55Fe in Pacific Ocean plankton

The specific activity of ⁵⁵Fe in plankton increased from below detectable limits at 20°N to a maximum of 130 nCi/g Fe at 20°S and then decreased to 5 nCi/g Fe at 50°S. Along an east-west track near 15°S a peak of 90 nCi/g Fe occurred at 135°W. It is suggested that this maximum is the result of fallout from French nuclear tests at Mururoa Atoll (22°S; 139°W). The increase in ⁵⁵Fe at high latitudes observed in previous studies in the North Atlantic Ocean and North Pacific Ocean did not occur in the South Pacific Ocean which supports the suggestion that belts of high tropospheric fallout accounted for the increase in the Northern oceans.     

The structure of plankton communities of the Indian Ocean

Species diversity and trophic structure of plankton communities have been considered as most important biological features. The Fisher () and Simpson () indices and the information index H=- pi log₂ pi (Margalef) were used for evaluation of species diversity. The indices have been calculated within the group of filter feeders by numbers ( and H) and biomass ( and H) of the species; the best results were obtained in the latter case. The index H=- pi log₂ pi is proposed for the quantitative evaluation of trophic structure, where a portion of an individual trophic group is taken as pi. The plankton material was collected by the R.V. Vityaz in the northern Indian Ocean in January through February, 1960. Analysis of characteristics of zooplankton distribution, species and trophic diversity in connection with characteristics of the water regime resulted in the definition of 3 types of the plankton communities: (a) those in intensive divergence zones; (b) those in poor divergence zones; (c) those in regions with stable water stratification or with feebly marked convergence, distiguished by different values of biomass, species diversity, and trophic group ratios.     

On the near-surface plankton of the eastern South Pacific Ocean

A study has been made of the plankton of the near-surface water layer (0 to 30 cm) of the eastern South Pacific Ocean in the region lying to the east of the meridian 90°W, between latitudes 5°N and 35°S. This region is influenced by the Peru Current: the current brings water from high latitudes, which results in a decrease in the number of species of the local fauna of copepods of the family Pontellidae, typical of tropical near-surface plankton. Some of the widely tropical and one bicentral species are absent or rare. Least affected by the Peru Current are the waters of equatorial structure in the northern part of the region. Here, 7 species of pontellids were recorded: the widely tropical Labidocera detruncata, Pontella tenuiremis, Pontellopsis regalis, the distant-neritic Pontella danae and Labidocera acuta, the bicentral Labidocera acutifrons, and the neritic Pontellopsis lubbockii. The dominant species among these are L. detruncata and L. acuta. To the west of the convergence, in the southern part of the region, live the southern central species Pontella valida and P. whiteleggei, with Pontellopsis regalis occurring occasionally. In these regions the copepod fauna is frequently dominated by pontellids. To the south of the boundary of the waters of equatorial structure, between the coast of South America and the line of convergence, lies a region most subjected to the effect of waters from high latitudes and of upwellings. It is inhabited by 2 pontellids only: Pontellopsis regalis and Labidocera acutifrons, but they too disappear close to the coast. In this particular region the copepods Calanus australis and Centropages brachiatus are common; they are found in a thicker water layer (0 to 200 m), and are often more abundant than the pontellids.     

Distribution and structure of heterotrophic protist communities in the northeast equatorial Pacific Ocean

