Decadal-scale changes in shallow-infaunal foraminiferal assemblages at the Porcupine Abyssal Plain,NE Atlantic

Trends in the abundance, diversity and taxonomic composition of ‘live’ (rose Bengal stained) foraminiferal assemblages (0-1 cm layer, >63-μm fraction) were analysed in replicate multiple corer samples collected at the Porcupine Abyssal Plain (48° 50’ N, 16° 30’ W, 4850 m water depth) over a 13-yr period (1989-2002). Total densities were significantly higher in 1996-2002 compared to 1989-1994, a change coincident with a spectacular rise in the density of the holothurian Amperima. However, total densities exhibited no significant relation to seasons or any significant correlation with modelled organic matter flux, the North Atlantic Oscillation (NAO) index, Amperima densities, or megafaunal assemblage composition. Over the same period, species richness and diversity measures decreased and dominance increased, although not significantly. Multivariate analyses revealed three assemblages represented by samples collected in 1989-1994, 1996-July 1997 and October 1997-October 2002. These reflected temporal changes in the densities of higher taxa and species. Trochamminaceans, notably a small undescribed species, increased from 5-9% (1989-1994) to 29-40% (1996-2002) of the assemblage with a corresponding rise in absolute abundance. Species of Hormosinacea and Lagenammina also tended to increase in density from 1996/1997 onwards. Rotaliids, dominated by Alabaminella weddellensis and Epistominella exigua, showed a bimodal distribution over time with peak densities in May 1991 (32%) and September 1998 (28%) and lowest densities in 1996-1997. Responses by these species to seasonal phytodetritus inputs probably explain the relative abundance of E. exigua, and to a lesser extent A. weddellensis, in 1989 and 1991 when phytodetritus was present. A qualitative change in the phytodetrital food, repackaging of food by megafauna, increased megafaunal disturbance of the surficial sediment, or a combination of these factors, are possible explanations for the dominance of trochamminaceans from 1996 onwards. The miliolid Quinqueloculina sp. was virtually absent in multicore samples (0-1 cm, >63-μm fraction) from 1989-1994, peaked in September 1996 (22%) when degraded phytodetritus was present on core surfaces, was less common in March 1997, and thereafter was relatively uncommon. However, horizontally sliced box-core samples (0-5 cm, >250-μm fraction) revealed that large specimens were more abundant in March 1997, and also were concentrated in deeper sediment layers, than in September 1996. We suggest that Quinqueloculina sp. migrated to the sediment surface in response to a 1996 flux event, grew and reproduced, before migrating back into deeper layers as the phytodetrital food became exhausted. Overall, the abyssal time-series revealed decadal-scale changes among shallow-infaunal foraminifera, more or less coincident with changes in the megafauna, as well as indications of shorter-term events related to seasonally-pulsed phytodetrital inputs.     

Variability of C02 distributions and sea-air fluxes in the central and eastern equatorial Pacific during the 199–1994 El Nin˜o

As part of the U.S. JGOFS Program and the NOAA Ocean-Atmosphere Carbon Exchange Study (DACES), measurements of C02 partial pressure were made in the atmosphere and in the surface waters of the central and eastern equatorial Pacific during the boreal spring and autumn of 1992, the spring of 1993, and the spring and autumn of 1994. Surface-water pC0₂ data indicate significant diurnal, seasonal, and interannual variations. The largest variations were associated with the 1991–1994 ENSO event, which reached maximum intensity in the spring of 1992. The lower values of surface-water ΔpC0₂ observed during the 1991–1994 ENSO period were the result of the combined effects of both remotely and locally forced physical processes. The warm pool, which reached a maximum eastward extent in January-February of 1992, began in September of 1991 as a series of westerly wind events lasting about 30 days. Each wind event initiated an eastward propagating Kelvin wave which caused a deepening of the thermocline. By the end of January 1992 the thermocline was at its maximum depth, so that the upwelled water was warm and C0₂-depleted. In April of the same year, the local winds were weaker than normal, and the upwelling was from shallow depths. These changes resulted in a lower-than-normal C02 flux to the atmosphere. The results show that for the one-year period from the fall of 1991 until the fall of 1992, approximately 0.3 GtC were released to the atmosphere; 0.6 GtC were released in 1993, and 0.7 GtC in 1994, in good agreement with the model results of Ciais et al. [Science,269,1098–1102;J. Geophys. Res.,100, 5051–5070]. The net reduction of the ocean-atmosphere C02 flux during the 1991–1994 El Nifio was on the order of 0.8 – 1.2 GtC. Thus, the total amount of C02 sequestered in the equatorial oceans during the prolonged 1991–1994 El Nin˜o period was about 25% higher than the severe El Nin˜o of 1982–1983.     

