Marine death assemblages typically reflect the ecological and environmental signatures of the settings in which they are deposited. Comparatively little is known, however, about the effects of environmental change on death assemblages or with what acuity past changes might be preserved, by proxy, in sedimentary deposits. This study assesses the response of present-day molluscan death assemblages, both among surface sediments and stratigraphically, to an instance of anthropogenic environmental change along a carbonate bank. Additionally, I investigate whether body size changes can be recognized among death assemblages as a component of a broader response to changing environmental conditions.
Environmental perturbation along Cross Bank, in Florida Bay, resulted from an experiment, during the early-mid 1980’s, where local benthic environments were subjected to nutrient enrichment. Elevated nutrient levels drove a transition between morphologically and sedimentologically distinct seagrass species, from Thalassia testudinum, the regionally dominant seagrass species, to Halodule wrightii. Altered localities from this previous investigation, occurring at regular intervals along the crest of Cross Bank, were used to form an experimental transect to investigate changes in molluscan death assemblages following environmental changes along the bank. A second, control transect was established at sites approximately 50 m N-NE of altered localities to capture the signature unaltered death assemblages. Data considered here were derived from two distinct sampling protocols: two shallow (~15 cm) push cores and a single (82 cm average length) piston core, later subsampled at 2 cm increments, collected at each of twelve study sites to assess the present state of molluscan death assemblages and their ability to record historical environmental change.
Results demonstrate that molluscan death assemblages can effectively retain, by proxy, evidence of environmental change and signals may persist in the sedimentary record on decadal time scales. A transition from Thalassia to Halodule at altered sites appears to be preserved stratigraphically among death assemblages as an envelope defined by assemblage-level properties and compositional variations, rather than a distinct turnover event. Specimen abundance, taxonomic richness, and calculated evenness display strong declines with depth below the sediment-water interface across study localities, likely a taphonomic modification of assemblages through compaction. Findings also indicate that death assemblages are capable of capturing within-taxon changes in mean body size and distribution over ecological time scales, which may elucidate underlying paleoecological shifts during an environmental change.