Engel A, Handel N, Wohlers J, Lunau M, Grossart HP, Sommer U, Riebesell U
sea surface warming, DOM, nutrient addition, C:N ratio, mesocosm, Kiel, Germany, 1.4 m3, Polysaccharides, Amino acids, Microbial loop
An experimental study was conducted to test the effects of projected sea surfacewarming (according to the IPPC scenarios) on the accumulation and composition ofdissolved organic matter (DOM) during marine phytoplankton blooms in cold seas(,48C). Eight mesocosms ( 1400 L) were filled with natural seawater, and two replicatemesocosms each were incubated by raising temperature by þ0, þ2, þ4 andþ68C, respectively. The enclosed water was initially fertilized with inorganic nutrientsto induce the development of phytoplankton blooms, which were then dominated bydiatoms. Over a 4-week period, dissolved combined carbohydrates (DCCHO) anddissolved 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, whennutrients became depleted. Rising temperature resulted in an earlier, faster andhigher accumulation of DCCHO and of combined glucose predominantly. DCCHOyielded a maximum percentage of 35, 40, 49 and 59% of total combined carbohydratesin the þ0, þ2, þ4 and þ68C treatments, respectively. Accumulation ofDAA occurred more continuously and at an average rate of 0.79+0.20 nmol L21 h21, but was not affected by rising temperature. Owing to the higheraccumulation of DCCHO, the C:N ratio of DOM increased strongly during thecourse of the bloom, with higher ratios in the warmer treatments. Our study suggeststhat warming increases the extracellular release of carbohydrates from phytoplanktonand, therefore, may affect the bottom-up control of the microbial loop in cold seas inthe future.