growth and mortality of coral transplants (pocillopora ...growth and mortality of coral transplants...

39

Growth and Mortality of Coral Transplants (Pocillopora damicornis)along a Range of Sediment Influence in Maui, Hawai‘i1

Gregory A. Piniak2,3,4 and Eric K. Brown5,6

Abstract: Fragments of the lace coral Pocillopora damicornis (Linnaeus, 1758)were transplanted to four sites on the south-central coast of Maui, Hawai‘i,to examine coral growth over a range of expected sediment influence. Coralsremained in situ for 11 months and were recovered seasonally for growth mea-surements using the buoyant weight technique. Average sediment trap accumu-lation rates ranged from 11 to 490 mg cm�2 day�1 and were greater at the wave-exposed reef site than at the protected harbor sites. Coral growth was highest atthe donor site and was higher in the summer than in the winter. A stepwise lin-ear regression found significant effects of sediment trap accumulation and lighton growth rates, but the partial correlation coefficients suggest that these factorsmay be only secondary controls on growth. This study did not show a clear linkbetween coral growth and sediment load. This result may be due, in part, to co-variation of sediment load with wave exposure and the inability of trap accumu-lation rates to integrate all sediment effects (e.g., turbidity) that can affect coralgrowth.

Twenty-two percent of the world’s coralreefs are threatened by sediment and land-based pollution (Bryant et al. 1998). InHawai‘i, the total sediment runoff fromagricultural, industrial, ranching, and urbansources exceeds 1 million tons per year(Gulko et al. 2000). With so much materialmoving through the system, coral reefs in

Hawai‘i can be vulnerable to sediment stress.High wave energy might be more importantthan anthropogenic factors in structuringcoral reef communities on exposed coastalreefs in some parts of Hawai‘i (Grigg 1983),but sedimentation could pose a particularthreat in populated areas such as Maui andO‘ahu (Gulko et al. 2000), in protectedembayments (Dollar and Grigg 2004), orin areas where land use patterns contributeto erosion. Some coral communities arenaturally exposed to considerable sedimenttransport and deposition, but anthropogenicactivities can alter the amount, timing, andtype (terrigenous versus carbonate) of sedi-ment delivered to the system.

One common approach to test the impactof water quality on coral health has been tomeasure the growth rate of coral transplants(Davies 1990). In recent years this techniquehas been employed to study the effects ofsewage (McKenna et al. 2001), aquaculturewaste (Bongiorni et al. 2003a,b), sedimenta-tion (Te 2001), and thermal stress (Smithand Birkeland 2003). Resource managersoften need to predict the impact of a stress,even when pre-stress baseline data are not

Pacific Science (2008), vol. 62, no. 1:39–55: 2008 by University of Hawai‘i PressAll rights reserved

1 This study was funded by a U.S. Geological SurveyMendenhall Postdoctoral Research Fellowship to G.A.P.Manuscript accepted 1 March 2007.

2 Corresponding author.3 U.S. Geological Survey Pacific Science Center, 400

Natural Bridges Drive, Santa Cruz, California 95060.4 Current contact information: NOAA Center for

Coastal Fisheries and Habitat Research, 101 Pivers Is-land Road, Beaufort, North Carolina 28516 (e-mail:[email protected]).

5 Hawai‘i Institute of Marine Biology, P.O. Box 1346,Kane‘ohe, Hawai‘i 96744.

6 Current contact information: Kalaupapa NationalHistoric Park, P.O. Box 2222, Kalaupapa, Hawai‘i 96742(e-mail: [email protected]).

available; comparing growth of coral trans-plants at impacted and control sites can pro-vide a sublethal stress indicator. Growth canbe measured in numerous ways (see reviewby Buddemeier and Kinzie 1976). A commongrowth index is linear extension, but the mostcommon methods in the literature (alizarinstaining, x-rays) require destructive sam-pling. Buoyant weight ( Jokiel et al. 1978) isanother common technique and has the ad-vantage of being noninvasive and allowing re-peated measurements of the same colonyover time.

