Polycyclic aromatic hydrocarbons in surface seawater and in indigenous mussels (Mytilus galloprovincialis) from coastal areas of the Saronikos Gulf (Greece)

Polycyclic aromatic hydrocarbons (PAHs) were identified and measured in surface seawater and in the tissues (gills and mantle) of indigenous black mussels, Mytilus galloprovincialis, collected from three coastal sites of Saronikos Gulf (Greece), a gulf that exhibits high levels of pollution. The total PAHs measured by spectrofluorometry in the surface seawater were found in the range of 425–459 ng L⁻¹ at the most polluted sites 1 and 2 (Elefsis Bay–Salamis Island) and in the range of 103–124 ng L⁻¹ at site 3 (Aegina Island). PAHs' sources in seawater were identified by application of specific PAH ratios, such as phenanthrene/anthracene and fluoranthene/pyrene. Levels of PAHs in soft tissues (gills and mantle) of indigenous mussels were much higher than those reported for seawater. Total PAH concentrations in mantle tissues were in the range of 1300–1800 ng g⁻¹ dry weight (dw) tissue at sites 1 and 2 and approximately 380 ng g⁻¹ dw at site 3. In gill tissues total PAH concentrations were in the range of 1480–2400 ng g⁻¹ dw at sites 1 and 2 and approximately 430 ng g⁻¹ dw at site 3. PAHs composition was dominated by two-, three- and four-ring compounds in seawater, where 17 different PAH compounds were identified and measured in mussel tissues. Mussels can be used as sentinel organisms to monitoring PAHs' contamination, since they concentrate PAHs from the surrounding water media and therefore making the chemical analysis simpler and less prone to error than that for water. In surface seawater possible weathering and photodegradation due to hot climates contribute to reduced PAHs concentrations.     

Vitellogenin as biomarker for estrogenicity in flounder Platichthys flesus in the field and exposed to 17α-ethinylestradiol via food and water in the laboratory

The ability of 17α-ethinylestradiol (EE2) to elevate vitellogenin levels were investigated in male flounder Platichthys flesus and vitellogenin concentrations in flounders from the Danish coastal environment were determined. Male flounders were exposed to 17α-ethinylestradiol (EE2) via food or water. Average vitellogenin concentrations in the control fish ranged between 25 and 100 ng mL⁻¹. Exposure to 5.1, 8.1 and 16.8 ng EE2 L⁻¹ in water and 500 and 5000 ng EE2 kg⁻¹ body weight (bw) every second day in the food increased the plasma vitellogenin concentration in a concentration and time dependent manner, whereas exposure to 2.7 ng EE2 L⁻¹ in water for 21 d and 5 and 50 ng EE2 kg⁻¹ bw for 12 days in the food did not. EE2 could be detected in liver and testes (but not in muscle) after exposure to 8.1 and 16.8 ng EE2 L⁻¹ in the water and 5000 ng EE2 kg⁻¹ bw in the food; the highest concentration was 6 ng g⁻¹ wet weight in liver. The majority of the male flounders collected from nine coastal Danish sites from 1999 to 2004 had vitellogenin concentrations below 100 ng mL⁻¹, and only at two sites moderate estrogenic inputs were indicated.     

Selective depressions of surface silicic acid within cyclonic mesoscale eddies in the oligotrophic western North Pacific

