Central Gyre Research Articles

Seasonal abundance of the microplankton population in the North Pacific central gyre

Population structure as numerical and biomas abundance was determined for the microplankton (∼ to 200 μm organisms) sampled throughout and just below the euphotic zone at five times of year other than fall in the North Pacific central gyre around 28°N, 155°W. Average total microplankton carbon of all strata varied by twofold or less over the cruises, conforming to the hypothesis of low heterogeneity of biomass in central oceanic gyral populations. For example, mean euphotic zone abundance ranged from 5.9 μg Cl −1 (late winter) to 10.5 μg Cl −1 (spring), averaging 7.9 μg Cl −1. Its variability and that of stations within cruises were similar. Microplankton did not show a maximum correlated with the chlorophyll α maximum seen near the base of the euphotic zone when there was thermal stratification within the euphotic zone. Nanoplankters consistently dominated the population. ‘Monads and flagellates’ and non-thecate dinoflagellates were the most abundant groups. Diatoms in the upper waters where temperature range was the widest were the most temporally variable. Statistically significant differences in the biomass of taxonomic groups between cruises were found in relatively few of the possible instances tested.

Zooplanktonic assemblages in three areas of the North Pacific as revealed by continuous horizontal transects

Long serial sequences of macrozooplankton samples from the North Pacific Central Gyre, the western California Current, and the nearshore area off southern California indicated a strong intraspecific pattern, in that copepodite stages of each of two species in both the nearshore and California Current samples covaried in space. At the same time we found interspecific associations in all three areas. In neither case were the associations very strong at particular spatial scales, however. The areas differed in intensity of patchiness of the dominant species but not in characteristic spatial scales of variation, although characteristic scales of distance often differed between day and night within the Central Gyre. There was little indication of pattern in comparisons of the distributions of chlorophyll or of temperature with those of individual categories of zooplankton. When the abundances of the dominant zooplankters in each area were summed. however, this index of the zooplanktonic biomass was related to the distribution of chlorophyll at certain scales in two of the three areas.

On the distribution of copper, nickel, and cadmium in the surface waters of the North Atlantic and North Pacific Ocean

Concentrations of copper, nickel, and cadmium have been determined for about 250 surface water samples. Nonupwelling open‐ocean concentrations of these metals are Cu, 0.5–1.4 nmol/kg; Ni, 1–2 nmol/kg; and Cd, less than 10 pmol/kg. In the equatorial Pacific upwelling zone, concentrations of Ni (3 nmol/kg) and Cd (80 pmol/kg) are higher than in the open ocean, but Cu (0.9 nmol/kg) is not significantly enriched. Metal concentrations are higher in cool, nutrient‐rich eastern boundary currents: Cu, 1.5 nmol/kg; Ni, 3.5 nmol/kg, and Cd, 30–50 pmol/kg. Copper is distinctly higher in the coastal waters of the Gulf of Panama (3–4 nmol/kg) and also higher in the shelf waters north of the Gulf Stream (2.5 nmol/kg); these copper enrichments may be caused by copper remobilized from mildly reducing shelf sediments and maintained by a coastal nutrient trap. In the open ocean, events of high‐Cu water (1.5–3.5 nmol/kg) are seen on scales up to 60 km; presumably, these are due to the advection of coastal water into the oceanic interior. The lowest copper concentrations in the North Pacific central gyre (0.5 nmol/kg; (Bruland, 1980)) are lower than in the Sargasso Sea (1.3 nmol/kg), while for nickel the lowest concentrations are 2 nmol/kg in both the North Pacific and the North Atlantic. Nickel and cadmium, while generally correlated with the nutrients in surface waters, show distinct regional changes in their element‐nutrient correlations. The residual concentrations of trace metals in the surface waters of the ocean can be explained if biological discrimination against trace metals relative to phosphorus increases as productivity decreases.

