchins, which occurred along the offshore margin of the kelp bed, were large, but relatively stationary. The white urchins rarely ate adult kelps, but grazed extensively on early developmental stages of kelps and evidently prevented seaward expansion of the bed. The spatial distribution of both types of red urchin aggregations appeared to be unrelated to predation pressure from fishes or lobsters.

Changes in the content and fatty acid composition of lipid in an isolated population of the capelin Mallotus villosus during sexual maturation and spawning

Abstract: Changes in the wet weights and lipid contents of muscle, liver and gonad were determined in male and female Mallotus villosus in Balsfjorden, northern Norway, in 1981, from January, when gonadal development becomes noticeable, until May when the fish are spawning. Fatty acid compositions of tissue lipids were also determined. Over 4 mo prior to spawning, the weight of muscle in female capelin decreased by 32% while the weight of the ovary increased exponentially by 830%. In males the weight of the muscle remained constant and that of the testis decreased slightly. The lipid contents of the muscle of both males and females decreased by 76% over the period and an inverse relationship existed between the water and lipid contents of muscle in both sexes. Male liver weight remained constant over the period of study whereas female liver weight increased transiently by 300% between January and March. 38% of the lipid lost from female muscle was accounted for by lipid deposited in ovary whereas negligible amounts of the lipid lost from male muscle was accounted for by lipid in the testis. Gonadal lipid was always richer in polyunsaturated fatty acids than muscle lipid and, immediately prior to spawning, 42% of the fatty acids in ovarian lipid were polyunsaturated. Muscle lipid of males and females showed a progressive increase in the percentage of the long-chain monoenes 20:1 and 22:1 between January and May. It is concluded that male capelin catabolise more of their muscle lipid reserves than females in the 4 mo prior to spawning and that most of the lipid catabolism in males is associated with physical activity. Conversely, females deposit much more of their muscle lipid in gonads than males, although considerable selectivity occurs in the mobilisation of fatty acids from muscle lipid into ovarian lipid. Additionally, biosynthesis of gonadal constituents accounts for a considerable proportion of the lipid catabolised in females.

Swimming pattern as an indicator of the roles of copepod sensory systems in the recognition of food

Abstract: The roles of copepod sensory systems in the recognition of food were investigated using the “Bugwatcher”, a video-computer system designed to track and describe quantitatively the swimming patterns of aquatic organisms. The swimming behavior of the copepodPseudocalanus minutus in the presence of phytoplankton is characterized by a decrease in average swimming speed and an increase in “pause” behaviors compared to its swimming behavior in filtered seawater. Copepods exposed to chemosensory stimulation alone (filtered phytoplankton exudate) exhibited an increase in average swimming speed and an increase in the number of “burst” swimming behaviors. When exposed to a novel, non-food chemosensory stimulus (morpholine), no change in swimming behavior was observed unless the copepods had been conditioned to this odor in the presence of phytoplankton. Copepods exposed to mechanosensory stimulation alone (plastic spheres) exhibited a decrease in swimming speed and an increase in pause behaviors. When exposed to both forms of stimulation simultaneously (phytoplankton exudate and plastic spheres), a further decrease in swimming speed and increase in pause behaviors occurs, yielding a swimming pattern similar to that found in the presence of phytoplankton. This analysis of swimming pattern indicates that both chemoreception and mechanoreception contribute to the recognition of food inP. minutus.

Metabolic and blood characteristics of the hydrothermal vent tube-worm Riftia pachyptila

Abstract: Specimens of the hydrothermal vent pogonophoran Riftia pachyptila Jones were collected by submersible at a depth of 2 600 m at the 21°N hydrothermal vent site on the East Pacific Rise (20°50′N, 109°06′W) in April and May of 1982. The worms were maintained in pressurized aquaria for up to 45 d for metabolic studies. Consumption of O2 was regulated down to low PO2 (oxygen partial pressure) values; O2 consumption rates were 0.63 and 1.12 μ mol g-1 wet wt h-1 at 2.5° and 8°C, respectively; such rates were comparable to those previously measured for other pogonophorans. Intact specimens of R. pachyptila (including bacterial symbionts) did not consume significant amounts of CH4 from the environment. The respiratory quotients, in the absence of added sulfide, indicated that metabolism was mainly heterotrophic. High rates of uptake of dissolved amino acids were recorded for one specimen. The total [CO2] in the vascular blood and the Hb-containing coelomic fluid were high. Under anaerobic conditions, there were equilibrium distributions of pH, total [CO2] and sulfide concentrations between the vascular blood and the coelomic fluid, apparently because these metabolites were readily exchanged between the two compartments. The vascular blood bound neither CH4 nor H2. However, sulfide was reversibly bound by both the vascular blood and coelomic fluid; because this binding depended strongly on pH (with a maximum at about 7.5), HS- was probably the molecular species bound. Under anaerobic, but not aerobic conditions, the trophosome bound substantial amount of sulfide; thus, the high concentrations of sulfide in the trophosome may have resulted mainly from sulfide bound to sulfide oxidases under anaerobic conditions. The coelomic fluid had a relatively low buffering capacity (2.2 mmol CO2ΔpH-1).

