ORIGINAL ARTICLE Annual dynamics of the mesozooplankton communities in a highly variable ecosystem (North Adriatic Sea, Italy) Elisa Camatti 1 , Alessandra Comaschi 1 , Alessandra de Olazabal 2 & Serena Fonda Umani 3 1 Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Venice, Italy 2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – Dipartimento di Oceanografia Biologica, Trieste, Italy 3 Dipartimento di Biologia, Universita ` di Trieste, Trieste, Italy Problem The northern Adriatic Sea is a very productive region at several trophic levels from phytoplankton to fish. Within this region, the delta of the Po River is particularly rich, with high but variable phytoplankton biomass and pro- duction, whose dynamics is mainly related to the spread- ing of its plume (Franco 1973; Gilmartin & Revelante 1981). Therefore, a marked west-to-east decreasing gradi- ent of phytoplankton biomass and production is often observed (Smodlaka & Revelante 1983), which can be reverted in case of severe and prolonged drought (Specchi & Fonda Umani 1983). The Gulf of Trieste is an area in which zooplankton is mostly studied, with monthly sampling of a coastal sta- tion since 1970 (Cataletto et al. 1995; Kamburska & Fonda Umani 2006). The whole basin has been sampled at different occasions during oceanographic surveys in different seasons on an irregular basis (e.g. Hure & Scotto di Carlo 1969; Comaschi et al. 1998; Hure & Krs ˇinic ´ 1998). Only recently spatial surveys have covered the whole area on a monthly basis in the framework of the MAT (Giani et al. 2005) and INTERREG cooperative pro- jects, allowing the collection of a reference dataset for the interpretation of zooplankton distribution and dynamics (Camatti et al. 2002; Fonda Umani et al. 2005a). In the northernmost part of the northern Adriatic Sea (Gulf of Trieste and northward of the river Po delta) me- sozooplankton is dominated by strictly neritic species with low diversity, and prevalence of Penilia avirostris in Keywords Acartia clausi; cladocerans; copepods; diatoms; mesozooplankton; nutrients; Penilia avirostris; river inputs; salinity; temporal dynamics. Correspondence Elisa Camatti, Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Castello 1364 ⁄ A, Venice I-30122, Italy. E-mail: elisa.camatti@ismar.cnr.it Accepted: 5 June 2008 doi:10.1111/j.1439-0485.2008.00256.x Abstract Mesozooplankton community composition at the species ⁄ taxa level was investi- gated from January to December 2001, at eight stations in the northern Adriat- ic Sea. Annual dynamics, taxonomic composition and spatial diversity in relation to different trophic conditions are discussed as related to previous studies and to different conditions, such as the presence of mucilage events. Zooplankton communities all over the northern sub-basin were dominated by the cladoceran Penilia avirostris in summer, and by the calanoids Paracalanus parvus, Acartia clausi and the poecilostomatoids Oncaea spp. during the rest of the year. Whereas coastal communities were more variable with time and location, we were able to identify a group of offshore stations with a similar species ⁄ taxa composition and annual dynamics. Comparing the present results with previous studies, we observed significant changes in community composi- tion in the time scale of 20 years, mostly due to a general decrease of A. clausi as the dominant species, being replaced by P. parvus. Also, P. avirostris swarms appear to have extended their temporal occurrence, and were present for longer periods of time when compared to past records. These changes might be related to the observed general increase of the average sea water temperature in the northern Adriatic Sea. Marine Ecology. ISSN 0173-9565 Marine Ecology 29 (2008) 387–398 ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd 387