Temporal fluctuations of two Mediterranean salp populations from 1967 to 1990. Analysis of the influence of environmental variables using a Markov chain model

Abstract

The weekly abundances of the sexual phase (aggregate zoold) of 2 salp populatlons (tunlcates Thaha democratlca and Salpa fuslform~s) were determined from 1967 to 1990 at a fixed statlon in the coastal waters of the Western Mediterranean, using a discrete scale of abundance For both specles the abundance tlme senes shows the occasional developn~ent of blooms of aggregate zoolds Several hydrologlcal and meteorological variables were recorded concormtantly, and thelr Influence on the occurrence of blooms was evaluated uslng a Markov reglesslon model for ordlnal tlme senes Abundance at Week t was found to depend on the slze of the population for the 2 prevlous weeks and on environmental vanables The Influence of hydrologlcal variables alr temperature and lrradlance corresponds to the relationship between seasonal strahficatlon of the water column and occurrence of blooms the stronger the stratlflcatlon the lower the probablllty of observing hlgh densities of salps Wlnd stress was also found to exert a slgnlflcant ~nfluence, wlth gusts of wlnd fachtatlng the development of blooms through the renewal of the pelagic production The lnfluence of easterly wlnds, w h ~c h lnvolves a process of surface waters accumulating near the coast, is h m t e d to T democrat~ca Conversely, the influence of westerly wlnds, whlch leads to local upwelhng, IS h ~t e d to S f u s r f o m s These results are consistent w t h known ddferences In the spatlal dlstnbuhon of 7 democratica and S fuslfonrus wlth respect to both dlstance to the coast and depth