Anthropogenic nitrogen pollution threats and challenges to the health of South Asian coral reefs

Abstract

Nitrogen pollution is a widespread and growing problem in the coastal waters of South Asia yet the ecological impacts on the region’s coral ecosystems are currently poorly known and understood. South Asia hosts just under 7% of global coral reef coverage but has experienced significant and widespread coral loss in recent decades. The extent to which this coral ecosystem decline at the regional scale can be attributed to the multiple threats posed by nitrogen pollution has been largely overlooked in the literature. Here, we assess the evidence for nitrogen pollution impacts on corals in the central Indian Ocean waters of India, Sri Lanka and the Maldives. We find that there is currently limited evidence with which to clearly demonstrate widespread impacts on coral reefs from nitrogen pollution, including from its interactions with other stressors such as seawater warming. However, this does not prove there are no significant impacts, but rather it reflects the paucity of appropriate observations and related understanding of the range of potential impacts of nitrogen pollution at individual, species and ecosystem levels. This situation presents significant research, management and conservation challenges given the wide acceptance that such pollution is problematic. Following from this, we recommend more systematic collection and sharing of robust observations, modelling and experimentation to provide the baseline on which to base prescient pollution control action.

Rapid seagrass meadow expansion in an Indian Ocean bright spot

“…We suggest that nutrient levels are below a critical threshold that would tip the seagrass habitats into eutrophic conditions with negative implications for seagrass health. Currently, it may be that the naturally low nutrient oceanic waters 41 , coupled with high advective flushing of reef platforms limit the flux of nutrients into the coastal system 42 . Indeed, seagrass growth and production, particularly at the seedling stage, is largely limited by the availability of nutrients in temperate and tropical regions and moderate increases in nutrient concentrations have been shown to stimulate their production and growth 43 , 44 .…”

“…Indeed, seagrass growth and production, particularly at the seedling stage, is largely limited by the availability of nutrients in temperate and tropical regions and moderate increases in nutrient concentrations have been shown to stimulate their production and growth 43 , 44 . Nutrient sources include sewage, domestic waste, fertilisers, and wastewater that can enter the marine environment through surface water runoff and/ or groundwater discharge through the porous reef island sediments 42 , 45 , 46 . In many cases, appropriate waste management is limited in the Maldives, and contamination of groundwater from anthropogenic waste is recorded on many inhabited islands across the country 26 , 45 .…”

“…On reef platforms adjacent to agricultural islands, trends of increasing fertiliser use may already lead to eutrophication in the coastal zone 26 , 42 , 45 . Indeed, our results showed that occupancy of seagrass habitat was lower adjacent to agricultural islands compared to inhabited and resort islands (Fig.…”

“…Additionally, the use of fertilisers and pesticides is unregulated in the Maldives and therefore any potentially negative effects of these chemicals on the marine environment are largely unknown. Increased urbanisation, population growth, and unregulated fertiliser use across the country could lead to more widespread nutrient loading into coastal systems 26 , 42 , 45 . For example, waste production in the Maldives increased by 56% between 2006 and 2016 45 and there have been reports of red tides and mass fish mortalities in 2007, 2008, and 2012 which could be indicative of eutrophication 26 , 58 .…”