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

Comparative study of methodologies to measure in situ the intertidal benthic community metabolism during immersion

International audienceMethods used to estimate community primary production and respiration in intertidal environment are still subject of controversy. Underwater community respiration (CR), net production (NCP) were calculated from simultaneous in situ measures of change in oxygen (O-2), dissolved inorganic carbon (DIC) and carbon dioxide (CO2) concentration in benthic chambers performed in February, April, July and November on a Zostera noltii bed. The CRQ (CRDIC/CRO2) and CPQ (NCPo2/NCPDIC) varied between 0.15 and 3.07 and between 0.03 and 6.83, respectively. Carbon fluxes calculated from CO2 measurement were greatly underestimated, representing only 0.4-5.9% of fluxes estimated from DIC measurement. Indeed, CO2 or HCO3- input or uptake by seagrass community affect the proportions of all the chemical components of dissolved inorganic carbon (DIC, the sum of free dissolved CO2, carbonic acid H2CO3, bicarbonate and carbonate ions, HCO3- and CO32-). Thus, CO2 method is not reliable. O-2 measurement does not take into account anaerobic respiration through chemical oxidation and simultaneous O-2 and DIC measurements should be favored to calculate CRQ and CPQ which need to be discussed in marine environment at community scale

Effect of canopy removal on a rocky shore community metabolism and structure

International audienceThe effects of the permanent removal of the canopy-forming alga Fucus serratus was studied in terms of both functioning and diversity on a mid–low rocky shore ecosystem in the south-west English Channel (48°N 43.686′, 3°W 59.282′). Ecosystem functioning was examined as net or gross primary productivity (NPP or GPP) and respiration (Resp) measured through CO2 fluxes. Diversity was examined as number and composition of species. Measurements were performed in situ, during emersion times, without altering target assemblages. The experiment was designed with two treatments [control (C) or canopy removed (CR)] and five replicates, and was conducted over an 18-month period (from February 2006 to August 2007) to integrate the seasonal variability. The mean GPP and Resp were severely reduced in CR treatment compared to control throughout the survey. The mean NPP was not affected at first, due to the development of opportunistic green algae, but was drastically reduced after 9 months of experiment. The canopy removal affected neither the number of species nor their distribution among trophic groups, and the algal community was only slightly affected. The abundance and biomass of mobile invertebrates, however, were greatly reduced in the absence of canopy. This indicates an important effect of the dominant alga on the higher trophic levels of the community. At this tidal level, the canopy did not seem to affect the community by dampening the environmental stress but by providing food, habitat or both

Functioning of a canopy-dominated intertidal community during emersion: highly productive but heterotrophic at the annual scale

In order to address lack of data regarding coastal carbon budgets, we estimated the annual metabolic carbon budget of an intertidal rocky reef macroalgal community during emersion. This budget is based on direct in situ measurements during emersion and establishes the seasonal variations of the photosynthetic parameters of such a community. CO2 fluxes were measured hourly to study the response of community gross primary productivity (GPP) to irradiance and the variation of GPP and community respiration (CR) over the emersion period at different times of the year. These were combined together with existing monthly measures of GPP and CR hourly rates to model the variations of these fluxes as a function of irradiance and the tidal cycle throughout an entire calendar year. Daily, monthly and annual values of GPP, CR and net primary productivity (NPP) were calculated with a relatively low sensitivity to any of the parameters used. While GPP fluxes show comparable orders of magnitude to those measured in other systems, higher CR fluxes lead to a heterotrophic system during emersion, both under measured (NPP = −299 gC m−2 year−1) and theoretical irradiances (NPP = −119 gC m−2 year−1). This heterotrophy is directly linked to the light availability, varying according to combined daily, tidal and seasonal cycles, and to temperature at the seasonal scale. Measurements performed in situ at the community scale integrated interactions that are otherwise absent at the individual scale. This gives access to aspects of the functioning that cannot be otherwise identified

Freshwater seepages and ephemeral macroalgae proliferation in an intertidal bay: II. Effect on benthic biomass and metabolism

International audienceIntertidal soft-sediments biomass and metabolism are naturally heterogeneous in time and space at different scales. Particular perturbations such as freshwater seepages and seasonal proliferation of ephemeral macroalgae can intermittently and/or locally create additional variability in these systems. Since the impacts of such environmental stresses on natural processes are not well understood, the hypothesis that they would affect the functioning of the benthic system was tested. An intertidal bay whose structure and functioning has been previously described and where a carbon budget has been calculated, was chosen. The results showed that the metabolism of the intertidal sediments was greatly impacted by the above perturbations. Freshwater seepage increased meiofauna and microalgae biomasses and enhanced the total benthic metabolism (increasing community respiration and gross primary production until 4 and 2 fold respectively) without altering its seasonal trend. Ephemeral macroalgae proliferation had a more important effect on the total benthic metabolism, increasing community respiration and gross primary production 8 and 12 fold respectively and leading to a change in the seasonal trend. (C) 2011 Elsevier Ltd. All rights reserved

