Maria Alexandra Chícharo

RNA:DNA Ratio and Other Nucleic Acid Derived Indices in Marine Ecology

Some of most used indicators in marine ecology are nucleic acid-derived indices. They can be divided by target levels in three groups: 1) at the organism level as ecophysiologic indicators, indicators such as RNA:DNA ratios, DNA:dry weight and RNA:protein, 2) at the population level, indicators such as growth rate, starvation incidence or fisheries impact indicators, and 3) at the community level, indicators such as trophic interactions, exergy indices and prey identification. The nucleic acids derived indices, especially RNA:DNA ratio, have been applied with success as indicators of nutritional condition, well been and growth in marine organisms. They are also useful as indicators of natural or anthropogenic impacts in marine population and communities, such as upwelling or dredge fisheries, respectively. They can help in understanding important issues of marine ecology such as trophic interactions in marine environment, fish and invertebrate recruitment failure and biodiversity changes, without laborious work of counting, measuring and identification of small marine organisms. Besides the objective of integrate nucleic acid derived indices across levels of organization, the paper will also include a general characterization of most used nucleic acid derived indices in marine ecology and also advantages and limitations of them. We can conclude that using indicators, such RNA:DNA ratios and other nucleic acids derived indices concomitantly with organism and ecosystems measures of responses to climate change (distribution, abundance, activity, metabolic rate, survival) will allow for the development of more rigorous and realistic predictions of the effects of anthropogenic climate change on marine systems.

Changes in a temperate estuary during the filling of the biggest European dam

This study aimed to determine whether and how the disruption of river flow, during the filling of the Alqueva dam, influenced the variability of abiotic and biotic factors in the Guadiana estuary, particularly the abundance and distribution of anchovy eggs. River inflow was found to be the most important factor in determining abiotic and biotic variability in the Guadiana estuary. Seasonal patterns were obscured by long periods of low inflow (mid April to early December 2002), which caused marked changes in the estuary. The estuarine turbidity maximum zone was displaced towards the upper estuary, to at least 38 km from the river mouth, 8 to 16 km upstream from previous records. The dynamics of nutrient stoichiometry was also affected. In the upper and middle estuary, P was more potential limiting than N and potential Si limitation was only frequent on the coast, with direct and/or indirect influence in changing phytoplankton dynamics and composition. Previously, the upper estuary alternated between potential P limitation during winter, Si limitation during spring and mid summer and N limitation during mid summer and autumn. The flooding of vast areas in the catchment of the dam probably caused the increase of DSi concentrations, as well as maximal N and P loadings. The abundance of larval stages of anchovy decreased, putatively because estuarine productivity has also decreased. In April 2002 there was an uncontrolled discharge from the Alqueva dam, which reduced the abundance of anchovy eggs by 99.99%. It is suggested that dam managers should mimic, as much as possible, the natural river flow, in order to minimize the impact on downstream ecosystems. Management efforts should not be restricted to the areas upstream of the dam, but should also encompass the estuary and adjacent coastal area.

Status of the Guadiana Estuary (south Portugal) during 1996-1998: An ecohydrological approach

The aims of this study were to monitor basic hydrological and ecological characteristics of the Guadiana Estuary (1996-1998) before the construction of the Alqueva dam. This work was carried out to determine how environmental factors affect seasonal and tidal variations of plankton populations in the estuary. The available information on the subcatchment of the estuary (e.g., urban, agricultural and forested areas) was integrated into a geographic information system-based software program. Mean monthly river flow varied markedly on a seasonal and yearly basis. River flow near Mértola (ca 50 km upstream from the mouth) reached 3400 2 10 6 m 3 in winter and decreased to 42 2 10 6 m 3 in summer. With respect to nutrients, nitrogen to phosphorus ratios indicated some limitation by phosphorus, except at the end of summer, when nitrogen limitation appeared. During this period, cyanobacterial blooms usually occurred in the upper/middle estuary. Estuarine Turbidity Maximum may significantly influence the retention of zooplankton in the estuary. The fish larval life cycle, especially sensitive to environmental alterations, showed high ratios of ribonucleic to desoxyribonucleic acids indicating good physiological condition. It was concluded that an ecohydrological approach, allowing integration of different elements from the cellular to the habitat level into a geographic information system, can contribute to a better understanding of the processes that influence the aquatic biota of the estuary. The approach will be a useful assessment tool for monitoring the estuary following dam completion.

