Estructura i dinàmica del poblament algal de les fulles de "Posidonia oceanica" (L.) Delile als herbeis de Tossa de Mar (Girona)

Research on leaf-epiphyte community structure and dynamics was carried out in Posidonia oceanica beds from Tessa de Mar (Western Mediterranean). Three sampling stations where chosen at 2, 9 and 23 m depth. Seasonal community structure was studied along the year 1982 in the 9 m station. Species composition, species covering, species richness, specific distribution, homogeneity (Kulczynski's similarity index), species diversity, pattern diversity and the qualitative and quantitative minimal areas were obtained from each sample. These parameters were estimated from the species/number of shoots curves, the diversity (Shannon index) /number of shoots curves and the similarity/number of shoots curves as indicated by Ballesteros (1986). Structural changes in the epiphyte community have been observed; there is an increase in species richness and a decrease in the specific distribution and pattern-diversity from autumn to summer. Two different phases can be distinguished: a settling phase, where different shoots have a rather different species composition, that begins in October, just after the fall of senescent Posidonia leaves; and a canalization phase, where there is a progressive concurrence in the qualitative and quantitative composition of leaf-epiphytes between shoots. Comparison with other Mediterranean algal communities shows the miniaturization, the great homogeneity and the scanty structure of the Posidonia leaves epiphytic community. Dynamics were studied in the three sampling stations. Species composition of Posidonia leaves is well characterized by the abundance of Fosliella species and the encrusting phaeophycean Myrionema magnusii. Other species such as Giraudia sphacelarioides, Castagnea irregularis, C. cylindrica, Giffordia mitchelliae, Myriactula gracilis, Ectocarpus siliculosus v. confervoides and Feldmannia globifera have a spring maxima in the shallow stations. Zooepiphytes are more abundant, in relative terms, in the deepest station (basically the hydrozoan Sertularia perpusilla and bryozoans Electra posidoniae and Fenestrulina joannae). Mean phytoepiphyte biomass varies from 12 to 495 mg dw/shoot (11-470 g dw/m2) in the shallow stations and 4 to 335 mg dw/shoot (1-82 g dw/m2) in the deepest station. Zooepiphytes biomass varies from 5 to 188 mg dw/shoot (2-180 g dw/m2). A time delay between the shallowest and the deepest stations has been found for all leaf features and phytoepiphyte composition and biomass, but not for zooepiphyte biomass. A time delay in terms of production and biomass maxima between shallow and deep phytobenthos communities is something general in the Mediterranean (Ballesteros, 1984). This appears to be an adaptation of different species and communities in order to couple their life-cycle to the seasonal pattern of the limiting factor (or factors), usually light (in deep communities) and nutrient availability (in shallow ones); this could be also the case of Posidonia and its epiphytes. Production estimates of leaf-epiphytes vary from 70 g C/m2 year in shallow stations to 12 g C/m2 year in the deepest one, that is, between the 10 and the 20 % of the whole Posidonia meadow production

Estructura i dinàmica del poblament algal de les fulles de Posidonia oceanica (L.) Delile als herbeis de Tossa de Mar (Girona)

