Award: OCE-1260296

Intellectual Merit The ocean regulates the atmospheric CO2 through several mechanisms, including a ?biological pump? ? the process of photosynthetic primary production that removes CO2 from surface and, converting it into organic matter, exports this carbon to deeper parts of the ocean, removing it from the atmosphere for thousand of years. Understanding the details of these dynamics is important in predicting the response of global ocean to perturbations in atmospheric CO2 that are being caused by human activities: close to a third of anthropogenic CO2 has been sequestered by the oceans yet the capacity of the oceans to further sequestration is unknown. Only a fraction of the carbon fixed through photosynthesis is exported, as most is rapidly recycled in the surface ocean. The exported fraction escaping in situ recycling determines the efficiency of the biological pump. This efficiency is often quantified as Net to Gross production ratio, N/G (= net carbon fixed and exported)/gross rate of photosynthesis). While the N/G ratio at open ocean sites has been measured previously, little work has been done in the coastal zone, where seasonal upwelling facilitates episodic algal blooms, resulting in globally significant carbon export. The major goal of the project has been to address a classic question: does the ratio of net (exported) and gross (photosynthetically fixed) carbon remains constant, being defined by some intrinsic biochemistry of marine primary producers, or it varies depending on the growth conditions, particularly, the supply of nutrients required by the phytoplankton for growth. This question was approached by making time-series measurements of in-situ (naturally occurring) tracers for both upwelling and production (net and gross) at a nearshore site (SPOT, San Pedro Ocean Time-Series), in the San Pedro Channel between Los Angeles and Catalina Island. Data was collected during the upwelling seasons (Jan-June) of 2013 and 2014. Tracers used include the following isotopes: 7Be balance to determine upwelling; 18O/16O and 17O/16O ratios to determine gross production; 234Th balances and short-term floating sediment traps to determine export of particulate carbon (net production), as well as O2/Ar ratios that reflect net biological O2 production. These tracers integrate over the timescale of weeks, the same timescale as biological bloom evolution. A model that incorporated the time-varying dynamics of upwelling was developed for interpreting the data to account for the transport terms critical in the oxygen budgets of the upwelling regimes. The main finding of the study was a clear temporal trend in Net and Gross Production, and the decoupling of the N/G ratios from the two productivity metrics, likely driven by the non-steady state dynamics of the spring bloom (Fig. 1 presents the productivity in O2 metric, as NOP and GOP, net and gross O2 production respectively). The maximum export efficiency, quantified as NOP/GOP ratios, occurred early in the bloom during both years of observation, even preceding the maximum in Net Production. The Gross Production ?catches up? later in the bloom, consistent with increasing contribution of biological recycling as the bloom progresses. While several possible mechanisms may explain this behavior, it is likely that this represents rapid early growth by diatoms, outstripping the ability of zooplankton and respiring bacteria to keep pace by metabolizing the organic carbon produced. To our knowledge, this is the first study to quantitatively examine the N/G ratio, demonstrating the role of a non-steady state bloom environment in regulating the biological carbon pump efficiency in a coastal upwelling setting. We also completed two seasons of continuous observations of dissolved O2, using an autonomous buoyancy-driven Slocum glider. Glider deployments largely overlapped with the timing of discrete cruises-based observations in San Pedro Channel. Using empirical relationship derived using discrete measurements of O2/Ar in the surface mixed layer, we were able to remove the influence of bubble injection and calculate near-continuous NOP rates (Fig. 2). Using estimates of vertical transport from wind-speed based parameterizations, validated by the results obtained in the earlier phases of this project, we were able to correct for the physical biases, which are known to significantly influence dissolved oxygen budgets in this region. The glider-based NOP agreed well with our discrete estimates, but also revealed higher-frequency variability that discrete sampling was unable to resolve, suggesting that this approach may be useful in other regions with well-constrained vertical transport rates. Broader Impacts At Pomona College, four undergraduate student research projects were supported by this grant. One undergraduate completed a senior thesis using training and funding from the project, and three students conducted summer research. Three students participated in cruises, getting hands-on experience and valuable training in oceanographic research. A postbac was partially supported with the funds, participating in cruises and analytical work. The scientific results add important insights that will be of use to those modeling the behavior of CO2 in the atmosphere, an essential approach for predicting the impact of anthropogenic CO2 on climate. Last Modified: 05/01/2019 Submitted by: Maria Prokopenko