Aline Migné

BENTHIC METABOLISM ON A SHELTERED ROCKY SHORE: ROLE OF THE CANOPY IN THE CARBON BUDGET1

While the importance of canopy‐forming algae in structuring ecosystems is recognized, their role in the carbon budget is still not well understood. To our knowledge, no measurements of rocky shores primary production and respiration under emersion periods have been carried out in situ. A benthic chamber coupled to a CO2‐infrared gas analyzer was used to measure gross primary production and respiration on the Ascophyllum nodosum (L.) Le Jol. zone of a sheltered rocky shore in Brittany, France. Over a year of monthly measurements on the zone with and without the A. nodosum canopy showed fairly high production and respiration values for the global community as well as carbon fluxes due to the canopy that largely dominated the benthic metabolism of the zone. The strong canopy respiration relative to the primary production also suggested a high metabolic activity by microscopic heterotrophs on the surface of the alga. Both the canopy and the understory annual primary production and respiration were under the control of light and temperature seasonal variations. Finally, the range of the amount of carbon produced on the A. nodosum zone during diurnal emersions was estimated. Additional measures accounting for the day–night cycles and seasonal light variations over an entire tidal cycle are, however, necessary to establish an annual carbon budget. Such measures using the benthic chamber together with complementary techniques would allow a better understanding of the functioning of sheltered rocky shores.

Large-scale variation in combined impacts of canopy loss and disturbance on community structure and ecosystem functioning

Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.

Impact of the Phaeocystis globosa spring bloom on the intertidal benthic compartment in the eastern English Channel: a synthesis.

From 1999 to 2005, studies carried out in the frame of regional and national French programs aimed to determine whether the Phaeocystis globosa bloom affected the intertidal benthic communities of the French coast of the eastern English Channel in terms of composition and/or functioning. Study sites were chosen to cover most of the typical shore types encountered on this coast (a rocky shore, an exposed sandy beach and a small estuary). Both the presence of active Phaeocystis cells and their degradation product (foam) did have a significant impact on the studied shores. The primary production and growth rates of the kelp Saccharina latissima decreased during the bloom because of a shortage of light and nutrient for the macroalgae. On sandy sediments, the benthic metabolism (community respiration and community primary production), as well as the nitrification rate, were enhanced during foam deposits, in relation with the presence of bacteria and active pelagic cells within the decaying colonies. In estuarine sediments, the most impressive impact was the formation of a crust at the sediment surface due to drying foam. This led to anoxic conditions in the surface sediment and resulted in a high mortality among the benthic community. Some organisms also tended to migrate upward and were then directly accessible to the higher trophic level represented by birds. Phaeocystis then created a shortcut in the estuarine trophic network. Most of these modifications lasted shortly and all the systems considered came back to their regular properties and activities a few weeks after the end of the bloom, except for the most impacted estuarine area.

Stressed but stable : Canopy loss decreased species synchrony and metabolic variability in an intertidal hard-bottom community

The temporal stability of aggregate community properties depends on the dynamics of the component species. Since species growth can compensate for the decline of other species, synchronous species dynamics can maintain stability (i.e. invariability) in aggregate properties such as community abundance and metabolism. In field experiments we tested the separate and interactive effects of two stressors associated with storminess–loss of a canopy-forming species and mechanical disturbances–on species synchrony and community respiration of intertidal hard-bottom communities on Helgoland Island, NE Atlantic. Treatments consisted of regular removal of the canopy-forming seaweed Fucus serratus and a mechanical disturbance applied once at the onset of the experiment in March 2006. The level of synchrony in species abundances was assessed from estimates of species percentage cover every three months until September 2007. Experiments at two sites consistently showed that canopy loss significantly reduced species synchrony. Mechanical disturbance had neither separate nor interactive effects on species synchrony. Accordingly, in situ measurements of CO2-fluxes showed that canopy loss, but not mechanical disturbances, significantly reduced net primary productivity and temporal variation in community respiration during emersion periods. Our results support the idea that compensatory dynamics may stabilise aggregate properties. They further suggest that the ecological consequences of the loss of a single structurally important species may be stronger than those derived from smaller-scale mechanical disturbances in natural ecosystems.

Long-term changes (1979-1994) in two coastal benthic communities (English Channel): analysis of structural developments

Abstract The long-term variations of two coastal benthic communities (Eastern English Channel, France) were studied between 1979 and 1994 by means of factor correspondence analysis (FCA), trophic structure and rank-frequency diagrams (RFD). FCA identified periods of relative homogeneity and periods of strong variation in the structure of the two communities. The first community, established on heterogeneous sand, showed a multi-annual cycle linked to grain-size variations in the sediment. As passive supplies of organic matter were not important and suspension feeders could hardly live in these conditions, the community remained poor and stable and was dominated by deposit-feeders and carnivores. The other one, a rich and diverse mussel ( Mytilus edulis ) bed, showed great variations in the abundance of the main species which generated the enrichment of the community by its suspension-feeding behaviour (biodeposition). Beyond these variations, the community remained stable until 1990 as no continuous trend or permanent change could be seen. Since 1990, no recruitment of M. edulis occurred and the relative dominance of suspension feeders, as well as the number of species, strongly decreased. During the following years, several species of tunicates (sessile suspension feeders with gregarious recruitment) recruited and partly took the ecological niche of M. edulis but they did not induce an increase of biodeposition and the previous richness was not restored. After a strong and rapid shift in the structure, a new equilibrium has been established within the community.

