Oceanica Seedlings Research Articles

Different thermal regimes and susceptibility to herbivory do not constrain seagrass seedling restoration.

Recovering seagrass ecosystems through restoration has become impellent to re-establish their functionality and services. Although the use of seedlings may represent an appropriate solution, little information is provided on the seedling-based restoration effectiveness with influence of biotic and abiotic interactions. Survival, morphological development and leaf total phenol content of transplanted Posidonia oceanica seedlings were evaluated under different origin, thermal regimes and herbivore pressure through a five-months field experiment in two MPAs, located on the west (cold) and east (warm) Sardinia coast to explore the effectiveness of seedling-based restoration. Seedlings originated from the two coasts responded differently to thermal regime site and herbivory pressure, as the warm-adapted ones survived less but developed more (and vice-versa) and resisted to the herbivory pressure increasing their phenol content, thus showing compensating responses. This study provided information on the P. oceanica seedling-based restoration by investigating abiotic and biotic interactions with the transplanted plants. It promotes the collection of beach-cast fruits from different coasts and their transplantation, regardless their origin, with no need of protecting seedlings from predators.

Tannins and copper sulphate as antimicrobial agents to prevent contamination of Posidonia oceanica seedling culture for restoration purposes.

Seed-based restoration methods are increasingly recognized as a relevant tool contributing to halt and reverse the loss of seagrass meadows while providing genetic and evolutionary benefit for the conservation of these habitats. Ad-hoc protocols aimed at maximizing the survival of plantlets obtained from seeds in cultivation systems are therefore required. Previous trials of seedling culture of Posidonia oceanica, the dominant seagrass of the Mediterranean Sea, recorded up to 40% loss due to mould development. In this study we aim to (i) identify the putative causal agents of seed decay and (ii) test the efficacy of copper sulphate (0.2 and 2 ppm) and of tannin-based products derived from chestnut, tara and quebracho in reducing seed and seedling decay, while assessing possible phytotoxic effects on plant development. Halophytophthora lusitanica, H. thermoambigua and a putative new Halophytophtora species were identified as possible causal agents of seed loss. The antimicrobial agents (copper and tannins) reduced seed contamination by 20%, although copper sulphate at 2 ppm strongly inhibited the root growth. Among tannins, chestnut and tara reduced seeds germination by up to 75% and decreased shoot and root development, while quebracho showed a less severe phytotoxic effect. The use of copper sulphate at 0.2 ppm is therefore recommended to prevent P. oceanica seedling loss in culture facilities since it reduces seed contamination with no phytotoxic effects. Our results contribute to improving the seedling culture of one the key species of the Mediterranean Sea, increasing propagule availability for restoration purposes.

Natural rock roughness as a key parameter for optimizing seedling adhesion and restoration of the seagrass Posidonia oceanica in its native environment

Seagrasses provide various ecosystem functions in coastal areas of the world. In the Mediterranean Sea, Posidonia oceanica is an endemic species threatened by several activities despite being protected by national and international laws. Currently, several transplanting initiatives have been carried out using different methods, among which those including seeds and seedlings are considered the most ecological and low‐cost ones. Beach‐cast fruits and seeds can be found in spring and their appearance can easily be reported, through a citizen science approach, by the community. One of the obstacles in using these methods is identifying the best substrate in which to place P. oceanica seeds to facilitate root adhesion of the seedlings prior to their transplantation into the sea. In the present study, we analyzed, using a 3D surface optical microscope, the roughness of natural rocks to identify the availability of specific roughness ranges suitable for adhesion and root anchoring of P. oceanica seedlings. Conventional roughness parameters and roughness power spectral density were calculated for the inner and outer surfaces of 9 different rock samples. Among the rock samples examined, the calcarenitic ones and in particular marsala calcarenite, due to the presence of the “ideal roughness for seedlings” can be considered one of the best consolidated substrates to be used for the construction of ad hoc devices on which plantlet of P. oceanica can grow for the purpose of restoration.

Field thermo acclimation increases the resilience of Posidonia oceanica seedlings to marine heat waves.

