  Quantifying the mechanisms of tracer dispersion in the ocean remains a
central question in oceanography, for problems ranging from nutrient delivery
to phytoplankton, to the early detection of contaminants. Most analyses have
been based on Lagrangian concepts of transport, focusing on the identification
of features minimizing fluid exchange among regions, or more recently on
network tools which focus on connectivity and transport pathways. Neither of
these approaches allows ranking the geographical sites of major water passage
and selecting them so that they monitor waters coming from separate parts of
the ocean. These are instead key criteria when deploying an observing network.
Here we address this issue by estimating at any point the extent of the ocean
surface which transits through it in a given time window. With such information
we are able to rank the sites with major fluxes that intercept waters
originating from different regions. We show that this allows us to optimize an
observing network, where a set of sampling sites can be chosen for monitoring
the largest flux of water dispersing out of a given region. When the analysis
is performed backward in time, this method allows us to identify the major
sources which feed a target region. The method is first applied to a
minimalistic model of a mesoscale eddy field, and then to realistic
satellite-derived ocean currents in the Kerguelen area. In this region we
identify the optimal location of fixed stations capable of intercepting the
trajectories of 43 surface drifters, along with statistics on the temporal
persistence of the stations determined in this way. We then identify possible
hotspots of micro-nutrient enrichment for the recurrent spring phytoplanktonic
bloom occuring here. Promising applications to other fields, such as larval
connectivity, marine spatial planning or contaminant detection, are then
discussed.
