Effects of naturally acidified seawater on seagrass calcareous epibionts

Surface ocean pH is likely to decrease by up to 0.4 units by 2100 due to the uptake of anthropogenic CO2 from the atmosphere. Short-term experiments have revealed that this degree of seawater acidification can alter calcification rates in certain planktonic and benthic organisms, although the effects recorded may be shock responses and the long-term ecological effects are unknown. Here, we show the response of calcareous seagrass epibionts to elevated CO2 partial pressure in aquaria and at a volcanic vent area where seagrass habitat has been exposed to high CO2 levels for decades. Coralline algae were the dominant contributors to calcium carbonate mass on seagrass blades at normal pH but were absent from the system at mean pH 7.7 and were dissolved in aquaria enriched with CO2. In the field, bryozoans were the only calcifiers present on seagrass blades at mean pH 7.7 where the total mass of epiphytic calcium carbonate was 90 per cent lower than that at pH 8.2. These findings suggest that ocean acidification may have dramatic effects on the diversity of seagrass habitats and lead to a shift in the biogeochemical cycling of both carbon and carbonate in coastal ecosystems dominated by seagrass beds.     

环境胁迫对海草非结构性碳水化合物储存和转移的影响

非结构性碳水化合物在海草体内的代谢对植株的生长有重要影响。为更好地跟踪非结构性碳水化合物在海草响应环境胁迫中所起的作用,根据国内外最新文献,重点综述了光强、营养盐、盐度、海洋酸化、温度、硫化物和动物摄食等环境胁迫对海草非结构性碳水化合物储存和转移的影响。光限制和富营养化均降低非结构性碳水化合物的合成,并使之从地下根茎转移到叶;而海洋酸化却促进非结构性碳水化合物合成并向地下组织转移;盐度变化改变海草体内渗透压,需要非结构性碳水化合物的新陈代谢来维持;温度通过影响光合作用、呼吸作用、氮代谢来影响非结构性碳水化合物的合成与储存;而硫化物和动物摄食则分别通过抑制海草酶的活性和啃食海草光合组织,减少非结构性碳水化合物的合成和储存。同时指出了一些今后关于海草非结构性碳水化合物的重点研究方向:(1)海草不同生命阶段(种子休眠和萌发,发育,繁殖等)非结构性与结构性碳水化合物之间,以及可溶糖与淀粉之间的转化分配机制;(2)双环境因子或者多环境因子对海草非结构性碳水化合物的耦合作用;(3)非结构性碳水化合物作为海草床生态系统健康评价指标的研究与应用。     

Seagrasses in an era of ocean warming: a review

Seagrasses are valuable sources of food and habitat for marine life and are one of Earth's most efficient carbon sinks. However, they are facing a global decline due to ocean warming and eutrophication. In the last decade, with the advent of new technology and molecular advances, there has been a dramatic increase in the number of studies focusing on the effects of ocean warming on seagrasses. Here, we provide a comprehensive review of the future of seagrasses in an era of ocean warming. We have gathered information from published studies to identify potential commonalities in the effects of warming and the responses of seagrasses across four distinct levels: molecular, biochemical/physiological, morphological/population, and ecosystem/planetary. To date, we know that although warming strongly affects seagrasses at all four levels, seagrass responses diverge amongst species, populations, and over depths. Furthermore, warming alters seagrass distribution causing massive die-offs in some seagrass populations, whilst also causing tropicalization and migration of temperate species. In this review, we evaluate the combined effects of ocean warming with other environmental stressors and emphasize the need for multiple-stressor studies to provide a deeper understanding of seagrass resilience. We conclude by discussing the most significant knowledge gaps and future directions for seagrass research.     

