Gérard Pergent

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 1.8lM) and phosphorous levels in the pore water (orthophosphates: 5.2–1.3 versus 1.7lM). 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. 2003 Elsevier Science B.V. All rights reserved.

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.

Primary production of Posidonia oceanica in the Mediterranean Basin

Primary production of the marine phanerogam Posidonia oceanica (Linnaeus) Delile was measured by lepidochronological analyses at 22 sites in the Mediterranean Sea (Corsica, France, Italy, Sardinia and Turkey), between 1983 and 1992, to determine spatial and temporal variations. Leaf production (blade and sheath) ranged from 310 to 1 540 mg dry wt shoot−1 yr−1, depending on site and depth. Rhizome production ranged from 24 to 120 mg dry wt shoot−1 yr−1 (6% of average leaf production). At some sites the results obtained by lepidochronological analysis were consistent with earlier results obtained by classic methods (e.g. leaf-marking). While primary production per shoot (mg dry wt shoot−1 yr−1) displayed no significant differences between sites, primary production of the P. oceanica meadow (g dry wt m−2 yr−1) decreased with increasing depth at all sites studied. This decrease correlated with reduced density of the meadow (number of shoots per m2) with increasing depth. Past primary production was also extrapolated at three sites at the island of Ischia (Italy) for a period of 5 yr in order to determine interannual variations over a period of several years. While major variations were recorded for the surface stations (5 and 10 m depth), production remained stable at the deepest station (20 m depth). Given the large geographical scale of the study (location, depth range), it would appear that while P. oceanica production remains considerable, the values recorded in the literature on the basis of classical analyses (surface stations) represent maxima, and cannot be generalised for meadows as a whole.

Lepidochronological analysis of the seagrass Posidonia oceanica (L.) Delile : a standardized approach

Abstract Leaf sheaths (scales) in the seagrass Posidonia oceanica (L.) Delile can persist for millennia within a “matte”, and they show cyclic variations in their thickness along the rhizome. Recent research has shown that these cycles are annual and may be changed by environmental parameters such as light, temperature or water movement. By analogy with dendrochronology, the study of cyclic changes in P. oceanica has been termed lepidochronology.

Fate of primary production in Posidonia oceanica meadows of the Mediterranean

Abstract Primary production of Posidonia oceanica meadows was estimated at two sites in the Mediterranean basin (Marseilles, France and Ischia, Italy), in the 10 to 20 metre depth range. At the five stations studied, foliar production (blade + sheath) ranged from 67.7 to 146.6 g C m−2 yr−1 and rhizome production from 8.2 to 18.0 g C m−2 yr−1. Based on litter input (rhythm of fall of leaves), stocks (biomass) and kinetics of litter decomposition, the rate of export of this necromass was estimated. The rate of export was strongly influenced by depth and hydrodynamic forces which ranged from 51 to 68% at Marseilles (stations exposed to strong currents) and from 37 to 49% at Ischia. Exclosures at Marseilles were used to assess the proportion of the foliar production consumed by macro-herbivores in this area. The values recorded ranged from 24.8 to 35.3 C m−2 yr−1, of which 44% is absorbed. On the basis of all these results, the fate of stocks and the flux of elements resulting from primary production were determined. For the Marseilles stations, carbon followed four major pathways: 10% was consumed by herbivores, 23% was decomposed in situ as the result of detritivore action, 35% was exported in the form of litter and 32% was stored in the matte (sheaths and rhizomes). For Ischia, the values were 3%, 34%, 27% and 36%, respectively. Extrapolation for the whole Mediterranean gave an estimated total production of P. oceanica meadows of 3.5 million tons of carbon, of which 2.4 million tons are lost to the community through export or storage in the mat.

Variations in caulerpenyne contents in Caulerpa taxifolia and Caulerpa racemosa

Caulerpenyne (CYN) contents was measured in two Chlorophyceae algae, Caulerpa taxifolia and Caulerpa racemosa, between July 1999 and July 2000. Sampling was performed at three stations exhibiting increasing levels of competition with the seagrass Posidonia oceanica. Significant differences were observed as a function of the Caulerpa species, the season, and the level of competition. CYN concentrations were always greater in C. taxifolia, regardless of either season or level of competition (35–80 times greater, according to the season). For a given species, maximum concentrations were recorded in autumn (September/November) and minimum values occurred in spring (April/May). CYN contents decreased with increasing level of competition, whereas frond length increased over this same gradient. It would appear that when the algae are in competition with P. oceanica, Caulerpa is more inclined to accelerate vegetative growth (competition for light) than to produce secondary metabolites.