Is long-term change in the abyssal Northeast Atlantic driven by qualitative changes in export flux? Evidence from selective feeding in deep-sea holothurians

The Porcupine Abyssal Plain (NE Atlantic) time-series has shown large, wide-scale, changes in the composition of the benthic community at 4800 m depth (48°50′N, 16°30′W). The abundance of holothurians has increased significantly since 1996 and one species in particular, Amperima rosea, has increased in abundance by three orders of magnitude. Environmental forcing in the form of phytodetrital food supply to the benthos is believed to be driving these changes. Chlorophyll and carotenoid pigments were determined from the gut sediments of seven species of abyssal holothurian, sampled from the Porcupine Abyssal Plain during Autumn 2000 and Spring 2002. These two samples fell either side of the main phytoplankton bloom in the NE Atlantic, providing an opportunity for seasonal comparisons. Significant inter-species differences in pigment profiles were observed among the seven species. Seasonal differences were noted among four species sampled in both time periods. All seven species were collected from the same geographical area and depth. As algal pigments cannot be synthesised by the holothurians, they provide good biomarkers for the composition of the phytodetritus. Differences in pigments from gut sediment profiles are indicative of selective feeding among the holothurians. A. rosea had a gut profile dominated by the pigments zeaxanthin, chlorophyll a/echineone and β-carotene; these pigments were all present in significantly smaller quantities in the other species. The high quantities of these pigments are indicative of a diet rich in cyanobacteria. The gut sediments of A. rosea also lacked many chloropigments characteristic of other phytoplankton groups, which were observed in the guts of other holothurian species. Ovarian tissue for the five species taken in the pre-spring bloom 2002 sample were examined. All species showed similar carotenoid profiles, dominated by zeaxanthin, echinenone and β-carotene, all of which are important compounds for reproductive success in echinoderms. The differences in gut pigment profiles highlight the potential for several species of deposit-feeding holothurians to partition the same phytodetrital food source, possibly providing a mechanism for maintaining the high diversity of deposit feeders at abyssal depths. The dominance of reproductively important carotenoids in the guts and gonads of A. rosea may highlight the ability of this species to rapidly utilise any change in the composition of the phytodetrital flux and translate that advantage into a successful reproductive and recruitment event. The results are discussed in relation to work on bathyal holothurians and the potential for food-driven regime shifts in both the abyssal and bathyal Northeast Atlantic.     

Factors influencing benthic bacterial abundance, biomass, and activity on the northern continental margin and deep basin of the Gulf of Mexico

As part of a larger project on the deep benthos of the Gulf of Mexico, an extensive data set on benthic bacterial abundance (n>750), supplemented with cell-size and rate measurements, was acquired from 51 sites across a depth range of 212–3732 m on the northern continental slope and deep basin during the years 2000, 2001, and 2002. Bacterial abundance, determined by epifluorescence microscopy, was examined region-wide as a function of spatial and temporal variables, while subsets of the data were examined for sediment-based chemical or mineralogical correlates according to the availability of collaborative data sets. In the latter case, depth of oxygen penetration helped to explain bacterial depth profiles into the sediment, but only porewater DOC correlated significantly (inversely) with bacterial abundance (p<0.05, n=24). Other (positive) correlations were detected with TOC, C/N ratios, and % sand when the analysis was restricted to data from the easternmost stations (p<0.05, n=9–12). Region-wide, neither surface bacterial abundance (3.30–16.8×10⁸ bacteria cm⁻³ in 0–1 cm and 4–5 cm strata) nor depth-integrated abundance (4.84–17.5×10¹³ bacteria m⁻², 0–15 cm) could be explained by water depth, station location, sampling year, or vertical POC flux. In contrast, depth-integrated bacterial biomass, derived from measured cell sizes of 0.027–0.072 μm³, declined significantly with station depth (p<0.001, n=56). Steeper declines in biomass were observed for the cross-slope transects (when unusual topographic sites and abyssal stations were excluded). The importance of resource changes with depth was supported by the positive relationship observed between bacterial biomass and vertical POC flux, derived from measures of overlying productivity, a relationship that remained significant when depth was held constant (partial correlation analysis, p<0.05, df=50). Whole-sediment incubation experiments under simulated in situ conditions, using ³H-thymidine or ¹⁴C-amino acids, yielded low production rates (5–75 μg C m⁻² d⁻¹) and higher respiration rates (76–242 μg C m⁻² d⁻¹), with kinetics suggestive of resource limitation at abyssal depths. Compared to similarly examined deep regions of the open ocean, the semi-enclosed Gulf of Mexico (like the Arabian Sea) harbors in its abyssal sediments a greater biomass of bacteria per unit of vertically delivered POC, likely reflecting the greater input of laterally advected, often unreactive, material from its margins.     

