Charles B. Miller

Life histories of large, grazing copepods in a subarctic ocean gyre: Neocalanus plumchrus, Neocalanus cristatus, and Eucalanus bungii in the Northeast Pacific

Life histories for the dominant, larger copepods of the subartic Pacific have been constructed by sampling from weatherships patrolling Ocean Station P (50°N, 145°W) during 1980 and 1981. Neocalanus plumchrus reproduced at depths below 250 m from July through February. Copepodite stages were present in surface layers from October through August with a large peak in numbers and biomass in spring. Fifth copepodites prepared for diapuse in 38 days during spring and descended to depths below 250 m. They commenced immediately to mature, and the females reproduced without renewed feeding. This schedule contrasts with that of the population in the Strait of Georgia, which remains in diapause from July to January and matures exclusively in January and February. There appears to be a difference between the coastal and oceanic habitats in preparing the diapausing individuals for maturation. Maturation of the diapausing stock of N. plumchrus maintained constant adult populations, averaging 714 males m−2 from June through October and 1,434 females m−2 from August through January. This constancy, together with the exponential pattern of decline in the diapause stock from September through February, suggests that density of adults may regulate maturation of fifth copepodites. Offspring of individuals delaying maturation and, thus, reproduction would benefit from the resulting moderation of intraspecific competition, probably that among copepodites. Reproduction of Neocalanus cristatus also occurred below 250 m, and, while spawning was continuous through the year, there was a substantial peak in November. That resulted in a peak of abundance for early copepodite stages in mid-winter, and a peak for the fifth copepodite stage in June. Stocking of the population of fifth copepodites in diapause below 250 m occurred from July through October. Some fifth copepodites were present in surface layers through the entire summer, and some younger copepodites persisted through the summer in progressively declining abundance just below the mixed layer. In autumn 1980 resurgence of early copepodite populations was rapid, occurring during the course of a prolonged October storm. The storm may have improved the habitat either by cooling the mixed layer or by resupplying nutrients to the euphotic zone. Eucalanus bungii reproduced in the mixed layer in early May and in early July. The first event was a spawning by females that had previously spawned in 1979 and then had returned to diapause. The second, heavier spawning (more females, more eggs per female) was by newly matured females from stocks that had overwintered as fifth copepodites. Nauplii peaked sharply in abundance on 19 July, one week after the peak in spawning. First and second copepodites peaked on 1 August, and all had advanced to the third copepodite stage by September. The diapause stock was established by September, principally between 250 and 500 m, and consisted of copepodite stages from third to sixth. Duration of the E. bungii life cycle appears to be typically two years. New nauplii develop as far as the third or fourth copepodite stage during their first summer, then enter diapause. The second summer they advance to the fifth copepodite stage and reenter diapause. Fifth copepodites mature in their third summer at two years of age. The males remain at depth and mate without subsequent feeding. Females migrate at night to the mixed layer where spawning occurs. About 20% of females that had already spawned in 1980 reentered diapause. They would reproduce again in their fourth summer at three years of age. All aspects of the life cycle suggest low mortality rates for copepodite stages, particularly at depth in the habitat occupied during diapuse. There can be no premium on rapid reproduction for E. bungii in the subartic Pacific, and there must even be benefit from spreading reproduction between years. This iteroparity may amount to a “bet-hedging” tactic, the young from a given mother having more than one chance to find sustaining conditions. It also produces gene flow between the year classes of the biennial life cycle.

Zonation and maintenance of copepod populations in the Oregon upwelling zone

Abstract Oregon has one of the smaller and best known coastal upwelling systems. It is about 50 km wide, but upwelling is most intense within 15 km of the shore, and episodes of active upwelling primarily affect the circulation and hydrography of the upper 20 m. It is in this nearshore, surface zone that phytoplankton and zooplankton are most abundant. Phytoplankton biomass is 5 to 20 mg chl- a m −3 , and zooplankton 50 to 200 mg dry weight m −3 . Vertically stratified sampling along transects perpendicular to the shore has produced a new picture of the upwelling process and suggests relationships between circulation and the population biology of planktonic animals. The zooplankton is dominated by five species of copepod. Each is distributed in a different pattern: Acartia clausii is almost completely restricted to the upper 5 to 10 m of the water column and the first 5 km from shore. Pseudocalanus sp. is abundant from 0 to 15 km and between 10 and 20 m depth, but it reproduces only within a few kilometers of the shore. Acartia longiremis lives and reproduces offshore (10 km) in the surface (0 to 10-m) mixed layer. Oithona similis is abundant offshore (10 km) between 10- and 20-m depths. Calanus marshallae lives offshore as older copepodite stages, but the females return shoreward and lay their eggs at about 10 km offshore. The nauplii and younger copepodites develop in the very nearshore zone. The patterns of animal distribution together with data on salinity, temperature, and chlorophyll- a lead to the following conclusions about the circulation in the Oregon upwelling zone: First, during active upwelling, the Ekman layer within 0 to 10 km is less than 5 m deep. Offshore of the frontal region, this layer is 10 to 15 m thick. Second, when upwelling is active, the surface layer offshore of the frontal region is not continuously transported offshore. Instead, this mass is moved offshore some fixed distance, no more than 20 to 40 km. During relaxation, this surface water returns shoreward to its former location. Third, we propose a two-cell zonal circulation scheme during initiation of active upwelling: Looking north in vertical section, a divergence is located 10 km from shore, with a clockwise rotating cell on the landward side and a counterclockwise cell on the seaward side. The population of each zooplankton species appears to be maintained within the upwelling zone by a specific relationship between its distribution and the circulation.

