Nematode fauna in the sulphide-rich brine seep and adjacent bottoms of the East Flower Garden,NW Gulf of Mexico

Nematodes sampled quantitatively in 1980 along a 96-m-long sulphide-rich brine seep gradient system in the NW Gulf of Mexico are characterized and compared with species from bottoms adjacent and beyond the influence of the brine. The age structure indicates permanent living populations within and outside the gradient system. Species composition shows a thiobiotic association and an oxybiotic association of nematodes, and each association is composed of two subgroups with a fifth group of species living in the ecotone. Such a distinction can also be shown in terms of dominance-diversity. The thiobiotic species are regarded as derived from their oxybiotic relatives and not the reverse. Nematodes describe environmental complexity since slenderness of the body correlates with the amount of dissolved sulphide in the environment. Body elongation, i.e. higher proportion of body surface area per unit body volume and shorter body radius, is suggested to be an adaptation to low oxygen tension in the environment as well as an adaptation to epidermal uptake of dissolved organic matter as additional nourishment of thiobiotic species. Analyses of buccal cavity structures and species distribution patterns show resource partitioning among most abundant species and congeneric species. Deposit feeders have a size diversity of buccal cavities similar to that of epistrate feeders and ominivore-predators. Deposit feeding prevails in the thiobios, but not in the oxybios.     

Oxygen/sulfide regime and nematode fauna associated with Arenicola marina burrows: new insights in the thiobios case

Profiles of oxygen and sulfide around the burrows of the lugworm, Arenicola marina, from a North Sea tidal flat were examined with microelectrodes, and the steep gradients were related to the microdistribution of nematodes. Around the tail shaft free oxygen penetrated only 2 mm into the burrow wall, coinciding with a bright zone sharply limited by the ambient black sediment. Contrastingly, in normal bottoms of the tidal flat (controls) only the surface of the bright zone was supplied with free oxygen. Here, the dark colouration coincided with the presence of free hydrogen sulfide. Around the tail shaft the nearest free hydrogen sulfide was detected 6 mm from the burrow wall leaving several millimetres of black sediment without measurable free sulfide. We discuss how these divergencies may relate to the stability of the oxygen/sulfide gradients and the course of time involved in their formation. A total of 54 nematode species were identified. Based on non-metric Multidimensional Scaling Ordination, four nematode assemblages corresponded to four microhabitats of the A. marina burrow: the funnel, the feeding pocket, the tail shaft and the feacal cast. The tail shaft assemblage (oxic plus partly anoxic zones) was similar to that of the anoxic zone of the control sediment. It was dominated by the most abundant nematode in the present study, Metalinhomoeus biformis (mean abundance in tail shaft 202 indx10 cm^-3). Adults of common nematode species from sulfidic microhabitats had a significantly higher length/diameter ratio than those inhabiting the oxic zone of the control sediment (p<0.001). The chemical recordings and metric analysis indicate that these slender nematodes around the A. marina tail shaft and in the reduced horizons of the reference sites represent thiobiotic assemblages, as compared to the shorter and stouter oxybiotic species characterising the assemblages from the surface zone and (partly) the funnel.     

Microplankton population structure in Southern California nearshore waters in late spring

Taxonomic composition, biomass as organic carbon, numerical abundance, and size distribution of the microplankton were determined at 6 Southern California nearshore locations in late May–early June, 1970. Samples were taken at approximately 5 m (10 m at one station) intervals through the upper 40 to 50 m to reveal some of the small-scale differences and levels of variability in the populations. Total microplankton biomass over all euphotic zone samples varied by more than two orders of magnitude (7.6 to 1,200 g C l^-1). Average biomass at comparable sites (n=5) ranged from 48 to 240 g C l^-1; biomass range within stations varied from about 5-fold to 120-fold. Total microplankton numbers varied approximately 22-fold (4.3×10⁵ to 9.5×10⁶ organisms l^-1) over all euphotic zone samples, but the range within stations was always less than an order of magnitude. At comparable stations, nanoplankton biomass had ranges extending from 3.7-fold to 12-fold; its average percentage contribution (±1 SD) to the total microplankton biomass varied from 39±5% to 54±13%. Netplankton biomass showed a similar minimal range, but its greatest range was more than two orders of magnitude. Ranges of abundance of major taxonomic groups within stations varied considerably from about 2-fold to more than three orders of magnitude. The small-scale variability of the populations probably affects the reliability of the microplankton as a food source for pelagic consumers.     