The response of Oneirophanta mutabilis (Holothuroidea) to the seasonal deposition of phytopigments at the Porcupine Abyssal Plain in the Northeast Atlantic

The impact of seasonal pulses of phytodetritus on the grazing behaviour of Oneirophanta mutabilis was assessed on the Porcupine Abyssal Plain (PAP) in the NE Atlantic. Sediment and sediment trap samples were analysed by HPLC to estimate the quantity and quality of the organic material in terms of phytopigments and nucleic acids. Food selection by Oneirophanta was estimated by analysing these constituents in the gut contents.The study area is characterised by large interannual variations in the deposition of fresh organic material. The mass fluxes at 10 m above bottom (mab) varied from 0.25 g DW m⁻² d⁻¹ in September 1996 to <0.1 g DW m⁻² d⁻¹ in March 1997. The material caught in the sediment trap in September 1996 had a relative fresh signature with a chlorophyll-a:phaeophorbide ratio of 1.33. During the other seasons (March 1997, July 1997 and October 1997) the chlorophyll-a:phaeophorbide ratio remained low. In sediment cores this ratio showed a similar seasonal and inter-annual pattern, and again September 1996 was the period of maximum abundance of fresh organic material in the surficial sediment. The analyses of the gut contents of Oneirophanta mirrored exactly the seasonal variation of the phytopigments in both the sediment and the sediment trap material. Concentrations of pigments in the fore-gut were 5 to 15 times higher than in the sediment and the nucleic acid concentrations were up to 80 times higher. This discrepancy between pigments and nucleic acids concentrations suggests that the latter are “indigenous” to the gut of Oneirophanta, either because the gut contains high numbers of actively-dividing bacteria or as a result of cell lysis of the gut epithelium. The seasonal differences in the pigment concentration factor suggest that Oneirophanta does not actively search for hotspots where pigment concentrations are enriched. By using the degradation rate of chlorophyll-a in the PAP sediments, the minimum residence time of chlorophyll in the sediment within the gut of Oneirophanta was calculated. In combination with gut volume and density data it was estimated that each year the Oneirophanta population skims a third of the sediment surface at the PAP site.     

Short-term seaward fish migration in the Messolonghi–Etoliko lagoons (Western Greek coast) in relation to climatic variables and the lunar cycle

In the present study we analysed the daily seaward migratory behaviour of four dominant euryhaline fish species (Mugilidae: Liza saliens, Liza aurata, Mugil cephalus and Sparidae: Sparus aurata) in the Messolonghi–Etoliko lagoon system (Western Greek coast) based on the daily landings' time series of barrier traps and assessed the relationship between their migratory behaviour and various climatic variables (air temperature and atmospheric pressure) and the lunar cycle. A 2-year time series of daily fish landings (1993 and 1994), a long time series of daily air temperature and daily temperature range (1991–1998) as well as a 4-year time series of the daily atmospheric pressure (1994–1997) and daily pressure range were used. Harmonic models (HM) consisting of annual and lunar cycle harmonic components explained most (R² > 0.80) of the mean daily species landings and temperature variations, while a rather low part of the variation (0.18 < R² < 0.27) was explained for pressure, daily pressure range and daily temperature range. In all the time series sets the amplitude of the annual component was highest. The model values of all species revealed two important migration periods (summer and winter) corresponding to the spawning and refuge migrations. The lunar cycle effect on species' daily migration rates and the short-term fluctuation of daily migration rates were rather low. However, the short-term fluctuation of some species' daily migration rates during winter was greater than during summer. In all species, the main migration was the spawning migration. The model lunar components of the species landings showed a monthly oscillation synchronous to the full moon (S. aurata and M. cephalus) or a semi-monthly oscillation synchronous to the new and full moon (L. aurata and L. saliens). Bispectral analysis of the model values and the model residuals' time series revealed that the species daily migration were correlated (coherencies > 0.6) to the daily fluctuations of the climatic variables at seasonal, mid and short-term scales.     

