Extreme rainfall events pulse substantial nutrients and sediments from terrestrial to nearshore coastal communities: a case study from French Polynesia

Caitlin R Fong,Robert C Carpenter,Camille J Gaynus

doi:10.1038/s41598-020-59807-5

Caitlin R Fong, Robert C Carpenter + Show 1 more

Open Access

https://doi.org/10.1038/s41598-020-59807-5

Copy DOI

Abstract

Rainfall mobilizes and transports anthropogenic sources of sediments and nutrients from terrestrial to coastal marine ecosystems, and episodic but extreme rainfall may drive high fluxes to marine communities. Between January 13th and January 22nd, 2017, the South Pacific Island of Moorea, French Polynesia experienced an extreme rainfall event. ~57 cm of rain was delivered over a 10-day storm. We quantified pulsed sediments and nutrients transported to nearshore reefs. We determined the spatial and temporal extent of the sediment pulse with estimates of water transparency. We quantified pulsed nutrients at multiple spatial and temporal scales. To determine if terrestrial nutrients were incorporated into the benthic community, we collected macroalgae over 10 days following the storm and measured tissue nutrient concentrations and δN15. Pulsed sediments impacted water clarity for 6 days following the storm, with greatest impacts closest to the river mouth. Nitrite +nitrate concentrations were >100 times the average while phosphate was >25 times average. Macroalgal tissue nutrients were elevated, and δN15 implicates sewage as the source, demonstrating transported nutrients were transferred to producer communities. Future climate change predictions suggest extreme rainfall will become more common in this system, necessitating research on these pulses and their ramifications on marine communities.

Highlights

Rainfall mobilizes and transports anthropogenic sources of sediments and nutrients from terrestrial to coastal marine ecosystems, and episodic but extreme rainfall may drive high fluxes to marine communities
Riverine transport is the primary conveyance of terrestrial anthropogenic impacts to nearshore coastal communities, and these pathways are influenced by both rainfall and land use patterns[1–3]
Extreme rainfall events can increase the discharge of nutrients and sediments into nearshore communities, creating pulsed events distinct from regular discharge[5,12]. These pulsed versus pressed subsidies probably have different impacts on coastal communities[11,13]. These extreme rainfall events likely are not captured by regular water sampling protocols, which typically occur at regularly scheduled intervals, over the course of weeks to months (e.g.14)

Summary

Introduction

Rainfall mobilizes and transports anthropogenic sources of sediments and nutrients from terrestrial to coastal marine ecosystems, and episodic but extreme rainfall may drive high fluxes to marine communities. Riverine discharge can increase both nutrient and sediment transport from terrestrial to coral reef communities, and extreme rainfall events likely result in pulsed transport of these materials. Research in Australia shows riverine discharge is the largest source of nutrients to the Great Barrier Reef Lagoon[5] These inputs are elevated in the wet season when rainfall can generate a large flood plume[18], likely from mobilized terrestrial sediments and resuspended sediments[19–21]. In Curaçao, researchers have documented river discharge nutrient pulses following rainfall events[12] Taken together, these studies show rain driven flood events elevate nutrient and sediment concentrations in the water column, which may have substantial impacts on coral reef benthic communities. Pulsed nutrient and sediment events from terrestrial communities to coral reefs are common, and changes to the water column have been documented globally, yet uptake of nutrients by the benthos following these events remains understudied

Methods

Results

Conclusion