Spatial variation of growth and gonadal developments of Maurolicus muelleri in the Norwegian Sea and in a Norwegian fjord

The epibenthic megafauna of the high-Arctic Northeast Greenland shelf was investigated by means of seafloor photography and Agassiz trawl catches. At 54 stations in water depths between 40 and 770 m, sequences of color slides, each depicting about 1 m² of the seafloor, were obtained along photographic transects of about 100 to 600 m length. The photographs were quantitatively analyzed for abundance of epibenthic organisms identified by comparison with specimens collected from trawl catches. Megabenthic biomass was estimated by multiplying density values with averge body mass figures. For five dominant brittle star species, the population oxygen uptake and, thus, organic carbon mineralization potential were approximated by applying individual respiration rates of average-sized specimens to density figures. Multivariate analyses of the megabenthic species distribution revealed a distinct depth zonation. Shallow shelf banks (<150 m), characterized by coarse sediments, many stones and boulders as well as negative bottom water temperatures, housed a rich epifauna (30 to 340 ind m^–2, 1.8 to 10.5 g AFDW m^–2), strongly dominated (80 to 98% by numbers) by the brittle stars Ophiocten sericeum and Ophiura robusta. The oxygen uptake by brittle stars ranged from 0.4 to 95 mol O₂ m^–2 h^–1 (i.e., assuming a respiratory quotient of 0.8, an organic carbon mineralization of 0.1 to 21.9 mg C m^–2 d^–1). At the bank flanks sloping to the shelf troughs (100 to 580 m), finer sediments prevailed, stones were rare, and bottom water temperatures were positive due to the inflow of Atlantic water. Compared to bank sites, total epibenthic abundances as well as carbon mineralization by brittle stars were roughly ten times and total biomass about four times smaller. In deep shelf depressions as well as at the continental slope (200 to 770 m), stones were completely lacking, and sediments very fine. Epibenthic standing stock and carbon mineralization were one to two orders of magnitude lower than on the banks. The estimation of brittle star oxygen uptake indicates that a considerable portion of the organic carbon produced in the polynya and partitioned to the benthos may be remineralized by epibenthic bank assemblages.     

Oxygen dependent sulfide detoxification in the lugwormArenicola marina

The lugwormArenicola marina L. oxidizes entering sulfide to thiosulfate. After 8 h of normoxic incubations with sulfide concentrations of 0.2 to 1.0 mmoll^-1 thiosulfate in the coelomic fluid amounted up to about 4 mmoll^-1 whereas sulfite concentrations were 100-fold lower and no accumulation of sulfate in the coelomic fluid was found. The sulfide oxidation was highly oxygen dependent. An increase of oxygen partial pressure ( ) in the medium was followed by enhanced thiosulfate production and by a decrease of sulfide concentration in the coelomic fluid. Under normoxia, the sulfide oxidation rate was sufficient to compensate the influx of sulfide into the coelomic fluid when the sulfide concentration in the medium was below 0.33 mmoll^-1. When external sulfide was raised beyond this level, sulfide up to 5 moll^-1 in the coelomic fluid appeared. Succinate in the body wall tissue was low as long as no sulfide appeared in the coelomic fluid, indicating the maintenance of an aerobic metabolism. The oxidation of sulfide to thiosulfate was localized in the mitochondria of the body wall tissue. The oxygen consumption of mitochondria was stimulated by the addition of sulfide. The mitochondrial sulfide oxidation rate depended on the amount of mitochondrial protein and followed a Michaelis-Menten kinetic. An apparentK _m of 0.68±0.29 moll^-1 and aV _max of 41.9±22.3 nmol min^-1 mg^-1 protein was calculated. Sulfide was stoichiometrically oxidized to thiosulfate with 1 mol sulfide consuming 1 mol oxygen. Sulfide oxidation was not inhibited by sulfide concentrations as high as 100 moll^-1. At low concentrations of cyanide or azide, when respiration without sulfide was already inhibited, sulfide oxidation could still be stimulated, tentatively indicating the existence of an alternative terminal oxidase. Specimens examined in the present study were collected near St. Pol de Leon, France, from 1989 to 1992.     

