Egg production and early development of the subarctic copepods Neocalanus cristatus,N. plumchrus and N. flemingeri

Reproduction and early development of the large subarctic copepods Neocalanus cristatus, N. plumchrus and N. flemingeri were investigated in the laboratory. All three species produced eggs at intervals of 5.0 to 14.7 days, depending on species and temperature. The females of N. flemingeri released the fewest clutches of the three species, but with the largest clutch size. Clutch size of N. cristatus was smallest with longest intervals between clutches. Mean±1 S.E. of total fecundities were 386±116 eggs for N. cristatus, 840±214 eggs for N. plumchrus, and 924±346 eggs for N. flemingeri. The egg laying period of a female at 2°C was estimated to be 91 days for N. cristatus, 60 days for N. plumchrus and 47 days for N. flemingeri. The color and outer characteristics of eggs and nauplii of these species were quite different. C : N ratios of the eggs were 9.3–10.5, which were slightly higher than that of females or CV. Egg hatching times for each species were 4.6–5.7 days at 2°C and decreased with increasing temperature at a Q₁₀ of 2.8–3.0. N. cristatus nauplii developed to copepodid stage I (CI) without feeding, and the developmental time in days from hatching to CI was expressed as a Bělehrádek equation, D_CI=17068×(T+14.7)^−2.05. Reproduction strategies of the three species of Neocalanus are discussed with reference to their life history strategies.     

High feeding rates on large particles by Neocalanus flemingeri and N. plumchrus,and consequences for phytoplankton community structure in the subarctic Pacific Ocean

The open subarctic Pacific Ocean is a high nitrate low chlorophyll (HNLC) system characterized by low concentrations of phytoplankton, a community dominated by small cells, and iron-limited growth of, especially, the larger phytoplankton. In such systems the main energy and material flow is through the microbial web, with large copepods considered primarily to be grazers on the larger microzooplankton occupying the top of this web. Consistent with this is the recognition that much of the nutrition of the dominant copepods in this system, Neocalanus flemingeri, N. plumchrus and N. cristatus, is derived from microzooplankton. Also, these copepods consume only a small fraction of the total phytoplankton production. In this paper, we show that the contribution made by N. flemingeri and N. plumchrus to establishing and maintaining the community structure of this ecosystem should be re-evaluated. Our experiments indicate these grazers have high clearance rates on large particles, including both large phytoplankton and microzooplankton, and this selective removal contributes to establishment and maintenance of the observed foodweb structure in the Gulf of Alaska. These high feeding rates combined with large populations of these two Neocalanus species concentrated in the upper layer of the ocean, result in population-based feeding rates approximately equal to the growth rates of large phytoplankton under iron-limited conditions. We conclude that N. flemingeri and N. plumchrus populations (a) directly prevent the accumulation of large phytoplankton cells by selectively feeding on them at high rates, and (b) indirectly stimulate the accumulation of the smaller phytoplankton by consumption of their major grazers, the microzooplankton.     

Geographical Variation of Body Size of Neocalanus cristatus, N. plumchrus and N. flemingeri in the Subarctic Pacific and Its Marginal Seas: Implications for the Origin of Large Form of N. flemingeri in the Oyashio Area

Size distributions of Neocalanus cristatus, N. flemingeri and N. plumchrus were investigated in the eastern and the western subarctic gyres and three marginal seas of the North Pacific during the diapause period to examine the geographical variation in body size of Neocalanus species and to clarify the origin of the large biennial N. flemingeri which has been observed in the Oyashio region. There were significant among region variations in body sizes for all three species of Neocalanus. Generally, the body sizes of the copepods were larger in the marginal seas and marginal areas of the open ocean. In the open ocean, the body sizes increased westward. These patterns of variation in the body sizes roughly correlated with local food availability. Distribution of biennial N. flemingeri was restricted to the Sea of Japan, the Okhotsk Sea and the Oyashio region. The large-sized biennial N. flemingeri were abundantly observed in the Okhotsk Sea, and the medium-sized biennial individuals were observed in the Sea of Japan. These facts strongly suggest that the large biennial N. flemingeri in the Oyashio region are advected from the Okhotsk Sea.     