Kinetics of light-intensity adaptation in a marine planktonic diatom

Abstract: The marine planktonic diatom Thalassiosira weisflogii was grown in turbidostat culture under both continuous and 12 hL: 12 hD illumination regimes in order to study the kinetics of adaptation to growth-irradiance levels. In both illumination regimes adaptation to a higher growth-irradiance level was accompanied by an increase in cell division rates and a decrease in chlorophyll a cell-1. The rates of adaptation for both processes, derived from first order kinetic analysis, equaled each other in each experiment. The results suggest that during the transition from low-to-high growth-irradiance levels chlorophyll a is diluted by cell division and is not actively degraded. Introduction of a light/dark cycle lowered the rate of adaptation. In transitions from high-to-low growth-irradiance levels there was a sharp drop in growth rates and a slow increase in chlorophyll a cell-1 under both continuous and intermittent illumination. In the 12 hL:12hD cycle there was a circadian rhythm in chlorophyll a cell-1, where cellular chlorophyll contents increased during the light cycle and decreased during the dark cycle. This circadian rhythm was distinctly different from light intensity adaptation. For kinetic analysis of light intensity adaptation in a 12 hL: 12 hD cycle, the circadian periodicity was separated from the light intensity response by subjecting the data to a Kaiser window optimization digital filter. Kinetic parameters for light-intensity adaptation were resolved from the filtered data. The kinetics of lightintensity adaptation of marine phytoplankton are discussed in relation to their spatial variations and time scales of mixing.

Predator and population density control of homing behavior in the Caribbean echinoid Diadema antillarum

Abstract: The frequency of homing behavior (percent returns to the same crevice) of Diadema antillarum Philippi varied from 30 to 84% among areas. Frequency of homing was positively correlated with predator abundance and negatively the protective quality of crevices and more readily vacates low quality crevices than more protective ones when a simulated predation attempt occurs. Previously grazed portions of the foraging range of homing individuals do not appear to be regrazed on successive nights and an avoidance of such areas is suggested. This predator-mediated behavior pattern and its effects on coral reef communities may be a tropical analog to predator-sea urchin-algal relationships in temperate marine communities.

Effects of delayed feeding and temperature on the age of irreversible starvation and on the rates of growth and mortality of Pacific herring larvae

Abstract: The time periods from exhausion of the yolk to the age of irreversible starvation for Pacific herring Clupea harengus pallasi larvae were 8.5, 7.0 and 6.0 d at 6°, 8° and 10°C, respectively. These periods are within the range perviously measured for Atlantic herring larvae and other temperature zone fish species; they are long compared to the periods for tropical species. The variation in the length of this period is due almost entirely to temperature; the natural logarithm of the time period from fertilization to irreversible starvation is highly correlated (r=0.91) with the mean rearing temperature for 25 species of pelagic marine fish larvae. The rates of growth and mortality, measured for 26 experimental populations of Pacific herring larvae reared at 6°, 8° and 10°C and ten ages of delayed first feeding, decreased and increased, respectively with increasing age of first feeding and increasing temperature. These rates, adjusted for the effects of rearing conditions, were compared with the rates for natural populations of herring larvae. Growth is generally faster in the sea than in experimental enclosures. Two of the eleven estimates of natural mortality rate were high enough to indicate possible catastrophic mass starvation. This is consistent with Hjort's critical period concept of year class formation and it suggests that mass starvation occurs in 18 to 36% of the natural populations of first feeding herring larvae.

