Current Research
The Ecology and Oceanography of Harmful Algal Blooms (ECOHAB) and Prevention, Control and Mitigation of Harmful Algal Blooms Program (PCMHAB)
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Summary:
The main objective of the project is to develop an integrated modeling system, carefully validated against available observations, to investigate the natural and anthropogenic drivers of Pseudo-nitzschia (PN) produced Harmful Algal Blooms (HABs) in the southern California Current System (CCS). We have two approaches: 1. Create a mechanistic model on domoic acid production, and 2. Predict when a HAB event will occur along the U.S. Western Coast and its severity. Project 1: The specific projects goals can be summarized as follows: (1) development of an end-to-end predictive capacity for the prediction of the PN-derived toxin, domoic acid (DA), in the southern CCS, evaluated against observations; (2) apply and use our integrated model to investigate the relative importance of anthropogenic inputs and other potential drivers on the frequency and severity of PN HAB events in the southern CCS; and (3) provide our findings to coastal zone managers to improve marine resource management and pollution control. Project 2: My goal is utilize machine learning techniques to predict when a HAB event will occur. My pipeline is relatively simple: (1) Quantify the presence or absence of a HAB events based on observations; (2) Determine which environmental parameters result in a HAB event; and (3) Utilize a regression random forest to predict the HAB concentration when an event does occur. |
Predicting Ocean Deoxygenation from Carbon-to-Oxygen Respiration Quotient
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Summary:
Models are our primary instrument for predicting future ocean deoxygenation. Current climate models assume a constant respiration quotient, but as demonstrated by my work respiration quotient is variable across ocean regions with predictable environmental relationships. This variability will likely have a significant impact on future subsurface oxygen concentrations 13. I postulate that the inclusion of these new observations will result in more accurate predictions of ocean deoxygenations in a future climate. The research will assess two main questions: RQ1) What is the impact of a variable respiration quotient on subsurface oxygen across different ocean biomes, in particular in hypoxic regions? RQ2) What is the impact of a variable respiration quotient on the progression of ocean deoxygenation under anthropogenic climate change? To address these, I will develop an ocean biogeochemical model that determines the impact of respiration quotient on the distribution of dissolved oxygen and hypoxic regions at present and future ocean temperatures. |
Dissertation Research
Latitudinal gradient in the carbon-to-oxygen respiration quotient and the implications for ocean oxygen availability
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Abstract:
Climate-driven depletion of ocean oxygen strongly impacts the global cycles of carbon and nutrients as well as the survival of many animal species. One of the main uncertainties in predicting changes to ocean oxygen levels is the regulation of the biological respiration demand associated with the biological pump. Derived from the Redfield ratio, the molar ratio of oxygen to organic carbon consumed during respiration (i.e., the respiration quotient,) is consistently assumed constant but rarely, if ever, measured. Using a prognostic Earth system model, we show that a 0.1 increase in the respiration quotient from 1.0 leads to a 2.3% decline in global oxygen, a large expansion of low oxygen zones, additional water column denitrification of 38 Tg N/yr, and overall an loss of fixed nitrogen and carbon production in the ocean. We then present direct chemical measurements of using a Pacific Ocean meridional transect crossing all major surface biome types. The observed has a positive correlation with temperature, and regional mean values differ significantly from Redfield proportions. Finally, an independent global inverse model analysis constrained with nutrients, oxygen, and carbon concentrations support a positive temperature dependence of in exported organic matter. We provide evidence against the common assumption of a static biological link between the respiration of organic carbon and the consumption of oxygen. Furthermore, the model simulations suggest that a changing respiration quotient will impact multiple biogeochemical cycles, and that future warming can lead to more intense deoxygenation than previously anticipated. |
ENSO Event Impacts California Coastal Elemental Stoichiometric Ratios and Macronutrient Concentrations
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Abstract:
El Niño Southern Oscillation (ENSO) influences multi-year variation in sea-surface temperature and the intensity of upwelling in many Pacific regions. However, it is currently unknown how El Niño conditions will affect the concentration and elemental ratios of particulate organic matter (POM). To investigate this, we have quantified POM weekly for 6 years (2012 to 2017) at the MICRO time-series station in the Southern California Bight. We found a strong influence of the 2015 El Niño on sea-surface temperature and phosphate concentration but to a lesser extent on nitrate availability. The 2015 El Niño also resulted in a short-term depression in POC and POP concentrations, whereas PON concentrations displayed an independent long-term decline regardless of the El Niño event. Reduced POM concentrations resulting from the 2015 El Niño occurred in parallel to high C:P and N:P ratios. Following the changes in PON, C:N continued to climb reaching ∼9.4 at the end of our sampling period. We suggest that an Eastern Pacific- vs. a Central Pacific-type El Niño as well as a switch in the Pacific Decadal Oscillation phase significantly altered the local response in POM concentrations and ratios. |
