Carbon and nitrogen translocation in response to shading of the seagrass Posidonia sinuosa

Translocation of carbon (C) and nitrogen (N) was investigated in response to shading of the seagrass Posidonia sinuosa in control (ambient light) and shade (below minimum light requirement) treatments after 10 d shading. A mature leaf was incubated in situ in ¹³C- and ¹⁵N-enriched seawater for 2 h and the appearance of the isotopes in the young leaf and adjacent rhizome monitored over 29 d. C and N isotopes gradually reduced in the mature leaf: of ¹⁵N contained in the entire shoot (mature leaf, young leaf and 4 cm rhizome), 95% (control) and 97% (shade) was found in the mature leaf after 2 h incubation and only 75% and 60% remained in the mature leaf after 29 d; 98% and 94% of ¹³C was found in the mature leaf after 2 h, and it had reduced to 36% and 44% after 29 d. This corresponded to an equal increase in the young leaf + rhizome indicating that the mature leaf is a source of these nutrients to the young leaf and rhizome. C translocation from mature leaves was not significantly affected by the shade treatment. In contrast, there was an increase in ¹⁵N taken up by the mature leaves (1.9× higher in the shade), the percent of ¹⁵N translocated to the young leaf and rhizome (24% in control and 40% in shade) and N concentration in the young leaf (1.24% control and 1.41% shade) and rhizome (0.86% control and 0.99% shade). Resorption of C and N was also estimated from changes in the total C and N content of the mature leaf over 29 d. N resorption from the mature leaf contributed up to 63% of young leaf N requirements in the control treatment but only 41% in the shade treatment. We conclude that uptake and translocation of N by mature leaves is a response to shading in P. sinuosa and would provide additional N to growing leaves, enhancing light harvesting efficiency.     

Abnormal embryo development in the seagrass Posidonia oceanica

In this study the occurrence of twin-like embryos and abnormal embryos in Posidonia oceanica, a rare phenomenon in seagrasses, was documented. The ability of additional embryos to develop seedlings was also demonstrated for the first time in seagrasses. Approximately 1750 fruits collected at three localities of the north-western Mediterranean on 2 years (1994 and 2004) were screened for abnormal embryo morphology. The frequency of embryo anomalies varied from 1.9 to 7.1% among localities, and no differences between years were detected within the same locality. Twin-like embryos were usually in contact and germinated to produce complete twin seedlings that could be separated. Abnormal embryos showed additional plumules but had a common hypocotyl and a single primary root; these embryos germinated and grew like “Siamese” seedlings.     

Detrital stocks and dynamics of the seagrass Posidonia oceanica (L.) Delile in the Spanish Mediterranean

Previous studies have shown that most leaf production (>90%) of the seagrass Posidonia oceanica is shed after senescence and that a substantial percentage (up to 80%) may thereafter be exported off the seagrass meadows by waves and currents. It has also been reported that P. oceanica meadows can accumulate large stocks of belowground detritus due to slow decomposition rates. However, the generality of these results across broad spatial scales is poorly known. In this report, we examine the fate of leaf production and the magnitude and dynamics of belowground detritus in 16 P. oceanica meadows distributed along the Spanish Mediterranean. Herbivores removed a small percentage of leaf production in all the meadows (≤13%), with most leaf production (>85%) being shed after senescence. Most shed leaves (>90%) were exported off the meadows by physical agents, such as waves and currents. The amount of belowground detritus stored within 10–15 cm from the sediment surface varied from ca. 70 to 7500 g DW m⁻² among the meadows examined, and they accumulated at rates ranging from ca. 65 to 650 g DW m⁻² per year. These values are large when compared to other communities of aquatic and terrestrial macrophytes. Our results show that P. oceanica meadows in the Spanish Mediterranean support high values of secondary production in other systems by exporting large amounts of leaf detritus as well as acting as substantial carbon sinks by accumulating large reservoirs of belowground detritus. Therefore, the increasing anthropogenic threats on P. oceanica could entail an important loss of secondary production and carbon storage in Mediterranean coastal ecosystems.     

