Epiphytic calcium carbonate production and facies development within sub-tropical seagrass beds, Inhaca Island, Mozambique

Seagrass beds have been widely recognised as playing an important role in influencing carbonate sediment facies development. This reflects their role not only as facilitators of fine sediment settling and stabilisation, but also as substrates for epiphytic organisms that, after death, contribute skeletal carbonate to the sediment substrate. In low latitude (reef-related) settings, epiphytic carbonate production rates are often high and this, in combination with the trapping of carbonate mud produced by a range of associated calcareous algal species, typically results in the development of carbonate mud-rich facies. Whilst such environments, and their associated sediment substrates, have been widely documented, studies of seagrass facies in marginal (sub-tropical/warm temperate) marine settings have not been conducted from a sedimentological perspective. This study determines rates of epiphytic carbonate production on two seagrass species Thalassodendron ciliatum and Thalassia hemprichii, and examines seagrass sediment facies from a sub-tropical reef-related environment in southern Mozambique. Dense seagrass beds colonise primarily siliciclastic sediment substrates and are characterised by low rates of epiphytic carbonate production (mean: 43.9 g CaCO3 m(-2) year(-1) for T ciliatum, and 33.4 g CaCO3 m(-2) year(-1) for T hemprichii). Epiphytic encrusters are dominated by thin, monostromatic layers of the crustose coralline red algae Hydrolithon farinosum, along with rotaliid smaller benthic foraminifera (including Asterorotalia cf gaimardi and Spirillina sp.) and the soritid Peneroplis sp., as well as rare encrusting acervulinid foraminifera, serpulids and bryozoans. Epiphytic calcium carbonate production rates are therefore low and this is reflected in the low (< 15%) carbonate content of the seagrass sediments, as well as the low (< 1%) sediment fine (< 63 mu m size fraction) content. This study suggests that mud-rich sediment facies do not necessarily develop in conjunction with seagrass beds and that mud-poor facies may develop in some seagrass-colonised environments. At these sites (sub-tropical and siliciclastic-sediment dominated) this is suggested to reflect both the low levels of epiphytic and benthic algal carbonate mud production as well as a net outflux of fine-grained sediment due to tidal current-induced sediment resuspension. In keeping with carbonate facies generally there are thus likely to be latitude and environment-related shifts in the composition and character of seagrass related sediment facies. (c) 2004 Elsevier B.V. All rights reserved.