Christian Hily

Contribution of soft-bottoms to the community metabolism (primary production and calcification) of a barrier reef flat (Moorea, French Polynesia)

Abstract The relative contribution of soft bottoms to the community metabolism (primary production, respiration and net calcification) of a barrier reef flat has been investigated at Moorea (French Polynesia). Community metabolism of the sedimentary area was estimated using in situ incubations in perspex chambers, and compared with estimates of community metabolism of the whole reef flat obtained using a Lagrangian technique (Gattuso et al., 1996. Carbon flux in coral reefs. 1. Lagrangian measurement of community metabolism and resulting air–sea CO 2 disequilibrium. Mar. Ecol. Prog. Ser. 145, 109–121). Net organic carbon production ( E ), respiration ( R ) and net calcification ( G ) of sediments were measured by seven incubations performed in triplicate at different irradiance. Respiration and environmental parameters were also measured at four randomly selected additional stations. A model of Photosynthesis–irradiance allowed to calculate oxygen (O 2 ), organic carbon (CO 2 ) and calcium carbonate (CaCO 3 ) evolution from surface irradiance during a diel cycle. As chlorophyll a content of the sediment was not significantly different between stations, primary production of the sediment was considered as homogeneous for the whole lagoon. Thus, carbon production at the test station can be modelled from surface light irradiance. The modelled respiration was two times higher at the test station than the mean respiration of the barrier reef, and thus underestimated sediment contribution to excess production. Sediments cover 40–60% of the surface and accounted for 2.8–4.1% of organic carbon excess production estimated with the modelled R and 21–32% when mean R value was considered. The sedimentary CaCO 3 budget was a very minor component of the whole reef budget.

Modifications of the specific diversity and feeding guilds in an intertidal sediment colonized by an eelgrass meadow (Zostera marina) (Brittany, France)

Abstract A sandy beach located in a bay of north Brittany (France) is partly colonized by an eelgrass meadow. The overall macrobenthic fauna, i.e. epifauna, endofauna and fish, living in this seagrass and in non-vegetated sands was compared in terms of species, abundance and biomass in the different feeding guilds. Seagrass makes this habitat more complex by increasing both its surface by 5.45–fold and volume by 4.79–fold. Eighteen species among the 23 living in the non-vegetated sands were observed in the seagrass assemblage. Their abundance and biomass were increased by 7.33–fold and 3.54–fold, respectively. The occurrence of Zostera was associated with the presence of 124 new species and 34 new feeding guilds. In the Zostera meadow, a spatial segregation exists among the taxa from the different microhabitats explored for food-quest. The balanced distribution of biomass within the various microhabitats may suggest that an optimization in the exploitation of feeding resources goes hand in hand with a higher complexity of the trophic web. It looks as if the architecturally increasing complexity of the habitat resulting from the development of a seagrass meadow on sediment both stimulates the abundance and biomass of the indigenous macrobenthic assemblage, increases the specific and functional biodiversity and favours the specialization of feeding guilds.

Macrofaunal diversity and habitat structure in intertidal boulder fields

Low-midshore boulder fields in Europe are increasingly subject to degradation related to beach fishing for crabs and abalones. The aim of the study was to better understand the structure and species diversity of macrofaunal assemblages living in a low-midshore boulder field in order to define conservation strategies for this biotope. Sampling strategy involved different spatial scales (‘macro-’ and ‘microstrata’) relative to the complexity and heterogeneity of the habitat. Assemblages of species and the corresponding habitats were identified by multifactorial analysis and compared by ANOVAs. The results show a hierarchical organization of the macrofauna within the boulder field, corresponding to three spatial levels of habitat structure: (i) eight ‘microhabitats’ at the lowest level of spatial organization, each defined by a specific assemblage (subcommunity); (ii) three ‘habitats’ at a middle level combining these microhabitats, each associated with a specific community (open rock, protected rock and sediment); and (iii) three ‘macrohabitats’ at the uppermost level (corresponding to the scale of the entire boulder field and including the main geomorphological features of the beach), each defined by a specific assemblage of species (boulders on boulders, boulders on bedrock, and boulders on sediment). Two microhabitats with particularly high species diversity were regarded as the most important ecological units of the field in terms of conservation of specific and functional biodiversity. Comparison of habitat/community parameters showed (i) that habitat heterogeneity was not an accurate indicator of faunal diversity, and (ii) that habitat complexity enhanced the species richness of the fauna, but only above a threshold value. This enhancement was due to semi-sheltered microhabitats, which were found only in the most complex areas of the boulder field. It is likely that this complexity affects species richness qualitatively more than by the diversity of microhabitats. In other words, a cross-scale effect is apparent in which high habitat complexity on the middle spatial scale creates microhabitats on the lowest spatial scale that are characterized by stable semi-sheltered environmental conditions conducive to a maximum of species.

