Linking phytoplankton primary production and chromophoric dissolved organic matter in the sea

The link between phytoplankton primary production (PPP) and chromophoric dissolved organic matter (CDOM) has been demonstrated indirectly in the laboratory, but not directly in the sea. Here, for the first time, we report a strong link between PPP and CDOM in a study carried out in coastal marine waters around Barcelona (NW Mediterranean) in contrasting seasons. We measured relevant correlations between a254 CDOM concentration and both PPP (r = 0.89, p < 0.001, n = 16; r = 0.76, p < 0.01, n = 10; and r = 0.68, p < 0.001, n = 26; for summer, winter–spring and both seasons together, respectively) and the specific production rate at optimal light intensity (PmB; r = 0.77, p < 0.001, n = 16; r = 0.86, p < 0.01, n = 10; and r = 0.85, p < 0.001, n = 26; for the same seasons). Based on our findings, we design a model that predicts PPP very well, using a254 CDOM, water temperature and photosynthetically active radiation (PAR) (R2 = 0.84, p < 0.001, n = 26).Relationships between CDOM concentration parameters (measured at wavelengths of 280, 300, 340, 355, 375, 412 and 460 nm) and variables associated to CDOM processes (chlorophyll-a, bacterial abundance, turbidity related to sediment resuspension and PAR) were also evaluated. Bacterial abundance is significantly correlated with CDOM concentration measured at higher wavelengths when all samples are considered (e.g., r = 0.82, p < 0.001, n = 26, for a355 CDOM). However, in seasonal data analysis, the correlations decrease slightly (summer season: e.g., r = 0.79, p < 0.001, n = 16, for a355 CDOM) or become non-significant (winter–spring season: e.g., r = -0.02, p = 0.95, n = 10, for a355 CDOM). Sunlight photobleaching and sediment resuspension processes significantly influence CDOM dynamics in the summer (for a300 CDOM: r = -0.72, p < 0.001, n = 16; and r = 0.67, p < 0.01, n = 16, respectively).