Blooms of the phytoplankton Phaeocystis can comprise 85% of total production and generate major biogeochemical signals across broad oceanic regions. The success of Phaeocystis may result from its ability to change size by many orders of magnitude when it shifts from small cells of 4 -6 m to large colonies of up to 30,000 m in diameter. Single cells are consumed by ciliates but not copepods, whereas colonies are consumed by copepods but not ciliates. We demonstrate that chemical cues associated with each of these grazers induce consumer-specific, but opposing, morphological transformations in the bloom-forming species Phaeocystis globosa. Chemical cues from grazing copepods suppress colony formation by a significant 60 -90%, a response that should be adaptive because copepods feed four times more on colonies versus solitary cells. In contrast, chemical cues from grazing ciliates enhance colony formation by >25%, a response that should be adaptive because ciliates grow three times faster when fed solitary cells versus colonies. Because size-selective predation fundamentally alters community structure and ecosystem function, this chemically cued shift may redirect energy and nutrients from food webs supporting fisheries to those fueling detrital pathways, thus potentially altering ecosystem-level processes such as productivity, carbon storage, and nutrient release.chemical signaling ͉ consumer-prey interaction ͉ inducible defense ͉ Phaeocystis ͉ size-selective predation

Species may compete indirectly by altering the traits of a shared resource. For example, herbivore-induced responses in plants may make plants more resistant or susceptible to additional herbivorous insect species. Herbivore-induced plant responses can significantly affect interspecific competition and herbivore population dynamics. These herbivore-herbivore indirect interactions have been overlooked in aquatic ecosystems where previous studies used the same herbivore species to induce changes and to assess the effects of these changes. We asked whether seaweed grazing by one of two herbivorous, congeneric snail species (Littorina obtusata or Littorina littorea) with different feeding strategies and preferences would affect subsequent feeding preferences of three herbivore species (both snails and the isopod Idotea baltica) and population densities of three herbivore species (both snails and a third periwinkle snail, Lacuna vincta). In addition, we measured phlorotannin concentrations to test the hypothesis that these metabolites function as induced defenses in the Phaeophyceae. Snail herbivory induced cue-specific responses in apical tissues of the seaweed Fucus vesiculosus that affected the three herbivore species similarly. When compared to ungrazed controls, direct grazing by Littorina obtusata reduced seaweed palatability by at least 52% for both snail species and the isopod species. In contrast, direct grazing by L. littorea did not decrease seaweed palatability for any herbivore, indicating herbivore-specific responses. Previous grazing by L. obtusata reduced populations of L. littorea on outplanted seaweeds by 46% but had no effect on L. obtusata populations. Phlorotannins, a potential class of inducible chemicals in brown algae, were not more concentrated in grazed seaweed tissues, suggesting that some other trait was responsible for the induced resistance. Our results indicate that marine herbivores may compete via inducible responses in shared seaweeds. These plant-mediated interactions were asymmetric with a specialist (L. obtusata) competitively superior to a generalist (L. littorea).

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The worldwide colony-forming haptophyte phytoplankton Phaeocystis spp. are key organisms in trophic and biogeochemical processes in the ocean. Many organisms from protists to Wsh ingest cells and/or colonies of Phaeocystis. Reports on speciWc mortality of Phaeocystis in natural plankton or mixed prey due to grazing by zooplankton, especially protozooplankton, are still limited. Reported feeding rates vary widely for both crustaceans and protists feeding on even the same Phaeocystis types and sizes. Quantitative analysis of available data showed that:(1) laboratory-derived crustacean grazing rates on monocultures of Phaeocystis may have been overestimated compared to feeding in natural plankton communities, and should be treated with caution;(2) formation of colonies by P. globosa appeared to reduce predation by small copepods (e.g., Acartia, Pseudocalanus, Temora and Centropages), whereas large copepods (e.g., Calanus spp.) were able to feed on colonies of Phaeocystis pouchetii; (3) physiological diVerences between diVerent growth states, species, strains, cell types, and laboratory culture versus natural assemblages may explain most of the variations in reported feeding rates; (4) chemical signaling between predator and prey may be a major factor controlling grazing on Phaeocystis; (5) it is unclear to what extent diVerent zooplankton, especially protozooplankton, feed on the diVerent life forms of Phaeocystis in situ. To better understand the mechanisms controlling zooplankton grazing in situ, future studies 123 should aim at quantifying speciWc feeding rates on diVerent Phaeocystis species, strains, cell types, prey sizes and growth states, and account for chemical signaling between the predator and prey. Recently developed molecular tools are promising approaches to achieve this goal in the future.

In high latitude planktonic ecosystems where the prymnesiophyte alga Phaeocystis pouchetii is often the dominant primary producer, its importance in structuring planktonic food webs is well known. In this study we investigated how the base of the planktonic food web responds to a P. pouchetii colony bloom in controlled mesocosm systems with natural water enclosed in situ in a West Norwegian fjord. Similar large (11 m 3 ) mesocosm studies were conducted in 2 successive years and the dynamics of various components of the planktonic food web from viruses to mesozooplankton investigated. In 2002 (4 to 24 March), 3 mesocosms comprising a control containing only fjord water; another with added nitrate (N) and phosphate (P) in Redfield ratios; and a third with added N, P, and cultured solitary cells of P. pouchetii, were monitored through a spring bloom cycle. In 2003 (27 February to 2 April) a similar set of mesocosms were established, but cultured P. pouchetii was not added. As expected, during both years, addition of N and P without addition of silicate resulted in an initial small diatom bloom followed by a colonial bloom of P. pouchetii (600 to 800 µg C l -1 ). However, the hypothesis that addition of solitary cells of P. pouchetii would enhance subsequent colony blooms was not supported. Interestingly, despite the large production of Phaeocystis colonial material, little if any was transferred to the grazing food web, as evidenced by non-significant effects on the biomass of micro-and mesozooplankton in fertilized mesocoms. Separate experiments utilizing material from the mesocosms showed that colonies formed from solitary cells at rates that required only ca. 1% conversion efficiencies. The results are discussed from the perspective of future research still required to understand the impact of life cycle changes of this enigmatic phytoplankter on surrounding ecosystems. KEY WORDS: Phaeocystis pouchetii · Mesocosms · Nutrients · Fjord · Biocomplexity Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 321: 2006 gelatinous polysaccharide 'skin' (Chen et al. 2002). Solitary cells may be either motile or non-motile, and are typically 3 to 9 µm in diameter (Rousseau et al. 1994). This unusually large range of sizes between colonies and solitary cells (ca. 6 to 11 orders of magnitude in biovolume) can significantly alter material flow among trophic levels and export from the upper ocean (Wassmann et al. 1990, Lancelot et al. 1998. Furthermore, each stage is thought to function in different ways in order to reduce losses to either small or large zooplankton and viruses, and thus Phaeocystis spp. effectively function as dual species (Weisse et al. 1994, Smaal & Twisk 1997, Hamm et al. 1999, Jacobsen 2000, Verity 2000, Jakobsen & Tang 2002, Tang 2003.The dual life history of colonial and solitary cell stages was described over 50 yr ago (Kornmann 1955), and the dominant morphology appears to alter the ecosystem function from a regenerative system (solitary cells) ...