Food-web structure and functioning of temperate and tropical lakes: A stoichiometric viewpoint

Abstract

-Difficulties to simply transfer trophic cascade theory from temperate to tropical lakes are now well recognized. Many mechanisms trying to explain top-down divergences between these systems have been proposed, such as lack of key species of herbivorous zooplankton, absence of seasonality in fish reproduction, cyanobacteria development, or differences in fish foraging behaviour. Very few studies have considered bottom-up mechanisms, in particular differences in nutrient recycling and nutrient limitation between the two types of ecosystems. According to the ecological stoichiometry theory, fish-induced alterations of food-web structure could modify the efficiency of consumer-driven nutrient recycling by changing the relative biomass contribution of species in food webs. Consequently, by mostly considering top-down processes, one could underestimate consequences on nutrient availability for phytoplankton growth. In this paper, we compared the results of two mesocosm experiments carried out in temperate and tropical areas, each manipulating foodweb structure via the presence or absence of fish. We found trophic cascades in both experiments, but differences between fishless and fish treatments were greater in temperate than in tropical systems. In the tropical experiment, the observed effects could not be supported by classical zooplankton community alteration or by cyanobacteria prevalence. Our results suggest a key contribution of fish nitrogen-excretion to phytoplankton growth in mostly nitrogen-limited tropical systems. Differences in stoichiometric response to food-web structure alteration between temperate and tropical lakes could thus represent a major difference between the two systems. Our study stresses the need for further studies that would allow robust generalization on the functioning of freshwater temperate and tropical ecosystems.

Cyanobacteria are controlled by omnivorous filter-feeding fish (Nile tilapia) in a tropical eutrophic reservoir

“…However, fish-mediated nutrient recycling did not enhance the cyanobacteria biomass, because phytoplankton growth in our study was P-limited, and tilapia release nutrients at high N to P ratios (Danger et al, 2009). This is confirmed by the higher ammonium/ nitrate and lower phosphate concentrations in the fish treatment in comparison to the control, from the middle to the end of the experiment.…”

Lower biodiversity of native fish but only marginally altered plankton biomass in tropical lakes hosting introduced piscivorous Cichla cf. ocellaris

“…Presumably, the cascading effects of predatory fish are weaker in tropical than in temperate lakes (Jeppesen et al 2010;Gelós et al 2010;Danger et al 2009), which has been attributed to: (1) higher abundance and diversity of small fish species with continuous reproduction, (2) higher fish aggregation within the vegetation, and (3) more omnivory by fish and less specialist piscivory (Lazzaro 1997;Jeppesen et al 2007Jeppesen et al , 2010. However, the trophic dynamics and role of fish in warm lakes are much less well understood than for temperate lakes (Jeppesen et al 2007).…”

Influence of food web structure on the biochemical composition of seston, zooplankton and recently deposited sediment in experimental freshwater mesocosms

“…However, a decrease in the biomass of planktonic primary producers has sometimes been observed without any increase in the grazing capacity by zooplanktonic communities (e.g. Danger et al 2009). This suggests that, in addition to modification of zooplankton communities, other processes could be involved in the reduction of phytoplankton biomass when fish are manipulated.…”

“…Indeed, several studies demonstrated that changes in fish biomass induced complex changes that can have important impacts on lake functioning. By manipulating the food web through the presence or absence of fish in a long-term mesocosm experiment, Danger et al (2008Danger et al ( , 2009) demonstrated that the stoichiometric characteristics (C:N:P ratios) of biofilms and zooplankton differed between fish and fishless enclosures. Such changes might modify the efficiency of predator-prey interactions and nutrient balance within aquatic ecosystems.…”