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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2013)

Fixed nitrogen (N) limits phytoplankton growth throughout the low latitude ocean. The oceanic reservoir of this essential nutrient is therefore a critical determinant of the fertility of the ocean as a whole, and the strength of the biological pump that sequesters carbon in deep waters out of contact with the atmosphere. Two biological processes dominate the ocean's N budget: a source of N through N2 fixation by diazotrophic plankton, and a sink through bacterial denitrification when oxygen is depleted. These processes are subject to external climate-related forces that push the N budget out of balance, but also coupled through internal feedbacks that strive to restore a balance and stabilize the N reservoir. The feedback mechanism can be thought to operate like a "nutrient thermostat", in which the nitrogen-to-phosphorous (N:P) requirement of phytoplankton represents a setpoint towards which the ratio of N and P reservoirs is restored. This dissertation comprises four projects that address different aspects of the general question: how flexible is the oceanic N reservoir?

Chapters 2 and 3 examine the flexibility of the setpoint towards which the N reservoir is restored. In Chapter 2, a diagnostic model is used to show that the N:P of nutrient drawdown by marine phytoplankton, long considered to have a universal "Redfield ratio" of 16:1, varies significantly at the scale of marine biomes due to stoichiometric diversity between taxa. Thus the mean N:P of marine phytoplankton, previously thought of as a fundamental biological property, only reflects the balance of high and low N:P biomes under modern-ocean conditions. Chapter 3 explores the role of this variability in the nutrient thermostat feedbacks. An ecosystem model is used to show that the ratio of nutrient reservoirs it is biased upwards towards the high N:P requirements of subtropical phytoplankton that cohabit and compete directly with N2-fixers. This resolves a discrepancy between the observed N reservoir and the predictions of "Redfieldian" models, which lose too much N before reaching equilibrium. We also demonstrate an important role in the nutrient thermostat for ocean circulation, which communicates stoichiometric signatures between biomes. Changes in the N reservoir may thus be driven, over millennial timescales, by a restructuring of plankton biomes and the circulation pathways that connect them.

Chapters 4 and 5 examine the potential for variations in the N reservoir via direct forcing of the oceanic N budget. Chapter 4 focuses on the process of N2-fixation, which may be limited either by an external supply of Fe through the atmosphere, or the internal generation of N deficits that allow diazotrophs to compete with faster-growing plankton, which are susceptible to N-limitation. Multiple geochemical constraints are used to show that N2-fixation in the modern ocean is limited by Fe inputs at the community scale, but strongly coupled to N loss through denitrification at the basin scale. These results reconcile biochemical evidence for Fe-stress in diazotroph communities with the existence of regulatory feedbacks that maintain a balanced the N budget. Within the regime of intermediate Fe-control, the oceanic N reservoir would respond weakly to the enhanced dust fluxes hypothesized for a glacial climate, but strongly to the reduced fluxes expected under climate warming.

Chapter 5 uses a biogeochemical model to derive a simple framework for understanding the strength of N cycle feedbacks. A feedback response to any N-budget forcing comprises two components: a fast initial adjustment period involving only the upper ocean, in which changes in the N reservoir are minor, and a slow millennial response involving deep ocean circulation. The magnitude of changes in the N reservoir depend on the fraction of forced anomalies that "leak" into the deep ocean, and the timescale over which those deep-ocean anomalies are removed. Feedbacks through N2-fixation, rather than denitrification, are most efficient at preventing leakage and removing deep-ocean anomalies, and are strong enough to prevent major perturbation to the N reservoir, unless Fe shortages confine diazotrophs to small regions of the ocean.

