A small-scale analysis of the spatial structure of a Posidonia oceanica meadow off the Island of Ischia (Gulf of Naples,Italy): Relationship with the seafloor morphology

Posidonia oceanica L. (Delile), an endemic species of the Mediterranean, forms extensive meadows which are continuously endangered by anthropic impacts. The availability of up-to-date information about interannual changes in shoot density of meadows and the knowledge of its expansion capabilities are crucial elements for the development of effective protection plans. Conversely, spatial ecology is becoming an increasingly important component of resource management, and the use of quantitative data for constructing prognosis maps of the dynamics of ecosystem degradation and restoration by nonlinear simulation methods is a topical field of landscape ecology. Unfortunately, little is known on spatial patterns of shoot density of P. oceanica on a small scale, despite their increasing use as indicators of the status and/or trends of meadows. The spatial structure of a continuous P. oceanica meadow, extending from 1 to 33 m depth in Lacco Ameno (Gulf of Naples, Italy), is investigated here by the “kriging” technique, a method widely used for geostatistical purposes. The analysis detected peculiar spatial patterns of shoot density and facilitated a small-scale (square meters) model of the distribution of P. oceanica. The highest shoot densities were found at the shallow stand (430 shoots m⁻², on the average, with a peak of 1000 shoots m⁻² in a relatively small area, at 1 m depth) and the lowest at the deep stand (average density <300 shoots m⁻² below 15 m depth). A high degree of patchiness was found in the shallow stand, down to 10 m depth. Nestlike patterns with a rounded shape, characterized by shoot density radially decreasing from the center, were demonstrated mainly in the shallow stand. An opposite trend was detected in one case, where the main nestlike pattern was characterized by a decrease of density towards the center. Nestlike patterns may be generated by the confluence and overlap of stolons expanding from proximal areas. The comparison of density and depth models indicated that the shape of the seafloor influences the density structure of meadows and the growth patterns of P. oceanica.     

Mediterranean warming triggers seagrass (Posidonia oceanica) shoot mortality

Rapid warming of the Mediterranean Sea threatens marine biodiversity, particularly key ecosystems already stressed by other impacts such as Posidonia oceanica meadows. A 6‐year monitoring of seawater temperature and annual P. oceanica shoot demography at Cabrera Archipelago National Park (Balearic Islands, Western Mediterranean) allowed us to determine if warming influenced shoot mortality and recruitment rates of seagrasses growing in relative pristine environments. The average annual maximum temperature for 2002–2006 was 1 °C above temperatures recorded in 1988–1999 (26.6 °C), two heat waves impacted the region (with seawater warming up to 28.83 °C in 2003 and to 28.54 °C in 2006) and the cumulative temperature anomaly, above the 1988–1999 mean annual maximum temperature, during the growing season (i.e. degree‐days) ranged between 0 °C in 2002 and 70 °C in 2003. Median annual P. oceanica shoot mortality rates varied from 0.067 year^?1 in 2002 to 0.123 year^?1 in 2003, and exceeded recruitment rates in all stations and years except in shallow stations for year 2004. Interannual fluctuations in shoot recruitment were independent of seawater warming (P>0.05). P. oceanica meadows experienced a decline throughout the study period at an average rate of ?0.050±0.020 year^?1. Interannual variability in P. oceanica shoot mortality was coupled (R²>0.40) to seawater warming variability and increasing water depth: shoot mortality rates increased by 0.022 year^?1 (i.e. an additional 2% year^?1) for each additional degree of annual maximum temperature and by 0.001 year^?1 (i.e. 0.1% year^?1) for each accumulated degree water temperature remained above 26.6 °C during the growing season. These results demonstrate that P. oceanica meadows are highly vulnerable to warming, which can induce steep declines in shoot abundance as well indicating that climate change poses a significant threat to this important habitat.     

