Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape

Abstract

Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that heterogeneity in nitrogen pollution can shape spatial patterns of coral bleaching across a seascape. Using island-wide surveys of coral bleaching and nitrogen availability within a Bayesian hierarchical modeling framework, we tested the hypothesis that excess nitrogen interacts with temperature anomalies to alter coral bleaching for the two dominant genera of branching corals in Moorea, French Polynesia. For both coral genera, Pocillopora and Acropora, heat stress primarily drove bleaching prevalence (i.e., the proportion of colonies on a reef that bleached). In contrast, the severity of bleaching (i.e., the proportion of an individual colony that bleached) was positively associated with both heat stress and nitrogen availability for both genera. Importantly, nitrogen interacted with heat stress to increase bleaching severity up to twofold when nitrogen was high and heat stress was relatively low. Our finding that excess nitrogen can trigger severe bleaching even under relatively low heat stress implies that mitigating nutrient pollution may enhance the resilience of coral communities in the face of mounting stresses from global climate change.

Keywords: Acropora; Pocillopora; climate change; coral reef; nutrient availability.

Fig. 1.

(A and B) Island-wide (A) and by site (B) temperature patterns from in situ temperature loggers at fringing reefs. (A) In 2016 (blue line) temperatures exceed the maximum monthly mean of 29 °C (red dashed line) during the Austral summer, and it was much warmer than the average long-term seasonal pattern (black line with 95% confidence intervals as dashed black lines). Bleaching surveys took place from 8 to 14 May 2016 (gray vertical bar). (B) Cumulative heat stress, measured as a 12-wk running sum for all temperatures exceeding the maximum monthly mean, peaked in April 2016 and was variable around the island as measured at LTER sites in Fig. 2. Temperature patterns from back reef sites were similar to those from fringing reefs.

Fig. 2.

Location of bleaching and nitrogen surveys (black circles, n = 167 sites) around the island of Moorea, French Polynesia. In situ temperature recorders were located at LTER sites (large boxes). Nitrogen is represented as a continuous surface where warmer colors represent greater percentage of nitrogen in the tissue of the brown alga T. ornata and cooler colors represent lower percentage of nitrogen in algal tissue. Land is displayed as a digital elevation model with a hillshade to show ridgelines and valleys.

Fig. 3.

(A–D) Distribution of coral bleaching prevalence (A and B) and severity (C and D) for Acropora (A and C) and Pocillopora (B and D). Continuous surfaces were created using inverse distance weighting; the extent of the surface represents reef area within the lagoons, while white areas represent nonreef area (e.g., deep lagoons, sand) within lagoons or offshore reefs beyond the reef crest. Note that color ramps are different for prevalence and severity.

Fig. 4.

(A and B) Relationship between predictors and coral bleaching severity (A) and prevalence (B) for Acropora (blue) and Pocillopora (red). Betas are coefficients, where colony size and depth are related to bleaching at the colony scale and nitrogen and heat stress are related to bleaching at the site scale (sites are represented by circles in Fig. 2). Thin lines are 95% credible intervals, and thick lines are 80% credible intervals. We consider there to be evidence for each effect when the 80% credible interval does not cross zero (dashed line).

Fig. 5.

Visualization of the negative interaction between nitrogen and heat stress on bleaching severity for Acropora (blue) and Pocillopora (red) in the back reef. Plotted are the mean values (circles) and 95% credible intervals (lines) predicted from the models for bleaching severity across the range of nitrogen values when heat stress was held to a low (1.84 °C weeks), a moderate (2.14 °C weeks), and a high value (2.81 °C weeks).