Selected interactions between phytoplankton, zooplankton and the microbial food web: Microcosm experiments in marine and limnic habitats

The experiments presented in this thesis elucidate selected
interactions between the phytoplankton, the zooplankton and the
microbial food web in aquatic ecosystems. The objective is to
provide a mechanistic understanding of classic general ecology
topics including competition, predator-prey relations, food web
structure, succession, and transfer of matter and energy. Special
relevance is attributed to the role of mixotrophic organisms,
marine cladocerans, and gelatinous mesozooplankton. Although they
may contribute substantially to plankton composition they have thus
far been neglected in common ecosystem models. All experiments were
based on enrichment with nutrients and organic compounds.
Enrichment with nutrients and organic compounds that influence
overall system productivity is one of the most pervasive human
alterations of the environment and profoundly affects species
composition, food web structure, and ecosystem functioning. In
order to predict the consequences of such enrichment, a better
understanding of the impact that trophic structure has on community
dynamics and ecosystem processes is required. The presented thesis
consists of two studies. The first study includes three experiments
in which I investigated the role copepods, cladocerans and
doliolids play in plankton interactions. Copepods, cladocerans and
doliolids are major mesozooplankton groups in marine systems. The
first experiment (Katechakis et al. 2004) showed that copepods,
cladocerans and doliolids have different food size spectra and
different assimilation efficiencies. According to my experiment,
copepods actively select for larger food items, whereas cladocerans
and doliolids passively filter medium-sized and small food items,
respectively, with doliolids being the only group that feeds
efficiently on bacteria and picoplankton. The results illustrate
that food niche separation enables copepods, cladocerans and
doliolids to coexist. In addition, they emphasize the fact that
doliolids are favored in low nutrient environments, characterized
by small food items, whereas cladocerans and copepods have
competitive advantages at moderate and high nutrient supplies,
respectively. Furthermore, copepods obviously utilize ingested food
best, gauged in terms of produced biomass, followed by cladocerans
and doliolids, which suggests that the different mesozooplankton
have different impacts on energy transfer efficiency within the
food web. In the second experiment (Katechakis et al. 2002), I
investigated how copepods, cladocerans and doliolids directly
influence the phytoplankton and the microbial food web over a
longer period of time by grazing. Furthermore, I investigated how
they indirectly influence the system's nutrient dynamics through
"sloppy feeding" and their excretions. According to my experiment,
in the long run, doliolids and cladocerans promote the growth of
large algae whereas copepods shift the size spectrum towards small
sizes with different consequences for food chain length. Doliolids,
cladocerans and copepods also affect the microbial food web in
different ways. Size-selective grazing may lead to differences in
the nanoplankton concentrations. These in turn can affect bacterial
concentrations in a trophic cascade. My findings offered the first
experimental evidence for the occurrence of top-down effects in
marine systems. Although top-down explanations of phytoplankton
size structure had been acknowledged for limnic systems before,
they had not been attempted for marine systems. In the last
experiment of this series (Katechakis and Stibor 2004) I sought to
complement the knowledge about the feeding behavior of marine
cladocerans. Marine cladocerans are difficult to cultivate in the
laboratory. Therefore, the three cladoceran genera found in marine
systems, Penilia, Podon and Evadne, had never before been compared
under similar conditions. Existing studies with single cladoceran
genera were to some extent contradictory. My experiments indicate
similar feeding characteristics for Penilia, Podon and Evadne, that
is to say, similar food size spectra, clearance and ingestion
rates. However, Evadne obviously has problems feeding on motile
prey organisms. The results generated by my first study have been
summarized and their importance has been hypothetically extended to
ecosystem level by Sommer et al. (2002) and by Sommer and Stibor
(2002). My second study includes two experiments that refer to the
ecological role of mixotrophs in aquatic systems. Mixotrophic
organisms combine phototrophic and phagotrophic production
dependent on the availability of light and nutrients. Although they
are common in aquatic systems, their function for nutrient cycling
and as a link to higher trophic levels has never before been
examined. In my first experiment (Katechakis et al. 2005) I
investigated if mixotrophs influence energy transfer efficiency to
higher trophic levels differently than predicted for purely
phototrophic organisms. My results indicate that compared to
phototrophic specialists mixotrophs may enhance transfer efficiency
towards herbivores at low light conditions and in situations when
limiting nutrients are linked to bacteria and to the picoplankton.
Additionally, the results suggest that mixotrophs may have a
stabilizing effect on variations in trophic cascade strength caused
by perturbations to light and nutrient supply ratios. My second
experiment (Katechakis and Stibor 2005a) served as a first step
towards analyzing if the results gained from the first experiment
have any ecological relevance in situ, that is, if mixotrophs in
nature-like communities can gain enough importance to relevantly
influence transfer efficiency and system stability. Competition
experiments revealed that mixotrophs may invade and suppress
plankton communities that consist of purely phototrophic and purely
phagotrophic specialists at low nutrient conditions while high
nutrient supplies prevent mixotrophs from successfully invading
such communities. In systems where mixotrophs suppressed their
specialist competitors they indeed had a habitat-ameliorating
effect for higher trophic levels, gauged in terms of plankton food
quality.