Seasonal isotopic shifts in fish of the Pantanal wetland, Brazil

Abstract

Seasonal inundations shape the floodplain characteristics of the Pantanal, a large wetland in Central South America. In the first study combining stable carbon and nitrogen isotope analysis with classical stomach content analysis in this region, we investigated the influence of the annual inundation on diet and isotopic composition of floodplain fish. Apart from potential food items, 33 fish species from the Coqueiro Lake were analyzed, 10 of which were present during both the wet and dry season 1999. A d 13 C and d 15 N plot of the floodplain ecosystem allowed us to assess a foodchain of 3 -4 trophic levels. However, the wide overlap of nitrogen values suggested that the organisms act on trophic continua rather than on distinct levels. The foodweb was based mainly on C 3 -plant carbon. However, fish species capable of feeding on terrestrial invertebrates (e. g., Brycon microlepis) had d 13 C values above -25 ‰, indicating 13 -30 % intake of C 4 -plant based carbon during the flooding period. The novel use of vector coordinates and 2-dimensional ANOVA showed that the seasonal isotopic shifts of d 13 C and d 15 N were highly significant for some feeding guilds. d 15 N values increased from the wet to dry season in most Aquatic Sciences fish species, and these shifts were highly significant for omnivores (Astyanax bimaculatus, Triportheus nematurus, Tetragonopterus argenteus and Moenkhausia dichroura), and significant for invertivores (Gymnogeophagus balzanii and Poptella paraguayensis) and carnivores (Serrasalmus spilopleura). Average carbon isotope ratios decreased at the same time in the herbivores (Methynnis mola: 3.4 ‰) and detritivores (Psectrogaster curviventris: 5.3‰), but they did not change in the hypostomatic herbivore Sturisoma robustum. We explain these shifts by abundant and variable food sources during the inundation period and increasing carnivory and starvation during the dry season when the lake is confined to its central basin. Isotopic shifts between seasons were more prominent in less specialized species of omnivores, invertivores and some carnivores, whereas more specialized herbivores and detritivores appeared to be more influenced by changes in the carbon isotope ratio of the diet affected by biogeochemical processes such as respiration and methanogenesis. A general model for the interpretation of isotope data of floodplain fish considering different timescales is given.

Aquatic food webs in mangrove and seagrass habitats of Centla Wetland, a Biosphere Reserve in Southeastern Mexico

“…Therefore, the large variation in δ 15 N values for the invertivorous fish species that dominated the San Pedrito Lagoon food web could be due to consuming different proportions of terrestrial and aquatic insects. Wantzen et al (2002) observed a comparable amount of variation in δ 15 N values of invertivorous fish species in the Pantanal, and suggested that such variation may be due to differences in consumption of aquatic and terrestrial resources, especially as influenced by seasonal flooding. Zeug & Winemiller (2008) found riparian vegetation to be the dominant carbon source utilized in food webs of a temperate floodplain river system, with invertebrates as the pathway by which riparian-derived carbon entered the aquatic food web.…”

Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments

“…These increases can, at least partially, result from the input of terrestrial material through the transport of terrestrial invertebrates (e.g., ants, spiders, grasshoppers) with the wind and flood waters into the waterways, where they subsidize the diets of carnivorous fish. These invertebrates can be of crucial importance to those fish diets (Nakano andMurakami 2001, Sullivan et al 2012), and seasonal variations in this importance have been reported for other tropical areas (e.g., Wantzen et al 2002, Balcombe et al 2005.…”

“…Rivers and estuaries are subjected to high human pressure, as their settings make them ideal for human settlement. This pressure has been intensified over the last century, especially in tropical areas where increasing population numbers place increasing pressure on these environments (Junk 2002). Impacts such as deforestation, urbanisation, agriculture and flow regulation can directly affect the energetic connectivity between land and rivers, and these impacts can be transported downstream to the estuaries.…”

Seasonal variability in baseline δ15N and usage as a nutrient indicator in Lake Poyang, China

“…As water-level fluctuates, the organic matter from terrestrial or macrophyte plants also may represent a large proportion of the basal food sources of floodplain lakes (Wantzen et al 2002;Wang et al 2011Wang et al , 2012. Major nitrogen cycling processes, such as nitrogen fixation, nitrification, and denitrification, which exert strong effects on isotopic fractionation during nitrogen transformation, also vary between dry and wet seasons (Hadas et al 2009).…”