Sea-land transitions in food supply across global deltas

River deltas represent globally important population centres and areas for food production, embedded on a nexus of land-water production. Yet pressures at the sea-land interface in these coastal areas are increasing from population growth, economic expansion and climate change. Over time, delta human populations have shifted from sea to land-based food production, the former more associated with local communities’ subsistence and the latter with exports of food products elsewhere leading to potentially irreversible land use-land cover changes (LULCC). While livelihood diversification by switching between or including both sea and land food production systems, could promote resilience and increase food security, food production based on sea and land-based resources has seldom been addressed in tandem, and potential unintended spill-over effects and LULCC may emerge which may jeopardise both. Here we examine the extent to which deltas have food production systems based on sea and land-based food sources (biomass, calories and protein) and which factors could drive these relationships, as transitions between sea and land food production in deltas may have local and global food security implications.

We use global datasets across 235 large deltas, which include information on food biomass, calories and proteins from sea (fishing and aquaculture) and land (crops and livestock) production systems. Based on these indicators of food production, we calculated a sea:land ratio to examine to what extent food production of each delta is weighted towards the sea or land. We find clear geographical patterns in the sea:land food production ratio in global deltas: the majority of the deltas exhibit a dominance of sea-based food production (particularly those at higher latitudes) while the deltas in most of Europe, Central and Southern America, show a dominance of land-based food production. We found similar geographical patterns for biomass, calories and protein, with the relationship being stronger for protein than biomass or calories. We then examined how the sea:land ratio changes along gradients of human population density, and condition of the local ecosystems. Surprisingly, we found no relationship between population density and the sea:land ratio, indicating other factors may be at play (e.g., local context). Indeed, we found that as food production is mostly due to land contributions (i.e., the lower the sea:land ratio) the stronger the negative relationships with ecosystem conditions such as biodiversity intactness, soil and water quality. These results suggest that transitions between sea and land food production and subsequent LULCC can therefore be both complex and problematic in deltas globally, as these may represent switches in not just biomass but also nutritional quality, and may have severe implications for local ecosystem health and functioning. Increasing our understanding of what drives these transitions and associated LULCC at the nexus between land-water, and their effects, will allow more sustainable management of coupled food production systems and associated earth system processes.