Acclimation is a response that results from chronic exposure of an individual to a new environment. This study aimed to investigate whether the thermal environment affects the early development of the seagrass Posidonia oceanica, and whether the effects of a field-simulated Marine Heat Wave (MHW) on seedlings change depending on acclimation. The experiment was done in the field using a crossed design of Acclimation (acclimated vs unacclimated) and MHW (present vs absent) factors. Acclimation has initially constrained the development of P. oceanica seedlings, but then it increased their resilience to the MHW, under both a morphological and biochemical (fatty acid saturation) level. This treatment could be considered in P. oceanica restoration projects in a climate change-impaired sea, by purposely inducing an increased resistance to heat before transplants.

Assessing Tolerance to the Hydrodynamic Exposure of Posidonia oceanica Seedlings Anchored to Rocky Substrates

Among a suite of abiotic and biotic factors, the hydrodynamic regime strongly influences the success of seagrass recruitment through sexual propagules. Uprooting of propagules by drag forces exerted by currents and waves is one of the main causes for the failed establishment and the consequent recruitment. Substrate type and stability play a key role in determining the success of colonization through sexual propagules, as seedling establishment probabilities proved to be significantly higher on rocky bottoms than on unstable unconsolidated substrates. In this research, the current and wave flow intensity thatPosidonia oceanicaseedlings anchored to rocky substrates can withstand before uprooting were evaluated and the influence of substrate complexity on seedling anchorage success and anchorage strength was investigated.P. oceanicaseedlings withstood the current velocity of 70 cm s–1and increased orbital flow velocities up to 25 cm s–1. Seedling adhesion strength ranged from 3.92 to 29.42 N. Results of the present study corroborate the hypothesis that substrate complexity at scales relevant to the size of propagules is a crucial feature forP. oceanicaseedling establishment. The intensity of unidirectional and oscillatory flow that seedlings can withstand without being dislodged assessed in this study support the hypothesis thatP. oceanicasexual propagules, once adhered to a consolidated substrate, are able to tolerate high hydrodynamic stress. The results of the present study contribute to re-evaluation of the habitat requirements ofP. oceanica, assessing the range of hydrodynamic conditions that this species can tolerate during the early stages of its life history.

Managing biotic interactions during early seagrass life stages to improve seed‐based restoration

Abstract Seagrasses are declining globally, and effective restoration actions to promote the recovery of degraded meadows are urgently needed. Harnessing positive plant interactions during early life stages is considered a valuable strategy to improve terrestrial and coastal habitat restoration. Yet, its application to seagrass restoration is still in infancy, and very little is known on the role played by biotic interactions in shaping newly established populations. We assessed the feasibility of manipulating intraspecific and interspecific plant interactions to enhance seed‐based restoration success using the seagrass Posidonia oceanica as a model. Specifically, we investigated in mesocosm whether seed germination increased with increasing seed density and whether increasing seedling density and planting a pioneer seagrass promoted facilitation among seedlings. To do this, seedlings were grown either as a single individual or in clumps of medium and high density, with and without Cymodocea nodosa, for 2 years encompassing their most critical growth stage. Germination of P. oceanica seeds was not affected by seed density. Posidonia oceanica seedlings planted at medium and high density showed higher survival than those planted individually but only in the presence of C. nodosa. Seedlings planted at medium and at high density with C. nodosa performed better than those grown at low density or without C. nodosa due to a positive joint effect of intraspecific and interspecific interaction. Cymodocea nodosa plants grown with P. oceanica seedlings at high density were larger than those grown alone, indicating a mutualistic relationship. Synthesis and applications. Our results show that in seagrasses positive interactions during early life stages can be promoted by planting seedlings in dense clumps in association with an early successional species. The incorporation of this novel nature‐based approach in seed‐based restoration could accelerate the recolonization of degraded seagrass habitats. In planning future seed‐based restoration interventions, managers should assess both intraspecific and interspecific interactions established by seedlings of target species for identifying proper planting density/spatial configuration and potential benefactor species promoting facilitative mechanisms to maximize seedling planting success.

Substrate Type Influences the Structure of Epiphyte Communities and the Growth of Posidonia oceanica Seedlings.

