Distribution of live benthic foraminifera in the Ría de Vigo (NW Spain)

The spatial and vertical distributions of live benthic foraminifera from the muddy central axis of the Ría de Vigo were examined under downwelling and upwelling conditions. The spatial distribution of the main benthic foraminiferal species is controlled by salinity variations (San Simón Assemblage), food availability (Outer Ría Assemblage) and oxygen concentrations and/or reducing microenvironmental conditions in the sediment (Inner Ría Assemblage).The microhabitat distribution of benthic foraminifera is mainly dependant on the oxygenation/redox conditions in the sediment and the vertical segregation of food. In the inner areas of the ría, low oxygen and/or reducing microenvironmental conditions prevent the vertical segregation of microhabitats. In outer areas oxygen penetration is deep but microhabitat partitioning only occurs in the presence of additional food resources (mainly fresh organic carbon from phytoplankton blooms) which triggers the colonisation of the surface layer by new species and the migration upwards and reproduction of opportunistic species.     

Renewal time and the impact of harmful algal blooms on the extensive mussel raft culture of the Iberian coastal upwelling system (SW Europe)

The extension and intensity of the upwelling season in the NW Iberian Peninsula (42°N–43°N) have decreased by 30 and 45% over the last 40 years, respectively. Accordingly, the renewal time (τ) of the Rías Baixas, four large coastal inlets where 15% of the World extraction of blue mussels occurs, has increased by 240%. We indirectly demonstrate here that the growing τ has caused the increasing occurrence of harmful microalgae in these embayments, dramatically affecting mussel raft cultivation. The equation D = 365(1 − exp(−τ/c₁)) explains 80% of the variability of the number of days per year that mussels cannot be extracted from the hanging ropes because of the occurrence of harmful microalgae (D). The coefficient c₁ = 37 ± 2 days indicates that an average τ over the upwelling season of >25 ± 1 or 50 ± 3 days reduce mussel extraction to only 50 or 25% of the year, respectively.     

Downwelling and dominance of autochthonous dinoflagellates in the NW Iberian margin: The example of the Ría de Vigo

Dinoflagellate blooms in coastal upwelling systems are restricted to times and places with reduced exchange and mixing. The Rías Baixas of Galicia are four bays in the NW Iberian upwelling with these characteristics where harmful algal blooms (HABs) of dinoflagellates are common. These blooms are especially recurrent at the end of the upwelling season, when autumn downwelling amplifies accumulation and retention through the development of a convergence front in the interior of Rías. Because oceanic water enters the Rías during downwelling, it has been hypothesised that dinoflagellate blooms originate by the advection and subsequent accumulation of allochthonous populations. To examine this possibility, we studied the microplankton succession in relation to hydrographic variability in the Ría de Vigo (one of these four bays) along an annual cycle making use of a high sampling frequency. The results indicated that upwelling lasted from May to August, with downwelling prevailing in winter. Microplankton during upwelling, although dominated by diatoms, evidenced a progressive increase in the importance of dinoflagellates, which achieved maximum abundance at the end of the upwelling season. Thus, diatoms characterised the spring bloom, while diatoms and autochthonous dinoflagellates composed the autumn bloom. Diatoms dominated during the first moments of the autumn downwelling and dinoflagellates were more abundant later, after stronger downwelling removed diatoms from the water column. Since the dinoflagellates selected by downwelling belonged to the local community, it is concluded that advection of alien populations is not required to explain these autumn blooms.     

Habitat engineering by the invasive zebra mussel <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas) in a boreal coastal lagoon: impact on biodiversity

Habitat engineering role of the invasive zebra mussel Dreissena polymorpha (Pallas) was studied in the Curonian lagoon, a shallow water body in the SE Baltic. Impacts of live zebra mussel clumps and its shell deposits on benthic biodiversity were differentiated and referred to unmodified (bare) sediments. Zebra mussel bed was distinguished from other habitat types by higher benthic invertebrate biomass, abundance, and species richness. The impact of live mussels on biodiversity was more pronounced than the effect of shell deposits. The structure of macrofaunal community in the habitats with >10³ g/m² of shell deposits devoid of live mussels was similar to that found within the zebra mussel bed. There was a continuous shift in species composition and abundance along the gradient ‘bare sediments—shell deposits—zebra mussel bed’. The engineering impact of zebra mussel on the benthic community became apparent both in individual patches and landscape-level analyses.     

Seasonal variations in upwelling and in the grazing impact of copepods on phytoplankton off A Coruña (Galicia, NW Spain)

The impact of grazing by copepods on phytoplankton was studied during a seasonal cycle on the Galician shelf off A Coruña (NW Spain). Grazing was estimated by measuring the chlorophyll gut content and the evacuation rates of copepods from three mesh-size classes: 200-500 (small), 500-1000 (medium), and 1000-2000 μm (large). Between February 1996 and June 1997, monthly measurements of water temperature, chlorophyll concentration, primary production rates, and copepod abundance, chlorophyll gut content, and evacuation rates were taken at an 80-m-deep, fixed shelf station. Additionally, the same measurements were collected daily during two bloom events in March and in July 1996. Small copepods were the most abundant through the seasonal cycle. The highest grazing impact, however, was due to the medium and large size classes. Grazing by small copepods exceeded grazing by medium and large copepods only during phytoplankton spring blooms. The impact of copepod grazing (considering all size fractions) was generally low. On average, 2% of the phytoplankton biomass and 6% of the primary production were removed daily by the copepod community. Maximum grazing impact values (9% of the phytoplankton biomass and 39% of the primary production) were found in mid-summer. These results suggest that most of the phytoplankton biomass would escape direct copepod grazing in this upwelling area.     

