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To examine the grazing effects of copepod-dominated mesozooplankton on heterotrophic microbial communities, four mesocosm experiments using gradients of zooplankton abundance were carried out at a coastal marine site. The responses of different protist groups (nanoflagellates, ciliates) and bacterioplankton in terms of abundance and additionally, for bacteria, diversity, production, and exoenzymatic activity, were monitored during 1 week of incubation. Independent of the initial experimental abiotic conditions and the dominating copepod species, zooplankton caused order-of-magnitude changes in microbial functional groups in a clear community-wide four-link trophic cascade. The strongest predatory effects were observed for protist concentrations, thus generating inverse relationships between mesozooplankton and ciliates and between ciliates and nanoplankton. Copepod grazing effects propagated even further, not only reducing the abundance, production, and hydrolytic activity of bacterioplankton but also increasing bacterial diversity. The overall strength of this trophic cascade was dampened with respect to bacterial numbers, but more pronounced with respect to bacterial diversity and activity. High predation pressure by heterotrophic nanoflagellates, realized at the highest copepod abundance, was probably the underlying mechanism for these structural changes in the bacterial assemblages. Our results thus suggest a mechanism by which changes in higher trophic levels of marine plankton indirectly affect prokaryotic assemblages and microbially mediated ecosystem functions.

1. Density gradients of cladocerans and copepods were generated in an enclosure experiment to compare the impact on the plankton of a filter feeder (Daphnia hyalina × galeata) with that of more selective feeders (calanoid and cyclopoid copepods). The experiment was conducted in situ over 25 days during spring in a mesotrophic lake, Schohsee, Germany. 2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (

Ephemeral blooms of filamentous bacteria are a common phenomenon in the water column of oligo- to mesotrophic lakes. It is assumed that the appearance of such morphotypes is favored by selective predation of bacterivorous protists and that filter-feeding zooplankton plays a major role in suppressing these bacteria. The phylogenetic affiliation of the important bloom-forming filamentous bacteria in freshwaters is presently unknown. Here we report the identification of dominant members of a filamentous bacterial assemblage during a bloom of such morphotypes in a mesotrophic lake. By molecular cloning and fluorescence in situ hybridization with specific oligonucleotide probes, up to 98% of filamentous cells in lake water could be assigned to a clade of almost identical (99% similarity) 16S rRNA gene sequence types, the cosmopolitan freshwater LD2 cluster. For a period of less than 1 week, members of the LD2 clade constituted >40% of the total bacterial biomass, potentially favored by high grazing of planktivorous protists. This is probably the most pronounced case of dominance by a single bacterioplankton species ever observed in natural freshwaters. In enclosures artificially stocked with the metazoan filter feederDaphnia, bacteria related to the LD2 clade formed a significantly larger fraction of filaments than in enclosures whereDaphniahad been removed. However, in the presence of higher numbers ofDaphniaindividuals, the LD2 bacteria, like other filaments, were eventually eliminated both in enclosures and in the lake. This points at the potential importance of filter-feeding zooplankton in controlling the occurrence and species composition of filamentous bacterial morphotypes in freshwater plankton.

1. We performed a mesocosm experiment to investigate the structuring and cascading effects of two predominant crustacean mesozooplankton groups on microbial food web components. The natural summer plankton community of a mesotrophic lake was exposed to density gradients of Daphnia and copepods. Regression analysis was used to reveal top-down impacts of mesozooplankton on protists and bacteria after days 9 and 15. 2. Selective grazing by copepods caused a clear trophic cascade via ciliates to nanoplankton. Medium-sized (20-40 mum) ciliates (mainly Oligotrichida) were particularly negatively affected by copepods whereas nanociliates (mainly Prostomatida) became more abundant. Phototrophic and heterotrophic nanoflagellates increased significantly with increasing copepod biomass, which we interpret as an indirect response to reduced grazing pressure from the medium-sized ciliates. 3. In Daphnia-treatments, ciliates of all size classes as well as nanoflagellates were reduced directly but the overall predation effect became most strongly visible after 15 days at higher Daphnia biomass. 4. The response of bacterioplankton involved only modest changes in bacterial biomass and cell-size distribution along the zooplankton gradients. Increasing zooplankton biomass resulted either in a reduction (with Daphnia) or in an increase (with copepods) of bacterial biovolume, activity and production. Patterns of bacterial diversity, as measured by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE), showed no distinct grouping after 9 days, whereas a clear treatment-coupled similarity clustering occurred after 15 days. 5. The experiment demonstrated that zooplankton-mediated predatory interactions cascade down to the bacterial level, but also revealed that changes occurred rather slowly in this summer plankton community and were most pronounced with respect to bacterial activity and composition.

