Silicoflagellate Species Research Articles

The spatial distribution of silicoflagellates in the region of the Gulf Stream warm-core ring 82B: application to water mass tracer studies

To delineate potential water mass affinities, we investigated silicoflagellates from the region of Gulf Stream warm-core ring (WCR) 82B in the northwestern Atlantic. Silicoflagellates from 202 samples from N-S and an E-W transects across WCR 82B during late April were analysed. Shelf to Sargasso Sea transects, one completed in early May and the other in June 1982 were also examined. Eight to 11 vertical profiles to 200 m comprised each of the transects. Six taxa of silicoflagellates were found in the samples studied and a total of more than 8000 specimens were encountered. Three major taxa dominated standing stocks: Distephanus speculum, Dictyocha messanensis (intermediate-sized form) and D. mandrai. D. speculum, considered a cold-water taxon in the literature, showed a higher standing stock in the cooler high-velocity region (HVR) of the warm-core ring, continental shelf (SH) and slope (SL) waters. Fewer were present in the wanner ring center (RC), Gulf Stream (GS) and Sargasso Sea (SS). D. mandrai showed a similar distribution to that of D. speculum, but its preference for slightly warmer waters (>~10°C) was noted. In contrast, Dictyocha messanensis (intermediate-sized) and Distephanus pulchra, known to be warm-water taxa, were relatively abundant in the warm ring center. In contrast to standing stock data, ratios between cold- and warm-water taxa correlate well with temperature and salinity in the warm-core ring. Since these ratios are not effected by convective loss, they are excellent water mass tracers in this system. Distribution of the silicoflagellate taxa suggests that WCR82B April had a higher affinity with the Gulf Stream than the Sargasso Sea. Scores derived from factor analysis indicate that silicoflagellate species distributions are highly correlative with water masses. This was evident from correlations with temperature, salinity and with distance from ring center. Nutrients were generally not correlated with species data. This may be due to deep vertical mixing in the ring center of 82B in April that resulted in loss of plankton cells below the euphotic zone which was responsible for low silicoflagellate standing stock. Silicoflagellate productivity, measured as % double skeletons of D. speculum, was approximately the same in all water masses. This result is consistent with the hypothesized plankton population loss due to the convection.

Silicoflagellates as productivity indicators: Evidence from long temporal and spatial flux variability responding to hydrography in the northeastern Pacific

To study the temporal variability of oceanic productivity, time series particle fluxes were monitored at pelagic Station Papa (50°N, 145°W) in the eastern subarctic Pacific from September 1982 through August 1986. Automated Particle Flux (PARFLUX) time series sediment traps were deployed at 3800 m and 1000 m in the 4200‐m deep pelagic ocean. The traps collected samples for 2‐week periods for an almost continuous 4‐year‐long particle flux record. In order to study spatial flux variability, an additional trap at Station C (49.5°N, 138°W) was occupied during 1985‐1986 where particle fluxes were measured at 3500 m in the 3900‐m deep water. Time series flux samples from Station C were synchronized with those at Station Papa for comparison with each other. In order to evaluate their value as productivity or temperature indicators, species of silicoflagellates and Actiniscus, a dinoflagellate genus with siliceous endoskeletons, were examined from the time series trap samples. Their fluxes were enumerated for a total of 119 samples from Station Papa and 24 samples from Station C. Among seven silicoflagellate taxa, Distephanus speculum and Dictyocha mandrai were dominant taxa which constantly represented >70 % of total silicoflagellate assemblages. All of the taxa showed significantly suppressed fluxes during 1984 which were correlated with the σt changes centered around 100 m depth. It is hypothesized that the suppressed fluxes were attributable to a decrease in convective nutrient (or trace elements) supply. The flux of Distephanus speculum, a productivity indicator, was negatively correlated with the diversity index of silicoflagellates which is considered to be another productivity measure. Seasonal patterns of D. speculum and D. mandrai percentages also follow patterns of D. speculum flux and silicoflagellate diversity, either parallel or inversely. Thus, D. speculum flux trends, diversity indices and percent contribution to silicoflagellate assemblages reflect variability in upper water silicoflagellate production rate which is a consequence of hydrographic conditions. It is concluded that Stations Papa and C, 600 km apart, are situated in the same oceanic province with similar ecosystems. This is based on their exhibiting the same species composition, and very similar seasonal patterns of fluxes, and the percentages of species in the assemblages from the two stations. Most silicoflagellate species fluxes at Station C were approximately one half of those measured at Station Papa which can be explained by differences in upper water σt values relative those below 200 m. Thus, silicoflagellate data clearly suggested lower fertility of the upper waters at Station C than that at Station Papa. Flux maxima at Station C generally lagged behind Station Papa by two weeks, resulting from lower σt at values in the seasonal mixed layer. The importance of silicoflagellate fluxes as an indicator of seasonal mixed layer σt is demonstrated in the regional comparisons of fluxes and hydrography. Generic ratios of Dictyocha/Distephanus were more dependent on production rates than temperature, on the contrary to a previous hypothesis in the literature. Apparent correlations between the ratios and temperatures which were obtained in previous basin‐wide studies may be indirect results of the fertility of upper water masses which are in part governed by temperature and hence stability of the waters on a global scale.

