Rolando Quirós

The Effects of Hydrology on Plankton Biomass in Shallow Lakes of the Pampa Plain

Climatic and hydrological variability is usually high in the Pampa Plain (Argentina). However it has not studied yet how this variability may affect the phytoplankton and zooplankton biomass and community structure in aquatic systems of this region. The main purpose of this study was to assess flushing effects on nutrient and plankton dynamics in two interconnected very shallow lakes of the Pampa Plain. In order to study the impact of hydrology on the plankton biomass and community structure, we compared the summer plankton community among three consecutive years with contrasting hydrological characteristics. Water residence time varied an order of magnitude among years and this variability was correlated to strong changes in physicochemical and biological lake characteristics. Depending on the water discharge level, the hydrological regime within the lakes ranged from lentic to more lotic conditions. Nutrient and phytoplankton biomass were positively related to water discharges. During high flushing periods, nutrients import from intensive agriculture lands leads to a dramatic increase in trophic conditions. On the other hand, macrozooplankton biomass was positively related to water residence time and showed a dramatic decrease during high flushing years. Rotifers biomass was not affected by interannual water discharge variability during the study period. Our results support that in case of lakes with high flushing rates, zooplankton development is dependent on water residence time and that hydrology may have stronger effects on macrozooplankton biomass than top-down control by planktivores.

Factors related to variance of residuals in chlorophyll — total phosphorus regressions in lakes and reservoirs of Argentina

Data from an extensive mid-summer survey of Argentinian lakes and reservoirs were used to study the effects of variability in internal processes on the residual variance in chlorophyll — total phosphorus (CHL-TP) regression models. These effects were compared with those related to the external characteristics of lakes, e.g. climate, morphometry and nutrient status. Zooplanktivorous fish biomass, mean macrozooplankton size, dissolved oxygen at the sediment-water interface, and submersed macrophyte development, were shown to be significant in explaining residual variance in CHL-TP for Argentinian lakes and reservoirs. The results suggest that those variables have discontinuous or threshold effects on CHL-TP residuals among lakes. Significant differences were also shown between CHL-TP regressions for lakes with and without zooplanktivorous fish and for lakes with small or large macrozooplankton. Lakes with zooplanktivorous fish, small body size macrozooplankton, and very low hypolimnetic oxygen also have higher chlorophyll levels than predicted from CHL-TP regression models.

Fish effects on trophic relationships in the pelagic zone of lakes

To examine the relationships between the biomass of pelagic communities and to study how these relationships differ between lakes with different fish assemblages, we used data on nutrients, chlorophyll, zooplankton, and fish from 96 Argentinean lakes and reservoirs. Significant differences in regressions between chlorophyll and total phosphorus, macrozooplankton and chlorophyll, and between total fish and macrozooplankton, in lakes with different fish assemblages, were found. Lakes with zooplanktivorous fishes but without piscivores, and lakes without planktivores, had the highest and the lowest phytoplankton biomass, respectively. The regression line for lakes with both types of fish were intermediate. Furthermore, lakes with planktivores, but without piscivores, had the lowest macrozooplankton. The analysis of the trend in simple regression residuals suggests that the changes in chlorophyll, macrozooplankton, and total fish with lake trophy, are related to changes in fish assemblages.

The hydrochemistry of the inland waters of Argentina: a review

The major ion chemistry of 639 aquatic localities in Argentina is presented and discussed. Salinities range from quite fresh ( 300 g L−1); a variety of ion dominances occur but NaCl predominates in highly saline waters. The principal mechanisms controlling water chemistry are rock dominance and evaporation; atmospheric contributions are less important.

Cascading trophic effects in pampean shallow lakes: results of a mesocosm experiment using two coexisting fish species with different feeding strategies

Planktivorous fish, both visual predators and filter feeders, enhance eutrophication processes in lakes. In pampean shallow lakes several planktivorous species may coexist but often two species dominate: silverside (Odontesthes bonariensis), a visual planktivorous fish when young adult, and sabalito (Cyphocharax voga), an omnivorous filter feeder. To assess the relative strength of the cascading trophic effects of the two species, a mesocosm experiment was conducted using different proportions of both species as treatments. Differences were found in water transparency, phytoplankton biomass, crustacean abundance, individual size and biomass. Our results suggest that visual predators intensify eutrophication effects more strongly than filter feeders do.

