Chlorophyll fluorescence measures of seagrasses <Emphasis Type="Italic">Halophila ovalis</Emphasis> and <Emphasis Type="Italic">Zostera capricorni</Emphasis> reveal differences in response to experimental shading

In coastal waters and estuaries, seagrass meadows are often subject to light deprivation over short time scales (days to weeks) in response to increased turbidity from anthropogenic disturbances. Seagrasses may exhibit negative physiological responses to light deprivation and suffer stress, or tolerate such stresses through photo-adaptation of physiological processes allowing more efficient use of low light. Pulse Amplitude Modulated (PAM) fluorometery has been used to rapidly assess changes in photosynthetic responses along in situ gradients in light. In this study, however, light is experimentally manipulated in the field to examine the photosynthesis of Halophila ovalis and Zostera capricorni. We aimed to evaluate the tolerance of these seagrasses to short-term light reductions. The seagrasses were subject to four light treatments, 0, 5, 60, and 90% shading, for a period of 14 days. In both species, as shading increased the photosynthetic variables significantly (P < 0.05) decreased by up to 40% for maximum electron transport rates (ETR_max) and 70% for saturating irradiances (E_k). Photosynthetic efficiencies (α) and effective quantum yields (ΔF/Fm′) increased significantly (P < 0.05), in both species, for 90% shaded plants compared with 0% shaded plants. H. ovalis was more sensitive to 90% shading than Z. capricorni, showing greater reductions in ETR_max, indicative of a reduced photosynthetic capacity. An increase in E_k, Fm′ and ΔF/Fm′ for H. ovalis and Z. capricorni under 90% shading suggested an increase in photochemical efficiency and a more efficient use of low-photon flux, consistent with photo-acclimation to shading. Similar responses were found along a depth gradient from 0 to10 m, where depth related changes in ETR_max and E_k in H. ovalis implied a strong difference of irradiance history between depths of 0 and 5–10 m. The results suggest that H. ovalis is more vulnerable to light deprivation than Z. capricorni and that H. ovalis, at depths of 5–10 m, would be more vulnerable to light deprivation than intertidal populations. Both species showed a strong degree of photo-adaptation to light manipulation that may enable them to tolerate and adapt to short-term reductions in light. These consistent responses to changes in light suggest that photosynthetic variables can be used to rapidly assess the status of seagrasses when subjected to sudden and prolonged periods of reduced light.     

Habitat use of a multispecific seagrass meadow by green turtles Chelonia mydas at Mayotte Island

We investigated the habitat use in green turtles exploiting a 13-ha multispecific seagrass meadow at Mayotte Island, south-western Indian Ocean. A phyto-ecological survey shows the occurrence of eight seagrass species, dominated by Halodule uninervis and Syringodium isoetifolium, distributed according to four distinct seagrass communities along the depth gradient. Direct underwater censuses show that green turtles occurred all over the meadow. Yet when community relative surface area was taken into account green turtles preferentially frequented the most seaward, biomass-richer S. isoetifolium-dominated community, suggesting that green turtles compensate for their intrinsically nutrient-poor herbivorous diet. Additionally, smaller (<80 cm standard curved carapace length, SCCL) individuals also preferentially occurred in the most shoreward H. univervis-dominated community where no larger (>80 cm SCCL) individuals were sighted, suggesting habitat use is indicative of diet selection and may reflect size-specific food requirements and physiology.     

Photosynthetic utilisation of inorganic carbon by seagrasses from Zanzibar, East Africa

Photosynthetic rates of eight seagrass species from Zanzibar were limited by the inorganic carbon composition of natural seawater (2.1 mM, mostly in the form of HCO₃ ⁻), and they exhibited more than three time higher rates at inorganic carbon saturation (>6 mM). The intertidal species that grew most shallowly, Halophila ovalis, Halodule wrightii and Cymodocea rotundata, showed the highest affinity for inorganic carbon (K _1/2 = ca. 2.5 mM), followed by the subtidal species (K _1/2 > 5 mM). Photosynthesis of H. wrightii, C. rotundata, Cymodocea serrulata and Enhalus acoroides was >50% inhibited by acetazolamide, a membrane-impermeable inhibitor of carbonic anhydrase, indicating that extracellular HCO₃ ⁻ dehydration is an important part of their inorganic carbon uptake. Photosynthetic rates of H. wrightii, Thalassia hemprichii, Thalassodendron ciliatum, C. serrulata and E. acoroides were strongly reduced by changing the seawater pH from 8.2 to 8.6 in a closed system. In H. ovalis, C. rotundata and Syringodiumisoetifolium, photosynthesis at pH 8.6 was maintained at a higher level than could be caused by the ca. 30% CO₂ concentration which remained in the closed experimental systems at that pH, pointing toward HCO₃ ⁻ uptake in those species. It is suggested that the ability of H. ovalis and C. rotundata to grow in the high, frequently air-exposed, intertidal zone may be related to a capability to take up HCO₃ ⁻ directly, since this is a more efficient way of HCO₃ ⁻ utilisation than extracellular HCO₃ ⁻ dehydration under such conditions. The inability of all species to attain maximal photosynthetic rates under natural conditions of inorganic carbon supports the notion that seagrasses may respond favourably to any future increases in marine CO₂ levels. Received: 19 March 1997 / Accepted: 31 March 1997     

