Eutrophication and overfishing in temperate nearshore pelagic food

Vasas et al.: Eutrophication and overfishing in food webs11Mixed trophic impact0.250.200.150.100.050.00–0.05–0.10–0.15–0.20–0.25–0.300.150.10a PlvFishb PiscFish–0.37010.050.00–0.05–0.10–0.15–0.4200–0.5267–0.4733PicoPlAutNanoFlLAutFlDiatInedAlgFreeBacPOM-BacHetNanoFlµZooPlMesoZooPlNoctilucaJellyfishPlvFishBentFishPiscFishVirDiatVirInedAlgVirBacVirNocLDOMSDOMPOMNH4PO4NO3SiFig. 6. Mixed trophic impact of fish tropho-species. Colors explained in Fig. 3. See Table 1 for abbreviations& Wyatt 2006), similarly to what is described abovefor jellyfish.In summary, overfishing of PiscFish (under conditionsof nutrient enrichment) may lead to blooms of Diatand LAutFl that remain ungrazed, but overfishing ofPlvFish may support harmful species (InedAlg, Noctiluca,Jellyfish). Even in its simplicity, our analysis tentativelyexplains why the consequences of overfishing incoastal seas were not visible for so long. Historically,overfishing of larger PiscFish was the first step (Paulyet al. 1998), but the subsequent phytoplankton biomassincrease seemed to be related to the increase in nutrientloads. However, after depletion of the larger Pisc-Fish, fisheries have turned towards the smaller, butnumerous PlvFish (e.g. anchovy). With time, the effectsof nutrient enrichment and overfishing started todevelop in synergy, resulting in a very visible degradationof the ecosystem, characterized by HABs and jellyfishblooms. One remarkable example is the outburstof Mnemiopsis leidyi in the Black Sea after strongoverfishing of anchovy in 1988 (Shiganova 1998).Positioning of harmful speciesThe position of harmful species in the food web is farfrom random, as they disrupt the food web at eachbasic trophic level: InedAlg increases the primary producers’biomass, Noctiluca that of primary consumersand Jellyfish to biomass of secondary consumers. Allthese species appear to benefit more from nutrientsthan their competitors, partly as a result of the absenceof predators. The increasing dominance of inedibleforms over edible ones in response to nutrient enrichmenthas been demonstrated both theoretically (e.g.Leibold 1996) and experimentally (e.g. Steiner 2001),but other mechanisms are also involved. Harmfulgroups are positioned in the network in a way thatenables them to exploit the most important bottom-upcontrollers of the food web (Table 3), according to thebottom-up component of the keystone index (K b ). Themost important bottom-up controls are reflected by thehighest values of K b , which gives an approximation ofthe number of species that would disappear due to