Nitrogen-and phosphorus-depleted or NP-balanced toxic haptophyte Prymnesium parvum was fed to 2 dominant copepod species of the northern Baltic Sea (Eurytemora affinis and Acartia bifilosa), and their ingestion, egg and faecal pellet production rates and mortality were measured. The copepods were incubated in 5 different cell concentrations of P. parvum for 3 consecutive days; the cryptophyte Rhodomonas salina was used as a control for non-toxic, nutritionally high-quality food. Toxicity (haemolytic activity) of P. parvum was measured before and after the incubations. The haemolytic activity of P. parvum was the highest in cultures grown under nutrient deficiency. The toxicity decreased after 1 d incubation in all treatments, in both the presence and absence of copepods. Neither of the copepod species ingested P. parvum, irrespective of the nutrient treatment (toxicity) or cell concentration, and the pellet and egg production rates were correspondingly low. Although there was no significant increase in mortality in P. parvum treatments, copepods that were exposed to P. parvum in any concentration or nutrient treatment soon became inactive. It was evident that the toxicity of even nutrient-replete P. parvum had an indirect and sublethal influence on copepods, although this could not be measured as short-term increased mortality. Our results suggest a strong reduction in secondary production of copepods in an event of a P. parvum bloom.

Nodularia spumigena Mertens ex Bornet & Flahault 1886 (Cyanophyceae) frequently forms harmful blooms in the Baltic Sea, and the toxin nodularin has been found in calanoid copepods during the blooms. Although nodularin has been found at higher trophic levels of the food web, no available information exists about the role of the microbial loop in the transfer of nodularin. We followed the transfer of nodularin to the copepod Eurytemora affinis during conditions that resembled initial 'pre-bloom' (Expt 1) and late stationary (Expt 2) phases of a N. spumigena bloom. The experiments were carried out using natural plankton communities spiked with cultured N. spumigena and grown in laboratory mesocosms, and E. affinis, which were isolated from the Baltic Sea and had no prior contact with nodularin. The plankton community was divided into 6 size fractions as follows: <150, < 45, < 20, <10, < 3 and < 0.2 µm, in which E. affinis was incubated for 24 h. Ingestion and clearance rates, food selection and faecal pellet production were based on microscopical analyses. Nodularin was measured with HPLC-MS with electrospray ionization in the copepods, as well as in dissolved and particulate fractions before and after incubation. We found that nodularin accumulated in copepods in all the plankton size fractions. The copepods contained nodularin concentrations of 14.3 ± 11.6 (mean ± SD) and 6.6 ± 0.7 pg ind.-1 after incubation in the <150 µm fraction in Expt 1 and Expt 2, respectively, while the range in the smaller size fractions was from 1.3 ± 2.8 to 5.7 ± 1.3 pg ind.-1 . Nodularin was transferred to the copepods through 3 pathways: (1) by grazing on filaments of small N. spumigena, (2) directly from the dissolved pool, and (3) through the microbial food web by copepods grazing on ciliates, dinoflagellates and heterotrophic nanoflagellates. The relative importance of direct grazing on small N. spumigena filaments varied from moderate to insignificant. The microbial loop was important in nodularin transfer to higher trophic levels. Our results suggest that the importance of the microbial loop in harmful algal bloom (HAB) toxin transfer may be underestimated both in marine and freshwater systems.

The calanoid copepod Eurytemora affinis from the northern Baltic Sea was exposed to cell-free filtrates of the toxic haptophyte Prymnesium parvum as well as to cell mixtures of P. parvum and Rhodomonas salina. To test the effects of P. parvum exudates and allelopathy on selective grazers, copepods were incubated (1) in increasing concentrations of cell-free filtrates of P. parvum in the presence of good food (R. salina), (2) in 1:1 cell mixtures at 2 cell concentrations of P. parvum and R. salina and (3) in R. salina cell suspension, which was used as a control for good-quality food. P. parvum cultures were grown in nutrient-balanced (+NP) or limited (-N or -P) media to obtain different levels of toxicity. Survival, ingestion, faecal pellet production rates and egg production were measured over 3 d, together with measurements of P. parvum toxicity (hemolytic activity) (HA). Most of the copepods incubated in high-filtrate concentrations died or became severely impaired, although (HA) in filtrates was under the detection limit. Further, the ingestion and faecal pellet production rates were suppressed in the highest filtrate concentrations in nutrient-limited treatments. Higher cell density in cell mixtures resulted in significantly lower faecal pellet production, although survival remained high. Our results show that HA is not a good overall indicator of the total harmful effects of P. parvum on grazers. Besides monospecific P. parvum diets, filtrates and cell mixtures have negative effects on grazers, and these effects are stronger under nutrient-depleted conditions; however, the presence of good-quality food lowers harmful effects for copepods. The negative effects caused either by direct intoxication or by food limitation following from strong allelopathic effects of P. parvum on other components of nano-and microplankton suggest that P. parvum blooms have a realistic potential to be deleterious for copepod secondary production, irrespective of the presence of alternative food sources.KEY WORDS: Prymnesium parvum · Exotoxins · Hemolytic activity · Eurytemora affinis · Viability · Feeding · Faecal pellet production · Egg production rateResale or republication not permitted without written consent of the publisher

Dense localized blooms of the toxic dinoflagellate Alexandrium ostenfeldii are a recent phenomenon in the low salinity waters of the Baltic Sea. This study reports results from laboratory experiments investigating the interaction between PSP toxin producing strains of A. ostenfeldii and two copepod species, Eurytemora affinis and Acartia bifilosa, from the northern Baltic Sea. Copepod grazing rates were studied by incubating the copepods with mixed cultures of A. ostenfeldii isolated from a bloom area. The effect of A. ostenfeldii on copepod condition and behavior was studied by exposing copepods to the mixed cultures and cell-free filtrates. Grazing on A. ostenfeldii cells was negligible for both copepod species. The 24 h incapacitation incubations indicated that the condition of both copepod species was negatively affected by the presence of A. ostenfeldii. The behavioral changes observed took place during the first hours of exposure to A. ostenfeldii cells, and little or no change was observed in the condition of the copepods after 6 h compared to 24 h. Similar observations were made for E. affinis incubated in cell-free filtrates of A. ostenfeldii, while the response of A. bifilosa was less pronounced. Our study shows that the copepods do not graze on A. ostenfeldii due to rapid behavioral disturbance and incapacitation by the algal cells and their exudates. This represents the first observation of a negative effect of the A. ostenfeldii on co-occurring biota in the northern Baltic Sea.

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