The role of leaf nitrogen content in determining turtlegrass (Thalassia testudinum) grazing by a generalized herbivore in the northeastern Gulf of Mexico

In shallow marine environments the variability in grazing on seagrasses has been hypothesized to be controlled, in part, by the nutritive quality (i.e., nitrogen content) of their leaves. The few existing studies of the relationship between leaf nitrogen content and seagrass grazing have all found a positive relationship between leaf nitrogen content and preference by selective vertebrate grazers (i.e., the bucktooth parrotfish, green sea turtles, and dugongs). However, most marine herbivores (both vertebrate and invertebrate) are thought to be extreme generalists with broad diets of variable nutritive quality (e.g., detritus, living plants, and animals), suggesting the currently held view on the role leaf nutrient content in explaining the variability of seagrass grazing is an oversimplification.In this study, we evaluated how leaf nitrogen content influenced grazing on turtlegrass by a generalist invertebrate herbivore (the pink sea urchin Lytechinus variegatus) in the northeastern Gulf of Mexico. Using a short-term laboratory test and a longer-term field experiment, we tested the hypothesis that leaf nitrogen content controls sea-urchin grazing on seagrass leaves. We hypothesized that if poor nutritive value of seagrasses is responsible for reduced rates of feeding, then increasing leaf nitrogen concentrations should lead to increased rates of seagrass consumption by sea urchins.In the field experiment, we significantly enriched seagrass leaf nitrogen concentrations (some 10-20% depending on month) in experimental plots with a commercial fertilizer and we manipulated grazing intensity by enclosing adult sea urchins at densities that bracketed the range of average densities observed in the region (i.e., 0, 10 and 20 individuals/m(2)). Comparisons of changes in aboveground seagrass production and biomass showed no evidence that sea urchins grazed significantly more in treatments where leaf nitrogen was enriched. Because the statistical power of our test to detect such differences was low and aboveground seagrass production varied significantly among treatments, we also used a mass balance equation to estimate sea urchin consumption of nitrogen-enriched and unenriched leaves. This showed that sea urchins compensated for low nitrogen levels in our unenriched treatments by eating more leaves than in treatments where leaf nitrogen was elevated. Using a laboratory test, we also found that sea urchins ate less nitrogen-enriched seagrass than unenriched seagrass. In combination, these results show that, in contrast to findings reported for vertebrate herbivores, sea urchins feed at higher rates when offered seagrass leaves of lower leaf nitrogen content, and that low levels of leaf nitrogen are not always an effective defense against herbivores.     

Effects of CO 2 enrichment on photosynthesis,growth, and nitrogen metabolism of the seagrass Zostera noltii

Seagrass ecosystems are expected to benefit from the global increase in CO ₂ in the ocean because the photosynthetic rate of these plants may be C_i‐limited at the current CO ₂ level. As well, it is expected that lower external pH will facilitate the nitrate uptake of seagrasses if nitrate is cotransported with H⁺ across the membrane as in terrestrial plants. Here, we investigate the effects of CO ₂ enrichment on both carbon and nitrogen metabolism of the seagrass Zostera noltii in a mesocosm experiment where plants were exposed for 5 months to two experimental CO ₂ concentrations (360 and 700 ppm). Both the maximum photosynthetic rate (P_m) and photosynthetic efficiency (α) were higher (1.3‐ and 4.1‐fold, respectively) in plants exposed to CO ₂‐enriched conditions. On the other hand, no significant effects of CO ₂ enrichment on leaf growth rates were observed, probably due to nitrogen limitation as revealed by the low nitrogen content of leaves. The leaf ammonium uptake rate and glutamine synthetase activity were not significantly affected by increased CO ₂ concentrations. On the other hand, the leaf nitrate uptake rate of plants exposed to CO ₂‐enriched conditions was fourfold lower than the uptake of plants exposed to current CO ₂ level, suggesting that in the seagrass Z. noltii nitrate is not cotransported with H⁺ as in terrestrial plants. In contrast, the activity of nitrate reductase was threefold higher in plant leaves grown at high‐CO ₂ concentrations. Our results suggest that the global effects of CO ₂ on seagrass production may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Under a CO ₂ increase scenario, the natural levels of nutrients will probably become limiting for Z. noltii. This potential limitation becomes more relevant because the expected positive effect of CO ₂ increase on nitrate uptake rate was not confirmed.     

