POTENTIAL ROLE OF SPONGE SPICULES IN INFLUENCING THE SILICON BIOGEOCHEMISTRY OF FLORIDA LAKES

Amorphous silica, e.g. biogenic silica (BSi), contained in diatoms and in sponge spicules was estimated by time course extraction from surfi cial sediment samples of 82 Florida lakes. Separation of diatom BSi fr om sponge BSi was based on the observation that diatoms completely dis solve within 2 h of digestion at 85-degrees-C in 1% Na2CO3 whereas spo nge 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 amon g 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 spon ge BSi comprising on average 40% of the total amorphous silica extract ed, The procedure for separating diatom BSi from sponge BSi underestim ates sponge BSi because smaller and/or lightly silicified components o f sponges are completely dissolved early in the digestion. However, be cause sponge spicules comprise a significant fraction of total amorpho us silica extracted, we hypothesize that sponge spicules, which on ave rage are larger than diatoms and require a longer time for complete di ssolution, may constitute an important sink for BSi in Florida lakes.