Prey selection by larval weakfish (Cynoscion regalis): the effects of prey size,speed, and abundance

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 itemsl^-1 weakfish, larvae always chose large rotifers (length = 216 m) over small rotifers (length = 160 m). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 m); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 itemsl^-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10000 items^-1. Lwimming speeds of prey items, which ranged from 1 to 6 mms^-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.     

Abundance and biomass of macrobenthos in the vicinity of Carson Submarine Canyon,northwest Atlantic Ocean

The abundance, biomass and mean weight of macrofauna in the Carson Canyon region (Lat. 45°30N, Long. 48°40W) of the Grand Banks were estimated from 40 quantitative infaunal samples taken at 15 stations in June 1980. In contrast to other areas, there were no significant differences in these values between samples taken in and outside of the canyon. Abundance ( = 716 individuals m^-2), biomass ( = 10.9 gm^-2), and mean weight ( = 0.03 g individual^-1) displayed no consistent relationship with depth, but stations grouped according to sediment type had differing dominant taxa. Biomass and abundance at the edge of the Grand Banks are unexpectedly low. Larger animals, including fishes, that forage there must depend more on pelagic than on benthic food sources.     

A survey of environmental features in a section of the Vellar-Coleroon estuarine system,south India

A survey was conducted on 15th November, 1970, in mangrove forests and backwater regions of a section of the Vellar-Coleroon estuarine complex; a total of 19 stations were occupied. Detailed investigations on nutrients, pigments, and plankton were carried out. The following ranges in values were recorded: salinity, 10.40 to 30.50; pH, 7.50 to 8.30; temperature, 29.50° to 30.50°C; total phosphorus, 0.72 to 3.34 g at/l; inorganic phosphate, 0.19 to 1.59 g at/l; ammonia, 0.34 to 0.36 g at/l; nitrite, 0.11 to 0.25 g at/l; nitrate, 2.85 to 6.94 g at/l; silicate, 18.49 to 134.92 g at/l. Dissolved oxygen content ranged from a minimum of 3.69 ml to a maximum of 5.44 ml/l. Chlorophyll a ranged from an undetectable amount to 1.01 mg/m³, chlorophyll b from 0.02 to 0.85 mg/m³, chlorophyll c from 0 to 0.41 mg/m³ and carotenoids from 0 to 0.74 MSPU/m³. The plankton displacement volume ranged from a negligible amount to 3.60 cm³/m³; seston varied between 0.29 and 0.91 g/l. Phytoplankton was abundant at 3 stations; at other stations zooplankton was abundant. Coscinodiscus, Asterionella and Ditylum were the dominant forms among the phytoplankton; Oithona, Acrocaanus, Euterpina, Centropages, Corycaeus, Lucifer and Oikocreura were dominant among the zooplankton. Phytoplankton and zooplankton populations, as percentage of the total plankton, varied between 3.70 and 89.00% and between 11 and 96.30%, respectively. Average gross production values in the mangrove and back-water stations were 7.56 and 3.33 g C/m³/day, and the net production values 6.29 and 2.67 g C/m³/day, respectively.     

Spring abundance and distribution of the ctenophore Pleurobrachia pileus in the Seine estuary: advective transport and diel vertical migration

Several sets of plankton and hyperbenthos samples were collected from the Seine estuary in late May-early June of 1991, 1992 and 1994, and the abundance, size-frequency, horizontal and vertical distribution, and short-term fluctuations of Pleurobrachia pileus (O.F. Müller, 1776) were determined. In the Seine estuary, P. pileus was more abundant than reported for any other European shallow waters. Its depth-averaged density was 33 individuals m^-3 in 1991 and 32 individuals m^-3 in 1992, with a maximum density of 808 individuals m^-3. The ctenophores displayed passive tidal advection and active diel vertical migration. In the outer estuary, high abundances were observed around low tide, and diel vertical migration was very pronounced. However, many individuals remained aggregated near the bottom at all times. In spring, the distribution of P. pileus coincided with the 33 and 150 S isohalines, with high abundances on the marine side of the estuary. It is suggested that aggregation of ctenophores near the bottom and diel vertical migration play an important role in the retention of P. pileus in the estuary. Two schemes of P. pileus life cycle in the Bay of Seine are proposed.     