To reveal spatial dynamics of silicic acid [Si(OH)₄] in the poorly sampled oligotrophic western North Pacific, we investigated the surface distribution of Si(OH)₄ and associated biogeochemical parameters by using an underway survey system with a highly sensitive nutrient analyzer along the 138°E transect (between 30 and 34°N) and the 155°E transect (between 10 and 35°N) during the summers of 2007 and 2008. Surface Si(OH)₄ concentrations ranged from the detection limit (11 nmol L⁻¹) to 2462 nmol L⁻¹. High Si(OH)₄ concentrations (>1000 nmol L⁻¹) and dynamic fluctuations were generally observed north of 23°N, while consistently stable low concentrations of 415–751 nmol L⁻¹ were observed south of 23°N. Surface nitrate+nitrite (N+N) and phosphate (PO₄³⁻) were typically depleted to <20 nmol L⁻¹, except for PO₄³⁻ in the area south of 16°N. The majority of the study area was characterized by high-Si(OH)₄ and low-N+N and PO₄³⁻. However, submesoscale/mesoscale depressions of Si(OH)₄ were locally observed in the cyclonic eddy fields north of 23°N. Among a total of six Si(OH)₄ depressions within the eddies, a complete Si(OH)₄ depletion (<11 nmol L⁻¹) was observed on the cyclonic side near the Kuroshio axis (33.1°N, 138°E). This depletion was closely coupled with a diatom bloom, suggesting that Si(OH)₄ was exhausted by diatoms. All of the Si(OH)₄ depressions were selective and not accompanied by local depressions of N+N and PO₄³⁻. This unique phenomenon might be driven by biogeochemical processes such as selective Si export (Si pump), anomalous Si uptake associated with diatom physiology, and/or Si uptake supported by N₂ fixation.     

Genotoxicity of copper oxide and silver nanoparticles in the mussel Mytilus galloprovincialis

Though there is some information on cytotoxicity of copper nanoparticles and silver nanoparticles on human cell lines, there is no information on their genotoxic and cytotoxic behaviour in bivalve molluscs. The aim of this study was to investigate the genotoxic impact of copper oxide and silver nanoparticles using mussels Mytilus galloprovincialis. Mussels were exposed to 10 μg L⁻¹ of CuO nanoparticles and Cu²⁺ and Ag nanoparticles and Ag⁺ for 15 days to assess genotoxic effects in hemocytes using the comet assay. The results obtained indicated that copper and silver forms (nanoparticles and ionic) induced DNA damage in hemolymph cells and a time-response effect was evident when compared to unexposed mussels. Ionic forms presented higher genotoxicity than nanoparticles, suggesting different mechanisms of action that may be mediated through oxidative stress. DNA strand breaks proved to be a useful biomarker of exposure to genotoxic effects of CuO and Ag nanoparticles in marine molluscs.     

Oxygen radical formation in anoxic transgression and anoxia-reoxygenation: Foe or phantom? Experiments with a hypoxia tolerant bivalve

Intertidal blue mussels, Mytilus edulis, experience hypoxia reoxygenation during tidal emersion and resubmersion cycles, and this is often suggested to represent a major stress for the animals, especially for their respiratory tissues, the gills. We exposed mussels to experimental short and prolonged anoxia and subsequent reoxygenation and analyzed the respiratory response in excised gill tissue and the effects of treatment on reactive oxygen species (mainly ROS: superoxide anion, O₂^·− and hydrogen peroxide, H₂O₂), formation using live imaging techniques and confocal microscopy. Our aim was to understand if this “natural stress” would indeed produce oxidative damage and whether antioxidant defenses are induced under anoxia, to prevent oxidative damage during reoxygenation. Exposure to declining pO₂ in the respiration chamber caused an increase of gill metabolic rate between 21 and 10 kPa, a pO₂ range in which whole animal respiration is reported to be oxyregulating. Exposure of the animals to severe anoxia caused an onset of anaerobiosis (succinate accumulation) and shifted high and low critical p_c values (p_c1: onset of oxyregulation in gills, p_c2: switch from oxyregulation to oxyconformity) to higher pO₂. Concentrations of both ROS decreased strongly during anoxic exposure of the mussels and increased upon reoxygenation. This ROS burst induced lipid peroxidation in the mantle, but neither were protein carbonyl levels increased (oxidative damage in the protein fraction), nor did the tissue glutathione concentration change in the gills. Further, analysis of apoptosis markers indicated no induction of cell death in the gills. To our knowledge, this is the first paper that directly measures ROS formation during anoxia reoxygenation in mussels. We conclude that hypoxia tolerant intertidal mussels do not suffer major oxidative stress in gill and mantle tissues under these experimental conditions.     