Buffer sensitivity of photosynthetic carbon utilisation in eight tropical seagrasses

Some of the mechanisms involved in inorganic carbon (Ci) acquisition by tropical seagrasses from the western Indian Ocean were described by Björk et al. (Mar Biol 129:363–366, 1997). However, since then, it has been found that an additional, buffer-sensitive, system of Ci utilisation may operate in some temperate seagrasses (Hellblom et al. in Aquat Bot 69:55–62, 2001, Hellblom and Axelsson in Photos Res 77:173–191, 2003); this buffer sensitivity indicates a mechanism in which electrogenic H⁺ extrusion may form acidic diffusion boundary layers, in which either HCO ₃ ^? –H⁺ is co-transported into the cells, or where HCO ₃ ^? is converted to CO₂ (as catalysed by carbonic anhydrase) prior to uptake of the latter Ci form. Because a buffer was used in the 1997 study, we found it important to reinvestigate those same eight species, taking into account the direct effect of buffers on this potential mode of Ci acquisition in these plants. In doing so, it was found that all seagrass species investigated except Cymodocea serrulata were sensitive to 50 mM TRIS buffer of the same pH as the natural seawater in which they grew (pH 8.0). Especially sensitive were Halophila ovalis, Halodule wrightii and Cymodocea rotundata, which grow high up in the intertidal zone (only ca. 50–65% of the net photosynthetic activity remained after the buffer additions), followed by the submerged Enhalus acoroides and Syringodium isoetifolium (ca. 75% activity remaining), while Thalassia hemprichii and Thalassodendron ciliatum, which grow in-between the two zones, were less sensitive to buffer additions (ca. 80–85% activity remaining). In addition to buffer sensitivity, all species were also sensitive to acetazolamide (AZ, an inhibitor of extracellular carbonic anhydrase activity) such that ca. 45–80% (but 90% for H. ovalis) of the net photosynthetic activity remained after adding this inhibitor. Raising the pH to 8.8 (in the presence of AZ) drastically reduced net photosynthetic rates (0–14% remaining in all species); it is assumed that this reduction in rates was due to the decreased CO₂ concentration at the higher pH. These results indicate that part of the 1997 results for the same species were due to a buffer effect on net photosynthesis. Based on the present results, it is concluded that (1) photosynthetic Ci acquisition in six of the eight investigated species is based on carbonic anhydrase catalysed HCO ₃ ^? to CO₂ conversions within an acidified diffusion boundary layer, (2) C. serrulata appears to support its photosynthesis by extracellular carbonic anhydrase catalysed CO₂ formation from HCO ₃ ^? without the need for acidic zones, (3) H. ovalis features a system in which H⁺ extrusion may be followed by HCO ₃ ^? –H⁺ co-transport into the cells, and (4) direct, non-H⁺-mediated, uptake of HCO ₃ ^? is improbable for any of the species.     

Photosynthetic utilisation of carbon and light by two tropical seagrass species as measured in situ

In situ measurements of seagrass photosynthesis in relation to inorganic carbon (Ci) availability, increased pH and an inhibitor of extracellular carbonic anhydrase were made using an underwater pulse amplitude modulated (PAM) fluorometer. By combining the instrument with a specially designed Perspex chamber, we were able to alter the water surrounding a leaf without removing it from the growing plant. Responses to Ci within the chamber showed that subtidal plants of the seagrasses Cymodocea serrulata and Halophila ovalis had photosynthetic rates that were limited by the ambient Ci concentration depending on the irradiance that was available during short-term photosynthesis–irradiance trials. Relative electron transport rates (RETRs) at light saturation (up to 500 μ mol photons m⁻² s⁻¹) increased by 66–100% when the Ci concentration was increased from ca. 2.2 to 6.2 mM. On the other hand, intertidal plants of the same species exhibited a much lesser limitation of photosynthesis by Ci at any irradiance (up to 1500 μ mol photons m⁻² s⁻¹). Both species were able to use HCO⁻ ₃ efficiently, and there was stronger evidence for direct uptake of HCO⁻ ₃ rather than extracellular dehydration of HCO⁻ ₃ to CO₂ prior to Ci uptake. Subtidally, H. ovalis and C. serrulata grew to 10 and 12 m, respectively, where ambient irradiances were approximately 16 and 11% of those at the surface. Maximum RETRs (at light saturation) were lower for these deep-growing plants than for the intertidally growing ones. For both species, the onset of light saturation of photosynthesis (E _k) occurred at approximately 100 μ mol photons m⁻² s⁻¹ for the deep water populations, which was four and two times lower than for the shallow populations of C. serrulata and H. ovalis, respectively. This, and the differences in maximal photosynthetic rates (RETR _max), reflects an acclimation of the deep-growing populations to the lower light environment. The results presented here show that photosynthesis, as measured in situ, was limited by the availability of Ci for the deeper growing plants in Zanzibar, while the intertidally growing plants photosynthesised at close to Ci saturation. The latter result is contrary to previous conclusions regarding Ci limitations for these intertidal plants, and, in general, our findings highlight the need for performing similar experiments in situ rather than under laboratory conditions. Received: 4 April 2000 / Accepted: 31 August 2000     

