Steven J. Bograd

Tracking apex marine predator movements in a dynamic ocean

Pelagic marine predators face unprecedented challenges and uncertain futures. Overexploitation and climate variability impact the abundance and distribution of top predators in ocean ecosystems. Improved understanding of ecological patterns, evolutionary constraints and ecosystem function is critical for preventing extinctions, loss of biodiversity and disruption of ecosystem services. Recent advances in electronic tagging techniques have provided the capacity to observe the movements and long-distance migrations of animals in relation to ocean processes across a range of ecological scales. Tagging of Pacific Predators, a field programme of the Census of Marine Life, deployed 4,306 tags on 23 species in the North Pacific Ocean, resulting in a tracking data set of unprecedented scale and species diversity that covers 265,386 tracking days from 2000 to 2009. Here we report migration pathways, link ocean features to multispecies hotspots and illustrate niche partitioning within and among congener guilds. Our results indicate that the California Current large marine ecosystem and the North Pacific transition zone attract and retain a diverse assemblage of marine vertebrates. Within the California Current large marine ecosystem, several predator guilds seasonally undertake north–south migrations that may be driven by oceanic processes, species-specific thermal tolerances and shifts in prey distributions. We identify critical habitats across multinational boundaries and show that top predators exploit their environment in predictable ways, providing the foundation for spatial management of large marine ecosystems.

North Pacific Gyre Oscillation links ocean climate and ecosystem change

Decadal fluctuations in salinity, nutrients, chlorophyll, a variety of zooplankton taxa, and fish stocks in the Northeast Pacific are often poorly correlated with the most widely-used index of large-scale climate variability in the region - the Pacific Decadal Oscillation (PDO). We define a new pattern of climate change, the North Pacific Gyre Oscillation (NPGO) and show that its variability is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are driven by regional and basin-scale variations in wind-driven upwelling and horizontal advection - the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. The NPGO pattern extends beyond the North Pacific and is part of a global-scale mode of climate variability that is evident in global sea level trends and sea surface temperature. Therefore the amplification of the NPGO variance found in observations and in global warming simulations implies that the NPGO may play an increasingly important role in forcing global-scale decadal changes in marine ecosystems.

Climate change and wind intensification in coastal upwelling ecosystems

Strong winds, upwelling, and teeming shores Climate warming has produced stronger winds along some coasts, a result of growing differences in temperature and pressure between land and sea. These winds cause cold nutrient-rich seawater to rise to the surface, affecting climate and fueling marine productivity. Sydeman et al. examined data from the five major world regions where upwelling is occurring. Particularly in the California, Humboldt, and Benguela upwelling systems, winds have become stronger over the past 60 years. These regions represent up to a fifth of wild marine fish catches and are hot spots of biodiversity. Science, this issue p. 77 Increasing greenhouse gas concentrations have caused windier conditions in most major coastal upwelling regions. In 1990, Andrew Bakun proposed that increasing greenhouse gas concentrations would force intensification of upwelling-favorable winds in eastern boundary current systems that contribute substantial services to society. Because there is considerable disagreement about whether contemporary wind trends support Bakun’s hypothesis, we performed a meta-analysis of the literature on upwelling-favorable wind intensification. The preponderance of published analyses suggests that winds have intensified in the California, Benguela, and Humboldt upwelling systems and weakened in the Iberian system over time scales ranging up to 60 years; wind change is equivocal in the Canary system. Stronger intensification signals are observed at higher latitudes, consistent with the warming pattern associated with climate change. Overall, reported changes in coastal winds, although subtle and spatially variable, support Bakun’s hypothesis of upwelling intensification in eastern boundary current systems.

The biological response to the 1977 regime shift in the California Current

Among the least understood interactions between physics and biology in the oceans are those that take place on the decadal scale. But this temporal scale is important because some of the greatest ecological events take place on this time scale. More than 50 years of measurement in the California Current System have revealed significant ecosystem changes, including a large, decadal decline in zooplankton biomass, along with a rise in upper-ocean temperature. The temperature change was a relatively abrupt shift around 1976–77, concurrent with other basin-wide changes associated with an intensification of the Aleutian Low-pressure system. This intensification generates temperature anomalies in the ocean by altering the patterns of net surface-heat fluxes, turbulent mixing, and horizontal transport. Changes in the mean abundance of zooplankton in the southern California Current have been attributed to variations in the strength of coastal upwelling, variations in the horizontal transport of nutrient-rich water from the north, or increased stratification due to warming, all of which could be affected by fluctuations in the Aleutian Low. Here we show that a deepening of the thermocline accompanied the warming and increased the stratification of the water column, leading to a decrease in the supply of plant nutrients to the upper layers. This is the most likely mechanism for the observed plankton decline, and subsequent ecosystem changes. A global change in upper-ocean heat content, accompanied by an increase in stratification and mixed-layer deepening relative to the critical depth for net production, could lead to a widespread decline in plankton abundance.

Long-term variability in the Southern California Current System

Abstract We summarize 50 years of physical oceanographic data in the southern portion of the California Current System (CCS) based on the California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrographic record, 1950–1999. The long-term mean water property and circulation patterns are described, and the local signature of the large-scale 1976–1977 North Pacific climate regime shift is characterized. Changes associated with the climate shift include (1) significant warming in the upper 200– 400 m of the water column, a decrease in salinity in near-surface coastal waters, and an increase in salinity in subsurface offshore waters; (2) deeper density surfaces and increased stratification throughout the region, particularly within the Southern California Bight; (3) a subtle reorganization of the geostrophic flow structure, including an offshore shift of the California Current and increased nearshore poleward flow; and (4) a cross-shore dichotomy, with the nearshore regime changes occurring primarily during the upwelling season and the offshore variability being of lower frequency than nearer the coast. Changes in the vertical structure of the water column are of particular significance, as they have likely rendered upwelling less biologically effective. This is evident in the temperature and salinity changes, which were greatest (warming and freshening) just above the peak stability changes. There is evidence to suggest that another North Pacific-wide regime shift occurred following the 1997–1998 El Nino event, and large ecosystem changes are anticipated. The CalCOFI record is ideally suited for revealing the mechanisms of interdecadal physical–biological interactions in the coastal ocean.