LIVING BENTHIC FORAMINIFERA FROM “LA GRANDE VASIÈRE”, FRENCH ATLANTIC CONTINENTAL SHELF: FAUNAL COMPOSITION AND MICROHABITATS

The French continental shelf of the Bay of Biscay is occupied by a large body of silty sediments called “la Grande Vasiere”. In the northern part of the Bay of Biscay, the living benthic foraminifera, of the 63–150 μm and >150 μm size fractions, inhabiting this outer shelf silt belt have been studied at four stations (100–130 m deep), together with some environmental parameters (grain size, carbon content, pore water oxygen and nitrate + nitrite content). The faunas of “La Grande Vasiere” are characterized by a high foraminiferal density in the 63–150 μm size fraction, which is often dominated by Nonionella iridea. Although the ecology of this species is not well known, its appearance seems to be related to pulsed organic supplies. In our study area, this species is dominant at three stations, but almost absent at a fourth one, where the organic carbon content is minimal. The foraminiferal microhabitat is, for most taxa, restricted to the top four centimeters: only Stainforthia fusiformis has a distinctly deeper microhabitat (mean average living depth = 4.5 cm). Although free oxygen is limited to the uppermost centimeter, a major part of the foraminifera occurs between 1 and 4 cm deep, in an interval with maximum pore water nitrate + nitrite values. The nitrate + nitrite profiles allow us to distinguish between stations A and B, both at a depth of about 100 m. In the days prior to sampling, the organic matter flux seems to have been less intensive at A and B than at stations C and D (depth of 130 m), where the nitrate + nitrite maxima are positioned just below the sediment-water interface. The distinction between these two pairs of stations is confirmed by the faunal composition, with much lower densities of supposedly opportunistic taxa (such as N. iridea ) at stations A and B, and by the faunal penetration depth, with much deeper penetration at stations C and D. Our results suggest a strong impact of seasonal outer-shelf phytoplankton bloom events on the composition and density of the benthic foraminiferal faunas.

First observations in a high rocky-shore community after the Erika oil spill (December 1999, Brittany, France)

The high mediolittoral rocky-shore community on the island of Groix (Brittany, France), was studied for the year immediately following the Erika oil spill (12/12/1999). The macrofaunal assemblages of three habitats (exposed bedrock, crevices, boulders) under three situations (non-impacted, impacted and not washed, impacted and washed) were monitored monthly. The assemblage of each habitat showed specific responses to the impacts and initiated various patterns of succession: the bed rock assemblage changed in terms of abundance but no species-richness variations occurred; in the crevices richness variations occurred both in the disappearance of some species and the immigration of opportunistic ones; the boulder assemblage lost many species and remained unstructured one year after the oil spill. These first results of this most recent oil spill in Europe, raised the question of the relative importance of habitat-species and species interactions on the community structure.

Spatio-temporal variability of Chaetozone setosa (Malmgren) populations on an organic gradient in the Bay of Brest, France

Abstract The Chaetozone populations studied live under various conditions of organic enrichment, but in only one type of sediment (sandy mud). Each population variable is related to the density and individual mean weight (IMW). The optimal ecological area is defined as corresponding to maximum density (500–1500 ind. · 0.1 m −2 ) and maximum IMW (0.7 mg), this area is in the polluted zone of the harbour of Brest (10–11% organic matter). The boundaries of the potential ecological area are defined: temporary population and low IMW (0.2 mg) corresponding to the normal zone (5–7% organic matter). Each population (recruitment, mortality) has particular dynamics different from each other and from year to year.

Spatio-temporal variability of wasting disease symptoms in eelgrass meadows of Brittany (France)

Abstract Eelgrass-leaf dynamics and ‘wasting disease’ symptoms were compared in three meadows of the Brittany shoreline. The agent responsible for these symptoms, Labyrinthula zosterae , was present in these three sites; its infestation was highly correlated with leaf-age. To distinguish the periods of increased Labyrinthula infestation from the periods of normal functioning of the plant with none or only few infestations, a procedure based on the association of a wasting index (WI) and leaf-age was developed. This procedure is capable of revealing the phases of early infestation and recession of the ‘disease’ by comparison of the normal infestation of Labyrinthula which develops only to a minor extent in the oldest leaves of the shoots. The overall seasonal dynamics of leaf production and leaf growth were masked by the local conditions of each Zostera bed. Over the period of study (1–2 years), no significant mortality due to Labyrinthula was observed. The variability of the WI appeared to depend essentially on the normal life-span of the leaves and local events, and appeared to be asynchronous. However, periods of ‘disease’ were observed; in which the WI increased from 5–10% (baseline) to 20–25% (maximum) at the end of the summer, when the leaf surface and leaf numbers were at their maximum. No die-off due to Labyrinthula was found.

Ecology of spionid polychaetes in the swash zone of exposed beaches in Tahiti (French Polynesia)

Abstract The high-energy black sand beaches around the island of Tahiti show low species richness. Most of the individuals collected in the swash zone belong to two recently described species of polychaete Scolelepis sp. A, and Scolelepis sp. B (Spionidae). Densities of the first species can reach up to 1900 individuals.m −2 and biomasses up to 47.5 g dry weight (gD.W.).m −2 . These high densities and biomasses occur because of their adaptation to this high-energy habitat, where they are restricted to the swash zone on the beach. Scolelepis sp. A appears to be a suspension feeder and feeding occurs only as the wave recedes. This suggests that the species is highly efficient at obtaining nutrients in this oligotrophic environment. Scolelepis sp. B, sampled in one station, exhibits the same patterns as Scolepis sp. A. Only three other species are present in this environment; carnivores or suspension feeders, they occur in low densities ( −2 ).

Can We use a Landscape Approach to Assess Natural and Anthropogenic Perturbations of the Rocky Shore Ecosystems

Intertidal zone is a marine area which can be reached easily at low tide periods and consequently allows observation at the landscape scale, much larger than that of the subtidal zone. Then an intertidalscape approach can be developed, particularly on the rocky shores where a macroalgal and faunal zonation based on emersion rates help the understanding of structure and evolution of the ecosystem. In this paper, we give the baseline of a new approach of intertidalscape ecology from examples on the Brittany rocky shores and particularly by developing a visual index (Intertidalscape Boulder Field Index, IBFI) using the opportunistic behavior of the green macroalgae to characterize the hand-fishing impacts on the boulder field habitat of the low eulittoral zone, on the rocky shores of Brittany, France.