Quantitative and dynamic aspects of a mixed seagrass meadow in Papua New Guinea

Aspects of production and biomass were studied from November 1981 to November 1982 in six seagrass species which together from the mixed seagrass meadows in Papua New Guinea. These species, viz. Thalassia hemprichi (Ehrenb.) Aschers., Cymodocea serrulata (R.Br.) Aschers. et Magnus, Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., Syringodium isoetifolium (Aschers.) Dandy, Halodule uninervis (Forssk.) Aschers. and Halophila ovalis (R.Br.) Hook. f. have been previously studied in monospecific seagrass beds. Thalassia hemprichii was the dominant species, followed by Syringodium isoetifolium. These two species were present in all samples and evenly distributed. Cymodocea serrulata and C. rotundata were recorded in 91 and 86%, respectively, of the quadrats sampled. The density, however, varied considerably. Shoots of the remaining two species were found in < 50% of the samples. The percentage presence increased when below-ground plant parts were taken into account.Significant differences in the shoot density were only found in Syringodium isoetifolium. The distribution of the five other species remained unchanged during the year. Annual mean shoot density amounted to 860 for Thalassia hemprichii, 2100 for Syringodium isoetifolium, 200 for Cymodocea serrulata, 250 for C. rotundata and 54 for both Halodule uninervis and Halophila ovalis. All species reached their maximum density from September to November. The mean aboveground production was 3.9 g ash-free dry weight (ADW) m⁻² day⁻¹, of which 64% was contributed by Thalassia hemprichii. Syringodium isoetifolium, which had the highest shoot density, contributed only 17%.The plastochrone interval of the leaves (PIL) was constant in all species and the mean ranged from 10.1 to 11.1 days. The PIL was virtually the same in this mixed meadows as in monospecific seagrass beds. Furthermore, the above-ground relative growth rate was constant during the year. Thalassia hemprichii was the most productive seagrass (mean 0.043 day⁻¹), whereas the lowest mean relative production was observed for Syringodium isoetifolium (0.030 day⁻¹). Total mean production was 6.4 g ADW m⁻² day⁻¹, of which 39% was contributed by the vertical axes, the rhizomes and the roots. The caloric production efficiency of the meadows was 0.58% of the total insolation at the water surface.Thalassia hemprichii was, because of its morphology, the stable element in the meadow. All other species were present at all times and exhibited a continuative process of recolonization.     

Some structural and functional aspects of the epiphytic component of four seagrass species (Cymodoceoideae) from Papua New Guinea

The epiphytic component of four monospecific seagrass beds from Papua New Guinea was studied structurally and functionally. The floristic composition and abundance of the epiphytes on leaves of four seagrass species (Cymodoceoideae) showed considerable variation, but on all four seagrass species, the same algae were among the five quantitatively most important epiphytes: encrusting coralline algae, Cyanophyta, Ceramium gracillimum (Harv.) Mazoyer, Polysiphonia savatierii Hariot and Audouinella spp. The temporal pattern of the epiphytic algae showed more or less the same features on the four seagrass species.Annual mean biomass of epiphytes and seagrass leaves ranged from 54 g ADW m^?2 in a community of Cymodocea rotundata Ehrenb. and Hempr. ex Aschers. to 169 g ADW m^?2 in a community of Syringodium isoetifolium (Aschers.) Dandy. The contribution of the epiphytic component to the total above-ground biomass ranged from 22 to 24%. Productivity of epiphytes was highest on leaves of Halodule uninervis (Forssk.) Aschers. (2.12 g ADW m^?2 sediment surface day^?1) and the epiphytic community contributed 35–44% of the total above-ground production of these four seagrass communities.     

