Phytoplankton (bars) and zooplankton (triangles) densities in fertilized (Fert) and unfertilized (NoFert) ponds during the grow-out of tambaqui Colossoma macropomum. Different lowercase letters indicate significant differences between treatments within each month for phytoplankton (treatment × time interaction, P < 0.05). For zooplankton, both treatment and time effects were significant (P ∼ 0.05), but since no interaction was detected, letters for monthly comparisons are not shown.