Myriophyllum spicatum (Eurasian watermilfoil; Haloragaceae), a submerged aquatic plant invasive in the Great Lakes region and much of the northern United States, produces phytochemicals that inhibit or reduce the growth of cyanobacteria and some aquatic insect herbivores. The native Myriophyllum sibiricum (northern watermilfoil) grows less densely and produces lower quantities of these inhibitory chemicals than the invasive Eurasian and hybrid watermilfoil (M. spicatum x M. sibiricum). Thus, increased chemical defenses or allelopathy, in addition to other competitive mechanisms, might have a role in native species displacement by invasive Eurasian and hybrid watermilfoil. Although the chemical profiles of M. spicatum and M. sibiricum have been described, it remains unknown how watermilfoil phytochemistry varies within and between lakes over the growing season and across environmental gradients. M. spicatum has been found to produce greater quantities of a phytochemical called tellimagrandin II when grown in low light, low nitrogen environments. This study aimed to determine the dynamics watermilfoil phytochemistry in the field over the growing season and whether there are relationships between lake water column light availability, resource availability, and M. spicatum production of tellimagrandin II, an ellagitannin known to reduce cyanobacteria and insect growth.