Primary production and vegetative cycle in Posidonia oceanica when in competition with the green algae Caulerpa taxifolia and Caulerpa racemosa

Vegetative annual cycle and primary production were described in the sea grass Posidonia oceanica in competition with two Ulvophycea, Caulerpa taxifolia (Cap Martin, France) and Caulerpa racemosa (Antignano, Italy). Sampling was performed at three stations exhibiting increasing levels of interaction with Caulerpa. Significant differences were observed as a function of the Caulerpa species, season and level of interaction. For each interaction, the different parameters have some seasonal variations. Any effects of the two Caulerpa species has been detected on the seasonal variability of our parameters. For all seasons, with increasing levels of interaction, the P oceanica adult and intermediate leaf lengths, leaf index and mean age of the leaves always decrease whereas percentage of leaves having lost their apices, epiphyte biomass per unit surface area, mean number of leaves produced per shoot and per year always increase. Conversely, mean number of leaves per shoot or below-ground tissue production do not seem to be affected by Caulerpa. Finally, primary production of P oceanica, by an increased turnover of the foliar tissues, was greater in the case of high interaction.

Leaf renewal cycle and primary production ofPosidonia oceanica in the bay of Lacco Ameno (Ischia, Italy) using lepidochronological analysis

Abstract The cycle of leaf renewal ofPosidonia oceanica (L.) Delile has been modelled around Ischia Island (Italy), using lepidochronological data. The three parameters taken into consideration are rhythm of fall, rhythm of formation and leaf longevity. As at other Mediterranean sites, thePosidonia oceanica meadow showed a high autumnal leaf fall at Ischia which confirmed previous hypotheses. Leaf formation was very high at the end of summer and in autumn (more than 60% of new leaves appear between August and January). Leaf longevity ranged from 7 to 12 months, depending on the formation date. Primary production was also measured using lepidochronological data. Net primary production of the meadow ranged from 203 to 708 g dry weigth m−2 year−1, depending on the year and depth. This new technique for estimating primary production is much faster than the classic methods (leaf marking, oxygen, carbon-14) and has the further advantage that primary production for previous years can also be estimated.

Seagrass meadows (Posidonia oceanica) distribution and trajectories of change

Posidonia oceanica meadows are declining at alarming rates due to climate change and human activities. Although P. oceanica is considered the most important and well-studied seagrass species of the Mediterranean Sea, to date there has been a limited effort to combine all the spatial information available and provide a complete distribution of meadows across the basin. The aim of this work is to provide a fine-scale assessment of (i) the current and historical known distribution of P. oceanica, (ii) the total area of meadows and (iii) the magnitude of regressive phenomena in the last decades. The outcomes showed the current spatial distribution of P. oceanica, covering a known area of 1,224,707 ha, and highlighted the lack of relevant data in part of the basin (21,471 linear km of coastline). The estimated regression of meadows amounted to 34% in the last 50 years, showing that this generalised phenomenon had to be mainly ascribed to cumulative effects of multiple local stressors. Our results highlighted the importance of enforcing surveys to assess the status and prioritize areas where cost-effective schemes for threats reduction, capable of reversing present patterns of change and ensuring P. oceanica persistence at Mediterranean scale, could be implemented.

Growth and primary production of Cymodocea nodosa in a coastal lagoon

Cymodocea nodosa meadows within the Urbinu lagoon showed unimodal growth during an annual cycle (1998–1999), reaching maximum development in summer, minimal development in winter, and a particularly active phase in spring. The below-ground biomass (446 g dry wt. m −2 )w as higher than the above-ground biomass (332 g dry wt. m −2 ). Leaf production was 2058 g dry wt. m −2 per year (84 4gCm −2 per year), rhizome production was 412 g dry wt. m −2 per year (144 g C m −2 per year) and the production/biomass ratio (P/B) was 3.2 per year. These meadows have high total biomass (778 g dry wt. m −2 ) and vegetative production (2470 g dry wt. m −2 per year), with the largest total primary production recorded to date. These parameters are higher than those generally observed for the same seagrass species within other Mediterranean lagoonal and marine environments.