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.     

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.     

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.     

Physiology and chemical composition of nitrogen-fixing phytoplankton in the central North Pacific Ocean

The magnitude and physiological characteristics of biological nitrogen fixation have been studied in the oligotrophic waters of the North pacific gyre. The filamentous blue-green algae Trichodesmium spp. and Richelia intracellularis were the important nitrogen-fixing phytoplankton. Most of the nitrogen fixation occurs in the upper 40 m of the water column, with detectable fixation as deep as 90 m, which corresponds to about the 1 % light depth. There was no evidence of photoinhibition of nitrogen fixation, although CO₂ reduction was depressed slightly at the highest light levels. The rate of nitrogen fixation in the water column varied throughout the day, being highest in mid-morning and in late afternoon. Relatively high fixation rates were also found during periods of darkness. Elevated oxygen concentrations had a marked inhibitory effect on rates of nitrogen fixation, a pO₂ of 0.4 atm causing a 75% inhibition. Data from studies of nitrogen fixation and assimilation rates of ¹⁵N-labelled nitrate, ammonium, and urea indicate that nitrogen fixation furnished about 3% of the total daily fixed nitrogen requirement for phytoplankton growth. Studies with isolated colonies of Trichodesmium spp. indicated that 100% of their nitrogen requirement was met by nitrogen fixation. Chemical composition of the Trichodesmium colonies showed that the C:N ratio was 4.1 and that their phosphorus content relative to carbon or nitrogen was much lower than that of the total particulate material in the water column. Elevated ratios of carbon: adenosine triphosphate (ATP) also suggest that phosphorus deficiency may be limiting the growth of Trichodesmium. The magnitude of nitrogen fixation in the gyre is seasonally dependent, with high rates in late summer and autumn. At these times the water column is stratified, with phosphate and nitrate barely detectable in the upper 100 m. Our data suggest that during these months of stratification, biological fixation of nitrogen amounts to about 33 g-at N/m²/day.     

Weight and chemical composition of some important oceanic zooplankton in the North Pacific Ocean

Wet and dry weight, total carbon, nitrogen, hydrogen, and ash contents were determined on 33 species of zooplankton distributed predominantly in the open sea region of the North Pacific. Sampling covered the waters from 44°N to the equator. Average percentage of dry weight to wet weight was about 19% of all samples from the whole area. Percentage dry weight of carbon in copepods was on an average 51.5%. The highest value, 66.6%, was obtained in eggs of the copepod Pareuchaeta sarsi. Mixed zooplankton was assumed to contain carbon comprising about 35 to 45% of the dry weight. Carbon contained in the zooplankton biomass existing in the upper 200 m in the western parts of the northern North Pacific and Bering Sea during spring and summer was estimated to range from 20 to 85 mg C/m³. Nitrogen content varied considerably with localities. Average ratio of carbon to nitrogen was 8.5 in subarctic copepods, and 4.1 in subtropic-tropic copepods. This ratio also varied with season. In the copepod Calanus cristatus the ratio was highest (10.0) in May, immediately after the spring bloom of phytoplankton, when the animals contained much fat. The ratio fell to 5.1 in December. There seemed to be a large seasonal variation in boreal zooplankton due to great fluctuations of environmental conditions, especially the amount of food available; in tropical species the range was small because of environmental uniformity. Average hydrogen content was about 6 to 10%. The percentage of ash to dry weight amounted to 39.3% in pteropods and 3.4% in copepods.     