The Effects of Fish Assemblage Composition on Lake Water Quality

ABSTRACT Current trophic cascade theories predict that changes in the piscivore to planktivore biomass ratio will cascade down the food web, and will cause both chlorophyll and water clarity to deviate from nutrient – predicted values in individual lakes. An extensive survey of mid summer observations for nutrients, phytoplankton, zooplankton, and fish in Argentinean lakes and reservoirs, were used to study the effects of planktivorous and piscivorous fishes on chlorophyll concentrations and Secchi disk transparency. Regression analysis for different subsets of the data was used in this study. Regressions of chlorophyll on total phosphorus and Secchi disk on total phosphorus differed significantly among lakes with different fish assemblages. At comparable total phosphorus concentrations, lakes with planktivores but without piscivores had the highest phytoplankton biomass and the lowest water clarity. In conclusion, the results show that the fish assemblage composition has a dramatic effect on chlorophyll ...

Fish diversity and ecology, habitats and fisheries for the un-dammed riverine axis Paraguay-Parana-Rio de la Plata (Southern South America)

The riverine complex Paraguay-middle Parana-Rio de la Plata extends more than 3700 km southwards from its sources in the western hills of the Brazilian Shield to its discharge into the Rio de la Plata River. The high variety of habitats is reflected in the large diversity of its fish communities, which are dominated by characiform and siluriform fish species. The potamic axis is evolutionarily and ecologically open to fish movements and there are more than 400 fish species listed for the whole system, but only 100 species are common to both upper and lower basins. However, data limitations in some portions of the system need to be addressed before creation of an ichthyogeographic classification. The river basins that make up the potamic axis are low to medium developed and environmental pressures are unevenly distributed. Chemical pollution is a concern throughout. In the Pantanal, small hydro-projects and sedimentation from agricultural activities have had adverse effects on fish habitat inducing a loss of fish diversity. Un-dammed but more regulated and developed lowland rivers, may be impacted by upstream dams that may create unsuitable habitats for fish adapted to normal main channel conditions because they increase river flows during periods that were formerly low waters or change flows at random. The fisheries are lightly to moderately exploited compared to other subtropical and tropical riverine fisheries, and retain several of their original characteristics in less developed river reaches, although changes are evident. Large potamodromous fish are usually present in the catch, but the abundance of large piscivores is lower and fish size at catch is smaller. The development of the riverine system is expected to continue throughout the basin. If it is implemented as it was executed in the past, a continual loss of fish habitats and a general decrease in ecosystem health can be predicted.

Relations between planktivorous fish and zooplankton in two very shallow lakes of the Pampa Plain

1t is widely accepted that predation plays a critical role in determining woplankton community structure in ponds and lakes. The relationship between fish planktivory and zooplankton species composition has been discussed since the early work of HRBACEK et al. (196 I) an d BROOKS & DODSON (1965). Size-selective predation by fish may produce changes in zooplankton community structure. Phenomena frequently attributed to heavy planktivory include the small individual size of cladocerans and the reduced representation of the vulnerable Daphnia sp. (BROOKS & DODSON 1965).

