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  • Introduction

    It has been argued that approaches which scale eco-logical performance using simple morphological vari-ables (e.g. Littler 1980; Poorter & Remkes 1990;Steneck & Dethier 1994; Nielsen et al. 1996) will pro-vide more useful generalizations than thoseapproaches which rely on defining numerous species-specific traits (Hay 1994). If this is the case, it shouldbe possible to construct models based on a limitednumber of assumptions that are capable of makinggeneral predictions about ecological phenomena.

    Vertical zonation patterns formed by canopy-forming fucoid algae on rocky shores have a longhistory of investigation. On sheltered shores in thenorth-east Atlantic, Pelvetia canaliculata (L.) Dcneet Thur. is typically considered to be the specieswhich is distributed highest on the shore, followedby Fucus spiralis L., Ascophyllum nodosum (L.) leJol. and Fucus serratus L. as the mean level of lowwater at spring tides is approached (e.g. Schonbeck& Norton 1978). Several hypotheses have beenadvanced to explain algal zonation, based on desic-cation tolerance (e.g. Zaneveld 1937; Schonbeck &

    Norton 1978; Dring & Brown 1982), competition(e.g. Lubchenco 1980; Hawkins & Hartnoll 1985;Chapman 1990) and interactions with grazers (e.g.Underwood 1980). None of the proposed mecha-nisms has been generally accepted (Chapman 1995).The search for generality is complicated by the factthat experimental approaches have often tested foreffects such as competition as a phenomenon with-out revealing anything of the underlying mecha-nisms (Tilman 1987). The demonstration ofcompetition as a phenomenon can say little aboutwhen a boundary between two species will form,where the boundary will lie or how dynamic theboundary might be.

    This paper examines whether differences in themorphology of canopy-forming algae are sufficient tocause vertical zonation on rocky shores and considersgeneral predictions about the nature of zonationpatterns that arise from such an approach. The growthrates of different algal species were based on morpho-logical parameters derived from harvested individualsand desiccation tolerances taken from the literature.Algal photosynthesis was modelled in air and in atidal water column of variable depth.

    FunctionalEcology 199812, 259269

    1998 BritishEcological Society

    ORIGINAL ARTICLE OA 000 EN

    The establishment of fucoid zonation on algal-dominated rocky shores: hypotheses derived from asimulation modelM. P. JOHNSON,* S. J. HAWKINS,* R. G. HARTNOLL and T. A. NORTON*School of Biological Sciences, University of Southampton, Biomedical Sciences Building, Bassett CrescentEast, Southampton SO16 7PX and Port Erin Marine Laboratory, Port Erin, Isle of Man IM9 6JA, UK

    Summary

    1. A model was developed for the growth of intertidal algae with photosynthesissimulated both in air and in a tidal water column. Morphological data on dry massper unit area and lengtharea relationships were used to separate the growth ofdifferent fucoid species. The relative growth rate of fronds at any height on the shoredepended on a trade-off between net photosynthetic performance and tolerance todesiccation.2. The simulated zonation patterns and growth rates were consistent with thoseobserved previously for Fucus spp. and Pelvetia canaliculata.3. The simulated growth of Ascophyllum nodosum was always slower than for the otherspecies. This species did not form its characteristic distribution zones in simulationswithout including further processes in the model. However, Ascophyllum collectedfrom the field could be separated into upper and lower shore morphologies whichformed separate zones when they were simulated in competition with each other.4. Several hypotheses were proposed concerning the relative locations and sharpnessof interspecies boundaries on the shore. Zonation patterns were relatively insensitiveto changes in most model parameters except the desiccation rate.

    Key-words: Canopy, desiccation, relative growth rates, thallus specific mass

    Functional Ecology (1998) 12, 259269

    259

  • Materials and methodsDESCRIPTION OF THE MODEL

    Photosynthesis has been frequently modelled in cropsusing an exponential attenuation of photosyntheticallyactive radiation (PAR, 400700 nm) in the canopy andphotosynthesis-irradiance relationships (P vs I curves)for individual leaves (Johnson & Thornley 1984;Loomis & Connor 1992). Given information on respi-ration, the net carbon fixation of a plant in a canopycan be calculated. By altering the description of pho-tosynthesis to take account of tidal fluctuations inwater column depth, a similar approach can be appliedto algal canopies.

