Ash-free Dry Wt Research Articles

Impact of Enteromorpha intestinalis mats on near-bed currents and sediment dynamics: flume studies

The influence of the macroalgal mats of Enteromorpha intestinalis on near-bed current velocities and sediment dynamics was quantified by placing relatively undisturbed cored sediments in annular flumes. Density-dependent relationships were established for E. intestinalis densities; these ranged from 10 to 60% cover when air-exposed (biomass of 4 to 40 g ash-free dry wt m -2 ) and in com- parison with bare sediment (0% cover). There was a significant increase in friction drag with increas- ing E. intestinalis biomass and percent cover, ranging from a mean 18% reduction in current velocities at 10% cover to 56% reduction at 60% cover. The net result of a reduction in currents (depth-averaged between 1 to 12 cm above the bed) and the physical protection of the bed by Enteromorpha was a marked reduction in sediment erosion of 60% at 10% cover to 90% at 60% cover. The presence of E. intestinalis also significantly enhanced sediment deposition measured as a flux from the water column to the bed. At 60% cover the deposition rate was 48% higher than on bare sediment during the first 30 min of slack water (<0.05 m s -1 ). The results show that E. intestinalis has a marked influence on water flow over the bed and the flux of particulate material across the sediment-water interface.

The amphipod Corophium multisetosum (Corophiidae) in Ria de Aveiro (NW Portugal). II. Abundance, biomass and production

The abundance and biomass of Corophium multisetosum Stock, 1952 were determined from benthic corer samples collected monthly over 1 yr in the upper reaches of Canal de Mira (Ria de Aveiro, Portugal). Both density and biomass over the sampling period were negatively correlated with water temperature and positively correlated with chlorophyll a concentration in the sediment. C. multisetosum density was significantly negatively correlated with plant biomass and positively correlated with salinity. The nature of the sediment, favourable environmental conditions, high availability of food and low interspecific competition allowed the population to reach a maximal density of 200 × 103 individuals m−2 and a maximal biomass (ash-free dry wt, AFDW) of 62 gAFDW m−2. The population was highly productive, especially during the autumn/winter period. Production, estimated by two different methods (Hynes method: 251 gAFDW m−2 yr−1; Morin–Bourassa method: 308 gDW m−2 yr−1), was much higher than the values reported for other Corophium species. The annual P:B¯ ratio (10) was high, but similar to values reported for Swedish populations of C. volutator and lower than the values estimated from Mediterranean populations of C. insidiosum.

Survey of the occurrence and morphology of sub-cuticular bacteria in shelf echinoderms from the north-east Atlantic Ocean

Echinoderms from the shelf seas around the British Isles were examined by epifluorescence and transmission electron microscopy for the presence of sub-cuticular bacteria (SCB). Information was obtained on SCB distribution in 63 of the 88 species known from this area, and 40 of these contained SCB. The SCB were present in all five classes. Only 39% of the holothurian species had SCB. In each of the other four classes, ≥60% of the species were associated with SCB. No correlation was noted between host ecology and SCB distribution or morphology. SCB distribution appeared to be related to host phylogeny. Congeneric echinoderms almost always had or lacked SCB, and this was usually also true for co-familial species. SCB showed limited morphological diversity, with all of them fitting into a classification of three general types. There was no morphological evidence suggesting that they are chemoautotrophs or methylotrophs. Some species had two morphological types of symbiont. SCB load was quantified for four species, three of which had symbiont densities >109 SCB g-1 ash-free dry wt. SCB are probably heterotrophic and may be co-evolving with their hosts.

