Coexistence of competitive species is severely limited by the availability of 
resources and the characteristics of the environment. In particular, the 
so-called ``competitive exclusion principle'' states that, at equilibrium, the 
number of coexisting species cannot be larger than the number of resources they 
compete for. On the other hand, many ``in situ'' observations have revealed 
prolonged coexistence of a large number of competitive plankton species, a 
phenomenon known as ``the paradox of the plankton''. Here we investigate this 
problem and show that ocean mesoscale vortices generate transport barriers and 
incomplete horizontal mixing, allowing for a prolonged survival of the less-fit 
species even for fully homogeneous resource distributions. In such a situation, 
the temporarily less-fit plankton species are protected from competition by the 
action of the vortices.