Journal of Sea Research 66 (2011) 172–180

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Journal of Sea Research

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Deposition of shallow water sponges in response to seasonal changes

Enrique Ávila a,b,⁎, José Luis Carballo b, Cristina Vega b, Leonardo Camacho b, José J. Barrón-Álvarez b,Claudia Padilla-Verdín b, Benjamín Yáñez-Chávez b

a Instituto de Ciencias del Mar y Limnología, Estación El Carmen, Universidad Nacional Autónoma de México, Carretera Carmen-Puerto Real km. 9.5,Ciudad del Carmen, Campeche, C.P. 24157, Mexicob Laboratorio de Ecología de Bentos, Unidad Académica Mazatlán, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México,Apdo. Postal 811, Mazatlán 82000, Mexico

⁎ Corresponding author. Current address: Instituto deEstación El Carmen. Universidad Nacional Autónoma dPuerto Real km. 9.5, Ciudad del Carmen, Campeche, M(01–938)383–18–45; fax: +52 (01–938)383 18 47.

E-mail address: kike@ola.icmyl.unam.mx (E. Ávila).

1385-1101/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.seares.2011.06.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 January 2011Received in revised form 16 June 2011Accepted 21 June 2011Available online 5 July 2011

Keywords:Sponge DepositionsSeasonal ChangesSponge AbundancePrevailing WindsResuspension/SedimentationTides

Removal of organisms from the subtidal zone plays an important role in shaping benthic communities inshallow bays. The main objective of this research was to quantify the biomass of sponges washed up on thebeach at Mazatlan Bay (Mexico, eastern Pacific Ocean), and to determine its relationship with local weatherand oceanographic conditions. To know whether this process has a significant effect on the spongepopulations, changes in abundance of the species washed into the beach were also quantified in adjoiningsublittoral areas. The sponges that were washed ashore were mainly branching (Mycale ramulosa), massive(Haliclona caerulea) and cushion-shaped (Callyspongia californica) species. Species with high content ofspongin in their structure (e.g. Hyattella intestinalis) were common in the subtidal zone but were rarely foundon the beach. Encrusting species were never found. Four-year data of sponge deposition on the beach showedthat the total annual sponge biomass ranged from 30 to 60 g DW m−2 with an inter-annual range from 0.1 to17.3 g DW m−2. The highest deposition of sponges was during the spring–summer transition (from April toJuly), which was associated with a change in wind direction (from NW to WSW). This change also matchedwith low tides and a high resuspension of bottom sediments, suggesting a high-energy environment duringthis transition. The increase in sponge biomass washed on the beach coincided with a decrease in the densityof adjacent sponge populations. A multiple regression analysis showed that 68.48% of the variation on spongebiomass on the beach could be statistically explained using a combination of environmental factors (windspeed, sediment resuspension and tides). Thus, seasonal changes in wind direction combined with the effectof low tides and sediment resuspension could serve to predict fragmentation/detachment events of benthicorganisms in shallow sublittoral areas worldwide. This study also provides insights to explain thepredominance of encrusting sponges in the Mexican Pacific Ocean.

Ciencias del Mary Limnología,e México, Carretera Carmen-éxico. C.P. 24157, Tel.: +52

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Dislodgement and subsequent depositions on the beach of detachedorganisms may be product of a combination of environmental factorssuch as prevailing winds, storms, currents, tides and waves (Gallucciand Netto, 2004; Ochieng and Erftemeijer, 1999). Some studies haveshown that the quantity and seasonality of these depositionsmay differat local scale and geographically, depending on local prevailingclimatological and oceanographic conditions (Ochieng and Erftemeijer,1999; Orr et al., 2005). However, at present we know relatively littleabouthowthese factors affect benthic community compositionon rocky

shallow ecosystems, andmost of our knowledge is biassed toward coralreefs, (Lough, 2008) fishes (Munday et al., 2007), mussels (Carrington,2002) or crabs (Winnie et al., 2003).