Seagrass response to CO2 contingent on epiphytic algae: indirect effects can overwhelm direct effects

Increased availability of dissolved CO₂ in the ocean can enhance the productivity and growth of marine plants such as seagrasses and algae, but realised benefits may be contingent on additional conditions (e.g. light) that modify biotic interactions between these plant groups. The combined effects of future CO₂ and differing light on the growth of seagrass and their algal epiphytes were tested by maintaining juvenile seagrasses Amphibolis antarctica under three different CO₂ concentrations representing ambient, moderate future and high future forecasts (i.e. 390, 650 vs. 900 µl l^?1) and two light levels representing low and high PAR (i.e. 43 vs. 167 µmol m^?2 s^?1). Aboveground and belowground biomass, leaf growth, epiphyte cover, tissue chemistry and photosynthetic parameters of seagrasses were measured. At low light, there was a neutral to positive effect of elevated CO₂ on seagrass biomass and growth; at high light, this effect of CO₂ switched toward negative, as growth and biomass decreased at the highest CO₂ level. These opposing responses to CO₂ appeared to be closely linked to the overgrowth of seagrass by filamentous algal epiphytes when high light and CO₂ were combined. Importantly, all seagrass plants maintained positive leaf growth throughout the experiment, indicating that growth was inhibited by some experimental conditions but not arrested entirely. Therefore, while greater light or elevated CO₂ provided direct physiological benefits for seagrasses, such benefits were likely negated by overgrowth of epiphytic algae when greater light and CO₂ were combined. This result demonstrates how indirect ecological effects from epiphytes can modify independent physiological predictions for seagrass associated with global change.     

Unexpected resilience of a seagrass system exposed to global stressors

Despite a growing interest in identifying tipping points in response to environmental change, our understanding of the ecological mechanisms underlying nonlinear ecosystem dynamics is limited. Ecosystems governed by strong species interactions can provide important insight into how nonlinear relationships between organisms and their environment propagate through ecosystems, and the potential for environmentally mediated species interactions to drive or protect against sudden ecosystem shifts. Here, we experimentally determine the functional relationships (i.e., the shapes of the relationships between predictor and response variables) of a seagrass assemblage with well‐defined species interactions to ocean acidification (enrichment of CO₂) in isolation and in combination with nutrient loading. We demonstrate that the effect of ocean acidification on grazer biomass (Phyllaplysia taylori and Idotea resecata) was quadratic, with the peak of grazer biomass at mid‐pH levels. Algal grazing was negatively affected by nutrients, potentially due to low grazer affinity for macroalgae (Ulva intestinalis), as recruitment of both macroalgae and diatoms were favored in elevated nutrient conditions. This led to an exponential increase in macroalgal and epiphyte biomass with ocean acidification, regardless of nutrient concentration. When left unchecked, algae can cause declines in seagrass productivity and persistence through shading and competition. Despite quadratic and exponential functional relationships to stressors that could cause a nonlinear decrease in seagrass biomass, productivity of our model seagrass—the eelgrass (Zostera marina)‐ remained highly resilient to increasing acidification. These results suggest that important species interactions governing ecosystem dynamics may shift with environmental change, and ecosystem state may be decoupled from ecological responses at lower levels of organization.     

Specificity in Mesograzer-Induced Defences in Seagrasses

Grazing-induced plant defences that reduce palatability to herbivores are widespread in terrestrial plants and seaweeds, but they have not yet been reported in seagrasses. We investigated the ability of two seagrass species to induce defences in response to direct grazing by three associated mesograzers. Specifically, we conducted feeding-assayed induction experiments to examine how mesograzer-specific grazing impact affects seagrass induction of defences within the context of the optimal defence theory. We found that the amphipod Gammarus insensibilis and the isopod Idotea chelipes exerted a low-intensity grazing on older blades of the seagrass Cymodocea nodosa, which reflects a weak grazing impact that may explain the lack of inducible defences. The isopod Synischia hectica exerted the strongest grazing impact on C. nodosa via high-intensity feeding on young blades with a higher fitness value. This isopod grazing induced defences in C. nodosa as indicated by a consistently lower consumption of blades previously grazed for 5, 12 and 16 days. The lower consumption was maintained when offered tissues with no plant structure (agar-reconstituted food), but showing a reduced size of the previous grazing effect. This indicates that structural traits act in combination with chemical traits to reduce seagrass palatability to the isopod. Increase in total phenolics but not in C:N ratio and total nitrogen of grazed C. nodosa suggests chemical defences rather than a modified nutritional quality as primarily induced chemical traits. We detected no induction of defences in Zostera noltei, which showed the ability to replace moderate losses of young biomass to mesograzers via compensatory growth. Our study provides the first experimental evidence of induction of defences against meso-herbivory that reduce further consumption in seagrasses. It also emphasizes the relevance of grazer identity in determining the level of grazing impact triggering resistance and compensatory responses of different seagrass species.     

