Recolonization dynamics in a mixed seagrass meadow: The role of clonal versus sexual processes

Tolerance to aerial exposure influences distributional patterns in multi-species intertidal seagrass meadows

Multi-specific seagrass meadow assemblages dominate most tropical intertidal regions but the relative role of environmental stress in determining distribution patterns is still uncertain. Here we combine observational and experimental approaches to examine aerial exposure as a factor driving species occurrence patterns in intertidal meadows of the Andaman archipelago, where up to 6 seagrass species co-occur. In the studied meadow, patterns of exposure did not map onto distance from the coast, instead creating a patchy matrix of exposure, based on fine-scale bathymetric differences. Distributional surveys showed that seagrass species were similarly patchy, often tracking the degree of aerial exposure during low tide. While some species (Halophila ovalis, Halophila minor, and Thalassia hemprichii) frequently occurred in submerged or subtidal areas and were rarely found in completely exposed areas, other species (Cymodocea rotundata, Halophila beccarii, and Halodule uninervis) also occupied areas that were subject to partial or complete aerial exposure during low tide. To confirm this pattern, we used field-based transplant experiments, employing a natural gradient of tidal exposure to subject six seagrass species to different desiccation exposure times. After a month, H. beccarii and H. uninervis transplants survived in areas that sustained more than 3 h of aerial tidal exposure without significant mortality, compared with other species (H. ovalis, H. minor, T. hemprichii, C. rotundata) that showed dramatic shoot mortality at the same exposure regimes. For all species, 4 h represented the upper limit of exposure, in both experimental and distributional studies. However, despite their wider tolerance of exposure to air, H. beccarii and H. uninervis did not dominate the entire meadow. This could be a result either of their poor tolerance to other environmental factors or their lower competitive abilities among other mechanisms. This suggests that in tropical multi-specific meadows, strong environmental filters could override clear intertidal zonation to create patchy matrices based on species tolerances.

The cycle of seagrass life: From flowers to new meadows

Understanding sexual reproduction and recruitment in seagrasses is crucial to their conservation and restoration. Flowering, seed production, seed recruitment, and seedling establishment data for the seagrass Posidonia australis was collected annually between 2013 and 2018 in meadows at six locations around Rottnest Island, Western Australia. Variable annual rates of flowering and seed production were observed among meadows between northern and southern sides of the island and among years. Meadows on the northern shore consistently flowered more intensely and produced more seeds across the years of the survey. Inter-site variation in clonal diversity and size of clones, seed production, wind and surface currents during pollen and seed release, and the large, but variable, impact of seed predation are likely the principal drivers of successful recruitment into established meadows and in colonizing unvegetated sands. The prolific but variable annual reproductive investment increases the probability of low levels of continuous recruitment from seed in this seagrass, despite high rates of abiotic and biotic disturbance at seedling, shoot, and patch scales. This strategy also imparts a level of ecological resilience to this long-lived and persistent species.

Poor success of seagrass Posidonia oceanica transplanting in a meadow disturbed by power line burial

Local disturbances drive the decrease of the area covered by Posidonia oceanica in the Mediterranean. Mechanical impacts during the development of coastal infrastructures alter sea floor and the recolonization of benthic community will depend on the recovery of pre-disturbance environmental conditions and on the intrinsic characteristics of the local community that was disturbed. We transplanted 468 rhizome fragments and 450 seedlings of P. oceanica in a meadow disturbed by the trenching and deployment of a power line to evaluate the suitability of the disturbed sea floor for rehabilitating P. oceanica meadows. We quantify and compare the survivorship and vegetative development of the transplanted/planted (i.e. fragments/seedlings) material in the two types of the unconsolidated substrata left after infrastructure deployment works finished: sand and burlap bags filled with coarse gravel. The latter was used as a corrective measure for topographic restoration. Three experimental plots with sixteen transplanted fragments or twenty-five seedlings were placed at each substratum type at three different depths (i.e. 15, 20 and 25 m). Our results show that the transplanting of P. oceanica rhizome fragments in the disturbed substrata had low survival rates (0-31%) after 40-48 months. The survivorship of seedlings was lower than that of fragments. Our results highlight the importance of substratum for P. oceanica recovery after mechanical impact; disturbed, non-consolidated substrata will preclude P. oceanica rehabilitation through planting. Preservation of meadow substratum (i.e. dead matte) is a critical element that coastal infrastructure projects should consider to enable future recovery of P. oceanica meadows.

