ALBERT

Description: 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°C, 4°C, and 6°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.

Description: 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.

Description: Environmental context. Laboratory incubation experiments and shipboard measurements in the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit monoterpenes including (–)-/(+)-pinene, limonene and p-ocimene, all of which were previously thought to be exclusively of terrestrial origin. Maximum levels of 100–200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom. Abstract. Laboratory incubation experiments and shipboard measurements on the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit at rates, expressed as nmol C10H16 (monoterpene) g [chlorophyll a]–1 day–1, from 0.3 nmol g [chlorophyll a]–1 day–1 for Skeletonema costatum and Emiliania huxleyi to 225.9 nmol g [chlorophyll a]–1 day–1 for Dunaliella tertiolecta. Nine monoterpenes were identified in the sample and not in the control, namely: (–)-/(+)-pinene, myrcene, (+)-camphene, (–)-sabinene, (+)-3-carene, (–)-pinene, (–)-limonene and p-ocimene. In addition, shipboard measurements of monoterpenes in air were made in January–March 2007, over the South Atlantic Ocean. Monoterpenes were detected in marine air sufficiently far from land as to exclude influence from terrestrial sources. Maximum levels of 100–200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom, whereas in low chlorophyll regions monoterpenes were mostly below detection limit.

Description: Environmental context. Carbon monoxide (CO) is a key component for atmospheric chemistry and its production in the ocean, although minor at the global scale, could play a significant role in the remote marine atmosphere. Up to now, CO production in the ocean was considered to mainly originate from the photo-production of dissolved organic matter (mainly under UV radiation). In this paper, we show evidence for direct production of CO by phytoplankton and we suggest it as a significant mechanism for CO production in the ocean. Abstract. In order to investigate carbon monoxide (CO) emissions by phytoplankton organisms, a series of laboratory experiments was conducted in Kiel (Germany). Nine monocultures, including diatoms, coccolithophorids, chlorophytes and cyanobacteria have been characterised. This was done by following the CO variations from monoculture aliquots exposed to photosynthetically active radiation during one or two complete diurnal cycles. All the studied cultures have shown significant CO production when illuminated. Emission rates have been estimated to range from 1.4 × 10–5 to 8.7 × 10–4 μg of CO μg chlorophyll–1 h–1 depending on the species. When considering the magnitude of the emission rates from the largest CO emitters (cyanobacteria and diatoms), this biotic source could represent up to 20% of the CO produced in oceanic waters. As global models currently mainly consider CO production from the photo-degradation of dissolved organic matter, this study suggests that biotic CO production should also be taken into account. Whether this biological production might also contribute to some degree to the previous observed non-zero CO production below the euphotic zone (dark CO production) cannot be deduced here and needs to be further investigated.

Description: 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, Schöhsee, Germany. 2. The plankton community was monitored regularly. Daphniids were able to graze on the phytoplankton present, which mainly consisted of small (〈1000 μm3) species, whereas copepods did not show any impact on algae. 3. At the end of the experiment, Daphnia and remaining cyclopoid copepods were harvested and sorted manually, prior to analyses for stable isotopes of carbon and nitrogen. Daphniids from mesocosms stocked purely with differing densities of Daphnia showed little variability in stable isotope values, whereas those that thrived in enclosure bags together with copepods exhibited lower δ13C values. 4. The change in Daphniaδ13C indicates a change of food sources, modified by the presence of the copepods: the higher the mean abundance of copepods in the enclosures, the more 13C-depleted the daphniids. Increasing abundance of high nucleic acid (HNA) bacteria in the copepod bags may account for the trend in Daphniaδ13C via increased grazing on the bacteria themselves, or via grazing on phytoplankton utilising isotopically light CO2 from respiratory release. 5. Cyclopoid copepod stable isotope signatures were related to Daphnia and copepod abundances in copepod bags, suggesting that cyclopoids preyed on the available zooplankton.