This study was part of a U.S. GeologicalSurvey project on the geologic and oceano-graphic processes that affect coral reefsystems in Hawai‘i (Ogston et al. 2004, Stor-lazzi et al. 2004, 2005, 2006, Bothner et al.2006, Presto et al. 2006, Stamski and Field2006). The objective of this study was to de-scribe the effects of sediment on the growthof scleractinian corals in situ. Fragments ofthe lace coral Pocillopora damicornis Linnaeus,1758, were transplanted to four sites for a pe-riod of 11 months. Transplant sites includedan exposed carbonate beach and a protectedembayment with terrestrial sediment input.Growth rates were measured seasonally usingthe buoyant weight technique. The specifichypothesis tested was that coral growth woulddecrease at higher sediment loads.

materials and methods

Pocillopora damicornis was used for this studybecause its branching morphology lends itselfwell to fragmentation and transplantation, itis present at all of the study sites, and muchis known about its energetics and responsesto environmental stress. Using clones for atransplant study minimizes genetic variabilityand increases confidence in attributing differ-ences in growth to environmental variables.However, the P. damicornis colonies at thestudy sites were too small (10–15 cm diame-ter) to furnish a sufficient number of clonalfragments from a single colony. Instead a sin-gle donor site was selected on the assumptionthat corals from a single donor site that are inclose proximity are genotypically similar (e.g.,Hunter 1993).

Study Sites

Four transplant sites (depth 2 to 2.5 m) wereestablished with a range of sediment loadingand wave exposure along the south-centralcoast of Maui, Hawai‘i (Figure 1, Table 1).Three sites were located in Ma‘alaea Harbor.Surface sediment on the harbor bottomranges from fine mud at the west end of theharbor to coarser carbonate sediment nearthe harbor entrance. The East mole site wasestablished near the harbor entrance wherecoral cover is the highest (30–40% [ Jokieland Brown 1998]). The Boat dock site wasnear a drainage culvert at the west end of theharbor. The Triangle reef site was in themiddle of the harbor, where a remnant reefgives way to small rocky outcrops within asandy plain. The Ma‘alaea sites are protectedfrom the prevailing southerly swell by twobreakwaters built in the 1950s ( Jokiel andBrown 1998). In contrast, the site just off-shore of Sugar Beach was a low-relief reefpavement with sparse coral cover. Surfacesediment was largely marine carbonate, andthe site was subject to heavy seasonal southswell. Pocillopora damicornis was the local dom-inant coral at the Triangle, which served asthe donor site for the study. Pocillopora dami-cornis was also present in low abundance atthe Boat dock and was rare at the East molesite and Sugar Beach.

Coral Transplants

In August 2003, P. damicornis colonies werecollected from Triangle reef and brought tothe laboratory at the Maui Ocean Center. In-dividual branches (3–4 cm tall) were trimmedfrom the colonies, attached to a polyvinylchloride (PVC) mount using epoxy (SplashZone) (average weight of mount plus epoxy2.985 g), and randomly labeled with color-coded numbered tags (FTF-69, Floy Tag,Inc.). Transplants were weighed to the near-est 0.001 g on a balance (Sartorius CP-153)using the buoyant weight method ( Jokielet al. 1978). After a 2-day recovery period inan indoor, flow-through seawater system(27�C, 36–38 ppt, 25 mE m�2 sec�1), thetransplants were deployed at the four experi-

40 PACIFIC SCIENCE . January 2008

mental sites. At each site, three cinder blocks(Figure 2) were placed 2–3 m apart, in sandyareas between coral heads. Each cinder block(19.4 by 19.4 by 39.7 cm) had two parallelPVC racks of four corals each (total n ¼ 24

corals per site). A sediment trap (see follow-ing section) was attached to each cinderblock. The cinder blocks were left uncagedto minimize hydrodynamic artifacts andchanges in light regime due to biofouling.

Figure 1. Location of coral transplant sites in Maui.

TABLE 1

Coral Transplant Sites on Maui, Hawai‘i

Site Latitude Longitude Depth (m) Wave Exposure

Sugar Beach 20� 47.222 0 N 156� 28.250 0 W 2 OpenMa‘alaea Boat dock 20� 47.460 0 N 156� 30.800 0 W 2 ProtectedMa‘alaea Triangle 20� 47.467 0 N 156� 30.688 0 W 2 ProtectedMa‘alaea East mole 20� 47.433 0 N 156� 30.607 0 W 2 ProtectedMa‘alaea East mole 2 20� 47.488 0 N 156� 30.599 0 W 2.5 Protected

Note: The Ma‘alaea Triangle was the donor site for all transplants. In February 2004 the location of the East mole site was changeddue to fish predation.

Maui Coral Transplants . Piniak and Brown 41

Corals were retrieved from the field every 3–4 months. In the laboratory, colony conditionwas noted (healthy ¼ 100% live tissue; partialmortality ¼ live tissue but less than 100%;dead ¼ no live tissue). Biofouling organismson the transplant platform were removed,and corals were weighed using the buoyantweight technique. Coral growth rates werereported as daily mass change (mg days�1)and percentage growth (% day�1, correctedfor the weight of the PVC stand and epoxy).Only corals with live tissue (i.e., the healthyand partial mortality classes) were used forgrowth rate analyses.