Filter feeders play a very important ecological role in theecosystem, and changes in their community may also intensify theeffects of increased turbidity, as this group plays an important role inregulating water quality (Przeslawski et al., 2008). Thus, in thisresearch we have focused on sponges, which are an importantcomponent of tropical marine ecosystems, and are heavily involved inecological processes such as bentho-pelagic coupling (see review inBell, 2008). Sponges are also very diverse in form, ranging fromencrusting sheets to cushion shape, massive or branching morphol-ogies, which vary in response to an array of biological and physicalfactors (Bell and Barnes, 2000). Likewise, it is also known that thesemorphologies may differ in susceptibility to be affected by hydrody-namic forces; species with erect growth forms are generally moresusceptible to be dislodged or broken during high wave conditionsthan encrusting ones (Wulff, 1995).

Fig. 2. Location of study area in the Bay of Mazatlan. The numbers indicate the samplingstudy areas at Venados Island (1) and Lobos Island (2). The asterisks show the locationof the beach zone at Venados Island.

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Preliminary surveys in different places along the Mexican PacificOcean have reported large deposits of marine organisms washed upalong the beach, some of them detached together with their substrata(Carballo et al., 2008) (Fig. 1). These deposits are generally presentafter intense swell periods (unpublished data), and are mainly com-posed of seaweeds and sessile invertebrates such as marine sponges(Carballo et al., 2008). However, it is unknown whether these eventsoccur throughout the year or seasonally and whether there is a sig-nificant effect on the shallow sponge populations.

Since sponges are very diverse in morphology, they will beselectively affected by water movement. Therefore, we might expectto find a prevalence of massive and branching forms washed up onthe beach after intense swell periods.

The objectives of the present study were 1) to identify the spongespecies that are commonly fragmented and deposited on the beach,2) to examine whether there is a relationship between dislodgedsponges, sponges living on littoral zones closer to the beaches, andweather conditions, and 3) to provide insights to explain thepredominance of encrusting sponges in the Mexican Pacific Ocean(Bell and Carballo, 2008; Carballo et al., 2008; Carballo and Nava,2007).

2. Material and methods

2.1. Study area and meteorological conditions in Mazatlan Bay

The study area is located inMazatlan Bay, which is a semi-enclosedembayment located in northwestern Mexico (East Pacific) (Fig. 2).In this site, two sampling localities were selected at depths between1 and 10 m: Venados Island (23° 13′ 57″ N, 106° 27′ 42″W) and LobosIsland (23° 13′ 34″ N, 106° 27′ 43″ W). In the two locations, the

Fig. 1. A) Close up of sponge remnants deposited on the beach at Mazatlan Bay, B) Partial sponge collection on the beach of Venados Island in summer 2006 (kid is included for acomparative purpose). Individuals of C) Haliclona caerulea, D) Geodia media and E) Mycale ramulosa washed into the beach together with the substrate.

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bottom in the shallower zone (from 1 to 2 m) is primarily con-solidated rock; between 2 and 5 m, the bottom consists of relativelyflat boulders between patches of sediment, which increase until therocks disappear almost completely at 7 to 8 m.

The climate is tropical/sub-tropical with two contrasting seasonsin the year, the rainy season (from June to October) and dry season(fromNovember to May). The average annual air temperature is 25 °Cand the average annual rainfall is 800 mm.

2.2. Environmental and meteorological variables measured

The physical and environmental factors were measured in situ inthe Venados Island, or obtained from the National Water Commission(CNA).Water temperature (°C) was recorded daily at 12:00 pmwith atemperature sensor (HOBO data logger), which was permanentlyplaced at 4 m depth on the seafloor. Resuspension/sedimentation wasmeasured using a trap system consisting of four sets of plastic bottles(1 l and 5.3 of height/internal diameter ratio), which were positionedat 1 m from the bottom at 4 m depth, together with the temperaturedata logger (see detailed method in Carballo et al., 2006). The trappedmaterial was repeatedly rinsed with distilled water to remove salts,and dried at 60 °C for 24 h before weighing. The total amount ofsedimented material was then expressed as kg m−2 d−1. Depositionof sediment was monitored monthly during the study period (fromJanuary to December 2007 and from January to December 2010) bychanging the bottles. Also, weather data (rainfall and intensity anddirection of prevailing winds) were obtained from the localmeteorological station (National Water Commission).

Mazatlan Bay is subjected to a mixed semidiurnal tide with amaximum height of 1.3 m (Aguirre-Gómez et al., 1999). The numberof times (frequency) the tide was below the mean lower low water(MLLW) as well as the number of times the tide was above the meanhigher high water (MHHW)was also quantified monthly by using thetide charts from Mazatlan Bay.

