Hydrobiologia 260/261: 345-351, 1993.A. R. O. Chapman, M. T. Brown & M. Lahaye (eds), Fourteenth International Seaweed Symposium.© 1993 Kluwer Academic Publishers. Printed in Belgium.

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Interaction mechanisms between Gracilaria chilensis (Rhodophyta) andepiphytes

Alejandro H. Buschmann & Patricio G6mezInstituto Profesional de Osorno, Depto. Acuicultura y Recursos Acuticos, Casilla 933, Osorno, Chile

Key words: Gracilaria farming, epiphytism management, light nutrients, exudates, weight addition, Chile

Abstract

Epiphytes can have a considerable effect on Gracilaria production, and Ulva is one of the commonestalgal species identified as an epiphyte, reaching loads of 60% (g of epiphytes per g of Gracilaria) in theintertidal cultures of southern Chile. This study evaluates the relative importance of light reduction,addition of weight to the host thalli and nutrient depletion, as mechanisms determining the interactioneffects of Ulva epiphytes on Gracilaria cultivation. Using field experiments undertaken during the mainGracilaria growth season (spring), we evaluate the mechanisms of epiphyte-host algae interaction bymanipulating artificial epiphytes. The results indicate that Ulva can significantly depress Gracilariabiomass production and that the addition of weight to the host algae and the consequent dislodgementincrease, appear to be the main mechanisms involved in the Ulva-Gracilaria interaction. However, thelight reduction caused by the epiphytes can also partially explain the reduction in Gracilaria production.Nutrients depletion would not appear to fully account for the detrimental effects of Ulva over Gracilariain intertidal farming areas of southern Chile.

Introduction

Gracilaria farming is an activity considered to beof economic and social importance in Chile (San-telices & Doty, 1989). However, there is still aneed to improve the production technology inorder to optimize productivity and lower produc-tion costs. Plague organisms can, to a large ex-tent, determine the levels of production ofGracilaria beds and need to be studied if presentlevels of production are to be sustained in the nextyears. Algal epiphytes can significantly affectGracilaria production in Chile (Pizarro, 1986;Kuschel & Buschmann, 1991). A knowledge ofthe mechanisms involved in the epiphyte-hostalgae interaction may help to improve the epi-

phyte management capacity of farmers. Epiphytescan reduce the amount of light reaching the hostalgae, compete for nutrients and gases dissolvedin the seawater, add weight to the host algae pro-moting the detachment of the entire, or part ofthe, host algae (e.g. Enright, 1979; Sand-Jensenet al., 1985; Kuschel & Buschmann, 1991) and/orby releasing exudates that can promote, or bedetrimental to, the host algae (see Harlin, 1987 fora review). Green algae can reach loads of 0.6 g ofepiphytes per g of Gracilaria in intertidal beds andtheir effects and interfering mechanisms are un-known. Using artificial algae, this study aims toestablish the importance of several possiblemechanisms of interference between the epiphyteUlva sp and Gracilaria in the field, such as light

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reduction, nutrients depletion, and addition ofweight promoting the detachment of the hostalgae.

Materials and methods.

Study siteThis study was conducted in Metri Bay(41 36' S: 72 ° 42' W), 30 km southeast of Pu-erto Montt in southern Chile. Metri Bay is awave-protected area with tidal amplitudes of 7 m,leaving extensive areas available for Gracilariachilensis Bird, McLachlan & Oliveira cultivation.The experiments were undertaken in an area usedfor Gracilaria cultivation between 0.6 to 0.8 mabove the zero tidal level.

