Calla Schmidt, a 2011 Delta Science Fellowship recipient, has landed a faculty position in the Department of Environmental Sciences at the University of San Francisco.
Prior to becoming an assistant professor at the Jesuit college, she was a post-doctoral researcher in the Department of Earth and Planetary Sciences at UC Santa Cruz.
Schmidt, who began her new position in January, says she enjoys the clarity of purpose of working at a small, private university and is pleased to be living in the Bay Area.
Her Delta Science Fellowship research, which is still in progress, examines the impacts of nitrogen pollution on the base of the food chain in the lower Sacramento River.
Below is a technical summary of her project to date, which California Sea Grant communications will distribute at the State of the San Francisco Estuary Conference this fall.
In-situ measurements of nutrient use by phytoplankton: impacts of nutrient loading on the base of the food web
Calla Schmidt, Delta Science Fellow 2011-2014
Scientists have observed a decline in phytoplankton productivity in the lower Sacramento River in the last decade, and it is theorized that ammonium in treated wastewater discharged into the river may be contributing to this decline. In particular, it is theorized that high levels of ammonium (NH4+) may limit the growth of larger phytoplankton (diatoms) by inhibiting nitrate (NO3-) uptake. Since these larger diatoms are relatively more important to the base of the food chain than smaller algal species, ammonium could potentially be contributing to the region’s pelagic organism decline.
The primary objectives of this project are to use stable isotope analyses to track the degree to which phytoplankton in the San Francisco Estuary absorb ammonium vs. nitrate and the sources of these nutrients (i.e., wastewater treatment plant effluent, dissolved organic matter, agricultural runoff or river inputs).
Of particular interest is whether the phytoplankton community switches from using NO3- to NH4+, as NH4+ concentrations rise, and whether diatoms are more sensitive to changes in NH4+ concentrations than smaller phytoplankton species.
The work to date has focused on developing an improved technique for analyzing stable isotope compositions of phytoplankton extracted from bulk organic matter in water samples and using the new approach to compare the isotope signatures of phytoplankton and bulk organic matter. The thought is that the organic matter (which is much easier to collect) might provide a robust proxy for extracted phytoplankton, and if this were the case, historical archives of organic matter samples could be used to reconstruct a timeline of the sources of nitrogen at the base of the food web over the course of the pelagic organism decline. The fellow’s preliminary results suggest that phytoplankton do indeed have a different isotopic composition than the bulk organic matter, and the fellow is currently analyzing additional samples to determine whether a consistent relationship between the two can be found, which would make it possible to “correct” the bulk organic isotope signature into a proxy for phytoplankton.
If no such relationship can be found (i.e., if and the bulk organic matter proves to be a poor proxy), the fellow will collect new, isolated phytoplankton samples and use the stable isotope technique to track the sources and transfer of nitrogen to the base of the food chain on these new samples.
Results from this project could have significant implications for managing nutrient pollution into the San Francisco Bay-Delta watershed and in identifying what management practices would do the most to protect the food web.
The research mentor on this project is Carol Kendall of the U.S. Geological Survey and the community mentor is Peggy Lehman of California Department of Water Resources.