The growth patterns of some hydroids that are obligate epiphytes of seagrass leaves

The growth patterns of the hydroids Campanularia asymmetrica, Sertularia perpusilla and Algaophenia harpago on the Mediterranean seagrass Posidonia oceanica, and of Laomedea angulata on the temperate seagrass Zostera marina, were examined. The seagrass leaves grow from a basal meristem and have a short life which presents problems for these long-lived obligate epiphytes. Downward growth onto younger leaf tissue, shown by S. perpusilla and C. asymmetrica, reduces the risk of interspecific competition with other epiphytes, and places the hydroids on substrates that survive longer than older distal leaf tissue, close to short young leaves which they can colonize by stolon transfer. S. perpusilla was found only on the basal areas of leaves, possibly because of the progressive degeneration of their most apical tissue. C. asymmetrica showed no such degeneration and was found over most of the length of the leaves. The hydrorhizae of most small A. harpago and L. angulata grew both upward and downward along the leaves. Both species were most abundant on the apical halves of older leaves. This distal distribution is considered to be advantageous for these large branched hydroids which are suited for capturing planktonic prey, and to enhance their probability of colonizing neighbouring plants. Its disadvantages include the reduced probability of maintaining the hydroids on the same plant and an increased risk of being eaten by seagrass grazers.     

Epiphyte grazing enhances productivity of remnant seagrass patches

Anthropogenic nutrient enrichment is increasingly modifying community structure and ecosystem functioning in terrestrial and aquatic ecosystems. In marine ecosystems, the paradigm is that nutrient enrichment leads to a decline of seagrasses by stimulating epiphytic algal growth, which shades and overgrows seagrasses. This ignores the potential for herbivores, which graze upon epiphytic algae, to partially or wholly counter such nutrient effects. We conducted a field experiment to assess the role that the trochid gastropod Calthalotia fragum plays in reducing nutrient impacts on the seagrass, Posidonia australis, in an urbanized Australian estuary, Botany Bay, Sydney. In a field experiment, where nutrient loading and grazer density were orthogonally manipulated, nutrient enrichment failed to promote epiphyte biomass or diminish growth and primary productivity of P. australis. To the contrary, nutrient enrichment enhanced photosynthesis of the seagrass in plots where the grazer was present at higher density. Epiphytic growth was negatively affected by increased C. fragum density, while P. australis shoot growth was positively influenced. Thus, in this study system, grazing appears to play a much greater role in determining seagrass primary productivity and above‐ground growth than moderate nutrient loading, suggesting that the interaction between grazers and nutrients depends on the relative levels of each. Our study contributes to a growing body of literature suggesting that effects of nutrient loading on benthic assemblages are not universally negative, but are dependent on the biotic and abiotic setting.     

Grazing effects of a gammaridean Amphipoda,Ampithoe sp., on the seagrass,Syringodium isoetifolium,and epiphytes in a tropical seagrass bed of Fiji

In the pure stand of tropical seagrass,Syringodium isoetifolium, in a small oceanic island, Fiji, grazing effects of the seagrass-associated gammarid,Ampithoe sp., on seagrass and epiphytes were assessed in October 1989, November 1991, November 1992. Density of the gammarid was estimated with two methods, mesh bag method and tuft method. During the three years surveyed the density of the gammarid increased remarkably from 1989 to 1991, with heavy epiphytism. Gut contents of the gammarid were examined. Grazing rates on seagrass leaf with and without epiphytic blue-green algae were measured in a bottle experiment. Litter bag experiments were conducted using different mesh sizes each containing seagrass only and seagrass and gammarids. The seagrass leaf biomass in the litter bag reduced abruptly in both bags. After one week, 78–86% of seagrass biomass disappeared from the bags. Enhancement of decomposition of seagrass leaf by the gammarid grazing was observed. Oxygen consumption and ammonium excretion rates were measured simultaneously in bottle experiments. Carbon budget in the seagrass bed was estimated as follows: 0.9 gC m⁻² day⁻¹ in seagrass growth, gammarid grazing was about a half of it and further assimilated a half of it, about 0.1 gC m⁻² day⁻¹, and more than half of it become CO₂ by respiration. Grazing effects on epiphyte and seagrass growth and production were discussed through the carbon budget and indirect interactions between seagrass, epiphytes and associated gammarids to explain the temporal change of seagrass and epiphyte dynamics.     

