ENCRUSTING BIONT GUILD DISTRIBUTION ON EUPHOTIC TO APHOTIC HARD SURFACES IN THE MODERN GULF OF MEXICO AND BAHAMAS

Analysis of assemblages of encrusting epibionts has great potential for the interpretation of paleonvironments. Patterns of encrustation on hard substrates have been fairly well documented in modern shallow water environments (<10 m), but little is known of encrusting communities in deeper water settings. Results presented here are drawn from eight-year experimental deployments of mollusk shell substrates at the sediment water interface in two separate oceanic regimes: (a) carbonate platform, escarpment and basin; depths of 15 to 290m: Lee Stocking Island, Bahamas, and (b) siliciclastic-dominated shelf/slope settings: 72 to 600 m in varied settings from muddy seafloor to petroleum seeps and a brine pool, in the Gulf of Mexico. Biont guilds with the potential to be preserved in the fossil record (organisms with hard skeletons or those that leave traces on the substrate) exhibit trends with respect to depth (light), substrate type, ocean basin, and even specific environment (such as petroleum seeps). In shallower areas (<100m), biont community composition was strongly controlled by the substrate upon which the experimental shells were deployed, being most diverse in sediment-bypassed hardgrounds. Depth/substrate related differences were found to be more than those between carbonate vs. siliciclastic-dominated basins. In the shallowest (<25m) euphotic zone attached mollusks (bivalves, vermetid gastropods) were common on shells deployed on hardgrounds. Other guilds specific to the shallowest sites, but not correlated to seafloor substrate type, include foraminifera that harbor photosymbionts, chitinous ciliates, and molluscan grazing traces. Shells on hard sea bottoms in the deeper photic zone (25-100m) possessed a large cohort of bionts, including coralline and endolithic algae, varied bryozoans, clionid sponges, serpulids, and foraminifera; corals are also present in the deeper photic zone, but showed no correlation with seafloor type. Lastly, the deeper dysphotic to aphotic zone (100-600m) regardless of seafloor type, is characterized by a sparse, low diversity assemblage of chitinous ciliates, endolithic fungi, foraminifera, and serpulid tubes. These results should have important applications to interpretations of paleoenvironments based on bionts preserved on fossil shell material.