A simulation analysis of continental shelf food webs

Abstract: Energy flow through continental shelf food webs was examined using a simulation model. The model structure expands the two traditional marine food chains of phytoplankton-zooplankton-pelagic fish and benthos-demersal fish into a complex web which includes detritus, dissolved organic matter (DOM), bacteria, protozoa, and mucus net feeders. Simulation of energy flux for different shelf systems using the expanded web revealed that heterotrophic microorganisms and their predators account for a significant component of the energy flux in the continental shelf ecosystem. Contrary to previous models, where all phytoplankton were considered to be grazed by zooplankton, our simulation results indicate that only slightly more than 50% of the annual net primary production is grazed. A substantial quantity of the phytoplankton production directly becomes detritus. Bacteria mineralize detritus and DOM produced by phytoplankton and other components of the food web, converting these to biomass with high efficiency. Consequently, the model predicts that planktonic bacterial production is equivalent to zooplankton production. Exclusion of the bacteria requires the assumption that all DOM is either exported from the system or consumed by another component of the food web. Neither of these assumptions can be supported by present knowledge of the dynamics of DOM in the sea. Model simulations were also employed to test the hypothesis that production exceeds consumption on continental shelves, resulting in exports of 50% of the annual primary production. Simulations of shelves with high rates of primary production resulted in a particulate export of 27% and realistic estimates of secondary production. Results of other simulations suggest that shelves with lower primary production cannot export production and still maintain the macrobenthos and their predators. General properties about continental shelves can also be inferred from the model. From simulations of shelves of differing primary production, nanoplankton are predicted to account for a greater proportion of the primary production in nutrient limited systems. Benthic production appears to be related to both the quantity of primary production and the sinking rates of the phytoplankton. The model indicates that zooplankton fecal inputs to the shelf benthos are only a small portion of the total detrital flux, leading to the prediction that fecal pellets are of little significance in determining benthic production. Finally, the model generates production efficiencies that are highly variable depending on the type of system and kind of populations involved. We argue that the assumed ecological efficiency of 10% should be abandoned for continental shelves and other ecosystems.

Differential ability of marine and freshwater macrophytes to utilize HCO3- and CO2

Abstract: Six marine macroalgae and two angiosperms were examined for their ability to use HCO 3 - and CO2 for photosynthesis. All species used HCO 3 - despite wide taxonomic differences. They also used HCO 3 - with high affinity: natural HCO 3 - concentrations (2.2 mM) were close to saturation, and the apparent half-saturation constants were low, i.e. K1/2 (HCO 3 - )=0.54 to 0.80 mM HCO 3 - . Expressed as a ratio, the affinity for CO2 relative to HCO 3 - under rate-limiting concentrations was about 2 for the marine species compared to much higher values, 5.4 to 101, among freshwater species examined previously. This difference was due to a higher affinity for HCO 3 - among marine species, whereas the affinity for CO2 was in the same range for marine and freshwater species, i.e. K1/2 (CO2)=0.08 to 0.30 mM. The high affinity for HCO 3 - of marine species is consistent with the high and constant availability of HCO 3 - and the low availability of CO2 in seawater. In freshwaters, availability of HCO 3 - and CO2 varies considerably, depending on habitat. The difference in HCO 3 - affinity may be due to different electrogenic ionpumps and thus mechanisms of HCO 3 - -use, operating in freshwater and marine species. Photosynthetic rates in natural seawater were close to maximum at atmospheric equilibrium (pH 8.2 to 8.4) and declined at high pH approaching zero between pH 9.5 and 10.5. This reduction may be due to the conversion of HCO 3 - to CO 3 - , followed by CaCO3 precipitation, and/or to a direct pH-effect. Ulva lactuca and Enteromorpha sp. retained photosynthetic activity at the highest pH tested (10.5), consistent with the high pH recorded in mats of these species.

The Wahlund effect and the geographical scale of variation in the intertidal limpet Siphonaria sp.