海草种子特性与海草床修复

海草床面积的急剧减少已引起人们对海草床生态修复的重视, 种子在海草床修复中具有较大的应用潜力。该文系统归纳了近年来有关海草种子的研究概况及其应用进展: 从种子生物学和生态学角度对海草种子的形态结构、发育、散布、休眠和萌发进行了归纳比较; 提出了在研究天然海草种子库的同时, 有必要建立人工海草种子库的观点, 并指出建立和完善人工海草种子库需基于种子生物学和生态学研究, 以指导种子的采集和保存; 对海草种子的播种方式和利用种子修复海草床的途径进行了阐述, 并指出了目前研究和应用中存在的一系列问题。最后对海草种子的研究和应用前景进行了展望, 提出海草种子生态学将成为今后的研究热点, 在应用方面, 仍然需要以提高种子萌发率和成苗率为研究重点, 并指出如果利用由种子获得的人工幼苗进行海草床修复, 需首先解决幼苗成活率低的难题。     

Trophic relationships among concentrated populations of small fishes in seagrass meadows

Changes in available food and utilized foods for a densely populated guild of animals can uncover periods of niche displacement among particular consumer species or their separate size classes. Dense populations of small fishes and their foods were çensussed for 13 months and studied experimentally in shoalgrass and turtlegrass meadows of Redfish Bay, Texas. Feeding habits were determined with respect to prey availability, and illustrated the extent to which seasonal partitioning of food corresponded to food depletion among these abundant consumers in seagrass meadow food webs. The darter goby, code goby, pinfish, and Gulf pipefish were the most common species throughout the year. Although the darter goby did not show distinct ontogenetic changes in food habits, the code goby, Gulf pipefish, and pinfish demonstrated major ontogenetic progressions of foods selected.Food availability in the seagrass meadows changed seasonally. When major prey such as amphipods were abundant, during spring, many fish species showed high overlap in food use. Regardless of food availability, the code goby and Gulf pipefish fed mainly on amphipods or copepods. The more common darter goby and pinfish seasonally changed their diets with changes in food availability; the darter goby and pinfish were more carnivorous during spring, but they largely consumed epiphytic algae during summer. Cage experiments were used to monitor foods confined with elevated densities of pinfish and darter gobies, relative to control cages at natural overall densities. Prey items in the former cages decreased sharply, with corresponding dietary shifts by these common fishes. During resource depletion, these changes in resource use by these naturally concentrated consumers appeared as temporary partitioning of available resources. This shift occurred during both natural (seasonal) and experimental depletions of food, and appears to result from increased interspecific and intraspecific competition during periods of depleted preferred foods.     

Strong leaf surface basification and CO2 limitation of seagrass induced by epiphytic biofilm microenvironments

Coastal eutrophication is a growing problem worldwide, leading to increased epiphyte overgrowth of seagrass leaves. Yet little is known about how epiphytes affect key biogeochemical conditions and processes in the seagrass phyllosphere. We used electrochemical microsensors to measure microgradients of O₂, pH, and CO₂ at the bare and epiphyte-covered leaf surface of seagrass (Zostera marina L.) to determine effects of epiphytes on the leaf chemical microenvironment. Epiphytes result in extreme daily fluctuations in pH, O₂, and inorganic carbon concentrations at the seagrass leaf surface severely hampering the plant's performance. In light, leaf epiphyte biofilms and their diffusive boundary layer lead to strong basification, markedly reducing the CO₂ and HCO₃^- availability at the leaf surface, leading to reduced photosynthetic efficiency as a result of carbon limitation and enhanced photorespiration. With epiphytes, leaf surface pH increased to >10, thereby exceeding final pH levels (~9.62) and CO₂ compensation points for active photosynthesis. In darkness, epiphyte biofilms resulted in increased CO₂ and hypoxia at the leaf surface. Epiphytes can lead to severe carbon limitation in seagrasses owing to strong phyllosphere basification leading to CO₂ depletion and costly, yet limiting, HCO₃^- utilization, increasing the risk of plant starvation.     

