Okadaic acid accumulation in macrofilter feeders subjected to natural blooms of Dinophysis acuminata
Introduction
Harmful algal blooms cause severe economical losses to aquaculture and adversely impact human health. Diarrhetic Shellfish Poison (DSP) in humans causes mainly gastrointestinal symptoms as a result of eating shellfish contaminated with okadaic acid (OA) group of toxins. OA and its derivatives are powerful cytotoxins that can block dephosphorylation of proteins in mammals and plants, but very little is known regarding the effect of these compounds on marine organisms (Bauder et al., 2001). The most important DSP toxin producers belong to the genus Dinophysis, which produce toxins consisting mostly of OA and its derivatives.
Thermaikos Gulf (N. Aegean Sea, Eastern Mediterranean) is enclosed and eutrophic, with reduced water flux, receiving high nutrient inputs from rivers, urban and industrial runoff (Gotsis-Skretas and Friligos, 1990, Balopoulos and Friligos, 1993, Moncheva et al., 2001). The shellfish farming activity in this area represents about 85% of the total production in Greece (€10 million annually; Zanou and Anagnostou, 2001), with 70–80% of the product exported (Karageorgis et al., 2005). Mussel rafts increase water stability and water residence time, and create microhabitats favouring dinoflagellate growth (Granéli et al., 1998). Many sources of nutrients such as sewage and animal wastes, agricultural and fertilizer runoff, as well as the growing aquaculture industry in coastal areas, can also contribute to stimulate harmful algal blooms (Anderson et al., 2002).
In Thermaikos Gulf since 2000, DSP outbreaks have been confirmed and the causative organism has been identified as Dinophysis acuminata. The most serious HAB event was recorded during the winter of 2000, when D. acuminata reached 8.5 × 104 cells l−1 at the end of January (Koukaras and Nikolaidis, 2004). During each bloom period (usually early spring), mussel cultures remain closed to harvest, resulting in a substantial socio-economic impact in the area (economic losses of ∼€3 million annually) (Karageorgis et al., 2005).
As filter feeder animals gather food by filtering the water, contamination with DSP toxins occurs after their feeding on toxic Dinophysis cells. The filtering capacity of the filter feeding communities may play an important role on the control of phytoplankton blooms (Granéli et al., 1998, Cloern, 2001). The accumulation of OA, which D. acuminata mainly produces (Vale et al., 1998), has been until now studied in shellfish populations, while for other benthic groups (ascidians and polychaetes), to our knowledge there is no available information in the published literature.
Two successive annual DSP toxin outbreaks in Thermaikos Gulf presented an opportunity to (1) assess, determine and compare the amounts of free okadaic acid in different filter feeding species naturally contaminated by D. acuminata; and (2) identify the most appropriate species to be used as an indicator of DSP contamination in the Gulf, as a tool in the shellfish stocks management in DSP-affected areas, such as Thermaikos Gulf.
Section snippets
Description of the study area
The investigation area, Thermaikos Gulf, is located in the nortwestern part of the Aegean Sea (Fig. 1). Five major and several minor rivers flow into the gulf, introducing significant amounts of particulate matter and nutrient loads (Lykousis and Chronis, 1989, Friligos et al., 1997, Karageorgis et al., 2005). The city of Thessaloniki, with its partly untreated domestic sewage effluents and adjacent industrial zone, contributes to the eutrophication of the Gulf (Anagnostou et al., 1997). The
Results
Fig. 2 illustrates the spatial and vertical distribution of D. acuminata cell densities and their cellular toxin content during the blooms of May 2003 and March 2004.
In May 2003, D. acuminata cell densities varied from 500 cells l−1 in stations DA3 and TP7 to 2200 cells l−1 at station TP2 and the average OA intracelullar concentrations in D. acuminata varied from 3.7 to 8.6 pg cell−1 (Fig. 2).
The highest D. acuminata cell densities were recorded during March 2004. Cell densities ranged from 600 cells l
Discussion
The intense toxic bloom of March 2004 in Thermaikos Gulf was associated with an increased OA accumulation in animal tissues and also a higher number of filter feeding species found to be contaminated, in comparison to the toxic bloom of 2003. During both toxic blooms, D. acuminata cell densities in the mussel culture area (st. DA3) were generally comparable, indicating that mussels are effective filter feeders and their dense populations, are able to reduce/or control Dinophysis cell densities.
Acknowledgements
We thank I. Varkintzi, A. Gremmenas and S. Mitsoudi for their help during the field surveys in Thermaikos Gulf. Financial support was provided by the European Commission through the FATE project “Transfer and Fate of Harmful Algal Bloom (HAB) Toxins in European Marine Waters” (contract EVK3-CT01-00055) as part of the EC-EUROHAB cluster.[SS]
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