Reorganization of phytoplankton communities linked to physical forcing

The performance of individual phytoplankton species is strongly governed by vertical mixing within the water column, which alters resource availability of light and nutrients.  The present study documents shifts in phytoplankton species composition linked to intensified stratification over the last decades in Lake Tahoe.  Analysis of a 23 year dataset indicated gradients in phytoplankton species composition and community properties.  A distinct community structure was identified in the 1980s associated with elevated phosphorus concentration. Since the early 1990s phosphorus concentrations have been at constant low levels; however phytoplankton structure and species turnover changed significantly since the late 1990s, which was strongly linked to intensified stratification of the water column.  The strongest response was observed within diatoms and chlorophytes. Especially smaller-sized diatoms were associated with increasing stability, suggesting that less turbulence provides a competitive advantage over large-sized cells.  Small centric planktonic diatoms with rapid growth rates have become dominant enough to reduce compositional turnover rates and average cell size within this functional group.  A different response was however observed in chlorophytes, which showed an increase in colonial and filamentous species.  The increase in these functional groups is reflective in increasing species turnover rates and cell size.  This suggests that bloom-forming chlorophytes likely increase under reduced mixing processes.  As the thermal stratification of lakes and oceans is strongly linked to climate, the present study indicates that climate warming affects the reorganization of phytoplankton structure and suggests that intensified stratification will favor the expansion of small-sized species and species with the capability of buoyancy regulation.