Estuarine, Coastal and Shelf Science 74 (2007) 528e538 www.elsevier.com/locate/ecss Quantitative variability of the copepod assemblages in the northern Adriatic Sea from 1993 to 1997 Frano Krsˇinic´ a,*, Dubravka Bojanic´ b, Robert Precali c, Romina Kraus c a Institute of Oceanography and Fisheries Split, Ivana Mesˇtrovic´a 63, 21000 Split, Croatia b Institute for Marine and Coastal Research, University of Dubrovnik, Kneza Damjana Jude 12, 20000 Dubrovnik, Croatia c RuCer Bosˇkovic´ 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. lÀ1 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 lower due to incoming southern currents carrying oligotrophic water masses from the central and southern Adriatic (Orlic´ The northernmost part of the Adriatic Sea is located be- et al., 1992). Phytoplankton blooms, which have inspired nu- tween the western coast of the Istrian Peninsula (Croatia) merous investigations, were frequently observed in the last and the northeastern coast of Italy. The area is estuarine in 30 years in the Adriatic, as summarized by Fonda-Umani character and is shallow with depths between 20 and 40 m. (1996). Amorphous mucous aggregates have appeared during The western part of this area is strongly influenced by the the summer months with very strong intensities during 1988, Po River and is therefore highly eutrophic (Degobbis et al., 1989, 1991, 1997 and 2000e2002 (Herndl and Peduzzi, 1995). Productivity in the eastern part of the basin is much 1988; Degobbis et al., 1991; Kaltenbo¨ck and Herndl, 1992; Precali et al., 2005). Zooplankton assemblages have received less attention, * Corresponding author. though they are important for the knowledge of pelagic ecosys- E-mail addresses: [email protected] (F. Krsˇinic´), [email protected] tems. Copepods, as the dominant organisms in any marine hab- (D. Bojanic´), [email protected] (R. Precali), [email protected] (R. Kraus). itat, are naturally very important to the overall grazing potential 0272-7714/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2007.05.036 F. Krsˇinic´ et al. / Estuarine, Coastal and Shelf Science 74 (2007) 528e538 529 of the northern Adriatic. The first data on the qualitative content of copepods in the northern Adriatic originate from the last decades of the 19th century and the first decades of the 20th cen- tury. More recently, studies have been made on the distribution of copepods in the northern Adriatic (Hure and Scotto di Carlo, 1969; Hure and Krsˇinic´, 1998), their seasonal and inter-annual variability (Mozetic et al., 1998; Fonda-Umani et al., 2005), as well as the distribution or population densities of their early development stages (Revelante and Gilmartin, 1983; Krsˇinic´ et al., 1988; Krsˇinic´, 1995). Nauplii and earlier copepodites were included in the micro-zooplankton category, and they were not considered in the evaluations of total copepod popula- tions. Comparison of the results of many years of research con- ducted on all copepod groups found in the open waters of the northern Adriatic which had previously been studied presented problems, primarily because of the variety in sampling methods used for different groups. In early copepod research, it was com- mon to use a 250 mm(Hure and Krsˇinic´, 1998) or 200 mm plank- ton net in vertical hauls from the bottom to the surface (Mozetic et al., 1998; Fonda-Umani et al., 2005). Such samples are not quantitatively relevant, as they allow almost all small copepods and their developmental stages to pass through (Krsˇinic´ and Lucic´, 1994). In addition, during phytoplankton blooms and when gelatinous matter is present in large quantities, the mesh aperture becomes blocked, and the net loses its filtration capacity. The objective of this paper is to determine the quantitative aspect of the population structure, seasonal abundance and vertical variability of all copepod assemblages based on iden- Fig. 1. Map of the northern Adriatic Sea with location of sampling stations. tical methods at two characteristic stations in the northern Adriatic Sea between 1993 and 1997. standard hydrographic levels. For details, see Degobbis et al. (1995). Samples (200 ml) for micro-phytoplankton (cells >20 mm) 2. Materials and methods and nano-phytoplankton (cells 2e20 mm) enumeration were preserved with Lugol’s solution buffered with sodium acetate The copepod assemblages in the northern Adriatic Sea were (2% final concentration). Micro-phytoplankton species identifi- investigated at two stations (Fig. 1) during 43 cruises (mainly cation and micro-phytoplankton and nano-phytoplankton cell monthly sampling), in the period from January 1993 to Octo- count were conducted with a Zeiss inverted microscope, follow- ber 1997. Samples were taken at 0.5, 10, and 20 m depth, as ing the U¨ termo¨hl settling technique of random fields (U¨ termo¨hl, well as near the bottom, using 5-l Niskin bottles aboard the 1958). Autotrophic and heterotrophic nano-phytoplankton were vessel ‘‘Vila Velebita’’ from the Center for Marine Research not discriminated in this study. in Rovinj (CMR). The non-parametric Spearman correlation coefficient (Rs) Zooplankton samples were preserved with a 2.5% final con- was used for correlations between all variables. Parameters centration of formaldehyde neutralized with CaCO3. Samples were compared between stations using the t-test; all data were sedimented for 24 h in plastic containers in the laboratory, were log transformed. Non-parametric multivariate analysis from which about 3/4 of the water was decanted. The remainder (Clarke, 1993; Clarke and Warwick, 1994) based on zooplank- was poured into a glass cylinder (10 cm diameter), and was sedi- ton communities was performed. Classification (group average mented for a further 24 h after which the superfluous volume sorting of the BrayeCurtis similarity measures based on 4th was decanted. This process reduced the original volume of 5 l root transformed relative seasonal abundance data) and ordina- to 30 ml in 72 h. Decanting was accomplished using a vacuum tion (multi-dimensional scaling (MDS) on the above similarity pump and a slightly curved pipette. The organisms were counted matrices) methods were used. MDS analysis would be consid- by means of an inverted microscope (Olympus IMT-2) at a mag- ered unsuccessful if the stress values were >0.3 (Clarke, nification of 100 and 400Â. Samples were counted in a glass 1993). Similarity percentage analysis (SIMPER) was used in cell, dimensions 7 Â 4.5 Â0.5 cm. order to reveal the contribution of single groups of copepods Temperature was measured using reversing thermometers, to dissimilarities between stations. Multivariate statistical while salinity was measured with a Yoo Kal MKII high preci- analyses (MDS) and SIMPER were performed using PRIMER sion salinometer. The hydrographic samples were collected at software (version 5.0; Clarke and Gorley, 2001). 530 F. Krsˇinic´ et al. / Estuarine, Coastal and Shelf Science 74 (2007) 528e538 3. Results 10 m, rather than at the surface as occurs usually, and an unusu- ally high micro-phytoplankton bloom in June at the eastern sta- 3.1. Hydrography tion (SJ107), followed by only one another in November, which was smaller than other autumn blooms. Nano-phytoplankton During the period 1993e1997, temperature in the investi- abundances showed fewer oscillations during the year, with gated area ranged from 7.40 to 28.93 C, and salinity from regular spring increase, which was rather high in 1993 and 21.00 to 38.77 (Table 1). The larger temperature range and 1997 at the western station (SJ108). minimal salinity range were observed at station SJ108, which is close to the Po River and is therefore directly influenced by 3.3. Ciliated protozoans its freshwater (Degobbis et al., 2000). The highest salinity was observed (Table 1) at station SJ107 at the opposite side of the Non-loricate ciliates were occasionally the dominant zoo- basin, which is more influenced by the southern currents that planktonic organisms in the northern Adriatic. In general, import saltier waters to the area (Artegiani et al., 1997). two annual peaks of abundance were registered, lower in the The investigated period was characterized by two events.