In situ filtering and ingestion rates of deep-sea benthic boundary-layer zooplankton in the Santa Catalina Basin

In situ rates of filtration, particulate ingestion, and carbon ingestion of deep-sea benthic boundary-layer zooplankton were determined in December 1984 in the Santa Catalina Basin, at 1 300 m depth in the California Borderland, by a short-term radioisotope-incorporation technique. Zooplankton were collected at 1 or 50 m above the bottom with an opening-closing net system on a submersible, and incubated at depth with labelled amino acids in special cod-end chambers. Concentrations of particulate material and particulate organic carbon in the ambient water were also measured. The zooplankton had a median weight-specific filtration rate of 12.4 ml (mg dr. wt)^-1 h^-1 and a median carbon ingestion rate of 5.4 g C (mg dr. wt)^-1 h^-1. Filtration rates were not significantly different from those in similar experiments in the north Atlantic at 2 175 m depth or Narragansett Bay in the winter, although the medians of the deep-sea experiments were lower than for the Bay. In the Santa Catalina Basin, rates from experiments at 1 m above the bottom in more turbid water were not significantly different from those at 50 m above the bottom in clearer water. These deep-sea benthic boundary-layer zooplankton may have the potential to respond to food pulses, and their relatively high ingestion rates suggest that they could have significant effects on particulate, chemical, and bacterial processes in the near-bottom water column.     

A method to determine in situ zooplankton grazing rates on natural particle assemblages

In situ zooplankton grazing rates on natural particle assemblages were stimated by measuring zooplankton uptake of labelled autotrophic (with Na¹⁴CO₃) and heterotrophic (with [methyl-³H]-thymidine) particulate matter in 1-h incubations in clear, Plexiglas, Haney chambers. The in situ grazing rates are in the same range as those measured for zooplankton in the laboratory using standard particle counting techniques. A negative selection coefficient for ³H-labelled particles indicated a lower filtration efficiency or avoidance of these particles by zooplankton.     

Microbial transformations in deep-sea sediments: free-vehicle studies

Radiotracer techniques can be used to assess eitherin-situ metabolic activities of natural microbial populations or potential turnover rates of specific substrates under the givenin-situ conditions. The second approach has been used in deep-sea sediment studies where steady-state conditions are unlikely to occur because of the heterogeneous distribution of nutrients as a result of a patchy input by depositing particulates. Recent sediment trap studies have shown that particulate matter reaching the deep sea does contain simple organic compounds. Therefore, the present experiments dealt with the measurement of carbon incorporation and respiration of¹⁴C-labeled acetate, glucose, glutamate and trimethylamine in deepsea sediments. Free vehicles, i.e. aluminum framed tripods that descend untethered to the deep sea floor and return automatically, were used for thein-situ injection and incubation of cores at depths of 2600 to 5330 m. The data were compared to those obtained from (1) parallel samples incubated at 1 atm andin-situ temperature and (2) a shallow-water station (Buzzards Bay, 12 m). Rates measuredin situ generally decrease with water depth as well as with depth in the upper 9 cm of top sediment. In most experiments, rates measured in the 1 atm controls were higher than those observedin situ. This fact supports the general notion that natural populations of deep-sea sediment contain barotolerant and barophilic microorganisms in varying proportions.     

Spatial changes in the distributions of deep-sea “Cerviniidae” (Harpacticoida, Copepoda) and their associations with environmental factors in the bathyal zone around Sagami Bay, Japan

To estimate species turnover rates on scales of several tens of km in deep-sea benthic animals, we analyzed spatial and inter-annual changes in species diversity and composition of cerviniids, a typical group of deep-sea harpacticoids, at stations in and around Sagami Bay, central Japan. Associations with environmental factors were also investigated. Generally, bathymetrical patterns in diversity of benthos are unimodal and peak at depths of 2,000–3,000 m. In Sagami Bay, cerviniid diversity did not follow this trend; both species richness and evenness were negatively correlated with water depth. Multivariate analyses [detrended correspondence analysis (DCA) and non-metric multi-dimensional scaling] suggested that temporal changes in species composition of cerviniids are smaller than spatial changes that occur on horizontal scales of several tens of km. Community structure does not change completely on these scales in the bathyal zone around Sagami Bay. DCA also showed that bathymetrical changes in species composition can be regulated by certain factors associated with water depth.     