Use of inhibitors for coastal bacteria and phytoplankton: Application to nitrogen uptake measurement

For several decades, prokaryotic and eukaryotic inhibitors have been used to exclude bacteria from microalgal cultures and for investigating prey-predator relationships. Recently there has been considerable interest in using specific inhibitors for studying the interactions between bacteria and phytoplankton, by selective repression of either organism’s activity. The effectiveness of chemical inhibitors must be tested before applying them to natural communities to partition metabolic activities between functional groups. Six different antibiotics selected from the most commonly reported in the literature were tested, at concentrations varying from 12.5 to 100 mg L⁻¹, for their effect on bacterial growth and functional diversity of natural communities from Mediterranean coastal waters. Penicillin and streptomycin each at a final concentration of 100 mg L⁻¹ significantly reduced bacterial growth within 2 h. There was a greater impact on bacterial functional diversity when both antibiotics were mixed together. This mixture did not have any significant effect on the growth of selected cultured phytoplankton strains, whereas the eukaryote inhibitor cycloheximide at 100 mg L⁻¹ reduced growth within 2 h of incubation. The penicillin–streptomycin mixture and cycloheximide alone successfully partitioned NH₄⁺ and NO₃⁻ uptake between bacteria and phytoplankton bi-weekly sampled in a coastal lagoon in Autumn, where bacterial contribution to total NH₄⁺ and NO₃⁻ uptake averaged 46 and 41%, respectively. The use of specific inhibitors may be a valuable method for studying interactions, such as competition and mutualism, or lack of interaction between the different components of microbial communities and could be used to study their relative importance in biogeochemical fluxes.     

Downward transport of organic carbon by diel migratory micronekton in the western equatorial Pacific:: its quantitative and qualitative importance

Using simultaneous sampling with a commercial-sized trawl, a zooplankton net, and a sediment trap, we evaluated the contribution of vertically migrating micronekton to vertical material transport (biological pump) at two stations (3°00′N, 146°00′E and 3°30′N, 145°20′E) in the western equatorial North Pacific. The gravitational sinking particulate organic carbon flux out of the euphotic zone was 54.8 mg C m⁻² day⁻¹. The downward active carbon flux by diel migrant mesozooplankton was 23.53 and 9.97 mg C m⁻² day⁻¹, and by micronekton 4.40 and 2.26mg C m⁻² day⁻¹ at the two stations. Assuming that the micronekton sampling efficiency of the trawl was 14%, we corrected the downward carbon flux due to micronekton respiration to 29.9 and 15.2mg C m⁻² day⁻¹, or 54.6 and 27.7% of the sinking particle flux at the two stations. The corrected micronekton gut fluxes were 1.53 and 0.97mg C m⁻² day⁻¹. The role of myctophid fish fecal matter as a possible food resource for deep-sea organisms, based on its fatty acid and amino acid analysis, is discussed.     

Anchoring metabolic changes to phenotypic effects in the chlorophyte Scenedesmus vacuolatus under chemical exposure

