Cladoceran populations in the open waters of Lake Huron declined abruptly in 2003 and have since remained at historically low levels. The two dominant cladocerans, Daphnia mendotae and Bosmina longirostris , have been nearly extirpated from the northern region of the lake and are present in only slightly greater numbers in the south. Average nonpredatory cladoceran biomass in the lake has declined over 90% between 1998–2002 and 2003–2006. In addition, historically unprecedented declines in cyclopoid copepods were seen in the lake in 2005. These changes have occurred against the backdrop of declining nutrient levels in the lake and have coincided closely with declines in the amphipod Diporeia . We speculate that a combination of reduced primary production in the open waters and intensified planktivory due to the continuing disappearance of Diporeia has accounted for the losses in crustacean biomass seen in recent years.