Evidence for bottom-up control of recent shifts in the pelagic food web of Lake Huron

The magnitude of the spring bloom in the open waters of Lake Huron has declined dramatically in recent years, beginning in 2003. May chlorophyll values, as estimated by SeaWiFS imagery, for 2003-2006 have been 50-60% of 1998-2002 values. April phytoplankton biovolumes have also declined; average biovolume in 2003-2004 was approximately 25% of 2001-2002. Most of this decline was due to reduction in biovolume of colonial diatoms. Reductions in the spring bloom have been closely associated with abrupt declines in cladoceran populations, as well as with declines in cyclopoid copepod populations. In addition, Daphnia population egg ratios in August exhibited a pronounced decrease between 2002 and 2003 and have remained depressed through 2005. Taken together, these data suggest a role for reduced food supply in the dramatic shifts seen in the Lake Huron crustacean zooplankton community since 2003. Additionally, summer chlorophyll values have shown signs of decline in 2005 and 2006 in spite of the historically low populations of cladocerans, suggesting that control of summer phytoplankton populations in Lake Huron is determined by nutrient supply rather than grazing pressure.