The contribution of siliceous sponges to the silicon cycle of a diatom-rich shallow bay

XVII International Symposium on Oceanography of the Bay of Biscay, 1 – 4 June 2021, Virtual Event

There is great interest in studying the cycling of silicon in marine environments because this nutrient is key to the functioning of marine ecosystems. Silicon is an essential nutrient for a wide variety of marine micro- and macro-organisms, the silicifiers, which need it to build their skeletal structures. In coastal waters, planktonic and benthic silicifiers have to share the pool of dissolved silicon. While the contribution of planktonic diatoms to the cycling of silicon in coastal systems is generally well studied, that of benthic silicifiers such as sponges remains largely unaddressed. Herein we present the silicon stocks and flux rates related to the sponge community of the Bay of Brest (France), a shallow-water, highly productive ecosystem the annual primary production of which is dominated by diatoms. A total of 45 siliceous sponge species living in the Bay provide a total standing stock of 1550 tons of silicon, which is about 50 times greater than that of diatoms. The sponge silicon reservoir accumulated in the superficial sediments of the Bay also largely exceeds diatom accumulation. These comparatively large stocks of sponge silicon are estimated to cycle 20 to 100 times slower than equivalent diatom stocks. While sponge silicon turnover occurs over years to decades, diatom turnover lasts only days. Our results highlight that the silicon budget in sponge communities is substantially different from that in diatom assemblages, indicating that the comparison between these two groups of silicifiers is not straightforward due to their contrasting biological features (benthic and long-lived vs. planktonic and short-lived).They also suggest that if the increasing humaninduced perturbations to marine benthic communities seriously affect sponge communities, their large silicon stocks and reservoirs will likely be impacted. This could eventually trigger imbalances in the silicon budget of coastal systems, which may require decades to restore the equilibrium of the benthic-pelagic environment

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