Eutrophication and overfishing in temperate nearshore pelagic food

Vasas et al.: Eutrophication and overfishing in food webs5RESULTS AND DISCUSSIONStructure of the generic food webBased on our literature review, 26 ‘trophic’ groups—19 tropho-species and 7 nutrient pools—were identifiedas relevant in temperate coastal ecosystems (Fig. 3,Table 1). The basic definition of tropho-species wasbased on their metabolism (autotrophic, heterotrophic)and size, since these are the main determinants oftheir trophic roles. This classification was expanded,as organisms of similar size may differ in prey selectionor trophic fate.We defined 5 phytoplankton groups, based on theirbottom-up and top-down characteristics (Fig. 3). Theseare picoplankton PicoPl (e.g. Synechococcus spp.,Prochlorococcus spp., picoeukaryotes), autotrophic flagellatesAutNanFl (nano-sized phytoplankton belongingto haptophytes, chrysophytes, cryptophytes, prasinophytes),large autotrophic flagellates LAutFl (e.g.non-toxic dinoflagellates Ceratium spp.), diatoms Diat,and inedible blooming (HAB) algae InedAlg. The latterwas defined on the basis of the ‘loophole’ hypothesis(Bakun & Broad 2003), i.e. blooming species aredefined as those capable of escaping predation controlsby zooplankton because of their size, shape ortoxicity (Irigoien et al. 2005). InedAlg thus include algaeforming high biomass blooms (e.g. Phaeocystis globosacolonies) and toxic species (haptophytes, e.g. Prymnesiumparvum, Chrysochromulina polylepis; raphidophytes,e.g. Heterosigma akashiwo; dinoflagellates,e.g. Alexandrium spp., Karenia brevis; cyanobacteria,e.g. Nodularia spumigena). Phytoplankton, on the otherhand, may remain ungrazed if zooplankton is heavilycontrolled by predators (Daskalov 2002). The harmfuleffects of InedAlg and ungrazed edible phytoplanktonwill be discussed separately (see ‘Structural analysis:Consequences of human impacts’).The microbial network (Fig. 3) rests on the bacterialutilization of organic matter in the water column andthe consumption of small auto- and heterotrophicgroups, generally one order of magnitude smaller thanthemselves (Sheldon et al. 1972). Free-living bacteriaFreeBac utilize dissolved organic matter (DOM), whileNH4InedAlgVirInedAlgVirBacLDOMPicoPlFreeBacSDOMPO4Aut-NanoFlHet-NanoFlPOM-BacJellyfishLAutFlVirNocµZooPlPOMNO3SiDiatVirDiatNoctilucaMeso-ZooPlPlvFishBentFishPiscFishFig. 3. Generic structure of the pelagic food web. Circles represent trophic groups and arrows represent mass transfers betweentrophic groups. Acronyms defined in Table 1. Red: harmful groups; purple: viruses; dark green: linear food chain; light green andlight brown: microbial food web (light brown: bacteria); blue: inorganic nutrients; brown: organic nutrients; black: exchange withthe sediment. Arrows have the same color as the recipient group