Population ecology and secondary production of the polychaete Loimia medusa (Terebellidae)

In soft-sediment marine and estuarine habitats, population dynamics of resident species are an important aspect of commnity structure and function, yet the population dynamics of many members of these communities remain poorly studied. The population dynamics and secondary production of the infaunal terebellid polychaete Loimia medusa (Savigny) were investigated in a shallow sand habitat of the York River, Chesapeake Bay, Virginia, between June 1989 and December 1990. Monthly sampling throughout 1 yr revealed that individuals were present year-round with maximum densities of 60 ind m-2 and a life span of 1 yr. Abundances were highest from August through September when two cohorts recruited to the population. Mean individual growth was highest in summer, and appeared to be a function of temperature rather than food limitation. Adults were reproductive from May through October and exhibited high fecundity; the number of oocytes segment-1 ranged from 201 to 15840 among worms. Larvae initially appeared approximately 1 mo after gametes were first observed in adult L. medusa, and these larvae appeared to spend less than 1 mo in the plankton. Abundances of larvae in the water column were significantly higher around new moon, suggesting lunar spawning periodicity. New recruits attained sexual maturity within 2 mo of settlement. Secondary production was 3.3 g ash-free dry wt m-2 yr-1 and the P/B (production/biomass) ratio was 3.0. Despite equilibrium species characteristics such as large size, high fecundity, and planktonic larvae, L. medusa exhibits some characteristics typically ascribed to opportunistic species, namely, the capacity for rapid growth and maturation, a short life span and a relatively high P/B ratio. This mixture of life history characteristics emphasizes that there is no simple dichotomy between opportunistic and equilibrium species. We caution that the dominance of large infaunal polychaetes in soft-bottom estuarine and marine environments may not be a useful indicator of undisturbed habitats.

Physiological responses to fluctuation in temperature of salinity in invertebrates. Adaptations ofAlpheus viridari (Decapoda, Crustacea),Terebellides parva (Polychaeta) andGolfinigia cylindrata (Sipunculida) to the mangrove habitat

The snapping shrimpAlpheus viridari (Armstrong, 1949), the polychaeteTerebellides parva Solis-Weiss, Fauchald and Blankensteyn 1990, and the sipunculanGolfingia cylindrata (Keferstein, 1865) are commonly found in the same mangrove habitat, where they experience frequent, acute fluctuations in temperature and salinity. Ecological studies indicate a temporal variation, including occasional absence, in the distribution of bothG. cylindrata andT. parva; this fed us to examine the physiological adaptations of the three species (collected at Western Bay, Twin Cays, Belize in 1985, 1986 and 1988). Each was subjected to acute, repeated exposure to either control (35‰ S) and decreased (25‰ S) salinity or to control and increased (45‰ S) salinity. Ability to regulate water and ion content (g H2O or μmol g-1 solute free dry wt) was examinedA. viridari behaved as a hyperosmotic conformer at decreased salinity but as an osmoconformer at increased salinity. Regardless of direction of salinity change,A. viridari regulated water content through change in Na+, K+, and Cl− contents. In contrast,G. cylindrata behaved as an osmoconformer and did not demonstrate ability to regulate water content.T. parva behaved as an osmoconformer, showed incomplete regulation of water content via change in Na+, K+, and Cl− contents but had limited survival following exposure to 45‰ S. Each species was also exposed to change in temperature. Species were subjected to acute, repeated exposure either to control (28°C) and decreased (21°C) temperature or to control and inereased (35°C) temperatureA. viridari regulated water and ion content under both experimental conditions. In contrast,T. parva did not regulate water and ion content under either experimental temperature.G. cylindrata did not regulate water and ion content during exposure to decreased temperature and did not survive exposure to increased temperature. ForA. viridari, weight specific oxygen uptake rates (mg O2 g-1 ash-free dry wt) were determined. Exposure to decreased salinity or to increased temperature resulted in a small sustained elevation in O2 uptake. It is concluded that, unlikeA. viridari, T. parva andG. cylindrata are only marginally adapted to withstand the salinity and temperature stresses, respectively, of the mangrove habitat. The inability ofT. parva andG. cylindrata to fully adapt to extremes in the mangrove habitat could well explain the temporal variation seen in the distribution of these two species.

