Linking climatic extremes to episodes of small-cell phytoplankton dominance in a large Southeastern reservoirJoseph M. Dirnberger
What processes make lakes good sentinels Graphic by by MICHAEL WHELAN
Lakes collect things! • Nutrients • Heat Biology happens fast in lakes!
Short generation times of phytoplankton can explain their paradoxical diversity
Unidentified cyanobacterium bloom from late summer of 1993 (132,000 cells/ml)
Diatom (Achnanthesminutissima) bloom from late summer of 2007 (99% of a phytoplankton cells and 74% of phytoplankton biomass) 10µm
Noticeable changes in water color and clarity, as well as significant problems with treatment of drinking water in both years
Central Questions What conditions in the lake and watershed were similar in both bloom years, and how might such conditions explain the periodic dominance of small cells.
Data acquisition: • 1992 to 1996 U.S. EPA Clean Lakes Phase I Diagnostic and Feasibility Study. • Periodic monitoring of several physical parameters including water temperature in subsequent years. Station 1E (dam) Station 28A
Late summer, epilimnetic temperatures tended to be highest in the two summers that blooms occurred.
Mean summer air temperature was hottest in 1993, while 2007 had the least summer tributary inflow. • Over all of the years analyzed in this study, summer air temperatures co-varied with lake hydrology P<0.05 indicated in bold face
Shift to nitrogen limitation: • Algal growth potential tests indicated nitrogen limitation in the late summer of 1993 at 8 of 11 lake sites • Ratios of nitrogen to phosphorus were lower throughout the lake in the summer of 1993
P limited N limited
Hot, dry condition can favor nitrogen limitation: • In drier years, exposed sediments are resuspendedby wave action increasing availability of phosphorus. • Strength of stratification differentially affect the entrainment of phosphorus and nitrogen. • Atmospheric deposition of nitrogen across the watershed is reduced in drier years.
So how can these conditions favor smaller phytoplankton cells? 1) Smaller cells have greater surface area relative to volume, increasing the rate of nutrient supply relative to cellular demand.
2) Smaller cells sink more slowly (Stokes’ Law) Loss of cells from the photic zone increases with warmer temperatures and with stronger stratification, differentially increasing loss of large cells.
Few phytoplankton would seem to be adapted to extreme shifts in physical conditions, leaving small cells with an advantage. We are left with the somewhat paradoxical statement that environmental change can enhance ordiminish diversity. This appears to depend on magnitude and periodicity.
Human impacts tend to alter environmental variation Shift in strength of stratification : - Eutrophication - Climate change
Lakes as sentinels • The plankton community is sensitive to temporal change (as observed in Allatoona with shifts to near monocultures of small cells during hot, dry summers). • Lakes “accumulate” broader scale watershed and atmospheric changes (as observed in Allatoona with shifts in nutrient dynamics and thermal structure).