Volume 52, Issue 5 - Special Issue: Mediterranean Seagrasses

No abstract available

Seagrasses form high-productivity coastal ecosystems, providing habitats for many organisms, including species of commercial value. They play a significant role as a direct food source for herbivores and also enter detrital food webs. In this review, the trophic ecology of Mediterranean seagrass ecosystems is revised, with major emphasis on Posidonia oceanica , the most extensively studied seagrass. The major topics are the role of herbivory and detritus, the contribution to littoral fish production, exportation to other ecosystems and the seagrass role as a vector of anthropogenic nutrients into food webs. Topics are discussed by analysing both conventional knowledge and new insights derived from the application of carbon and nitrogen stable isotope analysis. In conclusion, perspectives for future research are highlighted, with particular emphasis on areas demanding further study and on the effects of seagrass decline on trophic organisation in coastal environments.

Seagrass meadows are considered to be among the most important marine ecosystems, with regard to both ecology and biodiversity and for the services they provide. Seven species occur in the Mediterranean Sea: Posidonia oceanica (the most common in the open sea), Cymodocea nodosa (particularly common in the eastern basin), Ruppia cirrhosa , R. maritima, Zostera marina and Zostera noltii (mainly in estuaries and brackish lagoons), and Halophila stipulacea (introduced from the Red Sea). Seagrass regression may be due to natural processes and/or natural or anthropogenic disturbances and stress. It can also be due to long-term climate trends, e.g., the post-Last Glacial Maximum rise in sea-level, the Little Ice Age (LIA) cooling and the post-LIA warming, resulting in possible misinterpretation. Human-induced losses of P. oceanica have been mainly related to coastal development, pollution, trawling, fish farming, moorings, dredging, dumping and introduced species. All other seagrasses have also undergone more or less dramatic regression events. In fact, accurate data are generally of very local value and they are lacking for most of the Mediterranean Sea. In the absence of a reliable baseline, some widely cited cases of regression are questionable. Relatively healthy P. oceanica meadows, whose limits have changed little since the 1950s, may thrive in highly anthropized areas. In addition, the decline of one species can benefit another, so that the overall seagrass balance may remain unchanged (e.g., Cymodocea replacing Posidonia ). However, to conclude that everything is for the best would be erroneous. First, the lack of data supporting the general regression hypothesis does not invalidate the hypothesis. Indisputably dramatic seagrass losses have been documented (e.g., P. oceanica and Z. marina ). Second, the Posidonia regression is irreversible at human scales, while other seagrasses can rapidly recover, and the expansion of some seagrasses (e.g., Cymodocea ) cannot counterbalance, in terms of ecosystem services, the decline of the P. oceanica meadows. Third, human pressure (demography, tourism, etc.) on Mediterranean seagrass ecosystems is destined to strongly increase in the coming decades. Finally, the rise in sea-level due to global climate change will automatically induce a withdrawal of the lower limit of seagrass meadows whenever the limit is beyond the compensation depth. So the regression trend observed in Mediterranean seagrasses, even if it proves to be currently weaker than postulated, will significantly increase and become a major concern in the future. There is therefore an urgent need for the adoption of a set of efficient indicators and the setting up of a robust comparative baseline in order to draw up an accurate assessment of the losses and, for seagrasses other than Posidonia , possible gains at Mediterranean scale. In addition, seagrasses and seagrass habitats should be granted legal protection and, where such protection already exists, it should be implemented.

It is widely accepted that light availability sets the lower limit of seagrass bathymetric distribution, while the upper limit depends on the level of disturbance by currents and waves. The establishment of light requirements for seagrass growth has been a major focus of research in marine ecology, and different quantitative models provide predictions for seagrass lower depth limits. In contrast, the influence of energy levels on the establishment, growth, and maintenance of seagrasses has received less attention, and to date there are no quantitative models predicting the evolution of seagrasses as a function of hydrodynamics at a large scale level. Hence, it is not possible to predict either the upper depth limit of the distribution of seagrasses or the effects that different energy regimes will have on these limits. The aim of this work is to provide a comprehensible methodology for obtaining quantitative knowledge and predictive capacity for estimating the upper depth limit of seagrasses as a response to wave energy dissipated on the seafloor. The methodology has been applied using wave data from 1958 to 2001 in order to obtain the mean wave climate in deep water seaward from an open sandy beach in the Balearic Islands, western Mediterranean Sea where the seagrass Posidonia oceanica forms an extensive meadow. Mean wave conditions were propagated to the shore using a two-dimensional parabolic model over the detailed bathymetry. The resulting hydrodynamics were correlated with bottom type and the distribution of P. oceanica . Results showed a predicted near-bottom orbital velocity of between 38 and 42 cm s -1 as a determinant of the upper depth limit of P. oceanica . This work shows the importance of interdisciplinary effort in ecological modeling and, in particular, the need for hydrodynamical studies to elucidate the distribution of seagrasses in shallow depths. Moreover, the use of predictive models would permit evaluation of the effects of coastal activities (construction of ports, artificial reefs, beach nutrient-input, dredging) on benthic ecosystems.