Long-term effects of ultraviolet radiation on growth and photosynthetic performance of polar and cold-temperate macroalgae

Long-term effects of artificial ultraviolet radiation (UV) and natural solar radiation on growth and photosynthetic activity, as measured by chlorophyll fluorescence, were investigated in 13 different polar and cold-temperate macroalgal species. Isolates of five different species from the Arctic and Antarctic were exposed to different light treatments of photosynthetically active radiation (PAR), PAR+UVA and PAR+UVA+UVB. Eight different species collected on the island of Helgoland, North Sea, Germany, were studied in the laboratory and under natural solar radiation conditions. Increase in fresh weight and changes in photosynthetic performance were monitored over a period of 3-4 weeks. The sublittoral polar species, particularly the Antarctic red algal species Gymnogongrus antarcticus and G. turquetii, the Arctic cold-temperate brown alga Alaria esculenta and, very drastically, the Arctic-endemic Laminaria solidungula, exhibited strong inhibiting effects of artificial UVB radiation on growth. In the cold-temperate sublittoral growth of the red algae Phycodrys rubens and, to a lesser extend, Membranoptera alata was substantially inhibited by UV radiation. In contrast, eulittoral species, e.g. Fucus serratus, did not show any differences in growth with respect to artificial irradiation conditions, with or without UV radiation. In the laboratory, some individuals of the green alga Codium fragile exhibited strong morphological changes of the whole thallus, particularly under UVB exposure. In the experimental outdoor set up, growth of most of the algal species was already inhibited by the full solar UV waveband, but, apart from Polyides rotundus, no additional UVB effect could be detected. Changes of in vivo fluorescence were not always consistent with the measurable changes in growth rate, indicating that physiological processes leading to an inhibition of growth may act independently of changes in photosynthetic activity. For the polar species, a general correlation between the natural vertical distribution in the field and the individual sensitivity towards UV radiation was indicated, while for eulittoral species from Helgoland no clear relationship was found. The obtained results show that measuring growth is a good ecological parameter to monitor long-term effects of UV radiation on single macroalgal species and the possible resulting changes of whole algal communities in coastal ecosystems.     

Irradiance-induced changes in the photobiology of <Emphasis Type="Italic">Halophila johnsonii</Emphasis>

The endangered seagrass Halophila johnsonii Eiseman, exhibits high-light adapted photophysiology consistent with its distribution in intertidal and shallow subtidal (0–3 m) coastal-lagoon habitats along 200 km of southeastern Florida. To examine the short-term responses of this seagrass to three controlled-irradiance treatments (PAR + UVA + UVB [full spectrum], PAR + UVA, and PAR only), greenhouse-acclimated plants were transferred to outdoor mesocosms during July–August 2002. Chlorophyll fluorescence, UV fluorescence, and samples for pigment extraction were collected in the greenhouse, prior to moving the plants outside and on days 1, 2, 3, 4, 6, 10, and 21 of the 24-day experiment. Typical of sun-adapted plants, effective quantum yields measured by pulse-amplitude modulated (PAM) fluorometry were relatively low in all treatments, ranging from 0.46 ± 0.09 (PAR only) to 0.58 ± 0.08 (PAR + UVA + UVB). In the PAR only treatments, there were strong effects on days 1 and 4, presumably because the irradiance in the greenhouse not only lacked all λ<400 nm, but also had low irradiance maxima (∼700 μmol photons m⁻² s⁻¹, compared with ∼1,500 μmol photons m⁻² s⁻¹ outside at midday). There were few treatment differences between PAR only and PAR + UVA treatments indicating little effect of UVA radiation on this species. Differences in effective quantum yields and relative electron transport rates between the PAR only and PAR + UVA + UVB treatments on day 4 indicated rapid acclimation to UVB radiation. Tissues of H. johnsonii contained compounds that absorbed strongly in the UV, with a λ_max at ∼345 nm (depending on the extraction solvent). Absorption peak maxima and minima changed over the course of the experiment but there were no significant light-treatment differences in any pigment parameters. Percent UV shield values, measured using a newly developed UVA PAM fluorometer, were highest the day after plants were transferred from the greenhouse to the outdoor mesocosms and declined significantly to pretreatment levels in all treatments by day 21. Percent UV shield exhibited a significant positive relationship with UV-absorbing pigment (UVP) absorbance, however, the absence of treatment effects suggests that the wavelengths inducing pigment synthesis must lie between 400 and 700 nm (PAR). The results indicate that H. johnsonii rapidly acclimates to high UVB and PAR which may largely explain its distribution in intertidal and shallow subtidal areas.     

