Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean

While the extension of aquaculture along the coasts of the Mediterranean islands is today an economic priority, no data is available on the impact of these facilities on the littoral environment in general, and in particular on the Posidonia oceanica meadows that are responsible for the richness and diversity of these biotopes. The impact of fish farming facilities in the marine littoral environment is assessed at three Mediterranean sites in Corsica and Sardinia. Various parameters are considered: (i) the amount of light available, (ii) the nature of the sediment, and (iii) the vitality of the Posidonia oceanica beds. The findings confirm the impact of the aquaculture facilities within the water column (increase in turbidity, enrichment of the sediment in organic matter and nutrients), and it would appear that the Posidonia oceanica beds also undergo significant alterations. The decline in density of the meadows in the area influenced by aquaculture farming and their total disappearance beneath the facilities is particularly disturbing, even if the areas affected are not very extensive, at least for the smaller farms (a few hundred square metres), and if certain beneficial effects may be recorded (increase in benthic primary production). Because of its ability to record environmental alterations caused by these facilities (light, nutrients, trace metals), the Posidonia oceanica meadow is a good bioindicator for use in monitoring studies.     

Spatio-temporal variation in the structure of a deep water Posidonia oceanica meadow assessed using non-destructive techniques

The Malta‐Comino Channel (Maltese islands, central Mediterranean), supports extensive meadows of the seagrass Posidonia oceanica that in some places extend to a depth of around 43 m, which is rare for this seagrass. To assess spatial and temporal variation in the state of the deeper parts of the P. oceanica meadow with time, data on the structural characteristics of the seagrass meadow at its lower bathymetric limit were collected during the summers of 2001, 2003 and 2004 from four stations (two stations within each of two sites) located at a similar depth, over a spatial extent of 500 m. Shoot density was estimated in situ, while data on plant architecture (number of leaves, mean leaf length, and epiphyte load) were successfully obtained using an underwater photographic technique that was specifically designed to avoid destructive sampling of the seagrass. Results indicated that P. oceanica shoot density was lower than that recorded from the same meadow during a study undertaken in 1995; the observed decrease was attributed to the activities of an offshore aquaculture farm that operated during the period 1995–2000 in the vicinity of the meadow. ANOVA indicated significant spatial and temporal variations in meadow structural attributes at both sites during the 3‐year study; for example, shoot density values increased overall with time at site A; a indication of potential recovery of the meadow following cessation of the aquaculture operations. Lower shoot density values recorded from site B (compared with site A) were attributed to higher epiphyte loads on the seagrass, relative to those at site A. The findings, which include new data on the structural characteristics of P. oceanica occurring at depths >40 m, are discussed with reference to the use of the non‐destructive photographic technique to monitor the state of health of deep water seagrass meadows.     

Effects of organic matter input from a fish farming facility on a Posidonia oceanica meadow

In the Mediterranean, the development of aquaculture along the coasts appears as a source of disturbance to the littoral ecosystems, and in particular to Posidonia oceanica seagrass meadows. Although the impact of fish farms in Northern Europe has been studied over the last few years, the data are more scarce in the Mediterranean. Thus, a number of physico-chemical and biological parameters have been examined here in order to evaluate the impact of a fish farm in a littoral bay of Corsica. The following values that were recorded in the vicinity of the fish farm are much higher than those at the reference station: organic content of the sediment (24–21 versus 2%), nitrogen concentrations (ammonium: 19.5–8.4 versus 1.8 μM) and phosphorous levels in the pore water (orthophosphates: 5.2–1.3 versus 1.7 μM). The seagrass meadow vitality also seems to be affected in the vicinity of cages, with densities that drop from 466 (reference station) to 108 shoots m⁻² (20 m from cages). Total primary production also varies from 1070.6 to 87.9 g m⁻² year⁻¹. The main impact factors seem to be the input of organic matter originating from the cages and the high epiphyte biomass caused by the nutrient enrichment. The high level of organic matter and the presence of mud seem to alter the physico-chemical characteristics of the bottom sediment; moreover, the plant/epiphyte competition seems to lead to a leaf fragility and, more importantly, to a decrease in available light.     