Persistent Clones and Local Seed Recruitment Contribute to the Resilience of Enhalus acoroides Populations Under Disturbance

Human-induced land use in coastal areas is one of the main threats for seagrass meadows globally causing eutrophication and sedimentation. These environmental stressors induce sudden ecosystem shifts toward new alternative stable states defined by lower seagrass richness and abundance. Enhalus acoroides, a large-sized tropical seagrass species, appears to be more resistant toward environmental change compared to coexisting seagrass species. We hypothesize that reproductive strategy and the extent of seedling recruitment of E. acoroides are altered under disturbance and contribute to the persistence and resilience of E. acoroides meadows. In this research, we studied eight populations of E. acoroides in four lagoons along the South Central Coast of Vietnam using 11 polymorphic microsatellite loci. We classified land use in 6 classes based on Sentinel-2 L2A images and determined the effect of human-induced land use at different spatial scales on clonal richness and structure, fine-scale genetic structure and genetic diversity. No evidence of population size reductions due to disturbance was found, however, lagoons were strongly differentiated and may act as barriers to gene flow. The proportion and size of clones were significantly higher in populations of surrounding catchments with larger areas of agriculture, urbanization and aquaculture. We postulate that large resistant genets contribute to the resilience of E. acoroides meadows under high levels of disturbance. Although the importance of clonal growth increases with disturbance, sexual reproduction and the subsequent recruitment of seedlings remains an essential strategy for the persistence of populations of E. acoroides and should be prioritized in conservation measures to ensure broad-scale and long-term resilience toward future environmental change.

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.

Identification of dispersal barriers for a colonising seagrass using seascape genetic analysis

Seagrasses are important habitats providing many ecological services. Most species have broad distributions with maximum dispersal distances of 100's of kms, however there is limited understanding of dispersal distances of colonising species like Halodule uninervis. It commonly grows in disturbed environments and could disperse to other meadows via clonal fragments. Effective conservation management requires greater understanding of genetic structure, dispersal barriers, and connectivity timescales to predict recovery following disturbance. Despite fragment viability of up to 28 days in a congenera, this theory remains untested in situ. Using 80 neutral single nucleotide polymorphisms, we investigated genetic diversity, gene flow patterns and structure among 15 populations of H. uninervis along 2000 km of Western Australian coastline. These data were combined with a multi-generational oceanographic dispersal model and a barrier dispersal analysis to identify dispersal barriers and determine which fragment dispersal duration (FDD) and timescale over which stepping-stone connectivity occurred, best matched the observed genetic structure. The 2-7 day FDD best matched the genetic structure with 4-12 clusters, with barriers to dispersal that persisted for up to 100 years. Modelling suggested greater fragmentation of metapopulations towards the southern edge of the species distribution, but genetic diversity did not decline. Several long-term boundaries were identified even with fragment viability of up to 28 days. This suggests H. uninervis dispersal is spatially limited by factors like oceanographic features and habitat continuity which may limit dispersal of this species. This study reiterates that potential dispersal does not equal realised dispersal, and management scales of 10's of kilometers are required to maintain existing meadows. Recruitment from distances further than this scale are unlikely to aid recovery after extreme disturbance events, particularly towards the range edge of H. uninervis distribution.

Natural recovery of a marine foundation species emerges decades after landscape-scale mortality

Globally, the conditions and time scales underlying coastal ecosystem recovery following disturbance remain poorly understood, and post-disturbance examples of resilience based on long-term studies are particularly rare. Here, we documented the recovery of a marine foundation species (turtlegrass) following a hypersalinity-associated die-off in Florida Bay, USA, one of the most spatially extensive mortality events for seagrass ecosystems on record. Based upon annual sampling over two decades, foundation species recovery across the landscape was demonstrated by two ecosystem responses: the range of turtlegrass biomass met or exceeded levels present prior to the die-off, and turtlegrass regained dominance of seagrass community structure. Unlike reports for most marine taxa, recovery followed without human intervention or reduction to anthropogenic impacts. Our long-term study revealed previously uncharted resilience in subtropical seagrass landscapes but warns that future persistence of the foundation species in this iconic ecosystem will depend upon the frequency and severity of drought-associated perturbation.

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.

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.

