Variação nictemeral do microfitoplâncton em um estuário do nordeste paraense, Brasil

Este estudo teve o objetivo de conhecer a diversidade do microfitoplâncton, assim como sua variação nictemeral relacionada aos fatores ambientais do estuário do rio Curuçá (Curuçá - PA). Foram coletadas 12 amostras de fitoplâncton, ao longo de 24 horas, em 3 pontos de amostragem localizados próximo a uma fazenda de cultivo camarão marinho Litopnaeus vannamei em marés de sizígia, nos dias 14 e 15 de agosto/2004 e nos dias 24 e 25 de janeiro/2005. Foram determinadas a composição específica e densidade do microfitoplâncton (org.L-1) e realizadas análises de frequência de ocorrência, diversidade e equitabilidade, agrupamento e componentes principais (ACP). Os parâmetros físico-químicos não apresentaram uma variabilidade significativa entre os meses de coleta, porém observa-se uma importante influência do regime pluviométrico sobre a variação dos valores de salinidade, registrando os menores valores durante o mês de janeiro/05 e os maiores em agosto/04. Foram registrados 170 táxons pertencentes às Divisões Bacillariophyta (149), Dinophyta (16), Chlorophyta (3) e Cyanobacteria (2).O filo Bacillariophyta foi predominante em número de espécies, frequência de ocorrência e densidade (97.59%). Poucas espécies apresentam elevados índices de abundância, sendo que no mês de agosto a comunidade microfitoplanctônica é dominada por Bacteriastrum hyalinum, Bellerochea horologicalis, Chaetoceros curvisetus, Dimerograma dubium, Dytilium brigtwelli, Pseudo-nitzschia seriata e Skeletonema costatum. No mês de janeiro predominam Chaetoceros pseudocrinitus, Chaetoceros curvisetus e Skeletonema costatum. Houve o predomínio de espécies marinhas planctônicas neríticas, marinha planctônica nerito-oceânica, e marinha-planctônica oceânica. A diversidade específica oscilou de 0.7591 bits.org-1 e 1.3314 bits. org-1, caracterizada, de um modo geral, por uma diversidade variando de muito baixa a baixa, apresentando uma estrutura pouco diversificada. A variação dos parâmetros físico-químicos e da densidade das espécies foi o fator determinante no agrupamento das amostras, formando-se dois grandes grupos, o primeiro composto por amostras do mês de agosto e o segundo grupo composto por amostras do mês de janeiro. A análise de componentes principais indicou que, apesar de os parâmetros físico-químicos apresentarem baixa variabilidade espacial e entre os meses de coleta, a variação do índice de pluviosidade e da salinidade foi muito importante na variação da densidade de grande parte das espécies, provocando um aumento da densidade fitoplanctônica no mês de janeiro.

Miocene to Holocene diatom biostratigraphy of ODP Leg 105 sites

Diatoms occur sporadically in lower Miocene to Holocene sediments recovered at ODP Site 645 and in upper Pliocene to Holocene sediments at ODP Site 646. The diatom assemblage at Site 645 contains rare stratigraphic indicators. Fragmentation of frustules and the occurrence of species characteristic of high-latitude shelf, upper-slope environments suggest current transportation from the shelf. The diatom abundance and preservation at Site 646 probably reflect climatic changes and are also affected by dissolution, but it is not possible to detect the dominant factor. Therefore, the stratigraphic ranges of the primary and secondary biostratigraphic indicators are often unreliable.

Abiotic characteristics and diatom abundance and biomass in different sectors of the Argentinien Shelf and antartic waters

A large spatial scale study of the diatom species inhabiting waters from the subantarctic (Argentine shelf) to antarctic was made for the first time in order to understand the relationships between these two regions with regard to the fluctuations in diatom abundances in relation with environmental features, their floristic associations and the effect of the Polar Front as a biogeographic barrier. Species-specific diatom abundance, nutrient and chlorophyll-a concentration were assessed from 64 subsurface oceanographic stations carried out during the austral summer 2002, a period characterized by an anomalous sea-ice coverage corresponding to a ''warm year". Significant relationships of both diatom density and biomass with chlorophyll-a (positive) and water temperature (negative) were found for the study area as a whole. Within the Subantarctic region, diatom density and biomass values were more uniform and significantly (in average: 35 and 11 times) lower than those of the Antarctic region, and did not correlate with chlorophyll-a. In antarctic waters, instead, biomass was directly related with chlorophyll-a, thus confirming the important contribution of diatoms to the Antarctic phytoplanktonic stock. A total of 167 taxa were recorded for the entire study area, with Chaetoceros and Thalassiosira being the best represented genera. Species richness was maximum in subantarctic waters (46; Argentine shelf) and minimum in the Antarctic region (21; Antarctic Peninsula), and showed a significant decrease with latitude. Floristic associations were examined both qualitatively (Jaccard Index) and quantitatively (correlation) by cluster analyses and results allowed differentiating a similar number of associations (12 vs. 13, respectively) and two main groups of stations. In the Drake Passage, the former revealed that the main floristic change was found at the Polar Front, while the latter reflected the Southern ACC Front as a main boundary, and yielded a higher number of isolated sites, most of them located next to different Antarctic islands. Such differences are attributed to the high relative density of Fragilariopsis kerguelensis in Argentine shelf and Drake Passage waters and of Porosira glacialis and species of Chaetoceros and Thalasiosira in the Weddell Sea and near the Antarctic Peninsula. From a total of 84 taxa recorded in antarctic waters, only 17 were found exclusively in this region, and the great majority (67) was also present in subantarctic waters but in extremely low (< 1 cell/l) concentrations, probably as a result of expatriation processes via the ACC-Malvinas Current system. The present results were compared with those of previous studies on the Antarctic region with respect to both diatom associations in regular vs. atypically warm years, and the distribution and abundance of some selected planktonic species reported for surface sediments.

