Spatial, seasonal and species variations of harmful algal blooms in the South Yellow Sea and East China Sea

The occurrences of harmful algal blooms (HABs), in terms of frequency and area in the Chinese coastal waters, have been increasing since 1980s and caused considerable economic losses. In the present study, we have analyzed spatial and seasonal characteristics of HAB events in the southern Yellow Sea and East China Sea along Chinese coast from 1933 to 2004. With a total 435 HAB records, the most frequent HAB occurrence area (FHA) is off the Yangtze River mouth and another two FHA areas are located south of the Yangtze River estuary along about isobaths of 30–60 m coastal water in the East China Sea. The time of HAB occurrence shifted during our study period: from autumn (August–October) before 1980s to July–August in 1980s, during May–July in 1990s, and May–June for the period of 2000–2004. Causative species were found to be different: Noctiluca scintillans and Skeletonema costatum were dominant causative species prior to 2000; and Prorocentrum donghaiense Lu was dominant from 2000 to 2004 and also caused large blooms in May. Trichodesmium sp. caused many HABs in autumn (August–October) prior to 1980s with only one HAB between 1980 and 2004. The changes of the dominant HAB species may have affected the timings of HAB occurrence, as well as the increasing HAB-affected areas in recent years.     

Ammonia may play an important role in the succession of cyanobacterial blooms and the distribution of common algal species in shallow freshwater lakes

With the human intensification of agricultural and industrial activities, large amount of reduced nitrogen enter into the biosphere, which consequently results in the development of global eutrophication and cyanobacterial blooms. However, no research had reported the effect of ammonia toxicity on the algal succession. In this study, we investigated the ammonia toxicity to 19 algal species or strains to test the hypothesis that ammonia may regulate the succession of cyanobacterial blooms and the distribution of common algal species in freshwater lakes. The bloom‐forming cyanobacterium Microcystis aeruginosa PCC 7806 suffered from ammonia toxicity at high pH value and light intensity conditions. Low NH₄Cl concentration (0.06 mmol L^?1) resulted in the decrease of operational PSII quantum yield by 50% compared with the control exposed to 1000 μmol photons m^?2 s^?1 for 1 h at pH 9.0 ± 0.2, which can be reached in freshwater lakes. Furthermore, the tolerant abilities to NH₃ toxicity of 18 freshwater algal species or strains were as follows: hypertrophication species > eutrophication species > mesotrophication species > oligotrophication species. The different sensitivities of NH₃ toxicity in this study could well explain the distributing rule of common algal species in the freshwater lakes of different trophic states. Meanwhile, the cyanobacterial bloom (e.g. M. aeruginosa) always happened at the low concentration of ammonia in summer, and disappeared with the decrease of ammonia. This may be attributed to the toxic effect of ammonia to M. aeruginosa in spring (the average and maximum ammonia concentration were 0.08 and 0.72 mmol L^?1 in 33 Chinese lakes), and the low level of NH₃‐N in summer and fall in the lakes might be used as preferred nitrogen nutrition by M. aeruginosa, rather than with toxicity. Therefore, ammonia could be a key factor to determine the distribution of common algal species and cyanobacterial bloom in the freshwater systems.     

Roles of mixotrophy in blooms of different dinoflagellates: Implications from the growth experiment

