Patterns of tetraspore discharge in Caloglossa and Murrayella (Ceramiales, Rhodophyta)

Caloglossa released tetraspores only during dark periods in contrast to a marked light-period release exhibited by Murrayella. Both could be readily converted to a reversed light-dark cycle after a lag period of 1 day. Maximum tetraspore discharge by Caloglossa occurred approximately 4–5 h after the onset of darkness and by Murrayella 4–5 h after the onset of light. Neither 8- nor 16–h light periods affected spore release patterns in Caloglossa, while in Murrayella the spore release pattern was briefly disrupted by short day lengths, but recovered after several days. In both genera, the rhythmicity was disrupted in periods of continual darkness or continual light and appeared to be independent of light quality; however, minimum irradiance levels were critical to maintenance of release patterns. Possible mechanisms of tetraspore discharge periodicity are discussed but since both plants typically have a limited habitat as epiphytes on mangrove trunks and pneumatophores, any speculations as to the adaptive advantages of each pattern tend toward mutual contradiction.     

Stand structure influences nekton community composition and provides protection from natural disturbance in Micronesian mangroves

Structurally complex mangrove roots are thought to provide foraging habitat, predation refugia, and typhoon protection for resident fish, shrimp, and crabs. The spatially compact nature of Micronesian mangroves results in model ecosystems to test these ideas. Tidal creek nekton assemblages were compared among mangrove forests impacted by Typhoon Sudal and differing in stand structure. Structurally complex Rhizophora spp. stands were predicted to support higher densities and different communities of nekton and to provide greater protection from typhoons compared to less complex Sonneratia alba/Bruguiera gymnorrhiza stands. Lift net data revealed that structural complexity did not support greater nekton densities, but did support significantly different nekton assemblages. The cardinalfish Apogon ceramensis and goby Oxyurichthys lonchotus had significantly higher densities in S. alba/B. gymnorrhiza mangrove creeks, whereas the silverside Atherinomorus lacunosus and diogenid crabs had significantly higher densities in Rhizophora spp. creeks. Similar nekton densities 17 and 4 months after the typhoon in Rhizophora spp. creeks provided indirect evidence that structural complexity increased protection for resident nekton from disturbances. Findings indicate that studies of structural complexity and nekton densities may be better served when individual species are compared and that diverse mangrove tree assemblages will support diverse nekton assemblages that may be more resilient to disturbance.     

Avicennia canopy effects on mangrove algal communities in Spencer Gulf, South Australia

Studies of mangrove algal communities at eight localities in Spencer Gulf, South Australia, provide evidence that the presence or absence of an Avicennia tree canopy may influence the frequency distribution of algal species on pneumatophores. Certain algae occurred with significantly higher frequencies on pneumatophores. Certain canopy, while other species were significantly more frequent on pneumatophores beyond the canopy. The distribution of total algal cover, total algal biomass, and total species diversity, however, does not appear to be correlated with the presence or absence of a tree canopy.     

Studies on Australian mangrove algae. III. Victorian communities: Structure and recolonization in western port bay

The algal communities associated with Avicennia pneumatophores in Western Port Bay, Victoria, Australia (145°E:38°S) are composed primarily of the red algae Bostrychia, Caloglossa and Catenella. Trends from seaward to landward indicate a decrease in frequency of occurrence, relative cover, and mean absolute biomass for all algal genera but differing trends occur for each genus in terms of relative biomass, and this is reflected in associated pattern analyses. In terms of vertical community structure, all algae occur most frequently in the 5–10 cm segment above mudline, whereas above 20 cm, values for all measured parameters decline. Vertical structure in the seaward and landward regions is compared, but pattern analyses indicate that no biologically distinctive pattern is present. Deliberately denuded pneumatophores quickly become recolonized; greater algal development occurs above 10 cm above the mudline than below 10 cm, and the new community is differently structured.     

