THE POWER OF BIVALVE ASSEMBLAGES AS INDICATORS OF ECOLOGICAL CONDITION OVER SPACE AND TIME

Biotic indices are often used to assess ecological condition using the abundance-weighted pollution tolerances of taxa. Applying such indices to recent fossil records – e.g., time-averaged death assemblages (DAs) – is a promising method to (1) characterize conditions from before whole-fauna monitoring began, and (2) detect otherwise unappreciated strain in systems using live-dead discordance. However, the robustness of biotic indices when applied to taxonomic subsets, such as shelly fauna, is under-explored. Here, we compare the power of indices calculated from a bivalve-only dataset using southern California’s Benthic Response Index (BRI), ATZI’s Marine Benthic Index (AMBI), and BENTIX.

Our test material is (a) a 45-year-long dataset of living macrobenthos from the Palos Verdes shelf in southern California, sampled annually at 44 sites by the Sanitation Districts of Los Angeles County to monitor benthic conditions influenced by treated wastewater discharge, and (b) bivalve DAs from the 2008 survey. The live time series was parsed into five temporal bins, and we calculated indices for the whole fauna, living bivalves, and bivalve DAs.

All indices indicate that benthic conditions improved with improving wastewater treatment, with the greatest changes close to the outfall. Bivalves underestimated whole-fauna AMBI because they occupy only low- and mid-disturbance AMBI groups (I-III), and thus cannot detect the most severely polluted sites. In contrast, BRI and BENTIX based on bivalves-only correlated strongly with the whole fauna: the binary group assignment used by BENTIX elevates the influence of AMBI group III bivalves.

Indices for bivalve DAs – which include shells >100s yrs old on this shelf – indicate less strain than was observed in early communities (1970s-80s) and either agree with or overestimate the strain in more recent communities (2000s-10s). This live-dead discordance suggests that time-averaging causes DAs to retain a signal from both pollution and pre-pollution benthic conditions that the shelf benthos is now re-attaining.

Bivalves are thus an effective subset of the whole fauna for detecting gradients in benthic conditions using biotic indices. Counterpart indices for DAs can reveal both the existence and direction of change in a community relative to its historic range of variation.