The influence of coastal dynamics on the upper limit of the Posidonia oceanica meadow

The recognized ecological importance of Posidonia oceanica, the most important seagrass of the Mediterranean Sea, makes it crucial to assess the state of health of its meadows, discriminating natural from anthropogenic impacts. In this paper, the hydrodynamic conditions at the upper limit of P. oceanica meadows along the Ligurian coast (NW Mediterranean Sea) were investigated. A relationship between the distance of the upper limit of the meadow from the shoreline and the morphodynamic domain of the beach (i.e. distinctive types of beach produced by the topography, wave climate and sediment composition) was found. A zonation of the state of the shallow portions of the meadows down the submerged beach profile was identified. Zone a, from the shoreline to the breaking limit, is naturally critical for the development of the meadow. Zone b, from the breaking limit to the closure depth, is subjected to natural and human impacts. Zone c, below the closure depth, is little influenced by coastal dynamics. This study quantifies for the first time how much the status of the shallow portions of P. oceanica meadows is dependent on coastal dynamics, which is important for their proper management.     

Influence of wave climate on architecture and landscape characteristics of Posidonia oceanica meadows

Seagrass meadow characteristics, including distribution, shape, size and within‐meadow architectural features, may be influenced by various physical factors, including hydrodynamic forces. However, such influences have hardly been assessed for meadows of the ecologically important and endemic Mediterranean seagrass Posidonia oceanica. The distribution of P. oceanica meadows at five sites in the Maltese Islands was mapped to a depth of c. 15 m using a combination of aerial photography and SCUBA diving surveys. Estimates of wind‐generated wave energy and energy attenuated by depth were computed using the hydrodynamic model WEMo (Wave Exposure Model). Metrics for P. oceanica landscape features were calculated using FRAGSTATS for replicate 2500 m² subsamples taken from the seagrass habitat maps in order to explore the influence of wave dynamics at the landscape scale. Data on within‐meadow architectural attributes were collected from five sites and analysed for relationships with wave energy. The results indicate that landscape and architectural features of P. oceanica meadows located within the 6–11 m depth range are significantly influenced by wave climate. Posidonia oceanica meadows tend to be patchier and have low overall cover, more complex patch shapes and reduced within‐patch architectural complexity along a wave exposure gradient from low to high energy. The findings from the present study provide new insight into the influence of hydrodynamic factors on the natural dynamism of P. oceanica meadow landscape and architecture, which has implications for the conservation and management of the habitat.     

Comparison between epiphyte assemblages of leaves and rhizomes of the seagrass Posidonia oceanica subjected to different levels of anthropogenic eutrophication

This paper aimed to compare epiphyte assemblages of leaves and rhizomes of Posidonia oceanica exposed to different levels of concentration of nutrients. The same design including a potentially impacted meadow and two reference meadows was used in each of two locations, characterized by the presence of a city or of suspended cages of a fish farm, respectively. This allowed to test for the consistency of responses of epiphytic assemblages to different sources of eutrophication. In both studies, results documented differences in patterns of composition and abundance of epiphytic assemblages on leaves between disturbed and reference meadows, while assemblages on rhizomes did not appear sensitive to this kind of disturbance. Moreover, in potentially impacted meadows, both assemblages showed different patterns of spatial variability compared to reference assemblages. Species composition and abundance of epiphyte assemblages seemed suitable for detecting moderate nutrient increases, even if adequate sampling designs are needed to separate patterns related to the large natural spatial variability of these systems from those related to changes in environmental conditions.     

Effects of fish farming on seagrass (Posidonia oceanica) in a Mediterranean bay: seagrass decline after organic loading cessation

The effects of fish farming on a seagrass (Posidonia oceanica) meadow at Fornells Bay, Minorca (Balearic Islands) were studied. Changes in plant and meadow features (e.g. shoot morphology, shoot density, biomass, rhizome growth, nutrient and soluble sugars concentrations…) in three stations along a transect from a disturbed (organic pollution due to fish cultures) to an undisturbed site were assessed. The fish culture had ceased in 1991; however, seagrass decline, already reported in a previous study for the period 1988–1990, was still taking place at the time of sampling (July 1994). Differences between stations were very clear; the station closest to the fish cages showed reduced shoot density, shoot size, underground biomass, sucrose concentration and photosynthetic capacities. In contrast, shoots from the most polluted station showed higher P-concentration in tissues and higher epiphyte biomass than the other two. Since water conditions had recovered completely by the time of the sampling mission, it is proposed that the persistence of the seagrass decline was due to the excess organic matter remaining in the sediment.     

