Journal of Coastal Research SI 39 633 - 638 ICS 2004 (Proceedings) Brazil ISSN 0749-0208

DUTRA, L.X.C.; KIKUCHI, R.K.P. and LEÃO, Z.M.A.N., 2006. Effects of sediment accumulation on reef coralsfrom Abrolhos, Bahia, Brazil. Journal of Coastal Research, SI 39 (Proceedings of the 8 International CoastalSymposium), 633 - 638. Itajaí, SC, Brazil, ISSN 0749-0208.

This paper evaluates the effects of sediment accumulation on reef corals from the Abrolhos area, at the southerncoast of the state of Bahia, Brazil. Sediment accumulation rate was measured with sediment traps that were fixed infive reef stations during the months of October/2001 and March/2002. Live coral cover, diameter of the coralcolonies and species diversity were measured in the same stations according to theAGGRA(Atlantic and Gulf RapidReef Assessment) protocol, along six belt transects (10 m x 1 m), and the number of recruits were counted in fivequadrats (25 cm) located at every 2 m along the transects. Pearson's correlation was used in order to confirm therelationship between biotic and abiotic parameters. Sediment accumulation rate of 10 mg.cm .day seems toconstitute a critical limit for coral survival. Bellow it, there is a significant inverse relationship betweensedimentation rate and the measured biotic parameters such as live coral cover, average diameter of

colonies, the total number of recruits and of the recruits of . In places wheresedimentation rate is higher than this value, there is a considerable decline in the coral community health indicators.Morphological and distribution variations of the reefs structures are, most probably, the major factors that expose thecorals to the effects of sediment influx. The stations located at the top of the bank reefs closest to mainland show thehighest values for the rate of sediment accumulation and the highest percentage of siliciclastic sediments, which alsoseems to influence reef vitality. Negative impact of terrigenous sediment content is shown by the inverserelationship among its content, which decrease relative to carbonate sediments from near shore to offshore, and reefvitality indicators, which increase from the inner to the outer reefs.

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Milleporaalcicornis Siderastrea stellata

Coral reefs, sedimentation, Abrolhos.ADDITIONALINDEX WORDS:

ABSTRACT

Effects of SedimentAccumulation on Reef Corals FromAbrolhos, Bahia, Brazil

L. X. C. Dutra†; R. K. P. Kikuchi† and Z. M.A. N. Leão†

INTRODUCTION

Sediment influx in coastal waters is one of the main factorsthat affect reef distribution, community structure, growth ratesand coral recruitment. Increasing rates of sedimentation can bethe result of natural and anthropogenic phenomena. Naturalprocesses include fluvial discharge, terrigenous run-off afterstrong rain periods, erosion of the coastline and ressuspensionof bottom sediments after storms ( 1976; 1990).The anthropogenic impacts may result from nearshore dredgingand the unplanned human occupation of the coastal zone( and 1993; and

, 2000; , 1993). These processeswill affect reef corals through several mechanisms, and some ofthem can be lethal and/or sublethal to the reef corals, such as:asphyxia, reduction on growth rates, abrasion, recruitmentinhibition, reduction on live coral cover, changes on coralzonation, greater abundance of branching corals or thedominance of colonies with small sizes ( and

, 1988; , 1992; and, 1999).

References about Brazilian reefs suggest that the probableincreasing of sediment influx in the coastal zone is due tonatural sources caused by the sea level regression, whichoccurred since the end of Quaternary period, approaching reefsto the coastline ( and , 1998; , 1996;and 1999; ., 1997; ., 2001). Thesereferences also suggest that this process has been increasing inthe Present due to deforestation in coastal areas causingnegative effects to the reef vitality.

In the present work we tested if the parameters of reef coralsvitality, such as coral cover, number of coral colonies, among

others, can be related with the increment of sedimentaccumulated in the reef tops.

The studied area is located at the south coast of the State ofBahia, where there are the largest and richest coral reefs in thewhole South Atlantic Ocean. Five reefs were selected and theyare located at varied distances from the coastline: Pedra deLeste Reef (LES, 12 km), Timbebas Reef (TIM, 18 km), LixaReef (LIX, 20 km), Pedra Grande Sul Reef (PGS, 32 km) andParcel dosAbrolhos Reefs (PAB, 60 km) (Figure 1).