海洋酸化效应对海水鱼类的综合影响评述

人类活动引起的大气CO2浓度的升高,除了使全球温度升高外,导致的另一个严重生态问题——海洋酸化(Ocean acidification,OA),受到社会各界包括科研界的高度重视,该领域的大部分研究结果都是在近十年才发表出来的,目前还有很多需要解决的问题。海洋酸化的研究涉及到很多学科的交叉包括化学、古生物学、生态学、生物地球化学等等。在生物学领域,海洋酸化主要围绕敏感物种,例如由碳酸钙形成贝壳或外骨骼的贝类,珊瑚礁群体等。鱼类作为海洋脊椎动物的代表生物类群,自身具有一定的酸碱平衡调节能力,但相关海洋酸化方向的研究并不是很多。尽管人们对于海洋酸化对鱼类的影响了解甚少,这并不说明海洋酸化对鱼类没有作用或者效应小,在相关研究逐步展开的同时,发现鱼类同样受到海洋酸化的危害,几乎涉及到鱼类整个生活史和几乎大部分生理过程,尤其是早期生活史的高度敏感。因此就目前国内外对此领域研究结果做综述,以期待业界同行能够对海水鱼类这个大的类群引起重视。     

海洋酸化生态学研究进展

工业革命以来,人类排放的大量二氧化碳引起温室效应的同时,也被海洋吸收使得全球海洋出现了严重的酸化。海洋酸化及伴随的海水碳酸盐化学体系的变化对海洋生物产生深远的影响。以海洋酸化对钙化作用和光合作用的影响为重点,总结了近年来关于海洋酸化的研究,介绍了海洋中不同生态系统对海洋酸化的响应。一方面,海水中CO23-浓度和碳酸钙饱和度的降低对海洋钙化生物造成严重损害,生活在高纬的冷水珊瑚和翼足目等文石生产者是最早的受害者;贝类和棘皮动物在钙化早期对海洋酸化尤其敏感,其幼体存活率受到海洋酸化的严重制约。另一方面,CO2浓度的增加能促进海洋植物的光合作用和生长,增加初级生产力,改变浮游植物的群落组成。此外,海洋酸化可以促进固氮和脱氮作用同时削弱硝化作用,改变溶氧浓度分布和金属的生物可利用性,从而对海洋生物产生间接影响。海洋酸化对海洋生态系统的影响机制复杂,影响程度深远。为了能准确的评估海洋酸化的生态学效应,需要更全面深入的研究。     

海草克隆性及其种群遗传效应

海草是适应在海洋环境中生存和繁殖的单子叶植物,由于所处环境常存在潮汐、风暴等的干扰,海草形成了一系列适应特征,克隆性是其中突出的一个.所有的海草都具有水平根状茎,许多海草也具有垂直根状茎,在一些海草中,也观察到无性生殖(无融合生殖).与克隆生长有关的参数(如节间长度、间隔子长度、分枝角度以及延伸速率和分枝率等)对于海草的克隆生长有着决定性影响,但繁育系统对克隆斑块大小也有较大影响.强烈的克隆性影响着海草的遗传变异.总体来看,海草种群内遗传多样性比陆生植物低,也低于另一类海洋高等植物-红树植物,利用DNA标记观察到的多样性高于等位酶标记.在一些海草植物种群中观察到较高的克隆多样性,但也有一些种群由单一基因型或少量基因型组成,其原因主要是由于奠基者效应和克隆生长.通常克隆植物中基因流有限,但是海草的克隆片段可能远距离扩散,从而提高种群间的基因流.就克隆生长对种群空间结构和交配系统的影响进行了综述.     