Vertical distribution and abundance of Acantharia and their symbionts

Microzooplankton was sampled during two cruises (Galápagos Vents, March 1985; Tongue of the Ocean and western edge of the Sargasso Sea, October/November 1985) by various collection methods (Niskin bottles, plankton nets, divers) to determine the vertical distribution and abundance of Acantharia. The larger size classes of Protozoa are dominated by the sarcodines, and Acantharia are frequently the most abundant of these in mesotrophic and oligotrophic oceans. The absolute densities of Acantharia have been consistently underestimated in many previous studies for two reasons: their skeletons dissolve in preserved samples, and they are undersampled by fine-meshed plankton nets. The previously identified dissolution problem may be less severe for concentrated samples because the dissolution of a portion of the Acantharia will raise the dissolved strontium concentration in the sample. Twenty five and 160 m-mesh plankton nets consistently underestimate the abundance of net plankton by one to two orders of magnitude. Possible reasons for this significant error are discussed. In the Equatorial Pacific Ocean, Acantharia were found at densities as high as 30 liter^-1 and integrated abundances of 1.58 to 5.34x10⁵ Acantharia m^-2. Up to 90% were concentrated near the surface; their abundance declined sharply below 20 m. At two stations in the Atlantic, peak densities reached 6.4 liter^-1. Wind-mixing may spread individuals more evenly through the euphotic zone, but they reestablish their surface maximum during period of calm. Acantharia generally have relatively few, but large symbionts. Small individuals average about 6 symbionts per host, larger hosts average 40 symbionts, and some individuals may have thousands of algal cells. Acantharia symbionts made up less than 1% of the chlorophyll in the water column, even at their host's peak abundances of 30 liter^-1. However, production estimates, using published sarcodine-symbiont production-rates, suggest that Acantharia could occasionally account for up to 20% or more of the carbon fixation in the upper euphotic zone of oligotrophic oceans.     

Effects of sulfide exposure history and hemolymph thiosulfate on oxygen-consumption rates and regulation in the hydrothermal vent crab Bythograea thermydron

The hydrothermal vent crab Bythograea thermydron is exposed to high environmental concentrations of sulfide and low levels of oxygen for extended periods of time. It has previously been shown that hydrogen sulfide is oxidized to the relatively non-toxic thiosulfate (S₂O ₃ ^2– ), which accumulates in the hemolymph. Hemolymph thiosulfate levels in freshly captured crabs vary significantly among crabs from different hydrothermal vent sites as well as between crabs from different microhabitats within the same site. Hemolymph thiosulfate concentrations were not significantly different between crabs captured at the same site 6 mo apart. Hemolymph thiosulfate concentrations ranged from 66 mol 1^–1 in a crab captured at a site with relatively low sulfide venting, to 3206 mol 1^–1 in an individual that was netted from an active smoker vent with much higher sulfide exposure. The differences in hemolymph thiosulfate between sites and the stability of hemolymph thiosulfate in crabs captured at the same site at different times suggest that sulfide exposure is significantly different between sites and that this exposure may not vary significantly over the course of a few months. B. thermydron experimentally exposed to sulfide had high levels of thiosulfate in their hemolymph and increased abilities to regulate oxygen consumption in conditions of low oxygen. This enhancement of regulatory abilities suggests that the previously demonstrated increased hemocyaninoxygen (Hc–O₂) affinity due to elevated thiosulfate may be adaptive in vivo. Average oxygen-consumption rates were much higher in crabs experimentally exposed to sulfide than in unexposed crabs. Crabs injected with isosmotic thiosulfate did not have increased oxygen-consumption rates as did the sulfide-exposed individuals, but did show a similar reduction in P _c (the critical partial pressure of oxygen at which crabs can no longer regulate oxygen consumption). This suggests that it is the sulfide exposure and/or detoxification rather than the elimination of thiosulfate that causes the increase in metabolic rate. Thiosulfate diffuses into dead crabs and into live crabs exposed to 15 mmol S₂O ₃ ^2- l^–1, indicating substantial permeability, and yet live crabs are able to eliminate thiosulfate when incubated in sea water containing 1.5 mmol S₂O ₃ ^2- l^–1, suggesting a process that has an active component.     

