Articles

Keywords: cell division; dinoflagellates; DNA; Gymnodinium nelsonii; Pyrrophyta; RNA; turbulence

Laboratory experiments were conducted to study the effects of agitation on growth, cell division, and nucleic acid dynamics of the dinoflagellate Gymnodinium nelsonii Martin. When cultures were placed on an orbital shaker at 100 rpm, cell division was prevented, cellular volume increased up to 1.5 times that of the nonperturbed cells, the form and location of the cell nucleus were modified, and the RNA and DNA concentrations per cell increased up to 10 times those of the controls. When shaking was stopped after 10 days, cells divided immediately at about 2/3 of the division rate of the unshaken populations, and all the altered parameters were restored. If the agitation continued for more than 20 days, total cell death and disintegration occurred. Several cellular types differing in size and shape were observed in the control and shaken cultures. One possible hypothesis for these results is that failure of the cell to divide results from physical disturbance of the microtubule assemblage associated with chromosome separation during mitosis. My study suggests that small-scale oceanic turbulence of sufficient intensity may inhibit growth of individual dinoflagellate cells, but immediate development of the population may continue when calm weather follows the active mixing period.

Keywords: Heterocapsa - Olisthodiscus - RNA/DNA - cell cycle

The division cycle of two phytoplankton species, Olisthodiscus luteus and Heterocapsa sp. was studied in relation to a 12:12 light:dark cycle. Batch cultures in exponential phase were sampled every three hours during 48 hours. Cell number, cellular volume and DNA and RNA concentrations were measured. Microscopic observations of the nuclei of Heterocapsa sp. were also performed. In both species, cell division took place in the dark. In Heterocapsa sp., DNA and RNA showed a similar diel variability pattern, with synthesis starting at the end of the light period, previously to mitosis and cytokinesis. In O. luteus. Major RNA synthesis occurred during darkness, and DNA was produced almost continuously. Both species presented different values and diel rhythmicity on the RNA/DNA ratios.

The notion that mesoscale phytoplankton heterogeneity results from hydrographic variability was tested, in the basis of data gathered in 39 cruises in diverse marine environments, to provide an explanation for cross-system differences in the extent of phytoplankton heterogeneity. This analysis showed that differences in the extent of phytoplankton heterogeneity are closely related to differences in the extent of hydrographic heterogeneity in moderately complex systems, but not in hydrographically complex systems (e.g. estuarine systems). Thus, phytoplankton heterogeneity was greatest in systems with intermediate hydrographic heterogeneity, and failed to increase in more complex environments. The limited phytoplankton heterogeneity in hydrographically complex systems, such as estuaries, can be partially attributed to delay in phytoplankton growth response following perturbations. We thus conclude that phytoplankton growth and, therefore, growing conditions appear to have a dominant bearing on the extent of mesoscale phytoplankton heterogeneity in moderately complex systems, whereas phytoplankton heterogeneity in systems with complex hydrography is primarily controlled by physical processes.

Keywords: mixing regime - Laboratory Scale Enclosures - phytoplankton - photosynthesis

The photosynthesis of plankton sampled from the eutrophic Lake Loosdrecht was studied in Laboratory Scale Enclosures (LSEs) with regard to the rate of mixing. First, two LSEs were operated at different mixing rates. No significant differences in photosynthetic performance were found, with the exception of a depressed photosynthesis in the afternoon in the LSE which had a low mixing rate. Secondly, when mixing was stopped, the phytoplankton which stayed in the dark due to the steep light gradient in the LSE responded by changing its maximal photosynthetic capacity. The results show that the filamentous cyanobacteria in the lake can respond rapidly to changes in the depth of the mixed layer by altering their photosynthetic performance.

Keywords: Heterocapsa sp. - Olisthodiscus luteus - cycles - pigments - HPLC

Photosynthetic pigment composition was studied in batch cultures of Heterocapsa sp. and Olisthodiscus luteus growing exponentially in a 12:12 light:dark cycle. Both species divided in the dark. The synthesis of pigments was continuous for both species. However for chlorophyll c and peridinin, in Heterocapsa sp., and chlorophyll c and fucoxanthin, in O. luteus, (pigments belonging to light harvesting complexes) the synthesis was significantly higher during the light period. Concentrations per total cell volume (TCV) of chlorophyll a, chlorophyll c, peridinin and diadinoxanthin in Heterocapsa sp., and chlorophyll a, chlorophyll c, fucoxanthin and violaxanthin in O. luteus, showed a maximum at the onset of light and decreased during the light period. The values of the chlorophyll a:chlorophyll c, chlorophyll a:peridinin and chlorophyll a:fucoxanthin ratios are compared with data reported in the literature.

Keywords: phytoplankton - respiration - diel changes - carbohydrate

The dark respiration of a natural plankton community from an eutrophic lake was studied in a laboratory scale enclosure (LSE), exposed to illumination which simulated natural light conditions in the water column. The dark respiration was measured continuously for 2 hours in samples obtained from the LSE each hour for 26 hours. The relationships between dark respiration rates, carbohydrate concentrations and other parameters were investigated. The dark respiration rate showed an exponential decrease with time in the dark in all light period incubations with a time coefficient of 0.3 h–1. The decrease in respiration rate in the dark period was much slower, reaching an approximately constant level at the end of the night. The overall dark period decline in respiration rate also exhibited an exponential pattern, but with a much lower time coefficient (0.04 h –1) than for the light period incubations. A linear relationship was found between dark respiration rate and carbohydrate concentration at night time but no relationship was apparent during the day. A comparison between these data and data from the literature show that this pattern of dark respiration rate decrease with time in the dark may have some general applications for dense phytoplankton communities.