Temperature and circulation changes in the eastern North Atlantic during the last 150,000 years: Evidence from the planktonic foraminiferal record

During the last 150,000 years the circulation of the subtropical gyre in the eastern North Atlantic has changed in response to glacial-interglacial variations in climate. Maps and time series constructed from planktonic foraminifera recovered from 18 piston cores show the nature of this response. Paleontological, chemical, and lithologic indices are used to establish a stratigraphy in the cores. Numerical analysis of the paleontological data yields estimates of the sea-surface temperatures (SST) and salinity. Maps of these estimates and of the interpreted circulation have been constructed for 18, 28, 73, 93, 125, and 150,000 years B.P. ( 18O Stages 2, 3, 4, 5-b, 5-e, 6). The analysis yielded six major results: 1. (1) In the southern Sargasso Sea, faunal abundances and SST have remained essentially constant for the last 150,000 years. 2. (2) In the northwestern part of the study area, between 30°N and 40°N, faunal variations were large but SST variations small (2–3°C) during the last 150,000 years. SST remained stable due to different ecological water types of similar temperature range exchanging dominance downcore. 3. (3) In the northern part of the study area, between 40–45N, SST remained warm in the west and decreased to glacial-level minima in the east between 120 and 80,000 years B.P. ( 18O substages 5-d to 5-a). The large west-east gradients may have been maintained by a meridional flow of the North Atlantic Current around the Azores Plateau. 4. (4) Along the Canary Current, there were generally monotonic decreases in SST during the last climatic cycle. 5. (5) The advance of ice that culminated in the Wisconsin maximum at 18,000 years B.P. was accompanied by rising SST in both the vicinity of the Azores and along the southern part of the Canary Current. 6. (6) Inferred increases in the velocities of the North Atlantic Current occurred at approximately the same time as all seven of the major polar advances of the last 150,000 years, and are associated with stable or rising temperatures. These SST patterns may have resulted from increased North Atlantic Current velocities advecting warm source waters more rapidly into the mid-latitudes. The increased velocities may have been due to a combination of reinforcing factors: increased wind stress, steepening of the thermocline due to increased cooling of 18°C water south of the Gulf Stream, and increased baroclinicity that resulted when the low-temperature North Atlantic polar front migrated southward toward the stable gyre center.

226Ra and 210Pb in the Weddell Sea

226Ra and 210Pb were measured in sections and profiles collected in the Weddell Sea during the International Weddell Sea Oceanographic Expedition in 1973. The results can be correlated with the circulation and mixing schemes deduced from hydrographic observations. Along the surface cyclonic gyre the Ra activities are fairly uniform at about 17 dpm/100 kg, quite similar to those of the Circumpolar surface water south of the Antarctic Convergence. The 210Pb activities in the northern flank of the gyre, probably influenced by the high 210Pb-bearing Circumpolar Deep Water in the north, are as high as 12 dpm/100 kg. At the central gyre and its southern flank, the surface water 210Pb activities are about 7 dpm/100 kg. The warmer surface water at the central gyre has a Ra activity of about 19 dpm/100 kg, slightly higher than the colder surface water at the flanks. Thus lower 210Pb/ 226Ra activity ratios are observed in the central gyre, and higher ratios in its flanks. Similar relationships between Ra and Pb are noted in the Weddell Sea Bottom Water (WSBW): lower Pb associated with higher Ra in the center; higher Pb with slightly lower Ra in the flanks. Vertical profiles along the cyclonic gyre show lower Ra and Pb activities in the southwestern Weddell Basin where lower temperature and lower silicate are observed. Similar to Ba, both Ra and Si are non-conservative in the Weddell Sea, with significant input from the bottom sediments and particulate dissolution during subsurface mixing. Each water mass or type in the Weddell Sea is well characterized by its Ra content, but not well by its Pb content. Ra and Si are crudely correlated with a slope of about 7 × 10 −4 dpm Ra per μmole of Si. The fact that the WSBW values fall on the slope suggests that the net input rate for Ra (corrected for the decay rate) is proportional to that of Si. The linear extrapolation to zero Si gives a Ra value of 13 dpm/100 kg. These relationships are quite similar to those observed in the Circumpolar waters.