Memorization of nuclear atmospheric tests by rhizomes and scales of the mediteranean seagrass Posidonia oceanica (Linnaeus) Delile

Determination of the age of rhizome sections of Posidonia oceanica (Linnaeus) Delile (Potamogetonaceae) by the examination of their dead leaf scales (lepidochronology), makes it possible to cut out the rhizomes in precisely dated 1- or 5-year sections. With increasing age of the section, the annual section length and the C:N ratio increase, while the dry weight per unit length and the ratio of dry weight:ash weight decrease. At a site where the only significant ¹³⁷Cs contamination can be ascribed to global nuclear fallout bombs, the activity of ¹³⁷Cs was measured i n 5-year age groups of rhizomes and scales separately. The maximum activity of ¹³⁷Cs in scales occurred in groups produced between 1960 and 1964, a period during which a peak of activity occurred in fallout. The distribution of ¹³⁷Cs activity in rhizome age groups indicates an apparent lag, perhaps owing to transport of material in the rhizome. In situ dead rhizome scales of P. oceanica thus constitute a memory, at least 30-years long, of an environmental chemical pulse. Posidonia oceanica could prove a valuable tool for marine-pollution surveys.     

An improved method to monitor the health of seagrass meadows based on kriging

This paper focuses on the spatial pattern of the shoot density of the seagrass Posidonia oceanica through kriging methods. Kriging is a group of geostatistical techniques to predict the value of a field at an unobserved location from observations of its value at nearby locations. The results of an ordinary kriging of both raw values of shoot density and normalized shoot density with reference to a density vs. depth model (Dn) were compared. Results confirmed the existence of spatially organized parameters other than depth that influence patterns of P. oceanica shoot density. Although still requiring additional testing for complete validation, Dn mapping appears to be a valuable tool to (1) infer causes for the meadow's condition and to design a cost-effective field survey; (2) define areas where protection policies should be undertaken.     

A comparison of hole punch and needle punch methods for the measurement of seagrass productivity

Seagrass productivity, as leaf extension, was measured using the hole punch and needle punch techniques. These methods have been widely implemented to determine seagrass leaf extension rates, yet there is no evidence in the literature of a comparison between methods. The hole punch method involves removing part of the basal area of a seagrass leaf and it was proposed that this measurement technique may affect the leaf extension rates that are being measured. Leaf extension rates were measured in Posidonia sinuosa meadows off the coast of Perth, Western Australia. There were no significant differences in seagrass leaf extension between the two methods. The hole punch method is favoured, as measurement of incremental leaf growth is facilitated by the obvious hole left by the punch. The needle punch method leaves lesions on seagrass leaves that are easily confused with other lesions, possibly left by invertebrate grazers. These findings are likely to be applicable to other straplike seagrass species, though a similar comparison is recommended in the initial stages of a study.     

Rapid recovery of the intertidal seagrass Zostera japonica following intense Manila clam (Ruditapes philippinarum) harvesting activity in Korea

Although the Manila clam (Ruditapes philippinarum) culture grounds are occasionally located in Zostera japonica beds along the coasts of Korea, plant responses to the clamming activity have not been reported for this seagrass species. Intense Manila clam harvesting activity took place in the intertidal Z. japonica bed during April 2004. The Z. japonica bed at the study site has been monitored since January 2003. Thus, this study provided a unique opportunity to compare the structure of the Z. japonica population before and after the clamming activity, which was conducted for approximately 1 week in April 2004. All Z. japonica shoots were removed and buried in the sediment immediately after the clamming activity. However, a few shoots were found at the disturbed area in July 2004, 3 months after the clamming activity. By September 2004, 5 months after the disturbance, shoot density and biomass were almost recovered to the levels reported before the clamming activity. No Z. japonica seedlings were observed when the shoot density rapidly increased in August and September 2004, 4-5 months after the disturbance, because revegetation of the disturbed seagrass bed has occurred before the seed germination time which is typically winter or early spring in this area. Thus, the initial rapid revegetation of the disturbed area occurred via asexual reproduction through new shoot formation from the buried below-ground tissues. The reproductive shoot density and reproductive efforts of Z. japonica were significantly higher after the disturbance relative to the levels recorded before the disturbance, and the duration of the fertile period was approximately three times longer following the clamming activity. The belowground biomass after the disturbance was also significantly higher than that before the disturbance. These results suggest that Z. japonica allocated more energy to sexual reproduction, as well as the maintenance of belowground tissues, to persist their population under unstable environmental conditions.     

