Warmer water makes seagrass carbon less durable, and blue-carbon math assumes durability

Warming seawater by 4 degrees Celsius cut the share of dissolved organic carbon released by seagrasses that resists microbial breakdown by an average of about 28 percent, according to a study in Communications Earth & Environment from researchers in Spain and Germany ^[4][1][2]. Blue-carbon accounting treats the durability of that carbon as a property of the ecosystem, and this experiment points to it being partly a property of the water temperature ^[3][18].

The work was done in mesocosms, large seawater tanks in which conditions can be held and varied, at the ZMT facility in Bremen ^[6]. The team ran 27 tanks for 40 days at three temperature levels spanning 24 to 28 degrees Celsius, with different combinations of species: the seagrasses Cymodocea nodosa and Zostera noltei and the macroalga Caulerpa prolifera, all native to the Bay of Cadiz, plus Halophila stipulacea, an invasive seagrass from the Indian Ocean ^[6][7]. They measured oxygen production, carbon turnover and the chemical composition of what the plants released, then watched for 60 days how fast microorganisms consumed it ^[8].

The 4 degree contrast is the top of that range against the bottom ^[17]. At the warm end the refractory fraction fell by roughly 28 percent while the easily degraded fraction rose, and much of the labile material was already decomposed by day 60 ^[4][5]. The invasive species, the variable that usually attracts the attention, had very little effect on the carbon balance; temperature dominated ^[9].

The reason this is not a marginal bookkeeping question is the size of the pathway. Seagrass carbon splits into structural tissue such as leaves and rhizomes, a portion buried in the seafloor for centuries to millennia, and a portion exported to the water as dissolved organic carbon ^[16]. Some of that dissolved carbon is eaten within days; the resistant fraction can stay in the ocean for weeks, years or centuries ^[15]. When the authors compared persistent dissolved organic carbon against sediment burial, the two came out at the same order of magnitude, and most previous work has concentrated on the sediment side ^[10][11]. The flow that responded most to heat is comparable in scale to the flow that gets counted.

Lead author Alba Yamuza-Magdaleno of the University of Cadiz said that seagrass beds and kelp forests are considered natural carbon sinks and that if the ocean keeps warming they could sequester significantly less carbon over the long term than previously thought ^[12]. She added that many climate models have so far assumed these ecosystems permanently absorb large amounts of carbon dioxide from the atmosphere ^[13]. Co-author Pedro Beca-Carretero of ZMT and the IIM-CSIC institute in Vigo said the long-lasting fraction, the portion that normally persists for extended periods, declined the most as temperature rose ^[14].

Three things constrain how far this travels. The persistence being claimed runs to centuries, but the evidence is a 40-day exposure followed by a 60-day incubation, so it is not yet clear whether warming destroys the durable fraction or merely reshuffles the timing of its release ^[6][8][15]. The tested macroalga was Caulerpa prolifera, not a kelp, so the extension to kelp forests in the framing is an inference rather than a result ^[7][12]. And the temperatures were 24 to 28 degrees, a subtropical band, which leaves open whether cooler meadows show the same slope ^[6].

Watch whether anyone runs the multi-year incubation, and whether crediting methodologies that assign a permanence factor to seagrass projects start treating that factor as temperature-dependent rather than fixed ^[3][13].