Revisiting silicon budgets at a tropical continental shelf: silica standing stocks in sponges surpass those in diatoms

Most of the silicon (Si) in marine coastal systems is thought to recirculate under the biological control of planktonic diatoms. We challenge this view after comparing the biogenic silica (bSi) standing stocks contributed by communities of planktonic diatoms and benthic sponges in five habitats of an extensive continental shelf area of the Mesoamerican Caribbean. In most habitats (outer reefs, patch reefs, sea grass beds, and mangroves), the sponge bSi stocks surpassed those of diatoms. Collectively, bSi in sponge communities was about 88.6% of the total Si pool. Diatoms represented 4.2% and ambient silicate about 7.2%. Consequently, when constructing future regional Si budgets in coastal areas, the Si standing stocks in sponge populations should be empirically examined before deciding that their contribution to the total is negligible. In order to understand Si fluxes in coastal areas where sponges are relevant, we need additional empirical approaches to set the timescale of sponge bSi turnover, which appears to be substantially slower than that of diatom bSi. doi: 10.4319/lo.2010.55.5.2001