Studies over the last two decades suggested that mixotrophy could be an important adaptive strategy for some bloom-forming dinoflagellates. In the coastal waters adjacent to the Changjiang River estuary in the East China Sea, recurrent blooms of dinoflagellates Prorocentrum donghaiense, Karenia mikimotoi and Alexandrium catenella started to appear from the beginning of the 21 century, but roles of mixotrophy in the formation of dinoflagellate blooms were not well understood. In the current study, mixotrophy-based growth of four selected bloom-causative dinoflagellate species, i.e. K. mikimotoi, A. catenella, P. donghaiense and Prorocentrum micans, were studied. Dinoflagellates were co-cultured with different prey organisms, including bacterium Marinobacter sp., microalgae Isochrysis galbana and Hemiselmis virescens, under a variant of nutrient conditions. It was found that growth of dinoflagellate K. mikimotoi was significantly promoted with the presence of prey organisms. Growth of P. donghaiense and P. micans was only slightly improved. For A. catenella, the addition of prey organisms has no effects on the growth, while both of the two prey microalgae I. galbana and H. virescens were killed, probably by allelochemicals released from A. catenella. There was no apparent relationship between nutrient conditions and the mixotrophy-based growth of the tested dinoflagellates. Based on the results of the growth experiment, it is implicated that mixotrophy may play different roles in the growth and bloom of the four dinoflagellate species. It can be an important competitive strategy for K. mikimotoi. For the two Prorocentrum species and A. catenella, however, the role of mixotrophy is much limited. They may depend more on other competitive strategies, such as phototrophy-based growth and allelopathic effect, to prevail in the phytoplankton community and form blooms.     

Recruitment of Quercus agrifolia in central California: the importance of shrub-dominated patches

Abstract. Many perennial plants strongly enhance the survival of seedlings of other species. We studied patterns of long-term recruitment of Quercus agrifolia (Coastal live oak) associated with shrub-dominated communities by counting Q. agrifolia recruits on a time sequence of historical aerial photographs and comparing recruitment among mapped patches of coastal sage scrub, chaparral, and grassland in an 1120-ha landscape. Because we could not identify new recruits in existing woodlands with aerial photographs, we studied the recruitment of Q. agrifolia in this vegetation type indirectly by comparing population size structures and the spatial relationships between shrubs and recruits among woodlands that varied in understory community type. At the landscape scale, recruitment was higher in coastal sage scrub vegetation than predicted by the extent of its coverage, commensurate with the spatial coverage of chaparral, and very low in grassland. Recruitment within woodland communities also varied considerably. In woodland communities on sheltered, north-oriented topography with understories dominated by shrubs, there were large numbers of small Q. agrifolia, and recruits were not significantly spatially associated with shrubs within plots. In woodlands with herbaceous understories there were few individuals in the small size classes, and recruits were strongly spatially associated with shrubs within plots. Woodlands with shrub-dominated understories have population structures that appear to be stable, but woodlands with herbaceous understories exhibit size structures associated with declining populations. Quercus recruitment into shrub-dominated patches corresponds with previous documentation of facilitative relationships between shrubs and oak seedlings, and suggests the occurrence of an unusual form of patch dynamics in these landscapes.     

The distribution of micro-organisms through a bunded lagoon in Rutland Water

The distribution of micro-organisms entering a bunded lagoon in Rutland Water from a sewage effluent outflow was assayed. The majority of micro-organisms were found to settle out in the immediate vicinity of the effluent outflow. In this area micro-organisms, especially spore formers, were present in numbers exceeding 1 × 10⁶ organisms per gram of sediment. The lagoon lies within a nature reserve and a reduction in the reservoir water level could lead to contact between shore feeding waterfowl and pathogenic microorganisms in the freshly exposed sediment.     

