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M236p089.Pdf MARINE ECOLOGY PROGRESS SERIES Vol. 236: 89–97, 2002 Published July 3 Mar Ecol Prog Ser Depth-acclimation of photosynthesis, morphology and demography of Posidonia oceanica and Cymodocea nodosa in the Spanish Mediterranean Sea Birgit Olesen1,*, Susana Enríquez2, Carlos M. Duarte3, Kaj Sand-Jensen4 1Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, 8240 Riskov, Denmark 2Unidad Académia de Puerto Morelos, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Apto. Postal 1152, 77500 Cancún, Quintana Roo, Mexico 3Instituto Mediterráno de Estudios Avanzados (SCIC-UIB), Miquel Marqués 21, 07190 Esporles, Islas Baleares, Spain 4Freshwater Biological Laboratory, University of Copenhagen, 51 Helsingørsgade, 3400 Hillerød, Denmark ABSTRACT: Depth-related changes in population structure, biomass partitioning and photosynthesis were studied in populations of Cymodocea nodosa and Posidonia oceanica on the NE Spanish coast. The population structure of both species changed much more with depth than leaf morphology and physiology. Leaf biomass declined 5- to 7-fold along the depth gradient reducing self-shading within the canopy, whereas the leaf area per unit leaf biomass and the photosynthesis-light response varied less than 1.5-fold among depths. Moreover, C. nodosa developed a greater proportion of leaves relative to rhizomes and roots at greater depths, thereby promoting the balance between photosyn- thesis and respiration in the shoots. C. nodosa, being a potentially fast-growing species compared to P. oceanica, had higher maximum photosynthetic and respiration rates as well as light compensation points for photosynthesis. Photosynthetic efficiency at low light, however, was almost the same for the 2 species as suggested by the relatively small differences in mass-specific light absorption. Only C. nodosa acclimated physiologically to depth as light-use efficiency increased, and light compensation point declined significantly from shallow to deep water. P. oceanica, however, possessed low respira- tion rates and slightly lower light compensation points values than C. nodosa throughout the depth range. Shoot mortality and recruitment rates were unaffected by rooting depth. C. nodosa stand experienced fast shoot turnover compared to P. oceanica, and shoot longevity of the former species decreased significantly with depth, suggesting higher risk of patch mortality at the depth limit. In contrast, P. oceanica shoot longevity was highest at great depths. Overall, these species differences in leaf metabolism and shoot dynamics suggest that C. nodosa responds faster to changing light con- ditions, whereas P. oceanica is able to survive longer at low irradiance due to low growth and respi- ratory maintenance rates. KEY WORDS: Seagrasses · Biomass partitioning · Shoot demography · Leaf production · Photosynthetic light response Resale or republication not permitted without written consent of the publisher INTRODUCTION constraints on seasonal biomass development (Sand- Jensen 1975, Marbà et al. 1996) and depth distribu- Light is often considered the principal factor regu- tion (Dennison 1987, Duarte 1991a). Seagrasses lating seagrass abundance and productivity through receive approximately 11% of surface irradiance at their maximum colonisation depth (Duarte 1991a) and thus, are able to live in a variable and low light *E-mail: [email protected] environment. © Inter-Research 2002 · www.int-res.com 90 Mar Ecol Prog Ser 236: 89–97, 2002 At low light seagrass performance is enhanced MATERIALS AND METHODS through a range of acclimative responses (Backman & Barilotti 1976, Dennison & Alberte 1986, West 1990, The study was conducted in June 1992 at the NE Olesen & Sand-Jensen 1993, Abal et al. 1994, Philip- coast of Spain. Cymodocea nodosa was collected in the part 1995) leading to altered resource allocation pat- shallow (mean depth 3.2 m) and relatively turbid (light terns that tend to maximise the efficiency of light har- attenuation coefficient, k: 0.57 m–1; Duarte 1991a) vesting and its conversion into chemical energy while Alfacs Bay (Tarragona) where C. nodosa grow from reducing respiratory costs (Björkman 1981). 