Effects of sea surface warming on the production and composition of dissolved organic matter during phytoplankton blooms: results from a mesocosm study

An experimental study was conducted to test the effects of projected sea surface warming (according to the IPPC scenarios) on the accumulation and composition of dissolved organic matter (DOM) during marine phytoplankton blooms in cold seas (< 4 degrees C). Eight mesocosms (similar to 1400 L) were filled with natural seawater, and two replicate mesocosms each were incubated by raising temperature by +0, +2, +4 and +6 degrees C, respectively. The enclosed water was initially fertilized with inorganic nutrients to induce the development of phytoplankton blooms, which were then dominated by diatoms. Over a 4-week period, dissolved combined carbohydrates (DCCHO) and dissolved amino acids (DAA) were determined as major components of freshly produced, labile to semi-labile DOM. In all mesocosms, the increase in DCCHO concentration occurred sharply after the peak of chlorophyll a concentration, when nutrients became depleted. Rising temperature resulted in an earlier, faster and higher accumulation of DCCHO and of combined glucose predominantly. DCCHO yielded a maximum percentage of 35, 40, 49 and 59% of total combined carbohydrates in the +0, +2, +4 and +6 degrees C treatments, respectively. Accumulation of DAA occurred more continuously and at an average rate of 0.79 +/- 0.20 nmol L-1 h(-1), but was not affected by rising temperature. Owing to the higher accumulation of DCCHO, the C:N ratio of DOM increased strongly during the course of the bloom, with higher ratios in the warmer treatments. Our study suggests that warming increases the extracellular release of carbohydrates from phytoplankton and, therefore, may affect the bottom-up control of the microbial loop in cold seas in the future.