Epiphytes colonizing adult seagrasses highly contribute to seagrass ecosystem functioning and plant growth. Yet, little information exists on epiphytic communities developing on seagrass seedlings. Moreover, for some species our knowledge about seedling performance is limited to early establishment phases, and the role of substrate type in affecting their growth is still unclear. These are considerable knowledge gaps, as seedlings play an important role in meadow expansion and recovery from disturbance. In this study, seedlings of Posidonia oceanica, a keystone species of the Mediterranean, were grown in a shallow (1.5 m deep) coastal area along the Tuscany coast (Italy). After five years of growth (July 2009), seedlings were collected and, through multivariate analysis, we examined whether the epiphytic communities of leaves (both internal and external side) and rhizomes, as well as the growth characteristics differed between rock and sand substrate. The epiphytic communities of seedlings largely reflected those found on adult shoots. Epiphyte cover was similar between the two leaf sides, and it was higher on seedlings grown on rock than on sand, with encrusting algae dominating the community. No differences in epiphyte cover and community structure on rhizomes were found between substrates. Seedling growth characteristics did not differ between substrates, apart from the number of standing leaves being higher on rock than on sand. No correlation was found among epiphyte communities and seedling growth variables (i.e., leaf area, maximum leaf length, number of leaves, total number of leaves produced, rhizome length, total biomass, and root to shoot biomass ratio). Results indicate that epiphytes successfully colonize P. oceanica seedlings, and the surrounding micro-environment (i.e., substrate type) can influence the leaf epiphytic community. This study provides new valuable insights on the biological interactions occurring in seagrass ecosystems and highlights the need for better understanding the effects of seedling epiphytes and substrate on the formation of new meadows.

Influence of biotic and abiotic factors of seagrass Posidonia oceanica recruitment: Identifying suitable microsites

The period between seed germination and successful seedling establishment is considered the most vulnerable phase for plant development. To better predict recruitment patterns within plant communities, it is essential to identify the abiotic constrains and biotic interactions that allow for the colonization of substrates by plant species. We evaluated which combination of factors are associated with successful survival and development of seedlings of the seagrass Posidonia oceanica in order to identify the most important microsite features acting together on recruitment success. Our results show that P. oceanica seedlings are rather specific in their environmental requirements during their first 18 months of life, when their development and survival are favored in microsites of consolidated substratum (solid rock, and to a lesser extent P. oceanica matte) covered by macroalgae (mainly crustose algae) and located in sheltered locations (with energy flux values not exceeding 7 × 105 kg s−2 m s−1). After this phase, their probability of surviving becomes more independent from external conditions.

Biological adhesion in seagrasses: The role of substrate roughness in Posidonia oceanica (L.) Delile seedling anchorage via adhesive root hairs

Seagrasses are marine flowering plants that developed several adaptive traits for living in submerged waters. Among this group, Posidonia oceanica (L.) Delile is the dominant species of the Mediterranean Sea, forming persistent meadows that provide valuable ecosystem services to human communities. P. oceanica seedlings can anchor to rocky substrates through adhesive root hairs. Here we investigate, for the first time, the bioadhesion process in seagrasses. Seedlings were grown on substrates provided with different roughness in order to identify mechanisms involved in the adhesion process. Root anchorage strength was measured through a peel test and hair morphology at different micro-roughness was analysed by electron and fluorescence microscopy. Maximum anchorage strength was recorded at roughness levels between 3 and 26 μm, while on finer (0.3) and coarser (52, 162 μm) roughness attachment was weaker. No attachment was obtained on smooth surfaces. Accordingly, root hair tip morphology strongly responded to the substrate. Morphological adaptation of the root hairs to surface topography and mechanical interlocking into the micro-roughness of the substrate appear the main mechanisms responsible for bioadhesion in the system under study. Substrate roughness at the scale of microns and tens of microns is pivotal for P. oceanica seedling attachment to take place. These findings contribute to identification of features of optimal microsite for P. oceanica seedling settlement and to the development of novel approaches to seagrass restoration that take advantage of species’ key life history traits.

