Eutrophication and overfishing in temperate nearshore pelagic food

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12Mar Ecol Prog Ser 336: 1–14, 2007food shortage after the extinction of the given species.According our calculation of the index of Jordán et al.(1999), the most important groups are: the inorganicnutrients NH3, PO4, NO3 (consumed by InedAlg); Diatand POM (consumed by Noctiluca), and MesoZooPl(consumed by Jellyfish). The crucial position of harmfulspecies in the food web explains their strong undesiredeffects in ecosystems.Use of qualitative structural analysis of coastalecosystemsSince the literature data required for the establishmentof the trophic web was scarce, the results of ourstructural network analysis are partly speculative.In addition, the qualitative structural approach failswhere dynamic features determine the relationshipsbetween trophic groups. Nevertheless, the agreementbetween our model results and the conclusions of elaboratecase studies supports the feasibility of the modeland underscores the overwhelming importance of thefood web structure in community dynamics.The effects of nutrient loading and overfishing onthe 3 harmful tropho-species and their interactions aresummarized in Fig. 7. InedAlg blooms support themicrobial network and inhibit the transfer of nutrientsup the food web, including to commercially importantfish species (Fig. 7). The effect of Noctiluca on fishes isless detrimental, since this dinoflagellate is involved inshort dynamic nutrient cycles at the base of the foodweb, and this cycling supports small phytoplanktonand bacteria. The developing longer pathway fromminute producers to fishes may partially substitute theshort chain from diatoms.In eutrophicated ecosystems, InedAlg and Noctilucablooms correspond to increased total phytoplanktonbiomass, supported by positive relationships betweenelevated chlorophyll a levels, harmful blooms andincreased nutrient loading (Cloern 2001, Daskalov2003). The InedAlg biomass directly contributes to thechlorophyll level, while Noctiluca indirectly supportsall non-diatom phytoplankton species (Fig. 4b) with itsintensive nutrient regeneration and intensive feedingon Diat. The shift from diatoms to non-siliceous algaein response to eutrophication has been repeatedlypointed out (e.g. Cloern 2001), but the possible contributionof Noctiluca blooms to this shift is new. InedAlgand Noctiluca are fundamentally interrelated in coastalecosystems, with many mechanisms ensuring the recyclingof nutrients in the water column instead ofexporting them to fishes.On the other hand, InedAlg and Noctiluca inhibitJellyfish blooming (Fig. 7), because they retain nutrientsin the water column. Jellyfish is likely to act asa buffer in eutrophicated and overfished systems,since it forms an efficient sink of matter. As a consequence,Jellyfish inhibits InedAlg and Noctiluca as well(Fig. 7). At the same time, Jellyfish blooms lead to amassive biomass accumulation of large phytoplanktonbecause of the reinforced trophic cascade.Anthropogenic nutrient enrichment favors harmfultropho-species (Fig. 7) more than it benefits desiredgroups; this has far-reaching effects, because harmfulspecies hold a key position in the food web by exploit-Fishery-PiscFishFishery-PlvFishTable 3. Bottom-up importance of each trophic group in thegeneric pelagic food web, calculated as the bottom-up componentof the keystone index (Jordán et al. 1999; cf. Fig. 2)JellyfishTrophic groupK bNH4, PO4, NO3 3.69Diat 3.17POM 2.69MesoZooPl 2.50FreeBac 1.69LDOM, SDOM 1.35LAutFl, µZooPl 1.17AutNanoFl, HetNanoFl 1.08Si 1.04InedAlg, Noctiluca 1.00PicoPl, POM-Bac 0.69PlvFish, BentFish 0.50Jellyfish, PiscFish, VirDiat, VirInedAlg, 0.00VirBac, VirNocNoctilucaIned-AlgNutrient loadingFig. 7. Diagram of relationships between harmful trophospeciesand anthropogenic impacts. Solid arrows: positiveeffects; dashed arrows: negative effects; double-headed arrows:reciprocal relationship. Width of arrows is proportionalto the strength of the calculated trophic impacts
Vasas et al.: Eutrophication and overfishing in food webs13ing the most important bottom-up controllers. Accordingto our calculations, the trophic level at which overfishingoccurs is of crucial importance, as previouslysuggested by e.g. Pauly et al. (1998). The earlieroverfishing of PiscFish appears to sustain blooms ofdiatoms and other large algae that remain ungrazed,while the present overfishing of PlvFish supports HABspeciesand jellies, acting synergistically with nutrientenrichment.Calculations with modified versions of the webemployed in this study (without viruses, without harmfulgroups, version for the North Sea: V. Vasas et al.unpubl.) indicate that the main conclusions and interactionpathways are robust. Still, we emphasize thatthe results in this study are not meant as absolute numbers,but rather as characteristic constraints inherent tothe structure of the generic food web. The relativeimportance of these pathways may be quite different,depending on the environmental conditions of theecosystem, and the next step is a comparison of systemswith different structural properties. Moreover,the various roles of nutrients and the effects of unbalancednutrient enrichment could be examined in thefuture, based on comparisons of the same web in termsof nitrogen, phosphorus and silica. Major changes toecosystem state often occur above certain thresholds ofstress (Scheffer et al. 2001), and a search for the structuralcauses behind them is promising. Ecosystems thatshow threshold behavior are a great challenge to management,and the context described here will serve asa basis to study possible mechanisms of thresholds inpelagic nearshore food webs.Acknowledgements. The authors thank 3 anonymous refereesfor constructive comments and suggestions. We aregrateful to W.C. Liu for technical help, T. Wyatt for commentson the manuscript, and K. Csepi for linguistic help. This studyis a contribution to Stream 3 (Nutrient-driven thresholdsof environmental sustainability) of the Integrated ProjectTHRESHOLDS (Thresholds of environmental sustainability),funded by the European Commission (Global Change andEcosystems) under contract 003933, and to the Belgianfederal policy project AMORE III. F.J. was supported bySociety in Science: The Branco Weiss Fellowship, ETH Zürich.LITERATURE CITEDArai MN (2001) Pelagic coelenterates and eutrophication: areview. Hydrobiologia 451:69–87Azam F, Cho BC (1987) Bacterial utilization of organic matterin the sea. In: Fletcher M, Gray TRG, Jones JG (eds)Ecology of microbial communities. Cambridge UniversityPress, Cambridge, p 261–281Azam F, Fenchel T, Field JG, Gray JS, Meyer-Reil LA,Thingstad F (1983) The ecological role of water-columnmicrobes in the sea. 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Eutrophication and overfishing in temperate nearshore pelagic food

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