Formation of preformed nitrate anomalies at the subtropical ocean time-series sites

The subtropical oceans, the so-called “ocean deserts”, maintain surface ocean biological productivity and the biological carbon pump throughout the annual cycle, in excess of that which could be supported by known nutrient fluxes to the system. This represents a large gap in our scientific understanding and ability to predict future changes for regions that are responsible for nearly half of all ocean productivity. Nutrient, carbon, and oxygen cycling in these regions also exhibit significant anomalies with respect to their assumed proportions in ocean biomass and metabolisms. We investigated the formation rates of oxygen to nitrogen anomalies in the subtropical North Pacific and North Atlantic using time-series data and provide a quantitative analysis of the potentially contributing biological mechanisms. We found that biological production and consumption patterns of organic matter at proportions enriched in carbon and depleted in nutrient content, as well as bacterial uptake of nutrients at depth to consume nutrient depleted organic matter, can explain a portion of the observed anomalies and their formation rates. However >50% of the anomaly formation remains unexplained by these mechanisms. We conclude that vertically migrating phytoplankton, which traverse ~100–150 meters vertically in the upper ocean over days to acquire nutrients from waters at depth returning to the high light waters at the surface for photosynthesis, possess the required biogeochemical signature and abundance to likely explain the observed anomaly formation rates and sustain surface ocean productivity throughout the annual cycle. Phytoplankton vertical migrators, although rare and easily overlooked, may play a large role in subtropical ocean nutrient cycling and the biological carbon pump.



Modeling Picoplankton with the CESM

An ongoing project involves development of the Community Earth System Model and the Energy Exascale Earth System Model to include representation of the picoplankton within the ocean ecosystem component. I’m adding the Prochlorococcus, Synechococcus, and pico-eukaryotes to join the nano-plankton, diatoms, and diazotrophs phytoplankton groups in these climate models. Each group will have varying carbon, phosphorus, and iron quotas which will allow for a more realistic assessment of ocean biology’s role in and response to 21st century climate change.


Lateral Nutrient Transport to the Subtropical Gyres

A recent study quantified the important role that the 3D ocean circulation and the lateral transfer of both organic and inorganic nutrients have on sustaining productivity in the remote “ocean deserts” of the central ocean gyres. The coupling of DON and DOP cycling model output with new tracer transport diagnostics from an offline ocean circulation model have allowed quantification of organic nutrient utilization rates in the subtropical gyres for the first time. Biological uptake of laterally supplied N and P dominates the nutrient budgets in the gyres and help explain seasonal carbon drawdown and nitrogen fixation patterns at the time-series sites. The results are published in the November 2016 issue of Nature Geoscience.



What Controls Marine DOC Production?

In a study published in Proceedings of the National Academy of Sciences, my colleagues Cristina Romera-Castillo, Dennis Hansell and I investigated the ultimate control on the biological production of marine dissolved organic carbon in the surface Atlantic ocean. We found that the introduction of new nitrogen, mostly in the form of nitrate, to the euphotic zone is the largest single determinant of marine DOC concentrations in the upper ocean. Any climate change related reductions in new nutrient input to the surface ocean will control the source term for this important component of the ocean carbon cycle.



Biogeochemical Modeling of Marine DOM

I’ve used data assimilation in a numerical modeling framework to investigate the many facets of DOM biogeochemistry. One study examined the differential recycling rates of dissolved organic phosphorus, nitrogen, and carbon and how this stoichiometry influences the spatial distribution of important global biogeochemical processes such as primary productivity, nitrogen fixation, and carbon export from the surface ocean. Dissolved organic phosphorus was found to have the fastest recycling rates with its cycling most important for shaping the spatial pattern of ocean productivity. See Letscher et al 2015 in Biogeociences and Letscher et al 2015 in Global Biogeochemical Cycles.



Global Dissolved Organic Nitrogen Biogeochemistry

One study examined a linkage between the marine nitrogen and carbon cycles by investigating the mechanisms by which dissolved organic nitrogen (DON) is recycled, shedding light on how DON can provide a nutrient source of nitrogen to sustain a portion of open ocean biological productivity. DON was found to be largely recalcitrant to biological consumption near the surface, instead requiring its transport to mid-depths where a small portion is remineralized releasing inorganic N back into the water column. See Letscher et al 2013 in Global Biogeochemical Cycles and a News & Views by Voss & Hietanen in Nature



Tracing the Fate of Terrigenous DOM in the Arctic Ocean

The Arctic region as a whole is undergoing rapid change as a result of climate warming including an increasing transfer of land-derived organic material to the Arctic Ocean via rivers. Previous studies of mine investigated the fate of this terrestrial organic carbon and nitrogen upon delivery to the Siberian shelf seas including its rate of degradation and the consequences of these processes on dampening the Arctic Ocean’s capacity for absorbing atmospheric CO2 (in the case of carbon) or recycling nutrients to sustain local productivity (for nitrogen) See Letscher et al 2011; 2013 in Marine Chemistry.



Ongoing Work

I am also actively working towards better understanding the environmental conditions and mechanisms that allow for the consumption or persistence of marine DOM for both the carbon and nitrogen pools from a combination of observational and numerical modeling approaches. One such project is testing hypotheses on refractory DOC removal in the deep ocean by building an OCIM of DOC-radiocarbon. One current collaboration is applying the inverse method to solve for the temperature sensitivity of microbial DOC decomposition in the deep ocean, which will inform on the fate of one of the largest pools in the global carbon cycle under the changing climate. Another collaboration is applying regional ocean circulation patterns to interpret rates of nitrous oxide (an important greenhouse gas) production and consumption within the Peru Current system from nitrogen stable isotope data.