fl oor Global Carbon Cycling on a HeterogeneousSea Opinion

Opinion

Global Carbon Cycling on a Heterogeneous Seafloor

Paul V.R. Snelgrove,

* Karline Soetaert,

Martin Solan,

Simon Thrush,

Chih-Lin Wei,

Roberto Danovaro,

Robinson W. Fulweiler,

Hiroshi Kitazato,

Baban Ingole,

Alf Norkko,

R. John Parkes,

and Nils Volkenborn

Diverse biological communities mediate the transformation, transport, and storageofelementsfundamentaltolife onEarth,includingcarbon,nitrogen, and oxygen.However, global biogeochemical model outcomes can vary by orders of magnitude, compromising capacity to project realistic ecosystem responses to planetary changes, including ocean productivity and climate.

Here, we compare global carbon turnover rates estimated using models groundedinbiologicalversusgeochemicaltheoryandarguethattheturnover estimatesbasedoneachperspectiveyield divergentoutcomes.Importantly, empirical studies that include sedimentary biological activity vary less than thosethatignoreit.Improvingtherelevanceofmodelprojectionsandreducing uncertainty associated with the anticipated consequences ofglobal change requiresreconciliationoftheseperspectives,enablingbettersocietaldecisions onmitigationandadaptation.

WhereHasAlltheCarbonGone?

Rapidand well-documented environmentalchange over thepast century hasaccelerated interestinquantifyingthecriticalroleoftheoceaninglobalcarbonandnutrientcycling[1].As humanpressures[e.g.,climatechangeandbiodiversity(seeGlossary)loss]alterphysicaland biologicalprocesses,wemustimproveourcapacity topredictthe consequencesofthese alterationsandtheirlinkstoglobalcycles[2].Divergentthinkinginevaluatingglobalcycles[3,4]

andtheroleofbiodiversity[5]hasledmostmarinestudiestocompartmentalizebiogeochemical versusbiological approaches,withlittle effortto integrate alternativeperspectives [6]. The functioningofmostoftheglobalseafloordependslargelyupontheadditionofoxygenand organicmattertothesediment–waterinterface[7](Figure1).Biogeochemicalandecological approachesbothhavevalueinassessingtheseprocesses,butremainpoorlyreconciled[8],an issuealso noted in geological [9] and paleobiological [10] studies. Previousauthors have highlighted the need for all types of model to improve how they represent sedimentary processes[8,11,12].Here,weillustratehowdifferentbiasesand/orperspectivesassociated withdifferentworldviews(Figure1)canleadtobothdifferentmodelprojectionsanddifferential abilitiestointerrogatemodeloutcomestounderstandbetterthecumulativeeffectsofdriversof change.The nature of the questions a model is expected to inform should influence the complexityofthemodel.However,theapplicationofmodelstobroad-globalscaleprojections oftenrequiressimplificationandaveraging[13],whichcanlosekeycomplexityorheterogeneity [14]essentialindetectingallbutthecoarsestchange.Biogeochemicalmodelersfocusonthe physicalandchemical processes[15] thataffectmicrobialactivityinawayhighlysuitedto developingglobalmodels[16],whereasecologistsfocusondevelopingoverarchingthemes governing ecosystems by studying different groups of organisms and how their activities

Trends

Climate-change models hinge upon understandinghowlivingecosystems influence carboncycling, but global modelsofoceanicsystemsproduce carbonturnoverestimateswithahigh degreeofuncertainty.

Environmental conditions, and tem- perature inparticular, strongly influ- ence rates of carbon and nutrient cyclingintheglobalocean

Recent studies demonstrate a link between seafloor biodiversity and organicmatterprocessingandnutrient efflux,suggesting thatthe functional groupcompositionofbiotaisthemost criticalaspectofbiodiversityforeco- systemfunctioning inthe contextof globalbiogeochemicalcycles.

Strongspatialvariabilityincarbonbur- ialandrecyclingratesoforganicmate- rialmayrelatetorecognizedvariation in seafloor functional group composition.

1DepartmentofOceanSciencesand BiologyDepartment,Memorial UniversityofNewfoundland,StJohn’s NLA1C5S7,Canada

2EstuarineandDeltaSystems, NetherlandsInstituteofSeaResearch andUtrechtUniversity,Yerseke,The Netherlands

3OceanandEarthScience,National OceanographyCentreSouthampton, UniversityofSouthampton,Waterfront Campus,EuropeanWay,

Southampton,SO143ZH,UK

4InstituteofMarineScience,The UniversityofAuckland,Auckland, 1142,NewZealand

5InstituteofOceanography,National TaiwanUniversity,Taipei106,Taiwan

96 TrendsinEcology&Evolution,February2018,Vol.33,No.2 https://doi.org/10.1016/j.tree.2017.11.004

©2017TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/

4.0/).