Optimizing Posidonia oceanica (L.) Delile shoot density: Lessons learned from a shallow meadow

Although shoot patchiness has long been studied in Posidonia oceanica meadows, small scale spatial structure of P. oceanica meadows is poorly known, as very few studies focused on this feature. In order to analyze spatial patterns within P. oceanica meadows that appear uniformly dense and undisturbed, we collected shoot density data at the Capo Rizzuto Marine Protected Area (NE Mediterranean, Italy). Intensive sampling was carried out within a square lattice at small spatial scale (i.e. in the 10⁻¹–10² m² range) and shoot counts were obtained from sample quadrats of different size (60, 40, 20 cm). Spatial data analysis highlighted high irregularity in shoot density from centimetric to larger spatial scales. Therefore the deviation between shoot density estimates obtained using conventional methods and the overall average of quadrat counts (assumed as the best estimate for true density) was never negligible even when larger counting quadrats or higher numbers of replicates were adopted. While shoot density is regarded as the most important property of a P. oceanica meadow and as an indicator of ecosystem health, uncertainty in density estimates and unknown expected errors impair the effectivity of this approach. However, we showed that error could be predicted based on sampling intensity and design in an apparently uniform meadow. Although results from a single case study cannot be generalized, our work is the first attempt at analyzing the problems related to density estimates obtained from shoot counts and it shows how sampling can be optimized to achieve any desired level of accuracy.     

Spatial,temporal and structural variations of a Posidonia oceanica seagrass meadow facing human activities

The Bay of Saint-Cyr (Provence, France, Mediterranean Sea) is the site of two harbours, coastal urban development, trawling, boat anchoring and a sewage outfall. The Posidonia oceanica seagrass distribution was mapped with the help of aerial photographs, side scan sonar and GIS. In addition, the temporal variations of its distribution were studied by aerial photographs and GIS from 1955 to 2000. Finally, coverage and shoot density were measured via scuba-diving. This work reveals (i) the regression of the P. oceanica meadow at sites where harbours have been built, (ii) the occurrence of spaces within the meadow free of live P. oceanica (“intermattes”), which account for 8% of its surface area, (iii) a deep area where P. oceanica coverage and shoot density are low and (iv) evidence of regression, although modest, of the meadow at its lower limit. Nevertheless, the study site also exhibits an extensive and on the whole relatively healthy meadow whose limits have changed little over time.     

Photoadaptation and growth of Zostera marina L. (eelgrass) transplants along a depth gradient

Photosynthetic and growth responses were assessed in Zostera marina L. transplants within and beyond the natural extent of an eelgrass meadow in Great Harbor, Woods Hole, MA. Transplant survival and rapid growth inshore of the shallow edge of the meadow (0.5 and 0.8 m water depth) indicated a periodic disturbance factor maintaining the shallow edge of the meadow. Transplant mortality, reduced growth, and a negative carbon balance of eelgrass transplanted offshore the deep edge of the meadow (7 and 10m) supported the hypothesis of light-limited eelgrass growth in the deep regions of the Great Harbor meadow. Photoadaptive responses occurred along the water depth gradient, and both photosynthesis and growth responses were used to assess the genetic vs. phenotypic components of eelgrass response to the water depth gradient. Reciprocal transplants between shallow (1.3 m) and deep (5.5 m) areas within the eelgrass meadow indicated photosynthetic and growth responses were primarily a result of growth habitat rather than genetic differentiation within the eelgrass meadow.     

Seasonal occurrence of coccoliths in sediment traps from West Caroline Basin, equatorial West Pacific Ocean

