Photoacclimation is a cellular process that allows phytoplankton to change the intracellular chlorophyll-a concentration (Chl) in relation to environmental factors such as light and nutrients. This ...process is currently overlooked by standard operational ocean colour algorithms used to retrieve information about both the phytoplankton standing stock and production. Here, we describe the photoacclimation effect on the phytoplankton seasonal cycle through the chlorophyll to carbon ratio (Chl:C) over the Mediterranean Sea, using SeaWiFS (1998–2007) monthly data. Chlorophyll was calculated through a regionalized empirical algorithm and Carbon was estimated from the particle backscattering, after subtraction of the non-algal contribution (bbpNAP). Historically, bbpNAP has been neglected or assumed constant, and only recently its space-time variability started to be taken into account. Here, bootstrapping simulations are used to compute the monthly bbpNAPs over a former bio-regionalization map of the Mediterranean Sea, all highlighting significant space-time variability. Resulting Chl:C varies within a factor of three, demonstrating the important role that photoacclimation plays at seasonal and basin scales. Lowest Chl:C values (~0.0013) are observed during summer; this is due to the combined effect of high irradiation, shallow mixing and low nutrient concentration. Maximum values (~0.031) are observed in winter and spring dominated by both low solar radiation, deeper mixing and high nutrient availability. It emerges that a better view of the phytoplankton space and time variability can be achieved through the joint use of Chl and C.
•Assessment of influence of photoacclimation on phytoplankton seasonal cycle•A new phytoplankton carbon algal biomass proxy for the Mediterranean Sea•Significant spatio-temporal variations of the non-algal particle coefficient•Photoacclimation dominates phytoplankton seasonal cycle in the Mediterranean Sea.•In late spring, phytoplankton carbon biomass is high while Chl drops quickly.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Understanding Alzheimer's disease (AD) heterogeneity is important for understanding the underlying pathophysiological mechanisms of AD. However, AD atrophy subtypes may reflect different disease ...stages or biologically distinct subtypes. Here we use longitudinal magnetic resonance imaging data (891 participants with AD dementia, 305 healthy control participants) from four international cohorts, and longitudinal clustering to estimate differential atrophy trajectories from the age of clinical disease onset. Our findings (in amyloid-β positive AD patients) show five distinct longitudinal patterns of atrophy with different demographical and cognitive characteristics. Some previously reported atrophy subtypes may reflect disease stages rather than distinct subtypes. The heterogeneity in atrophy rates and cognitive decline within the five longitudinal atrophy patterns, potentially expresses a complex combination of protective/risk factors and concomitant non-AD pathologies. By alternating between the cross-sectional and longitudinal understanding of AD subtypes these analyses may allow better understanding of disease heterogeneity.
Our knowledge of subsurface structures often derives from seismic velocities that are measured during seismic acquisition surveys. These velocities can greatly change due to lithological, fracture ...frequencies and/or effective pressure/temperature variations. However, the influence of such intrinsic lithological properties and environmental conditions at the large scale is poorly understood due to the lack of comprehensive datasets. Here, we analyze 43 borehole-derived velocity datasets of 3 end-member tight carbonate sequences from Central Italy, including massive pure limestone (Calcare Massiccio, CM), thick-layered (20-50 cm) pure limestone (Maiolica, MA), and thin-layered (2-20 cm) marly limestone (Calcareous Scaglia, CS). Our results show that the main rock parameters and environmental conditions driving large scale velocity variations are bedding and paleostresses, while mineralogical composition and current tectonic stress also play a role. For each of the 3 end-members, measured V
values vary differently with depth, as the thin-layered CS units show a clear increase in Vp, while velocity slightly increases and remains constant for the thick-layered MA and massive CM units, respectively. Such observations show that velocities are affected by specific characteristics of lithological discontinuities, such as the thickness of bedding. Counterintuitively, larger Vp values were recorded in the deformed mountain range than in the undeformed foreland suggesting that higher paleo-stresses increase velocity values by enhancing diagenesis and healing of discontinuities. Our results thus demonstrate that large scale velocity variations are strictly related to variation of lithological properties and to the geological and tectonic history of an area. We suggest that such lithological and environmental controls should be taken into account when developing velocity and mechanical models for tectonically active regions of the Mediterranean Area, where earthquakes mostly nucleate and propagate through carbonate formations, and for resource exploration in fractured carbonate reservoirs.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
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