Intrinsic and forced interannual variability of the Gulf of Alaska mesoscale circulation

The response of the Gulf of Alaska (GOA) circulation to large-scale North Pacific climate variability is explored using three high resolution (15 km) regional ocean model ensembles over the period 1950-2004. On interannual and decadal timescales the mean circulation is strongly modulated by changes in the large scale climate forcing associated with PDO and ENSO. Intensification of the model gyre scale circulation occurs after the 1976-1977 climate shift, as well as during 1965-1970 and 1993-1995. From the model dynamical budgets we find that when the GOA experiences stronger southeasterly winds, typical during the positive phase of the PDO and ENSO, there is net large-scale Ekman convergence in the central and eastern coastal boundary. The geostrophic adjustment to higher sea surface height (SSH) and lower isopycnals lead to stronger cyclonic gyre scale circulation. The opposite situation occurs during stronger northwesterly winds (negative phase of the PDO).Along the eastern side of the GOA basin, interannual changes in the surface winds also modulate the seasonal development of high amplitude anticyclonic eddies (e.g. Haïda and Sitka eddies). Large interannual eddy events during winter-spring, are phase-locked with the seasonal cycle. The initial eddy dynamics are consistent with a quasi-linear Rossby wave response to positive SSH anomalies forced by stronger downwelling favorable winds (e.g. southwesterly during El Niño). However, because of the fast growth rate of baroclinic instability and the geographical focusing associated with the coastal geometry, most of the perturbation energy in the Rossby wave is locally trapped until converted into large scale nonlinear coherent eddies. Coastally trapped waves of tropical origin may also contribute to positive SSH anomalies that lead to higher amplitude eddies. However, their presence does not appear essential. The model ensembles, which do not include the effects of equatorial coastally trapped waves, capture the large Haïda and Sitka eddy events observed during 1982 and 1997 and explain between 40% and 70% of the tidal gauges variance along the GOA coast.In the western side of the GOA basin, interannual eddy variability located south of the Alaskan Stream is not correlated with large scale forcing and appears to be intrinsic. A comparison of the three model ensembles forced by NCEP winds and a multi-century-long integration forced only with the seasonal cycle, shows that the internal variability alone explains most of the eddy variance. The asymmetry between the eddy forced regime in the eastern basin, and the intrinsic regime in the western basin, has important implications for predicting the GOA response to climate change. If future climate change results in stronger wintertime winds and increased downwelling in the eastern basin, then increased mesoscale activity (perhaps more or larger eddies) might occur in this region. Conversely, the changes in the western basin are not predictable based on environmental forcing. Eastern eddies transport important biogeochemical quantities such as iron, oxygen and chlorophyll-a into the gyre interior, therefore having potential upscale effects on the GOA high-nutrient-low-chlorophyll region.     

A synoptic survey of young mesoscale eddies in the Eastern Gulf of Alaska

Eddies in the Gulf of Alaska are important sources of coastal water and associated nutrients, iron, and biota to the high-nutrient, low-chlorophyll central Gulf of Alaska. Three primary eddy formation regions along the eastern boundary of the gulf have been identified, (from south to north, Haida, Sitka, and Yakutat). In the spring of 2005, three eddies (one of each type) were sampled soon after their formation. The subsurface eddy core water in all three eddies was defined by high iron concentrations and low dissolved oxygen compared with surrounding basin water. The Sitka and Yakutat core waters also exhibited a subsurface temperature maximum (mesothermal water) coincident in depth with the iron maximum, suggesting that eddies may play a role in the formation of temperature inversions observed throughout the Gulf of Alaska. The data suggest different formation regions, with the Yakutat eddy forming in shallow shelf water with riverine input, while the Sitka and Haida eddies appear to form in deeper water.     