Impact of the brine from a desalination plant on a shallow seagrass (Posidonia oceanica) meadow

Although seawater desalination has increased significantly over recent decades, little attention has been paid to the impact of the main by-product (hypersaline water: brine) on ecosystems. In the Mediterranean, potentially the most affected ecosystems are meadows of the endemic seagrass Posidonia oceanica. We studied the effect of brine on a shallow P. oceanica meadow exposed to reverse osmosis brine discharge for more than 6 years. P. oceanica proved to be very sensitive to both eutrophication and high salinities derived from the brine discharge. Affected plants showed high epiphyte load and nitrogen content in the leaves, high frequencies of necrosis marks, low total non-structural carbohydrates and low glutamine synthetase activity, compared to control plants. However, there was no indication of extensive decline of the affected meadow. This is probably due to its very shallow situation, which results in high incident radiation as well as fast dilution and dispersion of the brine plume.     

Long-term effect of beach replenishment on natural recovery of shallow Posidonia oceanica meadows

The recovery capacity of shallow Posidonia oceanica meadows degraded by beach replenishment eighteen years before was assessed in two impacted meadows and compared with other two undisturbed localities. Inside each locality, we selected randomly three sites separated by 500–1000 m. At site level we study the vitality of P. oceanica meadow assessing the vegetative growth, leaf characteristics, and non-structural carbohydrates of the plants. Additionally, at locality level, silt-clay fraction, organic matter, pH and light intensity incident on the sea bottom were measured to evaluate the environmental conditions. Covering of P. oceanica was significantly lower at the impacted localities while amount of dead “matte” was higher. Leaf production of horizontal rhizomes (14.6 ± 1.11 vs 19.47 ± 1.45 leaves y⁻¹), net total rhizomes recruitment (2.33 ± 0.17 vs 4.3 ± 0.33 branches y⁻¹) and starch concentration (43.625 ± 0.67 vs 54.45 ± 0.74 mg per g of rhizome) at impacted meadows were significantly lower than controls. Leaf features, epiphytes biomass, colonization, elongation and horizontal and vertical rhizome production did not show significant differences. Sediments at impacted localities contained higher silt-clay fraction and higher organic matter load while pH was lower. Light intensity on the sea bottom measured at all localities was over the minimum light requirements estimated for P. oceanica. Our results show that the press impact produced by beach replenishment was enduring in the time slowing natural recovery by 45%. This impact may be related with changes in the sediment features.     

Human influence on seagrass habitat fragmentation in NW Mediterranean Sea

Habitat fragmentation in meadows of Posidonia oceanica, the most important and abundant seagrass in the Mediterranean Sea, was investigated at a region-wide spatial scale using a synthetic ecological index, the Patchiness Index (PI). We tested the hypothesis that human impacts are the major factor responsible for habitat fragmentation in P. oceanica meadows contrasting fragmentation of meadows located in “anthropized” areas with that of meadows located in areas with low anthropization and considered as virtually “natural”. We also related fragmentation of meadow with the morphodynamic state of the submerged beach (i.e. distinctive types of beach produced by the topography, the wave climate and the sediment composition) in order to investigate the influence of one natural component on the seagrass meadow seascape. Results demonstrated that fragmentation in the P. oceanica meadows is strongly influenced by the human component, being lower in natural meadows than in anthropized ones, and that it is little influenced by the morphodynamic state of the coast. The use of landscape approaches to discriminate natural disturbance from human impacts that affect seagrass meadows is thus recommended for the proper management of coastal zones.     