Fine-scale genetic structure and flowering output of the seagrass Enhalus acoroides undergoing disturbance

Seagrass are under great stress in the tropical coast of Asia, where Enhalus acoroides is frequently the dominant species with a large food web. Here, we investigate the question of the fine-scale genetic structure of this ecologically important foundation species, subject to severe anthropogenic disturbance in China. The genetic structure will illuminate potential mechanisms for population dynamics and sustainability, which are critical for preservation of biodiversity and for decision-making in management and restoration. We evaluated the fine-scale spatial genetic structure (SGS) and flowering output of E. acoroides, and indirectly estimated the relative importance of sexual versus asexual reproduction for population persistence using spatial autocorrelation analysis. Results reveal high clonal diversity for this species, as predicted from its high sexual reproduction output. The stronger Sp statistic at the ramet-level compared with genet-level indicates that clonality increases the SGS pattern for E. acoroides. Significant SGS at the genet-level may be explained by the aggregated dispersal of seed/pollen cohorts. The estimated gene dispersal variance suggests that dispersal mediated by sexual reproduction is more important than clonal growth in this study area. The ongoing anthropogenic disturbance will negatively affect the mating pattern and the SGS patterns in the future due to massive death of shoots, and less frequency of sexual reproduction.

Negative effects of warming on seagrass seedlings are not exacerbated by invasive algae

The observed and projected rise in sea surface temperature challenges marine biodiversity worldwide, and particularly in temperate ecosystems dealing with the arrival of novel species of tropical provenance. When the impacted biota are early life stages of ecosystem engineers, the effects of those impacts are of major concern for ecologists and coastal managers. We experimentally examined the individual and potential additive effects of seawater warming and the presence of the invasive algae on the development of seedlings of the seagrass Posidonia oceanica in a three-month mesocosm experiment. Whereas the presence of the invasive algae (Caulerpa cylindracea and Lophocladia lallemandii) did not result in detrimental effects on seedlings, warming negatively affected seedling development. Interestingly, the presence of both invasive algae may ameliorate the negative effects of warming.

Disturbance Is an Important Driver of Clonal Richness in Tropical Seagrasses

Clonality is common in many aquatic plant species, including seagrasses, where populations are maintained through a combination of asexual and sexual reproduction. One common measure used to describe the clonal structure of populations is clonal richness. Clonal richness is strongly dependent on the biological characteristics of the species, and how these interact with the environment but can also reflect evolutionary scale processes especially at the edge of species ranges. However, little is known about the spatial patterns and drivers of clonal richness in tropical seagrasses. This study assessed the spatial patterns of clonal richness in meadows of three tropical seagrass species, Thalassia hemprichii, Halodule uninervis, and Halophila ovalis, spanning a range of life-history strategies and spatial scales (2.5-4,711 km) in Indonesia and NW Australia. We further investigated the drivers of clonal richness using general additive mixed models for two of the species, H. uninervis and H. ovalis, over 8° latitude. No significant patterns were observed in clonal richness with latitude, yet disturbance combined with sea surface temperature strongly predicted spatial patterns of clonal richness. Sites with a high probability of cyclone disturbance had low clonal richness, whereas an intermediate probability of cyclone disturbance and the presence of dugong grazing combined with higher sea surface temperatures resulted in higher levels of clonal richness. We propose potential mechanisms for these patterns related to the recruitment and mortality rates of individuals as well as reproductive effort. Under a changing climate, increased severity of tropical cyclones and the decline in populations of mega-grazers have the potential to reduce clonal richness leading to less genetically diverse populations.

Salinity and temperature significantly influence seed germination, seedling establishment, and seedling growth of eelgrass Zostera marina L

Globally, seagrass beds have been recognized as critical yet declining coastal habitats. To mitigate seagrass losses, seagrass restorations have been conducted in worldwide over the past two decades. Seed utilization is considered to be an important approach in seagrass restoration efforts. In this study, we investigated the effects of salinity and temperature on seed germination, seedling establishment, and seedling growth of eelgrass Zostera marina L. (Swan Lake, northern China). We initially tested the effects of salinity (0, 5, 10, 15, 20, 25, 30, 35, and 40 ppt) and water temperature (5, 10, 15, and 20 °C) on seed germination to identify optimal levels. To identify levels of salinity that could potentially limit survival and growth, and, consequently, the spatial distribution of seedlings in temperate estuaries, we then examined the effect of freshwater and other salinity levels (10, 20, and 30 ppt) on seedling growth and establishment to confirm suitable conditions for seedling development. Finally, we examined the effect of transferring germinated seeds from freshwater or low salinity levels (1, 5, and 15 ppt) to natural seawater (32 ppt) on seedling establishment rate (SER) at 15 °C. In our research, we found that: (1) Mature seeds had a considerably lower moisture content than immature seeds; therefore, moisture content may be a potential indicator of Z. marina seed maturity; (2) Seed germination significantly increased at low salinity (p < 0.001) and high temperature (p < 0.001). Salinity had a much stronger influence on seed germination than temperature. Maximum seed germination (88.67 ± 5.77%) was recorded in freshwater at 15 °C; (3) Freshwater and low salinity levels (< 20 ppt) increased germination but had a strong negative effect on seedling morphology (number of leaves per seedling reduced from 2 to 0, and maximum seedling leaf length reduced from 4.48 to 0 cm) and growth (seedling biomass reduced by 46.15–66.67% and maximum seedling length reduced by 21.16–69.50%). However, Z. marina performed almost equally well at salinities of 20 and 30 ppt. Very few germinated seeds completed leaf differentiation and seedling establishment in freshwater or at low salinity, implying that freshwater and low salinity may potentially limit the distribution of this species in coastal and estuarine waters. Therefore, the optimum salinity for Z. marina seedling establishment and colonization appears to be above 20 ppt in natural beds; (4) Seeds germinated in freshwater or at low salinity levels could be transferred to natural seawater to accomplish seedling establishment and colonization. This may be the optimal method for the adoption of seed utilization in seagrass restoration. We also identified seven stages of seed germination and seedling metamorphosis in order to characterize growth and developmental characteristics. Our results may serve as useful information for Z. marina habitat establishment and restoration programs.

