Scripps Institution of Oceanography

The effects of climate on abyssal megafaunal populations are largely unknown. Photo-transecting techniques have been used at a long-term abyssal study site in the NE Pacific to examine the population dynamics of mobile echinoderms on a roughly seasonal basis from 1989-2004. At 4,100 m depth, these animals live in a relatively stable environment with no light and relatively invariant temperatures. The primary habitat variables at the site are the quantity and quality of the particulate organic carbon (POC) that sinks from the surface waters above. Significant time-lagged correlations indicate that climatic variations related to the El Niño Southern Oscillation can influence surface ocean productivity and the amount of POC that is exported to the abyssal benthos.

All of the ten most dominant fauna at the site had order of magnitude or greater fluctuations in abundance with the most prominent differences occurring between 1989-1998 and 2001-2004. Increases in abundance were often linked to increases in smaller individuals suggesting that population fluctuations were the result of recruitment, biological interactions, and mortality. Significant correlations also exist between changes in the POC food supply and the abundance of several mobile megafauna with measurable changes in abundance lagging changes in food supply by 6 or more months. Some species, such as the holothuroid Elpidia minutissima, increased in abundance during periods of low food supply while other echinoderms thrived during periods of higher food flux.

One assumption of the line-transect methodology used here is that non-random aggregations are not present at scales or patterns that might bias density estimates. To help verify this assumption a three-term local quadrat variance technique was employed and found few aggregations that might bias the abundance estimates presented. A significant shift in megafauna community structure occurred between the 1989-1998 and 2001-2004 periods. An analysis of the rank abundance distributions of megafauna indicated that the overall species dominance changed between the two periods. This shift in equitability was also linked to the POC flux further indicating that these animals have differential responses to the food supply. This and other longterm research have indicated that interannual-scale climatic fluctuations could be influencing abyssal populations thousands of meters beneath the ocean surface.