Cumulative human impacts on marine predators

Stressors associated with human activities interact in complex ways to affect marine ecosystems, yet we lack spatially explicit assessments of cumulative impacts on ecologically and economically key components such as marine predators. Here we develop a metric of cumulative utilization and impact (CUI) on marine predators by combining electronic tracking data of eight protected predator species (n=685 individuals) in the California Current Ecosystem with data on 24 anthropogenic stressors. We show significant variation in CUI with some of the highest impacts within US National Marine Sanctuaries. High variation in underlying species and cumulative impact distributions means that neither alone is sufficient for effective spatial management. Instead, comprehensive management approaches accounting for both cumulative human impacts and trade-offs among multiple stressors must be applied in planning the use of marine resources.

Anticipated Effects of Climate Change on Coastal Upwelling Ecosystems

Ecosystem productivity in coastal ocean upwelling systems is threatened by climate change. Increases in spring and summer upwelling intensity, and associated increases in the rate of offshore advection, are expected. While this could counter effects of habitat warming, it could also lead to more frequent hypoxic events and lower densities of suitable-sized food particles for fish larvae. With upwelling intensification, ocean acidity will rise, affecting organisms with carbonate structures. Regardless of changes in upwelling, near-surface stratification, turbulent diffusion rates, source water origins, and perhaps thermocline depths associated with large-scale climate episodes (ENSO) maybe affected. Major impacts on pelagic fish resources appear unlikely unless couples with overfishing, although changes toward more subtropical community composition are likely. Marine mammals and seabirds that are tied to sparsely distributed nesting or resting grounds could experience difficulties in obtaining prey resources, or adaptively respond by moving to more favorable biogeographic provinces.

Impacts of the 2015–2016 El Niño on the California Current System: Early assessment and comparison to past events

The 2015–2016 El Nino is by some measures one of the strongest on record, comparable to the 1982–1983 and 1997–1998 events that triggered widespread ecosystem change in the northeast Pacific. Here we describe impacts of the 2015–2016 El Nino on the California Current System (CCS) and place them in historical context using a regional ocean model and underwater glider observations. Impacts on the physical state of the CCS are weaker than expected based on tropical sea surface temperature anomalies; temperature and density fields reflect persistence of multiyear anomalies more than El Nino. While we anticipate El Nino-related impacts on spring/summer 2016 productivity to be similarly weak, their combination with preexisting anomalous conditions likely means continued low phytoplankton biomass. This study highlights the need for regional metrics of El Ninos effects and demonstrates the potential to assess these effects before the upwelling season, when altered ecosystem functioning is most apparent.

Transport of mass, heat, salt, and nutrients in the southern California Current System: Annual cycle and interannual variability

Net fluxes of mass, heat, salt, nutrients, oxygen, and chlorophyll into a control volume within the southern California Current System (CCS) were computed from data collected on 55 cruises over a 14 year period (1984–1997). This analysis builds on an earlier work [Roemmich, 1989] by using an additional 39 cruises over 10 years, allowing for reliable estimates of the temporal variability in the fluxes on seasonal and interannual timescales and a reduction in the corresponding error budgets. A close balance was found between geostrophic convergence and Ekman divergence for the 14 year, seasonal, and interannual cruise subsets using three different wind products. Wind data taken concomitantly with the hydrographic sampling provided the best balance and hence the best flux estimates. The southern CCS was found to be a region with higher evaporation over precipitation and net heat gain by the ocean from the atmosphere (86 W m−2 in the 14 year mean) in all seasons. Significant variability in both the Ekman and geostrophic transports and the net property fluxes was found to be related to low-frequency (interpentadal and El Nino-Southern Oscillation timescale) changes in the dominant wind and circulation patterns in the CCS. Variability in primary productivity, estimated from the derived nutrient fluxes, accompanied the environmental changes. Application of this model to the ongoing data collection will further reduce the error bars on the fluxes and will allow for continued monitoring of changes in the physical and biological structure of the southern CCS.

Movement patterns for a critically endangered species, the leatherback turtle (Dermochelys coriacea), linked to foraging success and population status.

Foraging success for pelagic vertebrates may be revealed by horizontal and vertical movement patterns. We show markedly different patterns for leatherback turtles in the North Atlantic versus Eastern Pacific, which feed on gelatinous zooplankton that are only occasionally found in high densities. In the Atlantic, travel speed was characterized by two modes, indicative of high foraging success at low speeds (<15 km d−1) and transit at high speeds (20–45 km d−1). Only a single mode was evident in the Pacific, which occurred at speeds of 21 km d−1 indicative of transit. The mean dive depth was more variable in relation to latitude but closer to the mean annual depth of the thermocline and nutricline for North Atlantic than Eastern Pacific turtles. The most parsimonious explanation for these findings is that Eastern Pacific turtles rarely achieve high foraging success. This is the first support for foraging behaviour differences between populations of this critically endangered species and suggests that longer periods searching for prey may be hindering population recovery in the Pacific while aiding population maintenance in the Atlantic.