Abstract
Tropical shelf, platform and reef-lagoon systems are dominated by calcium carbonate (CaCO ) sediments. However, data on habitat-specific CaCO sediment production rates by different sediment producing taxa are sparse, limiting understanding of where and in what form CaCO sediment is produced, and how overall sediment budgets are influenced by habitat type and scale. Using novel census methodologies, based primarily on measures of plant biovolumes and carbonate content, we assessed habitat-scale production by two ubiquitous biogenic CaCO sediment producers, calcareous green algae and seagrass epiphytes, across southern Eleuthera Bank, Bahamas (area -140 km ). Data from species-specific plant disaggregation experiments and from X-ray diffraction (XRD) analysis of calcified plants also allowed us to resolve questions about the size fractions and mineralogies of the carbonates produced. Production rates varied significantly among habitats (range: 1.8-237.3 g CaCO m yr ), collectively totaling -0.98 M kg annually across the study area. Outputs comprise similar amounts of aragonite and high Mg-calcite, with -54% of the CaCO produced being contributed as mud-grade (<63 μm) sediment. Our analysis also reveals that habitat type and extent - especially of medium and high density seagrass beds - strongly influence the amounts and types of carbonate sediment generated. Dense seagrass beds were identified as the dominant per unit area production sites, contributing -17% of total CaCO despite covering only 0.5% of the study area. These findings have direct relevance for quantifying present-day sediment budgets and for predicting changes in sediment generation at the system scale in responses to modified habitat extent and productivity.
Highlights
Calcium carbonate (CaCO3) dominated tropical and sub-tropical coral reef, lagoon, and shallow shelf and platform systems cover an area estimated at > 11 M km2 (Milliman and Droxler, 1996; Green et al, 2000)
The platform itself comprises a range of sub-tidal and intertidal habitats, and whilst there is inevitably some gradation between habitat boundaries, the following major habitat types were delineated: mangrove fringed tidal creeks and embayments; hardgrounds sparsely colonized by green algae, sponges and corals; large expanses of patchily bioturbated sands colonized by sparse green algae and seagrass; and beds of T. testudinum seagrass
We present a methodology designed to support estimates of the rates, grain sizes and mineralogies of carbonate sediment generated by several species of calcareous green algae and by seagrass epiphytes that are ubiquitous substrate colonizers across many tropical and subtropical marine systems
Summary
Calcium carbonate (CaCO3) dominated tropical and sub-tropical coral reef, lagoon, and shallow shelf and platform systems cover an area estimated at > 11 M km (Milliman and Droxler, 1996; Green et al, 2000). These environments are focal points of marine CaCO3 accumulation, but they provide sediments to sustain tropical shorelines and islands (Perry et al, 2015) and export material to adjacent deeperwater deposits (Berelson et al, 2007). Complex cycles of carbonate precipitation and dissolution may occur within permeable reef and platform sediments (Eyre et al, 2018) and may influence net carbonate production rates (Brock et al, 2006)
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