Seston and Microplankton Sampling Protocol

The six large mesocosms and the 12 microcosms were sampled (1 L) for sestonic C, N, and P and for the microplankton abundance, Chl a concentration, and PP. Samples for C, N, P, and Chl a, were immediately transported to the laboratory in cold, dark, and thermally insulated containers. In the laboratory, a volume of 300 mL from each sample was filtered onto pre-combusted GF/F Whatman filters (450 °C for 1.5 h) for seston elemental analysis, filters were dried at 60 °C for 48 h and stored at −20 °C until analysed. C and N were analysed on a Perkin Elmer 2400 (Perkin Elmer, USA) (La Caldera) and Thermo Finnigan EA1112 (Thermo Finnigan, Italy) CHN (Los Cántaros) elemental analyzers and P was analysed with persulfate digestion followed by molybdate reaction⁶⁰ . Chl a was measured by filtering a volume of 300 mL at <100 mm Hg onto Whatman GF/F glass microfibre filters (25 mm in diameter) and extracting in a 90% acetone solution (at 4 °C for 24 h in the dark). Determinations were made in a Turner AU 10 fluorometer in Los Cántaros and LS 55 Perkin Elmer fluorometer in La Caldera⁶⁰ . To quantify abundance of phytoplankton, heterotrophic nanoflagellates (HNF), and small ciliates, we followed the procedures described in Medina-Sánchez et al.^{12 (link)} (Supplementary text S4).