18 páginas, 7 tablas, 7 figuras.Research on leaf-epiphyte community structure and dynamics was carried out in Posidonia oceanica beds from Tossa de Mar (Western Mediterranean) . Three sampling stations w here chosen at 2, 9 and 23 m depth . Seasonal community structu re was studiea along the year 1982 in the 9 m station. Species composition, species covering , species richness , specific distribution , homogeneity (Kulczynski 's simi larity index) , species diversity, pattern diversity and the qualitative and quantitative minimal areas were obtained from each sample. These parameters were estimated from the species/ number of shoots curves, the diversity (Shannon index ) / number of shoots curves and the similarity/ number of shoots curves as indicated by BALLESTEROS (1986). Structura l changes in the epiphyte community have been observed; there is an increase in species richness and a decrease in the specific distribution and pattern-diversity from autumn to summer. Two different phases can be distinguished : a settling phase, where different shoots have a rather different species composition, that begins in October, just after the fall of senescent Posidonia leaves; and a cana li zation phase, where there is a prog ressive concurrence in the qualitative and quantitative composition of leaf-epiphytes between shoots. Comparison with other Medi t erranean algal communiti es shows t he miniaturization, the great homogeneity and the scanty structure of the Posidonia leaves ep iphytic community. Dynamics were studied in the three sampling stati ons. Species composition of Posidonia leaves is well cllaracterized by the abundance of Fosliella species and the encrusting pllaeophycean Myrionema magnusii. Other species such as Giraudia sphacelarioides, Ca stagnea irregularis, C. cylindrica, Giffordia mitchelliae, Myriactula gracilis, Ectocarpus siliculosus V . confervoides and Feldmannia globifera llave a spring maxima in tlle shallow stations. Zooepiphytes are more abundant, in relative terms , in the deepest station (basically the hydrozoan Sertularia perpusilla and bryozoans Electra posidoniae and Fenestrulina joannae) . Mean phytoepiphyte biomass vari es from 12 to 495 mg dw/ shoot (11 -470 9 dw/ m2) in tlle slla llow stations and 4 to 335 mg dw/ shoot (1 -82 9 dw/ m2) in the deepest station. Zooepiphytes biomass varies from 5 to 188 mg dw/ shoot (2-180 9 dw/ m2). A t ime delay between the sha llowest and the deepest stations has been found for all leaf features and phytoepipllyte composition and biomass, but not for zooep iphyte biomass. A time delay in terms of production and biomass maxima between shallow and deep phytobenthos communities is something general in the Mediterranean (BA~LESTEROS, 1984) . This appears to be an adaptation of different species and communities in order to couple their life-cycle to the seasonal pattern of the limiting factor (or factors) , usually light (in deep communities) and nutrient avai lability (in shallow ones); this could be also the case of Posidonia and its epiphytes. Production estimates of leaf-epiphytes vary from 70 9 C/ m2 year in shallow stations to 12 9 C/ m2 year in the deepest one, that is, between the 10 and the 20 % of the whole Posidonia meadow production .Peer reviewe

Fish and benthos communities in regenerated dock systems on Merseyside.

Restored docks have high amenity, tourism and recreational value. Polluted and unaestheticw ater is a major factor potentially inhibiting redevelopmento f docklands, particularly if the source of water suffers from severe pollution problems. The aims were to examine spatial and temporal patterns of hydrography and ecology of Merseyside Docks. Strategies to develop benthic filter-feeders on both the walls and in the sediments were considered important in maintaining good water quality. In the South Docks, algal blooms were mainly small and short-lived and anoxia was infrequent. In Princes Dock (Central chain) water quality was very good. Morpeth Dock (Wirral chain) suffered from poor water quality. Anoxia resulted in high mortalities of dock fauna and fish and the release of hydrogen sulphide gas. Algal blooms were very large and were maintained over long periods. The dinoflagellate, Prorocentrum minimum, was particularly prevalent. Zooplankton have remained at relatively low densities in the South Docks, particularly in Albert Dock (South chain), probably because of strong competition for food with benthic filter-feeders. In contrast, Morpeth Dock has maintained much higher numbers, probably because of the plentiful food supply and absence of benthos. The benthos of the walls was surveyed. Mytilus edulis is most prominent in Albert and Queens Docks; Ciona intestinalis and Ascidiella aspera are most abundant in Albert and Princes Docks; and, Molgula manhattensis dominant in Brunswick and QueensD ocks. Tiles have been used to follow the pattern of annual successiona nd the effect of timing of available space on this pattern of succession. In contrast to previous years when there had been very little recruitment of Mytilus to the South Docks, Mytilus settlement occurred in Queens Dock during autumn 1995. Experiments aimed at increasing the filter-feeding capacity by introducing Cerastodermae dule (common cockle) and manipulating particle size of the sediment concluded that cockles do not survive in the dock, probably due to occasional low oxygenc on centrations in the hypolimnion, and modification of sediment has very little effect on natural colonization. Experiments examined the impact of filter-feeders on phytoplankton numbers and regeneration of nutrients. Mytilus was shown to have higher clearancer ates( cell mV h' int) than its major competitors( Ciona intestinalis, Styela plicata) on the dock wall. The experiment investigating nutrient release by Mytilus indicates release of phosphate. Fish communities in the South Docks were relatively diverse, with a total of fifteen species of fish being caught. The size/frequency analyses suggest that the majority of fish were juveniles. The occurrence of two sea trout (Salmo trutta) indicates a return of salmonids to the estuary. The use of a capacity model as a management tool is proposed; this has been fitted to the South Docks system with some success