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by: Ethyl Terry


Ethyl Terry
GPA 3.63


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This 4 page Class Notes was uploaded by Ethyl Terry on Thursday October 22, 2015. The Class Notes belongs to LAIS 10 at University of California Santa Barbara taught by Staff in Fall. Since its upload, it has received 53 views. For similar materials see /class/226917/lais-10-university-of-california-santa-barbara in Latin American&Latino Studies at University of California Santa Barbara.

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Date Created: 10/22/15
E CD i gush Spreading Dead Zones and Consequences for Marine Ecosystems Robert J Diaz et al Science 321 926 2008 DOI 101126science1156401 Science The following resources related to this article are available online at wwwsciencemag org this information is current as of May 6 2009 Updated information and services including highresolution gures can be found in the online version of this article at httpwwwsciencemagorgcgicontentfull321i5891926 Supporting Online Material can be found at httpwwwsciencemagorgcgicontentfull321i5891926DC1 This article cites 38 articles 5 of which can be accessed for free httpwwwsciencemagorgcgicontentfull321i5891926otherartices This article has been cited by 17 articles on the ISI Web of Science This article has been cited by 4 articles hosted by HighWire Press see httpwwwsciencemagorgcgicontentfull321i5891926otherartices This article appears in the following subject collections Ecology httpwwwsciencemagorgcgicollectionecology Information about obtaining reprints of this article or about obtaining permission to reproduce this article in whole or in part can be found at httpwwwsciencemagorgaboutpermissionsdtl Science print ISSN 00368075 online ISSN 10959203 is published weekly except the last week in December by the American Association for the Advancement of Science 1200 New York Avenue NW Washington DC 20005 Copyright 2008 by the American Association for the Advancement of Science all rights reserved The title Science is a registered trademark of AAAS Downloaded from wwwsciencemagorg on May 6 2009 926 Spreading Dead Zones and Consequences for Marine Ecosystems Robert Diazlquot and Rutger Rosenberg2 Dead zones in the coastal oceans have spread exponentially since the 19605 and have serious consequences for ecosystem functioning The formation of dead zones has been exacerbated by the increase in primary production and consequent worldwide coastal eutrophication fueled by riverine runoff of fertilizers and the burning of fossil fuels Enhanced primary production results in an accumulation of particulate organic matter which encourages microbial activity and the consumption of dissolved oxygen in bottom waters Dead zones have now been reported from more than 400 systems affecting a total area of more than 245000 square kilometers and are probably a key stressor on marine ecosystems ication is the greening of the water column as the algae and vegetation in coastal areas grow in direct response to nutrient enrichment The most serious threat from eutrophication is the unseen decrease in dissolved oxygen DO levels in bottom waters created as planktonic algae die and add to the ow of organic matter to the seabed to fuel microbial respiration 1 Hypoxia occurs when DO falls below 52 ml of Ozliter at which point benthic fauna show aberrant behavior for example abandoning burrows for exposure at the sediment wa er interface culmi nating in mass mortality when DO declines be low 05 ml of Ozliter 2 In most cases hypoxia is associated with a semi enclosed hydrogeomor phology that combined with water column strati cation restricts water exchange More recently dead zones have developed in continental seas such as the Baltic Kattegat Black Sea Gulf of Mexico and East China Sea all of which are major shery areas Although the anthropogenic fertilization of marine systems by excess nitrogen has been linked to many ecosystem level changes there are natural processes that can lead to nutrient enrichment along continental mar 39 s that produce similar ecosystem responses Coastal upwelling zones associated with the western boundary of continental landmasses are highly productive but are associated with severe hypoxia lt05 ml Ozliter These oxygen minimum zones OMZs occur primaril in the eastern Paci c Ocean south Atlantic west of Africa Arabian Sea and Bay of Bengal and are persistent oceanic features oc curring in water column at intermediate depths typically 200 to 1000 m 3 Where they extend to the bottom the benthic fauna is adapted to DO concentrations as low as 01 ml of Ozliter This is in stark contrast to the fauna responses seen dur The visible ecosystem response to eutroph JVirginia institute of Marine Science College of William and Mary Gloucester Point VA 23062 USA ZDe artment of Ma rine Ecology University of Gothenburg Kristineberg 566 450 34 Fiskebatkskil Sweden To whom correspondence should be addressed E mail d diazvimse u 15 AUGUST 2008 VOL 321 ing recent eutrophication induced hypoxic events 39n oasta an es arine areas where DO concen trations this low led to mass mortality and major changes in community structure 2 Global Nature of