Climate in the region is characterized by summertemperatures (December to April) ranging from 25 to 27ºC andan average winter temperature (June to August) of 23ºC.Average annual precipitation is 1392 mm and monthly averagesrange from 66 mm (February and August) to 175 mm(November) ( , 2003). The main oceaniccurrent influencing the study area is the Brazil Current, whichflows to south with an annual average speed of 0.7 knots (DHN,1993).

Tidal currents have average speeds of 1.0 to 1.5 knots( , 2001) and tidalrange varies from about 2 m during spring tide to 0.5 m duringneap tide (DHN, 1993).

The Abrolhos region has the most developed reefs of thewhole Brazilian coast ( and , 2001; ,1969; , 1982). They are mainly constituted by bank reefsparallel to the coastline forming an inner arc, which is affectedby the processes acting in the mainland, and an outer arc,shorter, and about 60 km off the coast, which is influenced bythe waters of the Brazil Current ( , 1999).According to (2003), the coral reefs of this region

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The Coral Reefs ofAbrolhos

†Centre for Ecological Economics andWater Policy ResearchUniversity of New EnglandArmidale NSW 2351 Australiatel: +61 2 6773 3986fax:+61 2 6773 3237leo.dutra@une.edu.au

Journal of Coastal Research Special Issue 39, 2006,

can be classified in four types: (i) bank reefs adjacent to thecoastline, which are discontinuous with varied shapes butalways elongated and usually parallel to the coastline; (ii) bankreefs detached from the coast, whose horizontal dimensionsrange from about 50 m up to some tens of kilometers and theirheights from the sea floor varies from 10 m (shallower banks) tomore than 20 m (deeper banks). They have varied shapes(irregular, elongated, circular, arc-like) and are flat topped,emergent during low tides, what favors their lateral growthinstead of a vertical development. Submerged bank reefs, fewmeters high, are found in depths up to 10 m, and; (iii) fringingreefs, quite continuous, formed by incrustation of calcareousorganisms in the rocky outcrops that border the islands presentin the region, and (iv) coral pinnacles, which can be of twotypes: 1) columnar, where the base is equally as wide as, orwider than the top of the reef, and 2) chapeirão where the flat topis wider, sometimes over three times larger than its base (Figure2B). Chapeirão (from the Spanish “chaperon”, pl. in Portuguese“chapeirões”) is a term introduced by Charles F. Hartt in 1870

1870), which alludes to hats with broad brims. It refersto the coral growth form unique of the Brazilian reefs andconsists of isolated narrow pillars whose tops are expandedlaterally, resembling flat topped mushrooms. The studied reefsare classified in two of the described types. LES and LIXstations are located in an area of isolated bank reefs withdimensions in the order of kilometers. They are located less than20 km away from the shoreline, and their position in relation tothe coast may function as a barrier, blocking the water fluxtowards the open ocean and thus acting as sediment traps (Fig.2A). The other reef stations (TIM, PGS and PAB) are located inan area of isolate coral pinnacles, where water circulates morefreely (Figure 2B).

The bottom intereefal sediments of Abrolhos arecharacterized by a transition from dominant siliciclasticcomponents along and near the coastal zone to pure carbonatesediments in the middle and outer shelves ( , 1982). Nearthe coastline the bottom sediments are constituted by 30 to 70%of quartz grains, rare micas, and terrigenous mud. In Parcel dosAbrolhos reefs (PAB station) siliciclastic sediments representless than 10% (Figure 3).According to and

(1997), these sediments are originated from two main sources(i) reworked material from erosion of the coastal cliffs (Tertiarysediments of the Barreiras Group) and (ii) fluvial dischargestransported to reef area by longshore currents.At the base of thereefs, carbonate sand and mud dominate, representing morethan 50% of the grains ( , 2001). The coexistence ofreef growth and an active muddy sedimentation in Abrolhos isdiscussed by (1982) and and (1997),who suggest that this fact is related to: (a) the absence of greatfluvial flows facing the reefs, (b) the short term events ofressuspension of fine sediments from the sea bottom, duringwinter storms, and (c) a probable functional adaptation of thecoral fauna to the presence of turbid waters.

The coral community parameters were measured according tothe AGRRA (Atlantic and Gulf Rapid Reef Assessment)protocol ( , 1998). Six belt transects (10 m x 1 m)were extended in the top of the reefs, where all coral colonieswere identified. Colonies larger than 20 cm were measured forthe maximum height and diameter, and dead coral cover wasevaluated. Living coral cover was estimated along a line of 10m. Recruits (coral colonies with less than 2 cm of diameter)were counted in five quadrats (25 cm), 2 m apart from eachother along each transect.