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.

Contemporary connectivity is sustained by wind- and current-driven seed dispersal among seagrass meadows

BACKGROUND

Seagrasses are clonal marine plants that form important biotic habitats in many tropical and temperate coastal ecosystems. While there is a reasonable understanding of the dynamics of asexual (vegetative) growth in seagrasses, sexual reproduction and the dispersal pathways of the seeds remain poorly studied. Here we address the potential for a predominantly clonal seagrass, P. australis, to disperse over long distances by movement of floating fruit via wind and surface currents within the coastal waters of Perth, Western Australia. We first simulated the dominant atmospheric and ocean forcing conditions that are known to disperse these seagrass seeds using a three-dimensional numerical ocean circulation model. Field observations obtained at 8 sites across the study area were used to validate the model performance over ~2 months in summer when buoyant P. australis fruit are released into the water column. P. australis fruit dispersal trajectories were then quantified throughout the region by incorporating key physical properties of the fruit within the transport model. The time taken for the floating fruit to release their seed (dehiscence) was incorporated into the model based on laboratory measurements, and was used to predict the settlement probability distributions across the model domain.

RESULTS

The results revealed that high rates of local and regional demographic connectivity among P. australis meadows are achieved via contemporary seed dispersal. Dispersal of seeds via floating fruit has the potential to regularly connect meadows at distances of 10s of kilometres (50% of seeds produced) and infrequently for meadows at distances 100 s km (3% of seeds produced).

CONCLUSIONS

The spatial patterns of seed dispersal were heavily influenced by atmospheric and oceanographic conditions, which generally drove a northward pattern of connectivity on a regional scale, but with geographical barriers influencing finer-scale connectivity pathways at some locations. Such levels of seed dispersal infer greater levels of ecological and genetic connectivity and suggest that seagrasses are not just strongly clonal.

Resilience of Zostera muelleri seagrass to small-scale disturbances: the relative importance of asexual versus sexual recovery

Resilience is the ability of an ecosystem to recover from disturbance without loss of essential function. Seagrass ecosystems are key marine and estuarine habitats that are under threat from a variety of natural and anthropogenic disturbances. The ability of these ecosystems to recovery from disturbance will to a large extent depend on the internsity and scale of the disturbance, and the relative importance of sexual versus asexual reproduction within populations. Here, we investigated the resilience of Zostera muelleri seagrass (Syn. Zostera capricorni) to small-scale disturbances at four locations in Lake Macquarie – Australia's largest coastal lake – and monitored recovery over a 65-week period. Resilience of Z. muelleri varied significantly with disturbance intensity; Z. muelleri recovered rapidly (within 2 weeks) from low-intensity disturbance (shoot loss), and rates of recovery appeared related to initial shoot length. Recovery via rhizome encroachment (asexual regeneration) from high-intensity disturbance (loss of entire plant) varied among locations, ranging from 18-35 weeks, whereas the ability to recover was apparently lost (at least within the time frame of this study) when recovery depended on sexual regeneration, suggesting that seeds do not provide a mechanism of recovery against intense small-scale disturbances. The lack of sexual recruits into disturbed sites is surprising as our initial surveys of genotypic diversity (using nine polymorphic microsatellite loci) at these location indicate that populations are maintained by a mix of sexual and asexual reproduction (genotypic diversity [R] varied from 0.24 to 0.44), and populations consisted of a mosaic of genotypes with on average 3.6 unique multilocus genotypes per 300 mm diameter plot. We therefore conclude that Z. muelleri populations within Lake Macquarie rely on clonal growth to recover from small-scale disturbances and that ongoing sexual recruitment by seeds into established seagrass beds (as opposed to bare areas arising from disturbance) must be the mechanism responsible for maintaining the observed mixed genetic composition of Z. muelleri seagrass meadows.