Abstract: Samples of Siphonaria sp. were collected between 1978 and 1982 from sites covering its known geographic range, from Kalbarri, Western Australia to Port Robe, South Australia. Geographic variation of 7 polymorphic enzymes was examined in this intertidal pulmonate limpet, and was found to be consistently small, indicating a large-scale influence of gene flow due to planktonic dispersal. Despite this large-scale uniformity, there is fine-scale genetic patchiness, which is repeated, rather than accumulated, on the larger scale. Throughout its geographic range, Siphonaria sp. shows deficits of heterozygotes for all 7 loci. The consistency among loci indicates that the causes of the deficits are populational, rather than locus-specific. A Wahlund effect, the departure from Hardy-Weinberg equilibrium due to mixing of individuals from groups with different allelic frequencies, is the simplest explanation of such deficits. The limited geographic variation of allelic frequencies, however, is grossly inadequate to produce these deficits through a Wahlund effect. Similarly, temporal variation in allelic frequencies in recruits does not explain the deficits. The largest contributor to a Wahlund effect appears to be binomial sampling variance among small local breeding groups. Thus, mixing of larvae on a scale of metres, rather than among geographical areas, apparently produces the deficits of heterozygotes.

Microhabitat-associated variability in survival and growth of subtidal solitary ascidians during the first 21 days after settlement

Abstract: Newly settled juvenile ascidians were transplanted into shaded and unshaded sites at two subtidal depths to test the hypothesis that photonegative settlement behavior enhances juvenile survival. Silt, filamentous algae and grazing gastropods (Margarites pupillus) were identified as sources of mortality associated primarily with non-cryptic microhabitats. Silt inflicted heavy mortality on all six species tested. Algal overgrowth was important in shallow water, where it increased mortality and depressed growth rates over a 14-d period. Laboratory experiments demonstrated that gastropods can remove juvenile ascidians from the substratum. The sources of post-settlement mortality investigated may help determine distributional patterns of ascidians in the San Juan Islands, Washington, USA, and may also represent selective pressures that maintain negative phototaxis in the behavioral repertoires of ascidian tadpoles.

Direct sampling and in situ observation of a persistent copepod aggregation in the mesopelagic zone of the Santa Barbara Basin

Abstract: Observations from a one-person submersible (“Wasp”) in fall, 1982, revealed a persistent aggregation of non-migrating, Stage V copepodites of Calanus pacificus californicus Brodsky in a band 20±3 m thick at a depth of 450 m, about 100 m above the bottom of the Santa Barbara Basin, California. Copepod abundances, calculated from nearest-neighbor distances measured directly from the submersible, yielded maximum densities of 26×106 copepodites m-3. Quiescent behavior, low laminarinase activity, low protein content, high lipid content and evidence of low excretion rate all suggest that these copepodites were in a state of diapause. Diapausing C. pacificus californicus at other locations along the eastern Pacific coast were also captured in discrete depth plankton tows. Both the submersible observations and the net collections suggest that the dense aggregation of diapausing copepods we observed in the Santa Barbara Basin was a phenomenon associated with seasonal upwelling cycles, and that such aggregations occur during non-upwelling periods when food is scarce in surface waters. Numerous predators, especially the deep sea smelt Leuroglossus stilbius, were observed feeding upon the aggregated copepods; thus, in contrast to the conventional picture of surface-dominated food distribution, deep-water aggregations of C. pacificus californicus may support the mesopelagic community during periods of low food availability in surface waters.

Abundance of picophytoplankton in the subsurface chlorophyll maximum layer in subtropical and tropical waters

Abstract: A distinctive chlorophyll maximum was detected around 60-m depth in the western North Pacific Ocean and the South China Sea, and almost 55% of the total chlorophyll in the entire water column was found within 50 m around the subsurface chlorophyll maximum (SCM) layer. More than 70% of the chlorophyll was contained in picoplankton which passed through a 3-μm Nuclepore but retained on 0.22-μm Millipore filters at the SCM as well as the surface layers. By transmission electron microscopic observations, the picoplankton were identified as aChlorella-like coccoid green alga having a section size of 1.2 to 1.5 μm and cyanobacteria of 0.5 to 2 μm. No obvious difference in these two dominant groups was observed in the SCM and the surface samples except in numerous and heavily stacked thylakoids in the former samples.