    Cover page: Biological and physical regulation of the oceanic fixed nitrogen reservoir
    • Article
    • Peer Reviewed
    Faculty Publications (2012)
    The conversion of dissolved nutrients and carbon to organic matter by phytoplankton in the surface ocean, and its downward transport by sinking particles, produces a “biological pump” that reduces the concentration of atmospheric CO2. Global rates of organic matter export are a poor indicator of biological carbon storage however, because organic matter gets distributed across water masses with diverse pathways and timescales of return to the surface. Here we show that organic matter export and carbon storage can be related through a sequestration efficiency, which measures how long regenerated nutrients and carbon will be stored in the interior ocean before being returned to the surface. For the first time, we derive global maps of the sequestration efficiency of the biological pump at different residence time horizons. These maps reveal how regional patterns of organic matter export contribute to the biological pump, and how the biological pump responds to changes in biological productivity driven by climate change.
      Cover page: The sequestration efficiency of the biological pumpCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2012)
      Biologically available nitrogen (N) limits phytoplankton growth over much of the ocean. The rate at which N is removed from the contemporary ocean by denitrifying bacteria is highly uncertain 1-3. Some studies suggest that N losses exceed inputs 2,4-6; others argue for a balanced budget 3,7,8. Here, we use a global ocean circulation model to simulate the distribution of N 2 gas produced by denitrifying bacteria in the three main suboxic zones in the open ocean. By fitting the model to measured N 2 gas concentrations, we infer a globally integrated rate of water-column denitrification of 66 ±6 Tg N yr -1. Taking into account isotopic constraints on the fraction of denitrification occurring in the water column versus marine sediments, we estimate that the global rate of N loss from marine sediments and the oceanic water column combined amounts to around 230 ±60 Tg N yr -1. Given present estimates of N input rates, our findings imply a net loss of around 20 ± 70 Tg of N from the global ocean each year, indistinguishable from a balanced budget. A balanced N budget, in turn, implies that the marine N cycle is governed by strong regulatory feedbacks. © 2012 Macmillan Publishers Limited. All rights reserved.
        Cover page: Global rates of water-column denitrification derived from nitrogen gas measurementsCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2018)
          Cover page: Biogeochemical Role of Subsurface Coherent Eddies in the Ocean: Tracer Cannonballs, Hypoxic Storms, and Microbial Stewpots?
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2024)
          The ability to anticipate marine habitat shifts responding to climate variability has high scientific and socioeconomic value. Here we quantify interannual-to-decadal predictability of habitat shifts by combining trait-based aerobic habitat constraints with a suite of initialized retrospective Earth System Model forecasts, for diverse marine ecotypes in the North American Large Marine Ecosystems. We find that aerobic habitat viability, defined by joint constraints of temperature and oxygen on organismal energy balance, is potentially predictable in the upper-600 m ocean, showing a substantial improvement over a simple persistence forecast. The skillful multiyear predictability is dominated by the oxygen component in most ecosystems, yielding higher predictability than previously estimated based on temperature alone. Notable predictability differences exist among ecotypes differing in temperature sensitivity of hypoxia vulnerability, especially along the northeast coast with predictability timescale ranging from 2 to 10 years. This tool will be critical in predicting marine habitat shifts in face of a changing climate.
            Cover page: Skillful multiyear prediction of marine habitat shifts jointly constrained by ocean temperature and dissolved oxygen.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2024)
            A key control on the magnitude of coastal eutrophication is the degree to which currents quickly transport nitrogen derived from human sources away from the coast to the open ocean before eutrophication develops. In the Southern California Bight (SCB), an upwelling-dominated eastern boundary current ecosystem, anthropogenic nitrogen inputs increase algal productivity and cause subsurface acidification and oxygen (O 2 ) loss along the coast. However, the extent of anthropogenic influence on eutrophication beyond the coastal band, and the physical transport mechanisms and biogeochemical processes responsible for these effects are still poorly understood. Here, we use a submesoscale-resolving numerical model to document the detailed biogeochemical mass balance of nitrogen, carbon and oxygen, their physical transport, and effects on offshore habitats. Despite management of terrestrial nutrients that has occurred in the region over the last 20 years, coastal eutrophication continues to persist. The input of anthropogenic nutrients promote an increase in productivity, remineralization and respiration offshore, with recurrent O 2 loss and pH decline in a region located 30-90 km from the mainland. During 2013 to 2017, the spatially averaged 5-year loss rate across the Bight was 1.3 mmol m -3 O 2 , with some locations losing on average up to 14.2 mmol m -3 O 2 . The magnitude of loss is greater than model uncertainty assessed from data-model comparisons and from quantification of intrinsic variability. This phenomenon persists for 4 to 6 months of the year over an area of 278,40 km 2 ( ∼ 30% of SCB area). These recurrent features of acidification and oxygen loss are associated with cross-shore transport of nutrients by eddies and plankton biomass and their accumulation and retention within persistent eddies offshore within the SCB.