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Intereefal Sediments

METHODS

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Figure 1. Location of studied reefs.

Figure 2. Top, major reef types in the study area. A) Large bankreefs from the coastal arc where the LES and LIX stations werelocated. B) The isolated chapeirões that form the reefs of theTIM, PGS and PAB stations. Bottom, the average sedimentaccumulation rates measured in each reef station.

Figure 3. Distribution of the intereefal bottom sediment in thearea of theAbrolhos reefs (Modified from , 1982).LE OÃ

Dutra et al.

Journal of Coastal Research Special Issue 39, 2006,

634

Sediment Accumulation Rate and The CaCO

Content

RESULTS

TheAccumulated Sediment

The Coral Community

DISCUSSIONS

Differences of the Rates of SedimentAccumulation Between theTwo Sampled Periods

The Processes Responsible for the Distributionof the Intereefal Sediments

3

Sediment accumulation rates were measured in sedimenttraps placed in the selected five reef stations, with threereplicates in each one. These sediment traps rested for 30 daysin October 2001 and in March 2002 on the top of the reefs atdepths varying from 1 to 6 m. They were built with PVC pipes(26 cm long and a diameter of 6 cm), having a reduction of itsdiameter in their tops (10 cm high and 4 cm diameter) in order toavoid ressuspension of the captured sediment. The collectedsediment was analyzed applying the Standard Methods (1980),where the material is filtered, dried and weighted. Thepercentage of the calcium carbonate content was calculatedadapting a soil method described in Embrapa (1997), in whichpart of the sample (1 g) is attacked with 50 ml of HCl, to digestthe calcium carbonate, and after that a titulation with NaOH isperformed in order to consume the excess of acid.

The values of the sediment accumulation rates variedbetween the two sampled periods. In October 2001 (afterwinter), these values were higher than the rates of March 2002(after summer), in all stations. LES and LIX stations had, inOctober, more than twice the values measured in March, andtheir rates werehigher than all other stations, in both periods, aswell(see figure 2).

The percentage of CaCO in the sediment accumulated onreef tops increases towards the reef stations located farther frommainland (LES < TIM < LIX < PGS < PAB) (Figure 4). Thissame distribution pattern was found by (1982) for theintereefal bottom sediments (see figure 3), which indicates aclear transition from predominant siliciclastic sediments nearthe coastline, to almost pure carbonate sediments towards theoffshore reefs.

Four hundred ninety eight colonies of corals andhydrocorals, larger than 20 cm in diameter, were counted duringthis investigation. A total of twelve species of corals and two ofhydrocorals were identified. Among the corals are

, , , ,, , ,

, , ,, . The two species of hydrocorals that

were identified are and Themost abundant species is (relativeabundance = 0,44), followed by (relativeabundance = 0,20) and (relative abundance = 0,19).

The reef coral communities in stations LES and LIX showthe lowest numbers for the biotic parameters, such as the

average percentage of live coral cover, the average diameter forthe colonies, as well as the total number of thecoral recruits and of the recruits of (Table 1). Whencomparing the rates of the sediment accumulated on the reeftops with these biotic parameters, one observes that there is aninverse tendency in their values as it is shown in figures 5 and 6.Yet, Pearson's correlations are significant between the sedimentaccumulation rates and: a) the number of coral species (r= 0.68;p= 0.03); b) the average diameter of the coral colonies (r=-0.66;p= 0.04), and c) the presence of the hydrocoral(r= 0.75; p= 0.013).

The highest rate of sediment accumulation in the reef topsduring the month of October (after winter) is probably related toclimate conditions. During the winter time there is a higherpercentage of winds coming from south (DHN 1993), which arerelated with cold fronts that bring more rainfall, contributing toan increase of sediment input to the sea. The waves associatedwith these cold fronts revolve the fine sediment from the oceanbottom causing more turbid waters. After this period, during acalmer weather time, the suspended sediment is deposited in themost protected areas, as occur in the back reef zone of the LESand LIX stations (see figure 2).

The difference in sediment accumulation rates from winter tosummer was also observed by (2003), in the Abrolhosarea. The highest values of the sediment trapped in the reefsfound by this author were measured in October, and the lowestrates were found during the month of March.

