G. Bertru

Fate of plant detritus in a European salt marsh dominated by Atriplex portulacoides (L.) Aellen

This paper deals with the organic matter dynamics of a dwarf shrub species, Atriplex portulacoides, which characterizes the majority of ungrazed European salt marshes. These dynamics were investigated by the simultaneous estimation of primary production, litter production, decomposition processes and accumulation of organic matter in sediment. We used harvested biomass techniques and the litter-bag method, as well as more recent techniques such as determination of natural isotopic compositions. The results of this study have been compared with those of other studies of salt marshes of the Eastern Coast of America dominated by Spartina alterniflora. Studies of the fate of halophytic organic matter in salt marshes have shown that processes such as consumption, decomposition, or flooding rapidly could distribute organic material to different compartments of the marsh system.Aboveground productivity of A. portulacides was 1700 g m-2 yr-1. Approximately 80% of this production was transformed rapidly in necromass. A large part of this dead material (86%) fell as litter directly onto the sediment at the sampling site. The remainder was washed away by tides, either out of the marsh, or into the upper marsh where it accumulated in drift lines. Decomposition of A. portulacoides on the sediment was followed using litter-bags. Decay constants, based on the first order decay function, ranged between 0.0404 d-1 and 0.0113 d-1 and appeared high when compare with data obtained in other American and European salt marshes. Nitrogen immobilisation was not detectable in this decomposition experiment. During decay, nitrogen concentrations increased, whereas absolute amount of nitrogen decreased. After microbial decomposition, plant detritus was rapidly incorporated into sediment. The carbon isotopic composition of organic matter in sediment showed that organic matter in the middle marsh came from both aboveground production of A. portulacoides and microalgae. The patterns for these two kinds of organic matter in different granulometric fractions depended on tidal submersion frequency. During the summer, when the salt marsh was not disturbed by tides, the clay fraction got richer in plant organic matter, whereas, during the other periods, this fraction was characterized by microalgal organic matter. This study provides new data about the dynamic of organic matter. It remains still difficult to establish comparisons between North American and European salt marsh functioning according to the variability of primary production, microbial decomposition and tidal range in these ecosystems.

Seasonal and spatial trends of nitrogen and phosphorus loads to the upper catchment of the river Vilaine (Brittany): relationships with land use

Influences of urban and agricultural activities on river water quality were analysed on the upper catchment of the River Vilaine (902 km2), France. Agricultural land covered 78% of the study area. Corn, grass and cereals were the main crops. Total Nitrogen (TN) and Total Phosphorus (TP) loads were determined for individual subcatchments (from 10 to 194 km2) during the hydrological year 1994/95.Relationships between water quality and catchment management were studied. Non-point source produced 95% of TN and 90% of TP of the whole river basin. Significant variation in TN and TP occurred, in time and space. The greatest part was discharged in January and February. Annual river exportation rates ranged from 20 to 65 kg TN ha-1 and from 1.15 to 3.05 kg TP ha-1. Reservoirs will held almost 15% of TN and TP loads. Cyanobacteria blooms occurred in summer and autumn, with significant production of organic matter here, and downstream, in the rivers.Greatest TP river retention rate were determined downstream a town of 15 000 people, and highest TN loss occurred in the subcatchment with the highest density of cattle and the most important percent of permanent grassland.

Combination of biomolecular and stable isotope techniques to determine the origin of organic matter used by bacterial communities: application to sediment.

Natural isotopic composition is a good tool to trace organic matter in ecosystems. Recent studies used a combination of molecular and stable isotope techniques to determine the origin of the organic carbon used by bacteria in the water column. In our study, we show that this procedure can be used for analysis of sediment bacterial communities with few modifications. In the water column, bacterial recovery is done before DNA extraction. In the sediment, we tested qualitatively and quantitatively a direct and indirect extraction of DNA. The direct extraction was the most efficient. It recovered between 3.1 and 15.8 microg DNA g(-1) dry sediment and the contamination of field samples by eucaryotic DNA was less than 13%. In this preliminary study of the salt marsh ecosystem, the delta(13)C values of DNA (-26 to - 24%) recovered from the sediment were close to the delta(13)C values of halophytic plants (-26.4 and - 25.3%) showing a relationship between plants and microorganisms. Thus, this procedure can be used to trace the flow of carbon through the sediment microbial biomass and to understand the variation of bacterial activity according to the inputs of allocthonous and autochtonous organic matter.

