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Effects of Propagule Settlement Density and Adult Canopy on Survival of Recruits of Sargassum Spp. (Sargassaceae: Phaeophyta)

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Effects of Propagule Settlement Density and Adult Canopy on Survival of Recruits of Sargassum Spp. (Sargassaceae: Phaeophyta) MARINE ECOLOGY PROGRESS SERIES Vol. 103: 129-140, 1994 Published January 6 Mar. Ecol. Prog. Ser. Effects of propagule settlement density and adult canopy on survival of recruits of Sargassum spp. (Sargassaceae: Phaeophyta) Gary A. Kendrick* Department of Botany, University of Western Australia. Nedlands, Western Australia 6009, Australia ABSTRACT The effects of increased densities of settled propagules and the presence of adults on sur- vlval of recruits were lnvest~gatedto determine whether patterns of propagule settlement would pel- slst and be expressed as distribution of recruits for Sargassum spln~lllgeru~nS d~stlchun~and S poda- canthum in a subtidal mixed specles bed at Rottnest Island, Western Australia The effect of highei densities of settled propagules (recruit-recruit lnteractions) was studied uslng l~mestonesettlement plates seeded with known densities of propagules Density-dependent n~ortal~tyoccurred at settlement densities < 105 propagules m A large component of recruit mortality was density independent (57 "/;> of total mortality) Vanabllity in recruit survival at any one settlement density was also great Growth of recruits was negatively density dependent Two distinct sizes of recru~tswere observed (< 10 and > 10 mm in height) The effect of adult canopy (recruit-adult interactions) on recruit sur\ilval was stud- ied using a cleanng expenment where adult thalli were removed from 4 m2areas For the flrst 6 mo the presence of adults had llttle effect on recruit survival Aftei 6 mo, the presence of adults leduced recruit suivival3-fold and surv~valwas highly vanable either under canopies or in cleared areas Distr~butions of dispersed propagules can persist as distnbutions of adults ivlthin beds of Sargassum spp at Rottnest Island More likely this would happen where patches within the bed are recolonized or where the bed is extended at ~tsedges, because orders of magn~tudedifferences in density of settled propagules would occur in these areas over dlstanccs of meters Recruit survival at densities < lo5 ind m-' are equally influenced by density-dependent and density-independent processes Presence of adult canopy reduces survival of recruits > 6 mo old, but the amount that survi\~alis reduced by canopy is not within the same range as differences in denslty of propagule settlement The causes of high density- independent mortality and its effect on patterns of propagule settlement need to be studied further KEY WORDS Sargassun~sp. Recruitment . Intraspecific competition INTRODUCTION lnteractions with adult thalli, and mortality from graz- ing, disturbance, and physiological factors. Kendrick & Dispersal distances of propagules of marine algae Walker (1991) observed that 98 % of dispersed propag- have usually been inferred from distributions of re- ules of Sargassum spinuligerum Sonder settled within cruits (Anderson & North 1966, Hruby & Norton 1979, 1 m of their source. In a similar species, S. muticurn Deysher & Norton 1982, Kennelly & Larkum 1983, (Yendo) Fensholt, the greatest densities of recruits oc- Hoffmann & Ugarte 1985, Reed et al. 1988).The distri- curred within 1 to 3 m of reproductive thalli (Deysher & bution of recruits, however, includes factors other than Norton 1982). Could these patterns of recruitment in variable dispersal (Harper 1977, Connell 1985). Such Sargassum spp. truly reflect patterns of dispersal? To factors are density-dependent mortality in recruits, further investigate the relationship between dispersal of propagules and distribution of recruits, I studied the 'Present address. CSIRO Marine Laboratories, PO Box 20, post-settlement survival of recruits. The experiments North Beach, Western Australia 6020. Australia presented here address the effect of propagule settle- @ Inter-Research 1994 Mar. Ecol. Prog. Ser. 103: 129-140, 1994 ment dens~tyon recruit mortality (recruit-recruit inter- Santellces & Ojeda 1984, Reed 1990) in n/lacrocystls actions), and the effect of a canopy of Sargassum spp. spp. populations nave shown that adult-juvenile inter- on survival of recruits (adult-recruit interactions). actions are major factors in determining patterns of Although there are many other processes that affect survival in kelp populations. The Sargassum species survival of young recruits, including grazing, distur- from Rottnest Island, Western Australia, are not bance, and physiological requirements, these density- expected to respond differently although this study related recruit-recruit and adult-recruit interactions hopes to document when and to what degree adult were chosen as they were most likely to respond in a thalli influence