Coccolith fluxes were investigated by sediment trap studies in the West Caroline Basin, which is located in the equatorial western Pacific. The investigation was conducted from June 1991 to March 1992 at two water depths, 1592 and 3902 m, as part of the Northwest Pacific Carbon Cycle Study (NOPACCS) program. Two seasonal maxima of coccolith fluxes were observed during September–early October and late December–January. The average coccolith and coccosphere fluxes at the depth of the shallow trap were 1800×10⁶ coccoliths m⁻² day⁻¹ and 1.9×10⁶ coccospheres m⁻² day⁻¹, respectively. The flux of coccoliths followed the same trend as the total flux, and was closely correlated with the flux of organic matter flux. Florisphaera profunda, Gladiolithus flabellatus, Gephyrocapsa oceanica, Umbilicosphaera sibogae var. sibogae, Emiliania huxleyi, and Oolithotus fragilis were the most abundant species together comprising more than 85% of the total flora. Observed seasonal changes of the species composition of coccolith flora, as well as analysis of the R-mode cluster, revealed that during the summer, the assemblage was marked by the dominance of G. oceanica and U. sibogae. However, during the winter, the assemblage was dominated by E. huxleyi and O. fragilis. These assemblage changes were influenced by monsoonal events, which were observed off the New Guinea coast. F. profunda dominated the community in the shallow trap throughout most of the year; peak values of this species were recorded during the winter. The coccosphere assemblage was dominated by G. oceanica at both water depths. In the deep trap, the sedimentation pattern was similar to that observed at the shallow depth. Mean coccolith and coccosphere fluxes at the deep trap were 2000×10⁶ coccolith m⁻² day⁻¹ and 0.08×10⁶ coccospheres m⁻² day⁻¹, respectively. The increase in coccolith flux with water depth suggests a lateral influx. The estimated average daily mass of CaCO₃ flux in coccoliths and coccospheres was 16.6 mg m⁻² day⁻¹ at the 1592 m trap and 17.9 mg m⁻² day⁻¹ at the 3902 m trap, respectively. These calculated values contributed only 23.3% to the total CaCO₃ flux at the shallow trap and 27.9% at the deep trap.     

Genetic identity and homozygosity in North-Adriatic populations of Posidonia oceanica: An ancient, post-glacial clone?

In the present paper we describe identityand homozygosity of genotypes in the seagrassPosidonia oceanica (L.) Delile(Posidoniaceae) in the Gulf of Trieste(Adriatic Sea). Forty shoots sampled off Kopercoast (Slovenia) and three samples from Grado(Italy) were analysed using microsatellitemarkers. The Koper meadow represents the onlyremarkable extension of Posidoniaoceanica recorded in the North-Adriatic regionsince 30 years. Results show the existence of asingle, homozygous genotype shared by allindividuals. Causes for the observed lack ofheterozygosity and for the presence of asingle, ancient P. oceanica clone in thisregion are discussed. Total absence of geneticvariability is cause of major concern forconservation of the species in this region.     

Comparing shoot population dynamics methods on Posidonia oceanica meadows

Prediction capacity of three main shoot population dynamics methods (age structure, net shoot recruitment per plagiotropic rhizome and shoot census) have been tested for a period of four years (2002-2006) on a Posidonia oceanica meadow. Accuracy of each method was checked by comparing measured and predicted densities at the end of the study period. Predicted densities came from the evolution of initial densities (measured in 2002) by a basic exponential model of population growth. The exponential model used the different net shoot recruitment rate estimates by each population dynamics method on three depths (upper, medium and lower limit) and three localities at each depth. Predictions performed by shoot census and net shoot recruitment per plagiotropic rhizome methods matched with measured densities at the end of the study period. Conversely, age structure method underestimated shoot densities at each depth, indicating an unreal decrease of shoot population in the meadow.     

Distribution,structure and phenology of Posidonia oceanica meadows along sicilian coasts

Abstract

Distribution, structure and phenology of Posidonia oceanica meadows along Sicilian coasts are reported. Posidonia beds are frequently found in Sicily, especially along the south-eastern, north-western and western coasts. Leaf surface per shoot is identified as the most important variable among the phenological parameters. The rhizomes annual mean primary production turned out to be among the highest values observed in the Mediterranean Sea. Flowering and fruiting of Posidonia oceanica are frequent and can occur every year.     

Are lulworthioid fungi dark septate endophytes of the dominant Mediterranean seagrass Posidonia oceanica?

