Award: OCE-1260692

Intellectual Merit The ocean has a "biological pump" which acts to remove carbon dioxide (CO2) from surface waters through photosynthesis, and export it, primarily as particulate organic carbon that sinks into the deep ocean. This activity sequesters the carbon into the deep ocean where it can no longer mix with the pool in the atmosphere. Consequently, understanding the details of these dynamics is important in predicting how our atmosphere, and its impact in regulating climate, will respond to perturbations in atmospheric CO2 that are being caused by human activities. Only a fraction of the carbon fixed through photosynthesis is exported, as most is recycled by organisms that metabolize the photosynthetic carbon. The fraction that is escapes metabolism is often described as 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 test a classic question, which is: Do surface ocean ecosystems adjust themselves to maintain a constant ratio of net and gross production (N/G), so that a constant fraction of the carbon fixed will be recycled regardless of the supply of nutrients, OR does the ratio of N/G vary with supply of nutrients in these upwelling areas? This question was approached by making time-series measurements of in-situ (naturally occuring) tracers for both upwelling and production (net and gross) at a nearshore site (SPOT), in the San Pedro Channel between Los Angeles and Catalina Island (Fig. 1). Data was collected during the upwelling seasons (Jan-June) of 2013 and 2014. These tracers integrate over the timescale of weeks, the same timescale as biological bloom evolution. Tracers used include the following isotopes: 7Be balance to determine upwelling; 18O/16O and 17O/16O to determine gross production; 234Th balances to determine export of particulate carbon (net production). A model that incorporated the time-varying dynamics of upwelling was developed for interpreting the data. This model offers a framework for evaluating the ratio of N/G in a non-steady state system, as it responds to changes in upwelling supply of nutrients. This model was applied to the SPOT time series station to estimate the N/G ratio as a function of time and the average for an annual cycle. The pattern observed (Fig. 2 presents these as equivalent O2 dynamics) indicated a time-varying ratio due to the non-steady state community dynamics during the spring bloom of each year. The ratio of (export)/(gross production) increases to high values early in the bloom, and then decreases as the bloom continues. 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. As the bloom proceeds, metablolism of the consumer groups increases, due to biomass increases and/or warmer temperatures. This phasing of the biology causes a large fraction of the particulate carbon to rain to the deep sea, early in the bloom; but at later times, most the particulate carbon produces is consumed much faster than it can settle out, causing N/G to decrease. To our knowledge, this is the first study that has been able to quantitatively examine this ratio, indicating that the behavior of the biota in this type of setting has a variable efficiency for exporting organic carbon as it is produced in a bloom event. These results offer insight into N/G ratios that may occur in similar settings. Broader Impacts At USC, five PhD students completed dissertations that utilized the shiptime and sampling opportunities provided by this research. One undergraduate completed a Senior Thesis project using training and funding from the project, and a second undergraduate received valuable training. All of these students have (or plan to) continued on in the field, offering a welcome addition to the intellectual resources of our society. Additional students from Pomona College, our partner institution in this study, also received training. 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: 01/15/2018 Submitted by: Douglas E Hammond