Relationship between the zooplankton,phytoplankton, particulate matter and dissolved free amino acids in the Celtic Sea

Estimates of the biomass of zooplankton, phytoplankton and particulate matter collected in the Celtic Sea during mixed-water conditions (on 8 and 9 April 1983) were compared to the concentration and diversity of sixteen dissolved free amino acids (DFAA) measured in seawater and in particles. During a day profile, variations of dissolved amino acids with depth reflected the feeding activity of copepods. The relationship was not apparent in a night profile and other processes, such as heterotrophic utilization of dissolved nitrogen by microorganisms, were thought to be involved. The ratios of total DFAA concentration (nM litre^-1) in the particulate phase over the concentration in seawater ranged from 1 to 200 within the water column. Of the sixteen amino acids measured, ornithine, a decomposition product of arginine, was responsible for more than 70% of the total concentration of DFAA in seawater. In the particles, phenylalanine ranged from 30 to 88% of total DFAA. In seawater this amino acid occurred in the 20 to 40 m depths (1.3 to 9.9% of total DFAA) in the day profile and at 5 m (12.4%) and 80 m (6.4%) in the night profile. Previously it has only been found in very low concentrations (<5%) in seawater, and its presence is considered to be the result of zooplankton feeding.     

Diel vertical migration bySalpa aspera and its potential for large-scale particulate organic matter transport to the deep-sea

In mid-summer 1975 throughout the Western Slope Water of the North Atlantic Ocean, massive numbers ofSalpa aspera performed a diel vertical migration of at least 800 m. This resulted in a movement of 85 to 90% of the total zooplankton biomass out of the upper 500 m during the day. Fecal pellet production and losses from this salp population were estimated to contribute approximately 12 mg C m^-2 day^-1 to the deep planktonic and benthic populations. If all this organic matter reached the deep-sea floor, it would represent over 100% of the daily deep-sea benthic infauna energy requirements.     

Description and use of an improved method for determining estuarine zooplankton grazing rates on phytoplankton

A method of rapidly determining zooplankton grazing rates on natural mixed phytoplankton populations using ¹⁴C is described. The method simplifies the design of grazing experiments as the grazing time can be kept short enough to prevent recycling of the isotope, and growth of the phytoplankton substrate. Very high specific activity, ¹⁴C-labelled phytoplankton concentrated either by centrifugation or sieving, may be used either as the sole grazing substrate, or as a tracer in natural mixed phytoplankton. Zooplankton, confined in glass jars at either ambient, or higher than ambient concentrations, are permitted to feed on the phytoplankton for periods of 30 min and 2 h, and are then separated by sieving. The zooplankton community grazing rate, or, if the samples are sorted into species, the individual species grazing rates, can be determined after scintillation counting of the zooplankton. The rate of appearance of ¹⁴C-labelled phytoplankton in the zooplankton is an estimate of the grazing rate, and the slope of the line joining the grazing rates at various phytoplankton concentrations gives an estimate of the grazing rate constant for the zooplankton population. The method provides a quick way of obtaining both zooplankton population, and individual species grazing rates on natural mixed phytoplankton. In two experiments, labelled phytoplankton was used as the sole grazing substrate in concentrations ranging between 0.4 and 5 times ambient levels. Grazing rate constants, for net-caught zooplankton concentrated to 46 times (Experiment 1) and 28 times (Experiment, 2) ambient estuarine levels were-0.14and-0.12 of the phytoplankton standing stock per day, respectively. There was a linear increase in the amount of phytoplankton grazed with an increase in phytoplankton concentration up to four times ambient phytoplankton levels. When tracer amounts of labelled phytoplankton were added to samples containing both phytoplankton and zooplankton at ambient concentrations the grazing rate constants were-0.28 and-0.42 of the phytoplankton standing stock per day. We conclude that zooplankton grazing was the major control factor of phytoplankton population size during October–November 1975 in South West Arm, Port Hacking, near Sydney, Australia.     