• A previous study from Sicily, Italy, indicated that the dominant Mediterranean seagrass Posidonia oceanica forms a dark septate endophytic (DSE) association with a lulworthioid fungus (‘Lulwoana sp.’). This conflicts with several other studies from the NW Mediterranean Sea that point at the recently described pleosporalean fungus Posidoniomyces atricolor.
• I collected P. oceanica roots at eight sites around Sicily and checked them for fungal colonization using light microscopy. At three sites, root fungal symbionts (mycobionts) were isolated into pure cultures and identified using ITS rDNA sequences.
• Posidoniomyces atricolor represented the most frequent mycobiont (56 isolates), closely followed by lulworthioid fungi (51). The obtained mycobiont spectrum also included Cladosporium (2), Alternaria (1), Corollospora (1), Fusarium (1), Penicillium (1) and Vishniacozyma (1) isolates. The characteristic DSE root colonization, similar to those occurring in terrestrial plants but not known from any other seagrass, was found in all investigated P. oceanica individuals. The microscopy screening suggests that P. atricolor is responsible for the observed DSE colonization.
• This study extends the known range of Pos. atricolor and the DSE association characteristic for P. oceanica to the southern Tyrrhenian Sea/Sicily. While lulworthioid fungi regularly occur in P. oceanica tissues, including terminal fine roots, their significance and functioning (e.g. parasitic, pathogenic, endophytic) are unknown and require further investigation. However, there is currently no proof that they belong among dark septate endophytes of this seagrass.

     

Depth and temporal distribution of vagile fauna associated with Posidonia oceanica meadows in Cap Zebib,north‐eastern Tunisian coastline

A study on plant features and motile fauna was carried out within a Posidonia oceanica (L.) Delile bed in Cap Zebib (north‐eastern Tunisian coastline). Fauna was sampled in two stations located at 3 and 12 m depth. A total of 154 species represented by 5592 individuals were collected. Taxa with the highest number of species included Crustacea (22), Mollusca (47) and Polychaeta (46). Among Crustacea, Leptocheirus pilosus, Elasmopus rapax and Lysianassa longicornis were recorded for the first time in Tunisian P. oceanica meadows. The species richness was found to be generally higher at the shallow station. Multivariate analyses showed a pronounced temporal variation in collected populations’ structure. Moreover, the distribution of some groups, in particular molluscs and crustaceans, appeared to be mainly influenced by ‘depth’ factor showing a clear zonation pattern for some species. It was also showed with Spearman test that the faunal features of the two latter taxa (molluscs and crustaceans) are related with the shoot density and Leaf Area Index. However, no correlation was found between the plant parameters and polychaetes community characteristics. In addition, environmental factors, i.e. temperature and dissolved oxygen were found to be correlated with species richness and Shannon–Wiener diversity for each zoological group.     

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.     

Size distribution of the amphipod Paramoera walkeri (Stebbing) along a depth gradient in Antarctica

Body weight distributions of the amphipod Paramoera walkeri colonizing litter-bags at different depths (2, 5, and 10 m) in Terra Nova Bay were compared. This species dominated the vagile benthic community and was found nearly alone at the shallow sites. At the deepest site it co-occurred with two smaller benthic species (the gastropod Laevilitorina antarctica and the isopod Munna antarctica) but still accounted for most of the animal biomass (80.2%). The frequency size-class distributions of the three dominant species were sharply distinct in a condition of stable coexistence. The total biomass was inversely related to the abundance of animals and diminished with increasing depth. Both species and size-class diversities were higher in deep water which offered a more diversified and favourable habitat than shallow water. At the shallow sites only the large size classes of P. walkeri were present. Differences in the mean individual ash content between samples support the hypothesis that different size individuals of P. walkeri segregated along the depth gradient on the basis of their metabolic requirements and the related ability to exploit sub-optimal environments.     

Influence of hydrodynamic forces on population structure of Pinna nobilis L., 1758 (Mollusca: Bivalvia): The critical combination of drag force, water depth, shell size and orientation