Growth patterns in some indo-west-pacific seagrasses

The plastochrone interval of the rhizomes (PIR) was determined in the monopodially branching seagrasses Cymodocea rotundata Ehrenb. et Hempr. ex Aschers. (1981), Cymodocea serrulata (R.Br.) Aschers. et Magnus (1981), Syringodium isoetifolium (Aschers.) Dandy (1982), Halodule uninervis (Forssk.) Aschers. (1982) and Halophila ovalis (R.Br.) Hook.f. (1982). The growth rate during 1981 was in all species, including Thalassia hemprichii (Ehrenb.) Aschers., constant during the year and the PIR amounted to 6.7 days (Cymodocea serrulata) and 3.9 days (Cymodocea rotundata). However, the PIR observed in 1982 showed in all species a unimodal pattern and a decrease during June, July and August. The PIR was 6.3 days in Halodule uninervis while an interval of 5.2 days was calculated in Syringodium isoetifolium.The growth rates of the horizontal and vertical axes (rhizomes and shoots) were correlated. A regression between the number of leaves on the shoot and the number of nodes on the rhizome resulted for most species in a linear correlation. The slopes of the calculated correlation were rather similar in the tree species with a comparable morphology: 0.44 in Cymodocea serrulata, 0.43 in Cymodocea rotundata and 0.49 in Halodule uninervis. In Syringodium isoetifolium, the longevity of the leaves increased during ageing of the shoot.The proliferation ratio (the ratio shoots/rhizome apices) was measured in all species. A maximum of 0.41 was observed in Thalassia hemprichii. This species and Cymodocea rotundata showed a unimodal pattern. In the last species, the ratio ranged from 0.03 to 0.13. The ratio was constant during the year in Cymodocea serrulata and Syringodium isoetifolium. Proliferation by means of monopodially branching of the rhizome was of little importance in Halodule uninervis. The pattern of proliferative and reiterative branching in the Cymodoceoideae is illustrated by an image of 123 days of growth in the fastest growing specimen.Most species showed growth in fronts. The expansion of monospecific seagrass fringes was monitored by repetitive mapping. The pattern of succession in subtidal meadows was determined.     

Aspects of production and biomass of four seagrass species (Cymodoceoideae) from Papua New Guinea

Biomass and production data of the seagrasses Cymodocea serrulata (R. Brown) Aschers. and Magnus, Cymodocea rotundata Ehrenb. et Hempr. ex Aschers., Halodule uninervis (Forssk.) Aschers. and Syringodium iksoetifolium (aschers.) Dandy were collectede in monospecific stands in Bootless Inlet, Papua New Guinea. Cymodocea serrulata and Cymodocea rotundata were studied from November 1980 to November 1981. Total annual mean biomass was 354 and 201 g ADW m⁻², respectively. The largest proportion of these biomass values was contributed by the rhizomes (49 and 36%, respectively) and leaf biomass was ± 30% for both species. Halodule uninervis was studied at an intertidal and a subtidal site. The highest total annual mean biomass (600 g ADW m⁻²) was recorded at the intertidal site, of which 85% was found below ground. The largest proportion of the biomass, at both sites, was contributed by the below-ground vertical axes of the shoots. The biomass of the rhizomes was relatively low (9–12%) for Halodule uninervis. Proportionally, the largest above-ground biomass (40%) was recorded for Syringodium isoetifolium, of which the annual mean biomass was 481 g ADW m⁻².Total production (above and below ground) was 4.9 and 3.0 g ADW m⁻² day⁻¹ for Cymodocea serrulata and Cymodocea rotundata, respectively. Approximately 70% was production of leaves. Total production amounted to 6.0 and 4.0 g ADW m⁻² day⁻¹ for Halodule uninervis at the intertidal and subtidal sites, respectively. The maximum production was recorded for Syringodium isoetifolium, 60% of the 9.0 g ADW m⁻² day⁻¹ was contributed by the leaves. All species reached the maximum production during February and March, when the water temperatures were highest and water was retained above all sites, at all times. The increase of leaf production was mainly due to the increase in biomass of the mature leaves. Significant changes in the plastochrone interval of the leaves were not observed during this period.     

A comparison of the annual production and biomass in three monospecific stands of the seagrass Thalassia hemprichii (Ehrenb.) aschers.