In vitro culture of the seagrass Halophila decipiens

The seagrass Halophila decipiens Ostenfeld was grown axenically in a culture medium consisting of 20% artificial seawater, f/4 nutrients (except that glutamic acid was the nitrogen source), and 1% sucrose (w:v). The culture medium was adjusted to pH 5.0. A root–rhizome layer was created by solidifying a portion of the medium with 0.9% agar (w:v) and 1% activated charcoal (w:v). The rhizome layer also contained the following vitamins: 0.5 mg l⁻¹ nicotinic acid, 0.5 mg l⁻¹ pyridoxine, 0.5 mg l⁻¹ biotin, 0.5 mg l⁻¹ cyanocobalamin and 0.1 mg l⁻¹ of thiamine HCl. The liquid overlay (without vitamins or charcoal) was poured onto the agar-solidified root–rhizome layer. Growth of H. decipiens was not improved by the addition of the auxins indoleacetic acid (IAA), indolebutyric acid (IBA) or naphthaleneacetic acid (NAA) at either of the tested concentrations (10 and 50 μM). At a concentration of 10 μM, the cytokinins 6-(γ,γ-dimethylallylamino) purine (2iP) and benzylaminopurine (BA) stimulated shoot and branch production compared to controls with no cytokinins. Among the tested nitrogen sources, growth was best on 1.7 mM glutamic acid. Cultures grown on 1.7 mM NH₄Cl showed the same growth rates as those grown on glutamic acid, but the leaves were smaller and curled, suggesting incipient ammonium toxicity. Use of nitrate or urea led to mortality of the cultures. Long term axenic culture of H. decipiens appears to require the added vitamins. Hence, H. decipiens is the first seagrass known to need exogenous vitamins. Cultures of H. decipiens died when grown suspended in liquid cultures or in a biphasic medium system without activated charcoal in the root–rhizome layer. The use of more highly charged κ-carrageenan could not replace the use of activated charcoal and agar in the root–rhizome layer.     

Effects of burial and erosion on the seagrass Zostera noltii

The effects of experimental burial and erosion on the seagrass Zostera noltii were assessed through in situ manipulation of the sediment level (− 2 cm, 0 cm, + 2 cm, + 4 cm, + 8 cm and + 16 cm). Shoot density, leaf and sheath length, internode length, C and N content and carbohydrates of leaves and rhizomes were examined 1, 2, 4 and 8 weeks after disturbance. Both burial and erosion resulted in the decrease of shoot density for all the sediment levels. The threshold for total shoot loss was between 4 cm and 8 cm of burial, particularly during the 2nd week. A laboratory experiment confirmed that shoots did not survive more than 2 weeks under complete burial. There was no evidence of induced flowering by burial or erosion. As well, no clear evidence was found of sediment level effects on leaf and sheath length. Longer rhizome internodes were observed as a response to both burial and erosion, suggesting a plant attempt to relocate the leaf-producing meristems closer to sediment surface or in search of new sediment avoiding the eroded area. The C content of leaves and rhizomes, as well as the non-structural carbohydrates (mainly the starch in rhizomes), decreased significantly along the experimental period, indicating the internal mobilization of carbon to meet the plant demands as a consequence of light deprivation. The significant decrease of N content in leaves, and its simultaneous increase in rhizomes, suggests the internal translocation of nitrogen from leaves to rhizomes. About 50% of the N lost by the leaves was recovered by the rhizomes. Our results indicated that Z. noltii has a high sensitivity to burial and erosion disturbance, which should be considered in the management of coastal activities.     