Contrasts in sedimentation flux below the southern California Current in late 1996 and during the El Niño event of 1997–1998

The vertical flux of particulate matter at 330 m depth in San Lázaro Basin off southern Baja California ranged from 63 to 587 mg m⁻² d⁻¹ between August and November 1996. Organic carbon contents were between 5.6 and 14.8%, yielding flux rates of 9–40 mgC m⁻² d⁻¹. In December 1997 and January 1998, at the height of the strong El Niño event, the respective fluxes (47–202 mg m⁻² d⁻¹ and 3–8 mgC m⁻² d⁻¹) were comparable. The February–June 1998 records, however, revealed sharply reduced mass (1–6 mg m⁻² d⁻¹) and organic carbon (0.2–0.8 mgC m⁻² d⁻¹) fluxes. The organics collected in 1996 were predominantly autochthonous (δ¹³C=−22‰; C/N=8). The variations in δ¹⁵N (8.3–11.0‰) suggest an alternation of new and regenerated production, possibly associated with fluctuations in the intensity of deep mixing during that autumn. The relatively high organic matter fluxes in December 1997 appear to be associated with regenerated production. The average composition from February to June 1998 (δ¹³C=−23.6‰; ¹⁵N=11.7‰; C/N=10.5) indicates degraded material of marine origin. The maximum δ¹⁵N value found (14‰) suggests that deeper, denitrified waters were brought to the surface and possibly advected laterally. Regime changes in the waters of the basin occur at 6–10 week intervals, evidenced by concurrent shifts in most of the measured parameters, including fecal pellet types and metal chemistry. The marine snow-dominated detritus collected showed a shift from a mixed diatom-rich-radiolarian-coccolith assemblage in late 1996 to a coccolith-dominated assemblage, including the contents of fecal pellets, during the 1997–1998 El-Niño period. T–S profiles, plankton analysis and chlorophyll contents of the upper water column indicated that the strong phytoplankton bloom, normally associated with seasonal upwelling along the Pacific coast of Baja, did not occur during the spring of 1998. The persistence of oligotrophic conditions during the 1997–1998 El Niño event favored the dominance of nanoplankton and reduced the vertical flux of particles.     

Distribution and abundance of the Pacific sardine (Sardinops sagax) in the Gulf of California and their relation with the environment

In 1989–90 the small pelagic fishery of the Gulf of California began to show a very marked decline in the catch of its main component, the Pacific sardine (Sardinops sagax). The catch plummeted from 292,000 t in 1988–89 to 7000 t in 1991–92 and 1992–93. This caused a serious economic crisis in the local fishery fleet and industry, and resulted in the loss of 3000 jobs. In 1993–94 the fishery showed signs of recovery as the abundance of the Pacific sardine began to recover. The catch improved to 128,000 t in 1993–94 and further to 215,000 t in 1996–97. In trying to understand this great variability, we proposed the hypothesis that the distribution and the abundance of the Pacific sardine of the Gulf of California is determined by the wind patterns (upwelling) and the sea surface temperature. The results of analyzing data from 25 cruises showed the period of low relative abundance between 1990 and 1993 and one of high abundance between 1993 and 1996. The range of the sardine's distribution expanded as its abundance increased and contracted when abundances were low. The relationship between the abundances of the sardine and environmental variables proved to nonlinear and bell-shaped. The adjusted pattern explained 78.8% of the variability of the sardine abundance. The highest abundance are produced by moderate upwelling (13–18 m³s⁻¹ per 10 m of coastline) and sea surface temperatures of between 19°C and 25°C.     

Seasonal variations in dimethylsulfoniopropionate and dimethylsulfide concentrations in relation to the plankton community in the St. Lawrence Estuary

Weekly variations in total dimethylsulfoniopropionate (DMSPt) and dimethylsulfide (DMS) were investigated in relation to the phytoplankton assemblage from spring to fall 1994 at a coastal fixed station in the St. Lawrence Estuary. DMSPt and DMS concentrations showed a strong seasonality and were tightly coupled in time. Maximum concentrations of DMSPt and DMS were observed in July and August, during a period of warm water and low nutrient concentrations. Seasonal maxima of 365.4 nmol l⁻¹ for DMSPt and 14.2 nmol l⁻¹ for DMS in early August coincided with the presence of many phytoplankton species, such as Alexandrium tamarense, Dinophysis acuminata, Gymnodinium sp., Heterocapsa rotundata, Protoperidinium ovatum, Scrippsiella trochoidea, Chrysochromulina sp. (6 μm), Cryptomonas sp. (6 μm), a group of microflagellates smaller than 5 μm (mf < 5), many tintinnids, and Mesodinium rubrum. The abundance of mf < 5 followed the general trend of DMS concentrations. The temporal occurrence of high P. ovatum abundance and DMSPt concentrations suggests that this heterotrophic dinoflagellate can either synthesize DMSP or acquire it from DMSP-rich prey. The calculated sea-to-air DMS flux reached a maximum of 8.36 μmol ⁻² d⁻¹ on August 1. The estimated annual emission from the St. Lawrence Estuary is 77.2 tons of biogenic sulfur to the atmosphere.     