Phytoplankton Stoichiometry Mediates Nonlinear Interactions between Phosphorus and Atmospheric CO2
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Abstract:
Marine phytoplankton stoichiometry links nutrient supply to marine carbon export. Deviations of phytoplankton stoichiometry from Redfield proportions (106C : 1P) could therefore have a significant impact on carbon cycling, and understanding which environmental factors drive these deviations may reveal new mechanisms regulating the carbon cycle. To explore the links between environmental conditions, stoichiometry, and carbon cycling, we compared four different models of phytoplankton C : P: a fixed Redfield model, a model with C : P given as a function of surface phosphorus concentration (P), a model with C P given as a function of temperature, and a new multi-environmental model that predicts C : P as a function of light, temperature, and P. These stoichiometric models were embedded into a five-box ocean circulation model, which resolves the three major ocean biomes (high-latitude, subtropical gyres, and tropical upwelling regions). Contrary to the expectation of a monotonic relationship between surface nutrient drawdown and carbon export, we found that lateral nutrient transport from lower C : P tropical waters to high C : P subtropical waters could cause carbon export to decrease with increased tropical nutrient utilization. It has been hypothesized that a positive feedback between temperature and pCO2, atm will play an important role in anthropogenic climate change, with changes in the biological pump playing at most a secondary role. Here we show that environmentally driven shifts in stoichiometry make the biological pump more influential, and may reverse the expected positive relationship between temperature and pCO2, atm. In the temperature-only model, changes in tropical temperature have more impact on the Δ pCO2, atm (∼ 41 ppm) compared to subtropical temperature changes (∼ 4.5 ppm). Our multi-environmental model predicted a decline in pCO2, atm of ∼ 46 ppm when temperature spanned a change of 10 °C. Thus, we find that variation in marine phytoplankton stoichiometry and its environmental controlling factors can lead to nonlinear controls on pCO2, atm, suggesting the need for further studies of ocean C : P and the impact on ocean carbon cycling. |
Ecological Stoichiometry of Ocean Plankton
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Abstract:
Marine plankton elemental stoichiometric ratios can deviate from the Redfield ratio (106C:16N:1P); here, we examine physiological and biogeochemical mechanisms that lead to the observed variation across lineages, regions, and seasons. Many models of ecological stoichiometry blend together acclimative and adaptive responses to environmental conditions. These two pathways can have unique molecular mechanisms and stoichiometric outcomes, and we attempt to disentangle the two processes. We find that interactions between environmental conditions and cellular growth are key to understanding stoichiometric regulation, but the growth rates of most marine plankton populations are poorly constrained. We propose that specific physiological mechanisms have a strong impact on plankton and community stoichiometry in nutrient-rich environments, whereas biogeochemical interactions are important for the stoichiometry of the oligotrophic gyres. Finally, we outline key areas with missing information that is needed to advance understanding of the present and future ecological stoichiometry of ocean plankton. |
Undergraduate Research
The Impact of Fish and the Commercial Marine Harvest on the Ocean Iron Cycle
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Abstract:
Although iron is the fourth most abundant element in the Earth's crust, bioavailable iron limits marine primary production in about one third of the ocean. This lack of iron availability has implications in climate change because the removal of carbon dioxide from the atmosphere by phytoplankton requires iron. Using literature values for global fish biomass estimates, and elemental composition data we estimate that fish biota store between 0.7–7×10^11 g of iron. Additionally, the global fish population recycles through excretion between 0.4–1.5×10^12 g of iron per year, which is of a similar magnitude as major recognized sources of iron (e.g. dust, sediments, ice sheet melting). In terms of biological impact this iron could be superior to dust inputs due to the distributed deposition and to the greater solubility of fecal pellets compared to inorganic minerals. To estimate a loss term due to anthropogenic activity the total commercial catch for 1950 to 2010 was obtained from the Food and Agriculture Organization of the United Nations. Marine catch data were separated by taxa. High and low end values for elemental composition were obtained for each taxonomic category from the literature and used to calculate iron per mass of total harvest over time. The marine commercial catch is estimated to have removed 1–6×10^9 g of iron in 1950, the lowest values on record. There is an annual increase to 0.7–3×10^10 g in 1996, which declines to 0.6–2×10^10 g in 2010. While small compared to the total iron terms in the cycle, these could have compounding effects on distribution and concentration patterns globally over time. These storage, recycling, and export terms of biotic iron are not currently included in ocean iron mass balance calculations. These data suggest that fish and anthropogenic activity should be included in global oceanic iron cycles. |