0.4 to 3.8 m depth. Posidonia oceanica was collected at Shade acclimation of aquatic plants is often accompa- Port Lligat (Girona) in a protected bay characterised by nied by increased light utilisation efficiency and re- low water turbidity (k < 0.2 m–1; Marbà et al. 1996). At duced respiratory rates of leaf tissue resulting in lower this study site a monospecific P. oceanica meadow minimum light requirement for photosynthesis (Spence extends from 0.7 to 15.6 m depth. Given the differ- & Chrystal 1970, Goldborough & Kemp 1988, Maberly ences in water light attenuation and seagrass depth 1993). This response pattern is typically achieved by distribution at the 2 locations, C. nodosa and P. ocean- the combined influence of higher pigment content and ica receive similar amounts of light (11.5 and 4.4% larger leaf area per unit leaf biomass, increasing light of surface irradiance, respectively) at the maximum absorption per unit weight of photosynthetic tissue sampling depth. (Enríquez et al. 1994, Markager & Sand-Jensen 1994). Posidonia oceanica and Cymodocea nodosa shoots Low light may also lead to a proportionally larger allo- were harvested along depth transects from 8 and 5 dif- cation of resources to leaves than to rhizomes/roots ferent rooting depths, respectively, with the maximum (Barko et al. 1982, Sand-Jensen & Madsen 1991). In sampling depth close to the depth limit of the 2 sea- combination with a larger biomass-specific leaf area grass species. P. oceanica shoot density was counted this leads to increased total leaf area relative to total within triplicate 0.09 m2 quadrats, and 50 to 100 verti- shoot weight (Björkman 1981). In addition to these mor- cal short shoots with associated horizontal rhizomes phological changes, reduced shoot density and stand- were harvested at each sampling depth. For C. nodosa, ing biomass are frequently observed (Chambers & all above and belowground biomass was harvested Kalff 1985a, Duarte 1991a, Krause-Jensen et al. 2000), within three to nine 0.04 m2 quadrats at each sampling thereby reducing self-shading within the canopy with depth. The number of samples was augmented as less light above the canopy (Via et al. 1998). shoot density declined at depth. Hence, light acclimation in seagrasses is a hierarchi- The biomass samples were transported to the labora- cal process, involving processes occurring in leaves, tory, where we counted the number of harvested individual shoots and the population structure. Al- shoots, the number of standing leaves and leaf scars on though seagrass depth acclimation has been assessed all vertical shoots, and measured the distance between in some detail, little information is available on the consecutive shoots on horizontal rhizomes. Shoot leaf relative importance of acclimation at each of these area was estimated from measurements of leaf width levels (viz. Dennison & Alberte 1985, 1986, Abal et al. and length of all standing leaves. Finally, the seagrass 1994, Lee & Dunton 1997). biomass, partitioned into leaves, rhizomes and roots, Here, we examine light acclimation in the 2 Mediter- was dried at 85°C for 24 h and weighed. Biomass per ranean seagrass species Posidonia oceanica (L.) Delile ground area for Posidonia oceanica was calculated as and Cymodocea nodosa (Ucria) Aschers growing along the product of average shoot weight and shoot density. natural depth gradients. The 2 species dominate the The age of all living and dead (without green leaves) seagrass vegetation within the Western Mediterranean, shoots was determined as the number of leaf scars on covering extensive areas down to depths of 40 to 50 m the vertical rhizome (plus the number of standing (Duarte 1991a) with P. oceanica dominating at larger leaves for living shoots) formed during the life span of depths (den Hartog 1970). The 2 species differ in the shoot. The average annual leaf formation rate or module size and growth dynamics, as P. oceanica is a plastochrone interval (PI, d leaf–1) was derived from larger, more long-lived and slower-growing species annual cycles in rhizome internode lengths on the 5 to than C. nodosa (Duarte 1991b), suggesting lower mor- 10 oldest short shoots harvested. This analysis allowed phological and physiological plasticity in response to us to translate the shoot age into absolute time units shading. Depth-related changes in shoot dynamics, (viz. Duarte et al. 1994). biomass partitioning and photosynthetic light response Shoot mortality and recruitment through clonal of leaves were analysed to elucidate their relative growth were estimated from the frequency distribution contribution to overall depth acclimation. Additionally, of shoot age as described by Duarte et al. (1994). Shoot species-specific differences in the acclimation to in- mortality rates (M, ln units yr–1) were derived from the creasing rooting depth are evaluated. exponential decline in shoot number with time, assum- Olesen et al.: Depth-acclimation of seagrasses 91 ing constant recruitment and mortality rates among gression analysis (e.g. Ledermann & Tett 1981), dark res- years and was calculated as: piration rate (R) was estimated from the intercept of the –1 regression line on the ordinate and the light compensa- M = [ln(Nt) – ln(N0)]t tion point (IC) from the intercept on the abscissa. where N0 is number of shoots with modal age
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