Inorganic carbon in the central California upwelling system during the 1997–1999 El Niño–La Niña event

Sea surface pCO₂ was monitored during 49 cruises from February 1997 to December 1999 along a section perpendicular to the central California Coast. Continuous measurements of the ocean–atmosphere difference of pCO₂ were made on a mooring in the same region from July 1997 to December 1999. The El Niño/La Niña cycle of 1997–1999 had a significant influence on local ocean–atmosphere CO₂ transfer. During the warm anomaly associated with El Niño, upwelling was suppressed and average sea surface pCO₂ was below atmospheric level. High rainfall and river runoff in the late winter and early spring of 1998 produced areas where pCO₂ was depressed by as much as 100 μatm. A flux ranging from 0.3 to 0.7 mol C m⁻² y⁻¹ from the atmosphere into the ocean was estimated for the El Niño period from wind and ΔpCO₂ data. Temperatures and upwelling returned to near normal in the summer of 1998, but a cold anomaly developed during autumn of that year. Temperature and pCO₂ data indicate that upwelling continued throughout much of the 1998–1999 winter and intensified significantly in the spring of 1999. During strong upwelling events, the estimate of ocean to atmosphere flux approached rates of 50 mol C m⁻² y⁻¹. The estimate for the average CO₂ flux from July 1998 to July 1999 was 1.5–2.2 mol C m⁻² y⁻¹ from the ocean to the atmosphere. While the flux estimate for the El Niño time period may be applicable to a larger area, the high ocean to atmosphere fluxes during La Niña might be the result of sampling near a zone of intense upwelling.     

A depocenter of organic matter at 7800 m depth in the SE Pacific Ocean

The Atacama trench, the deepest ecosystem of the southern Pacific Ocean (ca. 8000 m depth) was investigated during the Atacama Trench International Expedition. Sediments, collected at three bathyal stations (1040–1355 m depth) and at a hadal site (7800 m) were analyzed for organic matter quantity and biochemical composition (in terms of phytopigments, proteins, carbohydrates and lipids), bacterial abundance, biomass and carbon production and extracellular enzymatic activities. Functional chlorophyll-a (18.0±0.10 mg m⁻²), phytodetritus (322.2 mg m⁻²) and labile organic carbon (16.9±4.3 g C m⁻²) deposited on surface sediments at hadal depth (7800 m) reached concentrations similar to those encountered in highly productive shallow coastal areas. High values of bacterial C production and aminopeptidase activity were also measured (at in situ temperature and 1 atm). The chemical analyses of the Atacama hadal sediments indicate that this trench behaves as a deep oceanic trap for organic material. We hypothesize that, despite the extreme physical conditions, benthic microbial processes might be accelerated as a result of the organic enrichment.     

Plankton response to El Niño 1997–1998 and La Niña 1999 in the southern region of the California Current

The IMECOCAL Program began in 1997, with the objective of sampling plankton systematically in the Mexican region of the California Current. We present results of chlorophyll a concentrations and zooplankton displacement volumes for the eight cruises from September 1997 to October 1999. The abundance of 22 zooplankton groups was also analyzed for the first four cruises. The response of plankton to the 1997–1998 El Niño was atypical. From September 1997 to January 1998, chlorophyll a and zooplankton volume were at typical values (median integrated chlorophyll was 27 mg/m² and zooplankton 100 ml/1000 m³ in 9801/02). After the peak of El Niño, the system shifted to cooler conditions. Integrated chlorophyll gradually increased to a median of 77 mg/m² in April 1999. In contrast, zooplankton volumes decreased from October 1998 onward, despite favorable phytoplankton availability in 1999. Zooplankton structure was dominated by copepods and chaetognaths through the ENSO cycle, but interannual changes were evident. In the fall of 1997 there was a higher proportion of copepods, chaetognaths, and other minor groups, while the fall of 1998 zooplankton was richer in salps and ostracods. Historical data from previous Baja California CalCOFI cruises indicated that zooplankton volumes measured during the IMECOCAL cruises were above the long-term mean for the period 1951–1984. This suggests a differential response of plankton to the El Niño of 1997–1998 compared to the El Niño of 1957–1959. Regional differences in zooplankton volumes were also found, with central Baja California having 41% higher biomass than northern Baja California. Volumes from both regions were larger than those recorded by CalCOFI off southern California during 1997–1998, but the situation was reversed in 1999. The higher biomasses in the 1997–1998 El Niño can be attributed to high abundance of salps, which showed an affinity with warm, saline water.     

