Variability in Top-down Grazing Pressure on Picoplankton
Landry et al. (2023, PNAS) synthesized more than a decades’ worth of experimental protistan grazing rate measurements from CCE LTER Process cruises to investigate grazing pressure on different prokaryote populations. Contrary to the “enhanced microbial loop” hypothesis, they found that patterns of mortality were distinctly different between similarly sized organisms. More specifically, while mortality rates due to grazing increased with increasing system productivity for heterotrophic bacteria they declined for Synechococcus. Results show that grazing rates covaried with prey growth rates, suggesting a “kill-the-winner” dynamic for protistan grazers that would serve to enhance prey diversity. Such dynamics may be driven by either selective grazing by specific protists or a shift in protistan taxa along the environmental gradient.
Acidification Impacts on Iron Acquisition
By combining functional metatranscriptomics analyses with Fe incubation experiments across the CCE environmental gradient, Lampe et al. (2023, Nat. Comm.) showed that phytoplankton communities respond to short-term ocean acidification exposure by upregulating iron uptake pathways and strategies that reduce cellular iron demand. Molecular-level responses showed prioritization of iron uptake pathways that are less hindered by acidification and reductions in iron utilization, thus allowing maintenance of growth and nutrient uptake during four-day experiments.
These are likely adaptations to life in a coastal upwelling biome, where intermittent upwelling introduces carbon dioxide-rich deep waters to the surface ocean.
A Multi-decade Ocean Acidification Time-series in the CCE
Wolfe et al. (2023, Comm E&E) presented a 37-year time-series, the longest in the Pacific Ocean, of quarterly pH measurements at one of our sampling sites in the offshore portion of our domain. Results show an unambiguous long-term acidification signal along with a seasonal cycle driven by temperature and total dissolved inorganic carbon.
The Multiple Pathways of the Biological Carbon Pump
Stukel et al. (2023, Nat. Comm.) combined nearly 20-years of CCE LTER process studies with modeling syntheses to quantify organic carbon transport and storage associated with the biological carbon pump (BCP). They showed that sinking particles are the most important BCP pathway in the region and sequester 3.9 Pg C in the system. The other two important pathways are active transport by diel vertically migrating zooplankton and fish and transport of organic matter within subducted water parcels. While subduction was found to transport more organic matter out of the surface ocean than active transport, active transport actually led to more carbon sequestration because the storage depth and duration associated with migrating organisms was greater.
Trophic Control of Anchovy Boom and Bust Cycles
Swalethorp et al. (2023, Nat. Comm.) used stable isotope analysis of the amino acids in anchovy larvae to deduce the changing trophic positions of these organisms over a 45-year time series. They found that when larvae displayed a low trophic position – implying a short food chain – survivorship increased and produced boom periods of adult biomass. Conversely, when larval food chain length increased, and energy transfer efficiency decreased, the population crashed. They proposed the “trophic efficiency in early life” hypothesis that larval fishes must consume prey that confer sufficient energy for survival to help explain natural boom-bust dynamics of coastal pelagic fishes.
Persistent Fe-limitation in Subsurface Chlorophyll Maxima
Hogle et al. (2018, PNAS) utilized experimental Fe-addition experiments, biogeochemical proxies from previous CCE research, and metatranscriptomic analyses to show that ubiquitous subsurface chlorophyll maxima in the CCE are consistently Fe-limited or Fe- and light-colimited. From proxies developed from CalCOFI measurements, they also show a multi-decadal increase in the size of the subsurface Fe-limited domain in the CCE.
Ecological and Biogeochemical Impact of Cross-shore Filaments
Chabert et al. (2021, J. Geophys. Res. Oceans) used satellite altimetry, wind products, and CCE process cruise measurements to understand cross-shore flows and particle export associated with coastal upwelling filaments, They found that cross-shore organic matter flux can reconcile spatial imbalances of new production and export in the CCE region and show how transport associated with mesoscale features is linked to interannual variability in climate forcing.