After weighing, corals were returned totheir specific transplant sites. When trans-plants suffered high mortality at a given site(storms, fish predation, etc.), all corals for

that site were replaced by new P. damicornistransplants from the donor site. The Ma‘alaeaEast mole site was relocated farther inside theharbor in February 2004 in an effort to avoidhigh mortality from fish predation at theoriginal site.

Physical Environment

The relative amount of sediment accumula-tion at each site was measured using sedimentaccumulation tubes. The tubes consisted ofPVC pipe 5.1 cm diameter by 15.2 cm height,with an endcap on the bottom (E.K.B., 1999,unpubl. data). A tube was tied with cableto each cinder block so that its opening was1–2 cm above the top of the cinder block(Figure 2). Traps did not include baffles or

Figure 2. Coral transplants at the Triangle reef donor site in Ma‘alaea Harbor. Each cinder block is 39.7 by 19.4 by19.4 cm.


other preventive measures to mitigate biotur-bation. All sediment accumulation tubes werechanged approximately monthly. The con-tents of the traps were rinsed with freshwater,filtered, and dried to a constant weight(mg cm�2 day�1). Representative sedimentsamples were analyzed for grain size and per-centage carbonate using methods standard-ized by the Western Coastal and MarineGeology team of the U.S. Geological Survey(modified from Carver 1971 and Folk 1974).Sediment particles 63–2,000 mm were ana-lyzed using 2-m settling tubes (modifiedfrom Theide et al. [1976]). Carbon and car-bonate analyses were conducted with UICCoulometrics systems CM 150 and CM5200.

Water temperature at each site was mea-sured at hourly intervals using a data logger(Onset HOBO Water Temp Pro). Lightlevels on the seafloor were measured hourlywith a light intensity data logger (HOBOStowAway) in a clear underwater housing.Four deployments were made that corre-sponded to each growth period. Light in-tensity plots within each 75-day samplinginterval (growth period) showed a decline indaily maximum intensity after 30–40 days,which was attributed to biofouling on thehousing. Consequently, only daily average ir-radiance for the first 30 days was used foreach of the four growth periods. Variation inseasonal irradiance was simplified using astandardized day length: December to Febru-ary, 11 hours; March to May and Septemberto November, 12 hours; June to August, 13hours. Average daily irradiance was estimatedas the average hourly rate times the numberof daylight hours (Anthony and Connolly2004). Logger data were converted fromlog(lumens m�2) to photosynthetically activeradiation (PAR) (mm quanta m�2 sec�1) bycalibrating the HOBO loggers against anunderwater light sensor (LiCor Li192SA)(equation PAR ¼ 3:4136 � e1:4898�intensity).

Statistical Analysis

Statistical analysis was conducted using Statis-tica 6.1 (StatSoft 2001). Data were testedfor equal variances using Levene’s test, and

normality assumptions were tested using aKolmogorov-Smirnoff test for goodness offit (Sokal and Rohlf 1995). Sediment trapaccumulation rates, light meter data, and cer-tain subsets of the growth data met paramet-ric assumptions after log transformation. Allother data that failed to meet assumptionsafter transformation were analyzed usingnonparametric statistics. Effects of time andlocation on physical data (sediment, light,temperature), growth, and mortality weretested with analyses of variance (ANOVAs)or nonparametric equivalents, using time andlocation as categorical factors. Effects ofphysical data on growth and mortality weretested using similar analyses.

results

Sediment Trap Data

Average accumulation rates for the cinderblock sediment traps at all sites ranged from11 to 490 mg cm�2 day�1 (Figure 3). Becausea two-way analysis was unbalanced (due toloss of traps, etc.), the effects of time andsite were clearer when tested separately.Trap accumulation rates differed significantlyamong sites (Figure 3; F ¼ 21:8; df ¼ 3; 94;P < :001). Sugar Beach had higher sedimentaccumulation rates than the Ma‘alaea Harborsites (Tukey HSD post hoc comparisons,P < :001). The Boat dock had the highesttrap accumulation rates in the harbor (Eastmole, P ¼ :006; Triangle, P ¼ :033). Sedi-ment accumulation rates varied significantlyover time at all four sites, but effects werenot consistent from site to site. The weakesteffects were at Sugar Beach (F ¼ 3:3; df ¼7; 13; P ¼ :031) and at the Boat dock(F ¼ 3:4; df ¼ 8; 16; P ¼ :014). However,Tukey HSD post hoc tests found no signifi-cant differences over time at either site. Tem-poral effects were stronger at the East mole(F ¼ 74:6; df ¼ 8; 16; P < :001) and Triangle(F ¼ 122:1; df ¼ 8, 16; P < :001) sites, wheresediment accumulation was highest in Febru-ary, March, June, and July (Figure 3).

