Abstract
Kelps are highly productive macroalgae that form habitats along one-quarter of the worlds’ coastlines. Emerging evidence suggests that kelps have the potential to sequester carbon through the export of detritus to deep marine sinks, yet how much of this detrital carbon is remineralized through grazing and microbial decomposition before it reaches these sinks remains a critical knowledge gap. We measured decay ofLaminaria hyperboreadetritus in shallow kelp forests (10 m) and adjacent deep fjords (300 m), and experimentally tested the effect of temperature and oxygen conditions similar to those at these habitats inex situexperiments. Initial decay rate (k) was high (-0.107 to -0.183 d-1) with 40-60% of the original carbon biomass being lost within few weeks, after which decay rates slowed down (k= -0.009 to -0.038 d-1). Temperature had little effect on the rate and extent of decomposition within the temperature range tested (4-10°C). Blade detritus decomposed almost completely in 300 d under aerobic conditions. Anaerobic decay of both blade and stipe detritus ceased, in contrast, after 150-200 d, leaving 20-30% of the initial biomass to decompose extremely slowly or not at all. Decomposition was followed by changes in chemical composition; C:N ratios increased substantially, while mannitol and phenolics disappeared almost completely from the detritus matrix. Slow and incomplete anaerobic decomposition suggest that the potential for long-term burial and sequestration of kelp carbon will be enhanced if detritus is exported to nearby deep areas with permanent or periodic hypoxia near the bottom.
Highlights
Blue carbon is defined as carbon (C) captured by marine living organisms (Nellemann et al 2009), and sequestration of blue C through long-term burial is considered one of several criteria for identifying marine habitats as being blue C ecosystems (Lovelock & Duarte 2019)
Any type of vegetation must meet some basic criteria to contribute meaningfully to C sequestration: (1) it must cover a substantial proportion of the Earth’s surface area, (2) it must have a high net production per unit area and time, (3) the consumption of live and dead biomass must be small enough to minimize mineralization of C through respiration at higher trophic levels, and (4) part of the detritus must be in a form such that it decomposes slowly and/or incompletely, because that will increase the probability of permanent burial in soils or sediments
Most kelp production is channeled to the detrital pool within or outside the kelp forest (Krumhansl & Scheibling 2012), and these systems are considered potentially important C donors to blue carbon sediments in adjacent ecosystems (Hill et al 2015, Krause-Jensen & Duarte 2016, Filbee-Dexter & Wernberg 2020)
Summary
Blue carbon is defined as carbon (C) captured by marine living organisms (Nellemann et al 2009), and sequestration of blue C through long-term burial is considered one of several criteria for identifying marine habitats as being blue C ecosystems (Lovelock & Duarte 2019). Any type of vegetation (terrestrial or aquatic) must meet some basic criteria to contribute meaningfully to C sequestration: (1) it must cover a substantial proportion of the Earth’s surface area, (2) it must have a high net production per unit area and time, (3) the consumption of live and dead biomass must be small enough to minimize mineralization of C through respiration at higher trophic levels, and (4) part of the detritus must be in a form (or under environmental conditions) such that it decomposes slowly and/or incompletely, because that will increase the probability of permanent burial in soils or sediments. Most kelp production is channeled to the detrital pool within or outside the kelp forest (Krumhansl & Scheibling 2012), and these systems are considered potentially important C donors to blue carbon sediments in adjacent ecosystems (Hill et al 2015, Krause-Jensen & Duarte 2016, Filbee-Dexter & Wernberg 2020). We expected decomposition to be relatively fast and complete in high temperature and aerobic conditions like in the kelp forest, and slower and less complete at lower temperature and low O2 concentrations as in deeper areas adjacent to kelp forests
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