Diets and bathymetric distributions of the macrourid fish of the Rockall Trough, northeastern Atlantic Ocean

Abstract: The Macrouridae are the most common fish caught in demersal trawls on the continental slope and rise of the Rockall Trough. They represented 41% of all fish caught, with Coryphaenoides rupestris amounting to 28% of the catch. No previous study of the trophic interaction of these fish has been made over a wide bathymetric range. Samples were obtained at 250-m intervals of depth between 400 and 2 900 m in the period 1975 to 1981. The stomachs of 5 326 fish belonging to 12 species were examined to define their diets. Eight species are primarily benthopelagic feeders while four are primarily epibenthic feeders. The bathymetric centres of distribution of the populations of the benthopelagic feeding Trachyrhynchus murrayi, Malacocephalus laevis, Coryphaenoides rupestis, C. guentheri, Nematonurus armatus, Chalinura brevibarbis, C. leptolepis and C. mediterranea are different from each other. Similar differences in bathymetric distribution occurred among the epibenthic feeding Nezumia aequalis, Coelorhynchus coelorhynchus, C. occa and Lionurus carapinus. These differences decrease competition among species that exploit similar resources.

Disturbance by ice and life-history adaptations of the seagrass Zostera marina

Abstract: The formation of winter ice, and its movement with the tides, has had a major influence on the life-history parameters of shallow-water populations of the rhizomatous marine angiosperm eelgrass (Zostera marina) in Nova Scotia, Canada. In this region, and annual form of eelgrass inhabits intertidal and shallow subtidal mudflats and a perennial form is common in subtidal areas. Where lowtide water depth was greater than winter ice thickness (∼25 cm), ice movement removed much of the aboveground biomass from perennial ramets, but did not influence the density of ramets. Measurements of primary production showed that perennial plants allocated a greater proportion of their total production to below-ground structures than measured in all previous studies on eelgrass. Thus perennials are well anchored in the sediment, are more able to withstand removal by ice, and have reserves available for production of new leaves when ice melts. In the spring, seed germination was greatest in areas where ice had removed whole plants (or their aboveground parts) than among mature ramets which had survived winter intact. Survival of seedlings (new genets) was not affected by shading from the adult canopy, but a shading experiment showed that competition for light with mature ramets had a significant negative effect on morphology, growth and the allocation of seedling biomass to below-ground parts, thus reducing the ability of new genets to survive ice disturbance in the next winter. The annual form ofZ. marina was restricted to areas where low tide water depth was much less than winter ice thickness. Annual plants did not survive winter, had small investment in below-ground parts and high reproductive effort, and overwintered as seeds. The genetic status of the two forms and the restriction of the annual to very shallow water are discussed in the light of previous work on eelgrass

Predation on zooplankton by the benthic anthozoans Alcyonium siderium (Alcyonacea) and Metridium senile (Actiniaria) in the New England subtidal

Abstract: The Alcyonacean octocoral Alcyonium siderium Verrill and the sea anemone Metridium senile (L.), the only common perennial zooplanktivores on shallow (≦16-m depth) subtidal rock walls in much of northern New England, USA, are of similar heights and overlap in their habitat and microhabit distributions. The coelenteron contents of both species were sampled at four-hour intervals over a diel cycle and were compared to zooplankton available in the water at 1 to 5 cm from the rock wall, the height at which the cnidarians held their feeding tentacles. Prey in coelenterons of A. siderium were significantly smaller (means of 256 to 345 μm), and those in coelenterons of M. senile were equal to or slightly greater in length (means of 415 to 1006 μm) than the available zooplankton. The diets of A. siderium and M. senile differed significantly from each other and from the available zooplankton. A. siderium showed strong positive electivites for ascidian larvae and for foraminiferans, and strongly negative electivities for most crustaceans. M. senile had strong positive electivities for barnacle cyprids, ascidian larvae, and gammarid amphipods, and strong negative electivities for invertebrate eggs, foraminiferans, calanoid and harpacticoid copepods, and ostracods. Electivities may reflect tentacle avoidance or escape by motile prey as well as predator preference. Substratum-associated organisms (e.g. demersal crustaceans, larvae of benthic invertebrates) were the most common items in the diets of both species, suggesting a tight benthic food web, similar to the situation for coral reef anthrozoans which rely on reef-generated zooplankton. A. siderium ate large numbers of ascidian larvae which, as benthic adults, compete for space with A. siderium and can overgrow small colonies. Predation on the larvae of a competing species may alleviate competition by decreasing the competitor's recruitment.

