The aim of this study was to determine the effects of the tidal dynamics and physical forcing on phytoplankton distribution and photosynthesis dynamics in the Chubut River estuary, Argentina. Physical, chemical, and biological variables measured at the surface and at the bottom of the water column (variable between 0.5-m and 6-m depth) were made every 30-45 d during tidal cycles (13 h of duration) from June 2007 to November 2008 (11 tidal cycles). There was a remarkable consistency among different tidal cycles throughout the year, with strong stratification during the flood and almost complete mixing during the period high tide-ebb-low tide. Strong stratification during the flood resulted in significant inhibition of photosynthesis of mostly nanoplankton cells at the surface, while microplankton sank out of this upper layer and, thus, were less inhibited. Mixing conditions during the ebb, together with relatively high concentration of dissolved organic matter and particulate material, resulted in partial protection for phytoplankton against solar radiation stress and, therefore, relatively high maximum electron transport rate values were determined under this condition. However, the lowest photoinhibition and higher maximum rate of production (P max ) values occurred at depth during stratified conditions, probably due to relatively low solar radiation in this condition. Tidal dynamics together with physical forcing are key factors that condition the distribution, dynamics, and photoinhibition of phytoplankton in the Chubut River estuary.

A mesocosm experiment was conducted over a 4 week period to investigate the response of a phytoplankton community in a mountain freshwater reservoir to solar UV-B exclusion. The mesocosms were filled with water taken from the depth of 0.30 m in the reservoir, and exposed to near-ambient solar UV-B (+UV-B) and solar UV-B exclusion ()UV-B). The resulting chlorophyll-a (Chl-a), carotenoids, soluble sugars, algal composition and algal abundance were analysed. The study results indicated that carotenoids, carbohydrates and species distribution were affected under the )UV-B treatment. The carotenoid concentration was generally higher under the +UV-B treatment than under the )UV-B treatment. The sucrose and glucose concentrations were affected differently by the UV-B treatments, although both sugars strongly decreased under the +UV-B treatment by the end of the experiment. The sucrose and glucose concentrations exhibited similar distribution patterns to those of carotenoids for the +UV-B treatment, suggesting that these sugars could be directly involved in secondary metabolism. In contrast, the fructose content did not exhibit significant differences between UV-B treatments. The Chl-a concentration exhibited a progressive decrease during the experiment for both UV-B treatments. Although no significant impact of UV-B exclusion on the Chl-a concentration was observed in this study, a slightly higher concentration was observed for the )UV-B treatment. The total phytoplankton biovolume (BV) exhibited a pattern similar to that of the Chl-a concentration. Analysis of the phytoplankton composition revealed 52 species. However, because some species and classes were extremely difficult to identify, only 46 species from seven classes (Cyanophyceae, Chlorophyceae, Bacillariophyceae, Euglenophyceae, Charophyceae, Dinophyceae, Xanthophyceae) were unambiguously identified and considered for the UV-B treatments. Chlorophyceae, Bacillariophyceae and Cyanophyceae comprised about 78% of the identified species. Aulacoseira granulata and Cyclotella meneghiniana, followed by Ceratium hirundinella, were the most abundant species. Principal component analysis, using sampling periods and algal densities (PCA-1), indicated a heterogeneous phytoplankton assemblage, whereas PCA-2, with algal densities and biochemical variables (soluble carbohydrates, Chl-a and carotenoids), indicated that biochemical parameters could provide qualitative information about La Angostura phytoplankton community responses to quality of the sunlight.

No abstract

A natural plankton population from the eutrophic lake Cacique Chiquichano, in the Argentine Patagonia, was monitored for one year to evaluate changes in photosynthetic parameters as a result of exposure to ultraviolet radiation (UVR, 280-400 nm), grazer abundance, and the taxonomic composition of the phytoplankton community. Both physical (temperature, solar radiation) and biological (grazers, taxonomic composition, photosynthetic parameters) variables fluctuated throughout the study. Crustacean zooplankton showed alternating dominance between cladocerans (Daphnia spinulata) and copepods (Metacyclops mendocinus). The phytoplankton community underwent concomitant changes throughout the year, with cyanobacteria and diatoms alternately dominating. In addition, although copepod abundance was not significantly related to changes in phytoplankton, the presence of D. spinulata was significant during periods of more transparent water; these periods were dominated by diatoms. On the other hand, cyanobacteria dominated the phytoplankton assemblage when the penetration of solar radiation into the water column was lower. Photosynthetic inhibition due to UVR decreased during the diatom-dominated periods. In contrast, inhibition increased along with the proportion of cyanobacteria, likely as a result of acclimation to low irradiance during the lake’s phase of lower transparency. Moreover, the presence of D. spinulata was associated with the increased penetration of solar radiation into the water column, resulting in an indirect increment in the inhibition of cyanobacteria photosynthesis. The results suggest that both solar radiation and grazing abundance strongly influence the dynamics and photosynthetic activity of the phytoplankton in Lake Cacique Chiquichano.

Resumo Se desarrolla un estudio matemático de oscilaciones amortiguadas centrado en el cálculo de las constantes de la ecuación de movimiento, su significado matemático y sus implicancias en el comportamiento de los sistemas. Se ejemplifica el método de los valores iniciales para el cálculo de las constantes, tanto en oscilaciones libres como amortiguadas. Se deducen expresiones algebraicas para el modelo convencional amortiguado y se discuten comparativamente con las del formalismo de Crawford. Se discute el significado de las constantes y se ofrecen respuestas a algunas preguntas frecuentes entre estudiantes y profesionales. Se introduce el concepto de “función exponencial secante” y se establece la diferenciación entre ésta y la envolvente de la ecuación de movimiento. Se dan ejemplos de los errores sistemáticos cometidos al imponer arbitrariamente las constantes, y se discute su significado y razón matemática. En el apéndice, se deduce la ecuación general de oscilaciones amortiguadas y se ejemplifican los tres casos posibles de sistema (sobreamortiguamiento, amortiguamiento crítico, y oscilaciones subamortiguadas). Se deduce el modelo convencional de oscilaciones amortiguadas como un caso particular de la ecuación general.