Quantifying climatological ranges and anomalies for Pacific coral reef ecosystems

Abstract

Coral reef ecosystems are exposed to a range of environmental forcings that vary on daily to decadal time scales and across spatial scales spanning from reefs to archipelagos. Environmental variability is a major determinant of reef ecosystem structure and function, including coral reef extent and growth rates, and the abundance, diversity, and morphology of reef organisms. Proper characterization of environmental forcings on coral reef ecosystems is critical if we are to understand the dynamics and implications of abiotic-biotic interactions on reef ecosystems. This study combines high-resolution bathymetric information with remotely sensed sea surface temperature, chlorophyll-a and irradiance data, and modeled wave data to quantify environmental forcings on coral reefs. We present a methodological approach to develop spatially constrained, island- and atoll-scale metrics that quantify climatological range limits and anomalous environmental forcings across U.S. Pacific coral reef ecosystems. Our results indicate considerable spatial heterogeneity in climatological ranges and anomalies across 41 islands and atolls, with emergent spatial patterns specific to each environmental forcing. For example, wave energy was greatest at northern latitudes and generally decreased with latitude. In contrast, chlorophyll-a was greatest at reef ecosystems proximate to the equator and northern-most locations, showing little synchrony with latitude. In addition, we find that the reef ecosystems with the highest chlorophyll-a concentrations; Jarvis, Howland, Baker, Palmyra and Kingman are each uninhabited and are characterized by high hard coral cover and large numbers of predatory fishes. Finally, we find that scaling environmental data to the spatial footprint of individual islands and atolls is more likely to capture local environmental forcings, as chlorophyll-a concentrations decreased at relatively short distances (>7 km) from 85% of our study locations. These metrics will help identify reef ecosystems most exposed to environmental stress as well as systems that may be more resistant or resilient to future climate change.

Figure 1. Map highlighting the 41 islands and atolls that comprise the coral reef ecosystems of the U.S. Pacific.

Individual locations are color-coded by region. Regions include Northwestern Hawaiian, Hawaíi, Mariana, Equatorial and Samoa. Table 1 provides additional information pertaining to each location, including the location name for each of the location codes.

Figure 2. Monthly composite of chlorophyll-a concentrations at Pearl and Hermes Reef in the Northwestern Hawaiian Region for September 2003.

A) Unfiltered data with contaminated information associated with shallow-water bottom reflectance; B) data filtered using the 30-m bathymetric contour (black line), although contaminated information still remains as a result of bottom reflectance; C) fully cleaned data set using an additional data removal filter (gray line) that is everywhere perpendicular to the 30-m contour, removing all contaminated data associated with bottom reflectance.

Figure 3. Chlorophyll-a concentrations (mg m⁻³±standard error) by data inclusion zone from the error-free data set (Fig. 2) at Pearl and Hermes Reef.

Values for each data inclusion zone were calculated by taking the long-term mean of each pixel from July 2002 to May 2011, and then averaging over all pixels within each zone. The numbers on the x-axis are associated with sequentially expanding data inclusion zone, separated by 0.0295° (∼3.27 km). The data inclusion zones are exclusive and nonoverlapping, and color-coded and numbered based on the inset of Pearl and Hermes Reef. The black line represents the 30-m isobath and the gray line represents the additional data removal filter.

Figure 4. Map of long-term means in A) SST, B) wave energy, C) chlorophyll-a and D) irradiance across each of the regions that comprise the coral reef ecosystems of the U.S. Pacific.

Regions indicated in panel A are the same for panels B –D. Please see Fig. 1 as a reference for individual island and atoll locations.

Figure 5. Climatological ranges and average annual anomalies for U.S. Pacific coral reef ecosystems.

Island- and atoll-scale metrics for A) SST, B) wave energy, C) chlorophyll-a and D) irradiance. In each of the panels (A – D), the black bar represents the climatological range, with the top and bottom of the bar representing the upper and lower climatological range limits, respectively. The yellow and blue bars signify average annual positive and negative anomalies, respectively, represented here as the average annual percentage of time above (positive) the upper climatological range limit and below (negative) the lower climatological range limit. Islands and atoll names are presented as a three-letter code (see Table 1 for full location names), grouped and color-coded by region (see Fig. 1 for map of locations), and oriented by decreasing latitude from left to right (see Table 1 for specific positions). The asterisks represent the islands that are oriented based on geographic proximity to other islands and atolls, as opposed to strict latitudinal orientation. See Tables S1, S2, S3, S4, S5, S6 for climatology and anomaly values presented in this figure.

Figure 6. Principle component analysis (PCA) of the A) upper climatological range limit, B) long-term mean and C) lower climatological range limit for all environmental forcings: SST, wave energy, chlorophyll-a, irradiance.

Island and atoll names are presented as a three-letter code (see Table 1 for full location names) and grouped and color-coded by region (see Fig. 1 for map of locations). The direction of loading for each of the parameters is indicated by the black line, with the direction of the line pointed towards increasing values. Similarity Profile (SIMPROF) results are represented by dashed lines and indicate islands with similar environmental forcings (p<0.0001) with respect to each of the metrics. See Table S1 for climatological range limit and long-term mean values presented in this figure.