Evidence of hypoxia in the eastern coast of the Gulf of California as induced by stable nitrogen isotopes in surface sediments

The Gulf of California is a highly biodiverse marine basin located in the northeast Mexican Pacific Ocean. In the past three decades, this basin has experienced increased hypoxia in shallow waters, which threatens its coastal ecosystems. The aim of this study is to analyze δ¹⁵N and δ¹³C isotopes of organic matter in coastal sediments to characterize sources of primary production and shifts in biogeochemical processes that reflect increasing oxygen deficiency in the shallow coast of the eastern Gulf of California. Surface sediments samples were collected from 8 to 47 m deep along the coastal margin of Sinaloa and Sonora. This region is characterized by the development of anthropogenic activities, which could be the main source of organic matter evidenced in the marine environment. The C:N ratio and δ¹³C of sedimentary organic matter suggest that the dominant source is of marine origin. Values of δ¹⁵N measured in organic matter of surface sediments below 15 m are larger than >8‰. This enrichment suggests denitrification and hypoxic conditions in the water column, which could be associated with an increase in the frequency, periodicity, and intensity of shallow hypoxic events in the Gulf of California.