William T. Peterson

Emergence of Anoxia in the California Current Large Marine Ecosystem

Eastern boundary current systems are among the worlds most productive large marine ecosystems. Because upwelling currents transport nutrient-rich but oxygen-depleted water onto shallow seas, large expanses of productive continental shelves can be vulnerable to the risk of extreme low-oxygen events. Here, we report the novel rise of water-column shelf anoxia in the northern California Current system, a large marine ecosystem with no previous record of such extreme oxygen deficits. The expansion of anoxia highlights the potential for rapid and discontinuous ecosystem change in productive coastal systems that sustain a major portion of the worlds fisheries.

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.

Life cycle strategies of copepods in coastal upwelling zones

Abstract Life cycles of copepods of coastal upwelling zones are of the multigenerational type—as many as 10 or more generations may be produced each year, depending upon water temperature, food concentration and length of the upwelling season. Abundant food resources and moderate temperature convey advantages to those copepods living in coastal upwelling zones, however, there is a clear disadvantage in that coastal upwelling zones are highly advective environments. Typically, water circulation patterns are such that surface waters are carried offshore, deeper waters carried onshore and most of the water column over the continental shelf is moving equatorward. The challenge to copepod species that inhabit upwelling systems is life cycle closure—how do eggs, nauplii, juveniles and adults avoid being swept out of these ecosystems in the face of persistent transport out of the system? In this review, I first list the species which dominate coastal upwelling ecosystems then discuss three variations on the multigenerational life cycle scheme that are observed in upwelling systems. The latter part of the review is devoted to discussion of how individuals are retained in the productive continental shelf waters within coastal upwelling ecosystems. The suggestion is made that the only copepod species that successfully achieve life cycle closure in such systems are those that are preadapted to upwelling circulation patterns. Our quantitative understanding of the relative importance of physical factors (such as advection) and biological factors (birth, growth, and mortality) on life cycle strategies and population dynamics is quite rudimentary. It would help our understanding if there were more field studies and more computer modeling studies that focused on seasonal cycles of abundance, development times and vertical distribution of life cycle stages, and measurements of water circulation patterns.

Patterns in stage duration and development among marine and freshwater calanoid and cyclopoid copepods: a review of rules, physiological constraints, and evolutionary significance

Studies of development time of marine and freshwater copepods have taken separate tracks. Most studies on marine copepods report development time of each individual development stage, whereas studies on freshwater copepods report only development time, from egg to nauplius and nauplius to adult. This bias allows comparison of total development time but prevents detailed comparisons of patterns in stage-specific developmental schedules. With respect to egg to adult development time, three general relationships are known: developmental rates are dependent upon temperature and food concentration but independent of terminal body size; freshwater calanoids develop significantly slower than marine calanoids; freshwater cyclopoids develop at the same rate as marine calanoids. Two rules describe stage-specific developmental rates: the equiproportional rule and the isochronal rule. The first rule states that the duration of a given life history stage is a constant proportion of the embryonic development time; the second rule states that the time spent in each stage is the same for all stages. This review focuses on the second rule. From the 80+ published studies of copepod stage-specific developmental times, no species follows the isochronal rule strictly: Acartia spp. come closest with isochronal development from third nauplius (N3) to fourth copepodite (C4). The only pattern followed by all species is rapid development of the first and/or second naupliar stages, slow development of the second and/or third nauplius and prolonged development of the final copepodite stage. Once adulthood is reached, males are usually short-lived, but females can live for weeks to months in the laboratory. Adult longevity in the sea is, however, on the order of only a few days. The evolution of developmental patterns is discussed in the context of physiological constraints, along with consideration of possible relationships between stage-specific mortality rates and life history strategies. Physiological constraints may operate at critical bottlenecks in development (e.g. at the first feeding nauplius, N6, and the fifth copepodite stage). High mortality of eggs may explain why broadcast eggs hatch 2–3 times faster than eggs carried by females in a sac; high mortality of adults may explain why adults do not grow rather they maximize their reproductive effort by partitioning all energy for growth into egg production.