Diatom biostratigraphy of ODP Holes 191-1179B and 191-1179C

Although the objective of Ocean Drilling Program Leg 191 was to install a seismic monitoring station and to test a hard rock reentry drilling system, several good, near-continuous sedimentary core sequences were recovered during the cruise. Two holes, 1179B and 1179C, yielded an upper Miocene to Pleistocene diatom record through an expanded section with excellent recovery. Because diatom species included in both low-latitude and high-latitude biostratigraphies are present, zonations for the equatorial Pacific and northwest Pacific are applied to the sediments. The oldest zones from each zonation that are represented in the cores are the Nitzschia miocenica Zone and the Rouxia californica Zone, respectively. Only one zonal boundary is not observed within the diatom assemblage, that being the top of the Nitzschia jouseae Zone and base of the Rhizosolenia praebergonii Subzone A (equatorial Pacific). Preservation is good overall, and sample abundances vary from rare to abundant. This would be an excellent section for further biostratigraphic, paleoclimatic, and paleoceanographic study.

The Global Plankton Database: an inventory and data from the Former Soviet Union expeditions

At present time, there is a lack of knowledge on the interannual climate-related variability of zooplankton communities of the tropical Atlantic, central Mediterranean Sea, Caspian Sea, and Aral Sea, due to the absence of appropriate databases. In the mid latitudes, the North Atlantic Oscillation (NAO) is the dominant mode of atmospheric fluctuations over eastern North America, the northern Atlantic Ocean and Europe. Therefore, one of the issues that need to be addressed through data synthesis is the evaluation of interannual patterns in species abundance and species diversity over these regions in regard to the NAO. The database has been used to investigate the ecological role of the NAO in interannual variations of mesozooplankton abundance and biomass along the zonal array of the NAO influence. Basic approach to the proposed research involved: (1) development of co-operation between experts and data holders in Ukraine, Russia, Kazakhstan, Azerbaijan, UK, and USA to rescue and compile the oceanographic data sets and release them on CD-ROM, (2) organization and compilation of a database based on FSU cruises to the above regions, (3) analysis of the basin-scale interannual variability of the zooplankton species abundance, biomass, and species diversity.

Siliceous microplankton in surface sediments and the water column of the Red Sea

We studied the siliceous microplankton assemblages (mainly diatoms) from plankton tows (mesh size 20 µm) and surface sediment samples collected along a N-S transect in the northern Red Sea (28-21°N). In addition, we analyzed differences/similarities between plankton and sediment assemblages within a brine-filled basin (the southern basin) of the Shaban Deep and compared these assemblages with those from outside the brine. Plankton samples revealed the overwhelming dominance of diatoms over other siliceous groups. Diatoms accounted for ca. 97% of all biosiliceous particles at 120-20 m (vs. 2.9% silicoflagellates and 0.4% radiolarians), and ca. 94% at 200-120 m (vs. 4.5% silicoflagellates and 1.6% radiolarians). In general, a marine, warm-water (tropical/subtropical) diatom assemblage characterizes the plankton samples. Representatives of the Nitzschia bicapitata group are by far the most abundant contributors at both depth intervals (average=43%), ranging from ca. 30% in the North to ca. 60% in the South. Biogenic opal content in non-brine surface sediments is very low, (below 0.2 wt.% SiO2); and concentration of siliceous microorganisms is also low and of the order of 5*10**3-10**4 microorganisms/g dry sediment. Diatoms are the main contributors to the opal signal in the 20-40 µm fraction, while they share dominance with radiolarians in the >40 µm fraction. Total diatom concentrations average 1.2*10**4 valves/g in the 20-40 µm fraction and 4*10**3 valves/g in the >40 µm fraction. Robust taxa of warm water affinity (Alveus marinus, Azpeitia neocrenulata, Azpeitia nodulifera and Roperia tesselata) characterize the surface sediments. In contrast, biogenic opal content in brine surface sediment samples is much higher than in the non-brine samples, ranging from 2.8 to 3.8 wt.% SiO2, and concentration of siliceous microorganisms is 3-4 orders of magnitude higher. In addition here, diatoms dominate the opal signal. The taxa found in these samples are a mixture of non-brine and plankton samples, and fragile forms (e.g., N. bicapitata group, Neodelphineis indica) are well preserved in these sediments. Thus, brine sediments in this region seem to offer a great potential for palaeoenvironmental studies.