Evidence of intensified biogenic silica recycling in the Black Sea after 1970

The Black Sea has been subject to increased levels of nitrogen and phosphorus loading and a decrease in silicate input after around 1970. Changes in phytoplankton community composition from diatoms to non-diatom groups have been attributed to the decrease in silicate. However, a discrepancy between the decreasing silicate input and the increasing silicate pool in the deep sea reported elsewhere implies that another silicate source exists which challenges the current paradigm of widespread silicate limitation. In this study, we investigate changes in the dissolution state of siliceous protists over the last 140 years and show that siliceous protists became significantly more dissolved after the late 1960s indicating a reduction of the silicate pool preserved in the deep sea sediment. We hypothesize that the decline in the dissolution state is caused by increased recycling of biogenic silica in the water column due to an increased annual production driven by nitrogen enrichment.