    A simplifying assumption of no underlying differ-ence between gross photosynthetic performance perquanta absorbed in different fucoids was adopted (seeMarkager 1993). Fucoids are thought to absorb closeto 90% of incoming light, regardless of species(Norton 1991). From similar gross photosyntheticyields per quanta absorbed, the parameters which canbe used to separate the net carbon fixation of differentspecies are thallus-specific carbon and carbon-specificrespiration rate (Markager & Sand-Jensen 1992). If itis assumed that different species of fucoids will havesimilar carbon specific respiration rates, the main dis-criminant of net photosynthesis in fronds becomes thethallus-specific carbon content (g C m2). To facilitatethe use of field data a standard conversion betweencarbon and dry mass was used. Hence the net photo-synthesis in different fronds was solely dependent onthe thallus-specific mass (TSM, g m2).

    Profiles of light attenuation in submerged algalcanopies show exponential declines in irradiance withdepth (Gerard 1984; Cousens 1985; Holbrook, Denny& Koehl 1991). Strictly speaking this decline in irradi-ance with depth does not occur evenly across the PARband. However, changes in light quality with depth areprobably unimportant as it is likely that fucoids have aflat action spectra for photosynthesis, similar to thatreported for Laminaria saccharina by Dring (1982). Insubmerged plants the attenuation owing to the watercolumn must also be taken into account. Assumingminimal surface reflection of incident irradiance:

    Iz = Ice(l z k TAI), eqn 1

    in which Iz is the irradiance at depth z (m mol photonm2 s1), Ic is the irradiance at the top of the canopy(either corrected for overlying water if the canopy isfully submerged or equal to the incident irradiancewhere the canopy reaches the water surface, m molphoton m2 s1), z is depth below the water surface(m), l is the attenuation coefficient of sea water (m1),k is the attenuation owing to algal canopy (TAI1,fronds assumed to be vertical while submerged) andTAI is the cumulative thallus area index from top of thecanopy to depth z (m2 thallus m2 sea bed).

    The rise and fall of the tide means that the atten-uation owing to water and the canopy are effectively

    calculated with respect to different reference points.Attenuation in the water column is calculated withrespect to depth from the water surface, whereascanopy attenuation is calculated with respect to theinterval between the current position and the top of thecanopy when fully extended. The frond surface areaavailable for photosynthesis at any depth depends onthe species-specific growth form.

    For a discrete approximation of the light availablefor photosynthesis at different depths, fronds weredivided into segments 001 m in length with areas persegment calculated using allometric relationships.This allowed a discrete irradiance profile to be con-structed with the light in 001 m steps becoming afunction of the depth of overlying water combinedwith the sum of areas in segments above and includingthe present one. The discrete version of eqn 1becomes:

    d max

    Iz(s) = Ice(l (s05)/100 k Aseg(n)N k Aseg(n)N) ,

    d + 1 s d + 1 eqn 2

    in which Iz (s) is the irradiance in segment s (m mol pho-ton m2 s1), s is segment number from the surface, dis the depth of water column rounded to the nearest001 m (if d > max then d = max), max is the totalnumber of segments in the canopy, Aseg (n) is the frondarea in segment n (1 n max) (m2) and N is thefrond density (m2). Equation 2 deals with situationswhen the height of the water column exceeds thecanopy height as well as when canopy height exceedswater depth.

    With irradiance values calculated in 001 m incre-ments, Smiths (1936) equation can be used to esti-mate the instantaneous gross photosynthesis for asingle frond in the canopy:

    d Pmaxa Iz(n)Psng = Aseg(d + 1 n) +

    1 Pmax2 + ( a Iz(n))2

    Pmaxa Iz(0) max Aseg(n) , eqn 3 Pmax2 + ( a Iz(0))2 d + 1

    in which Psng is the instantaneous gross photosynthe-sis of a single frond (m mol C s1), Pmax is the maxi-mum gross photosynthetic rate (m mol C m2 thalluss1) and a is the initial slope of P vs I curve [m mol C(m mol photon)1].

    When plants are exposed to the air by a falling tidethere is some evidence that the photosynthetic rate canbe enhanced (e.g. Johnson et al. 1974; Johnston &Raven 1986; Madsen & Maberly 1990). Exposure inair, however, causes algae to desiccate and eventuallyphotosynthesis ceases as individuals dry out. Thewater content of exposed fronds was modelled as anexponential function of the time in air (Maberly &Madsen 1990):

    W = 100e(DE) eqn 4

    260M. P. Johnsonet al.

    1998 BritishEcological Society,Functional Ecology,12, 259269

  • in which W is the remaining water content of the frond(%), D is the desiccation rate (s1) and E is the timethat the frond has been exposed in air (s).

    The initial enhancement of photosynthesis causedby exposure in air followed by the decline owing toexcessive desiccation was modelled for all speciesusing the empirical equation presented for F. spiralisby Madsen &

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