Trophic interactions in pampean shallow lakes: evaluation of silverside predatory effects in mesocosm experiments

Planktivorous fish direetly suppress some prey species and indireetly affeet species that interaet with these prey (HRBACEK 1962, BROOKS & DonsoN 1965, HALL et al. 1970, DRENNER et al. 1986). Visually feeding rooplanktivorous fish are size-seleetive predators, direetly suppressing large rooplankton, and indireedy temporarily enhaneing small zooplankton and phytoplankton (HURLBERT & MuLLA 1981, DRENNER et al. 1990). The intensity of the direet and indireet effeets of planktivores is dependent on fish density (THRELKELD 1988). Planktivorous fish are sometimes eonsidered to promote eutrophieation proeess in lakes (QUIRós 1998). lts trophie effeets sometimes caseade through rooplankton to the base of the food web, promoting temporary algal growth by means of the suppression of large herbivorous dadoeerans (CARPENTER et al. 1985). The young pampean silversides (Odontesthes bonariemis), dominant speeies in pampean shallow lakes, are visual rooplanktivore feeders. Moreover, pampean silverside abundanee was shown to be indireetly related to phytoplankton biomass, after nutrient effeets have been taken into aeeount (QuiRóS 1995). To evaluate the direet and indireet effeets of this planktivore, two ourdoor mesoeosm experiments were eondueted, simulating average eonditions of salinity, nutrient status and plankton in the pelagie rone of a typieal pampean shallow lake.

Patterns in fish species composition and assemblage structure in the upper Salado River lakes, Pampa Plain, Argentina

The Pampa Plain, in the central region of Argentina, is mostly drained by the Salado River. The fish fauna of this river is mostly known from field collections in lower reach lakes. Consequently, we aimed to explore the composition and structure of the fish assemblages in the upper Salado River lakes. Patterns in dominance and persistence of species and assemblage level attributes were correlated with environmental and human activity-derived (NO 3 :NH 4 ) variables. Overall, 19,913 individuals of 17 species included in 5 orders and 11 families were collected. Several species are first records for the upper Salado River lakes. There was a marked proliferation of species in the family Characidae. Conversely, the remainder 10 families were only represented by one or two species. The species composition along the study lakes changed slightly but their relative contribution to the total fish collected (dominance) varied greatly. Rather few species were present in all collections suggesting an important interannual variability in assemblage stability. The gradient in water conductivity was an important factor for the persistence of particular species in the lakes. The gradients in water conductivity and NO 3 :NH 4 ratio were associated with particular fish communities dominated by different groups of species. These results suggest that even when broad management and conservation strategies should encompass the system as a whole, lake-specific approaches must also be addressed. Overall, our results highlight that human impacts on surface waters may interact with environmental factors to influence the dynamics of fish species and the structure of their communities. A planicie pampeana, na regiao central da Argentina, e em grande parte drenada pelo rio Salado. A ictiofauna desse rio e conhecida principalmente pelos estudos conduzidos nos lagos dos segmentos inferiores do rio. Em consequencia, procuramos examinar a composicao e a estrutura das assembleias de peixes dos lagos do alto rio Salado. Os padroes na dominância e persistencia das especies e os atributos das assembleias de peixes foram correlacionados com variaveis ambientais e variaveis indicadoras de perturbacoes antropogenicas (NO 3 :NH 4 ). Foram coletados 19913 individuos de 17 especies pertencentes a 5 ordens e 11 familias. Varias especies foram registradas pela primeira vez para as lagoas do alto rio Salado. Houve uma marcada proliferacao de especies da familia Characidae. Em contraste, as 10 familias de peixes restantes foram representadas por uma ou duas especies. A composicao de especies variou pouco ao longo dessas lagoas mas sua contribuicao ao total de peixes coletados (dominância) foi muito variavel. Poucas especies estiveram presentes em todas as coletas, sugerindo uma importante variacao interanual na estabilidade da assembleia de peixes. O gradiente na condutividade eletrica da agua foi importante fator para a persistencia de algumas especies nessas lagoas. A condutividade eletrica da agua e o gradiente na relacao NO 3 :NH 4 estiveram associados com assembleias de peixes particulares dominadas por diferentes grupos de especies. Nossos resultados sugerem que, embora as estrategias de conservacao e manejo deveriam considerar o sistema por completo, questoes particulares a cada lagoa devem tambem ser consideradas. Em geral, nossos resultados ressaltam que os impactos humanos nas aguas superficiais podem interagir com fatores ambientais para influenciar a dinâmica das especies de peixes e a estrutura de suas comunidades.