Revised life history analysis for large grazing copepods in the subarctic Pacific Ocean

Abstract Discovery that the subarctic Pacific copepods previously grouped as Neocalanus plumchrus belong to two species required reanalysis of the life histories of both. After correction of the abundance estimates for N. plumchrus s.str., our concept of its life history remains much as previously described, because it makes up about 90% of the summed populations. Fifth copepodites of the new species, Neocalanus flemingeri, descend from the surface layer in late May to early June and mature immediately. Males are only present for about two months, and females carrying spermatophores are found during that period. Throughout the summer and autumn the entire population is constituted of females with small, dormant ovaries. This appears to be a diapause phase. Ovarian development begins in November, and spawning occurs at the end of January. Copepodite stages develop in surface layers from February through May.

Vertical habitat partitioning by large calanoid copepods in the oceanic subarctic Pacific during Spring

Abstract The copepods Neocalanus plumchrus, N. flemingeri, N. cristatus, and Eucalanus bungii dominate the net zooplankton throughout the subarctic Pacific Ocean. All four species have an extensive seasonal ontogenetic vertical migration, completing most or all of their feeding and somatic growth in spring and early summer. We used stratified tows with MOCNESS and BIONESS instrumented net systems to resolve their upper ocean vertical distributions in May and June of 1984, 1987 and 1988. In each year the feeding copepodite stages of all four species were concentrated above the permanent halocline (roughly from 0 to 150m). However, the four species showed strong vertical species zonation and segregation within this layer. We consistently found a near-surface pair (N. plumchrus and N. flemingeri) and a subsurface pair (N. cristatus and E. bungii). The boundary between these groups shifts vertically, but was sharply defined and was very often coincident with a weak and transient thermocline marking the base of the layer actively mixed by surface wind and wave energy. Diel vertical migration was very limited during our sampling periods. The data suggest that the vertical distribution patterns of the copepods could be set by responses to the local intensity of turbulent mixing in the watercolumn. N. plumchrus and N. flemingeri occupied a stratum characterized by strong turbulence. N. cristatus and E. bungii occupied a stratum that was a local minimum in turbulence profiles. The depth of the boundary between the species pairs was deeper when winds and surface energy inputs were strong. The vertical partition pattern may also be determined by a difference in feeding strategy between the species pairs. N. plumchrus and N. flemingeri may feed on the enhanced protozoan population of the mixed layer, while N. cristatus and E. bungii feed on particle aggregates settling from above.

Blooms of large diatoms in the oceanic, subarctic Pacific

Abstract Sampling with fine mesh (73 μm) nets from the weatherships patrolling at Sta. P (50°N, 145°W) in the northeastern Pacific revealed prolonged population pulses of large diatoms. Density of Corethron criophilum Castracane exceeded 5000 cells l −1 through three weeks in July 1980. Density of a mixture of species including Thalassiothrix longissima, Chaetoceros atlanticus, Chaetoceros concavicornis , and Rhizosolenia alata was sustained at several hundred to over 1000 cells l −1 from late November 1980 to mid-February 1981. While truly massive diatom blooms are not observed in the nutrient-rich, oceanic subarctic Pacific, the observations simply that waters of the region are not utterly unsuitable as a medium to sustain diatom growth.

Effects of sediment microtopography on small-scale spatial distributions of meiobenthic nematodes

Abstract The influence of sediment microtopography on the small-scale spatial distribution of meiobenthic nematodes was investigated using time series statistical techniques to analyze data obtained from numerous small cores. Two transects, each 1 m long and comprised of contiguous 6 mm cores, were collected from an intertidal sand flat characterized by regularly spaced, asymmetrical sediment ripples of 8 cm wavelength. Autocorrelation coefficients and power spectral estimates showed a statistically significant periodicity at the wavelength of the sediment ripples in the fluctuation of total nematode abundance. Within each transect, several separate series of density oscillations were present, all with successive density peaks spaced 8 cm apart, but phase-shifted with respect to each other. Cross-correlation coefficients between nematode abundance and ripple height revealed that the highest densities of nematodes tend to be associated with ripple crests. These results indicate that nematodes can respond to small-scale environmental heterogeneity produced by sediment microtopography, although the exact mechanism by which this occurs remains unknown. A sequence of events extending over several tidal cycles probably generates this distribution pattern. Observations of stained sediments and fecal pellets showed that organic material accumulates in troughs at low tide and is subsequently buried by migrating ripple crests during the succeeding high tide. As this process is repeated, multiple series of organically enriched zones are laid down, all with the same spacing. Each series is displaced from the one laid down previously by a distance equal to the total movement of the ripples between their times of deposition. It is hypothesized that nematodes are attracted to this buried organic material.

