Ivona Onofri

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.

Zooplankton Community in the Boka Kotorska Bay

This study includes a review of recently published results regarding zooplankton in the Boka Kotorska Bay (since 2009), and comparison of these results with earlier investigations. Non-loricate ciliates were the most numerous microzooplankton in spring 2013 (37%). Loricate ciliates (tintinnids) values were low and similar to those recorded in the open Adriatic Sea. However, their diversity was quite high: 20 estuarine–neritic and 26 species typical of the open sea were identified. Copepod nauplii were dominant metazoan microzooplankton component (32%). Seven phylums and 81 mesozooplankton taxa were determined. Copepods were the most dominant group. Among them, Oncaea–cyclopoids and Oithona nana were the dominant copepod taxa at all stations. Heterotrophic dinoflagellate Noctiluca scintillans and cladocera Penilia avirostris were often extremely numerous during warm seasons. Changes in the zooplankton community noted during recent investigations can be linked with the observed climate changes identified in the Adriatic and Mediterranean Sea since the 1990s, which are reflected in: (1) zooplankton high densities in the winter period with a lack of spring peak; (2) domination of small-size cyclopoid copepods and decreasing of contribution of neritic calanoid species; (3) prolongation of high Penilia avirostris abundance in the autumn period; (4) spreading of invasive siphonophorae Muggiaea atlantica in the inner part of the Bay; (5) frequent outbreaks of gelatinous zooplankton; and (6) The first registered bloom of the ctenophore Bolinopsis vitrea in the Mediterranean in spring 2014, having a major impact on the regular planktonic food web system.