              Cover page: Cross-shore transport and eddies promote large scale response to urban eutrophicationCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2020)
              Climate warming is expected to intensify hypoxia in the California Current System (CCS), threatening its diverse and productive marine ecosystem. We analyzed past regional variability and future changes in the Metabolic Index (Φ), a species-specific measure of the environment's capacity to meet temperature-dependent organismal oxygen demand. Across the traits of diverse animals, Φ exhibits strong seasonal to interdecadal variations throughout the CCS, implying that resident species already experience large fluctuations in available aerobic habitat. For a key CCS species, northern anchovy, the long-term biogeographic distribution and decadal fluctuations in abundance are both highly coherent with aerobic habitat volume. Ocean warming and oxygen loss by 2100 are projected to decrease Φ below critical levels in 30 to 50% of anchovies' present range, including complete loss of aerobic habitat-and thus likely extirpation-from the southern CCS. Aerobic habitat loss will vary widely across the traits of CCS taxa, disrupting ecological interactions throughout the region.
                Cover page: Climate-driven aerobic habitat loss in the California Current System.
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2022)
                Carbonate mud represents one of the most important geochemical archives for reconstructing ancient climatic, environmental, and evolutionary change from the rock record. Mud also represents a major sink in the global carbon cycle. Yet, there remains no consensus about how and where carbonate mud is formed. Here, we present stable isotope and trace-element data from carbonate constituents in the Bahamas, including ooids, corals, foraminifera, and algae. We use geochemical fingerprinting to demonstrate that carbonate mud cannot be sourced from the abrasion and mixture of any combination of these macroscopic grains. Instead, an inverse Bayesian mixing model requires the presence of an additional aragonite source. We posit that this source represents a direct seawater precipitate. We use geological and geochemical data to show that "whitings" are unlikely to be the dominant source of this precipitate and, instead, present a model for mud precipitation on the bank margins that can explain the geographical distribution, clumped-isotope thermometry, and stable isotope signature of carbonate mud. Next, we address the enigma of why mud and ooids are so abundant in the Bahamas, yet so rare in the rest of the world: Mediterranean outflow feeds the Bahamas with the most alkaline waters in the modern ocean (>99.7th-percentile). Such high alkalinity appears to be a prerequisite for the nonskeletal carbonate factory because, when Mediterranean outflow was reduced in the Miocene, Bahamian carbonate export ceased for 3-million-years. Finally, we show how shutting off and turning on the shallow carbonate factory can send ripples through the global climate system.
                  Cover page: The origin of carbonate mud and implications for global climate
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2021)
                  Global change is leading to warming, acidification, and oxygen loss in the ocean. In the Southern California Bight, an eastern boundary upwelling system, these stressors are exacerbated by the localized discharge of anthropogenically enhanced nutrients from a coastal population of 23 million people. Here, we use simulations with a high-resolution, physical-biogeochemical model to quantify the link between terrestrial and atmospheric nutrients, organic matter, and carbon inputs and biogeochemical change in the coastal waters of the Southern California Bight. The model is forced by large-scale climatic drivers and a reconstruction of local inputs via rivers, wastewater outfalls, and atmospheric deposition; it captures the fine scales of ocean circulation along the shelf; and it is validated against a large collection of physical and biogeochemical observations. Local land-based and atmospheric inputs, enhanced by anthropogenic sources, drive a 79% increase in phytoplankton biomass, a 23% increase in primary production, and a nearly 44% increase in subsurface respiration rates along the coast in summer, reshaping the biogeochemistry of the Southern California Bight. Seasonal reductions in subsurface oxygen, pH, and aragonite saturation state, by up to 50 mmol m-3, 0.09, and 0.47, respectively, rival or exceed the global open-ocean oxygen loss and acidification since the preindustrial period. The biological effects of these changes on local fisheries, proliferation of harmful algal blooms, water clarity, and submerged aquatic vegetation have yet to be fully explored.
                    Cover page: Coastal eutrophication drives acidification, oxygen loss, and ecosystem change in a major oceanic upwelling system.
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2019)
                    A copy-number variant (CNV) of 16p11.2 encompassing 30 genes is associated with developmental and psychiatric disorders, head size, and body mass. The genetic mechanisms that underlie these associations are not understood. To determine the influence of 16p11.2 genes on development, we investigated the effects of CNV on craniofacial structure in humans and model organisms. We show that deletion and duplication of 16p11.2 have "mirror" effects on specific craniofacial features that are conserved between human and rodent models of the CNV. By testing dosage effects of individual genes on the shape of the mandible in zebrafish, we identify seven genes with significant effects individually and find evidence for others when genes were tested in combination. The craniofacial phenotypes of 16p11.2 CNVs represent a model for studying the effects of genes on development, and our results suggest that the associated facial gestalts are attributable to the combined effects of multiple genes.
                      Cover page: Oligogenic Effects of 16p11.2 Copy-Number Variation on Craniofacial Development
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