Mesoscale eddies dominate surface phytoplankton in northern Gulf of Alaska

The HNLC waters of the Gulf of Alaska normally receive too little iron for primary productivity to draw down silicate and nitrate in surface waters, even in spring and summer. Our observations of chlorophyll sensed by SeaWiFS north of 54°N in pelagic waters (>500 m depth) of the gulf found that, on average, more than half of all surface chlorophyll was inside the 4 cm contours of anticyclonic mesoscale eddies (the ratio approaches 80% in spring months), yet these contours enclosed only 10% of the total surface area of pelagic waters in the gulf. Therefore, eddies dominate the chlorophyll and phytoplankton distribution in surface pelagic waters. We outline several eddy processes that enhance primary productivity. Eddies near the continental margin entrain nutrient - (and Fe) - rich and chlorophyll-rich coastal waters into their outer rings, advecting these waters into the basin interior to directly increase phytoplankton populations there. In addition, eddies carry excess nutrients and iron in their core waters into pelagic regions as they propagate away from the continental margin. As these anticyclonic eddies decay, their depressed isopycnals relax upward, injecting nutrients up toward the surface layer. We propose that this transport brings iron and macro-nutrients toward the surface mixed layer, where they are available for wind-forced mixing to bring them to surface. These mesoscale eddies decay slowly, but steadily, perhaps providing a relatively regular upward supply of macro-nutrients and iron toward euphotic layers. They might behave as isolated oases of enhanced marine productivity in an otherwise iron-poor basin. We note that much of this productivity might be near or just below the base of the surface mixed layer, and therefore poorly sampled by colour-sensing satellites. It is possible, then, that eddies enrich phytoplankton populations to a greater extent than noted from satellite surface observations only.     

Coupling global and regional circulation models in the coastal Gulf of Alaska

As part of the US GLOBEC NE Pacific program, we are simulating currents in the Coastal Gulf of Alaska (CGOA) to explore sources of interannual and interdecadal variability. To do so, we have developed a coupled modeling system composed of linked regional and global circulation models. The regional model, configured with 13–22 km resolution in the CGOA, is forced at the surface by observed heat fluxes and wind stresses, at the continental boundaries by observed runoff, and at the open ocean boundaries by a combination of tracer climatologies and sub-tidal velocity and tidal elevation provided by a global finite element model. In this communication, we describe the coupled system, including its present method of intermodel coupling, describe a series of multi-year model hindcasts, compare hindcast results with Eulerian and Lagrangian field data obtained in the CGOA in fall 1996, and assess the impact of global information (barotropic sub-tidal velocities and tidal elevations) on the regional model under the present coupling strategy. We find that the regional model produces appropriate current systems (Alaskan Stream, Alaska Coastal Current) and scalar fields, but with mesoscale variability (of SSH and velocities) at somewhat reduced strength relative to data, and with temperature gradients somewhat larger than those observed. Barotropic sub-tidal information from the global model penetrates the regional model interior, supplying additional mesoscale variability, and modifying regional velocity and scalar fields in both shallow and deep areas. Tidal information exerts a significant influence on sub-tidal scalar and velocity structure only in specific shallow areas, where the tides (and tidal mixing) are strongest. Pending the exploration of alternate coupling schemes, we infer from these results that on a time scale of months, purely barotropic information from outside the CGOA will have a modest impact on its mean regional circulation, but a potentially stronger impact on the statistics and details of mesoscale eddies.     

Modeling iron limitation of primary production in the coastal Gulf of Alaska

A lower trophic level NPZD ecosystem model with explicit iron limitation on nutrient uptake is coupled to a three-dimensional coastal ocean circulation model to investigate the regional ecosystem dynamics of the northwestern coastal Gulf of Alaska (CGOA). Iron limitation is included in the NPZD model by adding governing equations for two micro-nutrient compartments: dissolved iron and phytoplankton-associated iron. The model has separate budgets for nitrate (the limiting macro-nutrient in the standard NPZD model) and for iron, with iron limitation on nitrate uptake being imposed as a function of the local phytoplankton realized Fe:C ratio. While the ecosystem model represents a simple approximation of the complex lower trophic level ecosystem of the northwestern CGOA, simulated chlorophyll concentrations reproduce the main characteristics of the spring bloom, high shelf primary production, and “high-nutrient, low-chlorophyll” (HNLC) environment offshore. Over the 1998–2004 period, model-data correlations based on spatially averaged, monthly mean chlorophyll concentrations are on average 0.7, with values as high as 0.9 and as low as 0.5 for individual years. The model also provides insight on the importance of micro- and macro-nutrient limitation on the shelf and offshore, with the shelfbreak region acting as a transition zone where both nitrate and iron availability significantly impact phytoplankton growth. Overall, the relative simplicity of the ecosystem model provides a useful platform to perform long-term simulations to investigate the seasonal and interannual CGOA ecosystem variability, as well as to conduct sensitivity studies to evaluate the robustness of simulated fields to ecosystem model parameterization and forcing. The ability of the model to differentiate between nitrate-limited, and iron-limited growth conditions, and to identify their spatial and temporal occurrences, is also a first step towards understanding the role of environmental gradients in shaping the complex CGOA phytoplankton community structure.     

