Awareness that sponges are relevant silicic acid (DSi) users is growing; however, understanding how their DSi consumption kinetics perform is still limited. We investigated the effects that seasonal changes in a temperate ecosystem (Bay of Brest, France) have on the DSi consumption of 2 dominant sponge species: Hymeniacidon perlevis and Tethya citrina. The results indicated that while both species increased their rate of DSi utilization with DSi availability following saturable Michaelis-Menten kinetics, only the kinetics of T. citrina shifted seasonally. This species consumed DSi at a higher rate in autumn than in summer. Surprisingly, the increase in DSi utilization did not involve an increase in net affinity for DSi but rather augmentation of both the half-saturation concentration and the maximum velocity of transport characterizing the kinetics. By quantifying the biomass of the 2 sponge species in the bay and the monthly availability of DSi over an annual cycle, a yearly DSi consumption of 2.50 +/- 3.21 x 10(6) mol Si was calculated for their populations. The oversight of the seasonal kinetic change would introduce inaccuracies of 10% into the global DSi consumption budget of the bay. Because the seasonal kinetic differences increased enormously with increasing DSi availability, the relevance of the kinetic shift into the budgets could increase to >30% for sponge assemblages other than those in the bay whenever they are characterized by higher DSi availability, as is typically occurring at high latitudes and in the deep sea.