The relationship between seagrass (Posidonia oceanica) decline and sulfide porewater concentration in carbonate sediments

In this study we test the hypothesized negative relationship between seagrass status and porewater hydrogen sulfide (H₂S) levels, through a comparative analysis within a range of seven Posidonia oceanica meadows growing over carbonate sediments in the NW Mediterranean Sea around Mallorca Island. The studied meadows range from meadows growing on sediments with very low sulfide porewater concentrations (4.6 μM) to those growing over higher sulfide conditions (33.5 μM). Organic matter content, sulfate reduction rates and sulfide porewater concentrations in the sediments were determined concurrently with the assessment of demographic plant dynamics (specific mortality and net population growth rates). Sulfide porewater concentration increased with increasing organic matter content in the sediment, while net population growth decreased significantly with low increases of sulfide concentrations. Our results confirm the previously suspected vulnerability of seagrass meadows growing on carbonate sediments to increased sulfide levels. An excess of 10 μmols H₂S L⁻¹ porewater is identified to already conduce P. oceanica meadows to decline, which this study identifies, particularly, as strongly sensitive to sulfides. The results reported here suggest that even moderate increases in organic carbon inputs may lead to enhancement of dissolved sulfides and may be an important factor for seagrass status in these iron-depleted carbonate sediments from the Mediterranean Sea.     

Relationship between sediment distribution and Posidonia oceanica seagrass

Relationships between sedimentary features and the distribution and structure of Posidonia oceanica meadows were investigated in the Gulf of Oristano (Sardinia) in a study site stretching from the mouth of the Tirso River in a more sheltered, inner sector of the gulf to an outer, more exposed sector near the rocky Cape San Marco. The results demonstrate that gradients in sediment texture and composition can be related to meadow distribution and areal coverage. We suggest that this involves the mechanical trapping of fine particles by the plants, enhanced mud deposition due to dampening of wave action in the meadows, and the admixture of bioclastic sediments linked to increased production by epiphytes and invertebrates. Independent evidence of seagrass erosion was recorded in an unvegetated breach in the meadow. The mass of fine sediment which would be resuspended in the wake of meadow destruction was estimated at 30–90 × 10³ tonnes km⁻², an amount which could substantially influence water quality and ecosystem stability in the gulf. Received: 27 May 1999 / Revision accepted: 2 February 2000     

Evolution of Posidonia oceanica seagrass meadows and its implications for management

Results of the monitoring network of the Posidonia oceanica meadows in the Valencia region in Spain are analysed. For spatial comparison the whole data set has been analysed, however, for temporal trends we only selected stations that have been monitored at least 6 years in the period of 2002–2011 (26 stations in 13 localities).At the south of the studied area, meadows are larger, and they have higher density and covering than that in the Valencia Gulf, excluding Oropesa meadow. Monitoring of P. oceanica meadows in the Valencia region in Spain indicates that most of them are stationary or they are increasing their density and covering while no decline was observed in the studied meadows. These results indicate that there is not a general decline of P. oceanica meadows and that the decline of P. oceanica, when it has been observed in other studies, is produced by local causes that may be managed at the local level. This study also reflects the importance of long series of direct data to analyse trends in the population dynamics for slow-growing species.     

Spatial pattern of coastal fish assemblages in different habitats in North‐western Mediterranean

Variability of fish assemblages across habitat structures can depend on spatial scales. A hierarchical sampling design was used to assess the spatial variability of temperate fish assemblages in different habitats and at multiple scales. Underwater visual censuses were carried out along the coasts of Elba Island (NW Mediterranean) on Posidonia oceanica beds, rocky algal reefs and sandy habitat at three spatial scales, namely tens of metres (individual replicates), hundreds of metres (sites) and tens of kilometres (locations). At the assemblage level, there was a clear relationship between fish and habitat type and the observed habitat‐related differences were largely dependent on species identity. Fish assemblages on P. oceanica beds and rocky reefs shared a high number of species, whereas overlap with sandy assemblages was negligible. Multivariate analyses revealed significant differences in fish assemblages among habitats, although there was also a significant habitat × site interaction. These differences relied mainly upon assemblage composition and species richness. Assemblages on rocky reefs and P. oceanica meadows usually harboured a higher number of species and individuals compared with sandy assemblages. Nevertheless, the patterns of habitat‐related differences in species richness and, especially, in the total number of fish, changed significantly from site to site. Eight species showed significant differences over habitats, but they were not consistent due to the interaction of habitat with site. Predictability of fish at both assemblage and population levels decreased with the scale of observation, and the spatial pattern of fish observed at the smallest scale was likely dependent on factors other than habitat type.     