Measurement of chlorophyll fluorescence reveals mechanisms for habitat niche separation of the intertidal seagrasses <Emphasis Type="Italic">Thalassia hemprichii</Emphasis> and <Emphasis Type="Italic">Halodule uninervis</Emphasis>

In Taiwan, Thalassia hemprichii dominates the upper intertidal zone, whereas Halodule uninervis occupies the lower intertidal zone. We tested the hypothesis that T. hemprichii is better adapted to high irradiance and more resistant to air exposure than H. uninervis. The photosynthetic efficiency, damage, and extent of recovery were determined by measuring chlorophyll fluorescence using pulse amplitude modulated fluorometry. Both species growing in tidal pools, in response to high irradiance alone, revealed a small depression in maximal quantum yield of photosystem II (Fv/Fm) at noon. The second experiment compared the effect of air exposure alone and the combined effect of air exposure with high irradiance by interposing a shading screen on both species, growing in the intertidal zone over a diurnal cycle. Values of Fv/Fm of both the shaded and irradiated T. hemprichii remained high at low tide. However, H. uninervis exhibited a marked depression following air exposure and a synergistic depression under the combined effect. The experimental manipulations of exposure time demonstrated that the tolerance of T. hemprichii to the combined effect was longer and the recovery from air exposure following re-submersion was better than those of H. uninervis. Both species were more susceptible to the combined effect in the dry season than in the wet season. Our results suggest that air exposure is more important than high irradiance in constraining the distribution of H. uninervis in the upper intertidal zone. This was confirmed by transplantation experiments in which a rapid decline of H. uninervis was observed after transplantation into the upper intertidal zone. In the lower intertidal zone, measurements of the response of the photosynthetic electron transport rate to irradiance demonstrated that the transplanted T. hemprichii exhibited a sun-type response and H. uninervis a shade-type response.     

Seasonal growth and biomass of the subtropical seagrassHalodule wrightii in relation to continuous measurements of underwater irradiance

Continuous year-round measurements of photosynthetically active radiation (PAR) were collected in relation to leaf elongation and plant biomass in the shoal-grass,Halodule wrightii Aschers., within three different estuarine systems on the south Texas coast (Laguna Madre: May 1989 to September 1993; Corpus Christi Bay: February 1990 to September 1993; San Antonio Bay; May 1990 to April 1991). Large differences in water transparency at all three sites masked seasonal variations in surface insolation as reflected in average diffuse attenuation coefficient (k) values ranging from 0.7 to 2.9 and differences in the maximum depth penetration ofH. wrightii, which varied from 0.6 to about 1.3 m. The continuous presence of a chrysophyte (brown tide) algal bloom in Laguna Madre since 1990 led to significant decreases in spring leaf elongation rates and a nearly 50% decline in below-ground biomass, which was reflected in root:shoot ratio (RSR) values that declined from 5.4 in 1989 to 2.3 in 1992. Increased turbidity and lower light levels in San Antonio Bay also corresponded with diminished plant biomass and the subsequent loss of plants; at both locations, the annual quantum flux ranged from 2200 to 2400 mol m^-2yr^-1, or about 18% of surface irradiance (SI). In contrast,H. wrightii populations growing at ca.1.2 m depths and characterized by high RSR values (4.0) were exposed to 5100 to 5700 mol m^-2yr^-1, or about 41 to 46% SI. Under these conditions, plants were exposed to daily saturating levels of PAR (H _sat) of 3 to 8 h during the spring/summer period of maximum growth, compared to an average of 2 h in Laguna Madre (after 1990) and San Antonio Bay based on field-derived measurements of photosynthetic parameters. Leaf elongation inH. wrightii exhibited a clear circannual rhythm at all sites, regardless of underwater light levels and therefore was not a sensitive indicator of light stress. Instead, chronic long-term reductions in underwater PAR were most strongly reflected in total plant biomass. The higher light demand (18% SI) forH. wrightii in relation to many other seagrasses (11% SI; Duarte 1991) may be related to its higher photosynthetic light requirement, but may also reflect the different methods used to evaluate the minimum light requirements of seagrasses. In estuarine and coastal waters, which are characterized by large and unpredictable variations in water transparency, continuous measurements of in situ PAR are invaluable in assessing the growth and photosynthetic response of seagrasses to variations in underwater irradiance.The University of Texas at Austin, Marine Science Institute Contribution No. 913     