In order to derive a causal understanding of toxic effects in organisms, ecotoxicology may benefit from linking molecular changes, evaluated by ‘omics’-techniques, to phenotypic observations. However, an approach to link these observation levels is still lacking.The aim of this study was to relate metabolic changes in the chlorophyte Scenedesmus vacuolatus to established parameters of toxicity. Therefore, synchronized cultures of the alga were exposed for 14 h to the phytotoxicant N-phenyl-2-naphthylamine (PNA) in the range of 0.00089 μmol L⁻¹ (environmental concentrations) up to 1.82 μmol L⁻¹. Cell growth and photosynthesis inhibition were evaluated but revealed no effect of PNA at experimental concentrations below 0.456 μmol L⁻¹. Changes in the biochemical composition of algae were measured by GC–MS in both polar and non-polar phases. PCA uncovered no separation in the multivariate pattern of mass spectral features at exposure concentrations below 0.00356 μmol L⁻¹ of PNA. However, a clear separation was detected at concentrations higher than 0.00713 μmol L⁻¹. A combined visualization of PCA results for metabolic changes and concentration–response relationships for growth and photosynthesis inhibition revealed (I) a two orders of magnitude higher sensitivity of metabolomics to detects changes after PNA exposure compared to the phenotypic parameters measured and (II) two types of metabolic responses: one group of features was reflecting pharmacological effects at low exposure concentrations and the second group corresponded to adverse effects along with conventional observations of toxicity.     

Oxygen distribution and aerobic respiration in the north and south eastern tropical Pacific oxygen minimum zones

Highly sensitive STOX O₂ sensors were used for determination of in situ O₂ distribution in the eastern tropical north and south Pacific oxygen minimum zones (ETN/SP OMZs), as well as for laboratory determination of O₂ uptake rates of water masses at various depths within these OMZs. Oxygen was generally below the detection limit (few nmol L⁻¹) in the core of both OMZs, suggesting the presence of vast volumes of functionally anoxic waters in the eastern Pacific Ocean. Oxygen was often not detectable in the deep secondary chlorophyll maximum found at some locations, but other secondary maxima contained up to ~0.4 µmol L⁻¹. Directly measured respiration rates were high in surface and subsurface oxic layers of the coastal waters, reaching values up to 85 nmol L⁻¹ O₂ h⁻¹. Substantially lower values were found at the depths of the upper oxycline, where values varied from 2 to 33 nmol L⁻¹ O₂ h⁻¹. Where secondary chlorophyll maxima were found the rates were higher than in the oxic water just above. Incubation times longer than 20 h, in the all-glass containers, resulted in highly increased respiration rates. Addition of amino acids to the water from the upper oxycline did not lead to a significant initial rise in respiration rate within the first 20 h, indicating that the measurement of respiration rates in oligotrophic Ocean water may not be severely affected by low levels of organic contamination during sampling. Our measurements indicate that aerobic metabolism proceeds efficiently at extremely low oxygen concentrations with apparent half-saturation concentrations (K_m values) ranging from about 10 to about 200 nmol L⁻¹.     

Algal concentration profiles above mussel beds

The distribution of phytoplankton was measured in vertical profiles at three stations in Limfjorden (Denmark) within and above Mytilus edulis beds (−5 to 200 cm above the bed surface) and related to the filtration capacity of the mussels and hydrodynamic conditions. The density of phytoplankton was 2690 to 10 600 cells cm⁻³ at 200 cm above the bottom (∼1.4 to 3.2 mg Chl-a m⁻³), declining towards the bottom to about 1000 cells cm⁻³ at two of the stations. This is a threshold of minimum algal concentration below which mussels are reported to stop feeding. The results of the present study further demonstrate that the near-bed phytoplankton concentrations observed in the field often remained below the concentration level at which mussels reduce their maximal filtration activity.     

Chronic hypoxia and low salinity impair anti-predatory responses of the green-lipped mussel Perna viridis

The effects of chronic hypoxia and low salinity on anti-predatory responses of the green-lipped mussel Perna viridis were investigated. Dissolved oxygen concentrations ranged from hypoxic to normoxic (1.5 ± 0.3 mg l⁻¹, 3.0 ± 0.3 mg l⁻¹ and 6.0 ± 0.3 mg l⁻¹), and salinities were selected within the variation during the wet season in Hong Kong coastal waters (15‰, 20‰, 25‰ and 30‰). The dissolved oxygen and salinity significantly affected some anti-predatory responses of mussel, including byssus production, shell thickness and shell weight, and the adductor diameter was only significantly affected by salinity. Besides, interactive effects of dissolved oxygen and salinity on the byssus production and shell thickness were also observed. In hypoxic and low salinity conditions, P. viridis produced fewer byssal threads, thinner shell and adductor muscle, indicating that hypoxia and low salinity are severe environmental stressors for self-defence of mussel, and their interactive effects further increase the predation risk.     