The pattern of distribution of the reefal sediments in theAbrolhos area, of siliciclastic dominant sands and muds in theinshore zone to pure carbonates towards the offshore reefs, is acommon occurrence along the whole coast of the State of Bahia.Similar trends are described, also, in other reef areas such as at

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Agariciaagaricites Siderastrea stellata Porites branneri P. astreoidesFavia gravida F. leptophylla Montastrea cavernosaMeandrina braziliensis Mussismilia harttii M. braziliensis M.hispida Scolymia wellsi

Millepora alcicornis M. nitida.Mussismila braziliensis

M. alcicornisM. hartii

M. alcicornisS. stellata

Millepora nitida

Effects of Sediment Accumulation on Reef Corals

Figure 4. The calcium carbonate content of the sedimentaccumulated in the studied reefs. (Data of PAB station isaccording to , 1982).LE OÃ

Figure 5. Average sediment accumulation rates of March 2002,average number of coral recruits, and average number of recruitsof .Siderastrea stellata

Journal of Coastal Research Special Issue 39, 2006,

635

Table 1. Average values of some measured biotic parameters of

the coral and hydrocoral colonies larger than 20 cm.

STATIONS LES LIX TIM PGS PABLive coralcover (%)

5.6±2.8 9.4±4.0 12.2±2.4 10.7±2.8 14.4±5.5

M. alcicornis

diameter (cm)42.1±9.0 42.9±11.3 52.3±8.8 61.1±24.7 53.5±17.1

Number ofcoral recruits

6.7±4.2 8.3±3.3 10.5±7.7 13.7±5.4 10.2±4.4

Number of S.

stellata

recruits1.0±1.1 0.7±0.8 1.5±1.4 1.0±1.7 2.8±1.7

0

20

40

60

80

100

LES TIM LIX PGS PAB

Stations

%

the north coast of the state ( , 2000; , 1987;and , 1986), and in the Todos os Santos Bay1985; , 1988).

The hydrologic processes acting in the coastal zone areresponsible for the transport and the deposition of thesesediments. The dominants winds from NE, in summer, and fromSE, during winter storms, originate waves that reach the shorediagonally, forming longshore currents that promote thedispersion of the coarse quartz sands along the coastline( , 2000). The continent derived muddysediment that reaches the inner shelf is blocked by thehydrodynamic barrier formed by the currents parallel to thecoast ( 1999), and then, is mostly deposited inthe deeper zones of the channel that separates the coastal reefsfrom the shoreline. The satellite image shown in figure 8pictures this process.

The reefs of the TIM station are located at almost the samedistance from shore than the LES and the LIX reefs, but it has amuch lower rate of sediment accumulation (see figure 2). Twofacts may respond for that: a) the morphology of the Timbebasreefs, which are isolated coral pinnacles that favor waves andcurrents to move more freely, removing the fine sediment thatreaches the reefs, and b) the location of the TIM station up northfrom the mouth of the Caravelas River, of which sediment input,is carried towards the south by the longshore currents.

This investigation found that the accumulation rate of 10mg.cm .day can be considered as a critical limit for the vitalityof the reefs. The LES and LIX stations, which exhibit sedimentaccumulation rates higher than these values, show the lowestpercentages for: a) live coral cover, b) average diameter of

colonies, c) average number of coralrecruits, and d) number of recruits of .

Similar results were found in other reef areas in the tropicalseas, where also critical numbers for the rate of sedimentaccumulation were suggested. For example, (1990)alludes to the number of 10 mg.cm .day for the rate ofsedimentation as a critical value for the reefs vitality;(1976) suggest that up to a rate of sediment deposition of 15mg.cm .day the reefs start to decline.

Reduction of living coral cover can be: a) a result ofinhibition of coral recruitment, b) an effect of reduction ofgrowth of the coral skeleton, and/or c) coral death (and , 1988; , 1993; and

1993). Coral recruitment can be affected by mobilesediment that removes and kills juvenile corals before theirfixation and 1991; ,1988; , 1992; and , 1999). Reduction ofcoral growth may occur when precipitation of calciumcarbonate to form the coral skeleton is altered by deficiency ofavailable light (due to excess of suspended sediment in watercolumn), to the endossimbiont algae (zooxanthellae) that makesphotosynthesis, or due to the excess of energy that the corals useto remove sediment from their polyps ( and

, 1988). Yet, corals can be killed by sediment due toasphyxia, abrasion, etc., as well as due to the presence ofpollutants carried by muddy solutions. In the Abrolhos area,

(1997) found an increase in metalconcentrations towards the reefs of the coastal arc.

