HARMFUL MARINE MICROALGAE

(algal blooms, red-tides, color tides, outbreaks)

FROM TURKISH SEAS

updated 9th October, 2001

It was in 1955's spring that fish mortalities associated with red-tides first reported by W. Nümann from a Turkish sea, Agean Sea. Since then, red-tides and other noxious algal blooms mainly due to progressive eutrophication from terrestrial inputs were observed almost each year by reporters and Turkish scientists. Phytoplanktonic members of the list clearly has showed a yearly succession and their total biomass increased oligotrophic toward the eutrophic zones of the bays. During the pronounced and widespread blooms of dinoflagellates (P. micans, N. scintillans, A. minutum, G. simplex, S. trochoidea) seawater is strikingly discolored red. Coccolithophorides, diatoms and euglenoids are also associated with different discoloration of seawater such as milky (E. huxlei) or green (all of the diatoms, euglenoids and prasinophytes). According to the results of these researches, since 1980, the most common species causing red-tides in the Turkish seas are presented in Table 1. Among these, only A. minutum have long been known as toxic and to be associated with bivalves and fish mortality. Although there is no clear evidence on PSP in the bay of Izmir (Eastern Aegean Sea), death of fishes due to this species is always characterized with visible yellowish color which can be observed on total body and gills. Levels of A. minutum exceeding 6-10 millions cells per liter when toxicity occurs. However, during the algal blooms of the bay of Izmir (Aegean Sea), demersal and pelagic fishes also exhibit anoxia symptoms. These symptoms are also followed both during non-toxic bloomings of the diatoms Thalassiosira anguste-lineata, T. allenii and euglenoid Eutreptiella gymnastica at nights. Thousands of the crap Carcinus mediterraneus migrate onto land at night when oxygen deficiency occurs. On the contrary, air bubbles are formed by high rate of photosynthesis during the day, seawater is supersaturated by dissolved oxygen and concentration frequently reach 17-22 ppm. where air bubbles are observed on surface during bloomings. This hyperoxia may be another risk factor for some marine consumers.

As summarized above, although the impact of algal blooms on some fishes (mullets, sardines, anchovies, gobiids) is frequently observed, little is known about which factor plays a major role on mortalities, anoxia or PSP reason respiratory paralysis, and how it influences the organisms. Both anoxic and hyperoxic layers (frequently overlaps with sub-surface cell maxima layer) that are respectively formed by decaying of the sedimented cells and high photosynthetic rate in the middle of bloomings are unfavourable environments for many marine pelagic organisms.

In examining the impacts of toxic or non-toxic red-tide organisms in the bay of Izmir shellfish populations (Mytilus galloprovincialis, Tapes decussata, Cardium edule, Venus spp.) it becomes clear that statistics are not correct. However, because little use of shellfishes as food is currently made during springs, toxicity is rarely a risk factor for inhabitants.

Ceratium fusus, Dinophysis acuminata, D. acuta, D. caudata, D. fortii, D. mitra, D. rotundata, D. sacculus, D. tripos, Gonyaulax grindleyi, Prorocentrum cassubicum and P. lima are the other risky species found in the plankton of Turkish seas.

Table 1: Harmful microalgae of Turkish seas.

abbreviations used: mc, maximum cell number in one liter; z, depth of maximum cell abundance; L, LLoyd patchines index; T, type of noxious effect: PSP, paralytic Shellfish Poison; DSP, Diarrhetic Shellfish Poison; ASP, Amnesic Shellfish Poison; HT, Hepatotoxic; HO, hyperoxia; AO, anoxia; NH₃, Ammoniac; ?, unknown.

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