The distribution and structure of heterotrophic protist communities and size-fractionated chlorophyll a were studied during the Korea Deep Ocean Study 98 (KODOS 98) research expedition (July 1998) in the northeast equatorial Pacific Ocean (5–11°N). Areas of convergence and divergence formed at the boundaries of the South Equatorial Current (SEC), North Equatorial Current (NEC), and North Equatorial Counter Current (NECC) during the expedition. Water column physicochemical characteristics significantly influenced the size structure of heterotrophic protist communities. Intense vertical mixing and high nutrient and chlorophyll a concentrations characterized SEC and NECC areas, which were affected by converging and diverging water masses, respectively. Nanophytoplankton dominated in SEC and NECC areas; both areas also had relatively high heterotrophic protist biomasses (average 743 µg C m^–2). NEC areas were characterized by a stratified vertical structure, low nutrient and chlorophyll a concentrations, and picophytoplankton dominance. The heterotrophic protist biomass in NEC areas averaged 414 µg C m^–2; nanoprotists (<20 µm) dominated the community. The nanoprotist biomass comprised 49–54% of the total heterotrophic protist biomass in SEC/NECC areas and 67–72% in NEC areas. The biomass of heterotrophic protists was higher in SEC/NECC areas than in NEC areas, but the relative importance of nanoprotists was greater in NEC areas than in SEC/NECC areas. Heterotrophic dinoflagellates were dominant components of the <20 µm and >20 µm size classes in both water columns. The biomass of heterotrophic protists significantly correlated with the net-, nano-, and picophytoplankton biomass in SEC/NECC areas and with the nano- and picophytoplankton biomass in NEC areas. Heterotrophic protists and phytoplankton also showed strong positive correlation in the study area. The size structure of the phytoplankton biomass coincided with that of heterotrophic protists; the heterotrophic protist biomass positively correlated with the protists prey source. These relationships suggest that the community structure of heterotrophic protists and the microbial food web depended on size classes within the phytoplankton biomass. Microzooplankton grazing and phytoplankton growth rates were higher in SEC/NECC areas than in NEC areas. In contrast, the potential primary production grazed by microzooplankton was relatively high in NEC areas (127.3%) compared with SEC/NECC areas (94.6%). Our results indicate that the relative importance and size structure of heterotrophic protists might vary according to two distinct water column structures.Communicated by T. Ikeda, Hakodate     

Nitrogen fixation in the North Pacific Ocean

Nitrogen fixation in the euphotic zone of the ocean was measured by C₂H₂ reduction and ¹⁵N₂ incorporation associated with Trichodesmium sp. and also with Richelia intracellularis occurring within the cells of Rhizosolenia styliformis var. longispina, and R. cylindrus. The vertical distribution of N₂ fixation activity, N₂-fixing species, particulate matter and dissolved nutrients was measured. The effects of light intensity, sample concentration, length of incubation, and nutrient enrichment on the rates of C₂H₂ reduction were determined. Estimates of the importance of N₂ fixation in adding previously uncycled nitrogen to the euphotic zone are given.     

Structure and temporal fluctuations of two intertidal seagrass-bed communities in New Caledonia (SW Pacific Ocean)

The structure and trophic organization of two intertidal seagrass-bed communities (Halodule uninervis and Thalassia hemprichii) were examined on the southwest coast of New Caledonia (SW Pacific Ocean), from April 1989 to March 1990. Five benthic samples were collected from each site at 2 mo intervals and various environmental parameters were simultaneously monitored. Animal:plant biomass ratios were close to 1 at both sites. Polychaetes dominated in number of species. The suspension-feeding bivalves Gafrarium tumidum and Anadara scapha constituted the greater part of the animal biomass. The evolution of the communities over an annual cycle displayed marked structural and organizational stability, resulting mainly from the absence of distinct recruitment periods for the dominant species, no mortalities during the brief low-salinity periods, and no temporal variations in the granulometry, the organic matter or the chlorophyll a contents of the sediments.     

Spring phytoplankton in the Subarctic North Pacific Ocean

The report includes quantitative and qualitative data on the phytoplankton from the First Canadian Transpacific Oceanographic Cruise from March to May 1969. Nanoplanktonic species (<20 m) were numerically dominant and, together with nertic diatoms, made up the larger portion of the biomass at all stations outside the Western Subarctic and Alaskan Gyres, where Denticulopsis seminae predominated. The nanoplankton consisted mainly of haptophycean, dinophycean and cryptophycean flagellates. Tropical oceanic dinoflagellates and diatoms were abundant in the net samples from Station 17 in the warm Kuroshio current but absent from Stations 24–27 in the Western Subarctic Gyre, Stations 2, 37–39 in the Alaskan Gyre, in which cold water oceanic species predominated, and Stations 31 and 33, south of the Aleutians. Subsurface maxima were common on the outward bound (westerly) leg in March.     

Vertical and horizontal structure in the picoplankton communities of a coastal upwelling system