Light adaptation and the role of autotrophic epiphytes in primary production of the temperate seagrass, Zostera marina L.

Photosynthetic features of Zostera marina L. and its autotrophic epiphyte community were investigated in a population inhabiting a shallow (1.3 m depth) water meadow in Great Harbor, Woods Hole, MA (U.S.A.). Photosynthesis versus irradiance (P-I) relationships were measured with respect to leaf age determined by the leaf position in the shoot bundle and by location of the tissue along the leaf axis. Therefore both age and light intensity gradients along the leaf axis were considered. The maximum photosynthesis (P_max) per dm² typically increased nearly two-fold along the leaf axis from leaf bases to apices. Photosynthetic rate on a chlorophyll (Chl) basis did not increase as dramatically along the leaf axis, and rates were usually lowest in tissues with the highest Chl content. The P-I relationships of leaves of different ages did not reveal photoinhibition even at light intensities > 1400 μE • m⁻² • s ⁻¹. Furthermore, no photoinhibition was observed in tissues from leaf blade bases, which never experienced high light levels (> 500 μE • m ⁻² • s⁻¹) in situ in Great Harbor. The initial slopes of the P-I curves and light compensation and saturation values varied along the leaf axis in relation to in situ light intensity gradients and in relation to leaf or tissue age. It appeared that leaf and/or tissue age was more important than light environment in determining P-I responses. The contribution of the autotrophic epiphyte community on Z. marina leaves to total photosynthesis per dm² was between 27 and 50%, and between 10 and 44% per mg chlorophyll. These levels of epiphyte photosynthesis can double the primary production of Z. marina leaves. No detrimental effects of epiphyte cover were realized in leaf maximal photosynthesis or P-I relationships. Non-epiphytized leaves and leaves from which epiphytes were removed showed essentially identical photosynthetic features. Light intensity and age gradients along the leaf axis control both the photosynthetic performance of the leaves and epiphyte biomass and photosynthesis.     

海草生态学研究进展

海草床生态系统是生物圈中最具生产力的水生生态系统之一,具有重要的生态系统服务功能。作者根据海草生态学及相关领域的最新研究进展,对世界范围内海草床的空间分布、海草床的生态系统服务功能以及外界因素对海草床的影响等研究进展进行了综述。海草床生态系统服务功能主要包括净化水质、护堤减灾、提供栖息地和生态系统营养循环等。对海草床影响较大的外界环境因素包括盐度、温度、营养盐、光照、其他动物摄食、人类活动和气候变化等。海草普查、海草生态功能研究,影响海草床的主要环境因素,海草修复研究等将是我国海草研究的主要方向。     

海草床恢复研究进展

海草床是热带和温带重要的海洋生态系统,是许多海洋动物的栖息地、生存场所和食物来源地,具有重要的生态系统服务价值。20世纪以来,全世界的海草床衰退严重,仅1993年到2003年间约有2.6×104km2消失,达到15%,开展海草床的生态恢复已迫在眉睫。总结了海草床衰退的原因及恢复的主要方法。海草床衰退的原因包括自然灾害和人类活动的影响。海草床恢复的方法包括生境恢复法、种子法和移植法。移植法是目前最常用的方法,可分为草皮法、草块法和根状茎法。比较了各方法的优缺点,即(1)生境恢复法投入少、代价低,但周期长;(2)种子法破坏小,但种子难收集、易丧失、萌发率低;(3)草块法成活率高,但对原海草床有破坏作用;(4)根状茎法节约种源,但固定困难。因此,海草床恢复的具体方法取决于种源地、移植地及恢复的目标。我国的海草研究与恢复尚处在起始阶段,移植法应是目前我国海草床恢复的首选方法。就未来我国海草恢复的关键技术和研究重点提出了建议。     

Food sources of two detritivore amphipods associated with the seagrass Posidonia oceanica leaf litter

