Marijana Miloslavić

Population structure and diel vertical migration of euphausiid larvae in the open southern Adriatic Sea (July 2003)

The composition, bathymetric distribution, and diel vertical migration of larval euphausiids in the oligotrophic southern Adriatic Sea were studied during the summer of 2003. Larvae of 11 species were identified. Of these, Thysanoessa gregaria is reported for the first time from the Adriatic Sea. Information is presented on the distribution and diel migrations of the five dominant species. Nematoscelis megalops furciliae had the widest bathymetric range, extending from surface to the bottom (1200 m); larvae of Stylocheiron maximum were found from 100 to 1200 m. Euphausia krohnii (calyptopes and furciliae), Stylocheiron abbreviatum calyptopes, and Stylocheiron longicorne furciliae ranged over 800 m. S. abbreviatum (furciliae) and S. longicorne (calyptopes) had more restricted bathymetric distributions. Different populations were associated with layers characterized by specific light intensities. Three migration patterns were observed: (i) nocturnal ascent to upper layers (E. krohnii, N. megalops, S. abbreviatum); (ii) scattered population through the water column (S. maximum); (iii) migration to upper layers at midday and night, and descent during the morning and evening (S. longicorne). Different stages of the same species showed different preferences for light intensity.

The importance of vertical habitat gradients on zooplankton distribution in an enclosed marine environment (South Adriatic Sea)

Abstract The spatio-temporal patterns of a zooplankton community structure in a strongly stratified enclosed marine environment were studied over a one-year period. Copepods were by far the most abundant group, accounting for 69–91% of the total zooplankton, with a numerical dominance of the genus Oithona. Cluster analysis revealed four different groups of samples and a comparison was made of the hydrographical and biological properties of the resulting groups. Water temperature was the main force discriminating the zooplankton community. The strongest vertical separation was recorded in summer, while a salinity impact was noted during the autumn after a strong intrusion of open sea water. Most of the dominant zooplankters exhibited a distinct vertical separation over the study period, where thermophilic taxa (Cladocera, Centropages kröyeri, C. typicus, Oikopleura fusiformis, O. longicauda, Euterpina acutifrons) kept to the surface and cryophilic species (Diaixis pygmaea, Mesaiokeras hurei, Calanus helgolandicus, Parasagitta setosa) were concentrated below the thermocline layer (20 m depth) for most of the year. Due to the unique physical, chemical and biological properties of the study site, this research is an important step towards better understanding the distribution of zooplankton during stratified conditions.

Temporal patterns of the calanoid copepod community in Veliko Jezero, an isolated marine lake (South Adriatic Sea): links to a larger-scale climate changes

We complied and analysed a long-term calanoid community data set (1952–2009) with respect to environmental variations in nearly closed seawater lake Veliko Jezero (South Adriatic Sea). Changes in hydrographic properties were influenced by deepening and broadening the channel between Veliko Jezero and the sea in the early 1960s, which was in particular reflected by increasing salinity. Calanoid abundance displayed high intra- and inter-annual fluctuations but lacked any regular and recurrent pattern. Calanoids in the lake comprised 22 coastal and estuarine species, with five of them present over the entire study period (Calanus helgolandicus (Claus, 1863), Paracalanus parvus (Claus, 1863), Centropages kroyeri Giesbrecht, 1893, Isias clavipes Boeck, Acartia (Acartiura) clausi Giesbrecht, 1889). Four sample groups were identified by cluster analysis, with the highest degree of dissimilarity between samples from the 1950s and those from the 1990s and onwards. Observed changes – low densities in early 1980s; a shift in the dominant species; reduction and, finally, disappearance of Pseudocalanus elongatus (Boeck); increases in Paracalanus parvus and Diaixis pygmaea (Scott T., 1894) – can be associated with synchronous changes across trophic levels that have occurred in other European seas. The results presented here underline the importance of long-term studies of semi-closed marine lakes and lagoons, as these are particularly sensitive to global climatic changes.

Ecology and population structure of the hyperbenthic copepod Mesaiokeras hurei Kršinić, 2003 (Calanoida: Mesaiokeratidae) from an isolated marine lake (Mljet island, Southern Adriatic Sea, Croatia)

The monogeneric family Mesaiokeratidae (Copepoda: Calanoida) is exclusively bentho-pelagic and comprises nine small hyperbenthic species. Previous knowledge of these species has been limited to the description of their morphology and habitat. We provide new information about the ecology of Mesaiokeras hurei Krsinic, 2003, one of the small-size species of the family and found only in an isolated marine lake on the southern Adriatic island of Mljet (Croatia). Genetic markers were used to investigate its phylogenetic relationships with other copepods. The abundance of M. hurei abundance was highly variable over the years and seasons sampled. Densities were higher in spring and autumn. It was particularly abundant in 2011, averaging 629 ± 800 individuals/m3, and with a maximum of 1971 individuals m−3 in November, but averaged 74 ± 71 individuals/m3 and 66 ± 71 individuals/m3 in 2001 and 2008, respectively. Specimens were found in a range of temperatures, from 10.5°C to 18.3°C, but a majority of individuals (67%) was found in the 12-14°C layer. In addition to temperature, oxygen was a significant predictor ( p < 0 . 001 ) of the vertical micro-distribution of M. hurei. The DNA barcode sequence for the species is reported here for the first time. Analysis of the nuclear small subunit rRNA gene fragment (18S) showed a genetic similarity with representatives of the superfamily Clausocalanoidea.