Benthic community respiration in the N.W. Atlantic Ocean: in situ measurements from 40 to 5200 m

Benthic community respiration was measured in situ at 9 stations along the Gay Head-Bermuda transect from depths of 40 to 5200 m. Three methods were used; bell jar respirometers, grab respirometers, and free vehicle respirometers. Benthic community respiration rates spanned three orders of magnitude, decreasing from 21.5 ml O₂ m^-2 h^-1 at 40 m in November to 0.02 ml O₂ m^-2 h^-1 at 5200 m. Rates decreased two orders of magnitude between 40 and 1800 m and then significantly declined again between the continental rise (3650 m) and the abyssal plain stations. Predictive equations for benthic community respiration along the transect reflect a strong correlation with depth of water. Of lesser significance are the correlations with water temperature, dissolved oxygen, benthic animal biomass, surface primary productivity and sediment organic matter. Calculations show that annual benthic respiration can utilize 1 to 2% of the surface primary productivity. Of the 2 to 7% organic carbon fixed at the surface which supposedly reaches the bottom, only 15 to 29% is utilized by the benthic community at 2200, 3000, and 3650 m. The energy requirements of other biological components of deep-sea benthic communities, such as benthopelagic and macro-epibenthic animals, not included in these measurements, must also be considered in calculating a balance of carbon.Contribution from Scripps Institution of Oceanography.     

An energy budget for Porites porites (Scleractinia)

An energy budget for Porites porites (Pallas) was determined for specimens from 10 m depth on the Fore Reef of Discovery Bay, Jamaica, between July 1984 and July 1985. Evidence for habitual zooplankton ingestion was not obtained, and P. porites appears to be largely autotrophic. Out of the daily photosynthetically fixed energy, 26% is used for animal respiration and growth, 22% for zooxanthellae respiration and growth, and <1% for colony reproduction as mature planulae; 45% remains unaccounted for. Colony respiration, net photosynthesis, colony skeleton and tissue growth, zooplankton ingestion, reproductive effort and energy content of tissues were measured. Energy loss as continuous mucus secretion was not detected, but may occur by an alternative route via mucus tunics, which occur periodically in situ and in the laboratory. The energy budget suggests that a considerable excess of photosynthetically fixed energy is produced on an ideal sunny day at 10 m depth. This surplus may be required for periodic rather than continuous energy demands, or may be essential to survive less-than-ideal days, when net photosynthetic input is reduced.Contribution No. 357 of the Discovery Bay Marine Laboratory, University of the West Indies     

Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Deep-sea caging of the mussel Mytilus galloprovincialis: Potential application in ecotoxicological studies

We experimented with caging the Mediterranean mussel (Mytilus galloprovincialis) at various depths for 69 d to measure basic physiological parameters, histological response and bio-accumulation of contaminants in a deep-sea contaminated area. In preliminary experiments, we demonstrated, under artificial pressure conditions, the ability of mussels Mytilus galloprovincialis to tolerate rapid immersion (at a speed of up to 120 m min^?1). In situ experiments were performed using submerged lines enabling mussels to be maintained at depths ranging of 40–1550 m with survival rates ranging from 80 to 38%, respectively. No significant differences in condition indexes were observed between treated and control specimens. However, histological observations demonstrated a clear reduction in thickness of the digestive epithelium with increasing depth exposure. By determining the contaminants in caged mussels, we found the following values for chromium accumulation: 27.4 μg g^?1 dry weight at 580 m depth and 9.8 μg g^?1 dry weight at 1550 m. Selected stations were located downstream of an industrial effluent at 420 m. The biological and environmental consequences of deep-sea contamination demonstrate the suitability of caged mussels for monitoring contaminant accumulation.     

Temporal patterns in diversity and species composition of deep-sea benthic copepods in bathyal Sagami Bay,central Japan

Little is known about temporal changes in the diversity and species composition of deep-sea metazoan meiofauna and their relationships with changes in the food supply. Those changes were studied for benthic copepod assemblages based on 2-year time-series data at a bathyal site in Sagami Bay (1430 m depth), central Japan, where annual fluctuation in the abundance of benthic foraminiferans was previously observed. Species diversity of benthic copepods at the site was as high as, or slightly higher than, that observed at other deep-sea sites, but did not fluctuate temporally through the study period. Multivariate analyses did not reveal any clear seasonal or directional change occurring over the longer term in their species composition, although there was some consistent pattern. These results indicate a lack of, or only weak, seasonality in the diversity and species structure of the deep-sea benthic copepod assemblages, even though the fresh organic food supply fluctuates seasonally. They also suggest that there are differences between copepods and foraminiferans in the response to changes in environmental factors, and that spatial differences in the composition of copepod communities are greater than temporal ones at this deep-sea site.Communicated by T. Ikeda, Hakodate     