Laboratory study of predation by Aurelia aurita on larvae of cod, flounder, plaice and herring: development and vulnerability to capture

Abstract: Capture success of the medusa Aurelia aurita preying on various developmental stages of fish larvae was measured together with larval reactivity and escape speed after being stung. These experiments were conducted in the spring of 1983 with A. aurita medusae collected from Loch Etive, Scotland and laboratory-reared larvae of Gadus morhua L., Platichthys flesus L., Pleuronectes platessa L. and Clupea harengus L. Capture success of the medusae increased with medusa size, but decreased with advancing larval development. Smaller species of larvae were more vulnerable to capture. Larval reactivity to encounters with medusae increased with advancing development, and larger species of larvae were more reactive to encounters. Larval escape swimming speeds also increased with advancing larval development and size. These results indicate that earlier stages of larvae within a species and smaller species of larvae at a given stage are more vulnerable to predation by medusae since they are less reactive to encounters. Apparently they are more susceptible to the effects of neurotoxins. Predation rates on different developmental stages of herring larvae are documented and compared with rates predicted by a predation model. Predictions fell within the range of observed predation rates, but tended to overestimate rates by larger medusae feeding on larger herring larvae. This indicates the possibility of predator satiation and/or behavioural avoidance.

Supply of organic matter to the sediment in the northern North Sea during a spring phytoplankton bloom

Abstract: Strings of moored sediment traps were deployed in a 150 m water column over a period covering the growth and collapse of the spring bloom (4 April–3 June 1976) in an area of the northern North Sea. The efficiency of collection of material in the moored traps was compared to collections in free-drifting traps in the same area of deployment. The ways in which the data from the trap collections may be interpreted was considered at some length and a best estimate of the flux of organic carbon and nitrogen to the sediment was made. For the period prior to the spring bloom (4–23 April) this flux was 50 mg C m−2 d−1 (about 20% of primary production). During the bloom (24 April–19 May) it was about 185 mg C m−2 d−1 (35% of production) and during early summer (20 May–3 June) it was 115 mg C m−2 d−1, about 25% of the overlying production. The organic carbon and nitrogen content of the material collected was measured and the material was examined microscopically. There was evidence of a large settlement of diatoms immediately after the spring bloom which was reflected in changes in the C:N and C:chlorophyll ratios of the material collected. This change in biochemical composition of the material may affect its nutritional quality and have a stimulatory effect on the growth and reproduction of the animals living in the sediment.

Close genetic similarity of Atlantic and Pacific skipjack tuna (Katsuwonus pelamis) demonstrated with restriction endonuclease analysis of mitochondrial DNA

Abstract: Restriction endonuclease analysis of mitochondrial DNA indicated a surprisingly high degree of genetic similarity between skipjack tuna (Katsuwonus pelamis) from the Atlantic and Pacific Oceans. The present results (1983) support the findings of previous morphological and electrophoretic studies. Evidently, since the uplift of the Panama land bridge about 3.1 million years ago, there has been continued genetic contact between Atlantic and Pacific skipjack tuna, presumably via the Southern Ocean.

Genetical structure within populations of the coral Pocillopora damicornis

Abstract: Genetic variation of allozymes within populations of Pocillopora damicornis from southwestern Australia was consistent with a primary role of local asexual proliferation of clones in population maintenance. Populations were composed typically of two to four multilocus genotypes accounting for 40 to 80% of individuals, with the remainder assigned to genotypes occasionally in twos or threes but more commonly singly. In the three populations where recruitment was examined genetically, 84% of all first-year recruits was assigned to clones represented in the population's resident adults. The majority of these recruits came from the most highly-replicated of the adult clones. The observed genotypic diversity was, on average, about half that calculated to occur for the same allelic frequencies in a sexually-reproducing population with free recombination. Despite the prevalence of asexual reproduction, both through planulae and fragments, the existence of a sexual mode of reproduction was inferred from the high level of variation produced by pooling populations, the existence of novel genotypes and the concordance of clonal gene frequencies at many sites with the predictions of Hardy-Weinberg equilibria.

