Martin Löder

Dinoflagellates and ciliates at Helgoland Roads, North Sea

A monitoring programme for microzooplankton was started at the long-term sampling station “Kabeltonne” at Helgoland Roads (54°11.3′N; 7°54.0′E) in January 2007 in order to provide more detailed knowledge on microzooplankton occurrence, composition and seasonality patterns at this site and to complement the existing plankton data series. Ciliate and dinoflagellate cell concentration and carbon biomass were recorded on a weekly basis. Heterotrophic dinoflagellates were considerably more important in terms of biomass than ciliates, especially during the summer months. However, in early spring, ciliates were the major group of microzooplankton grazers as they responded more quickly to phytoplankton food availability. Mixotrophic dinoflagellates played a secondary role in terms of biomass when compared to heterotrophic species; nevertheless, they made up an intense late summer bloom in 2007. The photosynthetic ciliate Myrionecta rubra bloomed at the end of the sampling period. Due to its high biomass when compared to crustacean plankton especially during the spring bloom, microzooplankton should be regarded as the more important phytoplankton grazer group at Helgoland Roads. Based on these results, analyses of biotic and abiotic factors driving microzooplankton composition and abundance are necessary for a full understanding of this important component of the plankton.

Facilitation of intraguild prey by its intraguild predator in a three-species Lotka–Volterra model

Explaining the coexistence of multiple species in the competition and predation theatre has proven a great challenge. Traditional intraguild predation (IGP) models have only relatively small regions of stable coexistence of all species. Here, we investigate potential additional mechanisms that extend these regions of stable coexistence. We used a 3-species Lotka-Volterra system to which we added an interaction term to model a unidirectional facilitative relationship between the two predators in the IGP. In this modelling study the IG predator was able to precondition a part of the common resource by an instantaneous manipulation, which resulted in the immobilization of the resource species. This mechanism of immobilization facilitated the resource uptake by the IG prey and thus increased its growth rates even in the presence of the common predator. The facilitative relationship of the IG prey by the IG predator produced a stable coexistence of both predators even though the IG prey was an inferior competitor for a common resource, which cannot be attained with the traditional IGP models. Furthermore, our model predicted a 3-species stable coexistence even at high enrichment where no coexistence was found in the basic IGP model. Thus, we showed that diversity of resource traits could significantly alter emergent community patterns via shifts in exploitative competition of IGP-related predators. The described mechanism could potentially lead to a higher efficiency in exploitation of common resources and thus promote higher diversity in a real community.

Microbial predators promote their competitors: commensalism within an intra-guild predation system in microzooplankton

This study elucidates the interspecific interactions between competing unicellular predators in an intraguild predation system. The organisms studied were two microzooplankton (MZP) predators competing for the phototrophic dinoflagellate prey Scrippsiella trochoidea. Since the smaller dinoflagellate predator Gyrodinium dominans was also potential prey for the larger predator, the tintinnid ciliate Favella ehrenbergii, the experimental system included the probability of intraguild predation (IGP). The development of the three species was studied in set-ups containing either one of the two predators or both together with their prey. The IG predator F. ehrenbergii grew at a mean rate of 0.77 d-1 independent of the presence of the IG prey G. dominans. High grazing of the IG predator on the smaller IG prey was detected in treatments containing only the two predators. However, when all three species were present, the IG prey displayed significantly higher growth rates (0.42 d-1) compared to treatments containing only the IG prey as predator (0.32 d-1). The results of further experiments allowed the exclusion of mechanical or chemical signals induced by the IG predator being responsible for the observed increase in growth rate of IG prey. Live observations revealed that the IG predator rejected a significant proportion of its S. trochoidea catch after initial uptake. This behavior led to an immobilization of around 26% of the caught cells. We tested if this prey immobilization by the IG predator facilitated prey uptake by the IG prey and thus could be potentially responsible for the higher growth rates of the IG prey. Indeed, the smaller predator selected positively for immobilized prey and reacted with higher grazing and growth rates. Consequently, the IG prey benefitted from this commensalism between IG predator and IG prey and the strength of this pattern predominated IGP in our model system. As both predators co-occur in the same environment their feeding relationship could increase exploitation efficiency of common mobile prey items. Furthermore, such commensalism potentially opens a loophole for a stable coexistence of MZP predators despite their competition.