2.3. Quantifying sponge deposition on the beach

Sponge deposition was monitored monthly (from January toDecember 2007 and from January to December 2010) in the beachlocated to the east side of Venados Island (Fig. 2).

All the sponge fragments were collected in a permanentlydelimited area (23 m×50 m=1150 m2) on the beach, parallel tothe shore, from the highest tide line to the lowest water levelexperienced during a typical low tide (Orr et al., 2005). Lobos Islandwas not monitored because it has a rocky coast without sandybeaches. Sponge fragments were shaken to remove sand and placedinto plastic bags. In the laboratory, the sponge species were separatedand identified, and the dry weight (DW) was obtained by drying thematerial in a stove at 60 °C for 24 h. Then, sponge biomass wasexpressed as g DWm−2.

In addition, data on sponge biomass obtained (in this same area)during summer 2005 and 2006 were also presented for comparativepurposes.

2.4. Quantifying sponge abundance in littoral areas close to the beach

Abundance was quantified in those species that are commonlyobserved washed on the beach of the study area (Carballo et al., 2008)which display different morphologies. Haliclona caerulea (as Sigma-docia caerulea by Hechtel 1964) and Callyspongia californica Dickinsonhave massive and massive-cushion shape growth forms, respectively,and Mycale ramulosa Carballo and Cruz-Barraza has a massive tobranching growth form. In addition, to know if the above environ-mental variables also affect encrusting species, abundance of Clathria(Microciona) sp., one of the most common and persistent species inthe bay was also monitored.

Abundance estimations were made by SCUBA diving in thesublittoral habitat close to the beach in front of Venados and Lobosislands. In the two localities, three 50 m transect lines were placedfrom 1 to 10 m depth, perpendicular to the shore and separated20 m from each other. To assess the abundance, a 1.5 m bar wasmoved along transect and all the individuals observed within thearea (75 m2) were counted (Carballo et al., 2004). The total sampledarea was 450 m2 (6 transects of 75 m2). In this study, an individualwas defined as being any sponge growing independently of itsneighbours (without contact). Abundance estimations were per-formed monthly during an annual cycle (from January to December2007).

In addition, the morphology of all the species present in thesubtidal zone was also recorded throughout the study period.

2.5. Data analysis

Spearman rank correlations were used to determine whetherinterannual variations in the biomass of sponges that are washed uponto the beach correlated to abundance of these same sponge speciesin the adjacent sublittoral ecosystem. Also, a multiple regressionanalysis was performed to determine whether two or moreindependent variables (environmental and meteorological variables)explain the temporal variations in sponge biomass on the beach(dependent variable). A two-way analysis of variance (ANOVA) wasused to test whether abundance of adjacent living sponges variedsignificantly through the year (12 months) and between sites (2sites), where time and sites were random factors. The assumption ofnormality and homoscedasticity in each variable was previouslytested by Kolmogorov–Smirnov's and Bartlett's tests, respectively(Sokal and Rohlf, 1995).

Comparisons of the different spongemorphologies (those from thesubtidal zone and those found on the beach) were undertaken using aChi-square test for differences in percentages.

3. Results

3.1. Environmental variability

Water temperature, sediment deposition and intensity and directionof prevailing winds varied significantly within the study period (Fig. 3).Water temperature showed seasonal variations, with maximum valuesin July–September (30 °C), and minimums in December (19 °C)(Fig. 3A). Maximum values of water temperature occurred during therainy season from June to October, peaking in August in 2007 and July in2010 (Fig. 3A). In 2007, the resuspension/sedimentation ratewas higherin April (5.9 kg m−2 d−1), while in 2010 the maximum value wasdetected in June (3.1 kg m−2 d−1) (Fig. 3B). Prevailing winds camefrom theNWfromOctober toMarch (average speed=2.1±0.1 m s−1),and from theWSW fromApril to September (2.7±0.1 m s−1) (Fig. 3B).Prevailing winds were stronger in September in both years (from 3.0 to3.5 m s−1), which came from WSW.

