In situ experimental study of reed leaf decomposition along a full salinity gradient

Abstract

An experimental study on Phragmites australis leaf litter decomposition was conducted in the estuarine environment, Ria de Aveiro,Western Portugal, using theleaf-bag technique, with fine- (1 mm) and coarse- mesh (5 mm) bags. The leaf bags were placed in the field sites at day 0, covering a complete salinity gradient, and replicates were collected over time, at days 3 (leaching), 7, 15, 30 and 60. The biomass loss through theleachingphase,about20% of theinitialleafmass,was independentof boththesalinityand the bag mesh size. The biomass decay pattern along the salinity gradient varied through time and presented strong similarities between the two mesh sizes, with the remaining biomass always lower in the 5 mm mesh-size bags. At days 7 and 15, the lowest remaining biomass was observed at the head of the estuary, the preferential distribution area of P. australis . At day 30, the remaining biomass was higher in the marine area and diminished under a direct relationship with salinity, reaching the lowest value in the freshwater environment, with values ranging from 66% to 44% of the initial weight in 5 mm bags, and from 79% to51% in 1 mm bags.The largestheterogeneity in the remaining biomass among the studyareas positioned along thesalinitygradientwasfoundclosetodays30(5 mm)and 40(1 mm). Theoverallresultsindicatethat the relationshipbetweenleafdecayrateandsalinitydependsonthedecaytimeconsidered( k 15 , k 30 or k 60 )and, for the later stages ( k 60 ), also on the leaf-bag mesh size. This implies that the use of leaf litterdecay rates as a functional indicator in transitional waters will need to take into consideration the factor location in the salinity gradient and leaf litter stage at which the decay rate is determined. The differences between the decay rates with the mesh size acted mainly at the level of the absolute k value and not at the level of the pattern along the salinity gradient. Even so, the data obtained at the mouth of the estuary, in the area closest to a fully marine environment, indicated that after the initial biomass loss through leaching, P. australis decayed either very slowly, in the 5 mm, or not at all, in the 1 mm mesh bags.