Gradients and patterns of sclerobionts on experimentally deployed bivalve shells: Synopsis of bathymetric and temporal trends on a decadal time scale

Abstract

Bivalve shells deployed experimentally in a variety of environments of deposition (EODs) and over a wide range of depths from 15 to 570 m in the Bahamas and Gulf of Mexico were assessed for encrusting and endolithic bionts (sclerobionts sensu Taylor and Wilson, 2002) that accumulated over 1-, 2-, 8- and 12/13-year intervals. Data include percent coverage and, when possible, identification to generic- or species-level taxonomic categories. Data were binned into two further categories based on guilds and whether or not sclerobionts were judged to be preservable. Each study site shows substantial variability in both percent coverage and species richness/composition, which we attribute to partial burial of different individuals or parts of arrays. Average values of taxonomic richness, guild number, and percent coverage, for a given EOD represent a combination of exposed and buried shells as would be typical of a time-averaged fossil assemblage; maximally covered shells give the potential coverage for each site. In both study areas there are characteristic patterns in terms of overall percentage of shells covered by sclerobionts, numbers and extent of coverage by different guilds as well as by different taxa. For most EODs, there is a progressive, though non-linear increase in both species richness and areal coverage through the twelve years; exceptions are in the 30 m site in the Bahamas, and Parker Bank and Flower Garden sites in the Gulf of Mexico, which show a significant decline in areal coverage (though not in richness). This effect may result from differences in sediment cover over time. These sites showed a tendency to have many or all experimental bags completely sediment covered in the later sampling. Cluster analysis and non-metric multidimensional scaling (NMDS) of various shell substrate species at all EODs from the Bahamas and Gulf of Mexico reveal discrete groupings that are related to both depth (light?) gradient and other factors, such as variations in sedimentation and salinity. Similar numbers of guilds are found at various EODs although the specific guild types represented differ. In general, highest average richness values are recorded for carbonate 70–90 m wall and deep reef sites (69 to 70 taxa) and lowest are for the EFG brine pool (no sclerobionts other than bacterial films after 12/13 years) and deeper dysphotic to subphotic sites (max values: 25 taxa); surprisingly, shallow water sites (< 30 m) yield only moderate diversities of 40–50 taxa. This results in part from the fact that certain encrusters, especially the foraminifer Gypsina and coralline algae tend to overgrow others and dominate space, in addition to burial effects. Certain taxa (notably the serpulid Pseudovermilia) are ubiquitous, while other taxa (e.g. larger foraminifera, vermetid gastropods and many bryozoans) are largely restricted to shallower depths. A combination of species richness, average coverage and key taxa permits discrimination of groups of EODs and potentially, in tandem with other evidence, of depth gradients.