Impacts of tropical cyclones on North Carolina estuarine and coastal carbon and nitrogen dynamics: Implications for biogeochemical cycling and water quality in a stormier and warmer world

Paerl, Hans

Abstract:

Coastal North Carolina has been impacted by 12 major hurricanes and 9 tropical storms in the past 2 decades; this frequency is forecasted to continue in the foreseeable future. Each of these storms exhibited unique hydrologic and nutrient loading impacts on the US’s second largest estuarine system, the Pamlico Sound System (PSS), its tributaries, and the nearby New River Estuary (NRE). Differing storm-associated rainfall amounts interspersed with record droughts resulted in highly variable freshwater, nutrient, and sediment discharge. Years with major storms caused up to a doubling of annual nitrogen and phosphorus loading compared to non-storm years. Phytoplankton-dominated primary production, biomass, and composition were strongly impacted by the combined effect of nutrient enrichment and altered flushing rates.

Major storms can also dramatically affect coastal carbon cycling. In 2011, Hurricane Irene released more CO₂ from the PSS over 48hrs than the cumulative CO₂ uptake over the prior year, during which the PSS had been a sink of atmospheric CO₂. In 2015, the combined effect of Hurricane Joaquin and nor’easters led to historic flooding in the PSS watershed. The CO₂ efflux during these storms was a factor of 3-4 times smaller than during Hurricane Irene, as the PSS was not directly impacted by hurricane-force winds. However, organic-rich floodwaters sustained high CO₂ emissions for months rather than weeks, significantly affecting the net C balance of the PSS on an annual scale. Despite differences in size and watershed characteristics, both the NRE and PSS were autotrophic and sinks for atmospheric CO₂ during dry, storm-free years. In years with at least one major storm, both systems were net heterotrophic and annual sources for atmospheric CO₂. Given current trends in storm intensity and frequency, storm-driven release of CO₂ may represent a positive feedback to the climate system via reduction in long-term C storage in estuaries.

Freshwater discharge, wind forcing, and warming are not manageable, but must be incorporated in management strategies for these and other estuarine and coastal ecosystems world-wide faced with a warmer climate, with increased frequencies and intensities of tropical cyclones, flooding, and droughts.