Mesopelagic microbes and zooplankton, degrade and attenuate >90% of the 10 billion tonnes of Particulate Organic Carbon (POC) that sinks into the oceans’ interior annually. Approaches such as particle interceptors/incubators (called C-RESPIRE) can isolate the microbial assemblage attached to particles from that of zooplankton, enabling quantification of microbially-mediated POC flux attenuation. This metric yields patterns of POC degradation by microbes through the upper mesopelagic (200-500 m depth). Here, we investigate the temporal sequence of POC degradation in two steps. First, we intercept sinking particle assemblages from different depths (180-300 m) and hence with varying degrees of exposure to microbial activity. Second, we incubate these intercepted particles shipboard for 12h (short-term) and track degradation using Apparent Respiratory Quotients (ARQ, dDIC/dDO2). We also incubate shipboard (12h) a particle assemblage previously incubated (36h) in situ using C-RESPIRE (long-term). At a subantarctic and two polar sites, ARQs from short-term incubations exhibited a significant decrease with depth, consistent with particles deeper in the upper mesopelagic being exposed to a longer period of degradation and flux attenuation (as they settle). ARQs from all long-term incubations had significantly lower ARQs, and smaller depth-dependent gradients, than the short-term incubations. We interpret these trends as being driven in part by sequential changes in the stoichiometry of the microbially-altered POC substrates. Several ARQs of <0.5 (less than the theoretical minimum) were observed in long-term incubations suggesting a role for incomplete oxidation of substrates. This temporal sequence is used to conceptually explore what sets the limits on microbially-mediated degradation of POC.

Seasonal patterns in the in situ ecophysiology of the common habitat-forming seaweeds Ecklonia radiata, Phyllospora comosa, and Macrocystis pyrifera were investigated at different latitudes and depths in southeastern Australia. We used multiple performance indicators (photosynthetic characteristics, pigment content, chemical composition, stable isotopes, nucleic acids) to assess the ecophysiology of seaweeds near the northern and southern margins of their range, along a depth gradient (E. radiata only), over a two year period (September 2010 – August 2012).