Seasonal studies on the relative importance of different size fractions of phytoplankton in Narragansett Bay (USA)

The composition and productivity of four different size-fractions (<20, 20 to 60, 60 to 100, >100 μm) of the phytoplankton of lower Narragansett Bay (USA) were followed over an annual cycle from November, 1972 to October, 1973. Diatoms dominated the population in the winter-spring bloom and in the fall, the summer population was dominated by flagellates. The nannoplankton (<20 μm) were the most important, accounting for 46.6% of the annual biomass as chlorophyll a and 50.8% of the total production. The relative importance of the different fractions showed a marked seasonality. During the winter-spring and fall blooms the netplankton fractions (>20 μm) were the most important. Nannoplankters domnated in the summer. The yearly mean assimilation numbers for the different fractions were not signfficantly different. During the winter-spring bloom, however, the assimilation numbers for the netplankters were significantly higher than those for the nannoplankton fraction. Temperature accounted for most of the variability in assimilation numbers; a marked nutrient stress was observed on only two occasions. Growth rates calculated from ¹⁴C uptake and adenosine triphosphate (ATP)-cell carbon were generally quite high; maxima were >1.90 doublings per day during blooms of a flagellate in the summer and of Skeletonema costatum in the fall. The series of short cycles observed in which the dominant species changed were related to changes in the physiological state of the population. Higher growth rates were generally observed at times of peak phytoplankton abundance while lower growth rates were observed between these peaks. The high growth rates and assimilation numbers usually found suggest that the phytoplankton in lower Narragansett Bay was not generally nutrient-limited between November, 1972 and October, 1973. Nutrient regeneration in this shallow estuary, therefore, must be very rapid when in situ nutrient levels are low.     

Activité biologique en domaine profond: transformations biochimiquesin situ de composés organiques marqués au carbone-14 à l'interface eau-sédiment par 2000 m de profondeur dans le golfe de Gascogne

In June 1983,in situ water-sediment interface experiments were conducted with the submersible Cyana at 2 000 m depth in the Bay of Biscay. Dissolved or particulate¹⁴C-labelled organic compound was injected into a compartment of a box-corer specifically adapted for use with the submersible. After incubation, the biochemical reactions were stopped by addition of Formalin. In the laboratory, the pathways of the labelled molecules in the various extracts were followed by means of chemical and granulometric fractionation and isolation of meiofauna. Various experiments run at 1 atm and atin situ pressures, for incubation times of between 3 and 144 h, revealedin situ biochemical transformation. After recovery of the samples, activity at 1 atm was low, with only 4% of the radioactivity being recovered in the macromolecules against 84% for thein situ experiments. These results comprise new evidence of the existence of a rapid biochemical transformation at great depth in the ocean and therefore of the activity of the microflora, microfauna and meiofauna which are living in the superficial sediment.

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Contribution No. 49 de l'IFREMER     

Benthic oxygen uptake and carbon cycling under aphotic and resource-limiting conditions in a submarine cave

To establish relationships between organic input to the benthos and decreases in benthic population biomass and density, benthic oxygen uptake was measured in an oligotrophic submarine cave in the northwestern Mediterranean Sea (Marseille, France), on seven separate occasions in 1987, using an in situ bell-jar respirometer. Oxygen uptake was measured in both the outer twilight section and the dark inner section of the cave during an annual survey (seven recording periods from February 1987 to November 1988). The mean annual benthic oxygen uptake was 80.9 litres O₂ m^–2 yr^–1 for the twilight outer section and 15.5 litres O₂ m^–2 yr^–1 for the dark inner section. The results are discussed and the biogeochemical budget for particulate organic carbon at the sediment-water interface calculated. Respiration rates (expressed as carbon equivalents), together with previously published data on vertical fluxes and burial of organic carbon, revealed that anaerobic pathways accounted for 14% and aerobic pathways for 86% of the total benthic metabolism in the outer part of the cave. In the inner section of the cave, degradation of organic carbon occurred only through aerobic degradation, indicating a strongly carbon-limited ecosystem. The low respiration rates recorded in the dark section were similar to values recorded for some oligotrophic deep-sea environments (1 000 to 2 000 m). Such budgets are essential preliminary steps in order to accurately model benthic metabolic pathways. The determination of annual fluxes linked to the acquisition of long-term data will yield better knowledge of the recycling processes at the water-sediment interface.     

