engleski

On phenology changes of two jellyfish species (Aequorea forskalea, Cotylorhiza tuberculata) in the northern Adriatic: comparison of recent and early 20th century data

In recent decades, jellyfish abundance has increased in several marine habitats, with significant ecological and socioeconomic consequences (Graham et al. 2014). There are few comprehensive temporal and spatial data series on jellyfish in the Adriatic Sea. Pestorić et al. (2021) noted in their review that pelagic Scyphozoa and Ctenophora taxa are significantly more abundant in the northern part of the Adriatic than in the rest of the Adriatic. Nevertheless, the differences between subregions have become less pronounced in recent years. Therefore, it is possible that the results of our analysis for the northern Adriatic can serve as projections for the whole Adriatic. The Adriatic Sea, especially its northernmost part, is warming ; Raicich & Colucci (2021) estimated a mean temperature increase of 1.1 ± 0.3 0C using the time series 1899 - 2015. In addition, several authors (e.g. Stiasny 1911) have published articles on monthly semiquantitative data on plankton community composition between 1900 and 1911. Thus, data from the beginning of the 20th century are available for sea temperature and gelatinous plankton. Therefore, we aimed to compare the results obtained at that time with the situation in recent decades. Two macrogelatinous species with a bipartite life cycle (benthic polyps and planktonic medusae) were selected for comparison: a temperate cold-water species Aequorea forskalea (Hyrozoa: Leptothecata) and a temperate warm-water species Cotylorhiza tuberculata (Scyphozoa: Rhizostomeae), for which semiquantitative data are available from both periods (1900-1911 ; 2010-2019). To obtain the monthly adjusted semiquantitative values for the last period, the following procedure was ensued: if no medusae were observed in a month, a value of 0 was assigned ; if individual organisms were observed sporadically, a value of 1 was ascribed to a month ; a value of 2 was assigned to a month in which we observed small aggregations of medusae and/or individual organisms were observed frequently ; and finally, a value of 3 was noted in a month in which we observed large aggregations frequently. These values (0, 1, 2, 3) correspond to 0, r (rare), c (common), cc (very common) used in publications from the beginning of the 20th century. A one-way ANOSIM test between seasons revealed significant differences in temperature in both time periods ; the exception was a nonsignificant difference between spring and autumn for the historical period. In addition, SIMPER analysis revealed significant seasonal differences between the two time periods (H vs. R). Spearman rank order correlation between semiquantitative abundance of A. forskalea and temperature was significant (r= - 0.4935, p < 0.05), while Mann-Whitney test revealed statistically significant differences between winter (p=0.014) and autumn (p=0.005) of historical data compared to current data. While A. forskalea was present and sometimes abundant in the historical autumn data set, it was completely absent in the recent data set. In contrast to the cold water affinity of A. forskalea, Cotylorhiza tuberculata is a typical warm water species.

jellyfish ; recent and historical data ; warming ; Adriatic Sea

nije evidentirano