Distributions of particulate and dissolved organic and inorganic phosphorus in North Pacific surface waters

Several operationally defined fractions of phosphorus (P) were measured along a surface water transect in the North Pacific. The P content in all fractions was found to increase northward from the edge of the subtropical to the subarctic region. Particulate organic P (POP) concentrations increased from 9 to 110 nmol L^− 1, whereas the particulate inorganic P (PIP) concentrations increased from 1 to 13 nmol L^− 1. A significant correlation between POP, PIP and chlorophyll a suggested that these P pools are associated directly or indirectly with phytoplankton cells. PIP comprised 10–20% of the total particulate P pool across the transect, indicating it is an important component of the marine P cycle in this region. Dissolved non-reactive P (assumed to consist predominantly of non-reactive organic P compounds, thus referred to as DOP) concentration increased from 0.10 to 0.22 μmol L^− 1, whereas soluble reactive P (SRP) concentration increased from 0.01 to 1.42 μmol L^− 1 along the transect. The proportion of DOP and SRP varied widely, with a large proportion of DOP in areas with low total dissolved P concentrations in lower latitudes and a large proportion of SRP in areas with high total dissolved P concentrations in higher latitudes. High demand for DOP in the lower latitudinal region would diminish the concentration of this pool relative to higher latitudinal regions where SRP is more abundant and would be preferentially utilized. The availability of SRP could have a significant impact on the concentration and probably on the composition of DOP. We show that P fractionation provides an important insight for discussing the marine P cycle.     

Copper complexing ligands and organic matter characterization in the northern Adriatic Sea

The study on dissolved organic ligands capable to complex copper ions (L_T), surface-active substances (SAS) and dissolved organic carbon (DOC) in the Northern Adriatic Sea station (ST 101) under the influence of Po River was conducted in period from 2006–2008. The acidity of surface-active organic material (Ac_r) was followed as well. The results are compared to temperature and salinity distributions. On that way, the contribution of the different pools of ligands capable to complex Cu ions could be determined as well as the influence of aging and transformation of the organic matter. The L_T values in the investigated period were in the range of 40–300 nmol l⁻¹. The range of DOC values for surface and bottom samples were 0.84–1.87 mg l⁻¹ and 0.80–1.30 mg l⁻¹, respectively. Total SAS concentrations in the bottom layer were 0.045–0.098 mg l⁻¹ in equiv. of Triton-X-100 while those in the surface layer were 0.050–0.143 mg l⁻¹ in equiv. of Triton-X-100. The majority of organic ligands responsible for Cu binding in surface water originate from new phytoplankton production promoted by river borne nutrients. Older, transformed organic matter, possessing higher relative acidity, is the main contributor to the pool of organic ligands that bind copper in the bottom samples. It was estimated that 9% of DOC in surface samples and 12% of DOC in the bottom samples are present as ligands capable to complex copper ions.     

Impact of macroalgal dredging on dystrophic crises and phototrophic bacterial blooms (red waters) in a brackish coastal lagoon

