39 research outputs found
Southern Ocean phytoplankton under climate change: a shifting balance of bottom-up and top-down control
Phytoplankton form the base of the marine food web by transforming CO2 into organic carbon via photosynthesis. Despite the importance of phytoplankton for marine ecosystems and global carbon cycling, projections of phytoplankton biomass in response to climate change differ strongly across Earth system models, illustrating uncertainty in our understanding of the underlying processes. Differences are especially large in the Southern Ocean, a region that is notoriously difficult to represent in models. Here, we argue that total (depth-integrated) phytoplankton biomass in the Southern Ocean is projected to largely remain unchanged under climate change by the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble because of a shifting balance of bottom-up and top-down processes driven by a shoaling mixed-layer depth. A shallower mixed layer is projected on average to improve growth conditions, consequently weaken bottom-up control, and confine phytoplankton closer to the surface. An increase in the phytoplankton concentration promotes zooplankton grazing efficiency, thus intensifying top-down control. However, large differences across the model ensemble exist, with some models simulating a decrease in surface phytoplankton concentrations. To reduce uncertainties in projections of surface phytoplankton concentrations, we employ an emergent constraint approach using the observed sensitivity of surface chlorophyll concentration, taken as an observable proxy for phytoplankton, to seasonal changes in the mixed-layer depth as an indicator for future changes in surface phytoplankton concentrations. The emergent constraint reduces uncertainties in surface phytoplankton concentration projections by around one-third and increases confidence that surface phytoplankton concentrations will indeed rise due to shoaling mixed layers under global warming, thus favouring intensified top-down control. Overall, our results suggest that while changes in bottom-up conditions stimulate enhanced growth, intensified top-down control opposes an increase in phytoplankton and becomes increasingly important for the phytoplankton response to climate change in the Southern Ocean.</p
Imprint of Southern Ocean mesoscale eddies on chlorophyll
Although mesoscale ocean eddies are ubiquitous in the Southern Ocean, their
average regional and seasonal association with phytoplankton has not been
quantified systematically yet. To this end, we identify over 100 000
mesoscale eddies with diameters of 50 km and more in the Southern Ocean and
determine the associated phytoplankton biomass anomalies using
satellite-based chlorophyll-a (Chl) as a proxy. The mean Chl anomalies,
δChl, associated with these eddies, comprising the upper echelon of
the oceanic mesoscale, exceed ±10 % over wide regions. The structure of
these anomalies is largely zonal, with cyclonic, thermocline lifted, eddies
having positive anomalies in the subtropical waters north of the Antarctic
Circumpolar Current (ACC) and negative anomalies along its main flow path.
The pattern is similar, but reversed for anticyclonic, thermocline deepened
eddies. The seasonality of δChl is weak in subtropical waters, but
pronounced along the ACC, featuring a seasonal sign switch. The spatial
structure and seasonality of the mesoscale δChl can be explained
largely by lateral advection, especially local eddy-stirring. A
prominent exception is the ACC region in winter, where δChl is
consistent with a modulation of phytoplankton light exposure caused by an
eddy-induced modification of the mixed layer depth. The clear impact of
mesoscale eddies on phytoplankton may implicate a downstream effect on
Southern Ocean biogeochemical properties, such as mode water nutrient
contents.</p
Green Wireless Access Networks
I Godor, P Frenger, H Holtkamp, M Imran, A Vidacs, P Fazekas, D Sabella, E Strinati, R Gupta, P Pirinen and A Fehsk
How much energy is needed to run a wireless network
Presenting state-of-the-art research on green radio communications and networking technology by leaders in the field, this book is invaluable for researchers and professionals working in wireless communication