In situ experimental decomposition studies in estuaries: A comparison of Phragmites australis and Fucus vesiculosus Marta Lobão Lopes, Patrícia Martins, Fernando Ricardo, Ana Maria Rodrigues, Victor Quintino * CESAM, Departamento de Biologia, Universidade de Aveiro, 3810-193 Aveiro, Portugal article info Article history: Received 20 October 2010 Accepted 19 February 2011 Available online 2 March 2011 Keywords: Phragmites australis Fucus vesiculosus decomposition rates functional indicators salinity gradient Ria de Aveiro abstract The decomposition rates of Phragmites australis and Fucus vesiculosus were experimentally determined in an estuarine system using the leaf-bag technique. The study was conducted in fifteen sites arranged in five areas, extending from freshwater, outside the tidal range, to the marine environment, near the mouth of the estuary. The leaf-bags (5 mm mesh), were set up with 3.0 g of dried substrate, submerged in the experimental sites at day 0 and collected at days 3, 7, 15, 30 and 60, to follow biomass loss. The biomass loss through the leaching phase (day 3) was about 16% for Phragmites australis and 33% for Fucus vesiculosus and was independent of salinity for both substrates. The difference in the remaining biomass between the two species increased with time and the decomposition rates differed along the salinity gradient. For F. vesiculosus, the decomposition rate was highest near the mouth of the estuary, corre- sponding to the preferential distribution area of the algae, and decreased towards freshwater. For Phragmites australis, the fastest decay was observed in the mid estuary, where Phragmites australis occurs naturally, confirming previous studies. The decomposition rates measured at different time intervals (0 e15, 0e30 and 0e60 days) were always higher for the algae and decreased with time for both species. These results indicate that the use of decomposition rates as a measure of ecosystem integrity or quality status in transitional waters will not be straightforward and must take into account, among others, the test species, the study area positioning along the estuarine gradient, and the time interval for the calculation of the decomposition rate. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Estuaries are extremely productive systems, with large quanti- ties of organic matter available to decomposition (among others, McLusky and Elliott, 2004), which enables the recycling of nutrients and chemical elements (Takeda and Abe, 2001; Cebrian and Lartigue, 2004). After physical, chemical and biological processes, detritus is reduced to elements which are released to the system and become available for uptake by organisms (Gessner et al., 1999). In aquatic ecosystems the decomposition of organic matter proceeds in three stages (Petersen and Cummins, 1974; Webster and Benfield, 1986): (i) leaching, during which a rapid weight loss is seen due to the washing out of soluble constituents; (ii) condi- tioning, which consists in the modification of the leaf matrix by microorganisms as a result of enzymatic activities; (iii) fragmen- tation, corresponding to the physical breakup of the coarse detritus, mostly mediated by invertebrates. The decomposition of organic matter, namely leaves, is affected by internal factors, such as the leaf species and its physico-chemical characteristics (Kok et al., 1990; Akanil and Middleton, 1997) and by external factors, which include abiotic and biotic factors. Biotic factors include the role of microfungi and invertebrates (Hieber and Gessner, 2002), and abiotic factors include water temperature (Carpenter and Adams, 1979; Reice and Herbst, 1982), salinity (Dang et al., 2009), pH (Thompson and Bärlocher, 1989), nutrients (Elwood et al., 1981; Bärlocher and Corkum, 2003), oxygen concentration (Chauvet, 1997) and regional characteristics (Denward et al., 1999; Lissner et al., 1999a, b). Abiotic factors can have a direct effect upon decomposition, affecting the leaching and fragmentation phases, but also an indirect effect, determining the conditions of the environmental niche and, consequently, filtering the traits of potential colonizers and affecting their metabolism (Suberkropp and Chauvet, 1995). The study of decomposition using the experimental leaf-bag technique (Petersen and Cummins, 1974) has been widely used in the freshwater environment (van Dokkum et al., 2002; Sangiorgio * Corresponding author. E-mail address: victor.quintino@ua.pt (V. Quintino). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2011.02.014 Estuarine, Coastal and Shelf Science 92 (2011) 573e580