Effect of algal deposition on acetate and methane concentrations in the profundal sediment of a deep lake (Lake Constance)

In the profundal sediment of Lake Constance (143 m depth) the temperature is constant at 4 degrees C. Despite the constant temperature, CH4 concentrations changed with season between about 120 micromolar in winter and about 750 micromolar in summer, measured down to 30 cm depth. The acetate concentration profiles also varied between seasons. In summer, acetate concentration reached a maximum at about 100 micromolar in 2 or 4 cm depth. In winter, maximal concentrations of about 5 micromolar were observed over the entire depth. Input of organic material in late spring may be the reason for the seasonal change of both compounds. To simulate such a sedimentation event, intact sediment cores were covered with suspensions of Porphyridium aerugenium or Synechococcus sp. The addition of the phytoplankton material resulted in a drastic increase of acetate concentrations with a maximum at 2 cm depth, similar to in situ acetate concentrations measured in summer. The same applies for CH4 for which increased concentrations were observed down to 6 cm depth. H2 concentrations, on the other hand, showed no distinct increase. Treatment of intact sediment cores with 14C-labeled Synechococcus cells resulted in the formation of 14C-acetate, 14CH4 and 14CO2. Maximum concentrations of 14CH4 were found at 4 cm depth, i.e. just above the depth to which 14C-acetate penetrated. The results show that phytoplankton blooms may cause a seasonal variation of acetate and CH4 in profundal sediments of deep lakes despite the constant low temperature. They also indicate that acetate is the dominant substrate for methanogenic bacteria in the profundal sediments of Lake Constance.