The Prévost lagoon (Mediterranean coast, France), was subject to annual dystrophic crises caused by the biodegradation of opportunistic macroalgae (Ulva lactuca) in the past. These crises result in anoxic waters with subsequent blooms of Purple Sulphur Bacteria (red waters) which, by oxidizing sulphide, contribute to the reestablishment of oxic conditions in the water column. Mechanical dredging of the macroalgal biomass has been carried out in the lagoon since 1991 with the aim of preventing the ecological and economic disturbances caused by such crises. Dredging began just before the phototrophic bloom when the water was already hypoxic (O₂ = 0.7 mg·L⁻¹) and contained sulphilde (H₂S = 7.3 mg·L⁻¹) and purple patches of phototrophic bacteria (Thiocapsa sp.) that were beginning to develop on decaying macroalgae at the sediment surface. The dredging prevented red water formation and drastically modified both phototrophic community structure and activity and biogeochemical sulphur cycling. The dredging permitted the reestablishment of oxic conditions for a short period only (1–13 August). Resuspension of the superficial sediment layers disturbed the phototrophic bacterial community, whose numbers decreased by one order of magnitude (from 2 × 10⁶ to 3.9 × 10⁵ CFU.mL⁻¹). The phototrophic community was no longer effective in reoxidizing the reduced sulphur compounds remaining in the sediments, as shown by a drastic sulphate depletion in the superficial sediment layers. Moreover, the increase in the specific bacteriochlorophyll a concentration of the phototrophic purple bacteria and the rapid development of Green Sulphur Bacteria (Prosthecochloris-like microorganisms) indicated that the phototrophic community was growing under severe light-limiting conditions due to the resuspension of sediment particles in the water. These conditions did not allow the phototrophic bacterial community to efficiently reoxidize the reduced sulphur compounds originating from the sediments. In consequence, hypoxic conditions (O₂ = 4.7 to 4.8 mg·L⁻¹) and low sulphide concentrations (H₂S = 0.4 to 0.7 mg·L⁻¹) were detected in the water column until September. The ecological balance in the lagoon was reestablished only in October, whereas, in previous years it had been restored in August.     

Dry atmospheric deposition and diazotrophy as sources of new nitrogen to northwestern Mediterranean oligotrophic surface waters

Atmospheric dry deposition of nitrogen (N) and dinitrogen (N₂) fixation rates were assessed in 2004 at the time-series DYFAMED station (northwestern Mediterranean, 43°25′N, 7°52′E). The atmospheric input was monitored over the whole year. Dinitrogen fixation was measured during different seasonal trophic states (from mesotrophy to oligotrophy) sampled during nine cruises. The bioavailability of atmospherically deposited nutrients was estimated by apparent solubility after 96 h. The solubility of dry atmospheric N deposition was highly variable (from ∼18% to more than 96% of total N). New N supplied to surface waters by the dry atmospheric deposition was mainly nitrate (NO₃⁻) (∼57% of total N, compared to ∼6% released as ammonium (NH₄⁺)). The mean bioavailable dry flux of total N was estimated to be ∼112 μmol m⁻² d⁻¹ over the whole year. The NO₃⁻ contribution (70 μmol NO₃⁻ m⁻² d⁻¹) was much higher than the NH₄⁺ contribution (1.2 μmol NH₄⁺ m⁻² d⁻¹). The N:P ratios in the bioavailable fraction of atmospheric inputs (122.5–1340) were always much higher than the Redfield N:P ratio (16). Insoluble N in atmospheric dry deposition (referred to as “organic” and believed to be strongly related to anthropogenic emissions) was ∼40 μmol m⁻² d⁻¹. N₂ fixation rates ranged from 2 to 7.5 nmol L⁻¹ d⁻¹. The highest values were found in August, during the oligotrophic period (7.5 nmol L⁻¹ at 10 m depth), and in April, during the productive period (4 nmol L⁻¹ d⁻¹ at 10 m depth). Daily integrated values of N₂ fixation ranged from 22 to 100 μmol N m⁻² d⁻¹, with a maximum of 245 μmol N m⁻² d⁻¹ in August. No relationship was found between the availability of phosphorus or iron and the observed temporal variability of N₂ fixation rates. The atmospheric dry deposition and N₂ fixation represented 0.5–6% and 1–20% of the total biological nitrogen demand, respectively. Their contribution to new production was more significant: 1–28% and 2–55% for atmospheric dry deposition and N₂ fixation, respectively. The dry atmospheric input was particularly significant in conditions of water column stratification (16–28% of new production), while N₂ fixation reached its highest values in June (46% of new production) and in August (55%).     