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalinaxgaleata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 microm3), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.

The oceans have absorbed nearly half of the fossil-fuel carbon dioxide (CO2) emitted into the atmosphere since pre-industrial times1, causing a measurable reduction in seawater pH and carbonate saturation2. If CO2 emissions continue to rise at current rates, upper-ocean pH will decrease to levels lower than have existed for tens of millions of years and, critically, at a rate of change 100 times greater than at any time over this period3. Recent studies have shown effects of ocean acidification on a variety of marine life forms, in particular calcifying organisms4, 5, 6. Consequences at the community to ecosystem level, in contrast, are largely unknown. Here we show that dissolved inorganic carbon consumption of a natural plankton community maintained in mesocosm enclosures at initial CO2 partial pressures of 350, 700 and 1,050 μatm increases with rising CO2. The community consumed up to 39% more dissolved inorganic carbon at increased CO2 partial pressures compared to present levels, whereas nutrient uptake remained the same. The stoichiometry of carbon to nitrogen drawdown increased from 6.0 at low CO2 to 8.0 at high CO2, thus exceeding the Redfield carbon:nitrogen ratio of 6.6 in today’s ocean7. This excess carbon consumption was associated with higher loss of organic carbon from the upper layer of the stratified mesocosms. If applicable to the natural environment, the observed responses have implications for a variety of marine biological and biogeochemical processes, and underscore the importance of biologically driven feedbacks in the ocean to global change.

An indoor mesocosm system was set up to study the response of phytoplankton and zooplankton spring succession to winter and spring warming of sea surface temperatures. The experimental temperature regimes consisted of the decadal average of the Kiel Bight, Baltic Sea, and three elevated regimes with 2 degrees C, 4 degrees C, and 6 degrees C temperature difference from that at baseline. While the peak of the phytoplankton spring bloom was accelerated only weakly by increasing temperatures (1.4 days per degree Celsius), the subsequent biomass minimum of phytoplankton was accelerated more strongly (4.25 days per degree Celsius). Phytoplankton size structure showed a pronounced response to warming, with large phytoplankton being more dominant in the cooler mesocosms. The first seasonal ciliate peak was accelerated by 2.1 days per degree Celsius and the second one by 2.0 days per degree Celsius. The over-wintering copepod populations declined faster in the warmer mesocosm, and the appearance of nauplii was strongly accelerated by temperature (9.2 days per degree Celsius). The strong difference between the acceleration of the phytoplankton peak and the acceleration of the nauplii could be one of the "Achilles heels" of pelagic systems subject to climate change, because nauplii are the most starvation-sensitive life cycle stage of copepods and the most important food item of first-feeding fish larvae.

We investigated the impact of copepods on the seston community in a mesocosm set-up, and assessed how the changes in food quantity, quality and size affected the condition of the grazers, by measuring the RNA:DNA ratios in different developmental stages of Calanus finmarchicus. Manipulated copepod densities did not affect the particulate carbon concentration in the mesocosms. On the other hand, chlorophyll a content increased with higher copepod densities, and increasing densities had a positive effect on seston food quality in the mesocosms, measured as C:N ratios and ω3:ω6 fatty acid ratios. These food quality indicators were significantly correlated to the nutritional status of C. finmarchicus. In contrast to our expectations, these results suggest a lower copepod growth potential on higher quality food. However, in concordance with earlier studies, we found that when copepods were in high densities the large particles (>1000 µm3) decreased and that the smaller particles (