Planktonic ciliates in Southampton Water: abundance,biomass, production,and role in pelagic carbon flow

The abundance and biomass of marine planktonic ciliates were determined at monthly intervals at two stations in Southampton Water between June 1986 and June 1987. The two stations, an outer one at Calshot and an inner one at N. W. Netley, were subject to differing marine and terrestrial influences. The potential ciliate production at cach station on each visit was estimated from these data. Enumeration of ciliates and measurements of biovolume were performed on Lugol's iodinepreserved samples and potential production was calculated using a predictive relationship based on temperature and cell volume. Heterotrophic ciliate abundance and biomass were greatest at both stations during spring and summer months, with respective maxima of 16x10³ organisms 1^-1 and 219 g Cl^-1 recorded at N. W. Netley. Estimates of the potential production of the ciliate community ranged from <1 to 18 g Cl^-1 d^-1 at Calshot and <1 to 141 g Cl^-1 at N. W. Netley, with annual values of 2 and 9 mg Cl^-1 yr^-1, respectively. Abundances, biomass and potential production estimates were generally greater at N. W. Netley than at Calshot. Carbon flow through the ciliate community was assessed using annual production values from both this study and the literature. The annual ciliate carbon requirement was equivalent to 9 and 11% of annual primary production at Calshot and N. W. Netley, and potential annual ciliate production was equivalent to 34% and >100% of the energy requirements of metazoan zooplankton at these locations, although comprising only 8 and 10% of their available food.     

Seasonal variations in distribution patterns of sympagic meiofauna in Arctic pack ice

During two expeditions of the R.V. Polarstern to the Arctic Ocean, pack ice and under-ice water samples were collected during two different seasons: late summer (September 2002) and late winter (March/April 2003). Physical and biological properties of the ice were investigated to explain seasonal differences in species composition, abundance and distribution patterns of sympagic meiofauna (in this case: heterotrophs >20 µm). In winter, the ice near the surface was characterized by extreme physical conditions (minimum ice temperature: –22°C, maximum brine salinity: 223, brine volume: 5%) and more moderate conditions in summer (minimum ice temperature: –5.6°C, maximum brine salinity: 94, most brine volumes: 5%). Conditions in the lowermost part of the ice did not differ to a high degree between summer and winter. Chlorophyll a concentrations (chl a) showed significant differences between summer and winter: during winter, concentrations were mostly <1.0 µg chl a l^–1, while chl a concentrations of up to 67.4 µmol l^–1 were measured during summer. The median of depth-integrated chl a concentration in summer was significantly higher than in winter. Integrated abundances of sympagic meiofauna were within the same range for both seasons and varied between 0.6 and 34.1×10³ organisms m^–2 in summer and between 3.7 and 24.8×10³ organisms m^–2 in winter. With regard to species composition, a comparison between the two seasons showed distinct differences: while copepods (42.7%) and rotifers (33.4%) were the most abundant sea-ice meiofaunal taxa during summer, copepod nauplii dominated the community, comprising 92.9% of the fauna, in winter. Low species abundances were found in the under-ice water, indicating that overwintering of the other sympagic organisms did not take place there, either. Therefore, their survival strategy over the polar winter remains unclear.Communicated by O. Kinne, Oldendorf/Luhe     