Annual production and biomass data were collected in three seagrass communities of Thalassia hemprichii (Ehrenb.) Aschers. from Papua New Guinea. Leaf growth rates, determined by the marking technique, resulted in a growth rate of 8.3 mm day^?1 for the youngest leaves. Production of above-ground plant parts was assessed by the plastochrone interval. The annual mean values were 9.3, 10.0 and 9.9 days for Sites 1, 2 and 3, respectively. Annual mean total above-ground production amounted to 2.1 mg ADW shoot^?1 day^?1 at Site 1, and 5.5 and 4.5 mg ADW shoot^?1 day^?1 for Sites 2 and 3, respectively; 73–89% of the total net production was contributed by the leaves. Rhizome production was correlated to the plastochrone interval of the leaves. Annual mean biomass of leaves amounted to 16–27% of the total biomass. The mean biomass of the other plant parts remained constant during the year. The annual mean turnover time of the different plant parts (above- and below-ground) varied considerably between the sites.     

The seasonal distribution and community structure of the epiphytic algae on Thalassia hemprichii (Ehrenb.) Aschers. from Papua New Guinea

Algae growing as epiphytes on leaves of Thalassia hemprichii (Ehrenb.) Aschers. have been studied from November 1980 to December 1981, in the Port Moresby area, Papua New Guinea. The epiphytic communities of 3 different monospecific seagrass meadows are compared for species richness, abundance and temporal pattern. Seagrass shoots were studied separately, using the method of Braun—Blanquet, as adapted by Boudouresque. By differentiating between the leaves of one single shoot, the inner- and outer-face of each leaf and the upper- and lower-part of each leaf, the epiphytic community was studied from its initial colonization (Leaf 1) to the final “climax” situation (Leaf 4). The diversity and abundance were strongly related to the age of the seagrass leaves. The Rhodophyta were best represented, with the Cryptonemiales dominating the community quantitatively; the Ceramiales predominated qualitatively. The Phaeophyta were negligible in terms of abundance and diversity. Differences between the 3 study sites are presented.     

Photosynthetic carbon metabolism in seagrasses C-labeling evidence for the c(3) pathway

The δ¹³C values of several seagrasses were considerably less negative than those of terrestrial C₃ plants and tended toward those of terrestrial C₄ plants. However, for Thalassia hemprichii (Ehrenb.) Aschers and Halophila spinulosa (R. Br.) Aschers, phosphoglycerate and other C₃ cycle intermediates predominated among the early labeled products of photosynthesis in ¹⁴C-labeled seawater (more than 90% at the earliest times) and the labeling pattern at longer times was brought about by the operation of the C₃ pathway. Malate and aspartate together accounted for only a minor fraction of the total fixed label at all times and the kinetic data of this labeling were not at all consistent with these compounds being early intermediates in seagrass photosynthesis. Pulse-chase ¹⁴C-labeling studies further substantiated these conclusions. Significant labeling of photorespiratory intermediates was observed in all experiments. The kinetics of total fixation of label during some steady-state and pulse-chase experiments suggested that there may be an intermediate pool of inorganic carbon of variable size closely associated with the leaves, either externally or internally. Such a pool may be one cause for the C₄-like carbon isotope ratios of seagrasses.     

