Silicoflagellates as productivity indicators: Evidence from long temporal and spatial flux variability responding to hydrography in the northeastern Pacific
Authors: Takahashi Kozo 
Published: February 2008

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.