Marie-Andrée Janquin

Simple standard procedure for the routine determination of organic matter in marine sediment

For the routine determination of organic matter inmarine sediment, a rough estimate is often obtained bymeasuring the loss of weight on ignition(Difference-On-Ignition method: D.O.I.). This D.O.I. is to beused in ecological studies where organic material insediment is used as an environmental variable amongothers. A review of papers using this method showed agreat variability within conditions used. Theseconditions are generally used without accuratejustification. We propose a simple standard procedurebased on grain-size fraction of sediment to determineoptimal temperature and time of ignition, and theinfluence of the sample weight. The method proposed, using the loss of weight on ignition, will give thesame accuracy in measurements of organic matter inmarine sediments.

Carbon and nitrogen content of Laminaria saccharina in the eastern English Channel : biometrics and seasonal variations

Fresh weight (FW), dry weight (DW), carbon and nitrogen content were measured for specimens of Laminaria saccharina (Heterokontophyta: Phaeophyceae) sampled in the eastern English Channel in order to conduct a biometrical study. The aim was to relate carbon and nitrogen masses of the algae to a simple and rapid morphological measurement of the total length of the sporophyte. These relationships were highly significant and appeared very useful to express the standing biomass of L. saccharina in terms of carbon or nitrogen and then to consider dynamic processes such as primary production. Variations in tissue carbon (C) and nitrogen (N) were examined over a complete seasonal cycle. Average carbon and nitrogen content ranged from 23·9 to 31·4% and 2·23 to 3·42% of the total dry weight, respectively. Variations in C/N ratio showed a clear seasonal pattern with an increase in the early spring corresponding to strong photosynthesis and growth.

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.

Ammonium excretion by the ophiuridOphiothrix fragilis as a function of season and tide

Ammonium excretion of a dense population (~1 500 individuals m−2) of the ophiuridOphiothrix fragilis (Abildgaard) was measured in the Dover Straits (French coast) between May 1989 and March 1990: the excretion rate varied from 4.8 µg N g−1 dry wt h−1 in November to 12.8 µg N g−1 dry wt h−1 in June. Mean individual ammonium excretion,E, wasE=0.019⋅t +1.26 (whereE=µg N individual−1 andt=time in min;r=0.80;N=81). Variations in the ammonium excretion rate during a tidal cycle appeared to arise from variations in the duration of the suspension-feeding activity ofO. fragilis, which was governed by the strength of the tidal current. During short-term starvation, excretion was low (E=0.009⋅t+1.47;r=0.91;N=17), increasing with increasing length of starvation [E=4.62⋅lnt−2.5;r=0.95;N=17], as observed for other echinoderms; this could be due to catabolism of tissue. The daily ammonia flux from thisO. fragilis population to the water column was estimated at 41 mg N m−2 d−1.

Photosynthetic response to light and temperature in Laminaria digitata gametophytes from two French populations

ABSTRACT Given the growing body of evidence on the general decline of kelp beds worldwide, it is crucial to understand the physiological responses of kelp gametophyte stages to environmental parameters. We investigated the physiological responses to light and temperature of gametophytes from two populations of Laminaria digitata in contrasting environments along the French coast of the English Channel. Gametophytes of both populations were highly tolerant of high light through an efficient down-regulation of photosynthesis triggered by the activation of the xanthophyll cycle. Temperature increases promoted photosynthesis and photosystem II showed high resistance to short-term exposure to high temperatures currently encountered in the field. Gametophytes from the two sites displayed some differences in their pigment content and photosynthetic characteristics, but low replication and difference in time of sampling precluded tests of potential local adaptation to the light conditions at each site, as observed in previously published results on adult sporophytes. Gametophytes of L. digitata appeared to be resistant to irradiation and temperature conditions currently experienced in the field, confirming their role in persistence of kelp species under stressful environmental conditions.

Biometrics, Carbon and Nitrogen Content in Two Cnidarians: Urticina Felina and Alcyonium Digitatum

A biometrical study was conducted on two common cnidarians: Alcyonium digitatum(Octocorallia) and Urticina felina (Hexacorallia). The aim was to relate both the carbon andthe nitrogen content of these species to a simple and rapid measurement. As the simplestmeasurement that can be done on A. digitatum is a measurement of size, relationshipswere determined between the height of a colony and its organic carbon (OC=00002 H