Photosynthesis and growth of red and green morphotypes of Kappaphycus alvarezii (Rhodophyta) from the Philippines

The effect of photosynthetic available radiation (PAR) levels, light quality, ultraviolet (UV) radiation, and temperature on photosynthesis, growth, and chlorophyll fluorescence was evaluated in red and green morphotypes of the rhodophyte Kappaphycus alvarezii (Doty) Doty under controlled conditions. Chlorophyll a and phycoerythrin (PE) levels were similar in the red and green morphotypes cultured under the same conditions, but phycocyanin (PC) and allophycocyanin (APC) levels were 2-fold greater in the green than in the red morphotype. Pigment characterization indicated that the overexpression of PC and APC masked the red pigmentation in the green morphotype. Maximum photosynthesis and photosynthetic efficiency were similar between the two morphotypes assayed at a wide temperature range, which was reflected in the similar growth rates observed in outdoor culture systems. In the green morphotype, photosynthetic efficiency increased 2-fold relative to the red morphotype when assayed with red light (λ > 600 nm), indicating that photosynthetic characteristics are modified as a result of pigment variation in these morphotypes. Such increase in photosynthetic efficiency in the green morphotype, however, did not result in greater growth rates when cultured under white light. Short exposure to high levels of solar radiation (UV-A + UV-B + PAR), and filtered solar radiation (UV-A + PAR or PAR) decreased effective quantum yield (ΔF/F _m′) in both morphotypes. The reduction of ΔF/F _m′ values in the red and green morphotypes was accounted for by high levels of PAR and not by the UV-A + UV-B + PAR and UV-A + PAR treatments. Photoinhibition caused by UV-A, UV-B, or PAR was completely reversed within 30 h after incubations. Recovery rates from photoinhibition, however, were significantly reduced in the green morphotype when incubated with UV-B radiation. The results here suggest that the overexpression of pigments do not necessarily increase photosynthesis and growth in these morphotypes. Received: 19 June 2000 / Accepted: 28 November 2000     

Some quantitative aspects of seagrass ecology in a coastal lagoon of Mauritius

Seagrass distribution was recorded by snorkel dives on a grid of stations in the waterfront of Club Méditerranée at Mon Choisy-Trou Aux Biches lagoon (NW Mauritius) and subsequently mapped using SURFER 6 computer software. Above-ground (AG) and below-ground (BG) standing biomass in terms of dry weight (DW) and ash-free dry weight (AFDW) as well as shoot density and shoot length were monitored monthly from June1997 to May1998 in a mixed stand of Halodule uninervis and Syringodium isoetifolium (dominant) at a shallow, nearshore station in the lagoon. Measurements of physical and chemical parameters [water temperature, current speed, salinity, pH, dissolved oxygen (DO), nitrate and phosphate concentrations] were made simultaneously, as well as at a reference station (ORE) outside the coral reef. The bottom sediment was analysed for grain size and type composition. Variation patterns were examined and statistical correlations drawn to relate plant performance to the environmental variables measured. The SURFER 6 programme generated a satisfactory contour map of seagrass distribution in the lagoon with a cover range of 0-60%. The densest patches occurred adjacent to the shoreline experiencing weaker water currents (3-13 cm s^-1) rather than near the reef (5-35 cm s^-1), where seagrasses were absent. Sand (0.063-2 mm grain size) constituted 97.2% and 77.6% of the nearshore and near-reef sediment, respectively. The dominant grain types were derived from corals (about 80%) and mollusc shells (about 14%). The recorded range of total standing biomass for H. uninervis was 243.1-468.2 g DW m^-2 (326.9ᇛ.7 g) or 71.7-141.2 g AFDW m^-2 (96.8ᆨ.1 g) and for S. isoetifolium it was 271.7-758 g DW m^-2 (460.4끯.1 g) or 119-220.5 g AFDW m^-2 (155.1ᆮ.5 g), with a maximum biomass increase during September-December in both species. AG:BG biomass ratios were generally <1 and approximated 1 during the warmest months of December-February only. Mean shoot density (1,077-4,364 shoots m^-2 in the overall range of 998-4,428 shoots m^-2) and mean shoot length (10.9-20.8 cm in the overall range of 7-31 cm) in S. isoetifolium were higher than in H. uninervis (1,732-4,137 shoots m^-2 in the overall range of 1,522-4,327 shoots m^-2 and 7.9-13.7 cm in the overall range of 6-20 cm, respectively). Temperature showed strong positive correlations with total AFDW biomass of both species (r=0.755, P<0.01 for H. uninervis; r=0.679, P<0.02 for S. isoetifolium) and with DO (r=0.925, P<0.01). High DO levels (10.7-11.2 mg l^-1) coincided with optimum standing biomass at 27.2°C. Correlations were also strong with shoot density (r=0.881, P<0.01 for H. uninervis; r=0.952, P<0.01 for S. isoetifolium) and shoot length (r=0.752, P<0.01 for H. uninervis; r=0.797, P<0.01 for S. isoetifolium). Under optimal environmental conditions, nutrient inputs from surface run-off or underground freshwater seepage in the lagoon due to heavy rainfall may boost up seagrass biomass, as suggested by positive significant correlations between phosphate levels and AG AFDW biomass (r=0.63, P<0.05 for H. uninervis; r=0.65, P<0.05 for S. isoetifolium) and shoot density (r=0.6, P<0.05 for H. uninervis; r=0.687, P<0.02 for S. isoetifolium). The results generated in this study suggest local seagrass standing biomass is comparable to that reported in monospecific stands from elsewhere. Anthropogenic activities increasingly draw down the resilience of the seagrass beds around Mauritius, and preventative measures are indispensable to achieve coastal ecological stability.     