Foraging by the stoplight parrotfish Sparisoma viride II. Intake and assimilation of food, protein and energy

Daily food intake by the herbivorous parrotfish Sparisoma viride, as well as assimilation efficiencies of algal food, protein and energy, were quantified through a combination of laboratory feeding trials and field observations. The intake of algal ash-free dry wt (AFDW) per bite increases linearly with fish wet wt (FWW, g) and algal biomass (mg AFDW CM-2), and is further determined by the skeletal density of the limestone substrate. Low-density substrates yield higher amounts of AFDW per bite than do high-density substrates. The percentage of the total food intake that is derived from endolithic and crustose coralline algae increases with the size of the fish, and can be > 70 % depending on the biomass of epilithic algae. The daily pattern of foraging activity is positively correlated with diurnal changes in food quality, while seasonal daylength variations result in 13 % variation in total daily bites taken. Daily number of bites of S. viride in the field decreases with fish size, and is further dependent on life phase and foraging depth. In experiments, fish attained an assimilation efficiency of ca 20 % from a natural diet of low algal biomass and high-density dead coral substrates that predominates in the shallow reef. Assimilation efficiency was ca 70 % from a diet of high algal biomass and low-density substrates that predominates on the deeper reef parts. In spite of lower daily foraging effort, territorial fish, living in deeper parts of the reef, ingest and assimilate higher amounts of AFDW, protein and energy per day than non-territorial fish foraging on the shallow reef. The difference is caused by increased availability of high-yield food and substrate types inside territories compared to the situation on the shallow reef. Daily assimilated energy (kJ d-1) is 0.85 x FWW 0.773 for fish foraging in the shallow reef zone, and 1.22 x FWW0.854 for S. viride foraging inside territories on the deeper reef.

Proximate and elemental composition and energy content of mesopelagic crustaceans from the Eastern Gulf of Mexico

The proximate and elemental chemical compositions of 25 species of pelagic decapod and mysid crustaceans collected from the eastern Gulf of Mexico (∼27°N; 86°W, 1984 to 1989) was examined. Water level ranged from 63 to 95% and increased slightly with species' increased depth of occurrence. Protein levels were generally high (1.5 to 18.3% wet wt, WW; 27.6 to 62.4% ash-free dry wt, AFDW) and comprised the primary organic component in the majority of species. Protein, both as % WW and % AFDW, decreased with increased depth of occurrence. In contrast to protein, lipid levels were low (0.5 to 8.9% WW; 5.7 to 60.9% AFDW), and increased with increased depth of occurrence. Carbon and nitrogen best mirrored measured lipid and protein levels when considered as non-protein carbon and non-chitin nitrogen, respectively. C:N ratios increased with increased depth, consistent with changes in protein and lipid with depth. Because of their compositional attributes, resident Gulf of Mexico species have a low total wet weight energy content relative to species from more productive regions. Energy content showed no significant trend with depth. Vertical migration patterns were distinctly different between shallow-and deep-living gulf species and these differences were largely responsible for the relationships observed between composition and depth. In migrating species, the protein and nitrogen content were higher, the lipid and carbon contents and C:N ratio lower, than in non-migrating species. Three deep-living species of the genus Acanthephyra were found to be compositionally atypical, resembling shallow, migrating types rather than other deep-living, non-migratory species.

Decomposition of senescent blades of the seagrass Halodule wrightii in a subtropical lagoon