Expansion of Rosa rugosa and Hippophaë rhamnoides in coastal grey dunes: Effects at different spatial scales

The study analysed the effects of shrub expansion on vegetation composition and plant species diversity in coastal grey dunes on the North Sea island Spiekeroog, comparing Rosa rugosa and Hippophaë rhamnoides. Species composition was recorded in plots of two spatial scales, 1 and 16 m², considering the full range of shrub cover from less than 10 to almost 100%. Although R. rugosa and H. rhamnoides established and spread in the same grey dune environment, the vegetation of the two shrubland types was much different. While the H. rhamnoides plots were relatively species-rich, characterised by remnant grey dune vegetation with many small, often annual, light-demanding species except in the densest shrubs, the R. rugosa plots were clearly species-poorer due to the loss of many typical grey dune species, including only few shade-tolerant taxa. The total number of species, the number of herbaceous species and of species typical for grasslands decreased with increasing cover of H. rhamnoides and R. rugosa at both spatial scales. For the number of shrubs and shrubland species, hardly any significant effects of shrub cover were observed in R. rugosa, while there were positive effects in H. rhamnoides. Both the Shannon index and evenness decreased with increasing cover of the two shrub species at both spatial scales. Here, the decline in species diversity was more improved in R. rugosa than in H. rhamnoides.     

Gene expression in the liver of rainbow trout, Oncorhynchus mykiss, during the stress response

To better appreciate the mechanisms underlying the physiology of the stress response, an oligonucleotide microarray and real-time RT-PCR (QRT-PCR) were used to study gene expression in the livers of rainbow trout (Oncorhynchus mykiss). For increased confidence in the discovery of candidate genes responding to stress, we conducted two separate experiments using fish from different year classes. In both experiments, fish exposed to a 3 h stressor were compared to control (unstressed) fish. In the second experiment some additional fish were exposed to only 0.5 h of stress and others were sampled 21 h after experiencing a 3 h stressor. This 21 h post-stress treatment was a means to study gene expression during recovery from stress. The genes we report as differentially expressed are those that responded similarly in both experiments, suggesting that they are robust indicators of stress. Those genes are a major histocompatibility complex class 1 molecule (MHC1), JunB, glucose 6-phosphatase (G6Pase), and nuclear protein 1 (Nupr1). Interestingly, Nupr1 gene expression was still elevated 21 h after stress, which indicates that recovery was incomplete at that time.     

Trace metal content in the seagrass Cymodocea nodosa: Differential accumulation in plant organs

Differences in the accumulation of seven metallic elements, including micronutrients (Cu, Fe, Mn, Ni and Zn) and non-essential elements (Cd and Pb) among plant organs (leaves, roots and rhizomes) were examined in the seagrass Cymodocea nodosa. Samples were taken from two coastal bays (Catalonia, Western Mediterranean), with a total of nine sampling sites encompassing different levels of metal availability. Metal content was generally higher in uptake organs (leaves and roots) than in rhizomes. However, accumulation in leaves and roots varied between elements. While Cd, Mn and Zn preferentially accumulate in leaves, Fe and Pb accumulate in roots and Cu and Ni in both. There were common spatial (between sites) trends in Cd, Mn, Cu and Zn accumulation in the three organs. However, these spatial trends varied according to the organ considered in the case of Fe, Pb, and Ni. Therefore, assessment of within-plant variability is strongly recommended prior to the use of C. nodosa for biomonitoring purposes, at least for Fe, Pb, and Ni.     

Male territorial aggression and androgen modulation in high latitude populations of the Sooty, Passerella iliaca sinuosa, and Red Fox Sparrow, Passerella iliaca zaboria