Biological communities in San Francisco Bay track large-scale climate forcing over the North Pacific.

Long-term observations show that fish and plankton populations in the ocean fluctuate in synchrony with large-scale climate patterns, but similar evidence is lacking for estuaries because of shorter observational records. Marine fish and invertebrates have been sampled in San Francisco Bay since 1980 and exhibit large, unexplained population changes including record-high abundances of common species after 1999. Our analysis shows that populations of demersal fish, crabs and shrimp covary with the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), both of which reversed signs in 1999. A time series model forced by the atmospheric driver of NPGO accounts for two-thirds of the variability in the first principal component of species abundances, and generalized linear models forced by PDO and NPGO account for most of the annual variability of individual species. We infer that synchronous shifts in climate patterns and community variability in San Francisco Bay are related to changes in oceanic wind forcing that modify coastal currents, upwelling intensity, surface temperature, and their influence on recruitment of marine species that utilize estuaries as nursery habitat. Ecological forecasts of estuarine responses to climate change must therefore consider how altered patterns of atmospheric forcing across ocean basins influence coastal oceanography as well as watershed hydrology.

Life history and population maintenance strategies of Calanoides carinatus (Copepoda: Calanoida) in the southern Benguela ecosystem

Analysis of quarterly cross-shelf distribution patterns of juvenile and adult Calanoides carinatus off the Cape Columbine upwelling centre in the southern Benguela ecosystem shows that this copepod has behavioural adaptations which result in clear patterns of inshore-offshore zonation. The combination of differential diel vertical movements of the various stages, seasonal ontogenetic migration and the resultant differential exploitation of cross-shelf advective processes and longshore current regimes play an important role in the maintenance of populations within the coastal upwelling areas of the southern Benguela ecosystem. At lower latitudes, C. carinatus utilizes true diapause in its life-cycle strategy to bridge the 8–10 month period between upwelling seasons. In the Benguela, however, the presence of an inshore active and an offshore resting component enables local C. carinatus populations to maintain a perennial presence. Because of a protracted upwelling season and occasional upwelling in winter, ...

The effect of a large cape on distribution patterns of coastal and oceanic copepods off Oregon and northern California during the 1998-1999 El Nino-La Nina

Abstract Hydrographic and ocean drifter measurements made along the Oregon coast indicate that the spatial structure of the coastal upwelling system differs in waters to the north and the south of Cape Blanco, Oregon. North of the Cape, a 10–30 km wide zone of coastal upwelling parallels the coast, but south of the Cape, increased wind stress leads to a seaward expansion of the upwelling system and cold upwelled water extends 50–100 km offshore. Because the hydrography and the transport differ, we hypothesize that zooplankton distributions will differ as well. In this paper we investigate differences in copepod distributions and copepod community composition between the waters north and south of Cape Blanco. Five cruises were conducted in 1998 and 1999, which were years of contrasting ocean conditions; there was a strong El Nino in 1998, which was followed by a strong La Nina in 1999. Copepod biomass did not differ between the El Nino and La Nina periods; however, species composition of the copepod assemblages differed vastly. During the 1998 El Nino, the copepod community was dominated by subtropical neritic and warm-water offshore species. During the 1999 La Nina, the zooplankton community was dominated by cold water boreal neritic species. The warm water species were widely distributed in shelf and slope waters in 1998, whereas in 1999, they were found primarily offshore of central Oregon, but over the shelf off northern California. During the summer upwelling season of both years, copepod community composition in shelf waters differed significantly from slope waters in the region to the north of the Cape, however, community composition was the same in shelf and slope waters in the region south of the Cape. These results lead us to suggest that offshore transport by the upwelling jet may be an important mechanism controlling copepod community structure south of Cape Blanco. When we examined these patterns in community composition on a species-by-species basis, among the dominant boreal copepod species, Pseudocalanus mimus and Acartia longiremis were displaced offshore and maintained high population densities in the waters south of Cape Blanco whereas densities of Calanus marshallae and Centropages abdominalis declined in the waters south of the Cape. Thus, the interaction between the boreal copepods and the waters north versus south of Blanco is species-specific. Species may be either lost or retained depending upon interactions between vertical current shear and their vertical distributions. Alternatively, there may be a differential ability among species to survive and reproduce in waters offshore and south of Cape Blanco.