Eutrophicationlnduced Hypoxia The worldwide distribution of coastal oxygen de pletion is associated with ma39or opulation cen ters and watersheds that deliver large quantities of nutrients Fig 1 and table S1 Most ofthese systems were not hypoxic when first studied but it appears that from the middle of the past c tury the DO concentrations of many coastal ecosystems have been adversely affected by eu trophication The observed declines in DO have lagged about 10 years behind the increased use of industrially produced nitrogen fertilizer that be gan in the late 19 0s with explosive growth in the 1960s to 1970s 4 For marine systems with data from the rst half of the 20th century de clines in oxygen concentrations were rst ob served in the 1950s in the northern Adriatic Sea 5 between the 1940s and 1960s in the north western continental shelf of the Black Sea 6 and in the 1980s in the Kattegat 7 localized declines of DO levels were noted in the Baltic Sea as early as the 1930s but it wasn t until the 1960s that hypoxia became widespread 7 Lo calized hypoxia had also been observed since the 1930s in the Chesapeake Bay 8 and since the 1970s in the northern Gulf of Mexico 9 and many Scandinavian coastal systems 7 Paleo indicators foraminifera ratios and organic and inorganic compounds show that hypoxia had not been a naturally recurring event in these ecosys tems 10 8 The number of dead zones has ap proximately 1 J L J J 39 1 1960s g S1 and table S1 Hypoxia tends to be overlooked until higher level ecosystem effects are manifested For ex ample hypoxia did not become a prominent environmental issue in the Kattegat until several years after hypoxic bottom waters were rst re ported and sh mortality and the collapse of the Norway lobster shery attracted attention 1 Although hypoxia in the northern Gulf of Mexico has affected benthic communities over the past several decades there is no clear signal of hy poxia in sh landings statistics 9 Ecosystem level change is rarely the result of a single factor and several forms of stress typ ically act in concert to cause change The shal low northwest continental shelf of the Black w1 other stressors including over shing and the introduction of invasive species all of which led to drastic reductions in demersal sheries Nutrient inputs declined in the 1990s hypoxia disappeared and ecosystem services recovered however nutrient inputs are again rising as agri culture expands and a return to hypoxic condi tions may be imminent 12 The key to reducing dead zones will be to keep fertilizers on the lan and out of the sea For agricultural systems in general methods need to be developed that close the nutrient cycle from soil to crop and back to agricultural soil 13 Degrees of Hypoxia The most common form of eutrophication induced hypoxia responsible for about half the known dead zones generally occurs once per year in the summer after spring blooms when the water is warmest and strati cation is strongest and lasts until autumn table S1 The usual ecosystem re sponse to seasonal oxygen depletion is mortality of benthic organisms fol 39hvn a hatnth northenn Adriatic Pomeranian Bay and the Ger man Bight Paleoindicators and models from the of commas 39 MM 2 of nutrient enrichment is a positive force in en hancing an ecosystems secondary productivity and t an m 39 mwmht 2N n and seasonal hypoxia favor only benthic species quot quot 39 39 39 hnMHf mm e of organic matter that reaches the sediments there is a tendency for hypoxia to increase in 39 ce In systems prone to persistent epletion may also er 9 cm a 3 E e in the world as well as many ftordic systems Progression of Hypoxia Coastal hypoxia seems to follow a predictable and smaller body sizes 2 Ecosystem Responses The tfect of seasonal hypoxia on biomass and REVIEW I result ofhypoxia have profound effects on eco sys em energetics an function as or 39sm r r r r b tem models for the Neuse River estuary 23 Chesapeake Bay 24 and Kaoegat 25 all show hypoxia anced diversion ofenergy ows into microbial pathways to the detriment of higher trophic levels Fig 2 Only under certain circum s es wil denersal sh predators be able to con r r 2 9 hat is not i ho hy poxia affects the habitat requirements of dif 4 m1 ofO2lita than that ofthe benthic fauna Thus v sion when hypoxia makes deeper cooler water unavailable in the summer 15 or overlaps with osition of organic matter which in turn promotes greater demand for oxygen DO levels become 39 column statue In e sec ond phase hypoxia occurs transiently accompa nied by mass mortalities ofbenthic animals With t39 er buildup of nutrients and organic matter in the sediments a third phase is initiated and hypoxia becomes seasonal or periodic char a o poxic zone expands and as the c n o continues to fall anoxia is established an microbially generated H28 is released This type of threshold response has been cumented in the Gulf ofMexico 17 ChesapeakeBay s and Danish waters 18 0 E S m an ecosystem to eutrophication is the appearance 39 E v OHypoxic system Human 39loolprinl 04 Fig 1 Global distribution oi 4007plu5 systems that have scientifically n s of being eutrophi atlunsassucl reported accou t c distribution matches the global human iootprtnt wwwsciencemagcrg