Osmoprotectors in some species of Japanese mangrove macroalgae

The macroalgae asSociated with the mangrove vegetation of the Japanese Islands Okinawa, Ishigaki and Iriomote were investigated. The flora includes members of the red algal genera Bostrychia, Caloglossa and Catenella, as well as the brown alga Dictyotopsis propagulifera Troll, which may be considered typical of mangrove forests. The distribution of the low molecular weight carbohydrates sorbitol, dulcitol, mannitol and floridoside was studied in the mangrove algae. Their physiological role as osmoprotectors was assessed by investigating the effect of salinity on the intracellular sorbitol and dulcitol concentration in Bostrychia pinnata J. Tanaka et Chihara and on the mannitol content in D. propagulifera. In both species the polyol values increased with increasing salinity.     

The community structure of macroalgae in a low shore mangrove forest in Selangor,Malaysia

The macroalgal communities associated with pneumatophores, basal area of tree trunks and sediment surface in the mangrove forest at Sementa, Selangor consisted of nine main species. Biomass, frequency of occurrence and relative cover of the species along a belt transect, showed two major trends, a decrease in these parameters in the landward direction for Colpomenia sp. Gracilaria blodgettii and Gracilaria crassa and an increase in the landward direction for Dictyota dichotoma, Catenella nipae, Rhizoclonium sp. and Bostrichia radicans. Algal dominance varied with substratum. Pneumatophores were dominated by Caloglossa lepreurii and sediment surface by D. dichotoma. The 40 cm zone at the base of tree trunks was dominated by two algal species. The 0–20 cm region above the sediment surface was colonized by C. nipae, while the 20–40 cm region was dominated by Rhizoclonium sp. The study identified the importance of substrate in macroalgal colonization.     

Interactive effects of shade and surface orientation on the recruitment of spirorbid polychaetes

A series of observations and an experiment were carried out to test hypotheses about the effects of shade on the densities of spirorbid polychaetes (Neodexiospira spp.) on intertidal pneumatophores (mangrove roots) of Avicennia marina. Densities of spirorbids were greater on pneumatophores surrounded by seagrass (Zostera mucronata) than patches without seagrass. Within patches of seagrass, the density and survivorship of spirorbids on pneumatophores was greater near the substratum (covered by seagrass) than high above the substratum (not covered by seagrass). The model that these patterns of abundance are explained by greater recruitment of spirorbids to shaded surfaces was assessed. This was done by experimentally testing the hypothesis that recruitment to patches without seagrass would not differ between the upper (unshaded) and lower surfaces (unshaded) of clear plastic sheets, but would be greater on the lower surfaces (shaded) than upper surfaces (unshaded) of black plastic sheets. Recruitment was consistent with these predictions and therefore provided evidence that differences in densities of spirorbids between substrata with and without seagrass may be due largely to differences in shading.     

Weak diurnal changes in the biochemical properties and benthic macrofauna of urbanised mangrove forests and mudflats

Diurnal changes in the biochemical properties and the benthic macrofaunal assemblage of sediments in urbanised mangrove forests and their adjacent mudflats in Sydney Harbour were investigated. Behavioural and physiological changes in the microphytobenthos between day and night were predicted to cause diurnal changes in the micro-scale depth distribution of chlorophylls a and b and colloidal carbohydrate. In addition, because macrofauna can alter sediment properties, diurnal changes in the macrofaunal assemblages were investigated. The microphytobenthos at the study sites were predominantly filamentous green algae, although diatoms were present. Samples for biochemical analysis were collected from the top 2 mm of sediment using mini-cryolanders, during low tide in the day and at night. Three biochemical properties of the sediments were measured spectrophotometrically: chlorophylls a and b (surrogate for microphytobenthos biomass) and colloidal carbohydrate. The amount of chlorophylls tended to be less at night than during the day, but site to site variability was large and these differences were generally small and not significant. Depth profiles indicated that there was some redistribution of pigments in the surface 2 mm between day and night, possibly due to migration of microphytobenthos or physiological changes. There was no significant difference in chlorophylls between the mangrove forest and adjacent mudflat, with the exception of chlorophyll b at one sampling time, which was larger in the mangrove forest than on the mudflat. Colloidal carbohydrate was significantly larger in the mangrove forest and significantly less on the mudflat during the day at one site at one time, but otherwise showed no significant differences between day and night or between the mangrove forest and mudflat. Whilst there were some differences in the benthic macrofaunal assemblages between day and night, these differences were only significant for spionids and polychaetes at one time. There were, however, significant differences in assemblages of benthic macrofauna between the mangrove forest and mudflat, probably due to structural differences between these habitats such as the presence of pneumatophores, shade and leaf litter. In summary, there was some minor diurnal variation in the measured biochemical properties of the sediment, but not in the macrofaunal assemblage. Diurnal changes should, therefore, be considered when investigating biochemical properties in these habitats, but they are not a major influence. These findings contrast to previous studies on diatom dominated mudflats in Europe, where stronger diurnal changes in biochemical properties were found. Diurnal changes in the macrofauna assemblages were largely insignificant and therefore could not explain the changes in the biochemical properties. Diurnal effects on the macrofauna in these habitats are more likely to be via altered behaviours and this requires further investigation.     

