Abstract
Laboratory experiments suggest that toxic Alexandrium spp. cells are unpalatable to zooplankton grazers, and that toxic cells should be selectively avoided by zooplankton when feeding in mixtures of different prey species. Such avoidance, if practised in the wild, might contribute to harmful bloom formation by reducing losses of Alexandrium spp. due to grazing. In the spring of 1998 and 1999, during 'red tide' outbreaks in the southwestern Gulf of Maine, weekly experiments were performed using field collected natural water samples with ambient phytoplankton and dominant mesozooplankton grazers. The feeding response of Acartia hudsonica, Semibalanus balanoides nauplii, and Calanus finmarchicus was tested during various weeks in natural water samples with low concentrations of Alexandrium spp. (~1000 cells l -1 , typical natural concentrations for this region). Semibalanus sp. nauplii consistently avoided toxic Alexandrium spp. and other dinoflagel- lates. C. finmarchicus selectively fed on diatoms when they were abundant, and fed non-selectively on all dinoflagellates (except Ceratium spp.) when the spring bloom declined and dinoflagellates dominated. A. hudsonica non-selectively cleared Alexandrium spp. throughout the study periods. During spring Alexandrium spp. bloom formation, if non-selective grazers such as A. hudsonica dom- inate the zooplankton, Alexandrium spp. losses from grazing depend on grazer abundance (biomass); if selective feeders such as S. balanoides nauplii dominate, then Alexandrium spp. benefits from reduced grazing losses relative to alternative prey.
Highlights
Toxic Alexandrium spp. in the southwestern Gulf of Maine produce potent neurotoxins known as paralytic shellfish poisoning (PSP) toxins
Turner & Anderson (1983) noted that the copepod Acartia hudsonica fed on tintinnid ciliates at higher rates than on co-occurring phytoplankton, but it was not reported in either study whether toxic Alexandrium spp. were selectively avoided or consumed relative to ambient non-toxic phytoplankton
Ciliates were often cleared at higher rates than other prey, and certain dinoflagellates (Ceratium spp., Dinophysis spp.) were frequently avoided, but Alexandrium spp. cells were in almost every case cleared at rates equivalent to those of ambient non-toxic phytoplankton (Fig. 1, Table 3)
Summary
Toxic Alexandrium spp. (predominantly Alexandrium fundyense Balech) in the southwestern Gulf of Maine produce potent neurotoxins known as paralytic shellfish poisoning (PSP) toxins. Harmful Alexandrium spp. blooms in this region are seldom monospecific. As Alexandrium spp. proliferation progresses, total abundance of diatoms often. The feeding behavior of zooplankton during Alexandrium spp. bloom development is not well understood. Mar Ecol Prog Ser 218: 213–226, 2001 erate to high Alexandrium spp. cell concentrations (Turner & Anderson 1983, Watras et al 1985). These studies concluded that zooplankton abundance was the primary factor affecting rates of Alexandrium spp. removal. Turner & Anderson (1983) noted that the copepod Acartia hudsonica fed on tintinnid ciliates at higher rates than on co-occurring phytoplankton, but it was not reported in either study whether toxic Alexandrium spp. were selectively avoided or consumed relative to ambient non-toxic phytoplankton
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