CHARACTERIZATION AND COMPARISON OF PROKARYOTIC EPIPHYTES ASSOCIATED WITH THREE EAST AFRICAN SEAGRASSES1

Prokaryotic epiphytes on leaves of three seagrass species, Thalassodendron ciliatum, Thalassia hemprichii, and Cymodocea rotundata, from two Kenyan coastal sites, Nyali (a high‐nutrient site) and Vipingo (a low‐nutrient site), were characterized genetically and morphologically. Denaturing gradient gel electrophoresis (DGGE) and clone libraries of PCR‐amplified 16S rRNA gene fragments were used to study prokaryotes associated with these seagrasses. In general, the epiphytic coverage was greater in the high‐nutrient site, while the microbial diversity was linked to seagrass species rather than the study sites. Cytophaga–Flavobacteria–Bacteroides (CFB) were associated with T. ciliatum and T. hemprichii mainly in the nutrient‐poor site, while α‐, β‐, and γ‐proteobacteria were associated with all three species at the two study sites. Some bacteria phylotypes were closely related to sequences of microorganisms previously recovered from wastewaters or other contaminated sources, indicating the influx of land‐based wastes into these coastal lagoon ecosystems. The abundance of potential nitrogen (N₂)‐fixing cyanobacteria on C.  rotundata, particularly in the low‐nutrient site, suggested that this association may have been acquired to meet N demands. Unicellular cyanobacteria were dominant and associated with C. rotundata and T. hemprichii (with those on T. hemprichii being closely related to cyanobacterial symbiotic species), while T. ciliatum was almost devoid of cyanobacterial associations at the same site (Nyali), which suggests specificity in the cyanobacteria–seagrass associations. The abundance of prokaryotic epiphytes was considered to be linked to water depth and tidal exposure.     

Chemometric Studies of Multielemental Composition of Few Seagrasses from Gulf of Mannar,India

This paper discusses the determination of minerals content (cadmium, cobalt, chromium, copper, nickel, lead, manganese, magnesium, iron, zinc, sodium, potassium and calcium) of six seagrass samples, Enhalus acoroides, Thalassia hemprichii, Halodule pinifolia, Syringodium isoetifolium, Cymodocea serrulata and Cymodocea rotundata using inductively coupled plasma optical emission spectrophotometry and flame photometer. Principal component analysis (PCA) and hierarchical cluster analysis revealed different mineral compositions of the seagrass samples. Among the 13 elements investigated, Ni 1.513, Na 690.167 and Ca 220.333; Cr 3.957; Mn 23.427, Zn 17.593 and Fe 156.567; Cd 0.357, Co 0.431, Pb 2.040, Mg 912.733 and K 300.9; Cu 7.8 mg/kg dry weight, respectively, were found at high concentrations in E. acoroides; T. hemprichii; H. pinifolia; S. isoetifolium and C. rotundata. PCA analysis confirmed the presence of three components with 91.28% of the total variance. The toxic elements Pb, Cr and Cd were also found in all six seagrasses, although the concentrations were below the permissible limits proposed by the World Health Organization.     

Resource translocation within seagrass clones: allometric scaling to plant size and productivity

The allometric scaling of resource demand and translocation within seagrass clones to plant size (i.e. shoot mass and rhizome diameter), shoot production and leaf turnover was examined in situ in eight seagrass species (Cymodocea nodosa, Cymodocea serrulata, Halophila stipulacea, Halodule uninervis, Posidonia oceanica, Thalassodendron ciliatum, Thalassia hemprichii and Zostera noltii), encompassing most of the size range present in seagrass flora. One fully developed shoot on each experimental rhizome was incubated for 2–3 h with a pulse of NaH¹³CO₃ (235 μmol) and ¹⁵NH₄Cl (40 μmol). The mobilisation of incorporated tracers across the clone was examined 4 days later. Carbon and nitrogen demand for shoot production across seagrass species scaled at half of the shoot mass, whereas seagrass leaves incorporated tracers (¹³C and ¹⁵N) at rates proportional to the shoot mass. The shoots of all seagrass species shared resources with neighbours, particularly with younger ones. The time scales of physiological integration and the absolute amount of resources shared by seagrass ramets scaled at 2.5 power of the rhizome diameter. Hence, the ramets of larger species were physiologically connected for longer time scales and share larger absolute amounts of resources with neighbours than those of smaller species. The different pattern of resource translocation exhibited by seagrasses helps explain the ecological role displayed by these species and the success of large seagrasses colonising nutrient-poor coastal areas, where they often dominate.     