La biodegradation des acides fulviques

The apparent difficulty of degrading fulvic acids in the aquatic environment was investigated, using samples taken during a whole annual cycle, and studying mineralisation processes caused by the action of different bacteria in samples with and without addition of benzoate and lactate. Qualitative changes in the respective ratio of various fulvic acid fractions produced by bacterial action, were studied with sephadex 25.

Bacteriolysis in the gut of Nereis diversicolor (O.F. Müller) and effect of the diet

Abstract The bacteriolytic activity in the polychaete Nereis diversicolor was investigated in extracts of faeces and sediment, and in three sections of the digestive tract: foregut, midgut and hindgut. Using agarose-gel tests, we found a lysozyme-like activity able to lyse Micrococcus lysodeikticus cells and stable to heat at acidic pH. The digestive extracts have also appreciable lytic action on plant leachate bacteria. In whole extract (tissues and contents), the lytic activity was higher in the hindgut, whereas in the contents the lytic activity was higher in the midgut. The presence of bacteria or the food intake seemed to stimulate bacteriolysis activity since very low lysis of leachate bacteria was found in the digestive tract of starved animals.

HPLC analysis of chlorophyll a breakdown products to interpret microalgae dynamics in a shallow bay

Phytoplankton production is determined by growth, senescence, sinking and zooplankton grazing. In an attempt to follow algal senescence and grazing, some authors have used HPLC fluorescence detection of chlorophyll a breakdown products. Laboratory grazing experiments have shown that copepods reduce chlorophyll a from diatoms leading to an increase in pheophytin a rather than pheophorbide a. However, field measurements only indicated a slight increase of pheopigment concentrations in summer. During this period, high heterotrophic activities (zooplankton and bacteria) seemed to be responsible for rapid pheopigment disappearance. On the other hand, highest chlorophyllide a levels appeared to be related to spring accumulation of nutrient-limited senescent algae. While increases in pheophytin a accounted for chlorophyll a consumption, changes in pheophorbide a concentrations could be linked to chlorophyllide a abundance. These results suggest that laboratory studies cannot be uncritically extrapolated to the field.

Phytoplankton community growth in enrichment bioassays : possible role of the nutrient intracellular pools

Abstract Examination of published experimental data showed that nutrient addition can sometimes inhibit growth rate of natural algal communities. Such reductions in algal growth might be due to toxic effects of some enrichments. This hypothesis could not however explain inhibitions following moderate additions of nitrate and phosphate. A new hypothesis is proposed to account for such “unexplained” results. It considers a limitation of algal community growth by several different resources. Experimental observations are consistent with resource competition determining the coexistence of several species competing for the same resources. Intracellular nutrient storage may reduce bioavailability of many resources involved in algal growth limitation. A simplified mathematical approach, derived from the Droop growth model (1973) , was developed to describe possible relationships between species diversity and algal community growth. Coexistence of several species competing for different resources might provide a partial buffer against large variations in algal community growth following nutrient enrichments.

Oceans, rivers, and lakes : energy and substance transfers at interfaces

Invited lectures session 1 - methodologies and modelling session 2 - interface between medium and living beings session 3 - interface between nutrients and physical medium session 4 - interfaces between pollutants, medium and living beings session 5 - fluxes at interfaces.

Endogenic development of sediments in a eutrophic lake

Sedimentation in the Gouet reservoir (France), measured for 2 years at 8 stations, was maximal during summer, when river inputs were minimal. Physical and chemical conditions in the deposits indicate that the endogenous part of sedimentation was about 70% and resulted from significant diatom production. The high sedimentation rate on the bottom was favoured by the funnel morphology of the reservoir, the chronic lack of oxygen in the water column, and the repeated copper sulfate treatment. The former river meanders of the reservoir were the preferential deposit sites.

A chemotaxonomic method to quantify phytoplankton groups in freshwater lentic mesocosms: an approach including chlorophyll a breakdown products.

The use of HPLC methods to determine and quantify phytoplankton population composition, is sometimes less time-consuming than microscopic identification. However, its general application poses problems since high discrepancies between chlorophyll a calculated using chemotaxonomic methods and direct measurements were noticed. For instance, chemotaxonomic protocols generally employed can lead to a poor estimation of total and relative abundance when high amounts of chlorophyll a breakdown products are present. Therefore, we propose a new approach to calculate relative abundance of algal groups in a phytoplankton population, based on integration of these degradation products in the chemotaxonomic assessment in lentic and shallow freshwater ecosystems.