Abstract
The spatial variability of plant organic matter processing was studied experimentally in a shallow coastal lagoon (Tancada lagoon, average depth: 37 cm, area: 1.8 km^2^) in the Ebro River Delta (NE Spain). To determine the effect of hydrology and sediment characteristics on plant organic matter processing, leaves of Phragmites australis at the end of its vegetative cycle and whole plants of Ruppia cirrhosa (Petagna) Grande, just abscised, were enclosed in litter bags. Two different mesh sizes (100 μm and 2 mm) were used to study the effect of macroinvertebrates on decomposition. The bags were placed in the water column and approximately 15 cm above the sediment at 6 different locations in the lagoon. The experiment was performed twice, in autumn‐winter and spring‐summer. The effect of macroinvertebrates on decomposition rate was not significant in Tancada lagoon. Breakdown rates showed spatial differences only in spring‐summer. In the autumn‐winter experiment, the effect of strong wind masked the effects of environmental variables and hydrology on decomposition rate. In the spring‐summer experiment, characterised by high stability of the water column, dissolved inorganic nitrogen (DIN) concentration in the water column and organic matter in the sediment were the main factors determining the variability of organic matter processing. A positive relationship was calculated between P. australis decomposition rate and dissolved inorganic nitrogen in spring‐summer (r^2^ = 0.92, p < 0.001). (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)