Experimental designAll the experiments were performed during theperiod of maximum Gracilaria growth (spring sea-son). Three independent experiments were car-ried out to test the effects of epiphytes, assessingthe importance of light, nutrients, and addition ofweight to the host algae between the green algaeUlva sp and Gracilaria. Three Gracilaria bundleswere tied to each sand-filled polyethylene tube(Fig. 1 a), which is a common method for anchor-ing the algae to the sandy substratum. Every ex-periment involved the installation of six plastictubes, two per square meter. The experimentswere installed at the begining of November 1991for an initial experimental period. After 1 monthin the field, the algal bundles were removed andtransported to the laboratory, where the wetweight of each Gracilaria bundle and Ulva wasdetermined separately with a portable digital bal-ance (1 g accuracy). Subsequently, the experi-ments were repeated for a second test period ofone month during December 1991, following thesame procedure as described for the first experi-

mental period. The results were then calculatedas wet weight of Gracilaria for each individualalgal bundle and separated by experimental pe-riod.

The first experiment tested the effect of lightintensity and epiphyte load on Gracilaria growth.In order to determine the effect of the epiphyte(Ulva sp), Gracilaria with and without epiphyteswere attached to the plastic tubes (Figs lc and d).In each experimental plot (1 m2 each) two tubes,one with and another without epiphytisedGracilaria, were installed. The same experimentwas repeated, but installing a roof with a greenplastic mesh (Fig. lb) which decreased the lightthat reached the host algae by 50 + 5 %. This lightreduction is fairly similiar to that produced byUlva over Gracilaria, although the effect is ho-mogenous in comparison to the effect of the realepiphytes. Observations made by diving duringhigh tide indicated that grazing was not importantin any treatment and the currents were not re-duced under the roofs since the algae showed thesame intensity of movements as without the roofs.This experiment was analysed separately for eachexperimental period, using a two factor analysisof variance (ANOVA), following Sokal & Rohlf(1979). Logarithmic transformation of the dataassured homogeneity of variances.

The second experiment tested the effect ofweight addition to the host algae and the result-ing enhancement of the Gracilaria biomass losses.To test this factor, bundles of Gracilaria free ofepiphytes, epiphytised by Ulva, and with artificialepiphytes, were installed in the field (Figs c, dand e). The artificial epiphytes were lead weightsfixed with a nylon string to the host algae, whichadded weight in the same proportion as the epi-phytes (0.6 to 0.7 g per g of Gracilaria). Theseartificial epiphytes added weight, but did not in-terfere with light or nutrients. In each plastic tube,one bundle of algae free of epiphytes, one epiphy-

Fig. 1. Diagram representing the different treatments used in this study. (A) Sand filled polyethylene tubes used for anchoringGracilaria in sandy substratum; (B) Sun roof intercepting 50% of the radiation; (C) Gracilaria bundles free of epiphytes;(D) Gracilaria epiphytised by Ulva sp; (E) Gracilaria with additional lead weights tied with a nylon monofilament and (F) Gracilariawith black and transparent plastic stripes simulating Ulva epiphytes adding weight, not affecting the availability of nutrients andreducing (black stripes) and not reducing the light (transparent stripes) reaching the host algae.

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tised with Ulva and another with the artificial ep-iphytes, was installed. Two plastic tubes were in-stalled in each plot and this experiment wasundertaken in triplicate and were distributed atrandom. The experiment was analysed using aOne Way ANOVA, and an a posteriori test (Tukeytest; Sokal & Rohlf, 1979) to detect differencesbetween pairs of means.

The third experiment was installed to test theeffect of light and the possible importance of nu-trients, using artificial epiphytes. Gracilaria free ofepiphytes, epiphytised by Ulva, and with artificialepiphytes consisting of transparent and blackplastic strips, were used (Figs c, d and f). Ifnutrients are important, at the end of a month inthe field we would expect the biomass of theGracilaria bundles to be significantly lower in thecase of the real epiphytes (nutrients + weight +light effects) as compared to the black artificialepiphytes (weight + light effects) and the trans-parent artificial epiphytes (only weight effects).The experiment was controlled as in the aboveexperiments and the data was statistically analy-sed using a One-Way ANOVA (Sokal & Rohlf,1979), considering the requirements of homoge-neity of the variances and normality of the data.An a posteriori test was used (Tukey test, Sokal &Rohlf, 1979) to detect differences between means.

