Potential Role of Sponge Spicules in Influencing the Silicon Biogeochemistry of Florida Lakes

Abstract

Amorphous silica, e.g. biogenic silica (BSi), contained in diatoms and in sponge spicules was estimated by time course extraction from surficial sediment samples of 82 Florida lakes. Separation of diatom BSi from sponge BSi was based on the observation that diatoms completely dissolve within 2 h of digestion at 85 °C in 1% Na2CO3 whereas sponge spicules, which are generally larger than diatoms, take longer to dissolve. Sponge samples from four lakes in northern Wisconsin ranged widely in the time required to dissolve completely (1.5–12 h), but no significant differences were observed in the rates of dissolution among the lakes. In Florida lake sediments, diatom BSi averaged 49.2 (± 48.4) mg∙g−1 and sponge BSi averaged 31.5 (± 35.8) mg∙g−1, with sponge BSi comprising on average 40% of the total amorphous silica extracted. The procedure for separating diatom BSi from sponge BSi underestimates sponge BSi because smaller and/or lightly silicified components of sponges are completely dissolved early in the digestion. However, because sponge spicules comprise a significant fraction of total amorphous silica extracted, we hypothesize that sponge spicules, which on average are larger than diatoms and require a longer time for complete dissolution, may constitute an important sink for BSi in Florida lakes.