In 2007, from January to June the tides below the mean lower lowwater (MLLW) were more frequent (mostly in March and April, 26and 22 times, respectively) than those tides above the mean higherhigh water (MHHW) (minimum in April, 4 times) (Fig. 3C). Incontrast, from June to December the tides that were above theMHHWwere more frequent (maximum in August and September, 25 and 27times, respectively) than those that were below theMLLW (minimumin October, 8 times) (Fig. 3C). Similarly, from January to August 2010the highest frequency of the MLLW was in March (30 times) and thelowest in September (3 times). In this same period, the highestfrequency of the MHHW was in August (26 times) and the lowest inFebruary (2 times).

Fig. 3. Seasonal changes in A) water temperature (line) and rainfall (open bars, secondary axis). B) Average speed (dotted line, primary axis) and direction (arrows) of prevailingwinds. Secondary axis shows the average (±SE) resuspension/sedimentation rate (grey bars) and the total dry biomass of sponges washed up on the beach (continuous line).C) Variation in the frequency of tides that were lower than themean lower lowwater (MLLW) (open circles, secondary axis), and those tides were above themean higher high water(MHHW) (solid circles, primary axis).

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3.2. Sponges washed up on the beach

The largest amountof sponge remnantswas recordedbetween springand summer. During 2007, the average annual biomass was 2.2±1.4 gDWm−2;withmaximumvalues inApril (17.2 gDWm−2) (Fig. 4A). Theaccumulated biomass in April represented 59% of the total biomassrecorded throughout the year (Fig. 4C). From January to December 2010the average biomass was 4.5±1.6 g DWm−2, with maximum values inJune (15.5 g DWm−2), and July (15.0 g DWm−2) (Fig. 4A). Theaccumulated total biomass of sponges during this period was 53.9 gDWm−2 (Fig. 4C). This figure also shows sponge cumulative biomassobtained during summer of 2005 and 2006, which are comparable withthose obtained in summer of 2007 and 2010.

The species that were found on the beach were M. ramulosa, C.californica,H. caerulea,Hyattella intestinalis Lamarck and occasionallywere also found a few specimens of Geodia media Bowerbank andHaliclona turquoisia (as Adocia turquoisia by de Laubenfels, 1954).

The dominant species were always M. ramulosa and H. caerulea(Fig. 4B).

The regression analysis showed a multiple linear regression modelto describe the relationship between sponge biomass and the threeenvironmental variables we analysed (resuspension/sedimentationrate, frequency of low tides [MLLW] and speed of prevailing winds)(Table 1):

Sponge biomass on the beach = 8:0068–2:8172 x wind speed + 3:4983 xresuspension=sedimentation rate−0:1621 x frequency of low tides:

Since the ANOVA p-value was less than 0.05 (Table 2), there is astatistically significant relationship between the variables at the 95.0%confidence level. The R-Squared statistic indicates that the model asfitted explains 68.48% of the variability in sponge biomass on thebeach (Fig. 5). The standard deviation of the residuals was 3.1, and themean absolute error (MAE) was 1.95 (average value of the residuals).

Fig. 4. A) Inter-annual variation in the total biomass of the sponges washed onto the Venados Island beach. B) Average monthly biomass of the sponges washed onto the beach.C) Accumulated biomass of sponges washed onto the beach through the year. This figure includes preliminary accumulative data from summer of 2005 (open rhombs) and 2006(open squares).

Table 1Summary of multiple linear regression analysis using sponge biomass on the beach asdependent variable. The asterisk indicates pb0.01.

Parameter Estimate Standard error T-statistic

p-value

Constant 8.00 4.98 1.60 0.12Wind speed −2.81 1.77 −1.58 0.12Resuspension/sedimentation rate 3.49 0.57 6.11 0.00*Frequency of low tides (MLLW) −0.16 0.11 −1.39 0.17

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The variance inflation factor (VIF) showed that there was nomulticollinearity among the independent variables: wind speed(VIF=1.6), resuspension/sedimentation rate (VIF=1.3) and MLLW(VIF=1.3).

Table 2Summary of the analysis of variance obtained from the multiple linear regressionanalysis. The asterisk indicates pb0.01.

Source Sum of squares Df Mean square F-ratio p-value

Model 400.44 3 133.48 13.76 0.00*Residual 184.31 19 9.70Total (Corr.) 584.76 22

Fig. 5. Fit of the observed and predicted values obtained for the sponge biomass(g DW m−2) washed into the beach to the multiple regression equation.