Biodiversity and trophic structure of invertebrate assemblages associated with understorey red algae in a Laminaria digitata bed

Although the composition of invertebrate assemblages associated with kelps has motivated several studies in the recent past, little is known about assemblages associated with smaller, understorey macroalgae in these ecosystems. Here, the composition of invertebrate assemblages living within understorey macroalgae of a kelp (Laminaria digitata) forest has been studied over two sampling dates, and the structure of the food web investigated using stable isotopes. A total of 145 species of mobile fauna, mainly amphipods and gastropods, were identified. Although differences were date-dependent, we showed that the three species considered (Palmaria palmata, Mastocarpus stellatus, Ellisolandia elongata) sheltered different associated assemblages, including high host-specificity, which suggests that the animal diversity associated with rocky shores is enhanced by a high algal diversity. Overall, a dominance of gastropods was observed for the two-dimensional, leaf-like P. palmata, while the three-dimensionally structured species (M. stellatus, E. elongata) were characterized by a dominance of amphipods. Stable isotopes highlighted different trophic structures; E. elongata-associated assemblages were most likely relying on a dominant food source, sediment organic matter, while other assemblages were based on a wider diversity of food sources (algae, sediment, suspended organic matter). Our results illustrate the need to consider the microhabitats formed by understorey macroalgae in order to thoroughly assess the diversity and understand the functioning of coastal rocky ecosystems.     

Plant-mediated methane emission from an Indian mangrove

Mangroves have been considered for a long time to be a minor methane source, but recent reports have shown that polluted mangroves may emit substantial amounts of methane. In an unpolluted Indian mangrove, we measured annual methane emission rates of 10 g CH₄ yr^?1 from the stands of Avicennia marina. This rate is of the same order of magnitude as rates from Northern wetlands. Methane emission from a freshwater‐influenced area was higher, but was lower from a stunted mangrove growing on a hypersaline soil. Methane emission was mediated by the pneumatophores of Avicennia. This was consistent with the methane concentration in the aerenchyma, which decreased on average from 350 ppm_v in the cable roots to 10 ppm_v in the emergent part of the pneumatophores. However, the number of pneumatophores varied seasonally. The minimum number occurred during the monsoon season, which reduced methane emissions largely. Ebullition from unvegetated areas may also be important, at least during monsoon season when measured bubble fluxes were occasionally about five times as high as pneumatophore‐mediated emissions.     