Radiolarian and silicoflagellate response to oceanographic changes associated with the 1983 El Niño

Strong seasonal signals in fluxes and composition of siliceous microfossils have been recorded in sediment traps from two sites in the equatorial Pacific, deployed from December 1982 to March 1984. During the early part of the sampling period, the 1982–83 El Nino event had a profound effect on the radiolaria and silico-flagellates within these two areas. During the El Nino, the radio-larian trap assemblages at Site C (1° N, 139° W) most resembled faunal assemblages in western equatorial Pacific sediments, whereas the trap assemblages resembled equatorial divergence sediments in the latter half of the period. At Site S (11° N, 140° W), the radiolaria and silicoflagellate species can be correlated with organic carbon fluxes. In general, silicoflagellate shell fluxes are correlated to total opal fluxes where radiolarian fluxes do not exhibit this relationship. Approximately 20–25% of the total count of radiolaria in traps are not present in underlying sediments with a significant loss of the weakly silicified forms. However, the comparison of trap sample assemblages with underlying sediments reported here shows that dissolution does not alter relative abundances of the fossil species used in palaeoclimatic reconstructions. A significant difference is observed between the silicoflagellate trap assemblage and the underlying sediment assemblage.

Seasonal variation of silicoflagellates in phytoplankton and varved sediments in the Gulf of California

Phytoplankton samples collected during two seasons (summer-autumn and winter-spring) in the Gulf of California were analyzed for their silicoflagellate species composition. Correlation of species abundances to measured temperature, salinity, chlorophyll, and distribution in high quality surface sediment samples revealed distinct correlation and distribution patterns for the following species: Distephanus pulchra is a central Gulf species being most common during increased productivity seasons, Dictyocha fibula var. robusta characterizes assemblages during decreased upwelling seasons (summer-autumn), and Dictyocha calida, D. californica and D. mandrai inhabit waters originating from outside the Gulf. The surface sediment distribution mirrors closely the averaged seasonal distribution as present in the phytoplankton with only minor indications of lateral displacement. The seasonal variability is best demonstrated in the importance and composition of factor 2 ( Q-mode factor analysis): Dictyocha fibula var. robusta is important during the summer-autumn season, whereas Dictyocha californica dominates this factor during the winter-spring season. A temperature prediction equation based on silicoflagellate abundances was developed with a 1.14°C standard error of estimate over the temperature interval 20°–28°C.

Paleogene paleoceanography of the Arctic Ocean is constrained by the middle or late Eocene age of USGS Core Fl-422: Evidence from silicoflagellates

Arctic Ocean Core Fl-422 has been of central importance in Arctic tectonics and paleoceanography because it provides the sole evidence for early Cenozoic marine conditions in the Arctic. The presence of several Eocene and Eocene or Oligocene guide species of silicoflagellates in samples from this core shows that it is no older than middle Eocene, and is not Paleocene as previously reported. There is no evidence of any Paleocene (54 to 65 Ma) siliceous microfossils from Core Fl-422. Paleoceanographic inferences conerning the Cretaceous-Tertiary boundary and the timing of silica deposition in the Arctic have been erroneous because of unsupported biostratigraphic correlations.

Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

Plankton and surface sediment samples from the Gulf of California were examined to determine the present geographic distribution of silicoflagellate species in this area. Variations in the species composition of the silicoflagellate assemblage were found to be related to water mass distributions. Eight species were identified in these samples. Octactis pulchra is associated with high levels of primary productivity in the surface waters and is found in greatest abundance in the central Gulf of California. Dictyocha messanensis dominates the silicoflagellate assemblage in stations outside the Gulf of California and increases in relative abundance with decreasing amounts of Octactis pulchra. Dictyocha calida, Dictyocha sp. A, and Dictyocha sp. B are associated with equatorial waters and have the highest relative abundance near the mouth of the Gulf. Dictyocha epiodon and Distephanus speculum are associated with cold California Current Water, and Dictyocha epiodon is present in minor abundance in Gulf samples. Dictyocha sp. 2 has a patchy distribution with low relative abundance.

Occurrences of Silicoflagellates from Central North Pacific Cores: ABSTRACT

Silicoflagellates are siliceous marine planktonic microorganisms which have never been a significant contributor to pelagic sediments. Consequently only limited investigations have been made on these microorganic remains from the deep-sea deposits of the world. Eleven taxa and one new species of silicoflagellates were recovered from the central North Pacific cores collected above the RV Thomas G. Thompson in 1968. The lower lithologic unit of a gravity core, TT28-25, was assigned a Miocene age after careful comparison of previous worldwide records. This is the first time that such a Miocene assemblage has been recognized from mid-latitude North Pacific sediments. From other cores, Quaternary assemblages were noted, including a limited occurrence of Mesocena cf. elliptica. The most recent extinction of this species seems to coincide with the last appearances of Dictyocha cf. ausonia and a proposed new species, D. subarctios. Their stratigraphic distribution was examined critically in connection with the results of paleomagnetic-reversal recor s and data on other microfossils from the area. It is suggested that silicoflagellates could become a useful tool for deep-sea biostratigraphy in the middle latitudes of the North Pacific Ocean. End_of_Article - Last_Page 558------------

Fossil Diatoms and Silicoflagellates from Newport Beach, California, Studied with Scanning Electron Microscope: ABSTRACT

With the advent of the scanning electron microscope (SEM) a new dimension was opened to the micropaleontologist. At its maximum useful magnification of 50,000 diameters, the SEM offers a depth of field of about 10 µ and a resolution of about 200 A, whereas the optical light microscope, at its maximum useful magnification of 1,300 diameters, offers a depth of field of about 0.5 µ and a resolution of about 2,00 A. Inasmuch as the classification of diatoms and silicoflagellates is based on the general and detailed characters observed with the light microscope, the use of the SEM must be viewed as an extension of the facilities offered by the light microscope. Ten species of diatoms and silicoflagellates from Miocene rocks, exposed at Newport Beach, California, have been studied with the light microscope and the scanning electron microscope. A comparison of the micrographs of the identical species taken with the two instruments clearly demonstrates the ability of the SEM to provide a more detailed description of taxa, a more precise circumscription of taxa, a separation of closely allied taxa, and a verification of many varieties of taxa. End_of_Article - Last_Page 558------------