Abstract
Estimates of dissolved organic carbon (DOC) release by marine macrophyte communities (seagrass meadows and macroalgal beds) based on in situ benthic chambers from published and unpublished are compiled in this study. The effect of temperature and light availability on DOC release by macrophyte communities was examined. Almost 85 % of the seagrass communities and all of macroalgal communities examined acted as net sources of DOC. Net DOC fluxes in seagrass communities increase positively with water temperature. In macroalgal communities net DOC fluxes under light exceeded those under dark condition, however, this trend was weaker in seagrass communities. Shading of a mixed seagrass meadow in The Philippines led to a significant reduction on the net DOC release when shading was maintained for 6 days compared to only 2 days of shading. Net DOC fluxes increased with increasing community respiration, but were independent of primary production or net community production. The estimated global net DOC flux, and hence export, from marine macrophytes is about 0.158 ± 0.055 Pg C yr-1 or 0.175 ± 0.056 Pg C yr-1 depending on the global extent of seagrass meadows considered.
Highlights
Dissolved organic carbon (DOC) represents only 2% of the carbon pool in the ocean, is the largest reservoir of exchangeable organic carbon in the marine carbon pool (685 Pg C; Hansell and Carlson, 2002)
The maximum net DOC uptake (−53.6 ± 10.8 mmol C m−2 d−1) and the maximum net release (81.6 ± 13.9 mmol C m−2 d−1) were both observed in C. nodosa communities (Table 1, Figure 1A)
The net DOC fluxes in seagrass communities were significantly correlated with the water temperature (r2 = 0.1, p < 0.05), increasing by 1.5 ± 0.7 mmol C m−2 d−1 by each ◦C temperature increase (Figure 2)
Summary
Dissolved organic carbon (DOC) represents only 2% of the carbon pool in the ocean, is the largest reservoir of exchangeable organic carbon in the marine carbon pool (685 Pg C; Hansell and Carlson, 2002). Bacteria consume preferentially newly synthesized DOC, which is highly labile (Kenworthy and Thayer, 1984), a large portion of the bulk DOC pool in the marine ecosystem consists of refractory material which is transported in the ocean (Hansell et al, 2004). Marine macrophyte communities are complex assemblages including microbes, metazoans and plants, all of which are involved in dissolved organic matter (DOM) fluxes. These communities can release DOM by different mechanisms, such as release from autotrophic organisms (seagrass, benthic macro- and micro-algae, phytoplankton, and epiphytes), release by leaching, and decomposition of detritus and excretion by organisms all of which can be considered to be of autochthonous origin. DOM can be produced from release and/or decomposition of organic matter produced elsewhere (i.e., of allochthonous origin), such as DOM derived from decomposition of sestonic material trapped in the sediments (Gacia et al, 2002; Hendriks et al, 2008; Vaquer-Sunyer et al, 2012)
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