Manganese in the North Pacific

A quantitative and precise method for determination of dissolved Mn at the nanomole(nmol)/kg level in seawater has been developed and used to study the distribution of Mn in the northeast Pacific. Mn concentrations in the surface mixed layer decrease from 1.0 to 0.6 nmol/kg between the central gyre and the western boundary of the California Current, then increase to values from 2 to 6 nmol/kg near the coastal boundary (in contrast to the distribution of 210Pb). Particulate Mn in the surface waters accounts for only about 1% of the total. Vertical distributions of Mn are characterized by surface maxima, minima near 300 m, maxima at mid-depth coinciding with the oxygen minimum and the labile nutrient maxima, and concentrations in Pacific bottom waters of approximately 0.2 nmol/kg. The oceanic distribution of Mn appears to be dominated by external inputs superimposed upon overall scavenging which can lead to Mn maxima in (1) the surface waters due to riverine and atmospheric sources; (2) the deep ocean as a result of hydrothermal injection and/or sediment resuspension; and (3) the oxygen minimum region resulting from in-situ breakdown of organic matter, in-situ MnO 2 reduction, and/or advective-diffusive transport of dissolved Mn from anoxic slope sediments.

Population structure and in situ division rates of Ceratium in oligotrophic waters of the North Pacific central gyre1

Species composition and division rates were examined for a population of the dinoflagellate genus Ceratium from oligotrophic waters of the North Pacific central gyre. Division rates of Ceratium were high relative to those in other oceanic regions despite the low nutrient levels. Doubling times were calculated to be 5.2–7.5 days for the genus and ranged from 4.0 to 9.2 days among the 10 most abundant species. Fifty‐one species of Ceratium were identified, and their rank order of abundance was remarkably constant over the largest temporal and spatial scales examined (9 months and 755 km). Ceratium's relatively high division rate coupled with its consistently low abundance supports the conclusion from other data that steady state conditions are approximated in this environment.

Oceanographic distributions of cadmium, zinc, nickel, and copper in the North Pacific

Vertical profiles of Cd, Zn, Ni, and Cu have been determined at three stations in the North Pacific and in the surface waters on a transect from Hawaii to Monterey, California. The distributions found are oceanographically consistent and provide a needed confirmation and extension of several recent studies on the marine geochemistries of these metals. Cadmium concentrations average 1.4 pmol/kg in surface waters of the central North Pacific and show a strong correlation with the labile nutrients, phosphate and nitrate, increasing to values of 1.1 nmol/kg at depths corresponding to the phosphate maximum. Zinc is depleted in surface waters of the central gyre to an average value of 0.07 nmol/kg and increases to a deep maximum of 9 nmol/kg exhibiting a strong correlation with the nutrient silicate. Nickel concentrations average 2.1 nmol/kg in surface central gyre waters and increase to a deep maximum of 11 nmol/kg. Nickel is best correlated with a combination of phosphate and silicate. Copper averages less than 0.5 nmol/kg in surface waters of the central North Pacific and increases gradually to values of 5 nmol/kg in bottom waters. The Cu profiles show evidence of intermediate and deep water scavenging. The involvement of these metals in the internal biogeochemical cycles of the sea is responsible for their distributions which are predictable on the basis of oceanographic parameters.

Fine structure measurements in a temperature‐compensated halocline

A study was made of the upper ocean region separating the winter water from the warm deep water of the northwestern Weddell Sea, using a series of 25 conductivity‐temperature‐depth casts. From 90 to 600 m the temperature and salinity increase by about 2°C and 0.2‰, respectively, while fine structure variations on both large and small scales are evident. The large‐scale variations appear to be primarily due to isopycnal mixing (horizontal interleaving) below about 450 m and primarily due to internal waves of 8‐to 10‐hour period above 300 m. There is also evidence of isopycnal mixing in the upper region, especially in the latter casts where inversions appear, and some evidence of internal waves in the lower region. The small‐scale structure, which shows steps in temperature, salinity, and density, is shown not to be wave induced. The steplike structures are most evident in the region of strongest vertical temperature and salinity gradients, and double‐diffusive effects appear to be important. Estimates of the upward heat and salt fluxes, obtained by comparison of the present data with previous laboratory experiments, are 7.65×10−5 cal cm−2 s−1 and 3.78×10−9 g cm−2 s−1, respectively. Additional measurements taken in the central Weddell Sea show larger step structures and an upward displacement of isotherms relative to the present data, supporting earlier conclusions of upwelling toward the gyre center.