Population structure of the planktonic copepod Calanus pacificus in the North Pacific Ocean

The pelagic copepod Calanus pacificus ranges nearly continuously across temperate-boreal regions of the North Pacific Ocean and is currently divided into three subspecies—C. pacificus oceanicus, C. pacificus californicus, C. pacificus pacificus—based on subtle morphological differences and geographic location. The relation between geography and genetic differentiation was examined for 398 C. pacificus individuals sampled from six widely distributed locations across the North Pacific, including an open ocean site and coastal sites on both sides of the North Pacific basin. For each individual copepod, the DNA sequence was determined for a 421-bp region of the mitochondrial coxI gene (mtCOI). A total of sixty-three different mtCOI sequences, or haplotypes, were detected, with a sequence divergence between haplotypes of 0.2–3.1%. The number and distribution of haplotypes varied with sampling location; 12 haplotypes were distributed across multiple sampling locations, and 51 occurred at only one location. Five genetically distinct populations were detected based on F _ST values. Haplotype minimum spanning networks, nucleotide divergence and F _ST values indicated that individuals from coastal sites in the North Pacific Ocean were more closely related to each other than to individuals from the open ocean site at Station P. These results provide genetic support for the designation of two subspecies—a coastal subspecies that consists of what is currently referred to as C. p. pacificus and C. p. californicus and an open ocean subspecies C. p. oceanicus. This work also indicates that planktonic copepods with potentially high dispersal capacity can develop genetically structured populations in the absence of obvious geographic barriers between proximate locales within an ocean basin.     

Temperature and metabolic rate in sedentary fish from the Antarctic,North Sea and Indo-West Pacific Ocean

Resting metabolic rate was measured in demersal stages of the teleostNotothenia neglecta Nybelin from the South Orkney Islands, Antarctica, from 1985 to 1987. The relationship between and body mass (Mb) conformed to the general relationship , wherea is a proportionality constant andb is the scaling exponent. (mg O₂ h^–1) was found to scale toMb ^{(0.82±0.011)} in the summer (November to April, 1.6 to 1 850 g,n=56) and toMb ^{(0.76±0.013)} in the winter (May to October, 0.9 to 1 850 g,n=57) (values ofb are means ± SD). Although the scaling exponents were significantly different (P<0.01), was similar in the juvenile stages of summer- and winter-caught fish matched for body mass. The effects of activity on oxygen consumption was studied using a Brett respirometer. Adult stages had a factorial aerobic scope for activity of 5.7, which is similar to that reported for demersal fish from temperate latitudes. The effects of temperature on resting metabolism was investigated in fish with similar sedentary lifestyles from the North Sea (Agonus cataphractus andMyoxocephalus scorpius) and the Indo-West Pacific (Paracirrhites forsteri, P. arcatus, Neocirrhites armatus andExallias brevis). Extrapolated values of for the tropical species approached zero at 5 to 10°C. For a standard 50 g fish, for the tropical species at 25°C was in the range 3.4 to 4.4 mg O₂ h^–1, compared with 1.3 mg O₂ h^–1 forNotothenia neglecta at its acclimation temperature. Thus, the maximum metabolic rate of sedentary tropical species at 24°C is likely to be 2 to 4 times higher than inN. neglecta at 0°C. This suggests that the energy available for sustained activity is significantly lower in cold- than in warm-water fish.     

Nitrogen assimilation by phytoplankton and other microorganisms in the surface waters of the central North Pacific Ocean

Assimilation rates of ¹⁵N-labelled ammonium, urea, and nitrate by plankton in the upper euphotic zone were measured in 2 summer, 2 winter, and 1 spring cruise in the central North Pacific Ocean. Average rates of ammonium plus urea assimilation could not be determined precisely, but were estimated to be 7 to 25 g-at. N m^-3 day^-1. Indirect evidence suggested that non-photosynthetic microorganisms contributed to these rates. Nitrate assimilation was negligible in the upper waters considered in this report (above the chlorophyll maximum and the nutricline). Potential, nitrate-saturated rates were in the range 1 to 8 g-at. N m^-3 day^-1. Seasonal comparison showed lowest rates of both carbon and nitrogen assimilation rates per chlorophyll a in winter.     

