A planktonic food-web model including sixty-three functional nodes (representing auto- mixo- and heterotrophs) was developed to integrate most trophic diversity present in the plankton. The model was implemented in two variants - which we named ‘green’ and ‘blue’ - characterized by opposite amounts of phytoplankton biomass and representing, respectively, bloom and non-bloom states of the system. Taxonomically disaggregated food-webs described herein allowed to shed light on how components of the plankton community changed their trophic behavior in the two different conditions, and modified the overall functioning of the plankton food web. The green and blue food-webs showed distinct organizations in terms of trophic roles of the nodes and carbon fluxes between them. Such re-organization stemmed from switches in selective grazing by both metazoan and protozoan consumers. Switches in food-web structure resulted in relatively small differences in the efficiency of material transfer towards higher trophic levels. For instance, from green to blue states, a seven-fold decrease in phytoplankton biomass translated into only a two-fold decrease in potential planktivorous fish biomass. By linking diversity, structure and function in the plankton food-web, we discuss the role of internal mechanisms, relying on species-specific functionalities, in driving the ‘adaptive’ responses of plankton communities to perturbations.

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This study explores vertical heterogeneities of in situ division rates (μ) of 3 species of Dinophysis and discusses its implications in modelling the population dynamics of these species. Based on a post-mitotic index approach, estimates of μ from vertical net-haul (integrated) samples (μ int ) were compared with those from a single depth (oceanographic bottle samples) (μ z ) at the cell maximum during 2 daily cell cycle studies in the Galician Rías Baixas (NW Spain). Additionally, vertical profiles of μ z were obtained during a field survey in Ría de Vigo. A 2-fold difference was observed between estimates of μ z from the cell maxima and μ int from integrated samples under stratified conditions. Differences were much larger when the minimum estimates of μ values from different depths (μ min ) were compared within single vertical profiles. An exponential growth model was applied to simulate the dynamics of a D. acuminata population during a daily cycle in June 1994. Results show that actively dividing cells of a target species may be restricted to narrow layers of the water column. Estimates of μ at different relevant depths during cell cycle studies may be key to determining whether if increased numbers of a target species are due to in situ growth or to the balance of imports and exports.KEY WORDS: Dinophysis spp. · Cell cycle · In situ division rate · Mitotic index · Cell maximum · Modelling approach Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 385: [87][88][89][90][91][92][93][94][95][96] 2009 and place, will be observed at its optimum depth in the water column, i.e. the depth where the combination of environmental conditions (resources and physicalbiological interactions) are the most suitable for that species' growth and survival.Dinophysis spp. and other toxin-producing algae (TPA) species that transmit toxins through the food web -even at moderate to low concentrations (10 2 to 10 3 cells l -1) -often constitute a small percentage of the total phytoplankton community. Bulk measurements using chemical indicators such as chlorophyll a concentration and primary production are unsuitable for growth studies of these rare TPA species since they reflect the physiology not of the target organisms but of the whole plankton community. Estimates of μ from in situ cell cycle studies are time-consuming, but provide realistic information on the division capabilities of the species in question (Reguera et al. 2003).Estimates of μ in field populations of Dinophysis spp. have been obtained either from incubations, on deck or in diffusion chambers (Garcés et al. 1997) or from direct in situ high frequency sampling with a mitotic index approach (Table 1). Dinophysis spp. often exhibit sharp heterogeneities in their vertical distribution (reviewed in , Velo-Suárez et al. 2008. To minimize sampling uncertainties derived from low cell concentrations, vertical migration or patchiness (which would result in samples with inadequate numbers of target cells), in situ ...

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Sea-surface discolorations due to high concentrations of phytoplankton are called red tides. Their ecological significance is a long standing puzzle, and they are sometimes considered pathological. Here we propose that many red tides, particularly but not exclusively those comprised of certain autotrophic dinoflagellates, are presexual/sexual swarms, essential links in their complex life cycles. This view provides a rationale for the appearance of these organisms in thin surface layers, and helps explain their ephemeral nature. We suggest that further understanding of this phenomenon, and of phytoplankton ecology in general, would benefit from attention to the 'net reproductive value' (r) over the whole life cycle as well as to the division rate (μ) of the vegetative phase. It is argued that r is strategically adapted to seasonal cycles and long term environmental variability, while μ reflects tactical needs (timing) and constraints (grazers, parasites) on vegetative growth.