福建中部近海浮游动物数量分布与水团变化的关系

根据2009—2010年在福建中部近海24°55'—25°13'N、119°11'—119°32'E水域冬、春、夏3个季节的调查资料,探讨了该水域浮游动物总丰度与生物量的平面分布、季节变化及其与台湾海峡水团变化的关系。结果表明,调查水域浮游动物的数量在冬、春之交变化较大,而在春、夏季变化较小。浮游动物冬、春两季的平均丰度分别为8.90 个/m³和245.65 个/m³,夏季为236.82 个/m³。冬、春两季,该水域浮游动物的分布特征相近。其数量在近岸较高,向外侧水域逐渐降低。冬季浮游动物的丰度最高为31.56 个/m³,春季最高达到831.67 个/m³。中华哲水蚤(Calanus sinicus)是冬、春季影响总丰度变化最主要的种类。与冬、春季不同,夏季浮游动物的数量在离岸水域较高,丰度最高达1053.13 个/m³,而在近岸较低,最低值仅19.17 个/m³。汉森莹虾(Lucifer hanseni)、双生水母(Diphyes chamissonis)是影响总丰度变化最主要的种类。浮游动物在各季的不同分布特征与台湾海峡的季节性水团变化有关。受季风转换影响,从冬季到夏季,海峡内沿岸流势力逐渐减弱,台湾暖流水势力逐渐增强,并影响到沿岸的水文环境。这导致调查水域内浮游动物的优势种类由暖温种向暖水种演替。由于冬、春季的重要优势种类中华哲水蚤与夏季的汉森莹虾、双生水母具有不同的温度适应性,受不同性质水团的影响,在近岸和离岸水域各自呈现出不同的数量高低。从而进一步影响到各季浮游动物总数量的分布。     

Geographical and seasonal distribution of the bluetongue virus vector, Culicoides imicola, in central Italy

Following the first incursion of bluetongue virus (BTV) into Italy, the geographical and seasonal distribution of the biting midge Culicoides imicola Kieffer (Diptera: Ceratopogonidae), the main vector of BTV and African horse sickness virus, was investigated in two regions of central Italy (Lazio and Tuscany). Surveillance of Culicoides was carried out between July 2001 and December 2002 using light traps: 1917 collections were made in 381 trap sites, well distributed across both regions. During the survey, bluetongue outbreaks were recorded in both regions. Culicoides imicola was found in 89 (23%) trap sites, distributed fairly continuously along the whole western coastline, between 41.2697 degrees N and 44.05724 degrees N. It was found only occasionally inland and usually in low abundance, with catches of more than 1000 specimens per night found in only two sample sites and 74% of catches numbering fewer than 10 specimens. Adults were caught from March to mid December, with peaks ranging from the end of August to mid November. The coastal distribution and the presence of only few sites with year-round records of adult vectors suggests that colonization may have occurred recently, by passive wind-dispersal from external source areas (Sardinia and Corsica). Alternatively, the species may occur in established, previously undetected, autochthonous populations that are limited from extension inland and northern-ward within Lazio and Tuscany by cool winter temperatures.     

Optimal seasonal schedules and the relative dominance of heteromorphic and isomorphic life cycles in macroalgae

Marine macroalgae (seaweed) show diverse life cycles. Species with a heteromorphic life cycle have a large multicellular algal body in one generation but have a very small body in the second generation of the same year. In contrast, the diploid and haploid life forms of isomorphic species have similar morphology, and these species often have more than two generations in a year. Here, we first study the optimal life cycle schedule of marine macroalgae when daily mortality changes seasonally, and then we discuss the conditions for coexistence and relative dominance of different life cycles. According to the optimal life cycle schedule, heteromorphic species tend to have a generation with a large algal body when mortality is low, and a microscopic-sized generation when mortality is high. In contrast, isomorphic species tend to mature when body size reaches a threshold value that is the same for different generations. We then examine the coexistence of the two life cycles when growth rate decreases with biomass. The model predicts that (1) at high latitudes (i.e., in strongly seasonal environments), heteromorphic species are likely to dominate over isomorphic species, and (2) species with a heteromorphic life cycle should dominate in the supratidal and upper intertidal zones where macroalgae tend to suffer high mortality, and also in the subtidal zone, where mortality is low, whereas isomorphic species are likely to be more successful when mortality is intermediate. These predictions are consistent with the observed distribution patterns of the two life cycles in macroalgae.     