A representative subset of the sedimentsamples was analyzed for grain size and com-position (Table 2). Sediment at the East mole


Figure 3. Sediment trap data for the study sites. Deployment periods were September (26 August to 28 September 2003), October (28 September to 18 Octo-ber 2003), December (18 October to 7 December 2003), February A (7 December 2003 to 2 February 2004), February B (2 February to 22 February 2004),March (22 February to 20 March 2004), April (20 March to 18 April 2004), June (18 April to 10 June 2004), and July (10 June to 18 July 2004). No data exist inFebruary A for Sugar Beach. For each panel, lower-case letters indicate groupings that are significantly different as determined by Tukey HSD post hoc com-parisons (P < :05). Error bars are standard deviation.

and Triangle was predominantly carbonatesand and gravel. In contrast, 90% of the sed-iment at the Boat dock was silt and clay thatwas primarily terrigenous in origin (only@22% carbonates, less than half the contentof the other two sites). Mean grain size wassmallest at the Boat dock, and sediments atall three sites were poorly or very poorlysorted. There was no correlation betweenmean grain size and sediment trap accumula-tion rate (r ¼ 0:07, P ¼ :802).

Temperature and Light

Light levels and water temperatures were re-ported only for the Ma‘alaea Harbor sites be-cause meters at Sugar Beach were disturbedduring strong wave events. Light (Figure 4)varied significantly with both site (F ¼ 240:8;df ¼ 2; 351; P < :001) and time (F ¼ 4:5,df ¼ 3; 350; P ¼ :004). Average light intensitywas higher at the East mole than at the Tri-angle (Dunnet post hoc test, P ¼ :005), whichin turn was higher than at the Boat dock(P < :001). Winter light levels (February toApril) were lower than those from Augustto October (P ¼ :004). There was also asignificant but weak negative correlation be-

tween sediment trap data and light levels( both data sets log transformed, r ¼ �0:33,P < :001).

Average daily water temperatures inMa‘alaea Harbor ranged from 23.8 to 27.7�C(Figure 5A; no data were available for theBoat dock and East mole sites for April–Julydue to errors with the temperature loggers).Temperatures differed seasonally (Kruskal-Wallis, df ¼ 3, N ¼ 764, H ¼ 337:0, P <:001); water was significantly warmer inAugust–October and significantly colder inFebruary–April (multiple comparison ofmean ranks, P < :001). Temperature was notsignificantly different between sites (Kruskal-Wallis, df ¼ 2, N ¼ 764, H ¼ 4:3, P ¼ :117).Diel variability in water temperature was ap-proximated by calculating the variance in thehourly temperatures used for each daily tem-perature average (Figure 5B). Variance wasgreatest near the harbor mouth and lowestat the Boat dock (Kruskal-Wallis, df ¼ 2,N ¼ 764, H ¼ 145:0, P < :001). There wasalso a significant seasonal effect on tempera-ture variance (Kruskal-Wallis, df ¼ 3, N ¼764, H ¼ 65:6, P < :001), which was highestduring the August to October period andlowest from February to April.

TABLE 2

Composition of Selected Sediment Samples in Ma‘alaea Harbor

Parameter East mole Boat dock Triangle

September 2003% Gravel (>2 mm) 4.9G 4.2 0.0G 0.0 0.0G 0.0% Sand (0.62–2 mm) 48.8G 13.5 7.5G 2.0 62.8G 8.3% Silt (0.04–0.62 mm) 41.9G 1.6 86.0G 1.2 33.6G 7.5% Clay (<0.04 mm) 4.4G 1.6 6.5G 1.2 3.7G 0.8Mean grain size (j) 0.13G 0.3 0.02G 0.01 0.10G 0.02Sorting (j) 3.25G 0.63 1.82G 0.05 2.54G 0.35% Carbonate 53.0G 8.7 21.7G 0.1 50.4G 6.8