Assessment of ecosystem natural variability is a major goal in ecology, not only to better understand fluctuations, but also as a basis for distinguishing such natural variability from changes caused by human activities. We document here spatial variability of several Cymodocea nodosa attributes relevant to ecosystem functioning and indicative of ecosystem health at three localities near its upper and lower latitudinal distribution limits. At each locality, we chose three healthy and unaltered meadows, and, at each meadow, three sampling areas. The attributes measured were shoot density, biomass allocation to leaves and roots, shoot size, carbohydrate reserves, element content (C, N, P, S) and stable isotope ratios (δ 13 C, δ 15 N and δ 34 S). Geographical differences (among localities) accounted for between 65 and 90% of the total variability, while variability within localities (both between meadows and within meadows) accounted for substantial but lower contributions in most attributes. This suggests a relatively important degree of geographic plasticity in the attributes examined. Results presented here contribute to our basic knowledge of C. nodosa and its variability over large areas, which can be especially relevant for coastal monitoring and management programs.

Information on dietary habits of mesograzers in Posidonia oceanica seagrass meadows is scarce and restricted to a few species. Here we provide data on the most likely food sources for eight gastropod species inferred from stable isotope data. We observed very similar isotopic signals for all species regardless of trophic guild category, indicating similar consumption behaviour with a main diet contribution from epiphytes. We also review the state of knowledge on gastropod-epiphyte-seagrass interactions, with particular emphasis on the scarcity of studies derived from Mediterranean systems. Laboratory experiments showed that under moderately nutrient-enriched conditions, two species of gastropods ( Bittium reticulatum and Jujubinus striatus ) were controlling epiphyte biomass at high grazer densities, with no consequences for seagrass performance. Finally, the results of a long-term in situ fertilisation experiment showed that the δ 15 N signal for seagrass, epiphytes and gastropods reflects experimentally induced eutrophication, thus cascading up nutrient effects throughout the food web.

A 6-month macrograzer exclusion field experiment was conducted in an altered Posidonia oceanica meadow affected by organic effluents from a fish farm. This experiment was carried out in order to ascertain whether or not overgrazing had contributed to the meadow degradation observed, because higher densities of macrograzers (mainly the sea urchin Paracentrotus lividus , 20 to 60 times higher density) were found in the degraded meadow than in an undisturbed reference meadow located nearby. The plant variables measured (shoot density, shoot biomass, leaf growth rate and epiphyte load) had consistently lower values in unmanipulated plots of the degraded meadow than in plots of the undisturbed reference meadow, with the exception of herbivore pressure, which was 2 to 5 times higher. In the herbivore exclusion plots established in the degraded meadow, shoot biomass and epiphyte load increased to values similar to those in the reference plots in the undisturbed meadow. At the end of the experimental period, shoot density had significantly decreased by up to 30–50% of the initial values in the control (unmanipulated) plots, while no changes were observed in the exclusion plots or in the reference meadow. Our results show that the impact of macrograzers on shoot features made a significant contribution to the reported decline of the P. oceanica meadow next to the fish farm. It is also suggested that organic effluents generated as a result of aquaculture activities may help to increase grazing activity in these seagrass communities, although the mechanisms by which nutrient loads control the behaviour of herbivores remain unclear.

The dominant Mediterranean seagrass Posidonia oceanica inhabits sublittoral environments with very stable salinity regimes; the species is considered highly sensitive to even moderate increases in salinity caused by hypersaline effluents (brine) from desalination plants. We analysed the effect of salinity increase on seagrass vitality and survival by means of an in situ mesocosm experiment. To this end, we used the brine (70–75 psu) produced by a pilot desalination plant, which was diluted with seawater to obtain brine solutions of 1 psu (high salinity increase, HS) and 2.5 psu (very high salinity increase, VS) over the mean natural salinity (37.5±0.16 psu) and then interspersed a set of experimental units in a nearby P. oceanica meadow. At the end of the experimental period (3 months), these treatments had produced differential effects on all seagrass descriptors, i.e., intense and significant in the VS experimental units and more modest (or even negligible) in the HS ones. Seagrass meadow declined through a significant decrease in shoot density in both HS (12.4±3.4%, mean±SE) and VS (18.5±3.05%, mean±SE) in comparison to control experimental units (<5%). Surviving shoots had reduced size and lower leaf growth rate than control plants, but more generally a reduction in the photosynthetic leaf surface caused by the increase in the proportion of necrotic leaf area (up to 60% in VS treatments). Non-structural carbohydrate (soluble and reserve) concentrations decreased in rhizomes, probably in relation to physiological osmoregulation processes. All these responses support the hypothesis that the threshold salinity tolerance of P. oceanica is very close to the upper limit of its natural salinity range (ca. 38 psu).

In March 2005, a preliminary Posidonia oceanica transplantation was carried out as a restoration measure prior to enlargement of a marina. Seagrass sods of approximately 1 m 2 surface area and 40 cm depth (including rhizome and sediment) were cut free from the area expected to be impacted by harbor expansion. These fragments were transferred to nearby areas lacking P. oceanica , viz. in bare areas within the nearest meadows and at the same depth as the donor site (∼10 m). The present work was carried out with the aim of evaluating the feasibility of this technique through estimating the vitality of transferred plants (measured as density and cover) for comparison with plants from a nearby meadow with similar conditions. There was a high level of mortality among transplants and a considerable degradation of the shoots that remained alive at the time of the sampling. When results were expressed in terms of global density, the rate of survival of transplanted shoots decreased to 15% (50 vs. 361 shoots m -2 of meadow in transplant and control plots, respectively). These results indicate that transplant techniques for P. oceanica on a large scale are not feasible.

No abstract available