SCIENCE VOL321 ated de the n or example the spawning success ofcod inthecentral 39 39 39 r V V r r39 L I L 39 y con eggs 20 Similar habitat compression occurs when sulphide is generated S m insedl39mmt In is Case benthos die microbial pathways quickly dominate enegy flows Ecologically important places such as nursery and recniitment areas suffer most from p in summer when growth Missing Biomass discontinuity layer is compressed close to the se imentw er in face deeper dwelling spe cies including the ke L39 L h l pore water chemistry 21 are eliminated The resence ofFe and Mn in the sediment may buffer the system and reduce th 3913 of hypoxia have low annual secondary produc tion and typically no benthic fauna Estimates of now pesistently hypoxic are 464000 metric tons ofcaibon MT C annually and represent 40 of total Baltic secondary production 26 Simi poisonous H28 Reduced bioturbation associ larly estimates for the Chesap e indicate ated with hypoxia also alters selim tary hab that 10000 MT C is lost because of hypoxia 39 39 39 quot 39 quot 39 each year p t Bay stotal Hence under hypoxic conditions instead ofni secondary production 27 If we estimate that trogen being removed as N2 by denitr39 tcation 40 ofbenthic energy should be passed up the monium together with phos phorus are the main fluxes out of reduced sedi ments 8 39 39 production ad Zones Their urmallzed human the Southern Hemisphere the occurrence of dead being reported Details on each system are in tables 51 an 15 AUGUST 2008 foo Chesapeake Bay 26 when hypoxic conditions prevail 106000 MT C of potential food energy sheries is lost in the Baltic and 6000 MT C in influence is expressed as a percent 41 in the Northern Hemisphere Fur zones is only recently d 52 927 Downloaded from www50iencemagorg on May 6 2009 J REVIEW 928 1 00 Mild periodic E nergy to mobile predators E nergy lo Seve re microb Seasonal Normoxia Fig 2 w 39 r 39 39 nnrinrlir hvnn i energy o preda ors ihis vh39ndiall is typically shunrlived and does n Hypoxia Perslslem H S o Anoxia typicallyZS o 75 oi c c c quot o always occur VWth declining E oxygen higherrlevel preda ion is suspended ben hic preda ion may con inue and he proporlion of bequot anoxia develops red he Chesapeake Bay respectively In areas of he nne enernv w is slill processed by oleran benthus Microbes process all ben hic energy as hydrogen sulphide and a prurg or and predao39on 29 Thus he shor er he in erval 39 39 th hypoxia ben hic biomass is reduced by as much as 14 MT 1ka 9 39 n f efficiency his is c 39 l a 39 l 17000 MT 1 of los prey o demersal fisheries made up by he ecosy em during normal condi produc ion ou side he dead zone The la er seems o occur in he Baldc where second as a resul of eutmphication 26 bu if he dead zones were elimina ed he al ic would be manta was 0 quotI mass for about a third of the Communin maturin hypoxia e ura ion of seasonal hy Fig 3 p declines to lt07 ml oi Ozliter and exlends through time mas greases 39 energy in he food chain During persis en M a production and microbes remineralize vinually all organic matter Recovery I 4 we dented n of marine sys ems had become a maior world Norrnoxia Hypoxia fac or affecting ecosys em ener gyflows 39 39 o hypoxia 34 A5 D0 s niorlalily oi bolh equir wide environmenorl problem wi h only a small frac ion 4 of he 400 plus sys ems ha had developed hypoxia exhibicing any signs of im provemen able 5 These improvemen s in o ere rela ed o reductions in hree fac ors 39 39 soadficacion s reng lc 90 he hypoxic zone on he nor hwes em con inen al shelf of he Black Sea had expanded o 40000 kmz wever since 1989 malized ecosys em funccion improved and he ben hic fauna s ar ed o recolonize bu have no recovered o prehypoxic levels In he Gulf of in an 4 39 column s ra ifica ion occurred be ween 1987 and 1994 which improved DO condi ions and facili a ed re urn ofben hic fauna 7 however wi h e e urn of s ra ifica ion condi ions have again de eriora ed In he nonhern Gulf ofMexico he occurrence and ex en of he dead zone are igh ly coupled wi h freshwa er discharge from he Mississippi from us agricul ural ac ivi ies During years wi h low river ow he area ofh oxia shrinks o lt 1 on y o increase o gt15000 kml when river flow is high 30 The man b in pu s has vir ually eliminated dead zones from several sys ems including heHudson and Eas Rivers in he Uni ed s a es an e ersey and Thames Esmaries in England 31 32 However in o her sys ems such as he Chesapeake Bay he 39 39 ha n t imnv I DO Nevenheless he managemen of sewage d pulp mill e luents has led o mal e ev sals in any s hypoxia able The key fac e enn in he de ee of ecosys em and succession escribed in Anoxia as d he Pearson Rosenberg model ofsp I 5 Emma is reached the 53m S thathave strong seasonal cycles ben hos are eliniinaled The recovery pa h from severe hypoxia is diiierenl han he pa em ofspemes lossdurmg 0 increases in pop 131139 4 4 Wh eri anion onse ly u ons axe re lated to recruitment events timed en exposed o niild hypoxia niorlalily is nioderale and he recovery pa h is closer o he r r n quotall 4 15 AUGUST 2008 VOL 321 Wh en exposed to SCIENCE wwwsciencemagorg Downloaded from www50iencemagorg on May 6 2009


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