Annual biomass and production of epiphytes in three monospecific seagrass communities of Thalassia hemprichii (Ehrenb.) Aschers

The biomass of epiphytes and seagrasses has been measured in relation to leaf age in three monospecific seagrass stands of Thalassia hemprichii (Ehrenb.) Aschers. in Papua New Guinea. From June 1981 through August 1982, biomass values for epiphytes at the three sites ranged from 5 to 70 g ADW m⁻² sediment surface at site 1, from 5 to 14 g ADW m⁻² at site 2, and from 3.5 to 7.0 g ADW m⁻² at the site 3. Annual mean epiphyte biomass values for the different sites were 1.3 g ADW m⁻² leaf surface at site 1, 1.7 g ADW m⁻² leaf surface at site 2, and 1.5 g ADW m⁻² leaf surface at site 3.

The annual mean standing crop of T. hemprichii leaves was highest at site 1 (103 g ADW m⁻². Values for site 2 and site 3 were 60 g ADW m⁻² and 41 g ADW m⁻², respectively.

Production of epiphytes was calculated in three different ways: firstly, by using biomass values for each specific leaf-age group, with corrections for colonization; secondly, by fitting the biomass values with a specific growth curve; and thirdly, by estimated the rate of biomass accumulation. On an area basis, production of epiphytes on leaves of T. hemprichii ranged from 0.55 to 3.97 g ADW m⁻² day⁻¹ at site 1, from 0.17 to 0.73 g ADW m⁻² day⁻¹ at site 2, and from 0.24 to 0.68 g ADW m⁻² day⁻¹ at site 3.     

The seagrass and associated macroalgal vegetation of Gazi Bay (Kenya)

The seagrass and macroalgal vegetation of Gazi Bay (at approximately 50 km south of Mombasa) have been studied by means of 88 relevés along 7 transects. Correlation between the distribution of the seagrasses and some abiotic factors (particle size fractions, chemical composition of the substrate) is not well marked. Nevertheless a general zonation and succession of seagrasses could be established:

1. A transition zone between the mangal and the seagrass beds is covered byBoodleopsis pusilla;
2. the pioneer associationHalophila ovalis +Halodule wrightii forms low sandy bumps at the upper limit of the seagrass beds, but also occurs in the whole midlittoral where sandlayers have recently been accumulated (e.g. on coral platforms);
3. the climax vegetation of the intertidal zone seems to beThalassia hemprichii which sometimes is associated withCymodocea rotundata andC. serrulata, certainly in deeper pools and close to low water mark;Halimeda opuntia,Gracilaria salicornia andG. corticata are also frequent in this vegetation type;
4. from low water at neap tide downwards patches of monospecificEnhalus acoroides vegetation can also occur;
5. from mean low water down to approximately ?1 m mixed meadows ofThalassia, C. serrulata, C rotundata andHalodule uninervis are well developed; the seaweedsHalimeda macrooloba andAvrainvillea obscura are also typical for this zone; locally patches ofSyringodium isoetifolium grow on small bumps andHalophila stipulacea grows as a pioneer on bare sand;
6. from ?1 m downwards the whole lagoon is covered by homogeneous, monospecificThalassodendron ciliatum meadows, locally replaced byE. acoroides.

     

Seagrasses in tropical Australia, productive and abundant for decades decimated overnight

Seagrass ecosystems provide unique coastal habitats critical to the life cycle of many species. Seagrasses are a major store of organic carbon. While seagrasses are globally threatened and in decline, in Cairns Harbour, Queensland, on the tropical east coast of Australia, they have flourished. We assessed seagrass distribution in Cairns Harbour between 1953 and 2012 from historical aerial photographs, Google map satellite images, existing reports and our own surveys of their distribution. Seasonal seagrass physiology was assessed through gross primary production, respiration and photosynthetic characteristics of three seagrass species, Cymodocea serrulata, Thalassia hemprichii and Zostera muelleri. At the higher water temperatures of summer, respiration rates increased in all three species, as did their maximum rates of photosynthesis. All three seagrasses achieved maximum rates of photosynthesis at low tide and when they were exposed. For nearly six decades there was little change in seagrass distribution in Cairns Harbour. This was most likely because the seagrasses were able to achieve sufficient light for growth during intertidal and low tide periods. With historical data of seagrass distribution and measures of species production and respiration, could seagrass survival in a changing climate be predicted? Based on physiology, our results predicted the continued maintenance of the Cairns Harbour seagrasses, although one species was more susceptible to thermal disturbance. However, in 2011 an unforeseen episodic disturbance – Tropical Cyclone Yasi – and associated floods lead to the complete and catastrophic loss of all the seagrasses in Cairns Harbour.     