The positive correlation found between the rates of sedimentaccumulation and the number of coral species (r=0.68; p=0.03)at the reef stations located closest to the coastline (Figure 9),may be roughly explained by the intermediate disturbancehypothesis proposed by (1978). In the coastal reefs(LES, LIX and TIM) there is a stronger seasonal variation in therates of sediment accumulation (almost two fold the valuefound in March compared to October in LES and LIX, seefigure 2), i.e., in this area a most severe disturbance occur atleast once a year, opening reef spaces for new species toestablish. At the stations PGS and PAB the difference betweenrates of the sampled periods is almost null, which means that thedisturbance is lower in intensity and in frequency, allowing themost adapted species dominate. In PAB,

dominate with 60% relative abundance, whereas inPGS, has an occurrence of 48% relative tothe other coral species.

he evaluation of the effects of sediment to the vitality of thereef corals ofAbrolhos leads to the following conclusions:

1. Rates of sediment accumulation higher than 10 mg.cm.day seem to be critical for the maintenance of the vitalconditions of the corals and hydrocorals of Abrolhos. The reefssubjected to sediment accumulation rates that reached values upto this limit (LES and LIX), showed an inverse relationship withrespect to some of their biotic parameters, such as thepercentage of live coral cover, the average diameter of coloniesof the hydrocoral , the number of coralrecruits and of the number of recruits of ;

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Millepora alcicornisS. stellata

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Mussismiliabraziliensis

Millepora alcicornis

Millepora alcicornisSiderastrea stellata

The Effects of the Sediment for the Vitality ofReef Corals

The Number of Coral Species Versus the Rate ofSedimentAccumulation

CONCLUSIONS

-2 -1

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Figure 7. A satellite image (Landsat coverage from the USErosdata Center, 1973) of the Abrolhos area illustrating theplume of sediment backward the coastal arc of reefs.

Dutra et al.

Journal of Coastal Research Special Issue 39, 2006,

636

Figure 6. This figure shows the average rate of sedimentaccumulation, the percentage of live coral cover and the averagediameter of the colonies in the sampled stations.M. alcicornis

2. The proximity of reefs to mainland constitutes the majorfactor that exposes them to the effects of continent derivedsediments. The stations LES and LIX, both located less than 20km away from coastline, have the highest values for the rate ofsediment accumulation and the highest percentages ofsiliciclastic sediments, which seem to influence negatively reefcorals. There is a clear transition in the deposition of terrigenousmaterial to pure carbonate sediments from the inner to the outerreefs, coincident with the pattern of the indicators of theintegrity of reef coral community, and

3. The morphological variation of the Abrolhos reefs, that is,bank reefs with great lateral dimensions in the range ofkilometers, and isolated narrow coral columns the Brazilianchapeirões , may control the differences found in the rate of thesediment trapped within the reef structures. The bank reefslocated closer to the shoreline, as the reefs of stations LES andLIX, are functioning as a physical barrier that block water flowstowards the open sea, acting, then, as sediments traps, whereasthe isolated coral pinnacles that form the reefs of stations TIM,PGS and PAB, allow a opener water circulation that preventsediment to accumulate in their surroundings. This explains thelowest sediment accumulation rates found in station TIM, inrelation to the LES and LIX stations, despite all being located insimilar distances from the coastline.

The authors are especially grateful to S. Spanó, C. Sampaio,B. Feitosa and A. Bonici Neto for their assistance duringfieldwork. They also acknowledge the administration of theAbrolhos National Marine Park for the logistic facilitiesprovided, and Dr. F. Barros for his constructive suggestions.This research was founded by CNPq (Brazilian Council for theDevelopment of Science) through projects to R.K.P. Kikuchi(Proc.#471309/01-0) and Z.M.A.N. Leão, (Proc.# 464029/00-7). L.X.C. Dutra received scholarship from CAPES (ScholarlySupport for Pos-Graduate Studies) during his MSc. Studies.

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Figure 8. The total number of coral species present in each reefstation, and the average number of coral species per transect.

Effects of Sediment Accumulation on Reef Corals

Journal of Coastal Research Special Issue 39, 2006,

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