Harpacticoid copepod response to epiphyte load variations in Posidonia oceanica (L.) Delile meadows

We conducted a field experiment to assess the response of phytal harpacticoids to nutrient‐driven increases of epiphyte load in Posidonia oceanica meadows. First, we evaluated differences in species richness, diversity and assemblage structure of phytal harpacticoids in P. oceanica meadows with differing epiphyte loads. Secondly, we conducted a field experiment where epiphyte load was increased through an in situ addition of nutrients to the water column and evaluated the responses of the harpacticoid assemblages. We predicted that there would be changes in the harpacticoid assemblages as a result of nutrient‐driven increases of epiphyte load, and that these changes would be of a larger magnitude in meadows of low epiphyte load. Our results show that the harpacticoid fauna (>500 μm) present in P. oceanica meadows in the Bay of Palma comprised taxa which are considered phytal and other less abundant ones previously described as sediment dwellers or commensal on other invertebrate species. Nutrient addition had an overall significant effect on epiphyte biomass and on harpacticoid abundance, diversity and assemblage structure, possibly as a response to the increased resources and habitat complexity provided by epiphytes. The abundance of dominant species at each location was favoured by nutrient addition and in some cases correlated with epiphytic biomass, although never strongly. This may indicate that structural complexity or diversity of the epiphytic cover might be more important than the actual epiphytic biomass for the harpacticoid species investigated. More species‐specific studies are necessary to ascertain this and clarify the relationships between harpacticoids and epiphytes in seagrass meadows. To our knowledge, this is the first account of harpacticoid species associated with P. oceanica leaves and the epiphytic community they harbour in the Mediterranean Sea.     

Experimental assessment of Posidonia oceanica‐associated gastropods grazing on an early‐successional biofilm community: nutrient availability and species‐specific effects

Although grazing is considered an essential process controlling epiphyte biomass on seagrass leaves, there is still a lack of fundamental knowledge about the species‐specific consumption rates of the most common grazers in Mediterranean meadows. This study experimentally assessed the effect of Posidonia oceanica‐associated gastropod grazing on early successional biofilm and the species‐specific relationship between biofilm consumption rates and biofilm biomass. Two biofilms on artificial substrata, both developed in situ (in a P. oceanica meadow), one under ambient conditions and the other under nutrient‐enriched conditions, were offered in aquaria assays to nine species of grazers found in P. oceanica meadows. Biofilm consumption rates and their association with biofilm biomass were assessed. It was found that: (i) there was a positive association between biofilm consumption and biofilm biomass up to 20 mg Chl a·m^?2 for Bittium reticulatum, Gibbula ardens, Jujubinus exasperatus and Tricolia pullus; (ii) Alvania montagui, B. reticulatum and Jujubinus striatus showed the highest consumption rates and are thus expected to be amongst the leading consumers in early‐successional epiphytic communities; (iii) there was not an increase of consumption rate when a substratum colonized under nutrient‐enriched conditions was offered to any of the nine studied species. This study provides species‐specific consumption rates knowledge that is useful for the assessment of the strength of grazer–epiphyte interactions and trophic fluxes in P. oceanica meadows.     

Comparison of the fish assemblages associated with Posidonia oceanica after the partial loss and consequent fragmentation of the meadow

An extensive Posidonia oceanica meadow was partially destroyed by excavation, resulting in areas of seagrass habitat of equal complexity (shoot density) but different heterogeneity (degree of fragmentation). The fish assemblages associated with differently fragmented beds were compared from a landscape perspective. Differences in the fish assemblages were detected, with several species showing different patterns: (1) species that increased their abundance along with the degree of bed fragmentation, (2) species that were more abundant in fragmented beds, but did not show differences between more or less fragmented beds, and (3) species that were mostly abundant in large seagrass patches or in the continuous meadow. The fish assemblages were also affected by depth, but further research is needed to determine properly this effect. Some ecological mechanisms are suggested to operate in the interaction between P. oceanica and the held fish assemblage.     