Diel and seasonal vertical distribution of meiobenthic copepods in muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay)

The vertical distribution of meiobenthic copepods was investigated within muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay, France). The aim of the study was to determine if in muddy sediments, as previously established in sandy sediments, meiobenthic copepods migrate vertically according to the seasons or diel periods. Two experimental approaches were used, viz: a three-season comparison was made of the diel vertical distribution of the harpacticoid Canuella perplexa T. & A. Scott (1893) and secondly the depth distribution of a meiobenthic copepod assemblage was followed for a 24 h period, in shallow water subtidal locations. The harpacticoid C. perplexa vertically migrated through the top three centimeters of the sediment, showing diel and seasonal variations in depth distribution. The differential vertical distributions shown by the dominant meiobenthic populations suggest that emergence into the water column may mainly concern surface dwelling copepods. The physical and biological factors affecting seasonal and diel changes in the copepod assemblage of the fish ponds are discussed.     

Modelling the distribution of a threatened habitat: the California sage scrub

Aim Using predictive species distribution and ecological niche modelling our objectives are: (1) to identify important climatic drivers of distribution at regional scales of a locally complex and dynamic system – California sage scrub; (2) to map suitable sage scrub habitat in California; and (3) to distinguish between bioclimatic niches of floristic groups within sage scrub to assess the conservation significance of analysing such species groups. Location Coastal mediterranean‐type shrublands of southern and central California. Methods Using point localities from georeferenced herbarium records, we modelled the potential distribution and bioclimatic envelopes of 14 characteristic sage scrub species and three floristic groups (south‐coastal, coastal–interior disjunct and broadly distributed species) based upon current climate conditions. Maxent was used to map climatically suitable habitat, while principal components analysis followed by canonical discriminant analysis were used to distinguish between floristic groups and visualize species and group distributions in multivariate ecological space. Results Geographical distribution patterns of individual species were mirrored in the habitat suitability maps of floristic groups, notably the disjunct distribution of the coastal–interior species. Overlap in the distributions of floristic groups was evident in both geographical and multivariate niche space; however, discriminant analysis confirmed the separability of floristic groups based on bioclimatic variables. Higher performance of floristic group models compared with sage scrub as a whole suggests that groups have differing climate requirements for habitat suitability at regional scales and that breaking sage scrub into floristic groups improves the discrimination between climatically suitable and unsuitable habitat. Main conclusions The finding that presence‐only data and climatic variables can produce useful information on habitat suitability of California sage scrub species and floristic groups at a regional scale has important implications for ongoing efforts of habitat restoration for sage scrub. In addition, modelling at a group level provides important information about the differences in climatic niches within California sage scrub. Finally, the high performance of our floristic group models highlights the potential a community‐level modelling approach holds for investigating plant distribution patterns.     

Diel and seasonal distribution of perch in Lake Constance: a hydroacoustic study and in situ observations

A near-shore belt 50 km in length was surveyed parallel to the shoreline of Lake Constance, central Europe, with a single-beam echosounder five times between July 1993 and February 1994. The species and age composition of fish in the survey area was investigated by gillnet fishing and SCUBA-diving. In summer, the horizontal distribution of perch was patchy. Population density declined from east to west, and highest densities occurred in one shallow bay and close to ports and jetties at steeper shores. During daytime, perch stayed in the sublittoral zone between 3 and 15 m depth and between 2 and 6 m above the thermocline. Within this layer age classes were separated spatially: the relative number of young-of-the-year perch declined with depth whereas the relative number of adult perch (2+ and older) increased with depth. At dusk the fish migrated to the littoral zone, where they spent the night resting on the bottom. In winter, under almost homothermal conditions, perch of all ages were located between the 35 and 70 m depth contours, where they performed pronounced diel vertical migrations. They rested on, or close to, the bottom during daytime and ascended up to 20 m below the surface at night. During this season, horizontal distribution of perch was much more homogeneous than in summer.     