The western Agulhas Bank: circulation, stratification and ecology

Studies of the physical oceanography of the western Agulhas Bank are reviewed, pointing out the unique position of this shelf region between the eastern boundary Benguela system and the western boundary Agulhas system. New observations from moorings off Quoin Point during summer 1986/87 permit a fuller description of the dynamics of circulation and stratification and the identification of three subregions. The inner shelf is dominated by wind-forcing, as in eastern boundary upwelling systems, whereas the outer shelf is dominated by oceanic forcing, as over western boundary shelves. The mid-shelf is characterized by strong stratification separating near-surface oceanic water from near-bottom upwelling Central Water. A lack of correlation between currents and temperatures in these three subregions of the western Agulhas Bank suggests that they function independently. This synthesis of the physical oceanography, which includes discussion of the hydrodynamic coupling of the western Agulhas Bank with shelf reg...

Six centuries of variability and extremes in a coupled marine-terrestrial ecosystem

Rings of ocean upwelling Coastal upwelling along the coast of California has become more variable than during nearly any period in the past 600 years. Black et al. used a 576-year tree ring record to construct a record of wintertime climate along the California coast. Because wintertime climate and coastal upwelling are so closely linked there, they were able to determine that upwelling variability has increased more over the past 60 years than for all but two intervals during that time. The apparent causes of the recent trend appear to be unique, resulting in reduced marine productivity and negative impacts on fish, seabirds, and mammals. Science, this issue p. 1498 Winter upwelling along the Pacific coast of North America became unusually variable during the 20th century. Reported trends in the mean and variability of coastal upwelling in eastern boundary currents have raised concerns about the future of these highly productive and biodiverse marine ecosystems. However, the instrumental records on which these estimates are based are insufficiently long to determine whether such trends exceed preindustrial limits. In the California Current, a 576-year reconstruction of climate variables associated with winter upwelling indicates that variability increased over the latter 20th century to levels equaled only twice during the past 600 years. This modern trend in variance may be unique, because it appears to be driven by an unprecedented succession of extreme, downwelling-favorable, winter climate conditions that profoundly reduce productivity for marine predators of commercial and conservation interest.

Jet stream intraseasonal oscillations drive dominant ecosystem variations in Oregon's summertime coastal upwelling system

Summertime wind stress along the coast of the northwestern United States typically exhibits intraseasonal oscillations (ISOs) with periods from ≈15 to 40 days, as well as fluctuations on the 2- to 6-day “weather-band” and 1-day diurnal time scales. Coastal upwelling of cool, nutrient-rich water is driven by extended periods of equatorward alongshore winds, and we show that the ≈20-day ISOs in alongshore wind stress dominated the upwelling process during summer 2001 off Oregon. These wind stress ISOs resulted from north–south positional ISOs of the atmospheric jet stream (JS). Upper-ocean temperature, phytoplankton, and zooplankton varied principally on the ≈20-day time scale as well, and these correlated with the ISOs in alongshore wind stress and JS position, even though there also were weather-band stress fluctuations of comparable magnitude. Such wind stress ISOs are typical along Oregon in the summer upwelling season, occurring in 10 of 12 years examined, including 2001. We present a previously unreported direct connection from the atmospheric JS to oceanic primary and secondary production on the intraseasonal time scale and show the leading importance of ISOs in driving this coastal upwelling ecosystem during a typical summer.