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Abstract The mesopelagic zone is a site of strong microbially driven particle attenuation with depth and thus plays a crucial role in controlling the transfer efficiency of the ocean's biological pump. However, little quantitative information exists on the dependency of decay processes on the source material. Here we followed the decay of¹⁴C‐labeled dead particulate organic carbon (POC) and dissolved organic carbon (DOC) from three different phytoplankton species, and two incubations of live diatoms, in mesopelagic water over 3 months. Commonly used first‐order kinetics failed to adequately describe the decay of organic material as rate constants varied from day to day. Over extended periods, decay rates for organic material exhibited two distinct phases, with rates in the second phase being inversely related to rates in the first phase. Microbial biomass (measured via adenosine triphosphate and cell counts) increased substantially during phase 1 and ebbed during phase 2. Decay rates were significantly different among the three algal sources; however, differences were even more pronounced among carbon pools and followed a distinct pattern (combined average per‐day decay rates at 12°C): fresh DOC (0.6) > fresh POC (0.1) > live cells (0.06) > aged DOC/POC (0.01). Separation of POC into four broad biochemical fractions showed that components in the operationally defined lipid fraction contained the most degradable compounds for fresh material. Our research highlights the need to include the dynamics of the most easily digestible fractions of freshly released organic material, and live plankton resilient to digestion, in calculations of vertical carbon flux budgets. 

Climate warming in the Arctic is thawing previously frozen soil (permafrost). Permafrost thaw alters landscape hydrology and increases weathering rates, which can increase the delivery of solutes to adjacent waters. Long-term river monitoring of the Kuparuk River (North Slope, Alaska, USA) confirms significant increases in solutes that are indicative of thawing permafrost. However, there is no evidence of an increase in total phosphorus (TP) or soluble reactive phosphorus (SRP), the nutrient that limits primary production in this and similar rivers in the region. Here, we show that Mehlich-3 extractable iron (Fe) and aluminum (Al) impart high P biogeochemical sorption capacities across a range of landscape features that we would expect to promote lateral movement of water and solutes to headwater streams in our study watershed. Reanalysis of a recently published pan-Arctic soils database suggests that this high P sorption capacity could be common in other parts of the Arctic region. We conclude that while warming-induced permafrost thaw may increase the potential for P mobility in our watershed, simultaneous increases in pedogenic secondary Fe and Al minerals may continue to retain P in these soils and limit biological productivity in the adjacent river. We suggest that similar interactions may occur in other areas of the Arctic where comparable biogeochemical conditions prevail. 

We consider the problem of numerically computing the quantum dynamics of an electron in twisted bilayer graphene. The challenge is that atomic-scale models of the dynamics are aperiodic for generic twist angles because of the incommensurability of the layers. The Bistritzer-- MacDonald PDE model, which is periodic with respect to the bilayer's moir\'e pattern, has recently been shown to rigorously describe these dynamics in a parameter regime. In this work, we first prove that the dynamics of the tight-binding model of incommensurate twisted bilayer graphene can be approximated by computations on finite domains. The main ingredient of this proof is a speed of propagation estimate proved using Combes--Thomas estimates. We then provide extensive numerical computations, which clarify the range of validity of the Bistritzer--MacDonald model.