The sterol and fatty acid compositions of seven tropical seagrasses from North Queensland,Australia

The sterol and fatty acid compositions of fresh leaves of the seagrasses Cymodocea serrulata, Enhalus acoroides, Halodule uninervis, Halophila ovalis, H. ovata, H. spinulosa and Thalassia hemprichii are reported. The major fatty acids were palmitic acid, linoleic acid and linolenic acid as expected. H. ovalis and H. ovata were characterized by the relatively high abundance (ca 5%) of the acid hexadeca-7,10,13-trienoic acid (16:3<7 > ). The sterol compositions were typical of higher plants, with sitosterol and stigmasterol accounting for 60–90% of the observed sterols. 28-Isofucosterol was a major sterol (20–30%) only in the Halophila spp. Cluster analysis of the sterol composition data clearly separated the Halophila spp. from the other seagrasses and enabled the distinction of Enhalus sp. from Cymodocea, Halodule and Thalassia spp. The seagrass species were clearly separated into five chemical groups using the combined fatty acid and sterol composition data and the need for a reappraisal of the taxonomic position of Halophila was indicated.     

Revised ranges of seagrass species in the Myeik Archipelago, Myanmar

In a survey of the Myeik Archipelago, we documented seven seagrass species in the southern region. Three seagrass species (Cymodocea rotundata, Enhalus acoroides, and Halophila ovalis) have previously been reported in the Myeik Archipelago; three species (Halodule pinifolia, Halodule uninervis, Syringodium isoetifolium) are new reports for the archipelago; and one species (Thalassia hemprichii) is a new report for Myanmar.     

What lies beneath: Why knowledge of belowground biomass dynamics is crucial to effective seagrass management

Conservation of seagrasses meadows is important, because these habitats are ecologically important and under threat. Monitoring and modelling are essential tools for assessing seagrass condition and potential threats, however there are many seagrass indicators to choose from, and differentiating between natural variability and declining conditions poses a serious challenge. Tropical seagrass meadows in the Indo-Pacific, in contrast to most temperate meadows, are characterized by a multi-species composition and a year-round growth. Differences in characteristics between species growing within one meadow could induce uncertainty in the assessment of the dynamics of these meadows if variation in productivity and related biomass turnover timescales are not taken into consideration. We present data on biomass distribution, production and turnover timescales of above- and belowground tissues for three key tropical seagrass species (Thalassia hemprichii, Cymodocea rotundata and Halodule uninervis) in two mixed-species meadows in the Spermonde Archipelago, Indonesia. Seagrass leaf turnover time scales were comparable for the three studied seagrass species and varied between 25 and 30 days. Variation in leaf and rhizome turnover timescales were small (or insignificant) between the two meadows. In contrast, rhizome turnover time scales were around ten times longer than leaf turnover timescales, and large differences in rhizome turnover time scales (200–500 days) were observed between the species. The late-successional species T. hemprichii had much slower rhizome turnover compared to the two early successional species. Furthermore, since rhizome biomass has a much longer turnover time compared to leaf biomass, changes in rhizome biomass reflect effects on seagrass meadows on a much longer timescale compared to changes in leaf biomass for these tropical meadows. We conclude that belowground biomass dynamics are an important proxy to assess long-term effects of environmental stressors on seagrass ecosystems and should be included in tropical seagrass management programmes.     