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.     

Characterization of carbon and nitrogen stable isotope gradients in the northern Gulf of Alaska using terminal feed stage copepodite-V Neocalanus cristatus

Individual terminal-feeding copepodite-V stage Neocalanus cristatus were collected systematically in the northern Gulf of Alaska (GOA) near 60°N from 1998 to 2004 from which the natural abundance of stable carbon and nitrogen isotopes was measured. The data confirmed the existence of an isotopic cross-shelf gradient such that values low in ¹³C content are diagnostic of oceanic production from the GOA when measured in organisms taken from Prince William Sound (PWS). The mean ¹³C/¹²C cross-shelf gradient of 3–4 delta units was relatively strong, with generally good separation between GOA and Prince William Sound observations, whereas the mean ¹⁵N/¹⁴N gradient of ∼2 delta units was relatively weak, with frequent overlap between GOA and PWS observations. There was a seasonal ¹⁵N/¹⁴N increase in the GOA. The ¹³C/¹²C values observed in PWS were more consistent over time than those observed in the GOA. Distinctively high ¹³C/¹²C values that were similar to those typical of PWS were observed at the continental slope during three of the Mays in the 1998–2004 period. The circulation pattern associated with mesoscale eddies, when they occurred just south of the sampling line based on satellite sea-surface height anomaly data, suggested that cross-shelf flow in the offshore direction drove high slope ¹³C/¹²C values. These observations led to positing that high ¹³C/¹²C values reflect coastal carbon isotope signatures and diatom blooms. Based on samples from May 1996, the three GOA Neocalanus congeners had concordant isotopic patterns with relatively small systematic species differences confirming that the isotopic patterns observed for N. cristatus apply to other zooplankton.     

Modeled transport of freshwater from a line-source in the coastal Gulf of Alaska

A set of multiply nested atmospheric (The Penn State/NCAR Mesoscale Modeling system—MM5) and oceanic (Regional Ocean Modeling System—ROMS) models has been developed to investigate ecosystem forcing as part of the US. GLOBEC program. This study focuses on the most finely nested oceanic model in the hierarchy, that of the coastal Gulf of Alaska (CGOA) during 2001–2002, and compares the model's results to data collected by GLOBEC investigators. The 3-km resolution model realistically generates two physical features needed to reproduce the CGOA ecosystem: the cross-shelf water mass structure on the Seward Shelf, and the seasonal cycle of vertical structure. In addition, the temporal variability of currents and tracer fields generated by the model is greatly improved compared to previous work, as is the resolution of the Alaska Coastal Current (ACC). However, the treatment of the line-source freshwater source along the coast of Alaska still presents difficulties, because the model cannot resolve the many inlets and fjords where mixing takes place initially. This issue is investigated by testing the model's sensitivity to various forcing mechanisms which could compensate for this weakness, such as the addition of tidal mixing, the use of finely resolved winds, and the use of brackish runoff rather than purely freshwater for the line-source.     

