Abstract
A large bloom of Prymnesium polylepis occurred in the Baltic Sea during the winter 2007 – spring 2008. Based on numerous reports of strong allelopathic effects on phytoplankton exerted by P. polylepis and its toxicity to grazers, we hypothesized that during this period negative correlations will be observed between P. polylepis and (1) main phytoplankton groups contributing to the spring bloom (i.e., diatoms and dinoflagellates), and (2) zooplankton growth and abundance. To test these hypotheses, we analyzed inter-annual variability in phytoplankton and zooplankton dynamics as well as growth indices (RNA∶DNA ratio) in dominant zooplankton in relation to the Prymnesium abundance and biomass. Contrary to the hypothesized relationships, no measurable negative responses to P. polylepis were observed for either the total phytoplankton stocks or the zooplankton community. The only negative response, possibly associated with P. polylepis occurrence, was significantly lower abundance of dinoflagellates both during and after the bloom in 2008. Moreover, contrary to the expected negative effects, there were significantly higher total phytoplankton abundance as well as significantly higher winter abundance and winter-spring RNA∶DNA ratio in dominant zooplankton species in 2008, indicating that P. polylepis bloom coincided with favourable feeding conditions for zooplankton. Thus, primary consumers, and consequently also zooplanktivores (e.g., larval fish and mysids), may benefit from haptophyte blooms, particularly in winter, when phytoplankton is scarce.
Highlights
In 2007–2008, a protracted bloom dominated by Prymnesium polylepis (Manton & Parke) Edvardsen, Eikrem & Probert, former name Chrysochromulina polylepis [1], occurred in the Baltic Sea [2]
Prymnesiales dynamics in 2007 and 2008 In summer 2007, P. polylepis was found at all open sea stations, increasing during autumn and reaching 0.36103 to 736103 cells L21 in November–December
The alternate stage of P. polylepis, which has not hitherto been found in phytoplankton samples, dominated Prymnesiales assemblages during winter (Figure S1)
Summary
In 2007–2008, a protracted bloom dominated by Prymnesium polylepis (Manton & Parke) Edvardsen, Eikrem & Probert, former name Chrysochromulina polylepis [1], occurred in the Baltic Sea [2]. The bloom started in late November – early December 2007, and the abundance increased over time, reaching maximum in March–May 2008 [2]. The bloom raised concerns regarding possible negative effects on phytoplankton competitors, grazers and secondary consumers in the food web. Various Prymnesiales, including P. polylepis, being a regular component of marine phytoplankton, are known to produce haemolytic and ichthyotoxic compounds [3], which have been reported to be harmful for a variety of primary and secondary consumers [4]. In field and laboratory studies, P. polylepis was found to suppress motility and growth in algae, heterotrophic protists and pelagic bacteria [5,6,7,8], and to inhibit growth and reproduction or even increase mortality in mesozooplankton grazers [5,9]. Severe toxic effects on benthic communities and farmed fish were observed in areas with high concentrations of P. polylepis [10] and in experiments with eggs and larvae of the ascidian Ciona intestinalis and the blue mussel Mytilus edulis [11]
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