ENSO and sandy beach macrobenthos of the tropical East Pacific: some speculations

Abstract. The influence of the ENSO cycle on marine fauna and flora has only recently been given the attention it deserves. The very strong 1997–1998 El Niño and its obvious effects on marine biota was a key point in ENSO research, but unfortunately few quantitative data about the 1997–1998 El Niño itself are available.

To gather information about the effect of ENSO on the macrobenthos, we performed a bi-weekly transect monitoring on an Ecuadorian sandy beach in 2000–2001, during the strong La Niña following the 1997–1998 El Niño, and in the normal period of 2002–2004.

In this paper, intertidal macrofaunal densities at higher taxonomic level are used to compare a La Niña phase with the 'normal' situation. The few existing documents about El Niño and sandy beach macrobenthos, and scattered data from previous and current research, were used to complete the picture.

Total macrobenthos densities were 300% lower during the La Niña phase compared with equal months in the normal phase. Especially Crustacea and Mollusca showed a marked increase in densities towards the normal situation (94% and 341% respectively). Polychaeta and Echinodermata, however, showed higher densities during the La Niña phase (22% and 73% respectively).

Two possible explanations are proposed. (1) Low densities during the La Niña could be due to the very strong preceding El Niño, suggesting the populations were still recovering. This hypothesis is supported by previous work done in the south of Peru. This is, however, a cold water system, compared to the Ecuadorian warm water system. (2) The second hypothesis states that a La Niña will have a very severe impact on the intertidal macrofauna of a warm water system like the Ecuadorian coast.