Genetic differentiation of Mytilus edulis in eastern North America

Abstract: There is significant differentiation at five polymorphic loci ofMytilus edulis among certain geographical areas of the Atlantic coast of North America. Non-metric multidimensional numerical methods distinguished three population groups: (I) populations south of Cape Cod, (II) populations throughout the Gulf of Maine, Gulf of St. Lawrence, areas of both southern and northern Newfoundland, and southern Hudson Bay, and (III) populations in southeastern Nova Scotia, northern Newfoundland and Hudson Strait, Quebec. Each subset consists of populations that exhibit characteristic multilocus, multiple allele genotypes. Populations in Groups II and III are spatially interdigitated among each other. At least one geographical area of mixing between genetically distinct populations occurs in northeastern Newfoundland. There is no evidence for interbreeding among genetically distinct individuals in mixed populations, suggesting the possibility that populations in the Atlantic Canadian Provinces and areas of northern Canada may consist of two distinct species.

13C/12C ratios and the trophic importance of algae in Florida Syringodium filiforme seagrass meadows

Abstract: Over 380 stable carbon isotope (δ13C) analyses made during 1981–82 showed that Syringodium filiforme Kutz seagrass meadows in the Indian River lagoon of eastern Florida have food webs based on algal rather than seagrass carbon. Seagrasses averaging approximately-8‰ were isotopically distinct from algae epiphytic on seagrass blades (X=-19.3‰) and particulate organic matter in the water column X=-21.6‰. δ13C values of most fauna ranged between-16 and-22‰, as would be expected if food web carbon were derived solely from algal sources. These results counter the idea that seagrass detritus is the dominant carbon source in seagrass ecosystems. Two factors that may contribute to the low apparent importance of seagrass in the study area are high algal productivities that equal or exceed S. filiforme productivity and the high rates of seagrass leaf export from meadows.

Analyse des paramètres hydrobiologiques dans la remontée de Cabo Frio (Brésil)

Abstract: South Atlantic Central Waters (SACW) upwell close to Cabo Frio (Brazil, Rio de Janeiro) shore. The resultant typical hydrobiological conditions were studied at an anchorage station over an annual cycle from February 1973–February 1974. Multivariate analyses of hydrological and planktonic data revealed the structure of the ecosystem and the factorial relations governing its dynamics. Alternation, superposition or mixing of the water masses of different origin (Brazil current, coastal, SACW) constitute the most important factors responsible for the great hydrobiological variability observed in the study area. Spasmodic changes in wind direction and force are superimposed on a seasonal trend of more frequent upwelling in summer than during the rest of the year. The deep water is characterized by temperatures of <18°C, nitrate contents of 10 μg-at l-1 and by organic matter mainly composed of detritic elements from the shelf. Temperature variations together with nutrient contents (NO3 or PO4) reflect variations in primary biomass at the surface but not at 50 m depth, where detrital matter precluded valid measurements. Water of the Brazilian Current (salinity ≧36.0‰) frequently mixes with deep water of the thermal front, or with coastal water (<35.0‰) which invades the area when south-west winds prevail. This lower-salinity water is rich in seston particles. During the study period, primary biomass was relatively low due to eutrophication. We observed less than 3 μg l-1 chlorophyll and 106 phytoplankton cells per litre: the phytoplankton populations were highly diversified, indicating an advanced degree of complexity and evolution of this ecosystem. The observed formation of a thermocline constitutes an important inducing factor for an algal bloom. Simultaneous phyto- and zooplankton maxima would induce an increased grazing rate by herbivorous zooplankton which would also partly explain the relatively low level of primary biomass. Zooplankton is as abundant here as in other great upwelling regions: 100 organisms l-1 and 200 mg organic matter m-3.

Predatory control of copepod seasonal cycles on Georges bank

Abstract: A stimulation model of copepod population dynamics (development rate, fecundity, and mortality) was used to compute the predatory consumption necessary to control population growth in three dominant copepod species (Pseudocalanus sp., Paracalanus parvus, and Calanus finmarchicus) on Georges Bank, given observed seasonal cycles of copepod and predator populations. The model also calculated secondary production of each species. Copepod development rate and fecundity were functions of temperature while mortality was a function of predator abundance and consumption rate. Daily inputs of temperature and predator abundance (chaetognaths, ctenophores, and Centropages spp.) were derived from equations fit to field data. Model runs were made with various consumption rates until the model output matched observed copepod seasonal cycles. Computed consumption rates were low compared with published values from field and laboratory studies indicating that, even at conservative estimates of consumption, predators are able to control these copepod populations. Combined annual secondary production by the small copepod species, Pseudocalanus sp. and P. parvus, was nearly twice that of the larger C. finmarchicus with P. parvus having the highest total annual production.