Effect of different substrata on rhizome growth,leaf biometry and shoot density of Posidonia oceanica

The effects of different substratum typologies on Posidonia oceanica growth and morphology were estimated in four Sicilian meadows using Generalized and Linear Mixed Models combined with retrodating and biometric analyses. Substratum exerted a multiple effect, resulting in different biometric features for P. oceanica shoots settled on rock from those growing on sand and matte. On rock, values for growth rate, leaf length and shoot surface were lower than those on other substrata, with 42%, 23% and 32% the highest degree of difference respectively. The present study may have interesting methodological consequences for the comprehensive understanding of the causative variables potentially affecting meadows features and their health status. The importance of substratum in the prediction of likely biometry changes in P. oceanica meadows, means that knowledge of substratum type should receive due attention in the future to derive reliable estimates of meadow status.     

Wave damping over artificial Posidonia oceanica meadow: A large-scale experimental study

An experimental study, conducted in the large wave flume of CIEM in Barcelona, is presented to evaluate the effects of Posidonia oceanica meadows on the wave height damping and on the wave induced velocities. The experiments were performed for irregular waves from intermediate to shallow waters with the dispersion parameter h/λ ranging from 0.09 to 0.29. Various configurations of the artificial P. oceanica meadow were tested for two stem density patterns (360 and 180 stems/m²) and for plant's height ranging from 1/3 to 1/2 of the water depth.The results for wave height attenuation are in good agreement with the analytical expressions found in literature, based on the assumption that the energy loss over the vegetated field is due to the drag forces. Based on this hypothesis, an empirical relationship for the drag coefficient related to the Reynolds number, Re, is proposed. The Reynolds number, calculated using the artificial P. oceanica leaf width as the length scale and the maximum orbital velocity over the meadow edge as the characteristic velocity scale, ranges from 1000 to 3500 and the drag coefficient C_d ranges from 0.75 to 2.0.The calculated wave heights, using the analytical expression from literature and the proposed relationship for the estimation of C_d, are in satisfactory agreement with those measured. Wave orbital velocities are shown to be significantly attenuated inside the meadow and just above the flume bed as indicated by the calculation of an attenuation parameter. Near the meadow edge, energy transfer is found in spectral wave velocities from the longer to the shorter wave period components. From the analysis it is shown that the submerged vegetation attenuates mostly longer waves.     

Modelling formation of complex topography by the seagrass Posidonia oceanica

Posidonia oceanica is a slow growing seagrass species that extends via growing rhizomes that grow only centimetres both horizontally and vertically each year. Posidonia oceanica forms topographically complex biogenic reefs of dead rhizome and sediments that are up to 4 m in height that are called “matte”. This study investigates the role of slow horizontal and vertical growth of rhizomes in the formation of topographic complexity in P. oceanica matte using agent-based modelling. The simulated infilling of landscapes by P. oceanica was run over 600 iterations (years) for 10 random starts of 150 agents each. Initial infilling rates were very slow and P. oceanica had limited cover after a century of growth. Growth accelerated after 100 years but plateaued after 400 years such that after 600 years only two-thirds of the landscape was occupied by P. oceanica. The pattern of spread of agents was initially random in direction but after larger patches were formed spread was radial from these patches. The seagrass landscape was initially highly fragmented with many small separate patches made up of a few agents each, with a Landscape Division index close to 1. Between 300 and 600 years Landscape Division declined sharply to 0.42, indicating patches had coalesced into larger more continuous meadows forming a less fragmented landscape. Perimeter to area ratio of seagrass patches declined exponentially from >1 to approximately 0.2 over 600 years of simulation. The matte developed from growth of patches and its greatest height occurred in more continuously occupied cells of the grid. The topography of the reef that occupied two-thirds of the landscape after six centuries of growth could be described as a pattern of channels between reef plateaus elevated 1–2 m above channels. These results demonstrate that development in P. oceanica meadows of three-dimensional structure, in the formation of biogenic reefs, can be explained by, and is an emergent property of, slow horizontal and vertical rhizome growth rates combined with the time it takes for the accumulation of rhizomes in any region of the landscape. As such, the model provides a parsimonious explanation for the development of complex matte topography.