Changes in nutrient content and stable isotope ratios of C and N during decomposition of seagrasses and mangrove leaves along a nutrient availability gradient in Florida Bay, USA

Abstract

The decomposition of the mangrove Rhizophora mangle and the seagrass Thalassia testudinum was examined using litterbags along a natural gradient in nutrient availability. Seagrass leaves had a higher fraction of their biomass in the labile pool (57%), compared to mangrove leaves (36%) and seagrass rhizomes (29%); the overall decomposition rates of the starting material reflected the fractionation into labile and refractory components. There was no relationship between the N or P content of the starting material and the decomposition rate.Nutrient availability had no influence on decomposition rate, and mass was lost at the same rate from litterbags that were buried in the sediment and litterbags that were left on the sediment surface. The dynamics of N and P content during decomposition varied as a function of starting material and burial state. N content of decomposing mangrove leaves increased, but seagrass rhizomes decreased in N content during decomposition while there was no change in seagrass leaf N content. These same general patterns held for P content, but buried seagrass leaves increased in P content while surficial leaves decreased. d 13 C and d 15 N changed by as much as 2‰ during decomposition.

Blue carbon stocks in Baltic Sea eelgrass (<i>Zostera marina</i>) meadows

“…Our estimated C org stocks for the study sites were generally lower (627-4324 t C km −2 ) than estimates (25 200-84 000 t C km −2 ) found in the literature (Nelleman et al, 2009;Fourqurean et al, 2012). Several of the studies were conducted in the Mediterranean P. oceanica meadows -a habitat with superior carbon sequestration and storage capacity (Duarte et al, 2005;Lavery et al, 2013).…”

“…Recent studies have shown considerable variation in the global estimates of carbon stocks (C org stocks) and carbon accumulation rates (C org accumulation) in seagrass meadows, indicating an incomplete understanding of factors influencing this variability (Fourqurean et al, 2012;Duarte et al, 2013a;Lavery et al, 2013;Miyayima et al, 2015). The Baltic Sea forms a key distribution area for eelgrass in Europe, but similarly to the global data sets, we have so far lacked estimates on seagrass carbon stocks and accumulation.…”

“…The importance of the Z. marina contribution to the C org stocks may be explained by slow decomposition rates of seagrass tissue (54). In particular, the high proportion of refractory organic compounds in the seagrass belowground parts and high C : N : P ratios of seagrass tissue in general make seagrasses less biodegradable than most marine plants and algae (Fourqurean and Schrlau, 2003;Vichkovitten and Holmer, 2004;Kennedy and Björk, 2009;Holmer et al, 2011). The slow decomposition rates are also a result of reduced sediment conditions commonly encountered in Danish seagrass meadows (Kristensen and Holmer, 2001;Pedersen et al, 2011).…”

“…Although seagrass meadows only cover globally about 300 000-600 000 km 2 of the ocean sea floor, corresponding to 0.1 to 0.2 % of the total area, their carbon sink capacity (the capacity of seagrasses to absorb and store carbon in living and dead biomass and in the sediments) may account for up to 18 % of the total oceanic carbon burial (Gattuso et al, 1998;Duarte et al, 2005;Kennedy et al, 2010;Fourqurean et al, 2012). A large portion of the carbon sequestered (captured and stored) by seagrasses is stored in sediments, with a conservative value of 10 Pg C in the top 1 m of seagrass sediments (Fourqurean et al, 2012). Con- sequently, recent global estimates imply that seagrass sediments store almost 25 200 to 84 000 t C km 2 (Fourqurean et al, 2012).…”

“…A large portion of the carbon sequestered (captured and stored) by seagrasses is stored in sediments, with a conservative value of 10 Pg C in the top 1 m of seagrass sediments (Fourqurean et al, 2012). Con- sequently, recent global estimates imply that seagrass sediments store almost 25 200 to 84 000 t C km 2 (Fourqurean et al, 2012). More importantly, carbon in submerged sediments is stored for timescales of millennia, while terrestrial soils are usually less stable and only sequester carbon up to decades (Hendriks et al, 2008).…”