Noctiluca scintillans MACARTNEY in the Northern Adriatic Sea: long-term dynamics, relationships with temperature and eutrophication, and role in the food web

Abstract

The first 'bloom' of Noctiluca scintillans in the Northern Adriatic Sea was recorded in 1977. The organism caused several red tides in the whole basin during the late 1970s, a period characterized by increasing nutrient loads. During the 1980s and early 1990s, there was no 'red tide', but the species was an almost constant summer presence, associated with high temperatures. Noctiluca scintillans was almost completely absent from 1994 until May 1997, concurrent with a general plankton decrease. From summer 1997, N. scintillans was recorded again in the whole basin, although there was no other signal of increasing eutrophication. In contrast to all previous observations, during winter 2002-2003, N. scintillans was continuously sampled in the Gulf of Tieste. We estimated experimentally growth and grazing rates of the dinoflagellate at 9-10°C in culture and consuming the natural assemblage. Noctiluca scintillans was able to reproduce actively at low temperatures, showing similar growth rates in both experiments (k = 0.2 day -1 ). The values found were close to those reported in the literature for higher temperatures. The natural diet was mainly composed of phytoplankton (ingestion = 0.008 μg C Noctiluca -1 day -1 ), micro-zooplankton (ingestion = 0.008 μg C Noctiluca -1 day -1 ) and bacteria (ingestion = 0.005 μg C Noctiluca -1 day -1 ) with an average carbon content of 0.138 + 0.020 leg C Noctiluca cell -1 .