February 2004% Gravel (>2 mm) 1.6G 1.4 1.2G 1.5 0.0G 0.0% Sand (0.62–2 mm) 62.8G 5.3 8.9G 4.6 85.7G 4.6% Silt (0.04–0.62 mm) 30.7G 4.3 82.1G 4.3 12.8G 4.1% Clay (<0.04 mm) 5.0G 3.2 7.8G 1.9 1.5G 0.5Mean grain size (j) 0.10G 0.40 0.02G 0.01 0.14G 0.01Sorting (j) 2.24G 0.72 1.93G 0.08 1.30G 0.09% Carbonate 49.3G 3.7 22.5G 0.7 66.5G 0.7

Note: Data are postacidified percentage composition for each size fraction, and carbonate composition is for the bulk sample. Dataare averagesG SD for the three cinder blocks at each site.


Coral Growth and Mortality

Growth rates were analyzed for corals withlive tissue only, in units of overall massgained (Figure 6A) and change relative tothe overall mass of the transplant (e.g., per-cent change, Figure 6B). Negative growthrates were possible; the most commonlyobserved case was tissue regrowth after aloss of skeletal mass ( breakage, predation,etc.). Growth rates were significantly dif-ferent among sites (Kruskal-Wallis, df ¼ 3,N ¼ 240; overall mass H ¼ 153:3, P < :001;percentage change H ¼ 1;452:7, P < :001).When daily mass changes were weighted forthe length of each growing ‘‘season,’’ the Tri-angle had the highest average growth (52.0mg day�1 or 3.1% day�1). The Boat dockaveraged a slight positive growth over thecourse of the year (3.1 mg day�1 or 0.3%day�1), and the East mole site lost weight onaverage (�16.5 mg day�1 or �1.1% day�1).However growth was not constant over thecourse of the year (Figure 6). Percentchange did not vary seasonally (Kruskal-

Wallis, df ¼ 3, N ¼ 240, H ¼ 6:7, P ¼ :082),but there was a significant effect of season onoverall mass gained (H ¼ 11:7, P ¼ :008).When sites were tested individually, seasonalgrowth patterns were not consistent amongsites. For example, August–October showedthe lowest growth at the Boat dock (posthoc multiple comparison of mean ranks,P < :002), but February–April showed thelowest growth at the Triangle (P < :002).

For all sites combined, there was a slightpositive correlation between growth ratesand sediment trap accumulation (r ¼ 0:14,P ¼ :03). Light and temperature data werenot collected at the Sugar Beach site, andthe East mole had poor survivorship (see fol-lowing paragraph). Therefore, effects of envi-ronmental parameters on growth rates weretested using the Boat dock and Triangle sites.A stepwise linear regression for growth ratesat these two sites (F ¼ 60:7; df ¼ 3; 151;P < :001) produced significant but weak re-gression coefficients for log photosyntheti-cally active radiation (b ¼ 0:772, P < :001,R2 ¼ 0:110) and log sediment trap accumula-

Figure 4. Average hourly daytime light levels at the study sites in Ma‘alaea Harbor. PAR, photosynthetically activeradiation.


Figure 5. Average daily temperature (A) and temperature variance (B) at the study sites in Ma‘alaea Harbor.

Figure 6. Daily growth rates of Pocillopora damicornis transplants at the study sites, in terms of overall mass gain (A)and percent of colony mass (B).

tion (b ¼ 0:161, P ¼ :023, R2 ¼ 0:387). Theregression coefficient for water temperaturewas not significant (b ¼ 0:126, P ¼ :062).

During the experiment high partial mor-tality and transplant death was observed atSugar Beach and the East mole sites (Table3). Mortality rates were tested statistically bydetermining the combined mortality (deathplus partial mortality) at each sampling inter-val for each cinder block at each site. A non-parametric two-way ANOVA (Scheirer-Ray-Hare extension of Kruskal-Wallis) showedthat mortality varied among sites (df ¼ 3,H ¼ 10:1, :025 > P > :01), but season andsite-season interactions were not significant.An analysis of covariance (ANCOVA) wasused to test the possible contribution of sedi-ment trap accumulation rates to mortality.The ANCOVA used arcsin–square roottransformed combined mortality as the re-sponse variable, with site as the categoricalpredictor and log-transformed sediment trapaccumulation as the continuous predictor.Site effects were significant (F ¼ 25:2; df ¼3; 43; P < :001), but trap accumulation rateswere not (F ¼ 0:1; df ¼ 1; 43; P ¼ :79). Mor-tality at the Sugar Beach and the East molesites commonly occurred from confoundingsources (predation, storm damage). However,repeating the ANCOVA using only the Boat

dock and Triangle sites gave similar results(site F ¼ 28:7; df ¼ 1; 21; P < :001; sedimentF ¼ 0:240; df ¼ 1; 21; P ¼ :63).