Chaetognaths and ctenophores in the holoplankton of the Bristol Channel

The geographical distributions, seasonal variations in numerical abundance and biomass (mg C m^-3) of the predators of the holoplankton of the Bristol Channel, between November 1973 and February 1975, are described. The predator numbers and biomass were dominated by the chaetognath Sagitta elegans Verrill. This species represented 96% of the holoplankton carnivore biomass in the outer, seaward region of the Channel and 60% in the inner region; the remainder being ctenophores. The maximum numerical abundance of S. elegans occurred in September at 129 individuals m^-3 (18 mg C m^-3). Juveniles (<5 mm) reached maximum numbers of 55 individuals m^-3 during June, August and September, demonstrating the reproductive activity of the population. The peak numbers were probably the result of the development of two major generations over the 90 d period from mid-June to mid-September. The tentaculate ctenophores were represented by Pleurobrachia pileus (O. F. Müller). The highest abundance was 81 individuals m^-3 (3.0 mg C m^-3) at a single site in July in the South Central Channel. However, June was the only month when the ctenophores dominated the carnivore biomass in all regions of the Channel; thereafter, S. elegans was more abundant. Reproduction of the ctenophore occurred from April to September, with juveniles reaching maximum abundance in June at 12 individuals m^-3. The estimated food demand of the population in May for the outer region of the Channel was approximately 31% of the daily production of copepods. When the population reached its peak abundance in June, the estimated food requirement outstripped the daily production of copepods and a decline in both the prey and predator standing stocks was observed. Similar estimations were derived for the inner region of the Channel. S. elegans increased from a standing stock of 0.038 mg C m^-3 in March to 6.35 mg C m^-3 in September. Estimates of the copepod production compared with the derived demand of the chaetognath population showed that the decline in the copepods in the late summer was the result of feeding by this predator. The holoplankton carnivore population was approximately 66% of the copepod standing stock for the 10 mo period November 1973 to September 1974 in the outer region of the Channel and 45% of that in the inner region. The carnivores formed the greater part of the total holoplankton biomass from September through the winter months to February, suggesting a predator-dominated community.     

Description of a Tetraclita stalactifera panamensis community on a rocky intertidal Pacific shore of Panama

The midlittoral zone at Paitilla Beach, Panama, Is occupied by a group of organisms closely associated with Tetraclita stalactifera panamensis. Upon death, this cirripede provides housing for a number of species bound by common substrata and interacting with each other due to their close physical proximity; the faunistic group associated with each barnacle qualifies then, as a small, well-defined community which represents a subsample of the more comprehensive midlittoral community dominated by T. stalactifera panamensis. In patches where it is abundant, T. stalactifera panamensis occupies 28% of the surface area available. It has a relative abundance of only 1% of the specimens found in the zone, but all the other 95 species (with an average of 26, 810 individuals/m²) identified from the area are associated either with the parietal canals, or the external or internal surfaces of T. stalactifera panamensis tests. Each one of these structures represents a habitat which provides surface for attachment, shelter, or both, to 37 species of polychaetes, 32 of mollusks, 20 of crustaceans, 3 of actiniarians, 2 of pycogonids, 2 of sipunculans and an unknown number of tanaid, nemertean and turbellarian species. The single most diverse space-niche is represented by the parietal canals (H=0.783), occupied mostly by polychaetes. dead barnacles harbor an average of 131 individuals per test, which compose a fauna of higher diversity (H=0.678) than that associated with live barnacles (H=0.163). The latter contain 53 specimens per test on the average. Although there is considerable overlapping of species associated with the different T. stalactifera panamensis space-niches, mollusks are clearly more abundant on internal surfaces, polychaetes in parietal, canals; Balanus spp. on external surfaces, and anemones at the base of the parapet on the external surfaces of the tests. Over 220 species of macro-invertebrates were identified from the rocky intertidal at Paitilla. Most of the species are concentrated in the lower (157 species, 1824 specimens/m²) and midlittoral (96 species, 26, 810 specimens/m²) intertidal zones. Splash zone is depauperate (6 species, 1578 specimens/m²). Of the entire fauna identified from the intertidal, 73 species (33%) occur in more than one zone and 147 (67%) are restricted to a particular belt. This last category of zone-restricted species breaks down into 42% for the lower intertidal, 17% for the midlittoral, 6% for the intermediate zone, and 2% for the splash zone.     

