Eutrophication and overfishing in temperate nearshore pelagic food

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6Mar Ecol Prog Ser 336: 1–14, 2007bacteria attached to detrital particles POM-Bac hydrolyzeparticulate organic matter POM and release DOMinto the surrounding water. Both bacterial groups,along with picoplankton, support the growth of heterotrophicnanoflagellates HetNanoFl, which are in turnconsumed by microzooplankton µZooPl (ciliates, dinoflagellates,copepod nauplii and copepodits I-II), togetherwith AutNanFl (Fig. 3).The higher trophic levels include mesozooplanktonMesoZooPl (mainly copepods and copepodits III & IV)that graze on large phytoplankton (LAutFl and Diat)and µZooPl (Fig. 3). In eutrophicated coastal waters,MesoZooPl may be outnumbered by the omnivorousdinoflagellate Noctiluca that forms red-tide (occasionallygreen-tide) blooms, which significantly influencethe functioning of the ecosystem (e.g. Yılmaz et al.2005). When blooming, Noctiluca is toxic to fish becauseof its release of ammonium (Okaichi & Nishio1976). The unsaturated feeding function determinedfor Noctiluca when growing under laboratory conditionswith different prey (E. Breton pers. comm.), suggeststhat Noctiluca has a great potential to benefitfrom an increased phytoplankton biomass in eutrophicatedwaters. Accordingly, Noctiluca has been reportedas an important indicator of anthropogenic influence(Daskalov 2003).The food requirements of top predators, like piscivorousfish species PiscFish, are fulfilled by planktivorousPlvFish and benthic-feeding BentFish fish. PlvFish competefor MesoZooPl with Jellyfish (members of theCnidaria and Ctenophora), the third harmful trophicgroup (Fig. 3). Jellyfish are undesirable for humans,since they compete with PlvFish and, being a trophicdead end, they do not transfer energy and nutrientstowards the top predators.At the base of the food web, viruses are responsiblefor much of the bacterial mortality and the decay ofphytoplankton blooms; their role in nutrient regenerationis thus far from negligible (Thingstad et al. 1993).In this study we consider 4 host-specific groups ofviruses susceptible of infesting Diat, InedAlg, FreeBacand Noctiluca. Diat and InedAlg were chosen on thebasis of their ability to form huge transient blooms.Viruses infesting attached bacteria were not considereddue to the generally low number of their hosts (Azamet al. 1983, Becquevort et al. 1998). Despite a lack of literatureevidence, Noctiluca-specific viruses wereincluded in this study because of the observed highdensity and sudden collapse of Noctiluca blooms thatmight indicate a viral attack (Beltrami & Carroll 1994).The generic food web included 7 nutrient pools(Fig. 3). All 4 inorganic nutrient pools (ammonium NH4,nitrate NO3, phosphate PO4 and silicic acid Si), whichsupport phytoplankton growth, exchange with the sediment,and NH4 and PO4 are remineralized by