Impacts of light penetration on the bathymetry of reef microboring communities: Implications for the development of microendolithic trace assemblages

Sediment samples recovered from three proximate reef environments in north Jamaica, characterised by different light penetration regimes, reveal marked variations in the bathymetric range of sediment microboring communities. In contrast to clear water environments, assemblages developed in turbid water conditions undergo both bathymetric restriction and compression in their range. In clear water sites, the upper photic - lower photic zone transition occurs between depths of 20-30 m. This is marked by a shift from a predominance of cyanobacterial (e.g., Eurygonum nodosum, Scolecia filosa, Fasciculus frutex and Fasciculus dactylus) and chlorophyte (e.g., Rhopalia catenata, Fasciculus grandis, Reticulina elegans and Scolecia botulifera) traces, to assemblages dominated by the traces E. nodosum and S. filosa (cyanobacteria), F. grandis (chlorophyte), Palaeoconchocelis starmarchii (rhodophyte) and Orthogonum fusiferum and Saccomorpha sphaerula (fungi). At sites characterised by intermediate levels of turbidity this same assemblage transition occurs at around 15^20 m depth and at the most turbid site at depths of only 5-10 m. Integration of assemblage data and light readings indicates that this transition occurs where percentages of surface light illumination are reduced to around 10-15%. Compression of the lower photic zone is so extreme at the most turbid site, that microboring assemblages at only 30 m depth show evidence of a transition to an assemblage associated with dysphotic (6 1% of surface illumination) conditions. This is dominated by fungal boring traces. Given that microendolithic assemblages have been widely considered to represent useful palaeobathymetric indicators, this study illustrates that it is more accurate to regard assemblages as indicative only of photic zones. The bathymetric range and extent of zones clearly varies significantly as light penetration levels change. Caution is thus needed in the extrapolation of assemblage or ichnocoenosis data to specific bathymetric ranges.