The population structure of planktonic protists and small metazoans in Mali Ston Bay (Adriatic Sea) - implications for determination of trophic state and shellfish culturing potential

Mali Ston Bay is located between a southeast-northwest oriented coast and Pelješac, the largest peninsula in the Middle Adriatic (Croatia). The bay is narrow, approximately 5 km wide, with a greatest depth of 29 m in the middle region of the outer bay. Its geomorphologic character, specific hydrographical and production features make it unique along the eastern Adriatic coastline. The effects of open seawaters on the bay are strongest during the winter season when highest salinity occurs due to the intrusion of bottom currents. Furthermore, Samplings in the Bay Mali Ston were performed between January 2002 and January 2003 at station Usko, conducted either once a week or twice a month, with 5-L Niskin water sampler at 1 m depth intervals from the surface to 13 m. The investigations of the population of planktonic protists and small metazoans covered the major assemblages: taxopodids, non-loricate ciliates, tintinnids, rotatorians, copepod nauplii, calanoids, oithonids, oncaeids, harpacticoids, bivalve larvae and juvenile appendicularians. The diversity and abundance of all assemblages on an annual basis is primarily dependent on variations in the hydrographic conditions. During periods of low temperature and high salinity, oncaea copepodites and adults show very high abundance, and together with non-loricate ciliates, tintinnids and planktonic harpacticoids form the winterspring zooplanktonic community. During the warmer period in conditions of higher temperature and salinity the summer-autumn zooplanktonic communities comprise high abundances of post-naupliar calanoids and oithonids, juvenile appendicularians and summer tintinnids. From the abundance of planktonic protists and small metazoans it can be concluded that the Bay Mali Ston has mesooligotrophic characteristics. This paper discusses the importance of the bio-deposition of faecal material from the shellfish that are cultured in the Bay and the potential negative impact on the ecological balance resulting from any significant increased capacity in aquaculture.