Impacts of CO2 Enrichment on Productivity and Light Requirements of Eelgrass

Seagrasses, although well adapted for submerged existence, are CO2-limited and photosynthetically inefficient in seawater. This leads to high light requirements for growth and survival and makes seagrasses vulnerable to light limitation. We explored the long-term impact of increased CO2 availability on light requirements, productivity, and C allocation in eelgrass (Zostera marina L.). Enrichment of seawater CO2 increased photosynthesis 3-fold, but had no long-term impact on respiration. By tripling the rate of light-saturated photosynthesis, CO2 enrichment reduced the daily period of irradiance-saturated photosynthesis (Hsat) that is required for the maintenance of positive whole-plant C balance from 7 to 2.7 h, allowing plants maintained under 4 h of Hsat to perform like plants growing in unenriched seawater with 12 h of Hsat. Eelgrass grown under 4 h of Hsat without added CO2 consumed internal C reserves as photosynthesis rates and chlorophyll levels dropped. Growth ceased after 30 d. Leaf photosynthesis, respiration, chlorophyll, and sucrose-phosphate synthase activity of CO2-enriched plants showed no acclimation to prolonged enrichment. Thus, the CO2-stimulated improvement in photosynthesis reduced light requirements in the long term, suggesting that globally increasing CO2 may enhance seagrass survival in eutrophic coastal waters, where populations have been devastated by algal proliferation and reduced water-column light transparency.     

Acclimation to ocean acidification during long‐term CO2 exposure in the cold‐water coral Lophelia pertusa

Ocean acidity has increased by 30% since preindustrial times due to the uptake of anthropogenic CO₂ and is projected to rise by another 120% before 2100 if CO₂ emissions continue at current rates. Ocean acidification is expected to have wide‐ranging impacts on marine life, including reduced growth and net erosion of coral reefs. Our present understanding of the impacts of ocean acidification on marine life, however, relies heavily on results from short‐term CO₂ perturbation studies. Here, we present results from the first long‐term CO₂ perturbation study on the dominant reef‐building cold‐water coral Lophelia pertusa and relate them to results from a short‐term study to compare the effect of exposure time on the coral's responses. Short‐term (1 week) high CO₂ exposure resulted in a decline of calcification by 26–29% for a pH decrease of 0.1 units and net dissolution of calcium carbonate. In contrast, L. pertusa was capable to acclimate to acidified conditions in long‐term (6 months) incubations, leading to even slightly enhanced rates of calcification. Net growth is sustained even in waters sub‐saturated with respect to aragonite. Acclimation to seawater acidification did not cause a measurable increase in metabolic rates. This is the first evidence of successful acclimation in a coral species to ocean acidification, emphasizing the general need for long‐term incubations in ocean acidification research. To conclude on the sensitivity of cold‐water coral reefs to future ocean acidification further ecophysiological studies are necessary which should also encompass the role of food availability and rising temperatures.     

High CO2 and silicate limitation synergistically increase the toxicity of Pseudo-nitzschia fraudulenta

Anthropogenic CO(2) is progressively acidifying the ocean, but the responses of harmful algal bloom species that produce toxins that can bioaccumulate remain virtually unknown. The neurotoxin domoic acid is produced by the globally-distributed diatom genus Pseudo-nitzschia. This toxin is responsible for amnesic shellfish poisoning, which can result in illness or death in humans and regularly causes mass mortalities of marine mammals and birds. Domoic acid production by Pseudo-nitzschia cells is known to be regulated by nutrient availability, but potential interactions with increasing seawater CO(2) concentrations are poorly understood. Here we present experiments measuring domoic acid production by acclimatized cultures of Pseudo-nitzschia fraudulenta that demonstrate a strong synergism between projected future CO(2) levels (765 ppm) and silicate-limited growth, which greatly increases cellular toxicity relative to growth under modern atmospheric (360 ppm) or pre-industrial (200 ppm) CO(2) conditions. Cellular Si:C ratios decrease with increasing CO(2), in a trend opposite to that seen for domoic acid production. The coastal California upwelling system where this species was isolated currently exhibits rapidly increasing levels of anthropogenic acidification, as well as widespread episodic silicate limitation of diatom growth. Our results suggest that the current ecosystem and human health impacts of toxic Pseudo-nitzschia blooms could be greatly exacerbated by future ocean acidification and 'carbon fertilization' of the coastal ocean.     

