MULTI-SCALE CHANGES IN DISSOLVED AND PARTICULATE CARBON AND NITROGEN DYNAMICS THROUGHOUT THE SAN FRANCISCO BAY AND SACRAMENTO-SAN JOAQUIN DELTA

Bulk particulate organic matter (POM) composition in estuaries can be a complex mixture of sources (phytoplankton, microzooplankton, bacteria, and detritus). While challenging, identifying sources and processes that regulate the bulk POM pool is useful for understanding the broader role of organic matter in food web dynamics. To better understand if and how the dominant sources of POM have changed through time and space in the highly-altered San Francisco Bay-Delta, we analyzed synoptic samples of POM C:N (at) ratios and associated δ¹³C values, collected coeval with other water quality data in select years spanning 2006 to 2016 and combined with historical data from the early 1990’s. We also used existing complimentary records of stable isotope values for dissolved inorganic N species to examine modern seasonal shifts in N biogeochemistry and sources along this freshwater-estuarine-marine gradient. Broadly, we found that both the quantity and quality of detrital POM have shifted in this system at large spatial and temporal scales; detrital POM C:N ratios and detrital POM concentrations declined across nearly all sites in months with available historical data for comparison. Changes in the quantity and quality of the bulk POM pool has likely impacted food web structure in this system, both directly and indirectly through changes to OM available for heterotrophic assimilation and remineralization. In our analysis of dissolved inorganic N stable isotope data, we found evidence of complex inorganic N cycling. A measurable fraction of NH₄⁺ in this system is regenerated in situ, and phytoplankton appear to be using predominantly NH₄⁺ across the entire freshwater to marine interface.