Seasonal Variations in Abundance and Size Composition of the Lobate Ctenophore Bolinopsis Mikado (Moser) in Tokyo Bay, Central Japan

Surveys of the abundance and size composition of the ctenophore Bolinopsis mikado were conducted in Tokyo Bay over a 5-year period from 1990 to 1994. B. mikado appeared throughout the year, and its mass occurrence was observed between August and November; annual maximum abundance ranged from 19 to 91 ind. m^–2. Water temperature seems to influence the seasonal variation of B. mikado abundance. Environmental conditions (e.g. rough waters due to a typhoon) and predation by the beroid ctenophore Beroe cucumis appear to affect annual variations of B. mikado abundance. Size frequency distributions of B. mikado indicated that its reproduction was most active in summer and fall but occurred throughout the year in Tokyo Bay. A sharp decline of the copepod population in August 1990 was probably due to predation by B. mikado which was very abundant at that time; its predatory impact was estimated to be 24 % day^–1.     

Downward carbon transport by diel vertical migration of the copepods Metridia pacifica and Metridia okhotensis in the Oyashio region of the western subarctic Pacific Ocean

Seasonal change in the downward carbon transport due to respiration and mortality through diel vertical migration (DVM) of the calanoid copepods Metridia pacifica and Metridia okhotensis was estimated in the Oyashio region, western subarctic Pacific during six cruises from June 2001 to June 2002. M. pacifica (C4, C5 and adult females) was an active migratory species throughout the year though its DVM amplitude varied among seasons and stages. The mean distribution depths of adult females during the daytime were positively related with the illumination level in the water column, being shallowest in April and deepest in January. M. okhotensis generally showed less-extensive migrations than M. pacifica. Therefore, together with their lower abundance, this species is considered to be a less-important mechanism of downward transport of carbon except for April when their DVM was more active and descended deeper than M. pacifica, which remained in the upper 150 m even during the daytime. The mean migrating biomass of the two Metridia species was 558 mg C m⁻² d⁻¹ and was high during summer to winter (263–1676 mg C m⁻² d⁻¹) and low during spring (59–63 mg C m⁻² d⁻¹). Total downward flux through DVM fluctuated between 1.0 and 20.0 mg C m⁻² d⁻¹ with an annual mean of 8.0 mg C m⁻² d⁻¹. Contribution of the respiratory flux was greater than the mortality flux and accounted for 64–98% of total migratory flux throughout the year except for January when contribution of both fluxes was equal. Overall the annual carbon transport by DVM of Metridia spp. was estimated as 3.0 g C m⁻² year⁻¹, corresponding to 15% of the annual total POC flux at 150 m at the study site, suggesting that DVM is a significant process for carbon export in the subarctic region as well as that in tropical and subtropical oceanic regions. Since DVM in M. pacifica is more active during the non-bloom season when the gravitational flux of particulate matter is low, this species plays an important role in driving the biological pump in the subarctic Pacific during summer to winter.     

Interannual variations of the occurrence of epipelagic fish in the diets of the seabirds breeding on Teuri Island, northern Hokkaido, Japan

The diets of breeding seabirds can be a good monitor of marine environmental changes. From 1984 to 2001 we monitored the diets of black-tailed gulls (Larus crassirostris) (“surface foragers”), rhinoceros auklets (Cerorhinca monocerata) (“epipelagic divers”), and Japanese cormorants (Phalacrocorax filamentotus) (“bottom divers”) that breed on Teuri Island at the northern boundary of the Tsushima Warm current in the Sea of Japan/East Sea. Between 1984 and 1987, both the gulls and the auklets foraged on the sardine (Sardinops melanostictus), but after 1992, they switched to the anchovy (Engraulis japonica). This change might reflect the collapse of the sardine stock in the late 1980s. In the 1990s, the year-to-year variations of the percentage of anchovy in the diets of the three seabird species showed similar trends: High in 1994 and 1998–2001; and low in 1992–1993 and 1995–1997. The estimated stock size of the anchovy population in the Tsushima Current area was positively correlated with the percentage of mass of anchovy in the seabirds’ diets. Thus, the short-term annual changes of the total anchovy availability, which might reflect SST or the volume transport of Tsushima Current, possibly affected the seabirds diets on this island.     

