Floods can cause large interannual differences in littoral net ecosystem productivity

Larmola, Tuula, Jukka Alm, Sari Juutinen, Sanna Saarnio, Pertti J. Martikainen, Jouko Silvola

Limnol. Oceanogr., 49(5), 2004, 1896-1906 | DOI: 10.4319/lo.2004.49.5.1896

ABSTRACT: Littoral wetlands comprise a terrestrial to aquatic continuum along which carbon dioxide is exchanged with the atmosphere and organic carbon is transferred to lakes. Net ecosystem productivity—the difference between atmospheric CO2 uptake and total ecosystem respiration—in these shore areas depends partly on the extent and duration of spring flooding. Ecosystem-atmosphere CO2 fluxes were studied at a boreal lake in Finland to analyze how flooding affects the dynamics of littoral net ecosystem productivity. Two shore transects with different hydrological conditions and vegetation distributions were studied during consecutive ice-free periods with contrasting flooding patterns. Net ecosystem productivity in different vegetation zones did not respond consistently to extended flooding; the response depended on the phenology of plant emergence during the flood and on the decrease in the water level after flooding. The decrease in the water level was steeper in silt-mud sediment than in fen peat. With an exceptionally high water level relative to the height of the vegetation, net ecosystem productivity decreased by 50-100% (net loss, 0.4-7.4 mol m-2 of CO2 during the wetter open-water period). However, the wetter season could also have a 60-140% higher net ecosystem productivity (net CO2 gain of 0.7 mol m-2 to net loss of 6.8 mol m-2) because of the decreased decomposition rate. An extended flooding period greatly reduces the amount of litter produced in a specific year. In both flooding patterns, the littoral zone was an overall net CO2 emitter, but the large variation in the decomposition rate suggests that there are differences in the load of organic matter from the littoral to the pelagic zone.

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