Results

The Ulva epiphytised Gracilaria biomass in thefirst (October-November) experiment was signif-icantly lower (F= 88.50; P< <0.01) than the un-epiphytised Gracilaria (Fig. 2). During the sameperiod, the light reduction under the roofs alsoproduced a significant (F= 17.23; P<0.01) re-duction in the Gracilaria biomass. However, nosignificant (F=0.914; P>0.05) interaction be-tween both factors (epiphytism and light) wasdetected. During the second experimental period(November-December) the Ulva epiphytism con-tinued to be the main (F= 153.28; P< <0.01)factor reducing the Gracilaria biomass (Fig. 2).During this period, the light factor did not signif-icantly affect (F = 3.363; P> 0.05) the biomass of

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Fig. 2. Mean ( 1 S.D.; n = 9) Gracilaria bundles wet weightof non epiphytised (G) and Ulva epiphytised (EG) Gracilariabundles during both experimental months under a sun roof(50% of light transmittance) and under the direct solar radi-ation, in both experimental months.

Gracilaria, but the interaction of Ulva epiphytismand light was, in this case, significant (F= 7.256;P<0.01). However, the effect of light in both ex-perimental periods, was not as strong as the ef-fect of Ulva indicating that light interference byUlva does not provide an adequate explanationfor the epiphyte effects.

The second experiment evaluated the effect ofthe addition of weight by the epiphytes (compar-ing artificial and natural Ulva fronds) (Fig. 3). Theresults obtained indicate, again, that Ulva has asignificant effect on the biomass of the Gracilariabundles in the first (November: F= 39.26;P< <0.01) and in the second experimental pe-riod (December: F= 71.77; P< < 0.01). An apos-teriori test indicates that the epiphyte freeGracilaria bundles had significantly higher weightvalues than the experimentally epiphytised bun-dles. The relative importance of the weight addi-tion and Ulva effect varies in time. During No-vember, the effect of Ulva was significantlystronger (Tukey test; P<0.05) than the weightaddition effects alone on the host alga biomass.However, during December, the effect of the

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Fig. 3. Mean ( 1 S.D.; n = 9) Gracilaria bundles wet weightof non-epiphytised (G), Ulva epiphytised (EG) Gracilaria andaddition of lead weights (W), in both experimental months.

weight addition was significantly stronger (Tukeytest; P< 0.05) than the detrimental effects of Ulva.

The third experiment tested the effect of lightreduction and nutrient depletion by epiphytes onthe host alga (Fig. 4). The analysis of varianceshowed that the treatments had a significant ef-fect both during the first experimental period(October-November: F= 60.38; P< <0.01) andduring the second experimental period (F = 50.96;P< < 0.01). An a posteriori test indicated that forboth experimental periods, the Gracilaria bundlesfree of Ulva and artificial epiphytes had signifi-cantly higher weights (November: Tukey test;P<0.05) (December: Tukey test; P<0.05). Thetreatments using transparent plastic stripes as ar-tificial epiphytes and authentic Ulva epiphytes,had the same effect on the Gracilaria weight(Tukey test; P<0.05) in both experimental peri-ods. The black stripes used as artificial epiphyteshad a significantly (Tukey test; P<0.05) strongereffect than the other treatments on the weight ofthe Gracilaria bundles.

G EG T B G EG T B

NOVEMBER DECEMBERFig. 4. Mean ( 1 S.D.; n = 6) Gracilaria bundles wet weightof non-epiphytised (G), Ulva epiphyted (EG) and with black(B) and transparent (T) artificial epiphytes.

