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Blooms of the Toxie Dinoflagellate Gonyaulax Excavata in a Faroese Fjord

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Blooms of the Toxie Dinoflagellate Gonyaulax Excavata in a Faroese Fjord International Couneil for the C.M. 1988/ L:6 Exploration of the Sea Biologieal Oeeanography Committee Ref. E and K Blooms of the toxie dinoflagellate Gonyaulax excavata in a faroese fjord . • by 1 2 Eili/ Gaard ) and Marita Poulsell ) 1)Fiskira1l1ls6knarstovan Fisheries Laboratory 0/ the Faroes FR-JOO T6rshavll. Faroe Islands. 2)Heilsu/r(}öiliga Starvsstovall Hygienic Institute FR-JOO T6rshavll. Faroe Islands • Abstract In the faroese fjord Trongisvägsfjorour, the toxie dinoflagellate GOllyaulax excavata has been observed every year since 1984. On the bottom sediment, high concentrations of resting cysts are found, and the algae seem to arise from germination of these cysts. The paper describes the blooms and aceumulations of PSP in shellfish during the periods of outbreaks. Environmental factors which may influence the blooms are discussed. - 2 - Introduction Blooms of GOllyaulax exca~'ata are described in several areal throughout the world. They mainly occur in inshore waters. Repeated blooms of GOllyaulax excavata has often been Iinked together with germination of benthic cysts (Dale, 1977; Anderson and Wall 1978; Anderson, 1980). The algae is weil known to produce Paralytic Shellfish Poisoning (PSP). (Prakash et al., 1971; Langeland et al., 1981; Tangen, 1983; Ogata et al., 1987). In 1984 a heavy bloom of GOllyaulax excavata caused mass mortalities of farmed fish in Trongisva;sfjordur, Faroe Islands. The cell concentrations were estimated to about 10 - 20xlO cells 1-1 (Mortensen, 1985 a). Arca of inl'cstigations The ocean surface temperatures around the Faroe Islands are relatively constant throughout the year from around 5°e in March to around lOoe in August. But in the • fjords the surface temperature may be higher during the summer. Around and between the islands there are very strong tidal currents, which cause strong vertical mixing of the watermasses. Nutrients therefore continously are transported up to the surface, and the sea surface around the islands is nutrient rich throughout the summer. The fjord Trongisvagsfjordur is about 6 km long (Fig. I). The outermost part of the fjord is relatively open to the ocean, while the innermost 3 - 4 km are weil protected from the waves and the strong tidal current outside the fjord. In this part of the fjord, wind is the strongest force to water movement. In calm weather the current normally is less than 2 cm sek-1 but it may be 10 - 15 cm sek-1 or more during periods with strong wind. The fjord is very shaHow compared to most other faroese fjords and most of the fjord is less than 20 m deep (Fig. 1). Material and methods Temperature was in 1984 and 1985 measured continously by a moored AANDERAA current meter and temperature profiles were in 1988 measured by a CTD. Salinity • profiles in 1984 were measured by titration with AgN03 and in 1988 by a eTD sonde. Nitrate concentrations are measured spectrophotometricaHy according to Strickland and Parsons (1972). Measurements of chlorophyll a were carried out spectro­ photometrically according to the recommentation of the Baltic Marine Biologists (BMB, 1979). When computing the results the equation of Jeffrey and Humphrey (1975) was applied. Cell counts were carried out on discrete sampIes and preserved with neutralized formalin. SubsampIes of 10 ml were counted after 24 hours sedimentation. Accumulation of Paralytic Shellfish Poisoning (PSP) was measured on rope-cultured musseIs (Mytilus edulis) taken from 1 - 3 meters depth at station 3 in 1984 - 87 and at station 1 in 1988 (fig. 1). PSP titres were determined using a method based on injection into mice of a hydrochloric acid extract of shellfish meat (Nordic committee on food Analysis, 1986). The measurements were made at Institutt for NWringsmiddelhygiene, Norges VetrinWrhVgskole, Oslo, Norway. The toxicity of the musseIs is represented as mouse units (MV) per 100 g musselmeat. One MV represents a dose of PSP needed to kill a 20 g mouse within 15 min. - 3 - 7'W 6rN TI\' Fig. 1. Chart showing the position ofTrongisvägsfjSllrilur (a) and the sampling stations within the fjord (b). Results Blooms O[ GOllvaulax excavata. In 1984 the first GOllyaulax excavata algae were found in the fjord in late May. In late June it occurred in heavy concentrations and on July the 10th it coloured the sea red. The cell numbers were not counted, but based on the red colour of the sea, the concentration can be estimated to around 10 - 20 millions cells 1- 1 at the sea surface. After that the concentrations again decreased (Table I). Since the sampies in table I were taken with a plankton net having a mesh size of 160;an, there may have been sampled to many colonies and to few unicellular or small algae. Table 1. Developement of Gonyaulax excavata bloom in 1984. Date Gonyaulax excavata Dominating species 16/5 none Leptocylindrus danicus. 