Looking Through Glass: Phytoplankton Players in Freshwater Silicon Cycles

Abstract

Spring dissolved Si (dSi) concentrations in Lake Michigan doubled from 1983 to 2016. Most recent budgets for Si were done in the 1980s, when Lake Michigan was much more eutrophic and prior to invasions by dreissenid mussels. To investigate possible causes of recent dSi increases, I investigated a wide range of phytoplankton taxa for dSi utilization and examined the rate of dSi release from diatoms in freshwater. These laboratory experiments were coupled with frequent nearshore and infrequent offshore sampling for dSi fluctuations over two years. The phytoplankton utilization experiments found only taxa known for creating biosiliceous structures were affected by Si limitation and were inhibited by the addition of GeO2 (a SiO2 analog) in a Si-replete growth medium. Only these taxa accumulated significant biogenic Si (bSi), examined by chemical digestion and using Si localization with scanning electron microscopy. To examine possible rates of dissolution of bSi into the dSi pool, jars of Lake Michigan sterile or filtered Lake Michigan water were inoculated with either killed diatoms or cleaned frustules. Jars were inverted to mix frequently. I tested the rate of dSi release from freshwater pennate diatom Synedra sp. at temperatures representing the Lake Michigan hypolimnion (4 °C) and summer epilimnion (18 °C). The dissolution of bSi was faster at 18 °C, but both temperatures showed almost 100% dissolution by day 176. In a second experiment, two different species, a pennate and a centric diatom, were examined for dissolution over a shorter period of 76 days at 4 oC. In this experiment, ~50% dissolution was observed for both diatom species. There was a significant difference in dissolution rates between species, with smaller, centric diatoms dissolving faster than a larger pennate, indicating that bSi dissolution in Lake Michigan is likely to be affected by the diatom species composition. With only siliceous species using dSi, the Si cycle is likely to be influenced by the growth of diatoms in Lake Michigan. Dissolution could also occur faster in the warmer nearshore regions where benthic beds of Cladophora with diatom epiphytes could foster bacterial communities efficient at dSi recycling.