discussion

Study sites were selected to representthe range of sediment influence to which Po-cillopora damicornis is exposed in Maui’s near-shore environments. Sediment loads weredetermined using sediment trap accumulationrates, a technique that has been criticized forits limited temporal resolution (Thomas andRidd 2005). Nevertheless, it is appropriatehere because seasonal coral growth rates inte-grate effects over a greater time scale than thesediment sampling interval (@monthly). Sed-iment traps have well-defined hydrodynamiclimitations and cannot capture all potentialsediment dynamics relevant to corals (e.g.,turbidity, abrasion, burial). Trap efficiencyincreases with the aspect ratio (height :diameter) of the traps, and Gardner (1980)reported maximum collection efficiency inflows <15 cm sec�1 for traps with an aspectratio of 2–3. The aspect ratio for traps inthis study was @3—this is appropriate forthe low to moderate flow environments inMa‘alaea Harbor, but sediment resuspensionby wave action at Sugar Beach could affect

TABLE 3

Percentage Survivorship and Mortality for Coral Transplants in South-Central Maui

Site Season % Healthy % Partial Mortality % Dead Mortality Source

Boat dock Aug.–Oct. 87.5 8.3 4.2 BurialOct.–Feb. 37.8 29.2 33.0 BurialFeb.–Apr. 41.7 58.3 0.0 BurialApr.–July 12.5 45.8 41.7 Burial

Triangle Aug.–Oct. 100.0 0.0 0.0Oct.–Feb. 100.0 0.0 0.0Feb.–Apr. 100.0 0.0 0.0Apr.–July 87.5 0.0 12.5 Burial

East mole Aug.–Oct. 0.0 29.2 70.8 FishOct.–Feb. 0.0 50.0 50.0 Fish, algaeFeb.–Apr. 0.0 45.8 54.2 Fish, algaeApr.–July 0.0 16.7 83.3 Fish, algae

Sugar Beach Aug.–Oct. 100.0 0.0 0.0Oct.–Feb. 0.0 0.0 100.0 StormsFeb.–Apr. 0.0 37.5 62.5 StormsApr.–July 37.5 25.0 37.5 Storms


collection rates. Given these caveats, trap ac-cumulation rates given here should be inter-preted as the relative rather than absolutesediment impact at a site.

Sediment trap accumulation rates in south-central Maui showed substantial variability,with average accumulation rates ranging from11 to 490 mg cm�2 day�1. This is compa-rable with other sediment trap data for themain Hawaiian Islands. Along the WestMaui coastline, sediment accumulation ratesrange from 6 to 510 mg cm�2 day�1

(E.K.B., unpubl. data). Sediment traps on theMoloka‘i forereef collected an average of50 mg cm�2 day�1 but reached up to 1,800mg cm�2 day�1 during storm events (Bothneret al. 2006). Sediment trap rates were 1 to12 mg cm�2 day�1 in Kane‘ohe Bay in O‘ahuand 2 to 250 mg cm�2 day�1 at morewave-exposed sites at Kawaihae, Hawai‘i (Te2001). Higher water motion has been shownto be correlated with trap collection ratesdue to resuspension of the sediment in thewater column (Kozerski 1994). Higher watermotion appears to partially explain the vari-able accumulation rates observed in thisstudy, because sediment traps at wave-exposed Sugar Beach collected significantlymore sediment than traps within protectedMa‘alaea Harbor.

Sediment accumulating at the Boat dockwas mostly terrigenous silt and clay with lowcarbonate content (Table 2); fine sedimentaccumulates at the Boat dock but is trans-ported out of the system by the higher waveenergy at the Triangle and East mole sites(which had predominantly coarser marinecarbonate sediment). The Boat dock may alsoreceive higher terrestrial input, because it isadjacent to two of the three drainage pipesthat enter Ma‘alaea Harbor. However, severerainfall events were not apparent in the Boatdock sediment traps but instead were corre-lated with sediment pulses at the Triangleand East mole sites observed in subsequenttrap collections. Dry periods did not decreasesediment accumulation rates at the Boat dockor any other site.