Heat budget in the Japan Sea

The long-term mean (31-year mean) surface heat fluxes over the Japan Sea are estimated by the bulk method using the most of the available vessel data with the resolution of 1^o×1^o. The long-term annual mean net heat flux is about –53 W m^–2 (negative sign means upward heat flux) with the annual range from 133 W m^–2 in May to –296 W m^–2 in December. The small gain of heat in the area near Vladivostok seems to indicate the existence of cold water flowing from the north. In that area in winter, the mean loss of heat attains about 200 W m^–2, and the Bowen's ratio is over the unity. The largest insolation occurs in May in the Japan Sea, and the upward latent heat flux becomes the largest in November in this area. The heat flux of Haney type is also calculated, and the result, shows that the constantQ ₁ has the remarkable seasonal and spatial variation, while the coefficientQ ₂ has relatively small variation throughout all seasons. Under the assumption of constant volume transport of 1.35×10⁶ m³s^–1 through the Tsugaru Strait, the long-term averages of the volume transport through the Tsushima and Soya Straits are estimated to be about 2.20 and 0.85×10⁶ m³s^–1 from the result of the mean surface heat flux, respectively.     

A seasonal carbon budget for the sub-Antarctic Ocean, South of Australia

Changes from winter (July) to summer (February) in mixed layer carbon tracers and nutrients measured in the sub-Antarctic zone (SAZ), south of Australia, were used to derive a seasonal carbon budget. The region showed a strong winter to summer decrease in dissolved inorganic carbon (DIC;  45 µmol/kg) and fugacity of carbon dioxide (fCO₂;  25 µatm), and an increase in stable carbon isotopic composition of DIC (δ¹³C_DIC;  0.5‰), based on data collected between November 1997 and July 1999.The observed mixed layer changes are due to a combination of ocean mixing, air–sea exchange of CO₂, and biological carbon production and export. After correction for mixing, we find that DIC decreases by up to 42 ± 3 µmol/kg from winter (July) to summer (February), with δ¹³C_DIC enriched by up to 0.45 ± 0.05‰ for the same period. The enrichment of δ¹³C_DIC between winter and summer is due to the preferential uptake of ¹²CO₂ by marine phytoplankton during photosynthesis. Biological processes dominate the seasonal carbon budget (≈ 80%), while air–sea exchange of CO₂ (≈ 10%) and mixing (≈ 10%) have smaller effects. We found the seasonal amplitude of fCO₂ to be about half that of a study undertaken during 1991–1995 [Metzl, N., Tilbrook, B. and Poisson, A., 1999. The annual fCO₂ cycle and the air–sea CO₂ flux in the sub-Antarctic Ocean. Tellus Series B—Chemical and Physical Meteorology, 51(4): 849–861.] for the same region, indicating that SAZ may undergo significant inter-annual variations in surface fCO₂. The seasonal DIC depletion implies a minimum biological carbon export of 3400 mmol C/ m² from July to February. A comparison with nutrient changes indicates that organic carbon export occurs close to Redfield values (ΔP:ΔN:ΔC = 1:16:119). Extrapolating our estimates to the circumpolar sub-Antarctic Ocean implies a minimum organic carbon export of 0.65 GtC from the July to February period, about 5–7% of estimates of global export flux. Our estimate for biological carbon export is an order of magnitude greater than anthropogenic CO₂ uptake in the same region and suggests that changes in biological export in the region may have large implications for future CO₂ uptake by the ocean.     

Copepod grazing on phytoplankton in the Pacific sector of the Antarctic Polar Front

Mesozooplankton abundance, community structure and copepod grazing on phytoplankton were examined during the austral spring 1997 and summer 1998 as part of the US JGOFS project in the Pacific sector of the Antarctic polar front. Mesozooplankton abundance and biomass were highest at the polar front and south of the front. Biomass increased by 1.5–2-times during the course of the study. Calanoides acutus, Calanus propinquus, C. simillimus, Rhincalanus gigas and Neocalanus tonsus were the dominant large copepods found in the study. Oithona spp and pteropods were numerically important components of the zooplankton community. The copepod and juvenile krill community consumed 1–7% of the daily chlorophyll standing stock, equivalent to 3–21% of the daily phytoplankton production. There was an increased grazing pressure at night due to both increased gut pigment concentrations as well as increases in zooplankton numbers. Phytoplankton carbon contributed a significant fraction (>50%) of the dietary carbon for the copepods during spring and summer. The relative importance of phytoplankton carbon to the diet increased south of the polar front, suggested that grazing by copepods could be important to organic carbon and biogenic silica flux south of the polar front.     