recruit survival. non-random way to increases in the density of dis- This paper addresses how settlement patterns result- persed and settled propagules. ing from dispersal can be modified during recruitment The effect of density-dependent interactions on the for Sargassum spinuligerum Sonder, S. distichum Son- structure of populations has been studied extensively der and S. podacanthum Sonder at Rottnest Island. The in terrestrial plants (Harper 1977) but not in rcarine effect of post-settlement processes on patterns of dis- algae. The few studies of density dependence in persal were studied at both recruit-recruit and adult- marine algae, mainly in kelps, have resulted in a range recruit levels of interaction by investigating (1) the of conclusions including negative and positive density- relationship between density of settled propagules and dependent and density-independent growth, repro- mortality of recruits of Sargassum and (2)the influence duction and survival. Black (1974) found negative of existing Sargassum thalli, both as canopy and as density-dependent survival and growth in intertidal juveniles, on settlement and survival of recruits. populations of the kelp Egregia menziesii. In subtidal stands of Macrocystis pyrifera and Pterygophora cali- fornica, increased density inhibited the growth of juve- MATERIALS AND METHODS niles (Dean et al. 1989, Reed 1990) and the amount of reproduction (Reed 1987).Sporophyte production was Two experiments were conducted at Nancy Cove, also found to decrease with increased gametophyte Rottnest Island, Western Australia (32' 02' S, 115' 28' E) densities (Reed et al. 1991).The southern hemisphere within a 6 m deep Sargassum spp. bed. The experiments kelp Ecklon~aradiata and Sargassurn sinclairii were addressed the effect of density of settled propagules reported to show positive density-dependent growth (recruit-recruit experiment) and the effect of an existing (Schiel & Choat 1980). Increased density of thalli of canopy of Sargassum spp. (recruit-adult experiment) Ascophyllurn nodosum from mixed cohorts of individu- on recruit survival. The bed was extensive (5000 m2) als from different locations had no effect on their and contained the species Sargassum decurrens (R. biomass (Cousens & Hutchings 1983) Cousens & Brown ex Turner) C. Agardh, S. tristichum Greville & C. Hutchings (1983) concluded that the variations in Agardh ex Sonder, S. spinuligerum Soncler, S. distichum density-dependence of Ascophyllum, Chordaria, Sac- Sonder and S. podacanthum Sonder growing together chorhiza, Sargassum and Ech-lonia were due to densi- (Kendrick 1993).Depth varied by less than 0.5 m within ties below those of the boundary condition defined the bed and the substratum was a friable low relief lime- by the -3/2 power, or self-thinning law (Harper 1977). stone platform. Density-dependent processes can be important in Sargassum in this paper is used to denote the species modifying patterns of settlement, as has been shown S. spinuligerum, S. distichum and S. podacanthum. It for marine barnacles (Connell 1985, Suther1an.d 1990, was not possible to differentiate between these i.ndl- Menge 1991). The influence of density-dependent vidual species as recruits. Adult thalli of S. spin- interactions on the survival of Sargassum spp. recruits uligerum, S. distichum and S. podacanthum are also iviI1 be studied over a range of settlement densities. nlorpholoyicdlly simild~dnd cdnnol be identified from For marine algae, a canopy of adults can also have each other without reproductive structures (Kendrick an effect on the recruitment and growth of juveniles 1993).Propagules of S. spinuligerum were used exclu- (Black 1974, Velimirov & Griffiths 1979, Dayton et al. sively to seed plates in the experiment studying 1984, Reed & Foster 1984, Santelices & Ojeda 1984, recruit-recruit interactions because large numbers of Paula & Eston 1987, Chapman 1989, de Eston & Bussab reproductive thalli of S. spinuligerum were available at 1990). Many of these studies have concluded that that time. S. spinuligerum also accounted for 63.5 X, of adults restrict growth and, survival of recruits. Canopy all reproductive thalli of S. spinuligerum, S. d~stichum removal experiments in Sargassum spp. beds have and S. podacanthum recorded during the period of the resulted in very high densities of Sargassum spp. experiment (Kendrick 1993). Recruits of all 3 species recruiting into areas without canopies (Paula & Eston were measured In the recruit-adult experiment. 1987). Surveys (Dean et al. 1989) and canopy thinning Settlement density and recruit mortality. The effect experiments (Dayton et al. 1984, Reed & Foster 1984, of settlement density on recruit mortality was assessed Kendrick: Propag~~lesettlement and survivdl of Sargassum spp. 131 by seeding settlement plates with different densities of Once propagules on the limestone blocks were Sargassum propagules and placing
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