heterotrophiccomponents of the food web (FreeBac, POM-Bac, HetNanoFl, µZooPl, MesoZooPl, Noctiluca). Largerheterotrophic organisms (Jellyfish, PlvFish, BentFish andPiscFish) remineralize nutrients only to a negligibleextent, because of their relatively low biomass (Schneider1990, Hudson et al. 1999).The detrital organic matter is composed of dissolved(DOM) and particulate POM forms. Two pools of DOMare distinguished on the basis of their biodegradability:labile LDOM and semi-labile SDOM (Fig. 3). Directexcretion by all phytoplankton groups flows to LDOMand is easily accessible for FreeBac. Other processes,namely overflow during nutrient limitation from phytoplankton,stress lysis or viral lysis, egestion, sloppyfeeding and fecal pellet production, contributes to all3 organic pools (LDOM, SDOM, POM).The structure of this generic pelagic food web oftemperate coastal ecosystems is basically the same indifferent ecosystems, but the dominance of trophicgroups or pathways changes in line with environmentalconditions.Structural analysisRole of harmful speciesThe trophic role of the 3 harmful species (InedAlg,Noctiluca, Jellyfish) was determined on the basis ofpairwise interactions between the components of thegeneric food web (Table 2).InedAlg (Fig. 4a) has a strong negative effect on allphytoplankton groups, due to competition for inorganicnutrients. On the other hand, InedAlg positivelyinfluences the lower food web (FreeBac, POM-Bac,HetNanoFl) through the release of organic matter afterviral and stress lysis, but unlike other large-sizedphytoplankters (LAutFl, Diat), it negatively affects thehigher food web (MesoZooPl, PlvFish, PiscFish; Fig. 4a).This result is supported by the low trophic efficiencyestimated for a Phaeocystis-dominated food web in theNorth Sea, and explained by the important flow of primaryproducts throughout the bacterial loop (Rousseauet al. 2000).The role of Noctiluca in the web is ambiguous(Fig. 4b). It impacts MesoZooPl negatively, but 1 orderof magnitude less than InedAlg (Fig. 4a,b); Noctilucaand MesoZooPl compete, however, for the same prey(Diat). Thus, we expect that any increase in Noctilucabiomass exerts a slightly negative effect on MesoZooPland—as a consequence—on fishes. This result canpossibly be explained with the intensive and shortnutrient cycles at the very basis of the food web, inwhich Noctiluca is involved. The direct nutrient catabolicrelease associated to Noctiluca growth, and the
Vasas et al.: Eutrophication and overfishing in food webs 7Table 2. Pairwise interactions between components of the generic food web, quantified as mixed trophic impacts (m ij values). See Table 1for abbreviationsTo PicoPl Aut LAutFl Diat InedAlg Free POM-Bac Het µZooPl Meso Noctiluca Jelly PlvFishFrom NanoFl Bac NanoFl ZooPl fishPicoPl –0.2570 –0.1208 –0.0689 –0.0393 –0.0608 –0.0292 –0.0672 –0.1758 –0.0397 –0.0122 –0.0070 –0.0122 –0.0091AutNanoFl –0.0818 –0.3272 –0.1070 –0.0460 –0.0693 –0.0402 –0.0594 –0.1743 –0.2347 –0.0145 –0.0276 –0.0145 –0.0109LAutFl –0.1388 –0.0058 –0.2130 –0.0641 –0.0762 –0.0079 –0.0288 –0.0373 –0.0958 –0.1115 –0.0278 –0.1115 –0.0836Diat –0.0899 –0.0118 –0.1180 –0.3770 –0.0276 –0.0495 –0.0532 –0.0530 –0.0487 –0.0811 –0.2635 –0.0811 –0.0608InedAlg –0.0930 –0.0674 –0.0557 –0.0388 –0.3052 –0.0413 –0.0915 –0.0194 –0.0061 –0.0179 –0.0234 –0.0179 –0.0134FreeBac –0.0438 –0.0106 –0.0309 –0.0178 –0.0344 –0.5940 –0.0123 –0.0896 –0.0352 –0.0149 –0.0281 –0.0149 –0.0112POM-Bac –0.0829 –0.0135 –0.0459 –0.0414 –0.0538 –0.0077 –0.2707 –0.1547 –0.0582 –0.0259 –0.0570 –0.0259 –0.0194HetNanoFl –0.4504 –0.0711 –0.0698 –0.0475 –0.0942 –0.0856 –0.1461 –0.4240 –0.1967 –0.0558 –0.0710 –0.0558 –0.0419µZooPl –0.3773 –0.4601 –0.0296 –0.0182 –0.0458 –0.0817 –0.1559 –0.3162 –0.4788 –0.0906 –0.0060 –0.0906 –0.0680MesoZooPl –0.0959 –0.2740 –0.3625 –0.0268 –0.0482 –0.0215 –0.0310 –0.1602 –0.2097 –0.5732 –0.0084 –0.4268 –0.3201Noctiluca –0.0512 –0.0315 –0.0480 –0.0718 –0.0348 –0.0479 –0.0691 –0.0049 –0.0149 –0.0016 –0.3038 –0.0016 –0.0012Jellyfish –0.0480 –0.1370 –0.1813 –0.0134 –0.0241 –0.0107 –0.0155 –0.0801 –0.1049 –0.2134 –0.0042 –0.2134 –0.1601PlvFish –0.0360 –0.1028 –0.1359 –0.0100 –0.0181 –0.0081 –0.0116 –0.0601 –0.0786 –0.1601 –0.0031 –0.1601 –0.3701BentFish –0.0120 –0.0343 –0.0453 –0.0033 –0.0060 –0.0027 –0.0039 –0.0200 –0.0262 –0.0534 –0.0010 –0.0534 –0.2100PiscFish –0.0240 –0.0685 –0.0906 –0.0067 –0.0121 –0.0054 –0.0078 –0.0401 –0.0524 –0.1067 –0.0021 –0.1067 –0.4200VirDiat –0.0086 –0.0014 –0.0332 –0.1226 –0.0155 –0.0258 –0.0599 –0.0121 –0.0193 –0.0125 –0.0212 –0.0125 –0.0094VirInedAlg –0.0216 –0.0263 –0.0281 –0.0150 –0.2216 –0.0219 –0.0400 –0.0163 –0.0116 –0.0097 –0.0237 –0.0097 –0.0073VirBac –0.0125 –0.0015 –0.0059 –0.0068 –0.0072 –0.1673 –0.0643 –0.0221 –0.0081 –0.0037 –0.0174 –0.0037 –0.0028VirNoc –0.0265 –0.0067 –0.0064 –0.0260 –0.0016 –0.0197 –0.0936 –0.0244 –0.0046 –0.0043 –0.2006 –0.0043 –0.0032LDOM –0.0219 –0.0053 –0.0155 –0.0089 –0.0172 –0.2030 –0.0062 –0.0448 –0.0176 –0.0075 –0.0140 –0.0075 –0.0056SDOM –0.0219 –0.0053 –0.0155 –0.0089 –0.0172 –0.2030 –0.0062 –0.0448 –0.0176 –0.0075 –0.0140 –0.0075 –0.0056POM –0.0573 –0.0292 –0.0698 –0.0055 –0.0712 –0.0162 –0.6947 –0.1522 –0.0657 –0.0251 –0.2911 –0.0251 –0.0188NH4 –0.1752 –0.1626 –0.1556 –0.0930 –0.1559 –0.0272 –0.0316 –0.0327 –0.0453 –0.0523 –0.0529 –0.0523 –0.0392PO4 –0.1752 –0.1626 –0.1556 –0.0930 –0.1559 –0.0272 –0.0316 –0.0327 –0.0453 –0.0523 –0.0529 –0.0523 –0.0392NO3 –0.1752 –0.1626 –0.1556 –0.0930 –0.1559 –0.0272 –0.0316 –0.0327 –0.0453 –0.0523 –0.0529 –0.0523 –0.0392Si –0.0225 –0.0030 –0.0295 –0.1558 –0.0069 –0.0124 –0.0133 –0.0133 –0.0122 –0.0203 –0.0659 –0.0203 –0.0152To –BentFish–PiscFish –VirDiat –Vir –VirBac –VirNoc –LDOM –SDOM –POM –NH4 –PO4 –NO3 –SiFromInedAlgPicoPl –0.0030 –0.0030 –0.0393 –0.0608 –0.0292 –0.0070 –0.0687 –0.0195 –0.0207 –0.0764 –0.0764 –0.0906 –0.0393AutNanoFl –0.0036 –0.0036 –0.0460 –0.0693 –0.0402 –0.0276 –0.0035 –0.0503 –0.0277 –0.0855 –0.0855 –0.1065 –0.0460LAutFl –0.0279 –0.0279 –0.0641 –0.0762 –0.0079 –0.0278 –0.0349 –0.0212 –0.0102 –0.1021 –0.1021 –0.1004 –0.0641Diat –0.0203 –0.0203 –0.6230 –0.0276 –0.0495 –0.2635 –0.0779 –0.1237 –0.0797 –0.0154 –0.0154 –0.0799 –0.6230InedAlg –0.0045 –0.0045 –0.0388 –0.6948 –0.0413 –0.0234 –0.0494 –0.0939 –0.1012 –0.0663 –0.0663 –0.0880 –0.0388FreeBac –0.0037 –0.0037 –0.0178 –0.0344 –0.4060 –0.0281 –0.3518 –0.3406 –0.0571 –0.0738 –0.0738 –0.0100 –0.0178POM-Bac –0.0065 –0.0065 –0.0414 –0.0538 –0.0077 –0.0570 –0.0295 –0.1020 –0.1934 –0.1147 –0.1147 –0.0143 –0.0414HetNanoFl –0.0140 –0.0140 –0.0475 –0.0942 –0.0856 –0.0710 –0.1407 –0.1785 –0.1419 –0.1574 –0.1574 –0.0620 –0.0475µZooPl –0.0227 –0.0227 –0.0182 –0.0458 –0.0817 –0.0060 –0.0506 –0.0205 –0.0022 –0.0867 –0.0867 –0.0097 –0.0182MesoZooPl –0.1067 –0.1067 –0.0268 –0.0482 –0.0215 –0.0084 –0.0393 –0.0565 –0.0491 –0.0802 –0.0802 –0.0326 –0.0268Noctiluca –0.0004 –0.0004 –0.0718 –0.0348 –0.0479 –0.6962 –0.0679 –0.0708 –0.0716 –0.0789 –0.0789 –0.0187 –0.0718Jellyfish –0.0534 –0.0534 –0.0134 –0.0241 –0.0107 –0.0042 –0.0196 –0.0282 –0.0246 –0.0401 –0.0401 –0.0163 –0.0134PlvFish –0.2100 –0.2100 –0.0100 –0.0181 –0.0081 –0.0031 –0.0147 –0.0212 –0.0184 –0.0301 –0.0301 –0.0122 –0.0100BentFish –0.2633 –0.2633 –0.0033 –0.0060 –0.0027 –0.0010 –0.0049 –0.0071 –0.0061 –0.0100 –0.0100 –0.0041 –0.0033PiscFish –0.5267 –0.4733 –0.0067 –0.0121 –0.0054 –0.0021 –0.0098 –0.0141 –0.0123 –0.0200 –0.0200 –0.0081 –0.0067VirDiat –0.0031 –0.0031 –0.1226 –0.0155 –0.0258 –0.0212 –0.0382 –0.0513 –0.0659 –0.0247 –0.0247 –0.0128 –0.1226VirInedAlg –0.0024 –0.0024 –0.0150 –0.2216 –0.0219 –0.0237 –0.0373 –0.0448 –0.0481 –0.0437 –0.0437 –0.0261 –0.0150VirBac –0.0009 –0.0009 –0.0068 –0.0072 –0.1673 –0.0174 –0.2297 –0.2464 –0.0533 –0.0153 –0.0153 –0.0018 –0.0068VirNoc –0.0011 –0.0011 –0.0260 –0.0016 –0.0197 –0.2006 –0.0311 –0.0525 –0.1057 –0.0047 –0.0047 –0.0030 –0.0260LDOM –0.0019 –0.0019 –0.0089 –0.0172 –0.2030 –0.0140 –0.1759 –0.1703 –0.0286 –0.0369 –0.0369 –0.0050 –0.0089SDOM –0.0019 –0.0019 –0.0089 –0.0172 –0.2030 –0.0140 –0.1759 –0.1703 –0.0286 –0.0369 –0.0369 –0.0050 –0.0089POM –0.0063 –0.0063 –0.0055 –0.0712 –0.0162 –0.2911 –0.0635 –0.1374 –0.2292 –0.1542 –0.1542 –0.0237 –0.0055NH4 –0.0131 –0.0131 –0.0930 –0.1559 –0.0272 –0.0529 –0.0693 –0.0449 –0.0479 –0.1139 –0.1139 –0.1485 –0.0930PO4 –0.0131 –0.0131 –0.0930 –0.1559 –0.0272 –0.0529 –0.0693 –0.0449 –0.0479 –0.1139 –0.1139 –0.1485 –0.0930NO3 –0.0131 –0.0131 –0.0930 –0.1559 –0.0272 –0.0529 –0.0693 –0.0449 –0.0479 –0.1139 –0.1139 –0.1485 –0.0930Si –0.0051 –0.0051 –0.1558 –0.0069 –0.0124 –0.0659 –0.0195 –0.0309 –0.0199 –0.0038 –0.0038 –0.0200 –0.1558
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Eutrophication and overfishing in temperate nearshore pelagic food

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