Quantitative variability of the copepod assemblages in the northern Adriatic Sea from 1993 to 1997 Frano Krs ˇinic ´ a, * , Dubravka Bojanic ´ b , Robert Precali c , Romina Kraus c a Institute of Oceanography and Fisheries Split, Ivana Mes ˇtrovic ´a 63, 21000 Split, Croatia b Institute for Marine and Coastal Research, University of Dubrovnik, Kneza Damjana Jude 12, 20000 Dubrovnik, Croatia c RuCer Bos ˇkovic ´ Institute, Center for Marine Research, 52210 Rovinj, Croatia Received 25 January 2007; accepted 23 May 2007 Available online 20 July 2007 Abstract Quantitative variability of the copepod assemblages in the northern Adriatic Sea was investigated at two stations, during 43 cruises, from January 1993 to October 1997. Samples were taken at 0.5, 10, and 20 m, as well as near the bottom, using 5-l Niskin bottles. For inter-annual variation in the density of copepod assemblages data were presented as total number of nauplii and copepodites with adult copepods of the fol- lowing groups: Calanoida, Cyclopoida-oithonids, Cyclopoida-oncaeids and Harpacticoida. Moreover, hydrographic conditions, both fractions of phytoplankton, non-loricate ciliates and tintinnids were taken into consideration. Nauplii are the most numerous fraction at both stations with an average over 74% in the total number of all copepod groups. Their numbers were significantly higher at the western eutrophic station, while at the eastern oligotrophic station, an absolute maximum of 693 ind. l 1 was noted. The maximum values of calanoids and oithonids occur gen- erally during summer and these copepods are always more numerous at the western station: 33e50% and 50e63%, respectively. The most abun- dant taxa identified were the calanoid Paracalanus parvus and the cyclopoid Oithona nana. Oncaeid species Oncaea waldemari and Monothula subtilis dominated during late autumn and winter. An atypical increase in the abundance of oncaeids during the summer of 1997 could be related to an invasion and mass occurrence of the calycophoran siphonophore Muggiaea atlantica. It can be concluded that these dominant copepods are responsible for the stabilization of very complex processes. Atypical appearances of major copepod groups and disturbances in the copepod population structure itself can significantly influence changes in the ecosystem of this very sensitive region. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: phytoplankton; zooplankton; copepods; Adriatic; northern Adriatic 1. Introduction The northernmost part of the Adriatic Sea is located be- tween the western coast of the Istrian Peninsula (Croatia) and the northeastern coast of Italy. The area is estuarine in character and is shallow with depths between 20 and 40 m. The western part of this area is strongly influenced by the Po River and is therefore highly eutrophic (Degobbis et al., 1995). Productivity in the eastern part of the basin is much lower due to incoming southern currents carrying oligotrophic water masses from the central and southern Adriatic (Orlic ´ et al., 1992). Phytoplankton blooms, which have inspired nu- merous investigations, were frequently observed in the last 30 years in the Adriatic, as summarized by Fonda-Umani (1996). Amorphous mucous aggregates have appeared during the summer months with very strong intensities during 1988, 1989, 1991, 1997 and 2000e2002 (Herndl and Peduzzi, 1988; Degobbis et al., 1991; Kaltenbo ¨ck and Herndl, 1992; Precali et al., 2005). Zooplankton assemblages have received less attention, though they are important for the knowledge of pelagic ecosys- tems. Copepods, as the dominant organisms in any marine hab- itat, are naturally very important to the overall grazing potential * Corresponding author. E-mail addresses: fkrsinic@izor.hr (F. Krs ˇinic ´), dbojanic@unidu.hr (D. Bojanic ´), precali@cim.irb.hr (R. Precali), kraus@cim.irb.hr (R. Kraus). 0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.05.036 Estuarine, Coastal and Shelf Science 74 (2007) 528e538 www.elsevier.com/locate/ecss