Distribution and seasonal biomass of drift macroalgae in the Indian River Lagoon (Florida, USA) estimated with acoustic seafloor classification (QTCView, Echoplus)

Three areas of the Indian River Lagoon, Florida (USA) were surveyed to show seasonal changes in the distribution and biomass of macroalgae and seagrass. Acoustic seafloor discrimination based on first and second echo returns of a 50 kHz and 200 kHz signal, and two different survey systems (QTCView and ECHOplus) were used. System verification in both the field and a controlled environment showed it was possible to distinguish acoustically between seagrass, sparse algae, and dense algae. Accuracy of distinction of three classes (algae, seagrass, bare substratum) was around 60%. Maps were produced by regridding the survey area to a regular grid and using a nearest-neighbor interpolation to provide filled polygons. Biomass was calculated by counting pixels assigned to substratum classes with known wet-weight biomass values (sparse algae 250 g m^− 2, dense algae 2000 g m^− 2, seagrass 100 g m^− 2) that were measured in the field. In three study areas (Melbourne, Sebastian Inlet, and Cocoa Beach), a dependence of algal biomass on depth and season was observed. Seagrass most frequently occurred in water less than 1 m deep, and in November, seagrass beds tended to be covered by dense algae that also extended up- and downstream of shoals in the Lagoon. In March, the pattern was similar, with the exception that some areas of previously dense algae had started thinning into sparse algae. Macrophyte biomass was lowest in May in the Melbourne and Cocoa Beach study areas, with the opposite situation in the Sebastian Inlet study area. In May, seagrass areas were largely devoid of dense algae and most algae accumulations were sparse. In August, dense algae covered large areas of the deep Lagoon floor while shoals were largely free of algae or had only sparse cover. We suggest this summer pattern to reflect moribund algae being washed from the shallows to deeper channels and from there being removed from the lagoonal ecosystem either through tidal passages into the open ocean or by degradation and breakdown in situ. The differences between the study areas indicate high spatial and temporal variability in biomass and distribution of macrophyte biomass in the Indian River Lagoon.     

Geographical and seasonal influences on the distribution of fungal endophytes in Quercus ilex

A systematic survey of the endophytic assemblages of Quercus ilex in central Spain has been performed, with the goal of evaluating the importance of geographical and seasonal factors on these fungal communities. Four sampling sites were selected; one of them was sampled twice, in the spring and the autumn. The collected plant material consisted of bark, twigs and leaves from eight trees per site. Fungal strains were isolated with the use of a surface-sterilization method with sodium hypochlorite. A total of 2921 fungal strains grouped into 149 'species' or morphological types were recovered. The 10 dominant species, with isolation frequencies >1.5%, were Pyrenochaeta sp., Periconiella anamorph of Biscogniauxia mediterranea (De Not.) Kuntze, Pseudonectria sp., Cryptosporiopsis quercina Petrak, Alternaria alternata (Fr:) Keissl., two undetermined coelomycetes, Penicillium funiculosum Thom, Diplodia mutila Fr. apud Mont. and Ascochyta sp. Medians of fungal species per tree were significantly different among the sampled sites. The isolation frequencies of the dominant species, as well as other less frequent species, were significantly dependent on the sampling site. The degree of endophytic infection and the diversity of fungal species were significantly higher in the spring. The frequencies of all dominant species at one of the sites depended significantly on the season, except for C. quercina , Acremonium sclerotigenum (F & V Moreau ex Valenta) Gams. and D. mutila . Cluster analysis of the whole endophytic mycoflora of the sampled trees suggested that the geographical factor affects the endophytic distribution patterns more significantly than the seasonal factor.     