Heat wave intensity can vary the cumulative effects of multiple environmental stressors on Posidonia oceanica seedlings

Climate change is introducing new stressors into already stressed ecosystems. Among these, extreme events such as heat waves play a crucial role in determining the structure of ecosystems. We tested single and combined effects of overgrazing, burial and heat waves on the seedlings of the habitat-forming species Posidonia oceanica. At current heat wave temperatures, overgrazing in isolation had more deleterious effects than seed burial, and effects were synergistic and additive when both factors co-occurred. The combined effect of overgrazing and seed burial with current heat waves could hamper P. oceanica seedling development, with similar or even higher levels than the sole effect of heat waves in the near future (29 °C). The effects of overgrazing and seed burial are expected to be overridden if heat waves temperatures exceed 29 °C. These results suggest that co-occurring environmental stressors, in combination with current heat waves, could compromise the sexual recruitment of this seagrass.

The perfect microsite: How to maximize Posidonia oceanica seedling settlement success for restoration purposes using ecological knowledge

The use of sexual propagules to restore seagrass meadows has raised increasing attention in the last years as seed-based strategies avoid impacts on donor beds while preserving genetic diversity in restored populations. However, the availability of suitable microsites for seedling establishment at transplantation locations is crucial in order to achieve positive outcome of restoration actions. In this study we develop ad-hoc holders that act as optimal microsites for Posidonia oceanica seedling establishment. Holders are intended to be transferred in the field for restoration purposes after few months of indoor seedling culture. Seedling ability to self-anchor to rocky substrates via adhesive root hairs was exploited. We tested rocky holders with different designs in order to maximize seedling survival and settlement. The effect of the holder design on seedling anchorage performances was evaluated. Holders were provided with different topographical complexity and substrate slope. Topographical complexity significantly influenced settlement success, as seedlings did not attach to flat holders, while anchorage reached 100% on holders provided with complexity at seed and the root scales. Substrate slope did not affect the percentage of anchored seedlings, conversely it influenced root growth pattern and thus anchorage stability. This study shows how ecological knowledge of species’ life history strategies and associated critical traits provides valuable hints to develop alternative approaches to seagrass restoration tailored to the biology of the system under study.

Field transplantation of seagrass (Posidonia oceanica) seedlings: Effects of invasive algae and nutrients

Seedlings are a key life stage in seagrasses, providing genetic diversity and being a useful tool for restoration. We examined the influence of increased sediment nutrients and the presence of the invasive macroalga Caulerpa cylindracea on the success of in situ transplanting Posidonia oceanica seedlings in a six-month experiment. Our results indicate that one-year old seedlings successfully survive in the field and their survival and growth are positively affected by the presence of C. cylindracea. Furthermore, nutrient addition in the sediment had positive effects on both C. cylindracea (increasing its cover) and seedlings (increasing leaf development), and the increased C. cylindracea cover did not result in detrimental effects on seedlings. Therefore, biological invasions and nutrient addition do not reinforce each other in the short term to negatively impact transplanted seedlings, which highlights facilitative interactions between invasive algae and native seagrass and provides useful information for successful strategies of seagrass restoration.

Future heat waves due to climate change threaten the survival of Posidonia oceanica seedlings

Extreme weather events are major drivers of ecological change, and their occurrence is likely to increase due to climate change. The transient increases in atmospheric temperatures are leading to a greater occurrence of heat waves, extreme events that can produce a substantial warming of water, especially in enclosed basins such as the Mediterranean Sea. Here, we tested the effects of current and predicted heat waves on the early stages of development of the seagrass Posidonia oceanica. Temperatures above 27 °C limited the growth of the plant by inhibiting its photosynthetic system. It suffered a reduction in leaf growth and faster leaf senescence, and in some cases mortality. This study demonstrates that the greater frequency of heat waves, along with anticipated temperature rises in coming decades, are expected to negatively affect the germination of P. oceanica seedlings.

Assessing Posidonia oceanica seedling substrate preference: an experimental determination of seedling anchorage success in rocky vs. sandy substrates.