This study focused on the ingestion and assimilation of Posidonia oceanica (L.) Delile litter by Gammarella fucicola Leach and Gammarus aequicauda Martynov, two dominant detritivore amphipods of the P. oceanica leaf litter. Scanning electron microscope observations indicated that leaf litter is highly colonized by diverse diatoms, bacteria and fungi, which may constitute a potential food source for the litter fauna. Gut content observations demonstrated that these species eat P. oceanica litter, and that this item is an important part of their ingested diet. Stable isotope analyses showed that the species do not experience the same gains from the ingested Posidonia. Gammarella fucicola displayed isotopic values, suggesting a major contribution of algal material (micro- and macro-epiphytes or drift macro-algae). On the other hand, the observed isotopic values of G. aequicauda indicated a more important contribution of P. oceanica carbon. The mixing model used agreed with this view, with a mean contribution of P. oceanica to approximately 50% (range 40-55%) of the assimilated biomass of G. aequicauda. This demonstrated that the two species, suspected to be detritus feeders, display in reality relatively different diets, showing that a certain degree of trophic diversity may exist among the detritivore community of the seagrass litter.     

The influence of prey identity and size on selection of prey by two marine fishes

Laboratory experiments were conducted with two predatory fishes, Lagodon rhomboides (Linnaeus) and Syngnathus floridae (Jordan & Gilbert), to determine if prey selection was a result of predator preference or prey accessibility. Prey consisted of two species of caridean shrimp, Tozeuma carolinense (Kingsley) and Hippolyte zostericola (Smith), that commonly inhabit seagrass meadows. Natural diets of both predators revealed that selection ofTozeuma and Hippolyte was not related to their field densities. My experiments demonstrated that natural prey selection was a consequence of prey accessibility, not predator preference. Experiments examining the role of prey size in predator preference revealed that large individuals were significantly preferred over small individuals. Observations of prey behavior indicated that prey motion affected predator choice. These results suggest that predator preference was primarily determined by prey visibility and that the combined effects of prey size and motion contributed to the visibility of these prey.     

海草床育幼功能及其机理

海草床是近岸海域中生产力极高的生态系统,是许多海洋水生动物的重要育幼场所。从生物幼体的密度、生长率、存活率和生境迁移4个方面阐述海草床育幼功能,并从食源和捕食压力两个方面探讨海草床育幼功能机理。许多生物幼体在海草床都呈现出较高的密度、生长率和存活率,并且在个体发育到一定阶段从海草床向成体栖息环境迁移。丰富的食物来源或较低的捕食压力可能是海草床具有育幼功能的主要原因,但不同的生物幼体对海草床的利用有差异,海草床育幼功能的机理在不同环境条件下也存在差异。提出未来海草床育幼功能的重点研究方向:(1)量化海草床对成体栖息环境贡献量;(2)全球气候变化和人类活动对海草床育幼功能的影响;(3)海草床育幼功能对海草床斑块效应和边缘效应的响应,以期为促进我国海草床育幼研究和海草床生态系统保护提供依据。     

海草床生态系统的退化及其恢复

海草床是生产力和生物多样性最高的典型海洋生态系统之一,目前全球海草床呈现退化趋势,自然干扰和人类活动的负面影响是其退化原因,以后者为主.海草退化的生理生态机制主要是光合作用速率、光合色素荧光强度和含量、酶活性等生理生态指标在胁迫(高温、光照、盐度、污染物等)下发生显著变化.海草床监测和保护计划已经在局部区域取得成效,使部分海草床得到恢复.中国的海草床恢复研究重点应放在生物多样性保护、海草移植和海草床生态系统服务价值评估.     

New distribution limits of seagrass beds in West Africa

The southern limit of the distribution of seagrass species along the west coast of Africa is not yet clearly defined. In March 2008 an expedition was organized in Senegal to search for seagrass beds from Dakar (14°45'15" N, 17°30'31" W) southwards to Tabakouta–Delta du Saloum (13°46'59" N, 16°28'42" W). Cymodocea nodosa and Halodule wrightii were observed in sandy areas of some protected bays of Dakar, around Sarène (14°16'18" N, 16°54'17" W), Joal Fadiouth (14°09'08" N, 16°50'03" W) and the Bamboung–Sourou area (13°50'08" N, 16°33'09" W), and they probably occur all along the coast of Senegal and beyond, perhaps much further south as well.     

