The biodiversity of plankton ecosystems is no longer a paradox. The mathematical mechanisms that determine the coexistence of competitors in a general class of models, which includes almost all theoretical and applied mass conserving ecosystem models in present use, are clear. Knowledge of these mechanisms simplifies the identification and construction of models with the structural property that all species coexist for all time, irrespective of environmental forcings, spatial interactions, and further model complexities. Here, we discuss the "paradox of the 'paradox of the plankton'" proposed by Record et al. (ICES Journal of Marine Science, 71: 236-240) and explain the mechanisms that underpin the solution.