Long-term monitoring of the sedimentary processes in the central part of Sagami Bay, Japan: rationale, logistics and overview of results

Deep-sea benthic ecosystems are mainly sustained by sinking organic materials that are produced in the euphotic zone. “Benthic-pelagic coupling” is the key to understanding both material cycles and benthic ecology in deep-sea environments, in particular in topographically flat open oceanic settings. However, it remains unclear whether “benthic-pelagic coupling” exists in eutrophic deep-sea environments at the ocean margins where areas of undulating and steep bottom topography are partly closely surrounded by land. Land-locked deep-sea settings may be characterized by different particle behaviors both in the water column and in relation to submarine topography. Mechanisms of particle accumulation may be different from those found in open ocean sedimentary systems. An interdisciplinary programme, “Project Sagami”, was carried out to understand seasonal carbon cycling in a eutrophic deep-sea environment (Sagami Bay) with steep bottom topography along the western margin of the Pacific, off central Japan. We collected data from ocean color photographs obtained using a sea observation satellite, surface water samples, hydrographic casts with turbidity sensor, sediment trap moorings and multiple core samplings at a permanent station in the central part of Sagami Bay between 1997 and 1998. Bottom nepheloid layers were also observed in video images recorded at a real-time, sea-floor observatory off Hatsushima in Sagami Bay. Distinct spring blooms were observed during mid-February through May in 1997. Mass flux deposited in sediment traps did not show a distinct spring bloom signal because of the influence of resuspended materials. However, dense clouds of suspended particles were observed only in the spring in the benthic nepheloid layer. This phenomenon corresponds well to the increased deposition of phytodetritus after the spring bloom. A phytodetrital layer started to form on the sediment surface about two weeks after the start of the spring bloom. Chlorophyll-a was detected in the top 2 cm of the sediment only when a phytodetritus layer was present. Protozoan and metazoan meiobenthos increased in density after phytodetritus deposition. Thus, “benthic-pelagic coupling” was certainly observed even in a marginal ocean environment with undulated bottom topography. Seasonal changes in features of the sediment-water interface were also documented.     

Organic matter assimilation and selective feeding by holothurians in the deep sea: some observations and comments

The selective feeding behaviour and assimilation efficiencies of deep-sea holothurians were investigated in order to assess their impact on carbon and nitrogen remineralisation on the Porcupine Abyssal Plain (PAP; 49°N 16°W, 4850 m water depth). Unfortunately, reliable determination of organic matter in the gut contents of the organisms proved to be difficult, because of the lysis of cells associated with the death of the animals on recovery. This was expressed in high levels of free fatty acids in the gut contents of Oneirophanta mutabilis, which we ascribe to unregulated lipolysis of phospholipids and triacylglycerides. It was not possible to estimate accurately the contribution that such material made to the gut contents, but based on the distributions of sterols in the gut sediments, it is likely to have been substantial. Therefore, all assimilation efficiencies calculated for holothurians in the deep sea should be treated with caution.Fortuitously, a bloom of holothurians that feed on the sediment surface (namely Amperima rosea and Ellipinion molle) during the period of study provided an opportunity indirectly to assess the impact of megafauna on organic matter cycling at the PAP. Observations suggest that the depletion of phytosterols from the surficial sediments between July and October 1997 resulted from the selective uptake of fresh phytodetritus by the blooming species. Deep-sea holothurians do not biosynthesise sterols de novo and an estimate of the sterol required by the increased population of A. rosea and E. molle is equivalent to the sterol flux to the seafloor during the spring/summer of 1997. The implications are dramatic. Firstly, these and other megafauna apparently turned over and selectively removed phytosterols from the freshly arrived phytodetritus and the surficial sediment (0–5 mm) at the PAP in less than four months. Secondly, their action impacted the food resource available to other organisms. Finally, as phytosterols are expensive to biosynthesise and are apparently an important resource for holothurians, we speculate that the supply of these compounds to the sedimentary community may be one important control on their population in the abyssal ocean.