Seasonal variation of autochthonous and allochthonous carbon sources for the first levels of the Beagle Channel food web

This study aimed to evaluate the relative importance of autochthonous (microplankton (i.e., phytoplankton and small heterotrophs), and Macrocystis pyrifera detritus) and allochthonous (terrestrial detritus) sources to the first levels of the planktonic trophic web supporting fish larvae in the Beagle Channel. Monthly samplings were conducted along the mid area of the Beagle Channel from March 2015 to February 2016, covering all seasons. Larval fish were present throughout the year, of which Patagonotothen spp. were dominant, particularly in spring. Bayesian stable isotope mixing models revealed that autochthonous carbon sources were more important for the particulate organic matter in surface (POM) and the particulate organic matter in sediment (SPOM) than allochthonous sources, which only slightly contributed to the POM. Also, seasonality in the contribution was observed. Microplankton was more important to the POM from the autochthonous carbon sources analysed, particularly in winter and spring. In addition, parts of the giant kelp M. pyrifera (blade, stipe and holdfast) equally contributed to the SPOM in all seasons. Microzooplankton organisms in the Beagle Channel depended on the POM and microplankton almost all the year, except in spring when SPOM gained relevance as a carbon source. Although fish larvae fed mainly on microzooplankton, a significant contribution of SPOM was also observed. Therefore, the giant kelp forests, in addition to providing refuge for fish larvae in the Beagle Channel, constitute an important carbon source for their planktonic prey during spring through the detrital pathway.