Impact of irrigation on oxygen flux into the sediment: intermittent pumping by Callianassa subterranea and “piston-pumping” by Lanice conchilega

O₂-flux into sediments attributed to the pumping behaviour of two macrofauna species, Callianassa subterranea (Decapoda) and Lanice conchilega (Polychaeta) was investigated. Samples were obtained from the North Sea near Helgoland in 1989 and 1990. The two species were found to transport roughly similar amounts (3 mmolm^-2d^-1) of oxygen into the sediment although they displayed markedly different pumping behaviours. Irrigation by C. subterranea was intermittent and characterized by regularly recurring breathing currents which lasted 2.6 min and were separated by 40-min pauses. In addition to this regular intermittent irrigation, an irregular mode was observed. C. subterranea constructed a complex burrow system. At least half of the burrow wall was not in contact with oxygenated water, however, and thus not effective as additional interface for O₂-exchange. Sediment expelled from the burrow increased the total oxygen uptake (TOU) relative to the surrounding sediment surface. L. conchilega moved water much more frequently (every 4 min) than C. subterranea. We suggest that L. conchilega acted as a piston when moving in its tube, exchanging burrow water with the overlying water. This mechanism, termed piston-pumping, is also potentially important in other smaller tube dwelling organisms. At a shallow water station in the southern North Sea 21 ind of C. subterranea constructed 1.6 m² burrow surface per m². L. conchilega (300 ind m^-2) created only 0.37 m²m^-2 tube surface. On the basis of the abundance and oxygen transport associated with pumping activity, it is calculated that the two species increase TOU by 85% compared to O₂-flux across the sediment-water interface.     

Symbiont photosynthesis increases both respiration and photosynthesis in the symbiotic sea anemone Anemonia viridis

Dark respiration of the symbiotic sea anemone Anemonia viridis (Forskäl) was observed to increase by 34% when anemones were exposed to hyperoxic sea water (150% oxygen saturation) overnight, and by 39% after exposure to 6 h in the light at a saturating irradiance of 300 E m^-2 s^-1 at normoxia (100% oxygen saturation). No increase due to light stimulation was observed in aposymbiotic control anemones. In darkness, the oxygen concentration of the coelenteric fluid was hypoxic. However, within 10 min of anemones being illuminated, coelenteric fluid was hyperoxic, and it remained elevated throughout a 12 h light period. When measured over a 24 h period (12 h light: 12 h dark), the dark respiration rate increased gradually over the first 6 h of the light period until it was 35% above the dark night-time resting rate. It remained elevated throughout the remaining light period and for 2 h into the following dark period, after which it fell back to the resting rate. Gross photosynthesis (P ^gross) increased significantly when anemones were exposed to either hyperoxia (150% oxygen saturation) or 300 E m^-2 s^-1 at normoxia. This increase was not observed when symbiotic anemones were illuminated at a low-light intensity of 100 E m^-2 s^-1. The results of this study suggest that respiration in the dark is limited by oxygen diffusion and that normal respiration is restored in the daytime by utilisation of the oxygen released by photosynthesis. Furthermore, it appears that the increased respiration following exposure to high-light intensities provides a CO₂-rich intracellular environment which further enhances the photosynthetic rate of the zooxanthellae.     

Phytoplankton in the Weddell Sea,Antarctica: composition,abundance and distribution in water-column assemblages of the marginal ice-edge zone during austral autumn