The reconstruction of sea surface temperatures in the Pacific Ocean of 18,000 B.P.

All the major microfossil groups were used to reconstruct the summer and winter sea-surface temperatures of an ice-age Pacific Ocean. The use of these four groups was necessary because of the varying degrees of preservation of siliceous and carbonate-rich sediments in the Pacific. Their use also permits comparisons of temperature estimates for samples in which more than one group is preserved. The standard error of estimate for the transfer function equations used in this study average about ± 1.5° C for the summer temperature estimates, and about ± 1.9° C for winter estimates. Laboratory (counting) errors result in an average error of estimate of about 0.6° C. Most of the individual estimates using different equations on the same samples agree within their pooled standard error. The reconstructions of ice-age temperature patterns show cooling in the subarctic region by about 4°C in both August and February. The equatorial region is 2–4° C cooler only in the winter season (August). Seasonality (August minus February temperatures) is stronger in the western Subarctic and Transition Zones at 18,000 B.P. than it is at present. Such changes in seasonality result primarily from increased winter cooling and equatorward shifts in the frontal zones. Temperatures within the centers of the subtropical gyres at 18,000 B.P. are generally as warm as, or warmer than, modern sea-surface temperatures. In particular, the Southern Hemisphere shows little or no cooling in tropical and subtropical latitudes except along the equator and in the eastern boundary current. The distribution of biotic assemblages and the derived temperature patterns indicate an intensified circulation at 18,000 B.P. Temperature gradients are steeper in the boundary currents, and divergence at the equator is increased. The “spin-up” of the subtropical gyres associated with this intensified flow appear to contain the warm tropical waters within the gyre centers, rather than allowing their dispersal to subpolar regions. These relatively warm gyre centers may act as loci for low pressure systems in the atmosphere which draw moisture away from the land masses.

Patterns of spatial and species abundance within the larval fish assemblage of the North Pacific central gyre during late summer

The larval fish assemblage in the epipelagic zone of the North Pacific central gyre near Lat. 28°N, Long. 155°W during late summer was sampled with stratified opening/closing bongo nets. This assemblage exhibited recurring patterns of spatial and species distributions. Spatial patterns included species vertical distributions, co-occurrence and pathchiness; species patterns included species composition and abundance relations. Both types of patterns changed with depth, the most dramatic change occurring in relation to the bottom of the summer mixed layer. An examination of the physical and biological environments of the epipelagic zone with relation to the summertime species and spatial patterns and to seasonal changes in larval fish species structure indicates that the thermal structure of the upper water column may exert a major influence on overall ichtyoplanton species and spatial structures within the gyre.

Coccolithophorids of the North Pacific Central Gyre with Notes on Their Vertical and Seasonal Distribution