Physical controls and mesoscale variability in the Labrador Sea spring phytoplankton bloom observed by Seaglider

We investigated the 2005 spring phytoplankton bloom in the Labrador Sea using Seaglider, an autonomous underwater vehicle equipped with hydrographic, bio-optical and oxygen sensors. The Labrador Sea blooms in distinct phases, two of which were observed by Seaglider: the north bloom and the central Labrador Sea bloom. The dominant north bloom and subsequent zooplankton growth are enabled by the advection of low-salinity water from West Greenland in the strong and eddy-rich separation of the boundary current. The glider observed high fluorescence and oxygen supersaturation within haline-stratified eddy-like features; higher fluorescence was observed at the edges than centers of the eddies. In the central Labrador Sea, the bloom occurred in thermally stratified water. Two regions with elevated subsurface chlorophyll were also observed: a 5 m thin-layer in the southwest Labrador Current, and in the Labrador shelf-break front. The thin layer observations were consistent with vertical shearing of an initially thicker chlorophyll patch. Observations at the front showed high fluorescence down to 100 m depth and aligned with the isopycnals defining the front. The high-resolution Seaglider sampling across the entire Labrador Sea provides first estimates of the scale dependence of coincident biological and physical variables.     

厦门海域小型浮游动物和桡足类的摄食对浮游植物生长的影响

2005年11月16日和27日,运用稀释法和桡足类添加法,对厦门宝珠屿海域小型浮游动物及桡足类的摄食对浮游植物生长的影响进行了研究.结果表明,各粒级浮游植物的生长率均大于小型浮游动物的摄食率,小型浮游动物对总的Chla和nano-Chla具有一致的显著的摄食作用(0.51～0.78d-1),当存在螺旋环沟藻等大型的异养甲藻时,亦能摄食micro-级浮游植物.所添加的桡足类主要摄食micro-级的浮游植物,也显著摄食小型浮游动物,16日,所添加的桡足类促进nano-级浮游植物的每天生长效应达0.03ind/dm3.说明了厦门海域小型浮游动物及桡足类的摄食共同控制着浮游植物的生长,由于桡足类的杂食性,可产生一定的营养级联效应.     

Short-term variability of the phytoplankton community in coastal ecosystem in response to physical and chemical conditions' changes

The short-term dynamics (time scale of a few days) of phytoplankton communities in coastal ecosystems, particularly those of toxic species, are often neglected. Such phenomena can be important, especially since these very species can endanger the sustainability of shellfish farming. In this study, we investigated the short-term changes in phytoplankton community structure (species succession) in two coastal zones in parallel with physical and chemical conditions. Mixing events with allochtonous waters could thus be distinguished from local processes associated with population growth when it was associated with a change in light or nutrient limitation. Mixing events and water advection influenced fluctuations in total phytoplankton biomass and concentration of dominant species, while local processes influenced delayed changes in community structure. The estuarine species Asterionellopsis glacialis increased in concentration when the water mass mixed with the nearest estuarine water masses. The biological response, measured as photosynthetic capacity, occurred after a time-lag of a few hours, while the changes in community structure occurred after a time-lag of a few days. Finally, the coastal water mass was constantly mixed with both the nearest estuarine and marine water masses, leading in turn to delayed changes in phytoplankton community structure. These changes in species composition and dominance were observed on a time scale of a few days, which means that some toxic species may be missed with a bi-weekly sampling strategy.     

Effects of physical phenomena on the distribution of nutrients and phytoplankton productivity in a coastal lagoon

Sea level, salinity, temperature, nitrate, nitrite, phosphate, silicate, chlorophylls a, b and c and their phaeophytins, phytoplankton abundance and phytoplankton productivity time series were generated for the mouth and three interior locations of Bahia San Quintin, Baja California, Mexico, for 10 days during summer of 1979. The samples were taken once every 2 h. This was done to describe space and time variability of these ecological properties and to elucidate the main factors that cause this variability. Upwelling events bring nutrient reach waters near the bay mouth and tidal currents propagate those waters throughout the bay. Nutrient remineralization at the sediments and the effect of turbulence induced by tidal currents and winds increase nutrient concentrations in the interiors of the bay. In comparison with available information on nutrients limited growth of planktonic algae, nutrients seemed not to be limiting to phytoplankton growth during the sampling period. Phytoplankton cell abundances at the extremes of the lagoon are an order of magnitude lower than at the mouth due to greater turbidity. Chlorophyll concentrations at the extremes are about one-third of those of the mouth. Primary productivity decreases from the mouth to the interiors in the same manner as chlorophyll does. There is not a significant difference in cell size between phytoplankton at the bay mouth and those at the extremes of the bay. Primary productivity in the bay is comparable to the productivity maxima of other upwelling areas. There is no clear permanent dominance of diatoms over dinoflagellates, or vice versa, at any location in the bay. The alternation of upwelling and non-upwelling played an important role, together with that of the spring-neap tide cycle, in producing low frequency (< 0.01 cycles h^?1) temporal variability of ecological properties throughout the bay.     