Fig. 6. A) Total average abundance of adjacent populations of the sponges Mycaleramulosa, Callyspongia californica, Haliclona caerulea and Clathria (Microciona) sp.during the study period. B) Average abundance of sponges at Venados Island and C) atLobos Island.

Table 3Percentage of the different sponge morphologies from the subtidal zone and thosewashed on the beach of the Venados Island. The total number of species was 27 and 4,respectively.

Morphologies Subtidal zone Beach of Venados Island

Arborecent 3.70 25Boring 7.41 0Cushion shape 33.33 25Encrusting 37.04 0Fistulose 3.70 0Globular 7.41 0Massive 3.70 50Repent 3.70 0

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3.3. Sponge density in adjacent areas

In general, H. caerulea was the most abundant species in thesubtidal zone (annual average=1.22±0.09 ind m−2) in comparisonwith the other three species studied (C. californica, Clathria (M.) sp.and M. ramulosa), which showed values below 1.2 ind m−2 (Fig. 6A).There was no evidence of variation in density of the four species overtime (ANOVA, pN0.05). However, two peaks of relatively highabundance (up to 0.8 ind m−2) were observed in the June–Julyperiod and in November (Fig. 6A). At species level, there weresignificant differences in the average abundance of C. californica,H. caerulea and Clathria (M.) sp. between sites (ANOVA, pb0.05).These species were in general more abundant in Venados Island thanin Lobos Island. The average abundance of M. ramulosa did not showsignificant differences between sites.

At Venados Island, the most abundant species was C. californica(1.92±0.2 ind m−2) (Fig. 6B), while at Lobos Island, H. caerulea wasthe dominant species (0.71±0.09 ind m−2) (Fig. 6C). Spearman rankcorrelations showed no significant correlation between spongedensity (both, total and individual species) in the adjacent subtidalzone and sponge biomass on the beach. However, in 2007, themaximum sponge biomass on the beach recorded in April coincidedwith a decrease in the average density of adjacent sponge populations(Fig. 6).

A total of 27 species were recorded in the subtidal zone, most ofthem were encrusting (37.04%) and cushion-shaped (33.33%) forms.Instead, sponge fragments found on the beach zone correspondedmainly to massive (50%), cushion shaped (25%) and branching (25%)species (Table 3). A Chi-square test indicated that the two character-istics that define the contingency table (percentage of morphologiesof the species of the subtidal zone and those washed ashore) were notsignificantly related (pN0.05).

4. Discussion

The sponges that were found washed ashore were massive(H. intestinalis and H. caerulea), cushion-shaped (C. californica) andmassive-branching (M. ramulosa) species, which are easily fragmen-ted or detached from the substrate during high wave conditions(Battershill and Bergquist, 1990; Wulff, 1991, 1995). Beside morphol-ogy, species such as M. ramulosa that have branches up to 40 cm tall,orH. caerulea that reach up to 15 cm in height, aremuchmore likely tobe ripped from the rocks than smaller ones (Denny et al., 1985;Wolcott, 2007). In contrast, encrusting species such as Clathria (M.)

sp. are less prone to fragmentation or detachment during adversewave conditions than elevated or massive forms, and are never foundwashed up on the beach in spite of being abundant in the subtidalzone (Carballo et al., 2006). In fact, sponges with encrusting growth

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forms are predominant in the Mexican Pacific in relation to cushion-shaped, massive, globular, branching and boring forms (Carballo et al.,2008; Carballo and Nava, 2007) (Table 3).

The effect of water flow on morphology and tissue mechanics hasbeen extensively documented in other benthic organisms and it hasbeen suggested that a reduced surface area: biomass ratio coupled withgreater strength may lessen wave-induced disturbance (Dudgeon andJohnson, 1992). In this sense, the skeletal composition also seems toplayan important role in the composition of sponge species on the beach.Species with a relatively high proportion of spicules in their structuresuch as M. ramulosa and H. caerulea (Ávila and Carballo, 2004; Vega,2002) may be relatively fragile and easily broken by wave actioncompared to species with a high proportion of spongin (Wulff, 1995).The presence on the beach of sponges whose skeleton is made entirelyby spongin (e.g.H. intestinalis)was relatively lowpossibly because thesestructures are more flexible and give the sponge more resistance towave action (Wulff, 1995).

