Fluctuating water table affects gross ecosystem production and gross radiation use efficiency in a sedge-grass marsh

The eddy covariance method was used for continuous measurement of the seasonal courses of the following parameters of the carbon cycle in a sedge-grass marsh type of wetland ecosystem (49A degrees 01'29aEuro(3)N, 14A degrees 46'13aEuro(3)E, South Bohemia, Czech Republic, Central Europe): gross ecosystem production (GEP), net ecosystem production (NEP) and ecosystem respiration. During a 3-year series of measurements, we recorded marked fluctuations of the water table, which affected the overall water regime of the wetland studied. Between-year differences in the water regime strongly influenced the total annual carbon sequestration. The lowest annual GEP and NEP of 996 and 152 g m(-2) of carbon, respectively, were recorded in 2006, a year with two large floods, one in the spring, the other in the summer. By contrast, in the dry year of 2007, with no flood, the highest annual GEP and NEP were recorded: 1,328 and 274 g m(-2), respectively. Significant differences were found in the efficiency of solar energy use for GEP [gross radiation use efficiency, GRUE = GEP/PhAR (photosynthetically active radiation), i.e., amount of carbon gained per energy unit]. The highest GRUE was recorded immediately after the 2006 summer flood. In 2007, the GRUE decreased linearly with rising water table. A variable water regime thus markedly affects the processes of carbon accumulation and the efficiency of solar energy use for organic matter production in freshwater wetlands of the sedge-grass marsh type.