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Article
<br /> a b Fig.3 1 Local land-sea human impacts and environmental factors that
<br /> 0.5 modified coral response to the 2015 marine heatwave.a,Historical
<br /> 29
<br /> N 04 (1986-2019)SSTsduringtheseasonalpeak(July-December)averagedacross
<br /> m the study region;2015 marine heatwave shown in red.b,Maximum DHW
<br /> 2
<br /> .,28 5 0.3 exposure in 2015,a common heat stress metric,among surveyed reefs.All
<br /> o reefs exceeded the eight DHW threshold expected to produce severe and
<br /> 0 0.2 widespread coral bleaching and mortality.c,Cora I cover before(2014-2015)
<br /> 27 d ° and one year following(2016)the marine heatwave among surveyed reefs
<br /> E a 0'1 (n=80,Fig.1b).The inset represents the distribution of absolute coral cover
<br /> ~ 0 change.d,The GAMM results(R2=0.79)showing key factors explaining coral
<br /> 26 9 10 11 12 13 14 response to the marine heatwave.Change accounts for starting condition,
<br /> DHW CC-weeks) defined as:percentage difference=((Aa.i-Ae,)/Abj)x100,whereAbandAaare
<br /> P5 d the mean coral cover values at each reef in 2014 or 2015,and 2016,respectively
<br /> Jul Aug Sep Oct Nov Dec 25
<br /> (Methods and Supplementary Fig.4).Positive and negative relationships
<br /> e`bRI fig^^^egtiootiootiootiotiootio^tio^tio^tio^tio^g o reduce or increase coral loss,respectively.Shaded regions represent 80%
<br /> confidence intervals.Factors with the strongest model averaged slopes are
<br /> Year
<br /> C 65 -25 shown.Total fish biomass and scraper biomass were also important factors in
<br /> v0.3 ou r mod e Is but had we a k s lopes(re p rese nt i ng less tha n 5%c ha nge;Exte nd ed
<br /> -50 Data Fig.4).Relative importance of factors among all models(that is,sum of
<br /> 60 C 02 Al Cc mode I we ig hts a cross a I I mode I s conta i ni ng e ach fa ctor)we re:sed i me nt
<br /> 0
<br /> o -75 -� i np ut(0.99),scra pe r bi omass(0.99),tota I fi sh bi omass(0.90),urba n r u noff
<br /> 55 0 0.1 1/�y (0.60),phytopl a n kton b ioma ss(0.38),wa stewate r pol l ut ion(0.28),peak
<br /> 2 t
<br /> 0.09 0.12 0.16 0.20 rainfall(0.20),nut rient load ing(0.19),grazerbiomass(0.16),DHW(0.08),wave
<br /> SOIL ho ao�o do.�o o 10 phytoplankton(mg m") power(0.07),depth(0.06)and fishinggear restrictions(0.05).See Extended
<br /> Coral cover change(/) 25 Data Table 1 for full listof factors included in the analysis,including those
<br /> 451 removed that were highIycorrelated(r>0.7,see Methods and Supplementary
<br /> H �
<br /> ;�►; w 0 Fig.5).See Supplementary Fig.6 for predictor variable distributions.
<br /> psi
<br /> 40,11 o
<br /> m-25
<br /> m 35 t
<br /> g -50 Coral bleaching involves the breakdown of the mutualistic rela-
<br /> U >
<br /> 0 tionship between the coral animal and its algal endosymbionts36
<br /> 6 30 U-75
<br /> I o ® A prolonged breakdown in this relationship often results in coral star-
<br /> 25,
<br /> " vation and death,as much of the energetic demands of corals are met
<br /> 2,500 10,000 22,500 by the photosynthetic activity of its endosymbionts36.We found that
<br /> Urban runoff(m2 her-') reefs with the highest levels of water column h to lankton biomass
<br /> 20 25 g P Y P
<br /> (that is,chlorophyll-a)during the marine heatwave showed reduced
<br /> 15' o coral mortality(Fig.3d).Productivity increases nearshore to tropical
<br /> islands such as Hawai'i37 and is further concentrated by small-scale
<br /> 10 -25 ocean processes that attract dense aggregations of plankton38.The
<br /> '
<br /> increase in nutritional subsidies to the coral animal may have helped
<br /> 5
<br /> -50 to reduce coral starvation during the heatwave or provided higher
<br /> ;
<br /> "_", 75 energetic reserves that promoted their recovery39.In other regions(for
<br /> 0 ` example,Great Barrier Reef),high levels of chlorophyll-a are anindica-
<br /> (2014i2015) (2016) for of poor water quality that drives negative outcomes for corals"
<br /> Before madne After marine 0 39 625 3,164 10,000
<br /> heatwave heatwave Sediment input(kg ha 1) Here,chlorophyll-a was uncorrelated to land-based human impacts
<br /> (Supplementary Fig.6)and probably reflective of natural gradients
<br /> in energetic subsidies that facilitated coral survival.Working towards
<br /> peaked at29.4°C(Fig.3a).Degree heating weeks(DHWs),a widely used locally relevant management strategies requires understanding how
<br /> heat stress metric for coral reefs,averaged 12 DHWs among surveyed human impacts superimpose on natural biophysical drivers,such as
<br /> reefs(Fig.3b),far exceeding the eight DHW threshold expected to phytoplankton biomass24,to influence reef ecosystem response to
<br /> cause severe and widespread coral bleachingandmortality35.Reef sur- acute disturbance.
<br /> veys performed one year following the marine heatwave showed that Coastal runoff can deliver a broad spectrum of land-based contami-
<br /> nearly one-quarter of reefs(19outof80)lost morethan20%coral cover nantsthatdegradenearshore water quality,with cascading effects on
<br /> whereas the hardest-hit reef lost 49%(Fig.3c).But not all reefs expe- coral health41.We found that reefs exposed to the lowest levels ofurban
<br /> rienced such catastrophic change.Coral cover remained unchanged runoff,and to a lesser extent sediment input,experienced a modest
<br /> or increased on 18%(14 out of 80)of reefs surveyed.This divergent reduction in coral mortalityfrom the marine heatwave(Fig.3d).Urban
<br /> ecological response was unexpectedgiven that all reefs were exposed runoff often contains heavy metals and petrochemicals that cause
<br /> to similarly extreme levels of heat stress(Fig.3b). coral tissue death42 and sediment input can impede the photosyn-
<br /> Interactions between heat stressand local conditions such asa high thetic capacity of corals and reducegrowthbyburyingcoralcolonies41.
<br /> abundance of competitive macroalgaecan exacerbatecoral bleaching Together,these stressors can underminethe natural defence abilities
<br /> and mortality22.However,we lack a detailed understanding of the land- of corals and increase the likelihood of mortality from heatstress40.
<br /> and sea-based factorsthat mediatecoral responseto marine heatwaves. Although turbid waters may shade corals from excessive sunlightthat
<br /> Using a generalized additive mixed-modelling framework,we identi- can exacerbate coral bleaching,high levels of heat stress can override
<br /> fied the land-sea factors that best explained variations in coral cover any protective benefits decreased light may provide43.Existing but
<br /> change(accountingfor starting cover)among reefs one year after the underused local and national policies such as the Clean Water Act in
<br /> 2015 marine heatwave in Hawaii(Fig.3d and Extended Data Table 2). the United States provide actionable pathwaysfor marine management
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