27/5 rare Phaeocystis pouchetii. 13/6 rare Phaeocystis pouchetii. 26/6 dominating Gonyaulax excavata. 10/7 red tide Gonyaulax excavata. Almost uniculture 2717 dominating Gonyaulax excavata. - 4 - The first GOllyaulax excavata algae therefore appeared in the fjord between May 16th and May 27th, although the first dense concentrations were found between June 13th and June 26th, 1984. The heaviest concentrations were on July 10th, when the water was colored red by the algae. Chlorophyll a concentrations and salinity at station I on June IIth are shown in Fig. 2. There was a marked stratification of the water at 2 - 3 meters depth and the chlorophyll a maximum was in or just below the pycnocline. At that date the water was not red any more, but the visibility was still very low with secchi depths of about I meter. Chl.a,mg·m-3 10 20 2 I 3 I I I E I I I i4c- I I 0• , I, • 5 ., I I I 6 I I I I \ 7 , 32 34 Salinity, 0/00 Fig. 2. Salinity (dashed line) and chlorophyll a (full-drawn line) at station I on July 11th, 1984. On May 15th and 17th 1985 GOllyaulax excavata again was found in the fjord. The average maximum concentrations were about 40000 cells r l (Fig. 3). After that the cell numbers decreased rapidly, and appeared at concentrations of around 2000 cells for the next 3 weeks. From June 8th and the rest of the summer no Gonyaulax excavata were found in the fjord but dense concentrations of diatoms, mainly • Skeletollema costatum and Thalassiosira spp. were found in the fjord from late May and the rest of the summer. In 1986 no algae countings were made in the fjord but measurements of PSP contents in farmed musseIs showed positive results in May 1986. The algae therefore probably have occurred in the water that year tao. In 1987 small concentrations of GOllyaulax excavata were found in the water on May Ist (Fig. 4). The following days to May 11th the algae still appeared in the l \vater at concentrations of 1000 - 2000 cells r . Eut from May 11th to 16th the l concentrations rapidly increased to about 400000 cells r . Altough GOllyaulax excarata had increased very rapidly in number in the period from May II th to May 16th, the water was dominated by the diatom Skeletonema costatum with l concentrations of 15 - 20 millions cells r . The numbers of Gonyaulax excarata then decreased very rapidly and on May 19th the average concentration was only about 7000 cells r 1 while Skelelollema coslalum was found in concentrations around 2 - 4 l miIlions cells r . In the following days both Gonyaulax excarata and Skeletonema costatum decreased in number and on May 25th the average concentrations of GOI/)'aulax excavata were about 1500 cells 1-1 while concentration of Skeletollema l costatum were around 300000 cells r . That low concentrations of GOllyaulax - 5 - excavata were found in the fjord for about a month after that, but the rest of the summer no GOl1yaulax excavata were found. Skeletol1ema costatum was the dominating algae in the fjord until the middle of June, when Thalassiosira spp. became dominating. Also in 1988 GOl1yaulax excavata was found. (Fig. 5). In the period from May 7th to June 11th the algae was found in the water at concentrations from less than 100 to about 2000 cells 1-1 and they dit not form blooms. In this period the water was dominated by diatoms, mainly Thalassiosira l1ordel1skioeldii and Rhizosolel1ia sp. which appeared' in very high concentrations. Also relatively high concentrations of Plzaeocystis pouchetii were found in the sampIes. From June 11 th to late July the algae was not found in the water (wich means than the concentration has been less 1 than 50 cells r ) but then a new bloom arose. This second bloom is not described in this paper. Accumulatiol1 0/ PSP il1 musseis. Since 1984 when the first bloom of GOl1yaulax exca~'ata was observed, musseIs (Mytilus edulis) have been analysed for PSP content every year. The concentrations of PSP toxin in musseIs in 1985, 1987 and 1988 are shown in fig. 3, 4 and 5 • respectively together with the average concentra"tions of GOl1yaulax exca~·ata. In 1986 no algae countings were made in the fjord but analysis of PSP showed 333 MV/IOO g musselmeat in May 30th and 241 MV/IOO g in July 23th. The algae therefore probably have been in the fjord that year too. In 1985 the highest PSP content was measured to be around 3500 MV/IOO g musseImeat. Dut since the analysis was made about 14 days after the highest cell counts that year, the maximum PSP content probably has been much higher. In 1987 the highest PSP content was 15000 MV/IOOg musselmeat 2 days after the 1 bloom.
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