Eutrophication and overfishing in temperate nearshore pelagic food

Vasas et al.: Eutrophication and overfishing in food webs9(Fernández et al. 2005). In addition, Jellyfish are likelyto impede InedAlg and Noctiluca, preventing theirblooms by taking up and storing nutrients, and disruptingnutrient cycling in the water column. Thus, from ahuman perspective, Jellyfish could play a beneficialrole in eutrophicated and heavily overfished ecosystems.However, Jellyfish are more voracious predatorsof MesoZooPl than are PlvFish, both because of theirfeeding characteristics and because they are not controlledby predators (Fig. 4c; cf. Fig. 6a). If Jellyfish outcompetePlvFish, the top-down cascade originatingfrom the upper trophic level is reinforced and leads toa massive accumulation of large phytoplankton biomass(Diat and LAutFl), if nutrient concentrations arehigh enough (Daskalov 2003). At low nutrient concentrations,the 4-step trophic chain AutNanoFl–µZooPl–MesoZooPl–Jellyfish prevails in the system and an increasedlevel of jellyfish predation leads to a decreasein total phytoplankton biomass. In eutrophicatedwaters, however, large algae and the 3-step chainDiat/LAutFl–MesoZooPl–Jellyfish are of greater importance,and the cascading effects of jellyfish predationon copepods lead to increased total phytoplanktonbiomass (Stibor et al. 2004). Mass sedimentation ofungrazed phytoplankton biomass may result in hypoxiaor anoxia near the bottom (Cloern 2001). These and thestrong negative effects on fish and fisheries due toresource competition (see Shiganova 1998 for field evidencefrom the Black Sea) make jellyfish an undesiredgroup, from a human perspective.Consequences of human impactsHuman eutrophication and overfishing affect thefood web at several nodes. An increased loading ofinorganic (NH4, PO4, NO3) and organic nutrients(LDOM, SDOM, POM) disturbs the food web at the bottom,while fishing pressure modifies top predators (Plv-Fish, PiscFish). In order to understand how thesehuman impacts may affect the ecosystem, we haveanalyzed the role these groups play in the food web,with a focus on harmful tropho-species.Increased nutrient load. Nutrients NH4, PO4 andNO3 are beneficial to all tropho-species except Bent-Fish (since it feeds on prey from outside the pelagiccommunity; Fig. 5a). InedAlg and Jellyfish seem to befavored, compared to Diat and PlvFish (Fig. 5a). Thissuggests that an increase in inorganic nutrient inputwill enhance a disproportional biomass developmentof these undesired species. Noctiluca indirectly benefitsfrom inorganic N and P enrichment and, in turn,contributes to this pool with remineralization processesassociated with its metabolism. This process is a generalfeature of heterotrophic tropho-species, includingMesoZooPl. The relative importance of these 2 nutrientcycles may lead to alternative stable states of theecosystem at high inorganic nutrient levels: one associatedwith high MesoZooPl, and one with high Noctilucabiomass. The dominance of Noctiluca can be explainedby the unexpected result that—even though bothgroups are direct competitors for Diat—MesoZooPlfavors Noctiluca growth through its contribution toPOM (Fig. 3), as reported by Kiørboe (2003).A confusing result is that inorganic N and P are increasinglymore beneficial to small phytoplankton(Fig. 5a), since every phytoplankton group supports itsheterotrophic grazer and the subsequent trophic levelsabove. Small phytoplankton organisms thus indirectlypromote the grazers of their larger competitors. Thepattern in our model contradicts the general observationthat small phytoplankton is dominant in oligotrophicwaters, while eutrophicated waters are dominatedby large phytoplankton. This may be an artifactresulting from the simplicity of our model, sincedynamical information is lost in structural networkanalyses. More specifically, the latter cannot accountfor time lags in the response of predators to changes inprey abundance. Even if the entire prey biomass isconsumed in the end, a predator responding with a significanttime lag to an increased abundance of preywill allow the prey to develop in a way that mitigatesthe predator’s effects; this feature is not captured bystructural network analysis. The observed fact thatsmall predators respond faster to increases in smallphytoplankters may counterbalance the advantagethat small algae have due to their high competitiveabilities for low nutrients (Fogg 1995) and their positionin the food web. Further research is needed forunderstanding this phenomenon.All harmful groups (InedAlg, Noctiluca, Jellyfish) benefitmore from organic nutrient enrichment (LDOM,SDOM, POM) than their respective ‘desired’ competitor,Diat, MesoZooPl, PlvFish (Fig. 5b,c). Thus, a sustainedincrease of organic matter may be responsiblefor the observed disproportional increase in the biomassof the undesired groups. A marked differencebetween these groups is the role of sink for organicmatter played by Jellyfish, while InedAlg and Noctilucaconstitute a source of organic matter throughout lysis(Table 1; Fig. 3). As a consequence, InedAlg, Noctilucaand DOM can increase simultaneously and constitutean alternative stable state of the ecosystem.Consequences of overfishing. Fish-driven trophiccascades may have large effects in ecosystems (Paceet al. 1999), and overfishing largely disturbs the foodweb (fishing down: Pauly et al. 1998). According toour calculations, PlvFish shows negative effects onJellyfish and, to a smaller extent, on InedAlg, Noctilucaand all nutrients (Fig. 6a), but it positively affects