At the compacted, north-south line of the ice edge, phytoplankton were sampled during early austral autumn of 1986 in the northwestern Weddell Sea. Cells from discrete water bottle samples from 12 stations on two east-west transects were counted to gain quantitative information on the composition, abundance, distribution, and condition of the phytoplankton in water-column assemblages. Over 70 species were found. The highest numbers of total cells (integrated through the top 150 m) were found in open water, well-separated from and to the east of the ice edge on the southern transect, with 6.01×10¹⁰ cells m^-2. The relative abundance of diatoms was low at ice-convered stations (< 35% of the total phytoplankton in preserved samples) and high at open-water stations (> 80%); however, the relative abundance of the prymnesiophyte Phaeocystis sp. was high at ice-covered stations (> 60%) and low at open-water stations (< 16%), with lower absolute abundances than during a previous austral-spring phytoplankton increase. In the open ocean, the dominants were the pennate diatoms Fragilariopsis cylindrus, Pseudonitzschia prolongatoides, F. curta, and a small form of the centric diatom Chaetoceros dichaeta in chains. Although the three pennate diatoms were frequently dominant in number, they represented less biomass than C. dichaeta in open waters. Mean phytoplankton abundance was low (0.2×10⁶ cells l^-1) but, overall, the diatom cell density (0.14×10⁶ cells l^-1) was similar to that found previously during a northward transect from ice-covered to ice-free water at the Weddell-Scotia Sea ice edge (spring 1983). The phytoplankton spatial patterns in the two autumn transects differed, with the more southerly transect exhibiting a higher abundance of diatoms and dinoflagellates. The ratio of full to empty diatoms was higher on the southern transect, indicating a healthy population, while lower ratios of full/empty frustules on the northern transect suggested a generally declining population. However, Phaeocystis sp. was more abundant on the northern transect.     

Photosynthesis-irradiance responses and photosynthetic periodicity in the sea anemone Aiptasia pulchella and its zooxanthellae

Sea anemones (Aiptasia pulchella) containing zooxanthellae (Symbiodinium microadriaticum) were maintained in a long-term laboratory culture on a 12 h light (100 E m^-2 s^-1):12 h dark cycle. Photosynthetic oxygen production was measured for the symbiotic association and for freshlyisolated zooxanthellae. Light utilization efficiencies () were similar for both sets of zooxanthellae, suggesting negligible shading of zooxanthellae by animal tissue in this association. Whereas freshly-isolated zooxanthellae were photoinhibited at high irradiances (800 to 1 800 E m^-2 s^-1), zooxanthellae in the host continued to function at photosynthetic capacity. Time of day may influence photosynthetic measurements in symbiotic organisms, as it was found that photosynthesis in A. pulchella followed a diel periodicity at both light-saturating (1 200 E m^-2 s^-1) and subsaturating (150 E m^-2 s^-1) irradiances. There was a peak period of photosynthesis between 12.00 and 14.00 hrs. Light stimulated dark respiration rates of A. pulchella. Dark respiration of sea anemones increased somewhat towards the end of the light cycle and was always greater after exposure to high irradiances.     

Habitat and reef-fish assemblages of banks in the Northwestern Hawaiian Islands

The moderately deep terraces and banks of the Northwestern Hawaiian Islands (NWHI) were surveyed to describe their habitat and reef-fish assemblages. These tracts of bottom at 30–40 m comprise more than 4,500 km² of the regions reef area. The habitat was found to be dominated by algal meadows (65% cover of exposed bottom), with infrequent relief features. Annual monitoring of select stations for 4 years at Necker Bank indicated that the relative difference in algal abundance between stations persisted from year to year (at least in summer). Temperature records from year-long deployments of archival thermistors in high-cover (>70%) and low-cover (<30%) algal biotopes were indistinguishable, providing no explanation of the algal differences between stations. At all banks, Microdictyon was the primary alga, averaging 1.22 kg/m². In spite of the extensive standing primary production, and a historical lack of fishing, bank reef-fish populations were impoverished. Mean densities, sizes, and biomass of trophic groups were considerably less than values reported for NWHI reef shallows. An overall mean biomass was estimated at 22.5 g/m², which is a fifth of that reported for shallow reefs of the region. Fish biomass of all trophic groups was associated with the few sources of relief available on the banks. Apex predators (sharks, jacks, and snappers), common on all surveys (with a mean of five per station), were proposed to constrain fish populations to sparse sources of relief resulting in a skewed size structure of the two primary fish trophic components. Sizes of lower-level carnivores were tightly correlated with sources of relief whereas the size of herbivores were not, indicating that herbivores more often venture out and risk the exposed algal meadows. These bank summits are a rare example of a near pristine reef system with high benthic primary productivity and low fish biomass, and are a stark contrast to shallower coral-reef ecosystems of the NWHI.Communicated by P.W. Sammarco, Chauvin