In the last decades the growing awareness of the ecological importance of seagrass meadows has prompted increasing efforts to protect existing beds and restore degraded habitats. An in-depth knowledge of factors acting as major drivers of propagule settlement and recruitment is required in order to understand patterns of seagrass colonization and recovery and to inform appropriate management and conservation strategies. In this work Posidonia oceanica seedlings were reared for five months in a land-based culture facility under simulated natural hydrodynamic conditions to identify suitable substrates for seedling anchorage. Two main substrate features were investigated: firmness (i.e., sand vs. rock) and complexity (i.e., size of interstitial spaces between rocks). Seedlings were successfully grown in culture tanks, obtaining overall seedling survival of 93%. Anchorage was strongly influenced by substrate firmness and took place only on rocks, where it was as high as 89%. Anchorage occurred through adhesion by sticky root hairs. The minimum force required to dislodge plantlets attached to rocky substrates reached 23.830 N (equivalent to 2.43 kg), which would potentially allow many plantlets to overcome winter storms in the field. The ability of rocky substrates to retain seedlings increased with their complexity. The interstitial spaces between rocks provided appropriate microsites for seedling settlement, as seeds were successfully retained, and a suitable substrate for anchorage was available. In conclusion P. oceanica juveniles showed a clear-cut preference for hard substrates over the sandy one, due to the root system adhesive properties. In particular, firm and complex substrates allowed for propagule early and strong anchorage, enhancing persistence and establishment probabilities. Seedling substrate preference documented here leads to expect a more successful sexual recruitment on hard bottoms compared with soft ones. This feature could have influenced P. oceanica patterns of colonization in past and present time.

Evidences of adaptive traits to rocky substrates undermine paradigm of habitat preference of the Mediterranean seagrass Posidonia oceanica.

Posidonia oceanica meadows are acknowledged as one of the most valuable ecosystems of the Mediterranean Sea. P. oceanica has been historically described as a species typically growing on mobile substrates whose development requires precursor communities. Here we document for the first time the extensive presence of sticky hairs covering P. oceanica seedling roots. Adhesive root hairs allow the seedlings to firmly anchor to rocky substrates with anchorage strength values up to 5.23 N, regardless of the presence of algal cover and to colonise bare rock without the need for precursor assemblages to facilitate settlement. Adhesive root hairs are a morphological trait common on plants living on rocks in high-energy habitats, such as the riverweed Podostemaceae and the seagrass Phyllospadix scouleri. The presence of adhesive root hairs in P. oceanica juveniles suggests a preference of this species for hard substrates. Such an adaptation leads to hypothesize a new microsite driven bottleneck in P. oceanica seedling survival linked to substrate features. The mechanism described can favour plant establishment on rocky substrates, in contrast with traditional paradigms. This feature may have strongly influenced P. oceanica pattern of colonisation through sexual propagules in both the past and present.

Use of Posidonia oceanica seedlings from beach-cast fruits for seagrass planting

Abstract Posidonia oceanica seedlings produced from beach-cast fruits were planted in an area where this Mediterranean seagrass was lost as a consequence of fish-farming. The effects of substratum type (dead matte vs. meadow) and planting level (above vs. below bottom surface) on seedling survivorship and leaf development were evaluated after 3 years. Seedling capacity to resist uprooting was also tested by comparing survivorship and leaf development after 2 years in seedlings firmly anchored in dead matte by artificial means (mesh-pot) with those planted without any anchoring other than their roots. Survivorship in dead matte was 44% after 3 years and was not affected by planting level. Seedling planting inside the meadow led to complete mortality after 3 years. Seedlings planted below the bottom surface branched during the second year and 50% had produced from one to five additional shoots after 3 years. After 2 years, survivorship and leaf development of seedlings planted with mesh-pot anchoring was similar to that of seedlings planted without anchoring. Current environmental conditions in the impacted area allow P. oceanica seedlings to survive, but natural recovery might be limited by propagule availability. Planting of seedlings in dead matte without anchoring does not affect survivorship and leaf development and might be a feasible technique for restoration of P. oceanica meadows.

Effects of salinity on seed germination and early seedling growth of the Mediterranean seagrass Posidonia oceanica (L.) Delile

Previous research has demonstrated the low tolerance of Posidonia oceanica mature shoots to salinity variability but there is no information about the response of its seeds and seedlings to this impact. In the present study, two independent experiments were conducted to investigate the effects of salinity variation on germination, development and survival of P. oceanica seeds and seedlings under laboratory-controlled conditions. Maximum P. oceanica seed germination occurred at the control salinity (37), while mortality was lowest for this treatment. However, no significant differences were detected with other salinity levels (39–49). In contrast, salinity appears to play an important role in seedling development. The number of blades, the length of the primary root and the maximum length of leaves of seedlings were significantly reduced with increased salinities. The results observed in the present study are consistent with those obtained for mature P. oceanica shoots, and suggest that hypersalinities, such as those associated with brine discharge of a desalination plant, may limit P. oceanica seedling recruitment, affecting the expansion and recovery of Posidonia meadows.