Neocalanus flemingeri, a new species of Calanidae (Copepoda: Calanoida) from the subarctic Pacific Ocean, with a comparative redescription of Neocalanus plumchrus (Marukawa) 1921

Abstract Copepodites of Neocalanus plumchrus dominate the macrozooplankton biomass of the subarctic Pacific during spring. Living specimens have patches of bright red-orange pigment. During May of 1984 it was found sharing dominance with another, related species with differently placed patches of more distinctly red pigment. This is an undescribed species, which is designated here as Neocalanus flemingeri sp. nov. In the Gulf of Alaska it is smaller than N. plumchrus , but in the Bering Sea where food supplies are greater it reaches dimensions as large or larger. The morphologies of the two species are distinct, and previous confusion in the taxonomy of these forms was caused by differences in their life histories.

Life history and vertical distribution of pelagic chaetognaths at Ocean Station P in the subarctic Pacific

Seven species of two genera of pelagic chaetognaths (Eukrohnia hamata, E. bathypelagica, E. fowleri, Sagitta elegans, S. macrocephala, S. scrippsae. and S. zetsios) were collected at Ocean Station P (50°N, 145°W) from February 1980 to January 1981. The most abundant species, E. hamata, E. bathypelagica, E. fowleri, and S. elegans comprised 71.0, 18.0, 7.2, and 3.8%, respectively, of the chaetognaths in the upper 2000 m. S. elegans inhabits the epipelagic layer (0 to 200 m), E. hamata the epipelagic and upper mesopelagic layers, E. bathypelagica the mesopelagic layer (200 to 1000 m), S. macrocephala the lower mesopelagic and bathypelagic layers (500 to 2000 m), and E. fowleri the layer below 1000 m. E. hamata and S. elegans both had three spawning periods during the year, separated by distinct gaps in both presence of mature individuals and appearance of new juveniles. Generation lengths were 8 to 10 months for E. hamata and 6 to 10 months for S. elegans. Factors that produce the overlapping, two-thirds year generations remain to be demonstrated. Mean growth rates determined by following their cohorts were 2.5 to 3.0 mm per month for E. hamata and 5 to 6 mm per month for S. elegans. Breeding and development were continuous through the year for E. bathypelagica and E. fowleri.

Development of large copepods during spring in the Gulf of Alaska

Abstract Development rate in Neocalanus plumchrus at Station P (50°N, 145°W) in the Gulf of Alaska was estimated from changing copepodite stage proportions. Apparent duration of the third copepodite stage (C3) was 13.4 days. The error imposed on this estimate by differential mortality was modelled numerically. If mortality rate declines with age, C3 duration as short as 11.7 days is compatible with the data. If it accelerates with age, durations up to 16.2 days are reasonable. These estimates are shorter than those made previously for May 1984. Dry weight and lipid-free dry weight estimates show that Neocalanus flemingeri C5 with full lipid loading were smaller in 1988 (270μg DW, 170μg LFDW) than in 1987 (400μg DW, 225μg LFDS). Resting stage N. flemingeri females from 1988 were smaller than those of any recorded year, while those of 1987 were of average size. Nutrient, floristic, and microbial conditions were unusual in May 1988, all possibly shifted by a late season storm in mid-April. This could have caused the poor growth of N. flemingeri and a delay in development of N. plumchrus of two weeks compared to 1984.

Abundance and distribution of ichthyoplankton in the upwelling zone off Oregon during anomalous El Niño conditions

The abundance and distribution patterns of nearshore ichthyoplankton were investigated during a year of anomalously high sea temperatures off Oregon. Samples collected from 2 to 18 km offshore from April through September of 1983 showed increased occurrences and higher abundances of taxa usually found at distances offshore of 37 km in other years. The dominant species collected, comprising more than half of the total larval fish abundance, was the northern anchovy (Engraulis mordax). Larval anchovy have rarely been collected inshore in previous studies. Many of the dominant taxa normally found inshore, especially osmerids, were present in reduced numbers in 1983. Changes in the hydrographic conditions associated with onshore surface drift and reduced summer upwelling during the 1983 El Nino could explain the distributional patterns observed. The warm inshore waters apparently provided a substantial spatial and temporal expansion of the spawning habitat for E. mordax.