This study considered the relationshipbetween growth and three physical factors:sediment trap accumulation, light level, and

water temperature. Growth rates in Ma‘alaeaHarbor were highest in the summer (Figure6A), when both light (Figure 4) and tempera-ture (Figure 5A) were at their maximum. Al-though these factors covary, they both havepositive effects on growth. For example, lightenhances calcification in P. damicornis (Rothet al. 1982). Skeletal growth in P. damicornisin Hawai‘i is greatest at about 26–27�C( Jokiel and Coles 1977), and average watertemperatures in Ma‘alaea Harbor were withinthat range from April to October (Figure5A). The lack of spatial variability in watertemperature within the harbor suggested thatthe relationship between temperature andgrowth was consistent among all harbor sites(Figure 5A) and why temperature was notsignificant in the stepwise regression for coralgrowth.

Distinguishing between the effects of lightand sediment on coral growth in situ is com-plicated, because the two factors can covaryvia turbidity. In this study, photosyntheticallyactive radiation was the first factor added inthe forward stepwise regression, whereas sed-iment explained a greater proportion of thevariance in P. damicornis growth. Both effectswere positive and significant but relativelyweak. Turbidity was not directly measuredbut can be inferred from the photosyntheti-cally active radiation and sediment trap data.The Boat dock and Triangle are close to-gether (@200 m). Light at the water surfacewas likely to be similar, yet there were con-siderable differences in light at the seafloordespite similar depths. Trap accumulationrates were not appreciably different for thetwo sites (yearly average 19.1 mg cm�2 day�1

for Triangle reef, 17.9 mg cm�2 day�1 for theBoat dock), but sediment size at the Boatdock is appreciably smaller (Table 2) alongwith lower light levels (Figure 4). Theseresults suggest that higher turbidity levelsexisted at the Boat dock due to suspensionof the finer-grained sediments, and the con-sequent reduction in photosynthetically activeradiation could have accounted for the lowergrowth rates observed at that site.

It is also possible that reduction in lightand increase in turbidity combined to shiftP. damicornis energetics. Sediment load de-


creases coral photosynthesis while increasingrespiratory losses and mucus production(Riegl and Branch 1995), and tissue could belost due to abrasion. These processes all re-duce the amount of energy available for coralgrowth. However, some corals can also derivean energetic benefit from sediment (Rose-nfeld et al. 1999, Mills et al. 2004). P. dami-cornis has been shown to feed readily onsediment (Anthony 1999) and may be able tocompensate for low photosynthesis : respi-ration ratios at elevated turbidity by increas-ing heterotrophic consumption (Anthony2000). These effects should be highest forthe muddy sediment at the Boat dock, be-cause fine sediments are likely to be both lessabrasive and more nutritious (Anthony andLarcombe 2002). Any possible energetic bene-fits of sediment were likely insufficient to off-set the low light at the Boat dock, becausegrowth and survivorship were much lowerthan at the Triangle reef site.

The predominant view in the literature isthat sediment negatively impacts coral reefs.This is likely to be the general case, and terri-genous sediment runoff has been implicatedin coral damage observed in Honolua Bay(Dollar and Grigg 2004) and on the southcoast of Moloka‘i ( Jokiel et al. 2004). Theseresults appear to be inconsistent with ourstudy, in which we did not observe a clearnegative effect of sediment on growth of P.damicornis. However, the results of our studyare consistent with other examples fromHawai‘i in which chronic sediment stress hadlittle effect on coral physiology. Turbidityof up to 1.0 nephelometric turbidity unit(NTU) was significantly and positively cor-related with growth of Montipora verrucosaand P. damicornis in Kane‘ohe Bay (Colesand Ruddy 1995) (P. damicornis growth wasalso significantly and positively correlatedwith sediment trap rates). A later study inthe same area found no relationship betweensediment trap accumulation and either coralgrowth or mortality for Porites compressa andM. verrucosa (Te 2001), and Maragos (1972)found that M. verrucosa growth was actuallyhighest at intermediate levels of turbidityand light extinction. It may be that chronic,adverse effects of sediment and turbidity

on corals are more likely to operate at thecommunity or population level instead ofthe physiological level (Anthony 2006). Sedi-ment clearly causes population-level mortality(Nugues and Roberts 2003), reduces diversity(Brown et al. 1990), and inhibits coral re-cruitment (Gilmour 1999), yet some coralscan physiologically adapt to sediment stress.Corals in highly turbid environments canmaintain elevated metabolic indices, includ-ing lipid levels (Anthony 2006) and RNA/DNA ratios (a proxy for protein synthesis[Bak and Meesters 2002]).