The Fox Sparrows, Passerella iliaca, include multiple groups and subspecies distributed at several latitudes from the Alaskan arctic to the southwestern United States. As such, this species represents a potential model for investigating latitudinal variation in androgen secretion and aggressive territoriality in male passerines. Breeding male Fox Sparrows from two subspecies within two groups, the Sooty Fox Sparrow, P. i. sinuosa, and the Red Fox Sparrow, P. i. zaboria, were assessed for aggressive territoriality and androgen responsiveness at multiple latitudes in arctic and subarctic Alaska. Subarctic Sooty Fox Sparrows had higher circulating androgen levels in the early (8.54 ng/ml) versus mid–late breeding season (2.44 ng/ml). Males in the mid–late breeding season did not up-regulate androgen secretion in response to social challenge, but were aggressive and spent more time within 5 m of a decoy during a simulated territorial intrusion (STI) than early breeding males. Male subarctic Red Fox Sparrows had slightly higher circulating androgen levels (2.29 ng/ml) than arctic males (1.10 ng/ml) in the mid–late breeding season. However, androgen levels were not correlated with blood collection time after a social challenge in either group, suggesting that neither arctic nor subarctic males up-regulate androgen secretion during the mid–late breeding period. Arctic males spent more time within 5 m of a decoy and sang less than subarctic males during an STI in the mid–late breeding season. These findings demonstrate that the Fox Sparrow is a tractable model for investigating the latitudinal regulation of aggressive territoriality and androgen responsiveness in passerines.     

Evidence of the active role of ligno-cellulosic enzymes of marine fungi in degradation of Posidonia oceanica leaves

Evidence is given of release of six enzymes activities generally associated with plant material degradation, during in vitro experiments with Corollospora maritima acting upon Posidonia oceanica leaves. The results obtained are discussed in the light of the present knowledge on the role of marine fungi in the biological breakdown and conversion of leaf material in marine ecosystem.     

Reproductive phenology of the Mediterranean seagrasses Posidonia oceanica (L.) Delile, Cymodocea nodosa (Ucria) Aschers., and Zostera noltii Hornem.

Investigations on the sexual reproduction of Posidonia oceanica (L.) Delile, Cymodocea nodosa (Ucria) Aschers. and Zostera noltii Hornem. were conducted in situ from 1979 to 1988 in beds around the island of Ischia. Seed germination and phenological features of seedlings of P. oceanica and C. nodosa were observed in situ and in the laboratory. The reproductive cycles showed, for each species, different timing of start and duration of flowering, fruiting and germination. Posidonia oceanica flowers always appeared in September in shallow stands and in November in stands deeper than 15 m. This time delay seems to be related to the different maximum summer temperatures found at those depths. This species did not show dormancy. Cymodocea nodosa flowering was always recorded after the minimum winter temperature (April–May); germination of the seeds followed after 8–10 months of dormancy. Zostera noltii flowers were always recorded in July, with fruiting in August. The life cycles of these species seem to be related to the annual temperature range rather than to other environmental parameters. The different reproductive strategies adopted by each species are discussed.     

Spectral response of the seagrass Zostera noltii with different sediment backgrounds

The efficiency of vegetation indices (VIs) to estimate the above-ground biomass of the seagrass species Zostera noltii Hornem. from remote sensing was tested experimentally on different substrata, since terrestrial vegetation studies have shown that VIs can be adversely influenced by the spectral properties of soils and background surfaces. Leaves placed on medium sand, fine sand and autoclaved fine sand were incrementally removed, and the spectral reflectance was measured in the 400–900 nm wavelength range. Several VIs were evaluated: ratios using visible and near infrared wavelengths, narrow-band indices, indices based on derivative analysis and continuum removal. Background spectral reflectance was clearly visible in the leaf reflectance spectra, showing marked brightness and spectral contrast variations for the same amount of vegetation. Paradoxically, indices used to minimize soil effects, such as the Soil-Adjusted Vegetation Index (SAVI) and the Modified second Soil-Adjusted Vegetation Index (MSAVI₂) showed a high sensitivity to background effects. Similar results were found for the widely used Normalized Difference Vegetation Index (NDVI) and for Pigment Specific Simple Ratios (PSSRs). In fact, background effects were most reduced for VIs integrating a blue band correction, namely the modified specific ratio (mSR₍₇₀₅₎), the modified Normalized Difference (mND₍₇₀₅₎), and two modified NDVIs proposed in this study. However, these indices showed a faster saturation for high seagrass biomass. The background effects were also substantially reduced using Modified Gaussian Model indices at 620 and 675 nm. The blue band corrected VIs should now be tested for air-borne or satellite remote sensing applications, but some require sensors with a hyperspectral resolution. Nevertheless, this type of index can be applied to analyse broad band multispectral satellite images with a blue band.