The period of maximum sponge biomass on the beach recorded inApril 2007 coincidedwith a decrease in the density of adjacent spongepopulations. However, we did not detect a significant relationshipbetween the two variables over the year. One possible cause could bethat most of the sponge remnants on the beach correspond tofragments of larger individuals. Previous studies conducted on H.caerulea revealed that individuals are continually fragmented by thewave energy and their biomass and size decreased considerablyduring periods of intense swell and high sedimentation (Ávila andCarballo, 2004). The monthly loss rate (individuals dislodgedmonthly) was between 25 and 50% and less than 3% of the spongeslabelled stayed in the place of origin after 1 yr. However, the densityremains almost constant in the zone due to the continuous settlementof the same fragments (in somemonths recruits were between 50 and70% of the total population) (Carballo and Ávila, 2004). During ourfield studies, detached fragments of this species were commonlyobserved in the subtidal zone, so it is probable that the populationdensity does not decrease significantly although biomass did, becausemany of these fragments adhere to the substrate before being washedashore.

It was also observed that small sponge remnants (e.g. H. caeruleaandM. ramulosa) are often seen remaining attached to the substratumafter individuals have been fragmented (Fig. 7), which holds thedensity practically at a constant level. This fact has been previouslydocumented, for example, after the Hurricane Gilbert (Cozumel,Mexico, in 1988), sponges such as Xestospongia muta Schmidt werebroken off at the base and washed up on the beach, but the base of thesponge remained attached and began regrowing (Fenner, 1991).

Fig. 7. Specimen of Haliclona caerulea fragmented after intense swell conditionsrecorded in April 2007.

Fusion and fission has also been documented in some species such asC. californica, H. caerulea, Microciona sp. and Mycale cecilia deLaubenfels, thus masking recruitment and mortality estimations(Carballo et al., 2008). Therefore, the different methodology used toestimate the abundance of the sponges (density vs. biomass) could beone of the causes of this apparently lack of fit.

The decrease in the abundance of H. caerulea and C. californicarecorded from March to April (2007) clearly matches the momentwhen a large biomass of sponges was washed on the beach, whichsuggests that the seasonal change had a negative impact on thepopulation of these two species. These dislodgement and reattach-ment events have beenwidely documented inmussels and it has beenregarded as a common mode of dispersion and patch formation inboth, soft substratum and rocky intertidal environments (Petrovic andGuichard, 2008; Reusch and Chapman, 1997).

On the other hand, the absence of correlation between the spongedepositions and the abundance of sponges in nearby populationscould be also because many individuals come from deeper areas. Itwas evident because many sponges were taken out to the beachtogether with the substrate (generally mollusk shells or smallboulders, b10 cm in diameter, Fig. 1), which only occur in softbottoms (pers. obs.).

Dislodgement and subsequent depositions of organisms on thebeach is also frequent in other places of the world and have beenattributed to a combination of environmental factors such as prevailingwinds, winter storms, currents, waves and tides (e.g. Griffiths et al.,1983; Kemp, 1986; Ochieng and Erftemeijer, 1999). It has also beensuggested that local differences in wrack composition and amount canbe explained, in part, by wave exposure and substrate type (Orr et al.,2005). In addition, some long-term studies have documented amarkedseasonality on this process (Piriz et al., 2003). For example, the largestaccumulationof seagrass beach cast along theKenyancoasts is generallyobserved during the South-East monsoon (March to October); andminimal amounts are observed during the North-East monsoon(November to March) (Ochieng and Erftemeijer, 1999).

In the tropics, hurricanes and tropical storms have been reportedto cause high losses of biomass on sponge assemblages (Fenner, 1991;Woodley et al., 1981). For example, Hurricane Joan, in 1988, removednearly 50% of the individuals and biomass of large, erect sponges onreefs in Panama (Wulff, 1995). Also, Hurricane Andrew of category 5on the Saffir–Simpson Hurricane Scale (in August 1992), causedconsiderable damage to the shallow benthic communities in BiscayneBay (Tedesco et al., 1995) and produced a loss of 50–100% of thecommercial sponges in areas exposed to scouring or covered bysediment (DiResta et al., 1995). Nevertheless, although Mazatlan Bayis also prone to these events (from May to November), the inter-annual variations in sponge depositions on the beach was not relatedto tropical storms. In 2007 and 2010 the first storm of the season (inthe eastern Pacific) was formed on late May and the higher stormactivity occurred between July and September.