The distribution of prokaryotic and eukaryotic picoplankton in the west coast upwelling-region off the South Island of New Zealand was investigated during midwinter (1988) the time of year when several commercially important fish species migrate into the region to breed. Picoplanktonic cells were major contributors to the autotrophic biomass, with > 80% of the particulate nitrogen and 39 to 55% of the total chlorophylla contained in the <2µm size-fraction. The prokaryotic picoplankton concentrations ranged from 6.3 × 10⁵ to 2.1 × 10⁷ cell l^–1, and the eukaryotic picoplankton between 3.9 × 10⁵ to 1.2 × 10⁷ cells l^–1. Picoplankton numbers increased with distance offshore to a maximum of ~ 3.0 × 10⁷ cells l^–1 at ~ 35 km from the coast, and then diminished towards the outer shelf and open ocean. The ratio of prokaryotic to eukaryotic cells varied between 1.01 and 4.71 in the mixed layer. Both groups declined substantially beneath the pycnocline, with no evidence of deep maxima. Prokaryotic cells dominated the planktonic cell concentrations at all but two stations, but eukaryotic cells dominated picoplankton biovolume as a result of their larger average cell size. The prokaryotic to eukaryotic picoplankton cell-number ratios in this system were considerably lower than often recorded elsewhere, and were inversely correlated with nitrate concentration. These observations show that a eukaryoticdominated picoplankton community makes a substantial contribution to autotrophic biomass in this nutrient-rich upwelling system, and may thereby play a major role in the food-web dynamics of this coastal fishery.     

Spectral analysis unmasks synchronous and compensatory dynamics in plankton communities

Community biomass is often less variable than the biomasses of populations within the community, yet attempts to implicate compensatory dynamics between populations as a cause of this relationship often fail. In part, this may be due to the lack of appropriate metrics for variability, but there is also great potential for large-scale processes such as seasonality or longer-term environmental change to obscure important dynamics at other temporal scales. In this study, we apply a scale-resolving method to long-term plankton data, to identify the specific temporal scales at which community-level variability is influenced by synchrony or compensatory dynamics at the population level. We show that variability at both the population and community level is influenced strongly by a few distinct temporal scales: in phytoplankton, ciliate, rotifer, and crustacean communities, synchronous dynamics are predominant at most temporal scales. However, in phytoplankton and crustacean communities, compensatory dynamics occur at a sub-annual scale (and at the annual scale in crustaceans) leading to substantial reductions in community-level variability. Aggregate measures of population and community variability do not detect compensatory dynamics in these communities; thus, resolving their scale dependence unmasks dynamics that are important for community stability in this system. The methods and results presented herein will ultimately lead to a better understanding of how stability is achieved in communities.     

Phytoplankton-zooplankton relationships in the Western Pacific Ocean and adjacent seas

Regressions of biomass and daily food requirements of herbivorous zooplankton on daily primary production were calculated, using assumptions based on data collected in various sea areas of the western Pacific Ocean and adjacent seas. A regression coefficient (1.470) of calculated herbivorous biomass on observed daily primary production is significantly higher than unity (P<0.01). This indicates that the herbivorous biomass sustained by unit amount of primary production is large in the more productive high latitudes, and small in the less productive tropical sea areas. This is attributed to relatively larger food requirements per unit biomass of the tropical herbivores as compared with those found in cold waters. Despite distinct areal differences in the herbivorous biomass-primary production ratios, the calculated daily food requirement of herbivores was in direct proportion to the daily primary production, when equilibrium had been established between phytoplankton and zooplankton. Under conditions of limited food supplies, the small body size of the tropical herbivores may be advantageous both in reducing the total energy consumption per individual, and in inducing rapid growth and reproduction. Therefore, the low ratio of biomass to primary production in the tropics could beregarded as a result of possible regulation of tropical herbivores to scarce food conditions rather than as evidence of failure of adaptation to such conditions.     

A simple plankton model for the Oregon upwelling ecosystem: Sensitivity and validation against time-series ocean data

Simple plankton models serve as useful platforms for testing our understanding of the mechanisms underlying ecosystem dynamics. A simple, one-dimensional plankton model was developed to describe the dynamics of nitrate, ammonium, two phytoplankton size-classes, meso-zooplankton, and detritus in the Oregon upwelling ecosystem. Computational simplicity was maintained by linking the biological model to a one-dimensional, cross-shelf physical model driven by the daily coastal upwelling index. The model sacrificed resolution of regional-scale and along-shore (north to south) processes and assumed that seasonal productivity is primarily driven by local cross-shelf Ekman transport of surface waters and upwelling of nutrient-rich water from depth.Our goals were to see how well a simple plankton model could capture the general temporal and spatial dynamics of the system, test system sensitivity to alternate parameter set values, and observe system response to the effective scale of potential retention mechanisms. Model performance across the central Oregon shelf was evaluated against two years (2000-2001) of chlorophyll and copepod time-series observations. While the modeled meso-zooplankton biomass was close in scale to the observed copepod biomass, phytoplankton was overestimated relative to that inferred from the observed surface chlorophyll concentration. Inshore, the system was most sensitive to the nutrient uptake kinetics of diatom-size phytoplankton and to the functional grazing response of meso-zooplankton. Meso-zooplankton was more sensitive to alternate parameter values than was phytoplankton. Reduction of meso-zooplankton cross-shelf advection rates (crudely representing behavioral retention mechanisms) reduced the scale of model error relative to the observed seasonal mean inshore copepod biomass but had little effect of the modeled meso-zooplankton biomass offshore nor upon phytoplankton biomass across the entire shelf.     