Spatial and temporal variability of CO2 fluxes at the sediment-air interface in a tidal flat of a temperate lagoon (arcachon bay, France)

International audienceThis study aimed to explore the spatial and temporal variability of benthic metabolism in a temperate mesotidal lagoon. This was achieved by measuring fluxes of CO2 in static chambers during emersion, both under light and dark conditions. Three sample sites were selected according to their tidal level (upper or mid), their sediment type (sand or mud) and the presence/absence of the seagrass Zostera noltei. The three sites were investigated at three seasons (end of winter, spring and beginning of autumn). At each site and each season, three benthic chambers were used simultaneously in successive incubations over the emersion period. The sediment chlorophyll-a content varied seasonally in the upper sands (reaching 283 mg.m− 2 in spring) but not in the mid muds (averaging 142 mg m− 2 in bare muds and 186 mg m− 2 in muds covered by seagrass). The maximum sediment CO2-uptake under light was 9.89 mmol m− 2 h− 1 in the mid bare muds, in early autumn. The maximum sediment CO2-release under darkness was 6.97 mmol m− 2 h− 1 in the mid muds covered by seagrass, in spring. Both CO2-fluxes measured in the light and in the dark increased over periods of emersion. This increase, not related to light nor temperature variations, could be explained by changes in the amount and chemistry of pore water during the air exposure of sediments. The benthic trophic state index, based on the maximum light CO2-flux versus maximum dark CO2-flux ratio, assigned to each site at each season indicated that the sediments were net autotrophic in spring in upper sands and in mid muds covered by seagrass and highly autotrophic in other cases. The most autotrophic sediments were the mid-level bare muds whatever the season. The relevance of this index is discussed compared to carbon annual budget

Photosynthetic activity and productivity of intertidal macroalgae: in situ measurements, from thallus to community scale

International audienceThe photosynthetic activity and productivity of four dominant canopy intertidal macroalgae were measured under emersion and immersion, at saturating light levels (PAR > 300 μmol photons m−2 s−1), and compared at two sites (eastern and western English Channel) in spring and summer. The photosynthetic activity of thalli was measured by the electron transport rate (ETR) using pulse-amplitude modulated (PAM) fluorescence and the productivity of individuals and of communities was measured by carbon fluxes in closed chambers. Under emersion, when thalli were still hydrated, the uppermost species Pelvetia canaliculata had higher photosynthetic activity (mean ETR between 327 and 460 μmol e− m−2 s−1) and individual gross productivity (between 60 and 212 μmol C gDW−1 h−1) than the lowermost species Laminaria digitata (mean ETR between 24 and 53 μmol e− m−2 s−1 and gross productivity between 2 and 38 μmol C gDW−1 h−1), whatever the site and season. P. canaliculata had higher ETR in air than underwater (averaged 146 μmol e− m−2 s−1) and L. digitata had lower ETR in air than underwater (averaged 112 μmol e− m−2 s−1), while they exhibited respectively 3 and 5 times higher gross productivity underwater. At the community scale, the low mid-shore zone of Fucus serratus had the highest mean gross productivity under emersion (47 mmol C m−2 h−1) while rates were higher for the uppermost than lowermost zone at the eastern site (average 20 and 6 mmol C m−2 h−1 respectively) and of the same order of magnitude for both zones at the western site (about 30 mmol C m−2 h−1). Finally, the variability of under emersion primary productivity among sites and seasons was reduced when the measurements were performed on entire communities compared to isolated individuals of the dominant species

Spatio-temporal variability of intertidal benthic primary production and respiration in the western part of the Mont Saint-Michel Bay (Western English Channel, France)

International audienceIn situ measurements of both community metabolism (primary production and respiration) and PAM fluorometry were conducted during emersion on intertidal sediments in the Mont Saint-Michel Bay, in areas where oysters and mussels were cultivated. Results highlighted a low benthic metabolism compared to other intertidal areas previously investigated with the same methods. Comparisons between gross community primary production and relative electron transport rates confirmed this statement. More specifically, primary productivity remained very low all over the year, whereas the associated microalgal biomass was estimated to be high. We suggest that the microphytobenthic community studied was characterized by a self-limitation of its primary productivity by its own biomass, as previously shown in Marennes-Oléron Bay for example. The almost permanent high biomass would represent a limiting factor for micromigration processes within the first millimetres of the sediment. This could be explained by very low resuspension processes occurring in the western part of the bay, enhanced by the occurrence of numerous aquaculture structures that could decrease tidal currents in the benthic boundary laye