Denitrification prevails over anammox in tropical mangrove sediments (Goa,India)

Denitrification, anammox (Anx) and di-nitrogen fixation were examined in two mangrove ecosystems- the anthropogenically influenced Divar and the relatively pristine Tuvem. Stratified sampling at 2 cm increments from 0 to 10 cm depth revealed denitrification as the main process of N₂ production in mangrove sediments. At Divar, denitrification was ∼3 times higher than at Tuvem with maximum activity of 224.51 ± 6.63 nmol N₂ g⁻¹ h⁻¹ at 0–2 cm. Denitrifying genes (nosZ) numbered up to 2 × 10⁷ copies g⁻¹ sediment and belonged to uncultured microorganisms clustering within Proteobacteria. Anammox was more prominent at deeper depths (8–10 cm) mainly in Divar with highest activity of 101.15 ± 87.73 nmol N₂ g⁻¹ h⁻¹ which was 5 times higher than at Tuvem. Di-nitrogen fixation was detected only at Tuvem with a maximum of 12.47 ± 8.36 nmol N₂ g⁻¹ h⁻¹. Thus, in these estuarine habitats prone to high nutrient input, N₂-fixation is minimal and denitrification rather than Anx serves as an important mechanism for counteracting N loading.     

Commercially-cultured oysters (Crassostrea gigas) exert top-down control on intertidal pelagic resources in Willapa Bay,Washington, USA

The capacity of filter feeders to reduce seston and phytoplankton concentrations in the water column has important implications for restoration and management of coastal ecosystems. We directly measured changes in chlorophyll a concentration on commercially stocked intertidal oyster beds (Crassostrea gigas) in Willapa Bay, Washington, USA by recording water properties near small drifters as they tracked parcels of water across tide flats. Chlorophyll declined 9.6% per half hour in water passing on-bottom adult oysters and 41% for longline adult oysters, whereas chlorophyll concentrations increased as water flowed across tide flats without adult oysters. Field filtration rates, which were fit to exponential declines in chlorophyll and accounted for oyster density and water depth, averaged 0.35 L g^− 1 h^− 1 (shucked dry weight) for on-bottom aquaculture and 0.73 L g^− 1 h^− 1 for longline culture, compared to values of 2.5–12 L g^− 1 h^− 1 reported from laboratory studies of C. gigas. Field filtration rates may be lower than laboratory rates due to unfavorable field conditions (e.g., low initial chlorophyll concentrations) or masked by resuspension of benthic microalgae. In addition to distinctions among on-bottom, longline, and no-oyster habitats, Akaike's Information Criterion analysis showed temperature, initial chlorophyll concentration, and depth related to chlorophyll decline. This research corroborates mathematical models suggesting that benthic suspension feeders are exerting top-down control of pelagic production in this estuary, with strong patterns in chlorophyll emerging across extensive tideflats populated by C. gigas despite low field filtration rates.     

Microbial degradation rates of small peptides and amino acids in the oxygen minimum zone of Chilean coastal waters

We found similar microbial degradation rates of labile dissolved organic matter in oxic and suboxic waters off northern Chile. Rates of peptide hydrolysis and amino acid uptake in unconcentrated water samples were not low in the water column where oxygen concentration was depleted. Hydrolysis rates ranged from 65 to 160 nmol peptide L⁻¹ h⁻¹ in the top 20 m, 8–28 nmol peptide L⁻¹ h⁻¹ between 100 and 300 m (O₂-depleted zone), and 14–19 nmol peptide L⁻¹ h⁻¹ between 600 and 800 m. Dissolved free amino acid uptake rates were 9–26, 3–17, and 6 nmol L⁻¹ h⁻¹ at similar depth intervals. Since these findings are consistent with a model of comparable potential activity of microbes in degrading labile substrates of planktonic origin, we suggest, as do other authors, that differences in decomposition rates with high and low oxygen concentrations may be a matter of substrate lability. The comparison between hydrolysis and uptake rates indicates that microbial peptide hydrolysis occurs at similar or faster rates than amino acid uptake in the water column, and that the hydrolysis of peptides is not a rate-limiting step for the complete remineralization of labile macromolecules. Low O₂ waters process about 10 tons of peptide carbon per h, double the amount processed in surface-oxygenated water. In the oxygen minimum zone, we suggest that the C balance may be affected by the low lability of the dissolved organic matter when this is upwelled to the surface. An important fraction of dissolved organic matter is processed in the oxygen minimum layer, a prominent feature of the coastal ocean in the highly productive Humboldt Current System.     