Photosynthetic activity of intertidal microphytobenthic communities during emersion: in situ measurements of chlorophyll fluorescence (PAM) and CO2 flux (IRGA)1

Photosynthetic microphytobenthic activity has increasingly been examined using pulse‐amplitude‐modulated (PAM) fluorescence techniques. Nevertheless, estimating carbon production rates from fluorescence measurements implies the establishment of reliable relationships. The aim of this study was to determine such a relationship from field measurements of both PAM fluorescence and CO2 fluxes. Three study sites of varying sedimentary features were investigated in different seasons. Both linear and with plateau relationships were obtained between the fluorescence parameter (relative electron transport rate [rETR]) and the community‐level carbon‐fixation rate (gross community primary production rate [GCP] in mg C · m−2 · h−1). The correlation calculated from the whole data set (i.e., all sites and all seasons) was very strong (n = 106; r = 0.928). Significant correlations were also obtained for light‐curve parameters assessed with the two methods: Pm (n = 8; r = 0.920) and Ik (n = 8; r = 0.818). Total community‐level carbon fixation for the emersion period was calculated from fluorescence measurements according to the relationship established between GCP and rETR, and between light‐curve parameters, and the results were compared to the estimation obtained directly from GCP measurements. The agreement between the two estimations was quite good for both ways of calculation (with a mean discrepancy of 30% for the first one and −2% for the second one). These results suggest the potential application of PAM measurements to calculate carbon‐fixation rates at large spatial and temporal scales, provided that a set of experiments coupled with CO2‐flux measurements are performed.

Multi-scale heterogeneity in a macrobenthic epifauna community

A quantitative study, based on monthly samples taken by divers with 0.1 m2 quadrats, was conducted on a benthic community settling coarse sediments in an area exposed to strong water currents.

Macrobenthic metabolism as carbon and nitrogen fluxes in a coastal area exposed to strong tidal currents (Dover Strait, eastern English Channel)

In view of recent studies of a Dover Strait macrobenthic community (eastern English Channel), an annual budget of carbon and nitrogen fluxes is proposed and discussed in order to understand the contribution of macrobenthic metabolism in this productive coastal area (phytoplanktonic production= 336 g C m−2 y−1). In this area typical of coarse sediments which are exposed to strong tidal currents in the English Channel, the macrobenthic compartment is suspected to be responsible for the major part of exchanges at the bottom-boundary layer. The macrobenthic biomass is high (mean annual biomass = 281 g afdw m−2) and dominated by three species (98.5%): the ophiurid Ophiothrix fragilis and the two cnidarians Urticina felina and Alcyonium digitatum. Carbon and nitrogen budgets including ingestion, production, respiration, excretion and egestion allow the estimation of annual fluxes from the water column to the benthic compartment (Ingestion = 820 g C m−2 y−1 and 130 g N m−2 y−1) and from the benthic compartment to the water column (Respiration + Egestion = 710 g C m−2 y−1 and Excretion + Egestion =100 g N m−2 y−1). The net metabolism, as the difference between primary production and consumption of organic matter, shows that this coastal ecosystem is heterotrophic. Its functioning is based on allochtonous material and advection and it appears to be an important source of carbon dioxide.

Sampling epifauna, a necessity for a better assessment of benthic ecosystem functioning: an example of the epibenthic aggregated species Ophiothrix fragilis from the Bay of Seine.

Sampling the sea bottom surface remains difficult because of the surface hydraulic shock due to water flowing through the gear (i.e., the bow wave effect) and the loss of epifauna organisms due to the gears closing mechanism. Slow-moving mobile epifauna, such as the ophiuroid Ophiothrix fragilis, form high-density patches in the English Channel, not only on pebbles like in the Dover Strait or offshore Brittany but also on gravel in the Bay of Seine (>5000 ind m(-2)). Such populations form high biomasses and control the water transfer from the water column to the sediment. Estimating their real density and biomass is essential for the assessment of benthic ecosystem functioning using trophic web modelling. In this paper, we present and discuss the patch patterns and sampling efficiency of the different methods for collecting in the dense beds of O. fragilis in the Bay of Seine. The large Hamon grab (0.25 m(-2)) highly under-estimated the ophiuroid density, while the Smith McIntyre appeared adequate among the tested sampling grabs. Nowadays, diving sampling, underwater photography and videos with remote operated vehicle appear to be the recommended alternatives to estimate the real density of such dense slow-moving mobile epifauna.

Ammonium excretion in two benthic cnidarians: Alcyonium digitatum (Linnaeus, 1758) and Urticina felina (Linnaeus, 1767)

Abstract As part of an evaluation of nitrogen fluxes between the water column and a macrobenthic community of the Strait of Dover (eastern English Channel), ammonium excretion was measured throughout the year in two common cnidarians (the octocoral Alcyonium digitatum and the sea anemone Urticina felina) which together accounted for 37% of the biomass. Mean ammonium excretion rates were 6.48 (±2.44 s.d.) μg N g−1 h−1 (ash-free dry weight) for A. digitatum and 4.12 (±l.38 s.d.) μg N g−1 h−1 for U. felina. Seasonal variations were observed in the excretion of A. digitatum, whereas such variations were less pronounced in U. felina. These trends are discussed in relation to temperature, food availability, nutritional state and nutritional habits of the two species. The results of this and a previous study indicate a nitrogen production of 18.9 g N m−2 y−1 for the entire benthic community.