Re-establishment of epibiotic communities in reforested mangroves of Gazi Bay, Kenya

Recolonization of epibiotic flora and fauna in two fringing Sonneratia alba reforestation plots was investigated and compared to a natural mangrove stand and a denuded site in Gazi Bay, Kenya. The reforested sites differed with respect to land history and planting density. Habitat availability in the form of pneumatophore surface differed among forested sites (P<0.001), and between landward and seaward zones (P<0.05). Eighteen algal species were found in the natural area compared to 23 and 10 in replanted sites. Only one species was encountered in the denuded area. SIMPER analysis distinguished Enteromorpha ramulosa, Polysiphonia sp., Hypnea sp. and Caloglossa leprieuri as the main algal species responsible for differences between sites. Algal biomass was positively correlated to pneumatophores area (P<0.001). Total algal biomass differed markedly between forested sites: 1.4 (matrix replantation), 28.6 (natural stand) and 44.3 g m⁻² (integrated replantation) in the seaward zones. The matrix replantation showed strong differences in algal community assemblages compared to the other forested sites, and this site also had significantly lower biomass of sessile benthic fauna (P<0.001). Statistical differences in algal (P<0.01) and sponge (P<0.05) community composition between landward and seaward zones were observed in all sites and trunk fouling fauna was distinctly different between sites. Reasons for the above patterns are discussed and it is suggested that zonation patterns affecting pneumatophore surface and inundation time, in combination with proximity of sites to natural seeding areas, are the most likely explanations for observed patterns of epibiotic community distribution in this study.     

西门岛人工秋茄林恢复对大型底栖生物的影响

选择乐清湾西门岛海域相同高程断面不同造林时间的人工红树林(秋茄林)、光滩和互花米草丛,采用空间代替时间的方法,分析我国分布最北界人工红树林造林过程对大型底栖生物的影响.大型底栖动物生活型分布基本表现为幼林(1、4、8a秋茄林)以底上附着型为主,而在光滩、50a秋茄林和互花米草中底下生活类群相对增加.并且穴居型动物只出现在发育成熟的生态系统内.各项指标显示50a老林群落生态稳定性较好,光滩和互花米草丛次之,但优于发育中的秋茄幼林.与以往研究结果不同,50a老林的大型底栖动物生物种类的丰度及群落的物种多样性最高,并不与红树林的发育状况呈负相关,也不比邻近光滩低.结合50a林下滩涂底泥情况,西门岛50a红树林林下滩涂的底质发育要落后于国内天然红树林土壤.这可能与当地红树林造林规模小以及强潮差海域有关.此外,红树林恢复过程中,大型底栖动物生物多样性与生态稳定性之间的线性关系,其适用的系统面积和演替时间的尺度范围有必要做更加深入的探讨.     

Analysis of diversity of chromophytic phytoplankton in a mangrove ecosystem using rbcL gene sequencing

Phytoplankton forms the basis of primary production in mangrove environments. The phylogeny and diversity based on the amplification and sequencing of rbcL, the large subunit encoding the key enzyme ribulose‐1, 5‐bisphosphate carboxylase/oxygenase was investigated for improved understanding of the community structure and temporal trends of chromophytic eukaryotic phytoplankton assemblages in Sundarbans, the world's largest continuous mangrove. Diatoms (Bacillariophyceae) were by far the most frequently detected group in clone libraries (485 out of 525 clones), consistent with their importance as a major bloom‐forming group. Other major chromophytic algal groups including Cryptophyceae, Haptophyceae, Pelagophyceae, Eustigmatophyceae, and Raphidophyceae which are important component of the assemblages were detected for the first time from Sundarbans based on rbcL approach. Many of the sequences from Sundarbans rbcL clone libraries showed identity with key bloom forming diatom genera namely Thalassiosira, Skeletonema and Nitzschia. Similarly, several rbcL sequences which were diatom‐like were also detected highlighting the need to explore diatom communities from the study area. Some of the rbcL sequences detected from Sundarbans were ubiquitous in distribution showing 100% identities with uncultured rbcL sequences targeted previously from the Gulf of Mexico and California upwelling system that are geographically separated from study area. Novel rbcL lineages were also detected highlighting the need to culture and sequence phytoplankton from the ecoregion. Principal component analysis revealed that nitrate is an important variable that is associated with observed variation in phytoplankton assemblages (operational taxonomic units). This study applied molecular tools to highlight the ecological significance of diatoms, in addition to other chromophytic algal groups in Sundarbans.     

