Influence of regional factors on zooplankton structure in a saline lowland river: the Salado River (Buenos Aires province, Argentina)

Abstract

Knowledge of regional factors influencing zooplankton structure in a semiarid river must include both the main channel and any tributaries in order to identify spatial and temporal patterns along with the main factor that affect the zooplankton community. Accordingly, seasonal samples were taken during 1997-1999 at 15 stations in the Salado River basin, where 172 species were identified (53 protozoans, 88 rotifers and 31 crustaceans). Conductivity and temperature optima and tolerances were calculated for the evident taxa. Different assemblages were recognized by cluster analysis, on the basis of their temperature and conductivity preferences. With respect to the zooplankton assemblages, three zones can be distinguished along the longitudinal axis of the basin: (1) the headwaters (the effluent from large saline shallow lakes), (2) the inter-tributaries, and (3) the lower basin and associated shallow lakes. The spatial distribution of the assemblages was a result of the combined action of factors always promoting the dominance of rotifers. The influence of floodplain waters (backwater ponds, waterlogged depressions, flushing lakes) was attenuated progressively downstream, thus producing disadvantages for the crustaceans. The food availability and high nutrient concentrations related to land use in the headwaters favoured the co-dominance of cladocerans and copepods along with the rotifers in that region.

Land Use and Basin Characteristics Determine the Composition and Abundance of the Microzooplankton

“…Shiel (2001, 2002) also observate its correlation with high trophic status. K. tropica was mainly abundant at Saladillos River, with a conductivity lower than Salado River, in agreement with Claps et al (2009) who also found it preference for conductivity values lower than 2,000 μS cm −1 . Dodson et al (2007), in a large-scale study showed that the land use has a significant adverse effect on zooplankton species richness in lakes.…”

“…However, the passage of the river through a plain with intense rural activity, determined that the strongly eutrophic character of the studied meandric river enables an increase in zooplankton abundance. This positive relationship between eutrophy and zooplankton abundance has been found in lakes (Whitman et al 2004) and rivers (Claps et al 2009). …”

“…However, zooplankton have rarely been included in biomonitoring schemes and investigations of land use effects. The link between watershed land use and zooplankton species richness has been recently explored, mainly in small streams, lakes and reservoirs (Ejsmont-Karabin and Kruk 1998;Lougheed and Chow-Fraser 2002;Dodson et al 2005;Dodson 2008;Dodson and Lillie 2001;Hoffmann and Dodson 2005), but references to the relationship with abundance of zooplankton, and even more, the case of plain rivers, are scarce (e.g., Olguin et al 2004;Claps et al 2009). …”

Zooplankton succession during extraordinary drought–flood cycles: A case study in a South American floodplain lake

“…During the LW phase, the zooplankton was represented by few species with extremely high abundances. Planktonic euryhaline species such as the rotifers B. plicatilis, B. ibericus, and B. dimidiatus, and the cyclopoid Metacyclops mendocinus, frequently found in saline lakes (Claps et al 2009;Cirós-Pérez et al 2001;Sterza and Loureiro Fernandes 2006) represented this assemblage. Evidently, the switch from parthenogenetic to sexual reproduction of some of the dominant species found in the lake during LW reflects their adaptation to highly fluctuating environmental conditions, as described by Gyllström and Hansson (2004).…”

“…As an example, the abundance of microcrustaceans is strongly affected, whereas that of rotifers shows a minor response (Rennella and Quirós 2006;Bonecker et al 2005;Paidere 2009). Since zooplankton species have different tolerance levels to conductivity and pH (Claps et al 2009), hydrology-driven changes of these variables affect species composition. Likewise, the fluctuation in the submerged vegetation, which constitutes a refuge for predators, causes great changes in the zooplankton community composition (Havens et al 2007).…”