Strontium/Calcium ratios in statoliths of the neon flying squid, Ommastrephes bartrami (Cephalopoda), in the North Pacific Ocean

Strontium to calcium ratios were observed along longitudinal sections of statoliths of nine neon flying squid, Ommastrephes bartrami (LeSueur, 1821), including three mature females (422 to 454 mm mantle length, ML; 207 to 306 d old) obtained from the North Pacific (27–35°N; 144–150°E) during winter and six immature males and females (187 to 226 mm ML; 126 to 164 d old) collected from 39°N; 145°E and 39°N; 169°W during summer. The distances between the nucleus (core) and the edge of the dorsal dome were approximately 660 to 690 μm in mature females and 450 to 510 μm in the immature squid. Sr/Ca ratios were determined at intervals of 30 μm between the nucleus and edge of the dorsal dome. Sr/Ca ratios were higher in areas near the nuclei and peripheral portions of the dorsal dome than in the middle portions of the statoliths (270 to 420 μm from the nuclei, corresponding to ages of 60 to 90 d) in mature females; thus a U-shaped pattern was evident. Sr/Ca ratios in the six immature squid decreased from nucleus to the dorsal dome; in three squid the ratios slightly increased toward the dorsal dome edge. The observed Sr/Ca ratios in immature squid were considered to represent younger portions of the U-shaped pattern. In the present study we discuss this pattern in relation to environmental and biological conditions of O. bartrami, which undertakes seasonal migrations between spawning grounds in the Subtropical Domain and feeding grounds in the Subarctic Domain and Transitional Zone in the North Pacific Ocean. Although Sr/Ca ratios are potentially affected by ambient water temperature and ontogenetic conditions, including somatic growth and statolith growth, it was impossible to evaluate each environmental and biological effect separately, as variations in these factors are complicated and effects could be interdependent. Received: 11 April 1997 / Accepted: 27 December 1997     

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.     

Nematode community composition in hydrothermal vent and adjacent non-vent fields around Myojin Knoll,a seamount on the Izu-Ogasawara Arc in the western North Pacific Ocean

In contrast to specific large benthic invertebrates in chemosynthetic ecosystems such as hydrothermal vents, meiofaunal communities in such habitats have been reported to have strong taxonomic overlap with meiofauna in the adjacent “normal” environments. However, meiofauna have only recently been included in studies of those environments and detailed information on these communities is still rare. This is especially true in the Northwest Pacific Ocean, even though there are many seamounts with active vents in the calderas of the region. Nematode community composition at the genus level in sediments from a hydrothermal vent field in the caldera of Myojin Knoll (32°06′N, 139°52′E, depth 1,300 m), a seamount on the Izu-Ogasawara Arc, Japan, was investigated for the first time and was compared with adjacent non-vent areas inside and outside the caldera. Multivariate analyses showed that the composition of nematodes in the hydrothermal field was significantly different from that in the non-hydrothermal fields around the caldera. However, the common genera, such as Oxystomina, Pareudesmoscolex, Desmoscolex, and Microlaimus were found in two, or all three vent fields while their rank contributions differed among the three fields. When the data from Myojin Knoll were compared with those from other deep-sea vent environments in different regions (e.g., North Fiji Basin, East Pacific Rise, Mid-Atlantic Ridge), the nematode composition in the vent field of the Myojin caldera was more similar to that of the non-vent fields around the caldera than the composition in vent fields of other regions. These data from the Northwest Pacific Ocean also suggest the absence of long-range transport systems and local adaptations for meiofauna in hydrothermal vent fields.     

Oceanographic influences on the dive behavior of juvenile loggerhead turtles (Caretta caretta) in the North Pacific Ocean

