Toxicity caused by trace metal elements in water is a major concern, leading to environmental disturbances and public health problems. The effect of cadmium on clonal macrophyte populations is poorly documented despite its high level of toxicity among aquatic organisms. Our aim here is to highlight the strong relationship existing between the physiological responses of Myriophyllum alterniflorum and the cadmium level over a long exposure period. Nine potential biomarkers of cadmium stress are tested, with three of them appearing to be highly sensitive: free proline, Hsp70, and malondialdehyde. Long-term follow-up analysis after metal exposure (27 days) also proves to be quite beneficial by providing a detailed overview of ecotoxicological events that is more complete and extensive than data recordings conducted over a few days. Taken together, these results support our initial hypothesis that leads to recommending biomarker analyses over at least 2 weeks of metal exposure.