Senescent blades from the seagrass Halodule wrightii Aschers were suspended in the water column of Laguna Madre (Texas, USA) for a period of 419 d , representing the longest seagrass decomposition study to date. The initial stage of decomposition was characterized by a rapid loss of organic matter (36 % in 24 d ) attributed to leaching. A total of 76 % of the organic matter from seagrass tissues was lost by the end of the decomposition period. Of the major bulk constituents measured, neutral sugars were most abundant and accounted for 23 % of the ash-free dry wt of the initial senescent material. A complete and early loss of the cyclitol, myo-inositol, a reduction in glucose yields, and a relative enrichment in mannose were the most dynamic features of the neutral sugar fraction. An overall stability series: mannose > fucose > arabinose = rhamnose = galactose = xylose > glucose, reflects patterns of selective degradation of the polymers from which these sugars are derived. Soluble and ester-bound phenolic acids were lost from tissues at relatively high rates. Susceptibility of individual phenolic acids to removal from bulk tissue upon treatment with weak base correlated well to initial losses in the field. Early changes in total CuO oxidation phenol yields from H. wrightii were caused pnmarlly by the relatively rapid loss of soluble and ester-bound phenolic acids. Therefore, we took a new approach to quantlfy lignin-derived phenols by subtracting soluble and ester-bound phenols from total CuO oxldat~on ylelds. When these losses were taken Into account, lignin-derived phenol yields were similar from freshly senescent and highly degraded detntus. Cutln, although initially selectively preserved relative to bulk tissue, was also found at near initial yields In the highly degraded detritus. Overall, the distribution of polymeric constituents (cellulose, hem~cellulose, lignin and cutin) was similar in freshly senescent and highly degraded tissues suggesting that the compounds which comprise the ultrastructure of H, wrightij are degraded at similar rates Photobleached H. wriglitii blades were characterized by a much higher neutral sugar content, slmilar levels of cutin acids and a nearly complete absence of phenols con~pared to freshly senescent tissue. Photobleaching appears to be an important degradative mechanism which yields a polysaccharide-rich detritus that is devoid of the lignin signature characteristic of vascular plant tissues.

Abundance and distribution of earthworms in relation to landscape factors on the Georgia Piedmont, U.S.A.

Abundance and distribution of earthworms were studied on the Georgia Piedmont of the Southeastern U.S.A., at sites representing various ecosystem types, management practices, landscape positions, soil textures and soil erosion status. Earthworm abundance showed distinct seasonal patterns, with winter/spring maxima and summer minima. Numbers and biomass ranged from zero in plowed. mono-cropped soil at an upland site to over 1000 m -2 ( > 25g ash-free dry wt m -2) in no-tillage, double-cropped soil on bottomlands. Numbers and biomass in plowed, double-cropped soil, in a bottomland forest, and in grass meadows at both upland and bottomland sites were intermediate. Soil texture, as influenced by water erosion, strongly affected earthworm abundance. Moderately and severely eroded sandy clay loam supported significantly higher earthworm numbers and biomass than did slightly eroded soil with higher sand content. This effect may have resulted from low organic content and water holding capacity of the sandy soils. Of the soil texture variables, silt content was most highly correlated with earthworm biomass. Earthworm abundance was also related to quantity and quality of plant residue inputs in the agroecosystems, and to standing stocks of soil organic carbon across all sites studied. At most agroecosystem and forest sites, the predominant earthworm species were European lumbricids; native Diplocardia spp were most prominent in a meadow soil with high organic content.

Spatial and seasonal variation in growth rate and secondary production of Yoldia notabilis in Otsuchi Bay, Japan, with reference to the influence of food supply from the water column

A 1 yr field study of a long-llved protobranch bivalve, Yoldia notahiljs Yokoyama, in Otsuchi Bay, northeastern Japan, revealed that there was a large seasonal and spatial variation in growth rate and secondary production, and that primary production in the water column and following organlc supply to the bottom were responsible for the vanation. Both shell and soft tissue growth of each of 9 year-classes were rapid in spring, and slow or even negative in other seasons. The rapid growth coincided well with the occurrence of spring phytoplankton bloom in the water column and the peak of organic flux to the bottom. Mean shell length and mean soft tissue weight of each year-class were larger, and the growth rates of shell and soft tissue as well as the secondary production were higher at the shallower station (10 m deep) than at the deeper station (14 m). The differences in slze, growth rate and production between the 2 stations were consistent with the local difference in food supply from the water column to the bottom, but not related to other environmental factors such as temperature, salinity, organic content in the sediment and density of Y. notabilis. Annual production was 1.38 and 0.89 g ash-free dry wt 0.1 m-'yr-', giving P/B ratios of 0.76 and 0.41 for the shallow and deep station respectively. Food intake by Y. notabilis between February and April 1991 was estimated to be 26.4 and 13.9 g C m-2 (shallow and deep respectively), which amounted to ca 10 % of the primary production and 30 to 40 % of the organic supply to the bottom. These estimates suggested that the Y. notabilis population plays an important role in the energy flow from pelagic to benthic communities.

