Contrasting fatty acids with other indicators to assess nutritional status of suspended particulate organic matter in a turbid estuary

Achieving a precise nutritional characterization of suspended particulate organic matter (SPOM) is key for understanding the efficiency of bottom-up trophic fluxes in estuaries. Assessing the nutritional status of SPOM in turbid estuaries is challenging due to the complex mixing and degradation dynamics of the autotrophic, heterotrophic and detrital sources configuring SPOM. Using a recently described index based on fatty acids (FA), SPOM nutritional value was quantitatively characterized in the Guadalquivir River Estuary (GRE). Over a two-year study, this FA-based nutritional index (FANI) discerned changes in the nutritional value of SPOM to a greater extent than other nutritional indices. Spatiotemporal results of FANI in the GRE indicated SPOM nutritional quality was 2.2-fold higher in the 20–35 salinity range respecting to the 0–5 salinity range and that SPOM in summer was 1.5-fold nutritionally richer than in winter. A mixing model using bacterial FA estimated a maximum 40–60% range of variation for a complementary contribution of autotrophs and heterotrophs to microbial biomass and was in agreement with results of carbon-based estimates of phytoplankton and total microbes. In the spatiotemporal scale, and referred to particulate organic carbon (POC), phytoplankton represented from 9.3% (winter) to 31.0% (summer), and from 15.0% in lower salinity (0–20) to 24.5% in higher salinity reaches (20–35). Phytoplankton was positively correlated with SPOM nutritional value. Specific FA markers indicated a low contribution of land plants to POC. The lipid-based approach here described can improve understanding of the nutritional status of basal food resources in turbid estuaries.