Bottle incubation experiments are widely used in mesozooplankton grazing studies. However, we have shown here that traditional particle removal experiments with Calanus finmarchicus and C. helgolandicus as grazers on natural plankton may yield low or even statistically significant (p < 0.05) negative grazing estimates, even though negative grazing rates are impossible. Low grazing rates are often reported, especially on smaller prey types, despite abundant food and significant egg production. Microzooplankton, such as ciliates, show higher biomass-specific grazing rates on algae than do copepods and other mesozooplankton. Instead, copepods often selectively feed on the microzooplankton. Thus, apparent negative rates would be expected when the release of microzooplankton grazing pressure outweighs the copepod grazing rates on the same food items in the incubation bottle. We show that this potentially large bias increases with microzooplankton community grazing pressure in the control. A simplified general method to correct for this bias is presented and compared with the original method (Nejstgaard et al. 1997, Mar Ecol Prog Ser 147:197-217). Although complexity and the need for taxonomic accuracy are reduced in the general method, the results are not significantly different between the 2 methods. Both methods also show a good fit with ingestion rates estimated from faecal pellet production. We suggest that the general method be combined with automated sample treatment in future studies. In addition, we argue that carefully estimated faecal volume production provides a simple and quick overall feeding estimate with important advantages over the common gut pigment technique, and it may be used as an independent method in bottle incubation experiments.KEY WORDS: Calanus finmarchicus · Calanus helgolandicus · Microzooplankton · Grazing methods · Clearance · Ingestion · Faecal pellet production · Natural plankton · Bottle effect Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 221: [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75] 2001 Peterson & Dam 1996, Verity & Paffenhöfer 1996, Nejstgaard et al. 1997, and this study). Quantification of copepod feeding rates in the field by methods based on gut pigment (Mackas & Bohrer 1976), 14 C-labeled algae, or plant pigment analysis by HPLC (e.g. Kleppel & Pieper 1984, MeyerHarms et al. 1999) is limited to herbivory, and may substantially underestimate total zooplankton ingestion rates and bias prey selectivity estimates. Such data must be treated with caution. The potential problem with food web interactions in incubation experiments as discussed here may further limit the value of phytoplankton-based methods, if not corrected.The dual labelling technique (Roman & Rublee 1981, Roman & Gauzens 1997) yields data on zooplankton omnivory in situ. However, this method does not give detailed data on feeding selectivity and has a number of potential problems, including the fact that grazing on algae ca...

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Mixotrophs combine photosynthesis with phagotrophy to cover their demands in energy and essential nutrients. This gives them a competitive advantage under oligotropihc conditions, where nutrients and bacteria concentrations are low. As the advantage for the mixotroph depends on light, the competition between mixo- and heterotrophic bacterivores should be regulated by light. To test this hypothesis, we incubated natural plankton from the ultra-oligotrophic Eastern Mediterranean in a set of mesocosms maintained at 4 light levels spanning a 10-fold light gradient. Picoplankton (heterotrophic bacteria (HB), pico-sized cyanobacteria, and small-sized flagellates) showed the fastest and most marked response to light, with pronounced predator-prey cycles, in the high-light treatments. Albeit cell specific activity of heterotrophic bacteria was constant across the light gradient, bacterial abundances exhibited an inverse relationship with light. This pattern was explained by light-induced top-down control of HB by bacterivorous phototrophic eukaryotes (PE), which was evidenced by a significant inverse relationship between HB net growth rate and PE abundances. Our results show that light mediates the impact of mixotrophic bacterivores. As mixo- and heterotrophs differ in the way they remineralize nutrients, these results have far-reaching implications for how nutrient cycling is affected by light.

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We studied the effects of future climate change scenarios on plankton communities of a Norwegian fjord using a mesocosm approach. After the spring bloom, natural plankton were enclosed and treated in duplicates with inorganic nutrients elevated to pre-bloom conditions (N, P, Si; eutrophication), lowering of 0.4 pH units (acidification), and rising 3°C temperature (warming). All nutrient-amended treatments resulted in phytoplankton blooms dominated by chain-forming diatoms, and reached 13–16 μg chlorophyll (chl) a l−1. In the control mesocosms, chl a remained below 1 μg l−1. Acidification and warming had contrasting effects on the phenology and bloom-dynamics of autotrophic and heterotrophic microplankton. Bacillariophyceae, prymnesiophyceae, cryptophyta, and Protoperidinium spp. peaked earlier at higher temperature and lower pH. Chlorophyta showed lower peak abundances with acidification, but higher peak abundances with increased temperature. The peak magnitude of autotrophic dinophyceae and ciliates was, on the other hand, lowered with combined warming and acidification. Over time, the plankton communities shifted from autotrophic phytoplankton blooms to a more heterotrophic system in all mesocosms, especially in the control unaltered mesocosms. The development of mass balance and proportion of heterotrophic/autotrophic biomass predict a shift towards a more autotrophic community and less-efficient food web transfer when temperature, nutrients and acidification are combined in a future climate-change scenario. We suggest that this result may be related to a lower food quality for microzooplankton under acidification and warming scenarios and to an increase of catabolic processes compared to anabolic ones at higher temperatures.