Oceanic productivity and high-frequency temperature variability—not human habitation—supports calcifier abundance on central Pacific coral reefs

Past research has demonstrated how local-scale human impacts—including reduced water quality, overfishing, and eutrophication—adversely affect coral reefs. More recently, global-scale shifts in ocean conditions arising from climate change have been shown to impact coral reefs. Here, we surveyed benthic reef communities at 34 U.S.-affiliated Pacific islands spanning a gradient of oceanic productivity, temperature, and human habitation. We re-evaluated patterns reported for these islands from the early 2000s in which uninhabited reefs were dominated by calcifiers (coral and crustose coralline algae) and thought to be more resilient to global change. Using contemporary data collected nearly two decades later, our analyses indicate this projection was not realized. Calcifiers are no longer the dominant benthic group at uninhabited islands. Calcifier coverage now averages 26.9% ± 3.9 SE on uninhabited islands (compared to 45.18% in the early 2000s). We then asked whether oceanic productivity, past sea surface temperatures (SST), or acute heat stress supersede the impacts of human habitation on benthic cover. Indeed, we found variation in benthic cover was best explained not by human population densities, but by remotely sensed metrics of chlorophyll-a, SST, and island-scale estimates of herbivorous fish biomass. Specifically, higher coral and CCA cover was observed in more productive waters with greater biomass of herbivores, while turf cover increased with daily SST variability and reduced herbivore biomass. Interestingly, coral cover was positively correlated with daily variation in SST but negatively correlated with monthly variation. Surprisingly, metrics of acute heat stress were not correlated with benthic cover. Our results reveal that human habitation is no longer a primary correlate of calcifier cover on central Pacific island reefs, and highlight the addition of oceanic productivity and high-frequency SST variability to the list of factors supporting reef builder abundance

A Model of Loggerhead Sea Turtle (Caretta caretta) Habitat and Movement in the Oceanic North Pacific

Habitat preferences for juvenile loggerhead turtles in the North Pacific were investigated with data from two several-year long tagging programs, using 224 satellite transmitters deployed on wild and captive-reared turtles. Animals ranged between 23 and 81 cm in straight carapace length. Tracks were used to investigate changes in temperature preferences and speed of the animals with size. Average sea surface temperatures along the tracks ranged from 18 to 23 uC. Bigger turtles generally experienced larger temperature ranges and were encountered in warmer surface waters. Seasonal differences between small and big turtles suggest that the larger ones dive deeper than the mixed layer and subsequently target warmer surface waters to rewarm. Average swimming speeds were under 1 km/h and increased with size for turtles bigger than 30 cm. However, when expressed in body lengths per second (bl s 21), smaller turtles showed much higher swimming speeds (.1 bls 21) than bigger ones (0.5 bl s 21). Temperature and speed values at size estimated from the tracks were used to parameterize a habitat-based Eulerian model to predict areas of highest probability of presence in the North Pacific. The model-generated habitat index generally matched the tracks closely, capturing the north-south movements of tracked animals, but the model failed to replicate observed east-west movements, suggesting temperature and foraging preferences are not the only factors driving large-scale loggerhead movements. Model outputs could inform potentia

Maps of the tracks from the longline (LL) bycatch (A, n = 28) and Japanese (B, n = 160) releases.

<p>Red dots indicate release locations.</p

Modeled habitat index for November 2004 release.

<p>The habitat index (color scale, between 0 and 1) is overlaid with portions of tracks (black segments). From A to D: Dec. 4, 2004; Mar. 3, 2005; Jun. 1, 2005; Sep. 12, 2005.</p

Modeled habitat index for May 2005 release.

<p>The habitat index (color scale, between 0 and 1) is overlaid with portions of tracks (black segments). From top to bottom: Jun. 13, 2005; Sep. 18, 2005; Dec. 17, 2005; Mar. 16, 2006.</p

Relationship between SST and size of loggerhead turtles.

<p><b>Smooth (solid line) and 95% confidence interval (dashed lines) of SST vs.</b> SCL estimated by the GAMs during A) the winter (Dec. to Mar.) and B) the summer (Jun. to Sept.). Ticks on the bottom axis represent values of SCL for which there is data; C) Mean latitude of observed turtles (29–38 cm, solid line) and of the 17°C SST isotherm at 190E longitude (black dots).</p

Values of swimming speeds (V, km/h) for different size groups.

<p>Values of swimming speeds (V, km/h) for different size groups.</p

Comparison between the modeled habitat index and TurtleWatch.

<p>The habitat index (color scale, between 0 and 1) is overlaid with the TurtleWatch region (purple area) for the May 2005 release. From top to bottom: Jun. 13, 2005; Sep. 18, 2005; Dec. 17, 2005; Mar. 16, 2006</p

Comparison between the temperature at the surface and in the euphotic layer.

<p>SST (in °C, blue line) and average temperature of the euphotic layer (noted as T-L1, in °C, red line) from Dec. 2004 to Sept. 2008.</p