Assemblages of peracarid crustaceans in subtidal sediments from the Ría de Aldán (Galicia,NW Spain)

Peracarid crustaceans inhabit many marine benthic habitats and are good indicators of environmental conditions. There is, however, a lack of information about diversity and distribution of peracarid crustaceans on the shallow subtidal sediments of the Galician rias. In the summer of 1997, 27 subtidal stations were sampled in the Ría de Aldán, a ria on the southern margin of the mouth of the Ría de Pontevedra (Galicia, NW Spain). A total of 16,191 peracarid individuals were collected, comprising 125 species belonging to five orders. Amphipods were dominant in number of species and individuals, followed by isopods and cumaceans. Multivariate analyses of these data indicated that depth and sediment granulometry were major determinants of distribution and composition of peracarid assemblages in the ria.     

Impact of suspended mussels (Mytilus edulis L.) on plankton communities in a Magdalen Islands lagoon (Québec, Canada): A mesocosm approach

The Grande-Entrée Lagoon (Magdalen Islands, Canada) has supported mussel (Mytilus edulis) cultivation for the last 25 years. Algal biomass in this lagoon is relatively low while heterotrophic plankton biomass is high. Although often considered herbivorous, it is known that filter-feeding bivalves can consume various types of food, from bacteria to zooplankton. We hypothesize that along with phytoplankton, heterotrophs constitute an important food resource for the Grande-Entrée mussels. In situ mesocosm experiments were undertaken at different seasons using short socks filled with mussels from the same cohort taken from an aquaculture farm, in order to determine the impact of cultured mussels on local plankton communities and assess the role of heterotrophs. Filtration activity by the mussels and associated epibionts present in the socks was expressed as clearance rates (CR). The average CR over all taxa was lowest in June and highest in October. Diatoms, dinoflagellates and heterotrophic protists constituted the bulk of planktonic carbon removed by mussels. While smaller-sized taxa contributed little (< 5%) to mussel carbon intake, large-sized heterotrophs (namely ciliates) contributed 69 to 88%. Taxon-marker pigment analyses generally confirmed these observations for groups containing phototrophic pigments. The high heterotrophic biomass retained by mussels indicates they are a major food source for mussels in this environment and should be considered both in the evaluation of mussel feeding and in assessing the influence of cultured mussels on local plankton ecosystems.     

Effects of Bucephalus sp. (Trematoda: Bucephalidae) on Perna perna mussels from a culture station in Ratones Grande Island,Brazil

This study reports the prevalence of Bucephalus sp. in Perna perna populations from a culture station of southern Brazil and its effect on the mussel reproductive tissue and immune system. The prevalence of Bucephalus sp. in P. perna (n = 1871) was considered low (3.1%) and did not seasonally vary. Histological sections of the mantle of infected mussels revealed a marked (80%) reduction of the reproductive tissue that was severe even in mussels exhibiting a moderate infection degree. The total (THC) and differential (DHC) hemocyte counts were lower in infected mussels (3.9 x 10(6) hem/ml; granular hemocytes = 33%) as compared with non-infected animals (5.5 x 10(6) hem/ml; granular hemocytes = 40%). The plasma protein concentration did not vary upon infection. Hemocyte infiltration was significantly higher only in mussels with a very heavy infection degree. The parasite sporocysts were never seen encapsulated by the host hemocytes. Our results indicate that Bucephalus sp. promotes a severe castration in its host and apparently evades the mussel immune system.     

Bioecology of the fiddler crab <Emphasis Type="Italic">Uca uruguayensis</Emphasis> and the burrowing crab <Emphasis Type="Italic">Chasmagnathus granulatus</Emphasis> (Decapoda,Brachyura) in the Refugio de Vida Silvestre Bahía Samborombón,Argentina

The aim of the present work conducted at the Refugio de Vida Silvestre Bahía Samborombón, Argentina, is to analyse the most relevant aspects of the life cycles of Chasmagnathus granulatus and Uca uruguayensis: their abundance, size distribution and sex ratio in order to be used for make management purposes. A total of 1200 individuals of U. uruguayensis (megalopae, juveniles and adults) and 957 individuals of C. granulatus (juveniles and adults) were collected from March 2001 to February 2003. U.␣uruguayensis had a maximun density of 42 ind./m² in February 2003, while the maximun density of C.␣granulatus was 52 ind./m² in June 2001. Minimun density was zero individual for both species in July 2002. Each environmental factor sampled (temperature of water, salinity and pH) was correlated with the abundance of each species by Pearson’s linear correlation analysis. Sex ratio did not significantly differ from the expected Mendelian ratio, except for U. uruguayensis in December 2002 (male-biassed) and C.␣granulatus in June and December 2001, April 2002 and February 2003 (female-biassed). Ovigerous females of U. uruguayensis and C. granulatus were found during the summer.