Impacts of Groundwater Discharge at Myora Springs (North Stradbroke Island,Australia) on the Phenolic Metabolism of Eelgrass,Zostera muelleri,and Grazing by the Juvenile Rabbitfish,Siganus fuscescens

Myora Springs is one of many groundwater discharge sites on North Stradbroke Island (Queensland, Australia). Here spring waters emerge from wetland forests to join Moreton Bay, mixing with seawater over seagrass meadows dominated by eelgrass, Zostera muelleri. We sought to determine how low pH / high CO₂ conditions near the spring affect these plants and their interactions with the black rabbitfish (Siganus fuscescens), a co-occurring grazer. In paired-choice feeding trials S. fuscescens preferentially consumed Z. muelleri shoots collected nearest to Myora Springs. Proximity to the spring did not significantly alter the carbon and nitrogen contents of seagrass tissues but did result in the extraordinary loss of soluble phenolics, including Folin-reactive phenolics, condensed tannins, and phenolic acids by ≥87%. Conversely, seagrass lignin contents were, in this and related experiments, unaffected or increased, suggesting a shift in secondary metabolism away from the production of soluble, but not insoluble, (poly)phenolics. We suggest that groundwater discharge sites such as Myora Springs, and other sites characterized by low pH, are likely to be popular feeding grounds for seagrass grazers seeking to reduce their exposure to soluble phenolics.     

Variation in multiple traits of vegetative and reproductive seagrass tissues influences plant–herbivore interactions

Plant–herbivore interactions have strong ecological and evolutionary consequences, but have been traditionally overlooked in marine higher plants. Despite recent advances in seagrass ecology that highlight the importance of herbivory, the mechanisms that regulate the feeding behaviour of seagrass consumers remain largely unknown. Herbivores have been shown to reduce the sexual reproductive success of seagrasses through direct consumption of inflorescences and seeds, but we know little about intraspecific variation in susceptibility to grazing of different seagrass tissues. We contrasted the relative palatability of reproductive and vegetative tissues of the temperate seagrass Posidonia oceanica in the field, and we assessed the feeding preferences among these tissues of the main consumers of the plant, the fish Sarpa salpa and the urchin Paracentrotus lividus. Moreover, we identified the plant traits that explained the observed feeding behaviour. We provide strong evidence for herbivore selectivity among seagrass tissues. In the field, 70–90% of inflorescences were damaged by herbivores compared to 3–60% of leaves of similar age. In feeding assays, the urchin P. lividus showed over a twofold preference for reproductive tissue at various stages of development. By contrast, we detected no feeding activity on either leaves or inflorescences from the fish S. salpa, which is known to migrate to deeper waters soon after flowering starts and during the period of fruit maturation. Despite being the preferred food of urchins, inflorescences were chemically defended, had higher levels of phenolics and lower nutrient and calorific content than leaves. We experimentally demonstrated that leaf structural defences are the primary factor in determining urchin feeding preferences. Removal of plant structure results in a drastic shift in urchin selectivity towards the most nutritious and less chemically defended leaf tissue, indicating that multiple mechanisms of defence to herbivory may coexist in seagrasses.     

