Temperature variation in the Mar Piccolo of Taranto (Italy, Mediterranean sea)

The aim of the present paper is to detect a possible temperature increasing trend in the Mar Piccolo basin (Taranto) in accordance to what observed in the western Mediterranean where a significant increase in the average temperature was registered. In fact, the increasing rate of global climate change observed in the last century is predicted to accelerate in the present century by the end of which temperature is expected to rise 1-3.5°C. In particular, enclosed coastal basins, like the Mar Piccolo, are more subject to temperature increase the effects of which can be enhanced by the confinement. Observing climatic fluctuations is of the most importance in that it is well known that they strongly affect the marine biota and there are evidence that some observed biodiversity changes in the Mediterranean Sea are related to increasing seawater temperature. Therefore, the study of temperature variations is important to better understand the marine communities changes and, sometimes, to predict such changes. To verify the existence of an increasing temperature trend in the Mar Piccolo basin, the historical data series, even though not continuous, in possession of the Institute of Coastal Marine Environment of Taranto was analysed. The above temperature data analysed come from a station (lat. 40°28'47" N, long. 17°15'38" E) located in the Primo Seno of the Mar Piccolo in the following periods: 1919-1923, 1932-1934, 1962-1969 and 1996-2006. A non parametric test (2) was performed on raw data to single out significative variations of each monthly mean. Successively, on the raw data series, a mobile mean was calculated, by a linear model, to obtain 12 parameters for those years more represented in the measures, so eliminating every variation due to seasonality. On this group, consisting of 12 parameters for each year, the ANOVA was carried out followed by the algorithm of Duncan test to single out years homogeneous groups. The cluster analysis was performed to detect years homogeneous groups. 2 analysis pointed out that the various years are not significatively different; only 10 months out 324 are significatively different from the expected mean value. The calculation of the yearly tendencies did not show specific linear trends (both slopes and r2 values being close to 0). Examining the groups of years with similar characteristics, a some uniformity among contiguous years is detectable with the exception of the years 1920, 1921 and 1922 which are similar to those of the period 1996-2006, and the year 1996 which is similar to the period 1932-1934. Moreover, it is also clear that the recent years, except 1996, are comprised in the clusters where the highest mean values fall. Therefore, it is possible to conclude that, in the Mar Piccolo of Taranto, the trend to temperature increase is confirmed although the available data series does not permit to exactly quantify such increase. The observed temperature increase is supported by the presence in the Mar Piccolo of numerous warm-water non-indigenous species, many of which are lessepsian migrants, which are spreading throughout the basin. On the contrary, the cold-temperate water species, even though usually reported as invasive, are undergoing a regression. In the light of these considerations, to know the temperature trend of a particular zone could permit to predict marine community changes and, in particular, the destiny of an introduced species on the basis of both its biogeographic element and affinity to temperature.