Biological Setting

One of the main aim of the cruise has been the study of the reponse of phytoplankton community. In coastal temperate ecosystems, the general pattern of the plankton dynamics shows a productive period with an important initial phytoplankton bloom during spring based on new production and characterized by large cells (.20 mm) on which the classical herbivorous food chain reposes. These spring blooms are a characteristic feature of the coastal and estuarine waters in temperate latitudes. However, over recent decades, the increasing quantities of nutrients produced by anthropogenic activities on the edges of coastal areas as well as the climatic variability have strongly modified the primary production calendar, its extent, and the nature of the phytoplankton communities. In a few coastal ecosystems, the phytoplankton dynamics pattern occasionally or recurrently shifts on the seasonal scale with early blooms starting by late winter.

Northern Adriatic is considered one of the most productive regions of the generally oligotrophic Mediterranean Sea RUSSO2002, BERNARDI2004, BERNARDI2006, BERNARDI2006-IRH, PUGNETTI2005, PUGNETTI2008, SOCAL2008, SOLIDORO2007, IANORA2008, SOLIDORO2009 although recent studies point to a decrease of the productivity during last decades MOZETIC2009. In particular, the largest phytoplankton blooms occur in surface layers on late winter and summer in the western part of the region close to the Po River delta.

The amount and the distribution of the diluted waters in the northern Adriatic basin are highly variable and they have a marked influence on phytoplankton communities, mainly through the supply of inorganic nutrients and seston, and through the control of the vertical stability of the water column. In particular, concurrent enrichment (mainly by river) and depletion (by phytoplankton uptake) of both DIN and P can cause rapid and marked variation of the N/P ratio DEGOBBIS2000. The meteo-ocenographic conditions, the patterns of currents, and the nutrient limitation (in particular by P) are considered to be the general environmental conditions that seem to favour mucilage formation in the Northern Adriatic basin GIANI2005. This phenomenon has been observed, at least at its early stage, almost every year since the 1990s and with a huge development in the years 1991, 1997, 2000, and 2002. Mucilage starts in late spring/early summer, when the stratification strengthens, and the exchange of water masses between the northern and the middle basin slows down GRILLI2005. However, there is no evidence of changes in the dominant species composition of microphytoplankton in the years when massive mucilage aggregates were observed TOTTI2005. The Utermöhl fraction of the phytoplankton community (cells $>$ 3 $\mu$m as a maximum linear dimension) has been extensively studied in the Northern Adriatic Sea in the past REVELANTE1976,SOCAL1989,HONSELL1989, FONDAUMANI1992, SOCAL1992, MOZETIC1998, MOZETIC2002, BERNARDI2004 . The community is mainly made up by diatoms (Skeletonema marinoi, previously identified as S. costatum, Chaetoceros spp., Thalassiosira spp., and Pseudo-nitzschia spp.) and by small flagellates (nanoflagellates and cryptophyceans). The following seasonal pattern has been generally recognized: a late-winter/early-spring diatom bloom, related to the increase in day length and irradiance and to high nutrient inputs from the rivers; a late-spring/summer decline, when the community is mainly sustained by nutrient regeneration; a late autumn/winter minimum, mainly related to the decrease in light and temperature. Although these seasonal fluctuations are common to other coastal seas MOZETIC1998, RIBERA2004, the dynamic of phytoplankton in the Northern Adriatic Sea shows marked spatial and temporal heterogeneity and both seasonal and inter-annual fluctuations related to the freshwater inputs and to their distribution in the basin.

Zooplankton are critical for the functioning of aquatic food webs, being major grazers and, therefore, providing the principal pathway for energy transfer from primary producers to consumers at higher trophic levels. Copepods, the most prominent zooplankton taxon, are the most abundant multicellular animals on the planet SCHMINKE2007 and in the northernmost part of the northern Adriatic Sea (Gulf of Trieste and northward of the river Po delta) mesozooplankton communities is dominated by strictly neritic species and prevalence of the cladoceran Penilia avirostris in summer, and by the copepods 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, a group of offshore stations with a similar species of taxa composition and annual dynamics was identified CAMATTI2008. The analysis of the mesozooplankton series in the Gulf of Trieste for the periods 1970-2005 showed extensive changes in the copepod community around the end of the '80s probably due to a large scale and abrupt change in the Mediterranean circulation at the end of the '80s and the 1warming in Summer KAMBURSKA2006, CONVERSI2009.

Figure 8: Structural setting of Montenegro.