Disturbance and recovery following catastrophic grazing: studies of a successional chronosequence in a seagrass bed

In August 1997, a large aggregation of the common sea urchin, Lytechinus variegatus , was discovered moving southward through a lush and productive seagrass monoculture of Syringodium filiforme in the Florida Keys, FL. Sea urchin densities at the grazing front were greater than 300 individuals m⁻² which resulted in the overgrazing of seagrasses and a complete denuding of all vegetation from this area. The steady rate of the grazing front migration permitted the estimation of the time since disturbance for any point behind this grazing front allowing the use of a chronosequence approach to investigate the processes early on in succession of these communities.
In May 1999, six north-south parallel transects were established across the disturbed seagrass communities and into the undisturbed areas south of the grazing front. Based on the measured rates of the migration of the grazing front, we grouped 60 sites into five categories (disturbed, recently grazed, active grazing front, stressed and undisturbed). The large scale loss of seagrass biomass initiated community-wide cascading effects that significantly altered resource regimes and species diversity. The loss of the seagrass canopy and subsequent death and decay of the below-ground biomass resulted in a de-stabilization of the sediments. As the sediments were eroded into the water column, turbidity significantly increased, reducing light availability and significantly reducing the sediment nitrogen pool and depleting the seed bank. The portion of the chronosequence that has had the longest period of recovery now consists of a mixed community of seagrass and macroalgae, as remnant survivors and quick colonizers coexist and jointly take advantage of the open space.     

Seagrass–sediment interactions,positive feedbacks and critical thresholds for occurrence: a review

This literature review summarizes the limiting factors for seagrass occurrence, and the effect positive feedbacks in seagrass systems have on these threshold levels. Minimum water depth is mainly determined by wave orbital velocity, tide and wave energy; and maximum depth by light availability. Besides these, other limiting factors occur, such as an upper current velocity threshold, above which seagrasses are eroded, or a lower water current velocity threshold below which carbon exchange is limiting. In some locations organic matter content, sulphide concentration or nutrient availability are limiting. N-limitation is mainly reported from temperate terrigenous sediments, and P-limitation from tropical carbonate sediments. However, limiting factors sometimes change over the year, switching from light limiting to N- or P-limiting, and show at times regional variation. The effect seagrasses have on current reduction, trapping sediment and decreasing resuspension can lead to several changes in both the sediment and the water column. In the sediment, an increase in nutrient availability has been reported, and increases in organic matter, sediment height increases, and burial of the seagrasses. In the water column the effect is a reduction of the turbidity through a decrease of the sediment load, decreasing the attenuation coefficient, thereby increasing light availability. Due to the large effect light availability has on seagrass occurrence, the effect of an improvement of the light conditions by a reduction of the turbidity by seagrasses is probably the most important positive feedback in seagrass systems. The latter effect should therefore be incorporated in models that try to understand or predict seagrass changes. Generalization are difficult due a lack of studies that try to find relationships between seagrass architecture and sediment trapping (studying both turbidity reduction and nutrient increase) on a global level under a variety of different conditions. Areas for research priorities are identified.     

海洋酸化条件下Cd2+和Hg2+对斧文蛤幼贝急性毒性效应

为研究在海洋酸化条件下重金属污染物对滩涂贝类的影响,采用半静态急性毒性实验的研究方法,利用海洋酸化人工模拟系统,分析了不同酸化条件下(对照组pH 8.20、酸化组pH分别为7.80、7.60和7.40)Cd2+和Hg2+对斧文蛤(Meretrix lamarckii)幼贝急性毒性效应的影响。实验结果表明:在实验设定的海洋酸化范围内,单一的海洋酸化对斧文蛤幼贝的存活没有显著性影响,但海洋酸化显著增强了Cd2+和Hg2+的急性毒性。与对照组相比,酸化组Cd2+和Hg2+的毒性随着酸化程度的加剧而呈现出逐渐增强的趋势; Cd2+和Hg2+均在pH 7.40时对斧文蛤的毒性最强,其96h半致死(96h LC50)浓度分别为4.068 mg/L(Cd2+)和10.332 mg/L(Hg2+),明显低于pH 8.20、7.80和7.60时其对斧文蛤幼贝的96h LC50浓度(其值分别为Cd2+ 6.458、5.947、4.728 mg/L和Hg2+ 12.027、11.169、10.595 mg/L)。研究有助于丰富海洋酸化与重金属毒理作用在海洋贝类中的研究内容,为斧文蛤资源恢复和海洋环境保护提供科学依据。