Meiofaunal recruitment to mimic pneumatophores in a cool-temperate mangrove forest: spatial context and biofilm effects

A field experiment was devised to test whether meiofauna that colonised mimic pneumatophores (artificial substrates) resembled the assemblage on adjacent live pneumatophores in three randomly chosen intertidal, estuarine sites. The experiment showed that the close proximity of particular biota on living pneumatophores did not reliably influence subsequent development of assemblages upon mimic pneumatophores within a scale of 10 m during a colonisation period of less than 20 weeks. There was some convergence of the composition of the colonising assemblage of meiofauna on mimic pneumatophores with the local assemblages in sites dominated by barnacles, or where the natural pneumatophores were free from macroscopic epibionts. However, tychopelagic meiofauna from algal epiphytes did not significantly colonise mimic pneumatophores during the 20-week trial, probably due a lack of growing algae. During the conditioning phase suspended in water at a marine site 20 km from the mangroves, mimic pneumatophores acquired an assemblage of meiofauna different from the estuarine assemblage that colonised mimics following implantation in the estuarine mudflat. Enhanced colonisation rates of mimics in suspended bags at the conditioning site may be explained by the absence of benthic macroinvertebrates, and the lack of intertidal exposure. Biofilms aged 2, 7, and 11 weeks had no consistent, different effect on the subsequent colonisation of meiofauna. We conclude that divergence of phytal-based assemblages of meiofauna depends upon the amount of coverage, as well as the type, of fouling macro-epibionts on the pneumatophores. Meiofaunal assemblages on artificial substrates after 20 weeks colonisation displayed less intrinsic patchiness than mature phytal assemblages on natural pneumatophores, and so present a potentially useful way of improving the power of biomonitoring applications using meiofauna.     

Microbial carbon oxidation rates and pathways in sediments of two Tanzanian mangrove forests

Temporal and spatial variations in benthic metabolism and anaerobic carbon oxidation pathways were assessed in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. The objectives were (1) to evaluate how benthic metabolism is affected by organic carbon availability; (2) to determine the validity of diffusive release of CO₂ as a measure benthic carbon oxidation; and (3) to assess the partitioning of anaerobic carbon pathways and factors controlling the availability of electron acceptors (e.g. oxidized iron). Microbial carbon oxidation measured as diffusive exchange of O₂ and CO₂ (32?C67 and 28?C115 mmol m^?2 day^?1, respectively) showed no specific temporal patterns. Low intertidal sediments at Mtoni fed by labile algal carbon of anthropogenic origin had higher diffusive CO₂ release than high intertidal sediments that primarily received less reactive mangrove detritus. Diffusive release of CO₂ apparently underestimated total sediment carbon oxidation due to CO₂ loss from deep sediments via emission through biogenic structures (i.e. crab burrows and pneumatophores) and porewater seepage into creeks. We propose that diffusive fluxes in the present mangrove sediments are roughly equivalent to depth-integrated reactions occurring in the upper 12 cm. Anaerobic carbon oxidation was dominated by FeR irrespective of anthropogenic influence in sediments where the oxidizing effects of biogenic structures increased the Fe(III) level. More than 80% of the anaerobic carbon oxidation in Mtoni and Ras Dege sediments was due to FeR when reactive Fe(III) exceeded 30 ??mol cm^?3. The anthropogenic influence at Mtoni was primarily noted as up to one order of magnitude higher denitrification than at Ras Dege, but this process always accounted for less than 1% of total carbon oxidation. It is noteworthy that organic and nutrient enrichment of anthropogenic origin in Mtoni has no measurable effect on microbial processes, other than carbon oxidation in the low intertidal area and denitrification throughout the forest, and indicates a strong resilience of mangrove environments towards disturbances.     