Modern benthic diatom distribution in a hypersaline coastal lagoon: the Lagoa de Araruama (R.J.), Brazil

Diatom assemblages identified in 75 surface sediment samples in a shallow hypersaline coastal lagoon (Lagoa de Araruama, R.J., Brazil) are mainly composed of holo-euryhaline and marine euryhaline benthic taxa. The lagoon is characterized by an assemblage dominated by Cocconeis placentula var. euglypta, associated with Catenula adhaerens and Cocconeis diminuta. However, different areas, characterized by specific assemblages, have been identified. Their distribution seems to be related to (1) fluctuations in bathymetry, influenced by water inputs from the ocean and from rivers adjacent to the lagoon; (2) fluctuations in salinity through the influence of marine water and the precipitation–evaporation rate. Moreover, the distribution of associations characterized by the presence of Nitzschia palea, N. pusilla and Fallacia cryptolyra suggests the influence of man-induced activities. Although these diatoms are not dominant, their wide distribution inside the lagoon, favored by local wind-hydrodynamics, suggest a large impact of anthropic activities (e.g. freshwater and organic sewages).     

Habitat suitability of Rhincodon typus in three localities of the Gulf of California: Environmental drivers of seasonal aggregations

The whale shark is an endangered species that usually feeds in coastal areas of highly productive seas such as the Gulf of California, Mexico. This study aims to describe the effect of sea surface temperature, chlorophyll a, bathymetry and slope on the habitat suitability of whale sharks in three important aggregation sites of the Gulf of California. A total of 2396 records of occurrence of whale sharks were obtained from international databases and scientific literature between 1996 and 2018. These records were used for the creation of a species distribution model using MaxEnt for each of the three aggregation sites. The concentration of chlorophyll a explained 71% of the habitat suitability, followed by bathymetry and slope with a combined 17%, and sea surface temperature constituting 10% of the model. Habitat suitability was related to areas where nontargeted fisheries may impact whale sharks through bycatch, entanglement and ship strikes. The implications for the conservation of whale sharks should be considered for management decisions in terms of marine protected areas, fishing refugees or bans, and other regulations regarding fisheries activities.     

Influences of sedimentation and hydrodynamics on the spatial distribution of Alexandrium catenella/tamarense resting cysts in a shellfish farming lagoon impacted by toxic blooms

Since resting cysts are a potential seeding source for blooms, the presence of these cysts in sediments is a marker of an established population for a number of harmful algal species. The spatial patterns of cyst density in relation to sediment characteristics and hydrodynamics are still largely misunderstood. This study investigated the spatial distribution of resting cysts belonging to the Alexandrium tamarense species complex (Dinophyceae) in sediments of a Mediterranean coastal lagoon (Thau Lagoon, France). This lagoon, hosting shellfish farming, is regularly impacted by toxic Alexandrium catenella blooms. The average cyst density across the whole lagoon was rather low, <20 cysts g⁻¹ of dry sediment (DS). However, densities varied widely among sampled stations, with the highest density (∼440 cysts g⁻¹ DS) recorded in a shallow cove named Crique-de-l’Angle, which is the only area where dense blooms of A. catenella and A. tamarense have been recorded in the years preceding this survey. An analysis using spatial autoregressive models demonstrated that cyst densities were highly spatially autocorrelated (indicating that close stations tended to have more similar cyst densities) with accumulation sites. With respect to sediment characteristics (5 granulometric fractions <2 mm and biochemical components), the highest densities were found in silty sediments containing high proportions of water and organic matter. Nevertheless, the linkage between cyst density and sediment structure was not always verified; this reflected the influence of hydrodynamics on the sedimentation of cysts and sediment particles, and on the dispersal of cysts away from the bloom area by wind-induced currents, suggesting that hydrodynamics was responsible for the spatially autocorrelated distribution of cyst densities.     

