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Seagrass meadows represent globally important stores of carbon. However, environmental heterogeneity in shallow, estuarine environments may shape the quantity, composition, and postdepositional processing of organic carbon stocks (Corg) in such meadows. Along a persistent gradient in total phosphorus concentrations in the water column and a parallel gradient in seagrass morphology, we measured bulk carbon parameters (Corg, dry bulk density, %Corg, Corg : N, δ13C) and lignin biomarkers in Thalassia testudinum tissues and in the sediments beneath these meadows in three coastal systems. We found Corg stocks and sources differed among coastal systems, but the aforementioned parameters were not consistently related to either standing stocks of seagrass or historical nutrient concentrations. We estimated that seagrasses contributed 30–53% of the total sedimentary Corg in these three coastal systems, with the remainder derived from allochthonous sources. The coastal system with intermediate phosphorus concentrations and aboveground seagrass stock had more Corg overall, more Corg from seagrass, and sediments with lower bulk density. A consistent negative relationship between dry bulk density and %Corg suggested hydrodynamics exerted a strong influence on stocks and sources of sedimentary Corg. Lignin biomarkers refined our understanding of sources of Corg and postdepositional processing of seagrass tissues. Phenolic acid‐to‐aldehyde ratios were high in fresh T. testudinum tissues. Lower values in sediments indicated seagrass tissues undergo extensive loss of acidic lignin phenols after deposition resulting in a need for lignin biomarker indices designed for estuarine sediments. Future studies of seagrass Corg should account for hydrodynamic setting, especially when investigating influences of environmental heterogeneity. [ABSTRACT FROM AUTHOR]

Bacteria decomposed damaged and moribund Chrysaora quinquecirrha Desor, 1848 releasing a pulse of carbon and nutrients. Tissue decomposed in 5-8 days, with 14 g of wet biomass exhibiting a half-life of 3 days at 22°C, which is 3× longer than previous reports. Decomposition raised mean concentrations of organic carbon and nutrients above controls by 1-2 orders of magnitude. An increase in nitrogen (16,117 μg l) occurred 24 h after increases in phosphorus (1,365 μg l) and organic carbon (25 mg l). Cocci dominated control incubations, with no significant increase in numbers. In incubations of tissue, bacilli increased exponentially after 6 h to become dominant, and cocci reproduced at a rate that was 30% slower. These results, and those from previous studies, suggested that natural assemblages may include bacteria that decompose medusae, as well as bacteria that benefit from the subsequent release of carbon and nutrients. This experiment also indicated that proteins and other nitrogenous compounds are less labile in damaged medusae than in dead or homogenized individuals. Overall, dense patches of decomposing medusae represent an important, but poorly documented, component of the trophic shunt that diverts carbon and nutrients incorporated by gelatinous zooplankton into microbial trophic webs. [ABSTRACT FROM AUTHOR]