Alternatively, it is possible that the weakpositive relationship between sediment andgrowth observed in this study would differ ifa growth metric other than mass change wereused. For example, coral tissue growth maybe more responsive to resource variationthan skeletal growth, because positive skeletalgrowth can be maintained even when tissuegrowth is negative (Anthony et al. 2002).Along a suggested sediment gradient in theGulf of Mexico, linear extension was in-versely correlated with skeletal density andcalcification rates, suggesting that coralsunder sediment stress generate higher linearextension rates by reducing skeletal density(Carricart-Ganivet and Merino 2001). Linearextension of Montastraea annularis in PuertoRico was negatively correlated with sedimenttrap data and percentage terrigenous sedi-ment (Torres 2001). This response can be in-terpreted as a mechanism for corals to avoidbeing smothered by high sedimentation rates.In contrast, Edinger et al. (2000) found thatlinear extension was negatively correlatedwith ‘‘downward sediment flux’’ (a misleadingterm when applied to sediment trap data).Still other studies found no correlation be-tween linear extension and sediment load(Brown et al. 1990, Barnes and Lough 1999).

The relationship between sediment andgrowth in our study was further confoundedby mortality from external factors at SugarBeach and the East mole sites. Wave energyis an important factor controlling coral reefdistribution and zonation in Hawai‘i (Dollar1982, Storlazzi et al. 2005), because stormevents cause mortality from breakage, scour,and abrasion (Grigg 1998). When wave


energy is low during late summer and earlyfall, corals at the Sugar Beach site gainedjust as much mass as corals at Triangle reef(Figure 6). This was not a function of linearextension; instead coral skeletons at SugarBeach had a higher density than those at theTriangle (G.A.P., unpubl. data). This is atypical response of corals to increased hydro-dynamic force (Scoffin et al. 1992). However,winter waves at Sugar Beach did cause coralmortality when the cinder blocks turnedover or moved out of their original sand holes.

The other site with high mortality was theEast mole site in Ma‘alaea Harbor. Fish graz-ing can exclude P. damicornis from sites thatwould otherwise be suitable (Neudecker1979, Cox 1994), and grazing pressure can in-fluence the outcomes of transplant studies(Coles and Ruddy 1995). Corallivorous fishescomprised <1% of the fish community inMa‘alaea Harbor ( Jokiel and Brown 1998),so it was assumed that the transplants wouldnot be subjected to substantial grazing pres-sure. However transplants in the first twoweighing intervals (August–October andOctober–February) suffered very high graz-ing mortality at the East mole site, so in Feb-ruary 2004 the site was relocated 100 m northto an area just off the edge of the reef flat.Pufferfish (Arothron hispidus and A. meleagris)were occasionally observed at the new site,but corals already at the site showed littleevidence of fish grazing. This new site, how-ever, was closer to shore, and the reef flat ispotentially impacted by septic tank leakagefrom coastal development ( Jokiel and Brown1998). Groundwater discharge could havecaused algal blooms of Ulva fasciata and U.reticulata observed at the site. Although P.damicornis may be less sensitive than othercoral species to spatial competition with algae(Tanner 1995), some transplants at the sitewere killed by algae dislodged by water mo-tion that became entangled with the trans-plants as the algae were being advected outof the harbor.

conclusions

The relationship between sediment and coralgrowth can be complicated, particularly forfield studies in which environmental effects

that covary with sediment (light, wave expo-sure, etc.) cannot be isolated. The intent ofthis study was to measure coral growth alonga range of sediment influence to test thehypothesis that coral growth would decreasewith increasing sediment load, which thisstudy did not observe. Instead there was a sig-nificant positive relationship between growthand sediment trap data, but the strength ofthe correlation (R2 ¼ 0:387) suggests that fac-tors other than sediment accumulation werethe primary controls of coral growth. Therelationship between coral growth and sedi-ment may depend on how growth is mea-sured, how sediment load is quantified, andthe intervals at which both are measured.Sediment traps are commonly used in fieldstudies but do not adequately capture the fullrange of sediment effects on coral growthprocesses (e.g., turbidity, abrasion, heterotro-phic particle capture, etc.).

acknowledgments

We thank the Maui Ocean Center for theirinvaluable logistic support, the Hawai‘i De-partment of Aquatic Resources for grantingthe scientific permits, and Charlene Tetlakand Mike Torresan at the U.S. GeologicalSurvey for the sediment analyses. The manu-script was improved by comments from MikeField, Caroline Rogers, Kimberly Yates, andthree anonymous reviewers. The use oftrademark names does not imply U.S Geo-logical Survey endorsement of products.

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growth and mortality of coral transplants (pocillopora ...growth and mortality of coral transplants...

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