Abstract
Rhodoliths are nodules of non-geniculate coralline algae that occur in shallow waters (<150 m depth) subjected to episodic disturbance. Rhodolith beds stand with kelp beds, seagrass meadows, and coralline algal reefs as one of the world's four largest macrophyte-dominated benthic communities. Geographic distribution of rhodolith beds is discontinuous, with large concentrations off Japan, Australia and the Gulf of California, as well as in the Mediterranean, North Atlantic, eastern Caribbean and Brazil. Although there are major gaps in terms of seabed habitat mapping, the largest rhodolith beds are purported to occur off Brazil, where these communities are recorded across a wide latitudinal range (2°N - 27°S). To quantify their extent, we carried out an inter-reefal seabed habitat survey on the Abrolhos Shelf (16°50′ - 19°45′S) off eastern Brazil, and confirmed the most expansive and contiguous rhodolith bed in the world, covering about 20,900 km2. Distribution, extent, composition and structure of this bed were assessed with side scan sonar, remotely operated vehicles, and SCUBA. The mean rate of CaCO3 production was estimated from in situ growth assays at 1.07 kg m−2 yr−1, with a total production rate of 0.025 Gt yr−1, comparable to those of the world's largest biogenic CaCO3 deposits. These gigantic rhodolith beds, of areal extent equivalent to the Great Barrier Reef, Australia, are a critical, yet poorly understood component of the tropical South Atlantic Ocean. Based on the relatively high vulnerability of coralline algae to ocean acidification, these beds are likely to experience a profound restructuring in the coming decades.
Highlights
Shallow water tropical benthic communities such as coral reefs are well known to be major carbonate producers in coastal areas [1,2,3], and significant progress has been made in understanding their calcium carbonate (CaCO3) production by mapping their global distributions [1,2,3,4] and by estimating mineralization rates [5,6,7,8]
Rhodolith beds are aggregates of nodules of non-geniculate crustose coralline algae (CCA) that generally occur in waters shallower than 150 m depth subjected to episodic wave or current movement, forming large expanses of hard bottom habitat [9]
The region known as the Abrolhos Bank (16u509 - 19u459S) is a,46,000 km2 expanse of the eastern Brazilian continental shelf; its inner and mid shelf encompass the largest and richest biogenic reefs in the South Atlantic [17,18,19], bearing coral assemblages dominated by Brazilian-endemic Neogene relicts of the genus Mussismilia
Summary
Shallow water tropical benthic communities such as coral reefs are well known to be major carbonate producers in coastal areas [1,2,3], and significant progress has been made in understanding their calcium carbonate (CaCO3) production by mapping their global distributions [1,2,3,4] and by estimating mineralization rates [5,6,7,8]. There have been few attempts to map the large-scale distribution of rhodolith beds [9], but studies from the early 1970s suggest that those occurring off Brazil, between 2uN and 27uS, potentially represent one of the largest marine CaCO3 deposits in the world, with estimates of 261011 tons of CaCO3 [14,15,16]. CCAs are recognized as the most important component of Abrolhos reefs [15,16,17,18], and rhodolith beds are recorded to the north and to the south of this region [20,21], the geographic extent and role of rhodolith beds on the Abrolhos Shelf have been largely overlooked
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