Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe Changes in the elemental (C, N) and isotopic (δ 13 C, δ 15 N) composition of estuarine plants during diagenesis and implications for ecological studies Marianna Lanari ⁎ , Marlucy Coelho Claudino, Alexandre Miranda Garcia, Margareth da Silva Copertino Institute of Oceanography, Federal University of Rio Grande (FURG), Av. Itália Km 08, Carreiros, Rio Grande – RS, CEP: 96.201-900, Brazil ARTICLE INFO Keywords: Decomposition Stable isotopes Detritus Salt marsh plants Seagrasses Opportunistic macroalgae ABSTRACT Stable isotopes have been used to quantify the contributions of plant-derived detritus in nutrient cycling and energy flow based on the assumption that mixtures reflect the isotopic composition of their sources. However, during the decay process, the plant detritus carbon and nitrogen isotopic signatures may vary from that of the original plant material. This fractionation during plant decomposition may impose limitations on the inter- pretation of results based on the stable isotopes approach. Studies investigating this process for estuarine pri- mary producers have shown contrasting results, and for some producers, they are still scarce. The present study investigated the temporal variations in the elemental (total organic carbon, total nitrogen, and C:N ratios) and isotopic (δ 13 C, δ 15 N) compositions of C 3 and C 4 salt marsh plants (Scirpus maritimus and Spartina densiflora, respectively), seagrass (Ruppia maritima) and opportunistic macroalgae (Rhizoclonium sp.) during decomposition. For this purpose, an in situ experiment using litterbag incubations was conducted in a shallow water (< 2 m) estuarine site over twelve weeks to investigate the plant decomposition process under the natural variability of environmental conditions (i.e., freshwater discharge, dissolved inorganic nitrogen availability, water salinity and temperature). Our results indicated that detritus biomass and nutrient mineralization rates varied among the different estuarine plant species. Overall decreases in the C:N ratios primarily resulted from total organic carbon losses. However, low variations in δ 13 C (< 2‰) were detected, possibly due to the short-term duration of our study. The observed lack of significant changes in total nitrogen content and the small changes in 15 N values during plant material degradation suggests, to some extent, N-limitation of microbial growth. The observed low levels of isotopic fractionation during decomposition support the usefulness of stable isotopes as natural tracers of organic matter. Nonetheless, to investigate the effects of several environmental and biotic factors on nutrient isotopic fractionation during plant decay and their implications for ecological studies, we emphasize the need for further experimental studies performed under distinct environmental conditions. 1. Introduction Salt marsh plants, seagrasses and opportunistic macroalgae are important estuarine primary producers, acting as both sources and sinks of carbon and other nutrients in shallow areas. First acting as a carbon and nitrogen sink during growth, estuarine plants slow down the sea- ward transport of dissolved nutrients in the water column, thus re- presenting a temporary stock in the system (Banta et al., 2004; Duarte et al., 2013; McGlathery et al., 2007). However, most of their produc- tion is not directly consumed by grazers, and large amounts of their biomass will ultimately enter detrital pools during senescence (Cebrian, 2004). The availability of plant-derived detritus will influence nutrient cycling and detrital food webs in coastal ecosystems according to the biochemical composition of the detritus (Bishop et al., 2010). Com- pared to macroalgae, salt marsh plant and seagrass tissues, with a high content of structural carbon fibers such as cellulose, hemicellulose and lignin (i.e., high C:N ratios), present higher resistance to microbial degradation and lower turnover rates (Kristensen, 1994). The high fraction of refractory material in salt marsh plants and seagrasses de- creases their digestibility, inhibiting their direct incorporation in the estuarine food chain (Cebrian, 2004; Galván et al., 2008; Kurata et al., 2001). Conversely, the nitrogen-rich tissue of ephemeral green macro- algae is highly palatable to consumers and readily consumed by grazers and detritivores (Rossi et al., 2011). Algal detritus is also rapidly mi- neralized by microbial activity (Fong et al., 2004), supporting further primary production in the water column at the sediment surface and in adjacent salt marsh areas (Hardison et al., 2013; Newton and Thornber, https://doi.org/10.1016/j.jembe.2017.12.013 Received 21 August 2017; Received in revised form 16 November 2017; Accepted 13 December 2017 ⁎ Corresponding author. E-mail address: marianna.lanari@gmail.com (M. Lanari). Journal of Experimental Marine Biology and Ecology 500 (2018) 46–54 0022-0981/ © 2017 Elsevier B.V. All rights reserved. T