Satellite telemetry data from 17 juvenile loggerhead turtles (43.5–66.5 cm straight carapace length) were used in conjunction with oceanographic data to analyze the influence of regional and seasonal oceanography on dive behavior in the North Pacific Ocean. Combined dive behavior for all individuals showed that turtles spent more than 80% of their time at depths <5 m, and more than 90% of their time at depths <15 m. Multivariate classifications of dive data revealed four major dive types, three representing deeper, longer dives, and one representing shallower dives shorter in duration. Turtles exhibited variability in these dive types across oceanographic regions, with deeper, longer dives in the Hawaii longline swordfish fishing grounds during the first quarter of the year, as well as in the Kuroshio Extension Bifurcation Region and the region near the Baja California Peninsula, Mexico. Turtles in the Kuroshio Extension Bifurcation Region also exhibited dive variability associated with mesoscale eddy features, with turtles making deeper, longer dives while associated with the strongest total kinetic energy. Turtles in the central North Pacific exhibited seasonality in dive behavior that appeared to reflect synchronous latitudinal movements with the North Pacific Subtropical Front and the associated seasonal, large-scale oceanography. Turtles made deeper, longer dives during the first quarter of the year within this region, the reported time and area where the highest loggerhead bycatch occurs by the longline fishery. These results represent the first comprehensive study of dive data for this species in this region. The increased understanding of juvenile loggerhead dive behavior and the influences of oceanography on dive variability should provide further insight into why interactions with longline fisheries occur and suggest methods for reducing the bycatch of this threatened species.     

Shifts in a Pacific Ocean Fish Assemblage: the Potential Influence of Exploitation

Abstract:  As in many regions of the world, marine fishes and invertebrates along the Pacific coast of the United States have long been subjected to overexploitation. Despite this history, however, we lack basic information on the current status of many fishes along this coastline. We used data from a quarter century of fishery-independent, coast-wide trawl surveys to study systematically the demersal fish assemblages along the U.S. Pacific coast. We documented fundamental shifts in this fish assemblage. Average fish size, across a diversity of species, has declined 45% in 21 years. There have been major shifts in the constituent species of the assemblage, with some species achieving annual population growth rates of >10% and others declining in excess of 10% per year. Annual rate of change in population size appeared to be a function of life history interacting with fishing pressure. Negative trends in population size were particularly apparent in rockfish ( Sebastes spp.). However, across all taxa examined, trends in population size were associated with size of maturity, maximum size, and growth rate. Trends in population size were associated inversely with harvest levels, but stocks that mature late tended to decline faster than would be predicted by catch rates alone. Our results are disquieting because they raise the possibility that fishing-induced phase shifts in fish communities may affect the recovery of fishes, even after the implementation of severe fishing restrictions.     

Bimodal vertical distribution and diel migration of the copepods Metridia pacifica,M. okhotensis and Pleuromamma scutullata in the western North Pacific Ocean

Diel changes in fine-scale vertical distributions of three calanoid copepods Metridia pacifica, M. okhotensis and Pleuromamma scutullata in the subarctic waters of the western North Pacific were examined. Sampling was carried out in June and August 1983, at two stations in Oyashio water using a Longhurst-Hardy Plankton Recorder (LHPR). Sampling, down to about 1 000 m, was repeated four to five times at intervals of several hours. Vertical resolution was 5 to 40 m. Copepods were concentrated in two strata, the surface (0 to 60 m) and the mesopelagic (200 to 300 m) layers, throughout the day at both stations. Younger M. pacifica (C III and C IV) were dominant in both strata. Although most female C V and adult females demonstrated diel vertical migration at 20 to 30 m h^-1, a significant number of females did not migrate upward but remained in the deep stratum at night. The same trend was evident in M. ohkotensis and P. scutullata. Foregut content observations indicated that feeding activities of the deep mode populations were as high as those of the surface mode, though food of deep individuals was different. Such a bimodal distribution may increase intraspecific diversity of copepod populations and is possibly why metridiid copepods dominate during late summer to winter in the relatively simple ecosystems of high latitudes.     