Phytoplankton spring bloom of the Gironde plume waters in the Bay of Biscay: early phosphorus limitation and food-web consequences

During the spring 1995 (2–25 May), a cruise was carried on the RV Poseidon (Germany) on the continental shelf of the south Bay of Biscay. The objective was a comprehensive study of the planktonic food web within the Gironde plume waters. In these waters phosphate was present at very low concentrations (undetectable to < 0.1 μmol.L⁻¹), whereas nitrate, silicate and ammonium concentrations were much higher (several μmol·L⁻¹ for nitrate and silicate and 0.5 to 1.0 μmol·L⁻¹ for ammonium). The size distribution of the phytoplankton biomass (estimated from chlorophyll a measurements by high performance liquid chromatography) and primary production (measured by ¹⁴C in situ method) showed a great proportion of small (40 to 70 % < 3 μm) and active autotrophic cells (growth rates estimated from 0.4 to 0.8 d⁻¹ for the entire euphotic layer). Considering the very high values of NO₃-N:PO₄-P ratios and the high C:P and N:P ratios for the particulate organic matter, it is suggested that an early phosphorus depletion limits the spring bloom phytoplankton and particularly the new production (nitrate uptake coming from the Gironde waters).From these results and other simultaneous observations on the heterotrophic processes (such as grazing of microzooplankton), we can conclude that the planktonic food web would be close to a maintenance system as defined by Platt et al. The possible generalisation of these results for each spring is discussed with respect to the scarcity of previous and reliable phosphate data.     

A role for nitrite in the production of nitrous oxide in the lower euphotic zone of the oligotrophic North Pacific Ocean

Understanding the role of the oceans in the Earth's changing climate requires comprehension of the relevant metabolic pathways which produce climatically important trace gases. The global ocean represents one of the largest natural sources of nitrous oxide (N₂O) that is produced by selected archaea and/or bacteria during nitrogen (N) metabolism. In this study, the role of nitrite (NO₂⁻) in the production of N₂O in the upper water column of the oligotrophic North Pacific Subtropical Gyre was investigated, focusing primarily on the lower euphotic zone where NO₂⁻ concentrations at the primary NO₂⁻ maximum reached 195 nmol L⁻¹. Free-drifting sediment trap arrays were deployed to measure N cycle processes in sinking particulate material and the addition of selected N substrates to unpreserved sediment traps provided an experimental framework to test hypotheses regarding N₂O production pathways and controls. Sinking particles collected using NO₂⁻-amended, unpreserved sediment traps exhibited significant production of N₂O at depths between 100 and 200 m. Subsequent stable isotope tracer measurements conducted on sediment trap material amended with ¹⁵NO₂⁻ yielded elevated δ¹⁵N values of N₂O, supporting N₂O production via a NO₂⁻ metabolism pathway. Experiments on seawater collected from 150 m showed N₂O production via NO₂⁻ metabolism also occurs in the water-column and indicated that the concentration of NO₂⁻ relative to NH₄⁺ availability may be an important control. These findings provide evidence for the production of N₂O via nitrifer-denitrification in the lower euphotic zone of the open ocean, whereby NO₂⁻ is reduced to N₂O by ammonia-oxidizing microorganisms.