Scanning electron micrographs supplement earlier observations on the living coccolithophorids of the North Pacific Central Gyre, and suggest that new forms may be present among the 44 taxa described. Vertical and seasonal distribution data obtained from 6 cruises during 1972-1974 confirm the existence of depth-dependent communities, particularly in the summer months. The populations become less structured in the winter months when some taxa, notably Emiliania huxleyi, expand their depth range, probably responding to a greater degree of mixing in the water mass. Coccolithophorids of the North Pacific Central Gyre with notes on their vertical and seasonal distribution INTRODUCTION In the course of an inverted microscope study of the total range of microplankton in a 400-sq. mile area of the North Pacific Central Gyre in the region of 28?N, 155?W, the coccolithophorid data proved to be most useful in testing the hypothesis of seasonal stability of populations. It was therefore important to identify as many forms as possible, and the SEM (scanning electron microscope) was used to supplement light microscope analysis. The following illustrations and descriptions resulted from these studies. Illustrations are introduced which further clarify existing taxa and new forms are described, although too few specimens were encountered to allow formal establishment of new taxa. The use of the TEM (transmission electron microscope) for morphological and taxonomic work in this group began in the 1 950's, resulting in papers such as those by Deflandre and Fert (1954) and Halldal and Markali (1955), and has continued to the present with major contributions from Gaarder (1962), Mcintyre and B6 (1967), Kamptner (1967), Gaarder (1970), Gaarder and Hasle (1971) and Okada and Mcintyre (1977). The last 3 include some SEM micrographs, but the publications based mainly on SEM work are those of Martini and Muller (1972), Heimdal (1973), Borsetti and Cati (1972, 1976), Kling (1975) and Gaarder and Heimdal (1977). Earlier, coccolithophorid taxonomists depended on the light microscope and made surprisingly accurate observations, e.g. Lohmann (1902, 1912, 1913b), Schiller (1930), Kamptner (1927, 1941), and Lecal-Schlauder (1951). The taxa illustrated and described herein were selected from a total of 53 named species and a group of unidentified forms found in the light microscope analysis of the gyre samples. Some previously welldescribed species were not included. Table 1 summarizes seasonal and depth distribution data for selected species derived from this analysis but does not include all the forms illustrated here, some of which were not recognized at lower magnifications. For further discussion of the total microplankton of the North Pacific Central Gyre refer to papers presently being prepared by Beers, Reid and E. Stewart and by Beers, Reid and G. Stewart. Data records for 5 cruises are published as IMR Report 75-6 (Beers, Reid and Stewart, 1975) and 77-1 (Beers, Reid and Stewart, 1977). Cato 1 data were incomplete due to sample loss. These records have quantitative data on all microplankton taxa identified, including many of the coccolithophorids described in this paper and notes on name changes that have been made during the years spent on this study. micropaleontology, vol 26, no. 2, pp. 151-176, pls. 1 -9, 1980 151 This content downloaded from 207.46.13.151 on Fri, 25 Mar 2016 07:39:58 UTC All use subject to http://about.jstor.org/terms F. M. H. Reid: Coccolithophorids of the North Pacific Central Gyre TABLE 1 Vertical and seasonal distribution of selected species based on numbers averaged over several stations for each cruise. Cruises arranged seasonally rather than chronologically. Intermediate group Lower layer group

Factor Analyses of a Hyperiid Amphipod Assemblage from the North Pacific Central Gyre

Factor Analyses of a Hyperiid Amphipod Assemblage from the North Pacific Central Gyre

Spatial and temporal feeding patterns of copepods from the North Pacific centra gyre

Spatial and temporal feeding patterns (determined from an index of gut fullness) are described for 10 typical species of calanoid copepods collected from the North Pacific central gyre (September 1968 to June 1977), an area where the zooplankton is food limited and there were a-priori reasons to suspect that feeding and competition for food were important in regulating zooplankton community structure. Over 100 samples from 11 cruises to the eastern part of the gyre were examined, and patterns of gut fullness were related to environmental variables and the copepod species structure. The copepods studied all tended to be omnivores and food generalists. Males had lower indices of gut fullness than females but both males and females of a species had similar spatial and temporal feeding patterns. Guts were usually fuller at night than during the day, even in nonmigrating species; however, within nighttime depth distributions, no depths were preferred for feeding. There were also differences between species in mean gut fullness, but different species tended to have similar spatial and temporal feeding patterns. There was considerable spatial variability, and locales could be identified in which most species had higher indices of gut fullness. The copepods were not necessarily more abundant in these locales, nor did these tend to be areas of above average chlorophyll concentration. These patterns were consistent with relatively nonselective feeding, and there was no evidence that these species separate their niches by feeding at differing places or times.