The effects of hydrodynamics on size, shape and distribution of benthic organisms are still not completely understood. Benthic organisms that inhabit wave-swept environments usually have small sizes and ethological adaptations to reduce drag and increase resistance force. Water speeds produced by waves in intertidal habitats can be more than 10 times higher than those in subtidal environments. However, comparatively small water speeds can produce high drag forces (F_d) on large subtidal organisms.Pinna nobilis is a subtidal epibenthic large bivalve-mollusc endemic to the Mediterranean Sea, a common inhabitant of Posidonia oceanica meadows. It lives partially buried in the seabed and shows a characteristic population structure. Small individuals are usually located at shallow sites whereas large individuals are only observed in deeper levels or sheltered locations. Also, some populations show a common orientation of the shell. These features are widespread throughout the Mediterranean, but their causes are unknown.The present work is a study on the relationship between population structure of P. nobilis and the habitat hydrodynamics. The main factor considered was Drag force due to the water flow produced by waves. Drag forces (F_d) supported by two populations located at different depths in the same P. oceanica meadow were estimated according to both the size and orientation of shells. Also, F_d acting on the individuals during the greatest storm recorded in the zone in the previous 9 years, were calculated. Drag coefficients (C_d), necessaries to estimate F_d, were estimated in the towing tank of the “Ecuela Técnica Superior de Ingenieros Navales (ETSIN)” of the Polytechnic University of Madrid.The results show significant differences in F_d acting in both populations. Despite the important increment of water speed with wave shoaling, individuals of the shallow population (SP), located at 6 m depth, withstand less F_d than those of the deep population (DP), at 13 m depth. The main reasons of this F_d reduction in SP are both the small size of the individuals and their common orientation, having the dorso-ventral side of the shell towards the main water flow. This fact, together with previous data showing higher mortality, less density of individuals, and less maximum asymptotic length, evidence that selective pressures regulate these population parameters, producing a trade-off between hydrodynamics, shell size and orientation, for each shore type and water depth.Combining the data of F_d supported by each population for different wave types, approximate values of the optimal mean F_d and the maximum dislodgement force withstood by P. nobilis were estimated (< 9 Newton (N) and ≈ 45 N respectively).     

Biomass and primary production of a 8–11 m depth meadow versus <3 m depth meadows of the seagrass Cymodocea nodosa (Ucria) Ascherson

Current knowledge about the abundance, growth, and primary production of the seagrass Cymodocea nodosa (Ucria) Ascherson is biased towards shallow (depth <3 m) meadows although this species also forms extensive meadows at larger depths along the coastlines. The biomass and primary production of a C. nodosa meadow located at a depth of 8–11 m was estimated at the time of maximum annual vegetative development (summer) using reconstruction techniques, and compared with those available from shallow meadows of this species. A depth-referenced data base of values at the time of maximum annual development was compiled to that end. The vegetative development of C. nodosa at 8–11 m depth was not different from that achieved by shallow (depth <3 m) meadows of this species. Only shoot density, which decreased from 1637 to 605 shoots m⁻², and the annual rate of elongation of the horizontal rhizome, which increased from 23 to 71 cm apex⁻¹ year⁻¹, were different as depth increased from <3 to 8–11 m. Depth was a poor predictor of the vegetative development and primary production of C. nodosa. The biomass of rhizomes and roots decreased with depth (g DW m⁻² = 480 (±53, S.E.) − 32 (±15, S.E.) depth (in m); R² = 0.12, F = 4.65, d.f. = 35, P = 0.0381) which made total biomass of the meadow to show a trend of decrease with depth but the variance of biomass data explained by depth was low. The annual rate of elongation of the horizontal rhizome showed a significant positive relationship with depth (cm apex⁻¹ year⁻¹ = 18 (±5.1, S.E.) + 5.0 (±1.33, S.E.) depth (in m); R² = 0.50, F = 14.07, d.f. = 14, P = 0.0021). As shoot size and growth did not change significantly with depth, the reduction of shoot density should drive any changes of biomass and productivity of C. nodosa as depth increases. The processes by which this reduction of C. nodosa abundance with depth occur remain to be elucidated.     

Genetic variation inOrchis papilionacea (Orchidaceae) from the Central Mediterranean region: Taxonomic inferences at the intraspecific level

The genetic differentiation ofOrchis papilionacea from Italy, Corsica, Sardinia, Sicily, and Greece was analyzed by isozyme studies. No significant differences at any of the 28 loci tested were observed between samples of the putative subspeciespapilionacea from Italy,grandiflora from Sicily andheroica from Greece, or by comparing sympatricpapilionacea-like andgrandiflora-like subsamples from Sardinia and Corsica. The genetic relationships found among the populations studied reflect more their geographic origin than their taxonomic assignment;papilionacea, grandiflora, andheroica are, therefore, at least in the study area, not to be considered as subspecies, but as simple morphs, controlled by few genes (possibly two) with Mendelian inheritance. The genetic divergence observed among the samples studied is very low and consistent with that of geographic populations of a single cohesive gene pool, linked by a high gene flow (Nm = 5.9).     