The increase in accumulation of sponge fragments on the beachrecorded in the March–April period (2007 and 2010) coincided with aseasonal change in wind direction (from NW to WSW) and speed,which was accompanied by an intense swell period that generated ahigh resuspension/sedimentation rate. This was also the period withthe highest frequency of tides below the lower low water (betweenMarch and April). Thus, it is possible that the combination betweenhigh swell and very low tides had a greater influence on the shallowsubtidal benthic community as well as on sediment resuspension. Inthis sense, Orr et al. (2005) suggested that the total regional windvelocity, regardless of direction, may produce wind waves, which, inturn, generate shear and stress forces on subtidal organisms, causingthem to be dislodged.

Sudden changes in wind direction have also been documentedduring the “spring transition” on the northern California continentalshelf (Lentz, 1987; Strub et al., 1987). This change typically occurs

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over a period of a few days, in March or early April, and had beenassociated with a sudden seasonal change in the large-scaleatmospheric pressure field over the North Pacific (Huyer et al.,1979; Lentz, 1987). At Mazatlan Bay, such changes in wind directionalso influence the direction of propagation of the waves (from NW toWSW) and the beach profile (pers. obs.). As it has been previouslydocumented (McDonald et al., 2003), these short-term changes inprevailing water flow direction may cause dislodgement, breakage orburial by sediment of many sessile organisms.

As was mentioned above, the intense swell observed in April wasaccompanied by a high sediment deposition and according tohistorical data in the study area, it almost always occurs betweenMarch and May (Carballo et al., 2008; Carballo and Ávila, 2004). Also,during this time particles collected in the sediment traps are mostlycomposed of medium-coarse sand (Carballo et al., 2008), suggesting avery high energy in the environment. Thus, the combined effect ofwater motion and resuspended material likely abraded and removedsponges from the rock surfaces.

In addition to the potential effects of changes inwind direction andin the wave pattern, the multiple regression analysis showed a model,which described a significant relationship between the biomass ofsponges on the beach and three environmental variables (wind speed,resuspension/sedimentation rate and frequency of very low tides).Thismodel explained 68.4% of the variability in sponge biomass. Thesefindings agree with previous studies that suggest that dislodgementand subsequent depositions of organisms on the beach may be due toa combination of environmental and meteorological variables ratherthan a single variable (Denny and Gaylord, 2002; Griffiths et al., 1983;Kemp, 1986; Ochieng and Erftemeijer, 1999). This kind of analysis hasalso been used to demonstrate that wind stress combined with tidalheight, shelter indices and water temperature explained about 90% ofthe variance in total benthic secondary production in shallow soft-sediment communities (Emerson, 1989).

Finally, the sponge biomass data obtained in summer 2005 and2006, which represent the accumulated biomass until that moment,support the most recent findings, since they were similar to thoseobtained for the same period in 2007 and 2010. This indicates that thedeposition of benthic organisms in this beach is an event that occursevery year, associated to the transition from spring to summer.

5. Conclusions

The present study provides evidence that depositions of sponges andother organisms on the beach at Mazatlan Bay occur throughout theyear, but the highest biomasswasdetected between springand summer.The largest deposition of sponges was attributed to a combination ofenvironmental and meteorological variables such as changes in winddirection (from NW to WSW), high swell conditions, changes in wavepattern, very low tides andahigh resuspension/sedimentation. Themostaffected sponge species were characterised by branching and massivegrowth forms, while encrusting species and thosewith a high content ofspongin in its structure were apparently less affected by these events(Fenner, 1991; Wulff, 1995). Although losses of biomass caused bychanges in local weather and environmental conditions may not becomparable to those caused by storms or hurricanes, they are alsoimportant as they occur throughout the year andmay directly influencethe life-history of many species. Seasonal changes in wind directioncombined with the effect of low tides and resuspension of sedimentscould serve to predict fragmentation/detachment events of benthicorganisms in shallow sublittoral areas.

Acknowledgements

We thank Clara Ramírez Jáuregui for help with the literature,German Ramírez Reséndiz and Carlos Suárez for their computerassistance, and Arturo Nuñez Pastén and Juan Toto Fiscal for their

assistance in the field samplings. The study was funded by theInstituto de Ciencias del Mar y Limnología (UNAM).

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