Understanding uncertainty in seagrass injury recovery: an information-theoretic approach

Vessel groundings cause severe, persistent gaps in seagrass beds. Varying degrees of natural recovery have been observed for grounding injuries, limiting recovery prediction capabilities, and therefore, management's ability to focus restoration efforts where natural recovery is unlikely. To improve our capacity for predicting seagrass injury recovery, we used an information-theoretic approach to evaluate the relative contribution of specific injury attributes to the natural recovery of 30 seagrass groundings in Florida Keys National Marine Sanctuary, Florida, USA. Injury recovery was defined by three response variables examined independently: (1) initiation of seagrass colonization, (2) areal contraction, and (3) sediment in-filling. We used a global model and all possible subsets for four predictor variables: (1) injury age, (2) original injury volume, (3) original injury perimeter-to-area ratio, and (4) wave energy. Successional processes were underway for many injuries with fast-growing, opportunistic seagrass species contributing most to colonization. The majority of groundings that exhibited natural seagrass colonization also exhibited areal contraction and sediment in-filling. Injuries demonstrating colonization, contraction, and in-filling were on average older and smaller, and they had larger initial perimeter-to-area ratios. Wave energy was highest for colonizing injuries. The information-theoretic approach was unable to select a single "best" model for any response variable. For colonization and contraction, injury age had the highest relative importance as a predictor variable; wave energy appeared to be associated with second-order effects, such as sediment in-filling, which in turn, facilitated seagrass colonization. For sediment in-filling, volume and perimeter-to-area ratio had similar relative importance as predictor variables with age playing a lesser role than seen for colonization and contraction. Our findings confirm that these injuries naturally initiate seagrass colonization with the potential to recover to pre-injury conditions, but likely on a decadal scale given the slow growth of the climax species (Thalassia testudinum), which is often the most severely injured. Our analysis supports current perceptions that sediment in-filling is critical to the recovery process and indicates that in order to stabilize injuries and facilitate seagrass recovery, managers should consider immediate restorative filling procedures for injuries having an original volume >14-16 m3.

Decoupled effects (positive to negative) of nutrient enrichment on ecosystem services

Eutrophication is a widespread phenomenon that disrupts natural ecosystems around the globe. Despite the general recognition that ecosystems provide many services and benefits to humans, little effort has been made to address how increasing anthropogenic eutrophication affects those services. We conducted a field experiment to determine the effect of nutrient enrichment on five ecological services provided by a model coastal system, a shallow seagrass community near Mobile Bay, Alabama (USA): (1) the provision of shelter for fauna; (2) the quality of food provided to first-order consumers; (3) quantity of food provision to first-order consumers and O2/CO2 exchange; (4) producer carbon and nitrogen storage, and (5) water clarity. The results showed a severe negative impact on seagrass density and biomass, which greatly reduced the structural complexity of the community and provision of shelter to fauna. Water clarity and the standing stock of producer carbon were reduced in the fertilized area in comparison with the control area. In contrast, nutrient addition did not affect in any consistent way the total quantity of food available for first-order consumers, the net exchange of O2/CO2, or the standing stock of producer nitrogen in the community. The nutritional quality of the food available for first-order consumers increased with fertilization. These results show that the impacts of nutrient enrichment on the services provided by natural systems may be disparate, ranging from negative to positive. These findings suggest that management policies for anthropogenic eutrophication will depend on the specific ecosystem service targeted. In the case of shallow seagrass beds, the loss of biogenic habitat and drastic impacts on commercially important fauna may be sufficiently alarming to warrant rigorous control of coastal eutrophication.

The impact of dredge-fill on Posidonia oceanica seagrass meadows: regression and patterns of recovery

Posidonia oceanica meadows can be severely damaged by dredge-fill operations. We report on the construction of gas pipelines that occurred between 1981 and 1993 in SW Sicily, Italy. A large portion of the meadow was mechanically removed, and the excavated trench was filled with a mosaic of substrates, ranging from sand to consolidated rock debris. Meadow loss and recovery were quantified over 7 years after the end of operations. We recorded an overall loss of 81.20 ha of meadow. Substrate strongly affected recovery as the percent cover by P. oceanica consistently increased on calcareous rubble, reaching values of 44.37 ± 3.05% in shallow sites after 7 years, whereas no significant increase occurred on other substrates. As in the Mediterranean Sea exploitation of coastal areas continues to grow with consequent impacts on P. oceanica meadows, this case study illustrates how artificial rubble-like materials could be employed to support the restoration of damaged meadows.

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.