Effects of the shoot density of seagrass on fish and decapods: are correlation evident over larger spatial scales?

Two conflicting models for the organisation of assemblages of fish and decapods associated with seagrass over large spatial scales, make contradictory predictions about the relationship between density of seagrass shoots and abundance, or diversity, of animals. We tested the predictions of both models by sampling small lish and decapods associated with two species of seagrass (Zostera capricorni Aschers and Posidonia australis Hook) at up to 16 sites within several estuaries in New South Wales, Australia, for 1.5 yr (December 1988 to March 1990). Variation in density of Z. capricorni shoots explained very little of the variation in abundance of animals. However, abundance of one species, the grass shirmp Macrobrachium intermedium, was more closely related to the density of shoots during non-recruitment seasons, suggesting that predation or emigration of individuals after settlement was greater in sparse beds. The effect of variation in density of P. australis shoots was confounded with consistent distribution patterns of most fish and decapod species. As a result, data from P. australis did not provide good tests of the hypotheses. We conclude that density of seagrass shoots explained very little of the large-scale variation in abundance of associated fish and decapods. The data do, however, support the inodel which predicts that the abundance of animals among separate seagrass beds will follow the supply of new individuals to them.     

Sensitivity and acclimation to UV radiation of zoospores from five species of Laminariales from the Arctic

Spores of five Laminariales from Arctic Spitsbergen were exposed in the laboratory to photosynthetically active radiation (PAR; 400–700 nm), PAR+UVA radiation (UVAR; 320–400 nm) and PAR+UVAR+UVB radiation (UVBR; 280–320 nm). Subsequently, germination was monitored over periods of 3, 6 and 9 days. The investigated species were the upper sublittoral Saccorhiza dermatodea, the upper to mid-sublittoral Alaria esculenta and Laminaria digitata, the mid-sublittoral L. saccharina and the lower sublittoral L. solidungula. The germination capacity decreased sharply after 16 h exposure to PAR+UVAR+UVBR in all species. However, S. dermatodea was able to recover from the damaging effects of UVBR. There was also a small increase in percentage germination of A. esculenta 6–9 days after the treatment. No recovery was evident in the other species. After 8 h exposure to PAR+UVA+UVB, L. digitata recovered completely, and L. saccharina and L. solidungula, partially. The only species susceptible to PAR+UVAR was L. solidungula. One prominent cytological feature of UVR-exposed spores was the enlargement of phenolic vesicles (physodes) (particularly seen in S. dermatodea and A. esculenta), which may have a protective function against UVR. Pilot experiments under natural irradiance conditions indicate that the PAR component of solar radiation exerts an additional stress. Overall the data show that zoospores of the species from the upper sublittoral are less sensitive to UVR or have the capacity to recover from UV stress in contrast to species from deeper waters, probably due to their UV protective and repair capabilities.Communicated by O. Kinne, Oldendorf/Luhe     

Structure and temporal fluctuations of two intertidal seagrass-bed communities in New Caledonia (SW Pacific Ocean)

The structure and trophic organization of two intertidal seagrass-bed communities (Halodule uninervis and Thalassia hemprichii) were examined on the southwest coast of New Caledonia (SW Pacific Ocean), from April 1989 to March 1990. Five benthic samples were collected from each site at 2 mo intervals and various environmental parameters were simultaneously monitored. Animal:plant biomass ratios were close to 1 at both sites. Polychaetes dominated in number of species. The suspension-feeding bivalves Gafrarium tumidum and Anadara scapha constituted the greater part of the animal biomass. The evolution of the communities over an annual cycle displayed marked structural and organizational stability, resulting mainly from the absence of distinct recruitment periods for the dominant species, no mortalities during the brief low-salinity periods, and no temporal variations in the granulometry, the organic matter or the chlorophyll a contents of the sediments.     

