A coral disease outbreak highlights vulnerability of remote high-latitude lagoons to global and local stressors

Abstract

Outbreaks of coral disease are often associated with global and local stressors like changes in temperature and poor water quality. A severe coral disease outbreak was recorded in the primary reef-building taxa Montipora spp. in a high-latitude lagoon at Norfolk Island following heat stress and pollution events in 2020. Disease signs suggest the occurrence of a Montiporid White Syndrome with four distinct phases and maximum measured tissue loss of 329 mm^-2 day^-1. In December 2020 and April 2021, 60% of the Montipora community were impacted and disease severity increased by 54% over this period. Spatial patterns in prevalence indicate the disease is associated with exposure to poor water quality in addition to size class of coral colonies. High prevalence levels make this event comparable to some of the most severe coral disease outbreaks recorded to date demonstrating the vulnerability of this system to combined impacts of warming and pollution.

Keywords: aquatic science; environmental science; global change; zoology.

Graphical abstract

Figure 1

Study context (A) Map of Norfolk Island. Point on the globe shows the position of Norfolk in the South Pacific. (B) Satellite image of the lagoonal reef system on the south side of the Island. The lagoon is split into two bays, Slaughter Bay and Emily Bay. Red dot marks a creek that over-flows into Emily Bay causing terrestrial-pollution events after high rainfall. (C) Timeline of data collection in this study. (D) Plot showing sea surface temperature (SST, gray line), degree heating weeks (DHWs, black line), and daily rainfall (blue line) during the monitoring period and in the 11 months prior to the observation of disease signs within the lagoon. Light and dark red hashed lines represent the maximum monthly mean (MMM) and maximum monthly mean +1°C over which temperature stress is expected to accumulate. Light and dark blue hashed lines represent DHWs of 4 (significant coral bleaching expected) and 8 (severe, widespread bleaching and significant mortality) respectively. Arrows point to significant events over reef prior to disease observation. TP stands for time point when ecological monitoring took place.

Figure 2

Disease signs (A) Purple color morph encrusting colony of Montipora spp. Scale bar represents 20 cm. (B) Brown color morph foliose colony with plates and pillar structures, Montipora spp. Scale bar represents 20 cm. (C) Brown color morph encrusting colony of Montipora spp. Scale bar represents 20 cm. (D) A large brown color morph of Montipora spp. showing plating and pillar structures. Scale bar represents 40 cm. (E) A foliose colony showing multiple irregular shaped tissue loss lesions on plating structures. Scale bar represents 20 cm. (F) A foliose colony with a single large irregular shaped tissue loss lesion. Scale bar represents 15 cm. (G) A foliose colony showing multiple tissue loss lesions present centrally and peripherally on plating structures. Scale bar represents 10 cm. (H–K) The four described phases of lesion progression identified (see results). Scale bar represents 0.5 cm. (L) Lesion transition dynamics over 7–11 days (based on time of re-survey). (M) Change in lesion area (mm⁻² day⁻¹) recorded for surveyed lesions that showed a decrease (i.e. colony tissue recovery, n = 4) or an increase in area (i.e. colony tissue loss, n = 14) over the monitoring period. (N) Change in lesion area (mm⁻² day⁻¹) recorded for surveyed lesions that showed no progression (n = 9), and progression (n = 9) in phase during the monitoring period. Boxplots upper and lower lines correspond to the first and third quartiles, and whiskers represent the minimum and maximum values. Points represent outliers. Asterisks indicate p < 0.01. (O) Lesion i shows the lesion with the highest measured tissue recovery (see M). Scale bar represents 0.1 cm. (P) Lesion ii shows the lesion with the highest rates of area increase (see M). Scale bar represents 0.1 cm.

Figure 3

Disease levels over the Austral summer Dot plots showing average disease prevalence (A) and severity (B) over time in sites Emily Bay (EB) and Slaughter Bay (SB). Black points and lines represent disease prevalence, whilst gray points and lines represent average total Montipora benthic cover. Lines represent SE. Asterisks indicate p < 0.01.

Figure 4

Drivers of disease risk (A) Results of model selection through the application of a LASSO penalty for disease occurrence (A) and disease severity (B) visualized as forest plots showing modeled coefficients and standard errors for remaining covariates after the LASSO is applied. Final model standard errors and confidence intervals do not condition on the model-selection process and should be taken as a guide only. For this reason, SE estimates were only included in the presentation of model coefficients as forest plots, but not in interaction and predicted effects plots. (C) Interaction plot showing the effect of size class on disease occurrence over time, and site (D).