Monitors with Memories: Death Assemblages Record a Century of Wastewater Pollution and Remediation

Abstract

Biotic indices are often used to assess ecological condition using the abundance-weighted stress 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 monitoring began, and (2) detect otherwise unappreciated strain using discordance with the living assemblage (LA). However, the robustness of regionally-specific biotic indices when applied to paleoecological data is under-explored. Here, I assess the power of three indices: Southern California’s Benthic Response Index (BRI), ATZI’s Marine Benthic Index (AMBI), and BENTIX. Our test material is (a) a 50-year-long dataset of macrobenthos from the Palos Verdes shelf in Southern California, sampled annually at 44 sites to monitor the effects of treated wastewater effluent, and (b) bivalve DAs from the 2008 survey. The time series was parsed into temporal bins based on wastewater treatment phases, and we calculated indices for the whole fauna, bivalve LAs, and bivalve DAs. All indices demonstrated that benthic conditions improved with remediation, and the greatest changes were close to the outfall source. Values generated for bivalves were strongly correlated to those of the whole fauna, indicating that bivalves are a strong surrogate for macrobenthic condition (second only to polychaetes when compared among other clades). Indices for bivalve DAs – which include shells >100s yrs old on this shelf – indicated less strain than was observed in early communities (1970s-80s) and either agreed with or overestimated the strain in more recent communities (2000s-10s). This live-dead discordance suggests that time-averaging causes DAs to retain a signal from pre-pollution benthic conditions that the shelf benthos is now re-attaining. Bivalve DAs, combined with long-term benthic time series data, can reveal both the existence and direction of change in ecological strain relative to historic conditions.