Pattern and Structure in an Oceanic Zooplankton Community

An analysis of pattern can reveal a great deal about the processes that regulate the species structure of a community, especially in those cases where experiments are not possible. It is predicted that differing mechanisms of regulation should result in different patterns in the distribution and abundance of potentially competing species and in the ways in which they utilize limiting resources. In this paper we describe these patterns in the zooplankton community of the North Pacific central gyre. The gyre community is old, stable and diverse and past studies have suggested that it is regulated by in situ processes. An analysis of samples collected over many years shows that the species are close to equilibrium. There are not large spatial, seasonal or year to year changes in abundance and disturbances do not seem to be large enough to perturb the species structure and prevent competitive exclusion. Food and predation are the two main limiting factors. Competition, by inference, appears to be strong, even between relatively dissimilar species. In spite of this the species do not seem to be specialists and niche separation is not apparent in either space or food utilization. This is surprising considering the age, stability and high diversity of the system. It is concluded that either some other mechanism, such as frequency dependent predation, regulates the species structure or that the mechanisms of niche separation are much more subtle than is suspected. Some of the implications of regulation through predation are discussed.

Larval fishes in the zooplankton community of the North Pacific Central Gyre

Patterns of species structure are described for the larval fish assemblage in the North Pacific Central Gyre. About 30,000 larvae, primarily of mesopelagic fish species, were identified for 7 cruises. Samples were collected using Isaacs-Kidd plankton trawls on 6 cruises over a 4 1/2 year period; stratified samples were taken with an opening/closing bongo net on one late-summer cruise. Data on absolute and relative abundances, size ranges and summertime depth distributions of over 150 species of larvae are presented. The depth distribution and abundance of the ichthyoplankton are compared to those of the total zooplankton community. The larval assemblage is also examined with respect to the known adult mesopelagic fish assemblage. Seasonal changes in ichthyoplankton species structure occur but, within seasons, species structure is remarkably constant from year to year. Despite seasonal changes in species abundance and rank order of abundance, a constant cumulative frequency structure was found to exist in the ichthyoplankton, both between seasons and between years. The implications of this result are discussed in light of similar findings in a terrestrial community and with respect to possible mechanisms of regulation.

Structure in the Copepod Community of the North Pacific Central Gyre

The North Pacific central gyre is a large, monotonous, and geologically old system. It (and other such gyres) is probably the nearest oceanic equivalent to an essentially self—regulating, climax ecosystem. We have taken a series of replicated, vertically stratified net tows for macrozooplankton and have replicated measures of a number of habitat variables. Copepods are the most numerous members of the macrozooplankton fraction of the community of the central gyre. It is likely that they are also the largest part of the biomass. There are at least 125 species regularly present and although we do not know what many of them eat, certain species can almost certainly be identified as herbivores or carnivores, and others as omnivores. We show that some species are significantly more frequent parts of each other's biotic environment than other species. These recurrent groups of species tend to occupy different depth zones. The members of the groups show strong concordance of abundance at the depth of high group coherence. Thus there is a dividing up of the water column and a clear tendency for vertical spatial structure in the copepod fauna. Within groups there is a significant constancy of dominance at the group's preferred depth over time. Thus there appears to be a large amount of species structure as well as spatial structure. A highly regulated species equilibrium is implied. There are congeners occurring within the same recurrent groups. The physical and biotic environment of each group differs with respect to both concentration (e.g., chlorophyll α) or magnitude (e.g., temperature) of habitat variables and with respect to the spatial variability of these. Our most diverse group tends to occupy a depth zone ranking high in primary productivity but does not have the greatest total environmental richness or greatest or least heterogeneity, and this seems contrary to what some current community theory predicts. Our evidence indicates that within groups many similar species coexist spatially and temporally. We speculate about the relative roles of competition and predation influencing the structure we see. Aspects of modern community theory, based on competitive equilibrium, seem inadequate to explain our results. Predator regulation of structure seems a more likely explanation. However, existing information indicates a lack of sufficient specialization of what we believe to be the main predators on copepods to account for the observed constancy of copepod species structure. We suggest that a more intensive study of the role of predation is in order.