Mediterranean Seagrass Growth and Demography Responses to Experimental Warming

We experimentally examined the effects of increased temperature on growth and demography of two Mediterranean seagrasses Posidonia oceanica and Cymodocea nodosa. Shoots of C. nodosa and seedlings and shoots of P. oceanica were kept in mesocosms for 3 months and exposed to temperatures between 25 and 32 °C encompassing the range of maximum summer seawater temperatures projected for the Mediterranean Sea during the twenty-first century. The response of P. oceanica seedlings to warming was evident with reduced growth rates, leaf formation rates and leaf biomass per shoot. Younger life stages of P. oceanica may therefore be particularly vulnerable to climate change and warming. Leaf formation rates in the shoots of P. oceanica declined with increasing temperature and the lowest population growth (−0.005 day−1) was found at 32 °C. Temperature effects on C. nodosa were variable. Rhizome growth increased with warming (0.07–0.09 cm day−1 °C of warming), whereas other indicators of plant performance (aboveground/belowground biomass, leaf biomass and population growth) appeared to be stimulated by increased temperature to a threshold temperature of around 29–30 °C beyond which they declined. P. oceanica and C. nodosa are likely to be negatively impacted by the effects of global warming over the next century and climate change poses a significant challenge to seagrasses and may stress these key habitat-forming species that are already suffering losses from anthropogenic impacts.

Posidonia oceanica and Cymodocea nodosa seedling tolerance to wave exposure

We studied the role of hydrodynamics in the establishment of sea grass seedlings for two Mediterranean sea grass species, Posidonia oceanica and Cymodocea nodosa, by combining flume and field studies. Flume measurements under both unidirectional and oscillatory flow showed that P. oceanica seedlings experienced higher drag forces than C. nodosa, which could be related to the larger total leaf area. Drag coefficients were between 0.01 and 0.1 for Reynolds numbers of 103 and 105. As a result, P. oceanica seedlings required 40‐50% of root length anchored to the sediment before being dislodged, whereas C. nodosa required ≈ 20%. To validate the flume results, seedling survival in sandy beds was evaluated for two depths (12 and 18 m) at two field locations. To calculate near‐bottom orbital velocities at the planting sites, deep‐water waves were propagated to shallow water using a numerical model. Our results showed that P. oceanica seedlings experienced high losses after the first autumn storms when near‐bottom orbital velocities exceeded 18 cm s‐1. The loss of C. nodosa seedlings was much lower and some seedlings survived velocities as high as 39 cm s‐1. Thus, flume and field results are consistent in explaining relative higher losses of P. oceanica seedlings than for C. nodosa.

Experimental Evaluation of the Restoration Capacity of a Fish‐Farm Impacted Area with Posidonia oceanica (L.) Delile Seedlings

Marine aquaculture is an activity that has induced severe local losses of seagrass meadows along the coastal areas. The purpose of this study was to evaluate the capacity of an area degraded by fish‐farm activities to support Posidonia oceanica seedlings. In the study site, a bay in the southeast coast of Spain where part of a meadow disappeared by fish‐farm activities, seedlings inside mesh‐pots were planted in three areas. Two plots were established in each area, one in P. oceanica dead matte and another inside a P. oceanica meadow. To evaluate if sediment conditions were adequate for the life of the seedlings, half of them were planted in direct contact with the sediment and the other half were planted above the surface of the sediment in each plot. Monitoring during 1 year showed that there were large differences in seedling survival between the dead matte and the P. oceanica meadow. While seedlings planted in dead matte had a high survivorship after 1 year (75%), seedlings planted in P. oceanica progressively died (survivorship of 20% after 1 year). The average leaf length of the seedlings surviving in the two substrata was not different, but the leaf area per seedling was lower in the seedlings growing inside the P. oceanica meadow during most part of the year. Seedling survivorship and vegetative development were not affected by the level of planting and suggest that the sediment conditions are adequate for the life of P. oceanica seedlings.