DIFFERENTIAL OXIDATION OF MANGROVE SUBSTRATE BY AVICENNIA GERMINANS AND RHIZOPHORA MANGLE

Both Avicennia germinans (black mangrove) and Rhizophora mangle (red mangrove) are commonly found in anaerobic substrate. Black mangrove roots create oxidized rhizospheres substantially larger than those described for other plant species. When air cannot enter the root system through the pneumatophores, the rhizospheres become as reduced as nearby un vegetated soil. The presence of red mangrove, in contrast, appears to have no effect on the oxidation state of surrounding anaerobic soils. It is likely that these data help to explain the distributions of both species in intertidal associations.     

The effects of habitat edges and trampling on understorey vegetation in urban forests in Helsinki,Finland

Question: How do habitat edges and trampling affect mesic (Myrtillus type) understorey vegetation in fragmented urban forests, and what are the relative strengths and spatial extent of these effects? Location: The city of Helsinki, southern Finland. Methods: Vegetation sample plots were placed at different distances from the forest edge. At each distance we selected plots on, next to and away from paths of different levels of wear, and the covers and frequencies of plant species were sampled. We used generalized linear mixed models and ordination techniques to test our hypotheses. Results: The effect of the edge penetrated up to 50 m into the forest interior. Only light effects of trampling were found in seemingly untrampled areas, in addition to the direct effects of trampling on the paths. Both trampling and edge effects changed the forest understorey species composition. Trampling changed plant species composition locally on paths, and provided opportunities for new species to establish in previously unbroken forest vegetation. Urban forest edges were characterised by species better adapted to sunny, warm and dry conditions. These species, such as grasses, replaced more sensitive forest species, such as dwarf shrubs and mosses. We observed a large number of light demanding deciduous trees at the edges, which may be one of the factors increasing soil fertility and consequently promoting the high abundance of fast growing species, such as grasses and herbs related to more fertile site types. Conclusions: We recommend that urban forest fragments left within urban development should be at least 2–3 ha in size and that the shape of the patch and the number of potential recreational users should be taken into account when preserving true forest understorey vegetation from the effects of edges and trampling.     

Issue Information

Collecting in New Ireland, Papua New Guinea. Upper left: coastal mangroves in Karu, Kaguso. Lower left: large mangrove trees distributed near Uparage village, and mangrove visitors. Right: Caloglossa thalli attached to an aerial root of a mangrove tree. Photos by M. Kamiya. See Kamiya et al. in this issue for details. Cover picture from: Article link here

     

Effect of diet and temperature upon muscle metabolic capacities and biochemical composition of gonad and muscle in Argopecten purpuratus Lamarck 1819

It has been difficult to understand the effects of habitat structure on assemblages because the different elements of habitat structure are often confounded. For example, few studies consider that the effects of structural components of a habitat (rocks, trees, pits, pneumatophores) may be separate from the complexity (e.g. surface area {SA}) they create. From prior observations and experiments, I developed three hypotheses about the effects of habitat structure on gastropods on rocky intertidal shores in Botany Bay, Australia. (1) The complexity of habitats positively affects the density and richness of gastropods. (2) The fractal dimension (D) represents elements of complexity that affect the density and richness of gastropods better than other indices of complexity. (3) The effects of specific structural components on the density and richness of gastropods are independent of their complexity. To test these hypotheses, treatments composed of pits and pneumatophores were used to independently manipulate complexity and structural components in experiments repeated at five different times on two shores. There was support for hypotheses (1) and (3) at most times and places but not for hypothesis (2). Richness, total density, and the densities of two of the three most common gastropods were greater in treatments with greater complexity. D was not definitively better than other indices of complexity, but D and SA were recommended for further consideration. When complexity was held constant, species richness and the density of most gastropods, except Austrocochlea porcata, was greater in treatments with pits than with pneumatophores. A common mechanistic explanation for the effects of habitat complexity on rocky intertidal gastropods relies on a specific characteristic of pits; they pool water and reduce desiccation stress. This assumption may be appropriate for many gastropods, but it was inappropriate for A. porcata. Habitat complexity affected its density, but this was not because of a characteristic specific to pits. The complexity and structural components of habitats have separate effects on assemblages, and it confuses the study of habitat structure to combine them.