Primary productivity and particulate organic matter flow in an estuarine mangrove-wetland in Hong Kong

Net primary productivity and organic matter flow of a mangrove-dominated wetland was estimated by following production and detritus dynamics in a tidal pond in north west Hong Kong in 1986–1988 (9.1 ha). Total productivity was 12.47 t dry wt ha−1 yr−1, of which >90% was from emergent macrophytes (the mangroveKandelia candel and the reedPhragmites communis). High turbidity and high summer temperatures probably limited respective production by phytoplankton and benthic macroalgae (dominated byEnteromorpha crinata). Despite the high total productivity, little detritus was exported from the emergent macrophyte stands, due to the low inundation frequency. This created a net water column carbon deficit which was provided for by the high organic matter import (mean = 4.42 g ash free dry wt m−2 d−1) from the incoming water. This same sediment and particulate organic carbon input giving a high accretion rate of 1.7 cm yr−1 was probably also the force behind progressive dis-coupling of emergent macrophyte production from water column consumers. This resulted in a tendency to retain production in the emergent macrophyte stands while the water column community increasingly relied on allochthonous carbon. This shift from a net exporter to a net importer of carbon in landward wetlands is probably characteristic of the transition into nutrient-conservative terrestrial systems.

Proximate composition of Antarctic mesopelagic fishes

Eleven mesopelagic fish species from the Weddell/Scotia Sea region of the Antarctic captured during the austral spring 1983, austral fall 1986, and austral winter 1988, were analyzed for proximate composition. Water, ash level, protein, lipid and carbohydrate were examined in relation to depth of occurrence and season. No depth-related trends were evident, primarily due to a low species diversity and minimal differences in those species' vertical distributions. The Antarctic speciesElectrona antarctica showed a significant increase in lipid level (% wet wt and % ash-free dry wt) between spring, fall and winter. The increase may signify an accumulation over the productive season, possibly as a reserve for the winter months. Lipid levels (% wet wt and % ash-free dry wt) were significantly lower in the Weddell Sea specimens examined in this study than in previously examined identical and congeneric species taken during the same season from a more productive near-shore Antarctic region. Comparisons with congeners and confamilials from tropical-subtropical and temperate systems revealed variable trends. The Antarctic speciesE. antarctica andCyclothone microdon had lower water and protein (% wet wt) levels than similar species from tropical-subtropical or temperate regions. Lipid levels of the two species are similar to temperate individuals, while energy levels are slightly higher. In contrast, species of the genusBathylagus show no trends in composition as a function of latitude. Differences in productivity, water-column temperature-structure, and seasonality are important considerations when examining trends among mesopelagic species.

Consumption of diatoms and diatom filtrates by the tentaculate deposit-feederEupolymnia nebulosa (Annelida: Polychaeta)

Rates of organic uptakes of three live diatoms (Nitzschia acicularis, Nitzschia sp. andNavicula incerta), and of three corresponding filtrates by the deposit-feeding polychaeteEupolymnia nebulosa (Montagu) were measured under similar experimental conditions. Worms used during this study were collected by SCUBA diving at Port-Vendres in shallow water (7 m deep) during the summer of 1986. Uptake rates of live diatoms were affected both by length of the experiments and by the nature of the food offered. The highest rate of uptake (11.8 10−4 mg algal ash-free dry wt mg−1 worm dry wt h−1) was recorded during a short-term experiment (4 h) with the smallest diatom (Nitzschia sp.). The lowest rate (1.1 10−4 mg algal ash-free dry wt mg−1 worm dry wt h−1) was recorded during a long-term experiment (48 h) with the largest diatom (Nitzschia acicularis). Filtrates ofN. acicularis were more readily utilized than those ofNitzschia sp. andNavicula incerta. Because of differences in uptake rates of algal filtrates as a function of species, it is not possible to evaluate the bias due to interaction with dissolved substances in experimental studies assessing ingestion of live benthic diatoms byE. nebulosa.