Characterization of earthquake sources in the Gulf of Alaska

Abstract

The characterization of earthquake sources in the Gulf of Alaska and the relative significance of earthquake sources for establishing seismic design inputs at a typical site for engineering purposes are discussed. Earthquake sources in the complex tectonic environment can be divided into two groups: (a) a subduction zone that underlies the entire region (maximum magnitude M = 8.5); and (b) individual thrust and strike‐slip faults associated with the plate motions (maximum magnitude M = 6 to 7.5). The sources of either group and individual earthquake events can be represented as planar surfaces for consistency with the physical process and a mathematically tractable computational scheme.

Although the area is very active seismically, the degree of activity of individual sources varies significantly. Therefore, even for sources with the same maximum earthquakes, different magnitudes may apply for a selected design return period. The area is considered to be a “seismic gap.”; No great earthquakes have occurred in nearly 80 years. Estimates based on a temporally varying seismic function such as the semi‐Markov model indicate that the probability of occurrence of a great earthquake in the near future is significantly higher than the average probability inferred from a statistical analysis of historical seismicity data of the entire region.

Separate attenuation relationships should be used for calculating ground motions due to earthquakes on the dipping subduction zone in the northern portion of the gulf. The dominant earthquake source for almost the entire Gulf of Alaska region is the subduction zone that contributes over 80 percent of the seismic exposure at a typical site. The dominant magnitude range is M_s = 6.5 to 7.5. “Gap filling”; earthquakes (M_s = 7.5 to 8.25) contribute a little over a third of the seismic exposure at a typical site. Deterministic assessments of ground motion values using the maximum earthquake on the subduction zone at the closest distance yield values significantly higher than those calculated for even 500‐year return periods. Estimated 100‐year return period accelerations in the area range from 180 to 340 cm/sec².     

Abundance and size composition of the summer phytoplankton communities in the western North Pacific Ocean,the Bering Sea,and the Gulf of Alaska

Chlorophylla concentrations (Chla) of size-fractionated phytoplankton samples were measured in the western North Pacific Ocean, the Bering Sea, and the Gulf of Alaska during the summer of 1986. Among samples collected in the upper 100 m (total of 210 samples), 207 samples were dominated by micro- (>10 m) or picoplankton (<2 m) and only three samples were represented by nanoplankton (2–10 m). These 207 samples were classified based on the total Chla content into three types: Type H (>1.0 g l^–1), Type M (0.5–1.0 g l^–1), and Type L (<0.5 g l^–1). These types further divided into two subtypes (-p and-m), depending upon dominancy of pico (-p) and microplankton (-m). The phytoplankton community was represented by Type L-p in the Gulf of Alaska, where 80% of the samples fell into this type. It was represented by Type M-p in the western North Pacific and the Oceanic Domain in the Bering Sea, where 53 and 41% of samples were identified as this type, respectively. In the Middle Domain of the Bering Sea, 68% of samples collected below the nitracline was Type H-m, which indicates blooms of microplanton. This type was also observed in the neritic waters near the Aleutian Islands. These types described above are consistent with a general trend that an increase in phytoplankton abundance is attributed to the growth of microplankton. An unusual type occurred above the nitracline of the Middle Domain, where microplankton prevailed, although the total Chla was less (Type L-m). This type represents a feature of late phase of an ice edge bloom. Another unusual type was found mainly in the Outer Domain of the Bering Sea, where the total Chla was high and picoplankton prevailed (Type H-p). The predominance of picoplankton seems to result from the heavy grazing intensity of large calanoid copepods upon microplankton but not upon picoplankton