Keywords: Physical oceanography, surface layer currents, microwave remote sensing. SAR, ERS-1.

The synthetic aperture radar (SAR) is an active sensor working in the frequency range of the microwaves. Its distinctive characteristic is the synthesis of a large antenna through the study of the phase of the received pulses over an integration time. The possibility of producing high resolution images regardless of the weather conditions makes SAR a very suitable sensor for the remote sensing of the ocean surface. In this paper we present the main characteristics of SAR, several techniques of image processing, and its applications to the study of sea surface structures, especially those related to mesoscale currents. We describe one of the first SAR images acquired by the European ERS-1 satellite in 1991 in the western Mediterranean.

The pattern of microgrowth bands in shell sections of the Mediterranean clam Chamelea gallina shows a seasonal variation in width. Widely spaced growth bands are formed in winter and late spring between January and June, whilst narrow growth increments are deposited in summer and early autumn between August and October, following the maximum summer seawater temperature. The warm summer seawater temperature of 33 "C in the western Mediterranean may be responsible for the suppression of shell growth, as evidenced both by the narrowing of the growth increments and by the later appearance of clefts in shell sections. There is no correlation between spawning and the formation of clefts in the shell of C. gallina. The annual pattern of narrow and wide growth increments has been used to estimate the age of C. gallina and to construct growth curves for this species from the eastern coast of Spain. Growth curves for the clams, determined from the internal microgrowth bands and from the spacing between the clefts in shell sections, differed from curve calculated using surface growth rings

The coast of Catalonia was divided into three zones on the basis of heterogeneity in their physical features and in the structure of the larval fish populations. Both physical and biological processes are normally more stable overall in the middle zone. However, in May 1983 an anomalous hydrodynamic event took place over the continental shelf in this zone and resulted in a major alteration in the structure of the larval fish population. An intense inverted density front changed the direction of coastal flow. The resulting density gradient confined larvae inshore in a zone in which the main larval concentrations are normally located over the shelf break during the rest of the year. This anomalous distribution of the larval fish population at that time was a consequence of the unpredictable hydrographic event and contrasted with the structure of the larval population over the rest of the year, produced by a more constant hydrodynamic pattern.

The coast of Catalonia was divided into three zones on the basis of heterogeneity in their physical features and in the structure of the larval fish populations. Both physical and biological processes are normally more stable overall in the middle zone. However, in May 1983 an anomalous hydrodynamic event took place over the continental shelf in this zone and resulted in a major alteration in the structure of the larval fish population. An intense inverted density front changed the direction of coastal flow. The resulting density gradient confined larvae inshore in a zone in which the main larval concentrations are normally located over the shelf break during the rest of the year. This anomalous distribution of the larval fish population at that time was a consequence of the unpredictable hydrographic event and contrasted with the structure of the larval population over the rest of the year, produced by a more constant hydrodynamic pattern.

Abstract--Water mass distribution and geostrophic circulation off Namibia are presented for April 1986, a month of relatively strong warm water intrusion from the north and west. The main result is that mesoscale processes develop in the area during the season when the coastal upwelling is relaxed, and these processes may be an alternative contribution to the fertilization of the region. The main features observed are the following. Angolan waters entered the region through the surface layer, with a maximum flow at 50 m depth. A frontal zone, centered between 20°S and 22°S separated Angolan and Benguela waters, at the surface layer. Anticyclonic eddies with a radius of 15-30 miles occurred off central and southern Namibia. Water upwelling off Walvis Bay appeared to derive from the core of the Angolan water, which laid at a depth of 50-100 m. Anticyclonic gyres might contribute to the upwelling off Walvis Bay and to the occurrence of a local sinking in its centre, about 50 miles offshore, thus allowing long residence time and recirculation. The occurrence of eddies may have been related to the general relaxation of the Benguela Upwelling System in the preceding 3 months and the penetration of the Angolan waters, in the region north of Walvis Bay, while in the south, the meanders of the oceanic front may remain most important.

Near-inertial motion on the shelf-slope front off northeast Spain was monitored using surface drifters and moored current meters. On the shelf, strong inertial currents were generated by a wind burst. The inertial current amplitude was about 70 cm/s at the surface, 30 cm/s at the base of the mixed layer, and 10 cm/s in the interior. The observed near-inertial frequency on the shelf was about 10% lower than the local inertial frequency, suggesting that the near-inertial motion was embedded in region of strong anticyclonic shear. Also, the phase of near-inertial motion increased through the water column, indicating that the energy propagation was downward. By contrast, the surface inertial currents were only about 10 cm/s in the center of the shelf-slope front. Indirect evidence suggests that the observed small surface inertial currents were the result of rapid downward transfer of near-inertial energy in the front.