In situ water motion and nutrient uptake by the giant kelp Macrocystis pyrifera

Rates of NO ₃ ^- uptake by individual blades of Macrocystis pyrifera (L.) C. Agardh were measured at different flow rates in the laboratory. Dissolution rates of hemispherical, plaster buttons attached to the blade surface provided a relative measure of flow rates over blades used in uptake experiments and also over intact blades of adult kelp plants in situ (Laguna Beach, California, USA; 1981). Laboratory results indicated that uptake was saturated at a flow rate equivalent to 2.5 cm s^-1 current velocity. Flow rates over intact blades in situ always exceeded this uptake saturation level. Wave surge and movement of plant surfaces relative to the surrounding water provided sufficient flow to saturate uptake, even in a dense kelp canopy during low-current and calm sea-state conditions.     

Scanning electron microscopy of dialysis tubes incubated in flowing seawater

Scanning electron micrographs (SEM) demonstrate the growth of an epiphytic diatom and bacterial community on the surface of dialysis membranes incubated In situ, Narragansett Bay, Rhode Island (USA). A comparison is made between brushed and unbrushed tubes in summer, and unbrushed tubes in winter. Detrital deposition and epiphytic growth may decrease surface area and, by uptake and regeneration, influence nutrient flux through the membrane.     

Measurement of copepod predation on nauplii using qPCR of the cytochrome oxidase I gene

A method to directly measure predation rates by older stage copepods upon copepod nauplii using species-specific primers for the mitochondrial cytochrome oxidase subunit one gene (mtCOI) and real-time quantitative PCR (qPCR) was developed. The general approach is to determine the mtCOI gene copy number of an individual prey organism and the copy number of the same gene in the stomachs of predatory copepods collected in the field. From the knowledge of DNA disappearance rates in the stomachs, ingestion rates can be calculated. In October 2006, laboratory experiments were carried out with Acartia tonsa N1 and N2 as prey and adult female Centropages typicus as predator. The copepods were collected in Narragansett Bay, USA. A. tonsa mtCOI copy numbers copepod⁻¹ were determined for stages N1–C1 and for adults. A. tonsa DNA was detectable in the guts of the predators for as long as 3 h. Exponential rates of decline in prey DNA from the stomachs of the predators are similar to those measured for gut pigments. Because of the very small amount of DNA in an individual N1 or N2 nauplius, procedures were developed to maximize the quantitative extraction and recovery of DNA and to increase the sensitivity of the method. Two quite divergent haplotypes of A. tonsa were found in Narragansett Bay, which required separate qPCR primers; one was present in summer (July) and the other in fall and winter (October and February). With modification, the methods in this study can likely be applied to a range of predator–prey systems.     

Stability of sea urchin dominated barren grounds following destructive grazing of kelp in St. Margaret's Bay,Eastern Canada

Kelp regeneration was observed for the first time in St. Margaret's Bay, Nova Scotia, Canada in an area known to have been devoid of macroalgae for several years. The regeneration was destroyed by sea urchins within 10 months. Experimentally induced kelp regeneration met a similar fate under normal grazing pressure. At the lowest sea urchin biomass and density encountered re-establishment of mature kelp stands seems highly unlikely. The sea urchin population required to suppress kelp regeneration is fed by benthic microalgae. Diatoms and other pioneer algal community species were found in the guts of sea urchins. The mean standing crop of benthic microalgae was found to be, 2.2 g C m^-2 and production estimated as ca 15 g C m^-2yr^-1 at 8m depth. Most of the primary production of St. Margaret's Bay has been lost with the disappearing kelp populations.     

In situ measurement of the urea decomposition rate and its turnover rate in the Pacific Ocean

The in situ decomposition rate of urea was measured using ¹⁴C-labelled urea at 3 areas in the North Pacific Ocean: Sagami Bay on the southern coast of central Japan, the northwestern Pacific central waters and the subarctic Pacific waters. The mean values of the decomposition rates of urea in surface waters of these areas were 44.5, 1.51 and 1.32 mol urea m^-3 d^-1, respectively. These rates decreased with depth. High rates of urea carbon incorporation into particulate matter and the CO₂ liberation from urea carbon into seawater were obtained in light bottles in the euphotic zone, while low rates were found in dark bottles. The turnover rates of urea in the 3 areas were calculated respectively as 12, 113 and 110 d at the surface, and the values increased with depth.