Résumé
The Marine Protected area of Mljet Island, South Adriatic Sea, embodies an almost completely enclosed marine lake, the Veliko jezero, which provides a unique opportunity to investigate responses of plankton food webs to climate change. We here compile long term data, from the period 1980-2011, to examine decadal patterns of water temperature and the plankton food web (nutrients, phytoplankton, zooplankton, microbial plankton) with emphasis on mesozooplankton and jellyfish. Significant interannual variations of temperature were found with a prevalence of positive anomalies since 1997, with 2003 and 2006-2008 characterized by particularly high values. Positive changes were mostly observed during winter whereas in other seasons the differences were smaller. Along with these temperature changes, thermophilic mesozooplankton taxa displayed marked variations. For instance, Cladocera were first noted in the Veliko jezero 89 in the 1980s, while their abundance reached significant numbers from the late 1990s. Also, Calanus helgolandicus showed an upward trend concurrent with enhanced water temperature after 2000. On the other hand, the boreal copepod Pseudocalanus elongatus, which before 1990 was one of the predominant species, disappeared from this ecosystem during the last 20 years. In addition, marked changes were also noticed in the size of the moon jellyfish, which showed a reduction in bell diameter as water temperatures rose. Since the Veliko jezero ecosystem is protected from anthropogenic disturbances such as pollution or fishing, the observed planktonic responses shed light on potential ecosystem changes occurring in the South Adriatic Sea.