Feeding responses of the tentaculate deposit-feederEupolymnia nebulosa (Annelida: Polychaeta): Influence of sexual maturity

Feeding responses of the tentaculate depositfeeding polychaeteEupolymnia nebulosa (Montagu) were studied by measuring rates of uptake of three different14C-labelled diatoms (unialgal cultures ofNavicula incerta Grunow,Nitzschia acicularis Wm Smith, andNitzschia sp.). Worms used during this study were collected in the harbor of Port-Vendre (Western Mediterranean) during August 1986 (immature worms) and December 1987 (mature worms). Uptake rates were affected both by the length of the experiments and by the nature of the food offered. The highest rate of uptake (2.98 10−4 mg ashfree dry wt of algae mg−1 dry wt of worms h−1) was obtained during short-term experiments (4 h) with the smallest diatom (Nitzschia sp.). The lowest rate of uptake (0.21 10−4 mg ash-free dry wt of algae mg−1 dry wt of worms h−1) was also obtained withNitzschia sp., but for a long-term (48 h) experiment. There was no significant difference between rates of uptake of immature and mature worms.

Population dynamics and production of Phyllorhiza punctata (Cnidaria: Scyphozoa) in Laguna Joyuda, Puerto Rico

Medusae of Phyllorhiza punctata were sampled in Laguna Joyuda from October 1985 through October 1986. Maximum numerical abundance (143 ind. per 103 m3) and population biomass (530.6 mg ash-free dry wt [AFDW] m-3) were observed during summer Net increments In population biomass were associated with growth of individuals from October to December and with individual growth and subsequent recruitment of small medusae from May to August. More than 75 ' ' , of the total recruitment into the smallest size classes ( 30 cm) during late summer in the following year suggests that P. punctata follows an annual cycle in Laguna Joyuda. Growth rates, calculated from increases In mean individual weight of held-collected specimens, were exponential during autumn (mean g,. = 0.077 d l ) , and fl-om June through August (summer; mean gw = 0.079 d-l). Individual growth rates declined with increasing size durlng both these growth stanzas, but faster growth rates were observed In autumn for medusae of equivalent or larger size. Total population biomass increased exponentially at rates of ca 0 063 and 0.028 d-l during October to December 1985 and May to August 1986 respectively. The slower growth rates in summer coincided with a marked reduction in abundance of non-gelatinous zooplankton, suggesting that medusae could have been food limited at this time. Nevertheless, estimated population production rates were much higher during summer (maximum 14.3 mg AFDW m-3 d.') due to the higher population biomass compared to autumn (maximum 1.6 mg AFDW m-%-').

Epilithon and dissolved oxygen depletion in the Manawatu River, New Zealand: Simple models and management implications

The relationship between epilithic biomass and oxygen depletion was studied above and below wastewater discharges to the Manawatu River in an attempt to guide river management by defining the maximum respiration rates and the corresponding epilithic biomass consistent with maintenance of the legal minimum DO concentration of 5 g m −3. A computer model, which simulated the river DO under lowflow conditions, predicted that the maximum allowable respiration rate giving a dawn DO concentration of 5 g m −3 ranged from 20 g O 2 m −2 d −1 at 21°C to 24.5 g O 2 m −2 d −1 at 12°C. A multiple regression model developed from chamber measurements made in situ showed that the total biomass density and water temperature accounted for 73% of the variation in the respiration rate observed. When adapted to allow for respiration occurring in the water column, the model gave reasonably accurate predictions of respiration when checked against the whole river respiration rate measurements. Biomass densities, at which DO levels reach 5 gm −3 predicted using these models, ranged from 34 g ash free dry wt m −2 at 21°C to 143 g ash free dry wt m −2 at 12°C. Analyses of historical hydrological data and the results of epilithon growth measurements under a variety of water quality conditions in the Manawatu River indicate that the present management strategy of limiting instream BOD 5 to below 5 g m −3 but not limiting nutrient (N and P) levels will not prevent the occurrence in summer of the predicted nuisance biomasses and hence unacceptable deoxygenation.