Discussion

An Ulva epiphytic load of 60%, as was previouslyreported in the case of the filamentous algae Ulva,Giffordia and Ceramiales (Buschmann & Kus-chel, 1988; Kuschel & Buschmann, 1991; Bus-chmann et al., 1992) can significantly depress theGracilaria biomass production during the mainproduction season in southern Chile. Light canalso be an important factor directly affecting theGracilaria biomass production, or having an in-teraction effect with weight addition of the epi-phytes on the host algae, depending on the monththat the experiment was performed. However,even with a light reduction of 50%, the reductionin biomass production over a period of one monthmay not be as pronounced as that produced bythe epiphyte effect (Fig. 2). The effect of weightaddition of epiphytes appears to account for theGracilaria biomass losses in the field (Fig. 3). In-tertidal systems appear to be particularly suscep-tible to the removal of biomass by wave action,especially during low tide (Buschmann et al.,1990). Previous evidence on the effect of the fil-

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amentous brown epiphyte Giffordia in the samearea, (Kuschel & Buschmann, 1991) further in-dicates that the addition of weight to the host algawill enhance the dislodgement of the Gracilariathalli from the sandy substratum. These experi-ments also suggest that Ulva did not depress thegrowth of Gracilaria to the same extent as theblack plastic stripes and that nutrients do notappear to be important during this experimentalperiod, as the intensity of the effects produced byUlva (light + weight + nutrients effects) was notgreater than that observed with both types of ar-tificial epiphytes.

Experiments like those described in this studymust be applied with caution as the introductionof artefacts can be misleading if not applied cor-rectly. The green plastic mesh used as a light fil-ter in this study reduced the light that reachedGracilaria by 50%, is similar to the effect pro-duced by the real Ulva epiphytes. The addition ofweight by the epiphytes to the host algae, is notidentical to that achieved with our artificialweights, even though the weight ratios (g of epi-phytes per g of Gracilaria and g of lead weightsper g of Gracilaria) are the same, because thenumber of attaching points of the epiphytes andthe real force exerted by the epiphytes are notnecessarily the same. However, as we addedweight with two different kinds of artificial epi-phytes (the lead weights and the transparent stripsthat added only weight to the host algae) withdifferent morphologies, that produced the sameresult, the suggestion that the weight addition isan important factor in this kind of environmen-tal systems appear plausible.

Furthermore, epiphytes attract mesograzers(e.g. Brawley & Fei, 1987) that could have anindirect positive effect by consuming the epiphytesor a negative effect if they also consume the hostalgae. Observations did indicate that the numberof mesoherbivores was very low during these ex-periments both on Ulva and Gracilaria and thatGracilaria did not present grazing marks duringthe experimental period. Also, the use of roofscan influence the numbers of herbivores if notapplied carefully. However, during this study, carewas taken to remove the gastropod herbivores

(Tegula atra Lesson) by hand during low tidewhich was an efficient method.

There are many indications that exudates frommacroalgae supress the growth of other algae(Harlin, 1987), and it has been reported that inbialgal cultures, Ulva lactuca Linnaeus, can de-press the growth of microalgae (Thorne & Harlin,1977). However, further studies are required toclarify this point that could also affect Gracilaria.

The results found in this study indicate that itis necessary to harvest Gracilaria during the pe-riods when wave action can be an important en-vironmental factor for decreasing the algal losses.The maintenance of the stocking biomass appearsto be the most important factor for ensuring ac-ceptable levels of productivity in time in the in-tertidal farms of southern Chile. For this reasonepiphyte abundance must be controlled. The har-vesting program is an alternative for reducing algallosses by keeping the algae short and thus mini-mizing the losses due to wave action.

Acknowledgements

This study was supported by a IFS-Sweden (A-1600/1) and FONDECYT-Chile (0888-90)Grants to the first author. Both authors appreci-ate the help of M. Werkmeister and C. Retamalesfor drawing the figures, S. Angus for reviewingthe English, Daniel L6pez, Juan Correa and ananonymous reviewer for commenting on themanuscript. The assistance in the field given byN. Pardo, C. Vargas, O. Mora, F. Contreras andA. Gutierrez is also acknowledged.

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