TEMPORAL FLUCTUATIONS OF 2 MEDITERRANEAN SALP POPULATIONS FROM 1967 TO 1990 - ANALYSIS OF THE INFLUENCE OF ENVIRONMENTAL VARIABLES USING A MARKOV-CHAIN MODEL

The weekly abundances of the sexual phase (aggregate zooid) of 2 salp populations (tunicates Thalia democratica and Salpa fusiformis) were determined from 1967 to 1990 at a fixed station in the coastal waters of the Western Mediterranean, using a discrete scale of abundance. For both species, the abundance time series shows the occasional development of blooms of aggregate zooids. Several hydrological and meteorological variables were recorded concomitantly, and their influence on the occurrence of blooms was evaluated using a Markov regression model for ordinal time series. Abundance at Week t was found to depend on the size of the population for the 2 previous weeks and on environmental variables. The influence of hydrological variables, air temperature and irradiance corresponds to the relationship between seasonal stratification of the water column and occurrence of blooms: the stronger the stratification, the lower the probability of observing high densities of salps. Wind stress was also found to exert a significant influence, with gusts of wind facilitating the development of blooms through the renewal of the pelagic production. The influence of easterly winds, which involves a process of surface waters accumulating near the coast, is limited to T. democratica. Conversely, the influence of westerly winds, which leads to local upwelling, is limited to S. fusiformis. These results are consistent with known differences in the spatial distribution of T. democratica and S. fusiformis with respect to both distance to the coast and depth.