Monthly averaged level-3 SeaWiFS chlorophyll concentration data from 1998 to 2001 are globally analyzed using Fourier's analysis to determine the main patterns of temporal variability in all parts of the world ocean. In most regions, seasonal variability dominates over interannual variability, and the timing of the yearly bloom can generally be explained by the local cycle of solar energy. The studied period was influenced by the late consequences of the very strong El Niño of 1997-98. After this major event, the recovery to normal conditions followed different patterns at different locations. Right at the equator, chlorophyll concentration was abnormally high in 1998, and then decreased, while aside from the equator, it was low in 1998, and increased later when equatorial upwelled waters spread poleward.

We examined the trophic contribution of pico-and nanoplankton to a patch reef dominated by scleractinian corals and located at Mayotte Island (Comoro Archipelago). Pico-and nanoplankton concentrations, as well as total particulate organic matter, were measured on a sandybottom and a patch reef transect. Results showed that particles <10 µm accounted for 74% of the chlorophyll a concentration and for 47% of the total living carbon. Synechococcus sp. represented 65% of the chlorophyll < 3 µm and 53 and 67% of the autotrophic carbon and nitrogen, respectively, followed by picoeukaryotes, nanoeukaryotes and Prochlorococcus sp. Concentrations of total chlorophyll a, as well as picoplankton groups, were depleted 30 to 45% above the reef compared to in the adjacent waters and in sandy-bottom samples. Concentrations of nanoflagellates and total particulate organic matter, by contrast, remained unchanged during their passage across the reef. These results suggest selective grazing of picoplankton by the benthic community and compound the importance of picoplankton for the benthic-pelagic coupling and trophic dynamics on coral reefs. KEY WORDS: Picoplankton · Nanoplankton · Benthos · Coral reef communities · Particle removal Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 44: [59][60][61][62][63][64][65][66][67][68][69][70] 2006 brèque et al. 2004b) and greatly benefit from zooplankton-derived energy (Houlbrèque et al. 2003(Houlbrèque et al. , 2004a. In this respect, pico-and nanoplankton should play a significant role in reef energetics -they can reach very high growth and production rates and they represent an important fraction of the reef planktonic biomass (Ducklow 1990).The aim of this study was to examine the in situ contribution of pico-and nanoplankton to the carbon and nitrogen removed by a patch reef, to gain a better understanding of the functioning of such ecosystems. In situ measurements avoid some of the artifacts known in flume experiments, such as limitation in size and food replenishment (Genin & Yahel 2002). They are, however, difficult to obtain, except under conditions of homogeneous or confined flows, such as those found in lagoons and channels (Genin & Yahel 2002). For this reason, we have chosen the reef of La Prévoy-ante (Mayotte Island) because it is located in one of the largest lagoons of the Indian Ocean. In this lagoon, flow rates are low and homogeneous; the reefs are considered healthy and are colonized by large colonies of Galaxea astreata and Acropora spp. (ORC 2003). Concentrations of phytoplankton (total and fractionated chlorophyll, Prochlorococcus sp., Synechococcus sp., picoeukaryotes) and heterotrophic microorganisms (bacteria, nanoflagellates, ciliates), as well as total and fractionated particulate organic matter, were measured on 2 transects located above a sand floor or above the reef. MATERIALS AND METHODSStudy site. The study was carried out in May in the lagoon of the French Comorian island of Mayotte (12°41' S, 45°...

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International audienceBiogeochemical processes in the sea are triggered in various ways by chlorophyll-containing phytoplankton groups. While the variability of chlorophyll concentration at sea has been observed from satellites for several years, these groups are known only from cruises which are limited in space and time. The Geochemistry, Phytoplankton and Color of the Ocean programme (GeP&CO) was set up to describe and understand the variability of phytoplankton composition on large spatial scales under a multi-year sampling strategy. It was based on sea-surface sampling along the route of the merchant ship Contship London which travelled four times a year from Le Havre (France) to Noumea (New Caledonia) via New York, Panama and Auckland. Observations included the measurement of photosynthetic pigments, counts of picoplanktonic cells by flow cytometry (Prochlorococcus, Synechococcus, and picoeucaryotes) and counting and identification of coccolithophores. The results confirmed that tropical areas have low seasonal variability and are characterized by relatively high divinyl-chlorophyll a and zeaxanthin concentration and that the variability is strongest at high latitudes-where the phytoplankton biomass and population structure are found to have large seasonal cycles. Thus, the spring bloom in the North Atlantic and an austral winter bloom north of New Zealand are marked by chlorophyll concentrations which are often higher than 0.5 mu g l(-1) and by high concentration of fucoxanthin (a pigment used as an indicator for diatoms), while summer populations are dominated by Prochlorococcus sp. and have low chlorophyll concentrations. Apart from this yearly bloom at temperate latitudes, fucoxanthin is scarce, except in the equatorial upwelling zone in the eastern Pacific Ocean, where it is found in moderate amounts. In this region, relatively high chlorophyll concentrations extend generally as far as 14 degrees S and do not respond to the seasonal strengthening of the equatorial upwelling during the austral winter. Prochlorococcus, which is known to dominate in oligotrophic tropical seas and to disappear in cold conditions, in fact has its minimum during the spring bloom in the North Atlantic, rather than during the winter. Coccolithophores are ubiquitous, showing a succession of species ill response to oceanic conditions and provinces. 19'Hexanoyloxyfucoxanthin, the pigment generally considered as all indicator of coccolithophores, is relatively abundant at all times and in all regions, but its abundance is generally not tightly correlated with that of coccolithophores. The regional differences revealed by these results are in overall agreement with Longhurst's division of the ocean into ecological provinces. (c) 2006 Elsevier Ltd. All rights reserved

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