Productivity in Coastal Filaments
Kranz et al. (2020, J. Geophys. Res. Oceans) conducted a comprehensive comparison of production metrics during Lagrangian experiments (following a moving frame of reference) on the CCE-P1604 and CCE-P1706 cruises (net and gross primary production, nitrate uptake, phytoplankton growth rates, net community production, sinking organic matter flux). Net community production and nitrate uptake exceeded sinking particle flux in the coastal regions of a filament but were lower than sinking particle flux in the offshore terminus of the filament as a result of cross-shore transport.
Biogeochemical Impact of Mesoscale Fronts
Stukel et al. (2017, PNAS) found that a combination of dense, Si-enriched, Fe-depleted diatoms with substantial zooplankton aggregations at a deep-water front led to extremely high vertical export of sinking particles. They showed that subduction of sinking particles contributes a substantial additional downward flux of organic matter into the deep ocean.
Conditional Top-Down Ecosystem Dynamics
Lindegren et al. (2018, Global Change Biol.) analyzed ~60 years of CCE, CalCOFI, and related population data to search for evidence of changing methods of pelagic ecosystem regulation during different climate states. Their nonlinear threshold models uncovered a particularly interesting novel pattern (that we term “Conditional Top-Down” forcing), where top-down predator control is expressed only during periods of weak upwelling, low nutrient concentrations, and low primary production.
El Niño Theme Issue
In a series of 5 papers introduced by Ohman (2018, Deep-Sea Res. I), these CCE authors show that the 2014-15 marine heat wave and subsequent 2015-2016 El Niño decreased the prevalence of oceanic fronts, surface chlorophyll, net primary productivity, and sinking organic matter export in the CCE. They also investigate the stationarity of functional responses between trophic levels and biogeochemistry (i.e., testing the “space-for-time exchange” hypothesis) and showed that El Niño events have a consistently strong impact on zooplankton biomass and composition and carbon export, establishing key relationships that form the basis for predictive ENSO modeling.
El Niño Impacts on Euphausiids
Lilly & Ohman (2021, Progress in Oceanography) utilized the unparalleled CalCOFI/CCE zooplankton time series to understand the effects of two major types of El Niño on spatial displacements of euphausiids (krill) in the CCE region. The study spans ~70 years of measurements, including 7 El Niños and the marine heatwave of 2014-15 to understand the CCE disturbance regime. The authors found differential effects of Eastern Pacific and Central Pacific El Niños on euphausiid habitat usage in the CCE region related to the biogeographic habitats of the animals and provide a framework for predicting future impacts of different types of El Niño.
Phenology of Blue Whale Migrations
Szesciorka et al (2020, Scientific Reports) documented a decadal-scale trend in the timing of arrival of blue whales in their CCE feeding grounds. This earlier arrival is linked to warming waters and a time-lagged response to the biomass of euphausiids, the blue whales’ exclusive prey in this region. The result has implications for conservation strategies, as the blue whales are exposed to increased anthropogenic threats at their feeding grounds.
Physical Drivers of Marine Heatwaves (MHW)
The 2019 MHW generated widespread concern among fishery and wildlife managers for sensitive marine ecosystems along the west coast of North America. Amaya et al. (2020, Nat. Comm.) showed that this MHW primarily resulted from a prolonged weakening of the North Pacific High-Pressure System, which reduced surface winds and decreased evaporative cooling and wind-driven upper ocean mixing. Warmer ocean conditions then reinforce the MHW through a positive low-cloud feedback.
Variability in Water Mass Contributions to Upwelled Nutrients
Bograd et al. (2019, Geophys. Res. Lett.) analyzed historical hydrographic data to quantify the variability of water mass contributions to the CCE region from the Subarctic, Subtropical, and Eastern Tropical Pacific. This analysis established that each of the major source waters has its own combination of temperature, salinity, nutrient, and O2 signatures that can be identified and quantified. This paper underscores the influence of remote source waters and the importance for CCE of oceanic changes occurring far afield in the larger Pacific Ocean.