The early life history of the clam <i>Macoma balthica</i> in a high CO₂ world

This study investigated the effects of experimentally manipulated seawater carbonate chemistry on several early life history processes of the Baltic tellin (Macoma balthica), a widely distributed bivalve that plays a critical role in the functioning of many coastal habitats. We demonstrate that ocean acidification significantly depresses fertilization, embryogenesis, larval development and survival during the pelagic phase. Fertilization and the formation of a D-shaped shell during embryogenesis were severely diminished: successful fertilization was reduced by 11% at a 0.6 pH unit decrease from present (pH 8.1) conditions, while hatching success was depressed by 34 and 87%, respectively at a 0.3 and 0.6 pH unit decrease. Under acidified conditions, larvae were still able to develop a shell during the post-embryonic phase, but higher larval mortality rates indicate that fewer larvae may metamorphose and settle in an acidified ocean. The cumulative impact of decreasing seawater pH on fertilization, embryogenesis and survival to the benthic stage is estimated to reduce the number of competent settlers by 38% for a 0.3 pH unit decrease, and by 89% for a 0.6 pH unit decrease from present conditions. Additionally, slower growth rates and a delayed metamorphosis at a smaller size were indicative for larvae developed under acidified conditions. This may further decline the recruit population size due to a longer subjection to perturbations, such as predation, during the pelagic phase. In general, early life history processes were most severely compromised at ,pH 7.5, which corresponds to seawater undersaturated with respect to aragonite. Since recent models predict a comparable decrease in pH in coastal waters in the near future, this study indicates that future populations of Macoma balthica are likely to decline as a consequence of ongoing ocean acidification

Influence of salinity and light climate on phytoplankton in the Schelde estuary

In this study, we tested the effects of light intensity and salinity on estuarine phytoplankton communities of the Schelde River and Estuary. We used HPLC in order to identify and quantify the different phytoplankton pigments. In two experiments (summer-spring), water was collected in the river and at two locations in the estuary (freshwater tidal and brackish). The water was mixed with an equal amount of filtered water (GF/C filters) from the same or a different site and was exposed to different light levels to test for the effects of the chemical composition of the water (including salinity) and the light climate on phytoplankton growth. In spring as well as in summer, diatoms dominated the phytoplankton community, while green algae were relatively more important in summer. A reduction in light intensity strongly reduced growth (as evaluated by changes in chl a concentration) of phytoplankton from the freshwater tidal estuary as well as the river. In spring, at high light intensity, a relative increase of green algae was observed although diatoms remain dominant (as evaluated by fuc/chl a and lut/chl a ratios). In summer, on the contrary, green algae were negatively influenced by the high light intensity. Exposure of phytoplankton from the river to water from the freshwater tidal estuary had no effect on phytoplankton biomass and community composition. Phytoplankton from the river as well as from the freshwater tidal estuary was negatively influenced by an increase in salinity. Phytoplankton from the brackish part of the estuary, on the other hand, was not significantly influenced by a decrease in salinity

Structural analysis of the meiobenthos communities of the shelf break area in two stations of the Gulf of Biscay (N.E. Atlantic)

In a first attempt to obtain information in order to characterize the shelf break zone, the meiobenthos communities of two fine sandy stations of 190 m and 325 m depth in the Gulf of Biscay off the Spanish coast were investigated in relation to some environmental characteristics (sediment composition, chlorophyll a content, redox values). The nematodes, which are the dominant taxon, were studied in detail. Their communities are relatively poor in densities (840 and 779 individuals per 10cm2 and biomass (0.137 and 0.334mg dwt/10cm2) compared to those of shallow coastal areas. The dominance of small species (mean individual biomass: 0.169 and 0.423 µg dwt) and a low total biomass can be related to the low chlorophyll a content (maximum of 1.14 µg/g). However, the number of nematode genera is much higher (46 and 62 genera per station) than on the continental shelves (often less than 40 species per 10 cm2). Sabatieria, Daptonema, Minolaimus, Richtersia and Halalaimus are the dominant genera in the two stations. These are also characteristic genera for the abyssal zone. The composition of the nematode communities is more similar to deep-sea communities than to those from subtidal zones. The low food supply is considered as an important structuring factor

The role of macrofauna in the functioning of a sea floor: is there any seasonal, density or functional identity effect?

Macrobenthos influences rates and intensities of benthic processes. The way in which these processes are affected depends on their densities and functional characteristics in terms of sediment reworking (bioturbation and bio-irrigation). This study focuses on the importance of three different functional groups (FG) of macrobenthos in the ecosystem processes of the Western Coastal Banks area (Belgian Part of the North Sea). Macrobenthic activity depends on temperature and food availability. Therefore two lab experiments were performed: one before sedimentation of the phytoplankton bloom (spring: low food availability and temperature) and one when organic matter had been settled on the sea bottom (late summer: high food availability and higher temperatures). Single - species treatments of key-species belonging to three different functional groups were added to microcosms at three density levels (average natural, lower and very low) to account for possible density declines. These species are the bivalve Abra alba (FG: biodiffuser), the tube-building polychaete Lanice conchilega (FG: piston-pumper) andthe predatory polychaete Nephthy sp. (FG: regenerator/gallery-diffuser).In both winter - and summertime, L. conchilega had a more pronounced influence on oxygen consumption and release Nephtys sp.. Abra alba appeared to be a more effective sediment reworker than Nephtys sp. in both seasons. In addition, ecosystem functioning (as oxygen consumption by the sediment community and bioturbation) seems to be related to animal densities. As such, a decline of densities (due to anthropogenic or natural disturbances) most probably will decrease the rates of ecosystem functioning in theWestern Coastal Banks area