Latitudinal changes in the structure of marine food webs in the Southwestern Atlantic Ocean

Abstract

Food chain length is known to increase with primary productivity, species richness and environmental stability. Primary productivity and species richness decrease poleward at mid and high latitudes in the Southwestern Atlantic, and hence food chain length of coastal ecosystems is expected to decrease poleward, unless seasonal changes in water temperature destabilise food webs at mid latitude. We used stable isotopes of carbon and nitrogen and SIBER (Stable Isotope Bayesian Ellipses in R) to determine the food chain length and other patterns of interconnections (i.e. topology) of coastal food webs from 3 contrasting regions: the temperate estuarine zone associated with the Rio de la Plata plume, the tidal zone off northern and central Patagonia, and the cold estuarine zone off southern Patagonia. Results indicate that food chain length decreases and trophic redundancy increases as latitude increases in all of the compartments of the food web considered. This is in agreement with results emerging from reconstructions of marine food web length during the late Holocene in the same regions, thus suggesting that they are not artefacts caused by the intense exploitation of top predators in the region since European settlement. Furthermore, differences in primary productivity do not fully explain the observed patterns. Instead, we argue that the latitudinal reduction of species diversity reported for the Southwestern Atlantic may be the primary reason for the observed pattern. The reduction in the diversity of forage fishes and squids at high latitude is proposed to result into a wasp-waist ecosystem, characterised by a high trophic redundancy at high trophic levels.