Benthic megafauna assemblage change over three decades in the abyss: Variations from species to functional groups

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Megafaunal seafloor assemblages on the Monterey Fan in the NE Pacific (Station M, 4000 m depth) were studied between 2006–2018 using remotely operated vehicles (ROVs) as part of a continuing time-series study that began in 1989. Since 2006 we identified nearly 120,000 individual animals representing over 142 morphospecies, and observed continuous changes in the megafaunal assemblage. This study, which tracked variation in observed morphospecies over a 13-year period, is one of the most detailed long-term records of megafaunal change for abyssal depths. Our investigation shows that new variations continued to emerge, reinforcing the concept that the deep-sea is dynamic over short time scales, rather than static over long periods. Some species were uncommon, but later observed in high numbers, then decreased to very low or undetectable levels (e.g. Elpidia sp. A), while others (e.g. Psychropotes longicauda) exhibited a relatively persistent presence with less fluctuation in abundance. Decreasing total echinoderm density from 2013–2018 did not correspond with the continued occurrence of large episodic POC flux events between 2016–2018. This may be attributed to the quality of food supply arriving at the seafloor and the varied ability of organisms to utilize it. Long-term tracking (30 years) of 10 specific epibenthic echinoderm species originally quantified from camera-sled images shows a pattern of assemblage structure, perhaps returning toward the composition observed in the 1990s and early 2000s. Many questions remain as to how this abyssal site and others will change with continued, and potentially increasing, anthropogenic change in the upper ocean. For example, the marine heat anomaly known as the ‘Warm Blob’ may have influenced major ecological processes at the abyssal seafloor in terms of morphospecies and functional group composition due to changes in POC flux. The degree of dynamism continues to indicate that ad hoc or short-term investigations provide a limited perspective for assessing community structure in conservation or resource exploitation impact assessment studies in the deep sea.