OSCILLATORIA (TRICHODESMIUM) THIEBAUTII (CYANOPHYTA) IN THE CENTRAL NORTH ATLANTIC OCEAN12

ABSTRACTPhysiological rate measurements were made with Oscillatoria thiebautii (Gom.) Geitler in the subtropical north Atlantic Ocean between Spain and Bermuda during May and June of 1975. The near surface C:N fixation ratios averaged 6.5, and the cellular composition ratio was 6.2, suggesting that N2 fixation is the major path of nitrogenous nutrition for this alga. Compared to other oceanic phytoplankters, it has a low affinity for orthophosphate at oceanic concentrations (ks= 9.0); however, it has a high potential for utilizing phosphomonoesters (170–300 ng atoms P ·μg chl a−1· h−1). Maximal photosynthesis occurred at 450–700 μ Einstein · m−2· s−1, and was inhibited by full sunlight. Calculated cell division rates (ca. 180 days) suggest that relative to other phytoplankters in this oceanic region, O. thiebautii must be subjected to negligible grazing pressure. No major differences in C, N, chl a or ATP were observed between the tuft (fusiform) and puff (spherical) colonies. ATP concentrations relative to other cellular constituents varied greatly between colonies, suggesting a general inter‐colony physiological variability in the open Atlantic. With increasing depth in the euphotic zone, there was no evidence for chromatic adaption. The observations that O. thiebautii represents only a small fraction of total phytoplankton biomass and that its growth rate is 10–100 times slower than that of the other indigenous phytoplankton, strongly suggest that N2 fixation by this alga is a virtually insignificant component of the nitrogenous nutrition for the phytoplankton of the North Atlantic central gyre in late Spring.

Distributional pattern and niche separation among North Pacific hyperiid amphipods

This paper describes aspects of pattern within the hyperiid amphipod fauna of the North Pacific central gyre. Strong patterns were found in interspecies associations, spatial distribution, and relative abundances. Pattern is described using recurrent group analysis, which separated 20 species (totaling 91% of captured individuals) into six groups. Members of a group co-occur in samples much more frequently than can be attributed to random co-ocurrences. Each group contains from two to seven species. The groups themselves are not strongly interconnected and seem to display different depth distributions and vertical migratory behaviors. Some recurrent groups contain sets of morphologically similar congeneric species. This result was unexpected; because the environment is physically unstructured and oligotrophic, similar species could well be competitors and therefore were not expected to co-occur on fine spatial scales. The data on three sets of congeners (two sets grouped together and one pair not so) were therefore examined more closely. Several niche or habitat axes have been identified for various hyperiid species, and congeneric species grouped together can be separated along some such axis. These differences among congeners may be significant in permitting their coexistence. However, of the 83 hyperiid species captured, only 20 are considered here; the remaining 63 (totalling 9% of captured individuals) failed to group.

Studies on Atlantic halocyprid ostracods: their vertical distributions and community structure in the central gyre region along latitude 30°N from off Africa to Bermuda

The vertical ranges and abundances of halocyprid ostracods in the surface 2000m of the water column are compared for two localities in the North Atlantic in the vicinity of 30°N, 23°W and 32°N, 63° 40′W (Ocean Acre). The samples collected at these two localities, positioned near the centre of the North Atlantic gyre, contained very similar assemblages of species. The vertical stratification of the species was similar between the two localities, although centres of abundance tended to be 100–200m deeper at Ocean Acre. Vertical migrations tended to be more extensive at Ocean Acre. The order of species abundances were very different at the two localities. A transect of 0–1000m oblique tows along 32°N linking the two positions showed no clear evidence of a faunal boundary between them despite the sudden occurrence of the 18°C water and the waning of the influence of Mediterranean Water to the west. The changes were clinal. Analysis of size spectra showed the trend towards more larger organisms with increasing depth was reversed below 800m. The results are discussed in relation to visual predation as a potentially important causal factor in effecting vertical distribution and diel migrations. In examining other theories of diel migration an examination of clutch sizes showed that the results were not consistent with diel migration conferring demographic advantage to the migrating species. Instead it seemed that the most favourable reproductive regime varied with depth and the degree of seasonality in the productivity cycle. The zoogeographic connotations of the results are discussed.