Benthic processes across mixed terrigenous-carbonate sedimentary facies on the central Great Barrier Reef continental shelf

The shallow continental shelf of the central Great Barrier Reef province is characterized by an across-shelf gradient of terrigenous to carbonate sedimentary facies, and is a region subjected to physical disturbances from commercial prawn trawling and, less frequently, from summer cyclones and storms. The benthic regime was examined during austral spring and summer to delimit changes in relation to the sedimentary transition. Standing stocks of bacteria ( x = 1.2;range= 0.4-2.7 × 10 10cells g −1 dry wt), protozoa ( x = 1.6;range= 1.0–2.1 × 10 6cells m -2) and meiofauna (abundances: x = 1955;range= 810–3255ind.10 cm −2; biomass: x = 2028;range= 694–3130μg dry wt 10 cm −2) generally did not vary significantly across the shelf and are very abundant compared to their counterparts on other shelves. Bacterial productivity ( 3H thymidine incorporation into DNA) and specific growth rates (μ) ranged from 0.4 to 2.6 g C m −2 d −1 ( x = 1.5) and from 0.1 to 0.8 d −1 ( x = 0.3), respectively, in surface sediments and generally did not decline significantly with sediment depth. Standing amounts of chlorophyll a ( x = 0.6;range= 0.1–2.0 μg g −1dry wt) and phaeopigments ( x = 3.9;range= 1.0–1.6 μ g −1dry wt) were low compared with pigment levels in adjacent slope and bathyal trough sediments, indicating low deposition rates of phytodetritus on the shelf proper. Macroinfaunal densities ( x = 3056;range = 2060–5406 ind. m −2) were moderate to high, but biomass ( x = 1.96;range = 0.94-3.37 g ash-free dry wt m −2) was low and decreased significantly across the shelf. Small, tube-building deposit and suspension-feeding polychaetes and amphipods dominated the infauna with a notable absence of bivalve molluscs. X-radiography revealed sedimentary profiles characteristic of oligotrophic seafloors experiencing low and variable rates of sedimentation and infaunal bioturbation, but subjected to episodes of physical reworking. The sediment fabric was extensively bioturbated mainly in the upper 2 cm with compact (<40% water) sediments devoid of large equilibrium infauna below this depth. Levels of organic carbon, total nitrogen, total phosphorus and porewater (NH 4 +, NO 2 − + NO 3 − Si(OH) 4, PO 4 3−) nutrients were low compared to other shelf sediments and generally varied little with sediment depth indicating some vertical mixing, but little burial of reactive organic matter. Dissolved inorganic nutrient fluxes measured in incubated ☐corers were low (ΣN = 0–2591; PO 4 3−; = 0–142; Si(OH) 4 = 1100–5790 μmol m −2 d −1) compared to other benthic environments suggesting low rates of detrital input. Oxygen consumption rates ranged from 13 to 40 mmol O 2 m −2 d −1. Nutrient fluxes were not directly related to surface respiration, bacterial growth or temperature. The average stoichiometric ratios of solid-phase (atomic C:N:P = 23:5:1) nutrients and measured fluxes (Si:P = 66:1; N:P = 9:1; O:N = 39:1) differed substantially from the Redfield ratio. Departure from the predicted ratios was attributed to denitrification and preferential retention of P due to binding with Fe and Mn oxyhydroxides and CaCO 3. Benthic standing crops and processes on the central shelf appear to be regulated by a variety of factors, including low and intermittent inputs of detritus, continually warm temperatures and physical disturbances caused by occasional cyclones and by frequent prawn trawling. Such disturbances and oligotrophic conditions perpetuate the dominance of a pioneering benthic regime as well as the cycle of low plankton production and sedimentation to the seafloor. Nutrient release from the benthos contributes, on average, 13 and 24% of the annual N and P requirements of the shelf phytoplankton, suggesting that a moderate coupling exists between benthic and pelagic regimes on this shallow tropical shelf.