Diel variation of the RNA/DNA ratios in Crassostrea angulata (Lamarck) and Ruditapes decussatus (Linnaeus 1758) (Mollusca: Bivalvia)

The aim of this study was to investigate the effect of time of day on RNA/DNA ratios among fed and starved Crassostrea angulata and Ruditapes decussatus juveniles. Sampling to investigate the day and night condition of juveniles was carried out for 48 h. A highly sensitive method for nucleic acid quantification was applied to bivalves. The results suggest that there is some variation in nucleic acid quantities with the time of the day. For the two species analysed, the RNA/DNA ratio was particularly high during the night and was higher in the fed animals. The results seem to indicate that there is some endogenous rhythm in the production of RNA. If there are diel changes in RNA/DNA ratios, it follows that average RNA/DNA ratios can be unrepresentative if there is any day or night bias in sampling.

Comparison of RNA/DNA ratios obtained with two methods for nucleic acid quantification in gobiid larvae

In this paper, RNA and DNA contents and RNA/DNA ratios obtained with two fluorometric methods for the extraction and quantification of nucleic acids in individual estuarine gobiid larvae are compared. A total of 141 gobiid larvae, collected in the Mira and Guadiana estuaries (Portugal), were analysed. This study indicated that the nucleic acids content and RNA/DNA ratios of individual larvae varied with the procedure used. The two methods yielded similar log DNA values but different log RNA and log (RNA/DNA). The differences could be due to a number of factors related to specific steps of tested protocols. Comparison of nucleic acids contents and calibration of the two methods by regression analysis provided a reasonable basis for comparison although differences between estimates were evident.

Phytoplankton dynamics in a coastal saline lake (SE-Portugal)

Abstract The aim of this study was to characterise phytoplankton dynamics in a coastal saline lake, pinpointing putative biotic and abiotic regulatory variables of its succession and productivity. Between February and September 1998, samples for the analysis of physical, chemical and biological variables were taken fortnightly (except in February and April). The phytoplankton community showed three distinct periods of evolution. The first period (February-March) was characterised by a chroococoid non-colonial cyanobacteria bloom (maximum abundance, 4.3 × 10 9 cells l –1 ) and also by its decaying. Long water residence and/or nitrogen limitation might have allowed cyanobacteria dominance; while its decaying could be associated to predation by aplastidic nanoflagellates and/or to the beginning of periodical partial renewal of lake water with water proceeding from an adjacent coastal lagoon (Ria Formosa). The second period (April-early August) can be differentiated, from the previous, by reduced abundances of phytoplankton (minimum abundance, 5.7 × 10 6 cells l –1 ) and plastidic nanoflagellates dominance. The overall low nutrient concentrations, likely as a consequence of periodical partial water renewal, could explain these results. In the last period (late August-September), increased phytoplankton abundance and the development of a diatom and mixotrophic dinoflagellate bloom was probably the result of a sudden increase in nutrient levels, occurring after a period of intense precipitation. In consequence, primary production reached a maximum value of 1367 mg C m –3 h –1 ; 36 times higher than a maximum value previously reported for Ria Formosa.

Chronic effects of dredging-induced stress on the clam (Spisula solida): nucleic acid and lipid composition

Responses of the clam Spisula solida to stress imposed by dredging were analyzed in terms of changes in chronic indices of biochemical conditions (RNA/DNA ratio and neutral/polar (N/P) lipid ratio). Cumulative stress on undersized (<25 mm) S. solida from repeated habitat disturbance by dredging was simulated in the laboratory and measured with in situ studies off the southern coast of Portugal, in April and July 1999. Laboratory simulations on undersized bivalves indicated decreases in RNA/DNA and N/P lipid ratios. Responses were sublethal; however, even though survival was not directly threatened, decreases in condition suggest that bivalves are more susceptible to predation when they have been left in the seabed after the dredging activity. Moreover, the in situ study revealed that this effect could be especially critical during spawning.