Growth of demersal fish species of the Mexican Pacific Ocean

The growth of 24 species of demersal fish caught off the Sinaloa-Nayarit coast and in the Gulf of Tehuantepec (Mexican Pacific Ocean) was determined by means of length-frequency analysis. Fishes constitute an important bycatch of the penaeid shrimp fishery, and 235 species were identified in catches made on research cruises in both areas in 1989 to 1992. Growth rates (cm yr^-1) ranged from 0.13 cm for lutjanids and serranids to 0.95 cm for the carangid Selene peruviana; the other species displayed growth rates within this range. Growth varied seasonally, with minimum growth in spring, and is probably related to seasonal changes in the waters of the area. By means of length-frequency analysis, we determined von Bertalanffy growth parameters for the primary species of this multispecies fishery. Such data is fundamental for the future commercial exploitation of these fishes.     

Data on biological productivity of the Western tropical Pacific Ocean

Vertical distribution and production of phytoplankton and planktonic bacteria were studied in the pelagic and neritic regions of the Western tropical Pacific Ocean and in temperate waters of the Japan Sea. The stable, layering structure of the community and its daily fluctuations caused by grazing are described. Data on the integral daily production of communities under 1 m² are presented. The data obtained are discussed with respect to the trophical relations in planktonic communities of the stratified sea basins.     

Long-term change in benthopelagic fish abundance in the abyssal northeast Pacific Ocean

Food web structure, particularly the relative importance of bottom-up and top-down control of animal abundances, is poorly known for the Earth's largest habitats: the abyssal plains. A unique 15-yr time series of climate, productivity, particulate flux, and abundance of primary consumers (primarily echinoderms) and secondary consumers (fish) was examined to elucidate the response of trophic levels to temporal variation in one another. Towed camera sled deployments in the abyssal northeast Pacific (4100 m water depth) showed that annual mean numbers of the dominant fish genus (Coryphaenoides spp.) more than doubled over the period 1989-2004. Coryphaenoides spp. abundance was significantly correlated with total abundance of mobile epibenthic megafauna (echinoderms), with changes in fish abundance lagging behind changes in the echinoderms. Direct correlations between surface climate and fish abundances, and particulate organic carbon (POC) flux and fish abundances, were insignificant, which may be related to the varied response of the potential prey taxa to climate and POC flux. This study provides a rare opportunity to study the long-term dynamics of an unexploited marine fish population and suggests a dominant role for bottom-up control in this system.     

A report of green algal pigments in the Central North Pacific Ocean

Thin-layer chromatography showed that chlorophyll b was present in acetone extracts of 6 samples of suspended matter collected in February 1973 from the Central North Pacific Ocean. Pigment patterns showed the presence of green algae and diatoms through most of the euphotic zone down to 200 m. Ratios of chlorophyll b:a ranged from 0.2 to 0.5 at 5 and 100 m depths, but were less than 0.05 at 200 m.     

Early life history of tropical Anguilla leptocephali in the western Pacific Ocean

In order to examine the early life-history characteristics of tropical eels, otolith microstructure and microchemistry were examined in leptocephali of Anguilla bicolor pacifica (27.6-54.1 mm TL, n=20) and A. marmorata (22.0-47.3 mm TL, n=8) collected during a cruise in the western Pacific. A. bicolor pacifica occurred between 10°N and 15°N in the west and between 5°S and 10°N farther to the east. A. marmorata also occurred in two different latitudinal ranges in the Northern (15-16°N) and Southern Hemispheres (3-15°S) of the western Pacific. The increment widths in the otoliths of these leptocephali increased between the hatch check (0 days) and about an age of 30 days in both species, and then gradually decreased toward the otolith edge. Otolith Sr:Ca ratios showed a gradual increase from the otolith center to the edge. The ages of A. bicolor pacifica and A. marmorata leptocephali ranged from 40 to 128 days and from 38 to 99 days, respectively. Growth rates of A. bicolor pacifica and A. marmorata leptocephali ranged from 0.33 to 0.71 mm day^-1 and from 0.45 to 0.63 mm day^-1, respectively. These leptocephali had estimated growth rates that were spread out throughout most of the reported range of growth rates of the leptocephali of the temperate species, the Japanese eel and the Atlantic eels. Differences in the spatial distribution in relation to current systems, and the age and size compositions of the leptocephali of A. bicolor pacifica and A. marmorata suggested different spawning locations for these two species.     