Altitudinal distribution and seasonal life cycle of drosophilid flies (Diptera: Drosophilidae) in central Japan,with reference to their biogeographic features

Altitudinal distribution and seasonal life cycle of drosophilid flies (Diptera) were studied on the southwestern slope of Mt. Higashi-Kagonoto in Tomi and at Ishi-no-yu in Shiga Heights, Nagano, central Japan. Collections of flies were carried out from early spring to late autumn using traps baited with fermenting banana. Based on the present results and previous faunal reports from central Japan, major native drosophilid species collected in this study were classified into migratory (5 species), high-altitude (6), mid-altitude (14) and low-altitude (14) species. The migratory species are assumed to perform extensive seasonal migration between low- and high-altitude areas. Among these native species, Drosophila alpina (a high-altitude species) and D. moriwakii (a mid-altitude species) are assumed to pass only one generation in a year, while the others pass two or more generations. Five of the six high-altitude species are assigned as the Palearctic elements, while four of the five migratory species and 12 of the 14 low-altitude species are assigned as the Sino-Japanese elements. The mid-altitude species consist of approximately equal numbers of the Palearctic and Sino-Japanese elements. Among the high-altitude species, D. alpina and Hirtodrosophila makinoi are also distributed in high-altitude areas in Hokkaido, northern Japan. The other high-altitude species and most of the mid-altitude species generally occur in low-altitude areas in Hokkaido or the further north. The Sino-Japanese elements occurring at high and mid altitudes in central Japan are also distributed at high altitudes in southern and southwestern China.     

Zooplankton-phytoplankton interactions: a possible explanation of the seasonal succession in the Kuršių Marios lagoon

The zooplankton-phytoplankton interactions inthe Kuršių Marios lagoon (southeastern Baltic Sea)were investigated in 1995. The objective was toevaluate the role of herbivores (crustaceanzooplankton) in the seasonal succession ofphytoplankton, as well as the influence of foodconditions on structure and dynamics of zooplanktoncommunity. Our results demonstrated that thecrustacean grazing pressure may restrict thedevelopment of small Chlorophyta and Diatomophyceaeand, in turn, favouring growth of Cyanobacteria.Blooms of filamentous Cyanobacteria possibly has aninhibitory effect for Daphnia, decreasing theirbiomass as well possibly explaining the shift ofdominant zooplankton species. The influence ofplanktivory on seasonal plankton succession remainsunclear because of lack of fish data. This revised version was published online in July 2006 with corrections to the Cover Date.     

Space and time distribution of zooplankton in a Mediterranean lagoon (Etang de Berre)

In the Berre lagoon, a large brackish and swallow area near Marseille, the environmental factors (temperature, salinity, oxygen, suspended particulate matter and chlorophyll) generally display strong space and time variations. The rotifer Brachionus plicatilis and the copepod Acartia tonsa constitute the bulk of the zooplankton population during all the year. Their space and time distributions were studied in 23 stations distributed all over the lagoon, during four seasonal cruises (February, June, October, November), at surface and bottom layers. There is no marked difference in the horizontal and vertical distribution of the two species, (except in November when rotifers were prevailing in surface and copepods at depth) and in their time occurence. When the four series of data are pooled, correlation analysis show that A.tonsa is positively correlated with temperature, salinity and seston and negatively to oxygen and chlorophyll. B. plicatilis is positively correlated with temperature and seston, but also with chlorophyll, while salinity has a negative effect. The specific eggs number of both species is chlorophyll dependent. Considering seasonal cruises separately, some differences appear in the sense or the significance of these different correlations. The respective distribution of the two species is only partly dependent on the variation of the environmental factors: most of the variance remains unexplained, as indicated by the result of a stepwise multiple regression analysis using the most significant factors (temperature, salinity and oxygen explain 33 to 42% of the variance in Acartia, while temperature and salinity explain 27 to 28% of the variance in Brachionus). Thus, internal behavioral factors could also play a role in the distribution of organisms, particularly in some cases of aggregations of organisms observed during this study. As the two species occupied the same space habitat most of the year, they are potentially in competition for food. A way to optimize the food utilization could be the time separation of their feeding activity, nocturnal in Acartia and diurnal in Brachionus. Another way could be selective feeding upon food particles depending on their size (Brachionus being able to use finer particles than Acartia) or their quality (Brachionus being more herbivorous than Acartia) as demonstrated in some grazing experiments carried out in parallel.