Exploring the acclimation to depth of Posidonia oceanica comparing the morphological variation to the histo-anatomical characteristics of the leaves

Posidonia oceanica is a marine plant that plays an important ecological role in marine ecosystems. Understanding the eco-physiology of this plant is fundamental for planning conservation and restoration actions covering this species and its habitat. The aim of this study was to explore the acclimation of P. oceanica to depth by examining the leaf morphological variation in a depth gradient. We measured leaf length, width and thickness, as well as the distance of fifth and sixth ribs from the margin and the central rib at different depths, comparing leaves morphology and histo-anatomy. Leaf length was the only morphological character clearly related to depth, whereas the overall morphological variation seems to be due to a change in the histo-anatomical characters. Depth-related characters showed a different variation pattern from that of not depth-related ones. Our results do not fit the model used to explain the seagrass responses to light variation; most morphological leaf traits are not directly linked to the acclimation of this species to depth. Finally, we hypothesise that the variation pattern of not depth-dependent characters is an expression of P. oceanica meadow genetic structure along the depth gradient.     

Presence of Megalorhipida leucodactyla (Lepidoptera Pterophoridae) in Sicily: new report for the Italian fauna and notes on the diffusion of its host plant Boerhavia repens L. ssp. viscosa (Choisy) Maire (Nyctaginaceae)

Despite its circum-tropical distribution, we report, for the first time in Italy and Sicily, the presence of Megalorhipida leucodactyla (Fabricius 1974), a species recorded only once before in Europe. Several adults, pupae and larvae of different instars were found on Boerhavia repens L. ssp. viscosa (Choisy) Maire (Nyctaginaceae), recorded for the first time in Italy (Sicily) in 1967. We provide both an update of Boerhavia repens ssp. viscosa’s distribution in Sicily and the results of our vegetational survey, based upon the phytosociological method by Braun-Blanquet Sigmatist School, on Boerhavia repens ssp. viscosa in Palermo city’s urban areas. This plant, naturalised especially in Palermo city, appears to have become an invasive plant, increasingly spreading in larger areas, by even replacing the native flora in urban ecosystems. This study also provides the results of laboratory bioethological observations on some specimen of both adults and larvae (the latter while feeding on Boerhavia repens ssp. viscosa’s fruits). Finally, using bibliographic data, we drew a map of the distribution of both Megalorhipida leucodactyla and Boerhavia repens ssp. viscosa in the Mediterranean basin.     

Effect of habitat degradation on competition,carrying capacity,and species assemblage stability

Changes in species’ trophic niches due to habitat degradation can affect intra‐ and interspecific competition, with implications for biodiversity persistence. Difficulties of measuring species’ interactions in the field limit our comprehension of competition outcomes along disturbance gradients. Thus, information on how habitat degradation can destabilize food webs is scarce, hindering predictions regarding responses of multispecies systems to environmental changes. Seagrass ecosystems are undergoing degradation. We address effects of Posidonia oceanica coverage reduction on the trophic organization of a macroinvertebrate community in the Tyrrhenian Sea (Italy), hypothesizing increased trophic generalism, niche overlap among species and thus competition and decreased community stability due to degraded conditions. Census data, isotopic analysis, and Bayesian mixing models were used to quantify the trophic niches of three abundant invertebrate species, and intra‐ and interspecific isotopic and resource‐use similarity across locations differing in seagrass coverage. This allowed the computation of (1) competition strength, with respect to each other and remaining less abundant species and (2) habitat carrying capacity. To explore effects of the spatial scale on the interactions, we considered both individual locations and the entire study area (“‘meadow scale”). We observed that community stability and habitat carrying capacity decreased as P. oceanica coverage declined, whereas niche width, similarity of resource use and interspecific competition strength between species increased. Competition was stronger, and stability lower, at the meadow scale than at the location scale. Indirect effects of competition and the spatial compartmentalization of species interactions increased stability. Results emphasized the importance of trophic niche modifications for understanding effects of habitat loss on biodiversity persistence. Calculation of competition coefficients based on isotopic distances is a promising tool for describing competitive interactions in real communities, potentially extendible to any subset of ecological niche axes for which specimens’ positions and pairwise distances can be obtained.