Monitoring the recolonization of the Mt Pinatubo 1991 ash layer by benthic foraminifera

Benthic foraminifera from the South China Sea were studied to assess mass mortality and to monitor the composition and recovery of the benthic communities following the 1991 Mt Pinatubo ashfall. Surface distribution data from monitoring stations in the eastern South China Sea that were occupied during four cruises between spring 1994 and summer 1998 display the following trends in recolonization patterns: (1) Suspension feeding epifaunal benthic foraminifera (i.e. Cibicidoides wuellerstorfi, Saccorhiza ramosa) and large xenophyophores (i.e. Syringammina (?) fragilissima) were absent in spring 1994 and only rare individuals were observed in June 1996, but in larger numbers in December 1996 and in summer 1998. Then, they were important recolonizers of the ash layer. (2) Diversity and population densities have changed significantly since 1994. Following an abundance maximum in winter 1996, the numbers of living individuals in summer 1998 decreased again and the deep sea benthic foraminiferal community started to return to a normal ecological structuring. However, infaunal foraminifera were still strongly dominated by several species of the genus Reophax. We interpret the changing abundance and diversity pattern during the recolonization process in two ways: (1) the markedly increasing activity of burrowing macrofauna observed since 1998 opened new ecological niches for infaunal benthic foraminifera but also intensified predator pressure; (2) competitive interactions within the recolonizing fauna began to play a major role. Opportunistic pioneer species, characterized by rapid reproduction rates and the capability to colonize disturbed environments, were outcompeted by non-opportunistic species.