Vertical distribution, life cycle, and developmental characteristics of the mesopelagic calanoid copepod Gaidius variabilis (Aetideidae) in the Oyashio region, western North Pacific Ocean

Vertical distribution, life cycle, and developmental characteristics of the mesopelagic copepod Gaidius variabilis Brodsky in the Oyashio region were investigated by combining analyses of field copepodite populations with laboratory-rearing data of egg hatching and naupliar development. Field samplings from five discrete depths between the surface and ≤2000 m were made approximately every month for 1 year. Most populations of G. variabilis occurred between 600 and 1000 m depth. A modest degree of reversed diel vertical migration behavior and some stage-specific depth-distribution patterns were noted. All copepodite stages were observed throughout the year, suggesting a year-round spawning of G. variabilis. From a prominent abundance peak of Copepodite Stage 1 (C1) seen in June to August, together with development times of eggs and nauplii obtained in laboratory-rearing experiments, the major spawning season was extrapolated to be April to June, the phytoplankton bloom season. Tracing the peak abundance of each copepodite stage (distinguishing males and females for C4 to C6), the generation times of males and females were deduced as 2 and 1 year, respectively. All between-stage increments in terms of wet-, dry-, and ash-free dry weights were greatest in C3/C4, and least in C5/C6 for both males and females. The increments in C3/C4 and C4/C5 were greater for males than for females, reflecting a longer stage duration of the males. These weights did not increase in C5/C6 males, possibly because feeding ceased in C6 males. These results for G. variabilis are compared with those for some mesopelagic copepods previously reported from other regions. Received: 25 October 1999 / Accepted: 20 March 2000     

Electrophoretic and biometric variability in the abyssal grenadier Coryphaenoides armatus of the western North Atlantic,eastern South Pacific and eastern North Pacific Oceans

Specimens of the abyssal grenadier Coryphaenoides armatus (Hector, 1875), from the western North Atlantic and eastern North Pacific Oceans were compared electrophoretically at 27 presumptive gene loci. At 6 of the 7 polymorphic loci there were only minor differences in allelic frequencies but a nearly fixed difference was found at one locus, phosphogluconate dehydrogenase. Eastern North Pacific grenadiers typically have a narrower interorbital space, a shorter dorsal interspace, more soft rays in the 1st dorsal fin (9–10 versus 8–9) and more pelvic fin rays (21–23 versus 18–21) than grenadiers from the western North Atlantic (as well as grenadiers from the eastern South Pacific, which were included in the biometric analysis). There is an apparent disjunction in the distribution of C. armatus in the eastern Pacific at the Gulf of Panamá which coincides with the change of morphology. It is suggested that North Pacific grenadiers comprise a subspecies, C. armatus variabilis Günther, 1878, which is morphologically distinct from the subspecies C. armatus armatus (Hector, 1875) of the other areas.     

Sea-bird aggregation at a deep North Pacific seamount

During June 1991, we studied sea birds at a mid-ocean seamount (Fieberling Guyot) in the eastern North Pacific Ocean. Avifaunal composition changed from small Procellariiformes [a storm-petrel; Oceanodroma leucorhoa (Vieillot)] away from the seamount to an assemblage dominated by larger tubenoses [mostly black-footed albatross Diomedea nigripes Audubon and Cook's petrel Pterodroma cookii (Gray)]. Compared to adjacent waters, sea-bird density and biomass within a 30-km radius centered on the seamount summit were 2.4 and 8 times higher, respectively. Individual sea-bird taxa were 2 to 40 times more abundant at the seamount relative to values reported previously from large-scale surveys of deep-ocean regions in the central North Pacific. In September 1991 we studied potential prey of sea birds in the upper water column using a neuston net and multiple opening-closing net system (MOCNESS) tows. Most potential prey types in the neuston exhibited no significant enhancement over the seamount. MOCNESS samples at 10 m depth, however, showed several prey types to be more abundant over the seamount, and the dominant size class of fish was slightly larger. We attribute the sea-bird aggregation observed at this seamount to changes in the abundance and/or behavior of pelagic organisms in the deep scattering layer (not adequately sampled in this study), perhaps augmented by migrations of seamount residents into the surface layers. Processes on and in the vicinity of seamounts may provide spatially-predictable prey to wide-ranging aerial sea birds foraging in this relatively austere environment.