Biomass and trophic structure of the macrobenthos in the south-west lagoon of New Caledonia

The biomass of the macrobenthic communities of the southwest lagoon of New Caledonia is analysed with special reference to tropical soft-bottom assemblages. The samples were obtained in 1986 from 350 Smith-McIntyre grabs as well as by diving over an area of 3 500 m2, and covered a wide size-range of benthic organisms. The classification of the biomass data into taxonomic groups revealed the following: (1) a zoobenthic biomass (mean=13 g ash-free dry wt m-2) that is high compared to values previously published for tropical systems, but fairly low compared to those recorded for medium and high latitudes; (2) a large macrophytic biomass (45% of ash-free dry wt); (3) dominance by weight of sponges, molluscs and cnidarians over the other faunal groups. The distribution of the biomass among the trophic groups revealed the following: (1) the dominance by weight of suspension-feeders, which account for 67% of the total zoobenthic ash-free dry wt; (2) the absence of trophic group amensalism between filter-feeders and deposit-feeders whatever the degree of bottom muddiness; (3) the existence of a relationship between the biomass of surface deposit-feeders and ATP at the surface layer of the sediment, suggesting that this group feeds preferentially on living particles. Differences in trophic structure between the muddy-bottom, greysand and white-sand communities of the lagoon reflect the functional characteristics of each bottom type.

Allocation of organic material and energy to the holdfast, stipe, and fronds inPostelsia palmaeformis (Phaeophyta: Laminariales) on the California coast

Postelsia palmaeformis were collected from the lower intertidal at Pigeon Point, California, USA, in May 1987, and the proximate composition and allocation of energy to the various body components were determined. The holdfast and stipe have a proximate composition (% dry weight) of ca. 40% ash, 5.3% protein, 1% lipid, 2% soluble carbohydrate, and 55% insoluble carbohydrate. The fronds have a proximate composition of ca. 25% ash, 6.5% protein, 2% lipid, 3% soluble carbohydrate, and 65% insoluble carbohydrate. The energetic level was ca. 12 kJ g-1 dry wt and ca. 19 kJ g-1 ash-free dry wt. The relative proportion of three plant components varied, comprising 26, 39, and 35% wet wt and 20, 42, and 38% kJ for the holdfast, stipe, and fronds, respectively. A plant with a basal stipe diameter of 33 mm contains 114 g wet wt and 266 kJ. The maximal density found in May 1987 was 826 plants, 49 301 g wet wt, and 106 157 kJ m-2.P. palmaeformis differs in these characteristics from another intertidal pheophyte,Durvillaea antarctica, that is found in a high-energy intertidal zone.

Ecology of amorphous aggregations (marine snow) in the Northern Adriatic Sea. II. Microbial density and activity in marine snow and its implication to overall pelagic processes

In the Northern Adriatic Sea, marine snow dry mass varied between 3.4 and 9.1 mg 1-' during summer. In situ measurements of 02-fluxes mediated by marine snow revealed a gross primary production (GPP) ranging from 3 to 23 pg 0 2 mg-' (marine snow ash-free dry wt, AFDW) h-' 0 2 consumption rates ranged from 1.35 to 3.7 pg O2 mg-' (marine snow AFDW) h-' and were ca 5 times the mean consumption obtained under laboratory conditions. Based on these 02-flux measurements, we eshmate that more than 90 % of the pelagic GPP is bound to marine snow aggregations and ca 70 % of the mineralization activity is mediated by marine snow associated microbes during summer. Total carbohydrate (TCHO) content in marine snow was found to be enriched by a factor of 304 as compared to the surrounding water (F = 0.82 &g TCHO ml-l). Batch cultures with marine snow were designed to investigate the influence of marine snow on the free-living microbial community. In the chambers containing marine snow, growth rates (v) of the bacterial community were 3 times the rates obtained for unenriched media (containing 1 pm filtered seawater only). Based on the batch culture experiment, it is concluded that marine snow enhances not only microbial growth on/in the organic matrix of the aggregations but also favors microbial activity of the surrounding water The degree of development of the microheterotrophic food chain was quantified using the ratio (nanoflagellate biomass/bacterial biomass) X 100. In the incubation media containing marine snow, ratios o! >l00 were maintained most of the time, indicating that additional food sources other than free-living bacteria were exploited by the nanoflagellate population. I t is suggested that more sensit~ve sampling methods should be used to resolve such nutrient patches which cause heterogeneity of the pelagic environment.