Phytoplankton species diversity control through competitive exclusion and physical disturbances

Competitive exclusion theory suggests that phytoplankton species number in an assemblage at equilibrium will be limited to the number of simultaneousl y limiting resources, generally three or fewer However, natural phytoplankt on assemblages usually exhibit high species diversity, hence the concept of Hutchinson's "paradox of the plankton." Recent works have suggested that t his apparent paradox is a result of disturbances intermediate in frequency relative to the time period necessary for species succession to lead to equ ilibrium conditions (sensu Connell's intermediate disturbance hypothesis [I DH]). Moreover, evidence indicates that disturbances of intermediate intens ities are also conducive to maintenance of high species diversity in phytop lankton communities. Using a long-term data record from hypereutrophic subt ropical Hartbeespoort Dam (South Africa) that was typically dominated by a single species, but annually subjected to physical disturbance, we demonstr ate here that disturbances can indeed enhance phytoplankton species diversi ty. However these data fail to support Connell's IDH per se, as moderate- a nd high-intensity disturbances yielded similarly high species diversity. Th ese data also suggest that community resilience (the ability or time to ret urn to predisturbed conditions) is negatively related to disturbance intens ity, such that higher intensity disturbances maintained high diversity for longer periods of time relative to lower intensity disturbances.