Associations between seabirds and water-masses in the northern Pacific Ocean in summer

Seabirds were systematically censused during more than 6 000 transect counts from research vessels in the North Pacific Ocean and Bering Sea during the summers of 1975 to 1984. Density indices were calculated for 1^o latitude-longitude blocks for 71 species. Blocks were assigned to oceanographic regions (current systems and domains) on the bases of geography, sea surface temperature and salinity. Bird abundances across regions were patchy; concentrations occurred at boundaries. Regional avifaunas overlapped as a function of three factors: similarity of water-types, geographic adjacency, and proximity to nesting areas. Four major avifaunas were apparent: the Bering Sea (and adjacent regions), the Subarctic Current System/Transition Domain (and adjacent regions), Upwelling Domain, and North Pacific Central Water. The subarctic boundary sharply separated different avifaunas. Ocean productivity may be the factor that ultimately affects avifaunal composition.     

Particulate organic carbon and nitrogen in the adjacent seas of the Pacific Ocean

Particulate organic carbon (POC) and nitrogen in sea water were measured in samples collected in the adjacent seas of the Pacific Ocean during the cruises of T. S. Oshoro-Maru (1969, 1970) and the R. V. Hakuho-Maru (KH-70-4, KH-72-1). High values were obtained in the northern North Pacific and the Bering Sea, the concentration of particulate carbon in the upper 50-m layer ranged from 35 to 550 g Cl^-1. In the deep waters of these area, values above 50 g Cl^-1 were frequently observed. The lowest values in the surface layer and deeper layers were obtained in the Japan Sea (23 gCl^-1) and in the South China Sea (7 g Cl^-1) respectively. A consistent minimum was located in the intermediate waters (100–400 m) throughout the entire region studied. Variation with depth was generally irregular with marked peak values in different layers. The POC distribution consited of these peak values and a relatively uniform background concentration. These background values slightly decreased with increasing depth and were different locally. Correlation analysis between carbon concentration and apparent oxygen utilization (AOU) of ambient water for the samples in the Japan Sea and the Sulu Sea showed that there was no systematic decrease of particulate carbon with increasing AOU. In these areas, the carbon concentration scattered in the higher AOU domain ranged from 10–100 g Cl^-1. These observations support the conception that downward transport of particulate matter from the overlying surface layer in the adjacent seas of the Pacific Ocean may be fairly rapid.     

Mega-epibenthic communities in Arctic and Antarctic shelf areas

Mega-epibenthic shelf assemblages were investigated off Northeast Greenland and in the Weddell, Bellingshausen and Amundsen Seas in the Antarctic using underwater video. In the Arctic a total of 94 taxa represented by more than 100 000 individuals were identified. Echinoderms, particularly brittle stars, were the most important elements of the mega-epibenthic fauna on the shelf off Northeast Greenland. Multivariate analyses of the species distribution revealed a conspicuous depth zonation in which an assemblage on the shallow banks can be clearly distinguished from that in the troughs. Between these a transitional zone with a heterogeneous benthic fauna was found. Physical disturbances are supposed to be responsible for the pronounced dominance patterns observed on the shallow banks. The fauna in the troughs, which consists of more than 50% suspension feeders, is diverse but low in numbers of individuals. In the Antarctic more than 115 000 individuals belonging to 169 taxa were analyzed. Obvious faunal differences exist between the stations in the Weddell Sea and the Bellingshausen/Amundsen Seas. Assemblages of suspension feeders dominated by sponges and bryozoans are prevalent on the shelf of the eastern Weddell Sea, but almost absent in the Bellingshausen and Amundsen Seas. These assemblages seem to be restricted to areas where bottom currents provide favourable feeding conditions. However, motile deposit feeders are more abundant in both regions where there is a soft bottom substrate with presumably slow bottom currents and reduced horizontal transport of organic particles. Received: 22 January 1999 / Accepted: 7 June 1999