Actions and interactions of two abiotic parameters--temperature (18, 21, and 24 degrees C) and photoperiod (8, 12, and 16 hr/day)--on mercury bioaccumulation (HgCl2 and CH3HgCl) by a rooted macrophyte species, Elodea densa, from the sediment source were studied using experimental designs. Comparative analysis of shoot growth (weight and length criteria) was made to look for a possible inhibiting action by the metal and to establish quantitative data for the interpretation of accumulation results. Total mercury accumulation by E. densa was quantified using two criteria--concentration and content. The amount of total mercury accumulated in the whole plant after a 28-day exposure was 40 times greater for the organic form, if the two compounds were initially added to the sediment at the same concentration. Temperature and photoperiod had a strong influence on methyl mercury accumulation by the plants, separately and in interaction. The interactions generally represent an amplification of effects produced by each abiotic factor, although this synergy varied according to the two accumulation criteria being considered or to the level of analysis (whole plant, leaves, and stems).