The value of patches of intertidal seagrass to prawns depends on their proximity to mangroves

Penaeid prawns were sampled with a small seine net to test whether catches of postlarvae and juveniles in seagrass were affected by the distance of the seagrass (mainly Zostera capricorni) from mangroves and the density of the seagrass in a subtropical marine embayment. Sampling was replicated on the western and eastern sides of Moreton Bay, Queensland, Australia. Information on catches was combined with broad-scale spatial information on the distribution of habitats to estimate the contribution of four different categories of habitat (proximal dense seagrass, distal dense seagrass, proximal sparse seagrass, distal sparse seagrass) to the overall population of small prawns in these regions of Moreton Bay. The abundance of Penaeus plebejus and Metapenaeus bennettae was significantly and consistently greater in dense seagrass proximal to mangroves than in other types of habitat. Additionally, sparse seagrass close to mangroves supported more of these species than dense seagrass farther away, indicating that the role of spatial arrangement of habitats was more important than the effects of structural complexity alone. In contrast, the abundance of P. esculentus tended to be greatest in sparse seagrass distal from mangroves compared with the other habitats. The scaling up of the results from different seagrass types suggests that proximal seagrass beds on both sides of Moreton Bay provide by far the greatest contribution of juvenile M. bennettae and P. plebejus to the overall populations in the Bay.Communicated by M.S. Johnson, Crawley     

Effects of wave-induced lightflecks in the intertidal zone on photosynthesis in the macroalgae Postelsia palmaeformis and Hedophyllum sessile (Phaeophyceae)

Intertidal algae are exposed to a highly variable photic regime because of crashing waves. We measured photosynthetic rate of whole fronds of the seaweeds Postelsia palmaeformis (Ruprecht) (at Bodega Marine Laboratory, 1991) and Hedophyllum sessile (Setchell) (Phaeophyceae) (at Friday Harbor Laboratories, 1990) in flashing and steady-state photosynthetically active radiation (PAR) of equal irradiance, using a recirculating metabolism chamber designed to minimize the thickness of diffusional boundary layers on the surfaces of algal tissues. The dimensionless ratio of photosynthetic rate under flecking PAR (P_fleck) and the sum of photosynthetic rate under steady state PAR (P_s) of high and low irradiance was computed for lightfleck periods from 0.2 to 100 s. P_fleck:P_s is a measure of the light-flash utilization-efficiency in flashing light, and was greater than unity at periods between lighflecks of 2 to 30 s, with a peak at 10 s. We used a novel optical fiber irradiance meter to measure PAR incident on fronds of P. palmaeformis as they were washed about by waves breaking in the intertidal zone, and compared the light records to that obtained by a stationary sensor under the canopy. PAR flashing period was closely correlated with the period of breaking waves in stands of P. palmaeformis. We estimated the seasonal spectrum of period between light flashes in stands of this species by examining the spectral density of ocean waves at Bodega Marine Laboratory, Bodega Bay, California. The wave spectrum peaks at a period (10 s) where light-flashes may theoretically enhance the light-flash utilization-efficiency of seaweeds. We calculate that the enhanced light-flash utilization-efficiency wrought by wave-induced light-flashes may contribute to significant gains in primary productivity of these macroalgae under some conditions.     

Diel variation of bacterial productivity in seagrass (Zostera capricorni) beds measured by rate of thymidine incorporation into DNA

Diurnal variations occurred in bacterial growth rates in the sediment and water column associated with seagrass (mainly Zostera capricorni Aschers) beds in Moreton Bay, Queensland, Australia. Studies were carried out in March and June 1981. Cell production rates increased by 5- to 10-fold during the morning and decreased during the afternoon. No nocturnal variation was observed. Daily bacterial cell production rate in the aerobic zones of the seagrass bed was estimated to be 43 mg C m^-2. A minimum of 100 mg C m^-2 d^-1 would be required to support the bacterial production. This represents about 10% of net primary production. The incorporation of tritiated thymidine into DNA was used to measure bacterial growth. The validity of the method is discussed.     