Inorganic Carbon Source for Photosynthesis in the Seagrass Thalassia hemprichii (Ehrenb.) Aschers

Photosynthetic carbon uptake of the tropical seagrass Thalassia hemprichii (Ehrenb.) Aschers was studied by several methods. Photosynthesis in buffered seawater in media in the range of pH 6 to pH 9 showed an exponentially increasing rate with decreasing pH, thus indicating that free CO₂ was a photosynthetic substrate. However, these experiments were unable to determine whether photosynthesis at alkaline pH also contained some component due to HCO₃⁻ uptake. This aspect was further investigated by studying photosynthetic rates in a number of media of varying pH (7.8-8.61) and total inorganic carbon (0.75-13.17 millimolar). In these media, photosynthetic rate was correlated with free CO₂ concentration and was independent of the HCO₃⁻ concentration in the medium. Short time-course experiments were conducted during equilibration of free CO₂ and HCO₃⁻ after injection of ¹⁴C labeled solution at acid or alkaline pH. High initial photosynthetic rates were observed when acidic solutions (largely free CO₂) were used but not with alkaline solutions. The concentration of free CO₂ was found to be a limiting factor for photosynthesis in this plant.     

Biomass accumulation and shading effects of epiphytes on leaves of the seagrass, Heterozostera tasmanica, in Victoria,Australia

A method is described for estimating the rate of accumulation of epiphyte biomass on leaves of the seagrass, Heterozostera tasmanica (Martens ex Aschers.) den Hartog and for estimating the effect of epiphyte biomass on photosynthesis of the seagrass. Epiphyte biomass was determined by comparison of the weight per unit area of epiphyte-covered and epiphyte-free leaf blades. Epiphyte weight increased as age of the seagrass leaves increased. Linear regression on epiphyte biomass vs. leaf age estimated the rate of biomass accumulation. Rates varied from 5.7 to 104 μg epiphyte dry weight per cm² of leaf surface per day at three sites in Western Port and Port Phillip Bay, Victoria. Rates of accumulation of epiphyte biomass were generally higher during December through March (summer) than in May (autumn), August (winter) or October (Spring). Light attenuation by epiphytes increase linearly with biomass. The rate of biomass accumulation of epiphytes was compared with leaf growth rate, ambient photon flux density in H. tasmanica beds and the photosynthesis—photon flux density curve of H. tasmanica. This comparison demonstrated that epiphyte biomass can accumulate fast enough to shade H. tasmanica leaves and significantly reduce the time (to less than one half of the leaf life span) in which positive net photosynthesis of the leaf blade is possible.     

High midday temperature stress has stronger effects on biomass than on photosynthesis: A mesocosm experiment on four tropical seagrass species

The effect of repeated midday temperature stress on the photosynthetic performance and biomass production of seagrass was studied in a mesocosm setup with four common tropical species, including Thalassia hemprichii, Cymodocea serrulata, Enhalus acoroides, and Thalassodendron ciliatum. To mimic natural conditions during low tides, the plants were exposed to temperature spikes of different maximal temperatures, that is, ambient (29–33°C), 34, 36, 40, and 45°C, during three midday hours for seven consecutive days. At temperatures of up to 36°C, all species could maintain full photosynthetic rates (measured as the electron transport rate, ETR) throughout the experiment without displaying any obvious photosynthetic stress responses (measured as declining maximal quantum yield, Fv/Fm). All species except T. ciliatum could also withstand 40°C, and only at 45°C did all species display significantly lower photosynthetic rates and declining Fv/Fm. Biomass estimation, however, revealed a different pattern, where significant losses of both above‐ and belowground seagrass biomass occurred in all species at both 40 and 45°C (except for C. serrulata in the 40°C treatment). Biomass losses were clearly higher in the shoots than in the belowground root–rhizome complex. The findings indicate that, although tropical seagrasses presently can cope with high midday temperature stress, a few degrees increase in maximum daily temperature could cause significant losses in seagrass biomass and productivity.