海草生物量和初级生产力研究进展

海草床是近岸重要的湿地生态系统,具有较高生物量和生产力。海草的生物量和生产力变化除了受到光照、无机碳源、营养盐、温度、盐度、水动力条件、铁限制和污染物等非生物因素制约外,还受到附生藻类和动物摄食等生物因素影响。非生物因素一般有最适合海草生长的范围,生物因素的影响具有两面性。海草生物量和生产力研究基本处于由定性向定量过渡阶段,准确便捷的方法、现场多因子围隔实验、更大时空尺度上的对比研究是今后研究的重点。     

濒危海草贝克喜盐草在海南东寨港的分布及其群落基本特征

贝克喜盐草( Halophila beccarii)是当前全球面临灭绝风险的10种海草之一,被国际自然保护联盟( IUCN)列为易危( VL)种。由于贝克喜盐草生境的特殊性(受涨退潮影响的潮间带)及个体十分纤小且易被沉积物覆盖,加之种群更新速率极快,导致通常难以被发现。据报道,贝克喜盐草仅在我国的广西、广东、台湾、香港、福建和海南6个省(区)有小面积的分布。该研究首次报道了位于海南东寨港的贝克喜盐草海草的分布、群落基本特征以及生境概况。结果表明：与国内其它地区的贝克喜盐草相比,调查期间东寨港的贝克喜盐草面积较大,为15.4 hm2;密度适中,为(10394±576)茎.m-2;覆盖度较低,为~5％;生物量较低,总生物量仅(9.224±1.151) gDW.m-2,地上生物量为(2.339±0.245) gDW.m-2,地下为(6.886±1.055) gDW.m-2,地上与地下生物量之比值较低(0.34);叶片较小,叶长为(4.83±0.87) mm,叶宽为(1.44±0.21) mm,调查时在植物体上未发现有花或果,但在沉积物中发现有一定数量的贝克喜盐草种子(2105±664)粒.m-2。该研究区的海水受到一定程度的水体污染。建议海南东寨港国家级自然保护区在今后的管理工作中,应把海草也作为重点保护对象。     

Nutritional diversity of symbiotic ascidians in a Fijian seagrass meadow

In a coastal lagoon of Dravuni Island, Fiji, at least six species of compound ascidians, some of them harboringProchloron as symbiotic algae, were found in aSyringodium-dominated seagrass meadow. Based on their heterotrophic (filrer feeding rates) and autotrophic (photosynthetic) activities, carbon gain of the ascidians was categorized into two groups: (i) supported by heterotrophic metabolism; and (ii) supported by both heterotrophic and autotrophic metabolisms.Didemnum molle, Lissoclinum bistratum andLissolinum voeltzkowi belong to the latter group, and the relative contribution of the autotrophic process was a significant portion of their carbon gain (52–74%). These symbiotic ascidians were found in light microhabitats, while the heterotrophic species occupied shady environments rich in suspended organic materials, such as the sheath surface of the seagrass.     

Food habits of fishes in azostera marina bed at aburatsubo, central japan

To clarify the feeding habits of seagrass fishes, the gut contents of 31 fish species collected in aZostera marina bed at Aburatsubo central Japan, were examined. Ontogenetic changes in food preference were recognized in 12 species. In general, juveniles of the latter preyed mainly on small crustaceans (e.g., gammaridean amphipods) or planktonic animals (e.g., calanoid and cyclopoid copepods), subsequently changing to other prey items (e.g., hard-shelled animals) with growth. Cluster analysis based on dietary overlaps showed that the seagrass fish assemblage comprised seven feeding guilds (small-crustacean, planktonic-animal, large-crustacean, polychaete, fish, hard-shelled mollusc and detritus feeders). Of these, small-crustacean feeders and planktonic-animal feeders were the most abundantly represented, including juveniles of several species, which, when adult, transferred to other feeding guilds. On the other hand detritivores were represented by a single species.