UV absorption by mycosporine-like amino acids in Phaeocystis antarctica Karsten induced by photosynthetically available radiation

Mycosporine-like amino acids (MAAs), which occur in diverse taxonomic groups, exhibit in vivo absorption maxima between 310 nm and 360 nm and may play a photoprotective role against ultraviolet (UV) exposure. Using cultures of colonial Phaeocystis antarctica, we examined the relationship between MAA concentration, in vivo UV absorption, photoprotective (carotenoid) and photosynthetic pigments, and photosynthetically available radiation (PAR, 350–700 nm). UV absorption was high; chlorophyll-specific absorption, a ^* _ph, at 330 nm ranged from 0.06 to 0.41 m²/mg chlorophyll a. Values of a ^* _ph (330) were 4–13 times greater than a ^* _ph (676). Mycosporine-glycine, shinorine, and mycosporine-glycine valine are responsible for the strong in vivo UV absorption. The sum of all MAAs increased with irradiance when normalized to chlorophyll a or carbon concentrations, whereas individual MAAs varied independently from each other. Mycosporine-glycine concentrations showed no statistically significant change over the range of light intensities, whereas mycosporine-glycine and shinorine concentrations increased at higher irradiances. The relative fluorescence yield for chlorophyll a was low in the UV region compared to the visible region, implying that absorbed UV radiation (<375 nm) is transferred inefficiently to chlorophyll a in the reaction center. Quantitative estimates of UV screening by MAAs are attributed to elevated MAA concentrations and increased diameter at high light. Received: 31 March 1999 / Accepted: 13 July 2000     

Effects of ambient levels of solar ultraviolet radiation on zooxanthellae and photosynthesis of the reef coral Montipora verrucosa

Paired flat plates of the hermatypic coral Montipora verrucosa from Kaneohe Bay, Oahu, Hawaii, were acclimated to photosynthetically active radiation (PAR) only and to full sunlight (PAR+UV) for several weeks in the summer of 1990. After the acclimation period, photosynthesis, both in PAR-only and PAR+UV as well as dark respiration were measured. Levels of the UV-absorbing compounds, S320, density of zooxanthellae, and chlorophyll a concentration were determined. Corals acclimated in PAR+UV had higher levels of the UV-protective compounds and lower areal zooxanthellae densities than corals acclimated in PAR-only. Chlorophyll a per unit volume of coral host and per algal cell did not differ between corals from the two acclimation treatments. Corals acclimated to PAR+UV displayed higher photosynthesis in full sunlight than corals acclimated to PAR-only, but when photosynthesis was measured in the light regime to which the corals had been acclimated, there were no differences in photosynthesis. Dark respiration was the same for corals from the two acclimation treatments regardless of the light quality immediately preceding the dark period.Contribution No. 902 HIMB     

The effect of light and heterotrophy on carotenoid concentrations in the Caribbean anemone<Emphasis Type="Italic"> Aiptasia pallida</Emphasis> (Verrill)

One of the consequences of ultraviolet radiation exposure in anthozoans possessing photosynthetic endosymbionts (i.e., zooxanthellae) is production of reactive oxygen species that can damage cellular components, especially lipids and photosynthetic membranes. It is well known that carotenoids are potent antioxidants that can mitigate oxygen radical damage, but the origin of these compounds in zooxanthellate anthozoans is obscured because they can potentially originate from endosymbionts, heterotrophic feeding by the host, or a combination thereof. We used Aiptasia pallida, a common Caribbean anemone, to investigate the effects of exogenous food sources, ultraviolet-A radiation (UVA, 320–400 nm), and photosynthetically active radiation (PAR, 400–700 nm) on carotenoid levels in zooxanthellate anthozoans. Anemones were exposed to one of three simulated light treatments in the laboratory for 38 days: PAR (60% below ambient)/UVA (similar to ambient), PAR/low UVA (42% below ambient), and low PAR (98.4% below ambient)/no UVA. In addition, anemones were either fed a carotenoid-rich diet of Artemia salina nauplii, or starved. Carotenoids identified in A. pallida included peridinin, diadinoxanthin, diatoxanthin and -carotene. While a diet of Ar. salina nauplii had no effect on the carotenoid composition of A. pallida, a two-way analysis of variance revealed that anemones exposed to ambient UVA levels had significantly greater diatoxanthin concentrations relative to the total xanthophyll pool [diato:(diato+diadino)] after 10 days of exposure. This difference among treatments was not present at 20 days, but reappeared as an effect due to starvation rather than UVA at days 30 and 38. These results suggest that carotenoids in A. pallida are not influenced by exogenous feeding and that photoprotective xanthophyll cycling is sensitive to stresses such as UVA and starvation.Communicated by P.W. Sammarco, Chauvin