The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of smallsized zooplankton in warm lakes.

In the subtropics, the effects of macrophytes on trophic interactions are more complex than in temperate lakes. Fish, particularly the smallest species and individuals, aggregate in high numbers in the vegetation, and a strong predation pressure on zooplankton by shrimps and invertebrates, such as Chaoborus, can occur in these systems. We studied seasonal and diel changes in zooplankton and their potential predators (both fish and invertebrates) and physical and chemical characteristics among open water and vegetated habitats (emergent and submerged plants (SP)), in the subtropical Lake Blanca (34°54¢ S; 54°50¢ W), a shallow system with an extensive and complex littoral area and high abundance of vertebrate and invertebrate predators on zooplankton. We found differential horizontal distribution of some zooplankton species under the scenario of high abundance of small omnivorous-planktivorous fish and Chaoborus, especially in the seasons with intermediate catch per unit effort of fish. We found indications of a diel horizontal migration (DHM) opposite than described for temperate systems, as the two main cladocerans Bosmina longirostris and Diaphanosoma birgei were found in higher densities in the submerged plant beds during night, in spring and autumn respectively. Although we need experiments to prove DHM, Chaoborus seemed to be the main trigger of the apparent DHM, followed by small omnivorous fish. During summer no spatial differences were found likely because of high densities of fish in all habitats. In absence of piscivorous fish, the distribution of fish Jenynsia multidentata seemed to be conditioned by food availability and by predation risk of waterfowl. The refuge capacity of aquatic plants for zooplankton in subtropical systems seems weak and with consequent weak or no cascading effects on water transparency, as under very high fish and invertebrate densities (summer) the refuge for zooplankton was lost.

Agricultural land covers approximately 40% of Earth’s land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea, and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks, and resulting in higher ecosystem productivity and CO2 outgassing.

Summary 1. Small cladocerans, copepod nauplii and rotifers often dominate the zooplankton community in tropical and subtropical lakes. This is probably because of high predation pressure by small omnivorous–planktivorous fish, but experimental evidence is scarce. 2. This study used two approaches to test the effect of the small omnivorous–planktivorous fish species Jenynsia multidentata, which is frequently abundant in (sub)tropical eutrophic lakes in South America, on the size distribution of zooplankton. In Lake Blanca (Uruguay), which lacks any piscivores, we sampled seasonally for both fish and zooplankton. We also conducted an outdoor mesocosm experiment with treatments containing or lacking J. multidentata. 3. Together, the empirical and experimental data suggest that J. multidentata predation plays an important role in modulating the size structure of the zooplankton community in subtropical lakes. In the absence of J. multidentata, stocked large‐sized zooplankters like Daphnia obtusa were abundant in the experiments, while small‐sized zooplankton dominated in the presence of fish, as they did in the lake itself from spring to the end of the season.