The seascape ecology of tropical islands: can we characterize and predict reef community patterns across scales?

Figure 1. The towed-diver survey method. Graphics credit: NOAA Ecosystem Sciences Division (ESD).

By Gareth J. Williams, Eoghan A. Aston, Andrew J. Davies, Jamison M. Gove

Landscape ecological theory, applied to the marine realm, can revolutionize our understanding of how biological spatial patterns are linked to ecological and physical processes. The challenge associated with collecting spatially broad, high-resolution data underwater is worth overcoming, since it can catalyze new ways in which we understand and manage marine ecosystems. We can now turn to methods that allow us to preserve the taxonomic resolution of our observations, while at the same time expanding the area we survey. One such method is the towed-diver survey (Fig. 1), whereby a diver is towed sub-surface behind a boat, using a board to guide themself at speeds of up to 3 km/h. As well as the diver making visual observations, the board can be equipped with high-resolution digital cameras that take photographs periodically of the reef floor, as well as other sensors that capture environmental conditions.

The Ecosystem Sciences Division (ESD) of the National Oceanographic and Atmospheric Administration (NOAA) uses this towed-diver technique to characterize the coral reef communities of numerous islands and atolls across the Pacific Ocean. They have done so during biennial surveys at each island since the year 2000, covering over 5000 linear km of reef, akin to swimming from London to New York. This heroic undertaking is matched by an equally heroic effort to then analyze the hundreds of thousands of georeferenced photographs to provide estimates of the amount of reef floor occupied by different benthic groups. By circumnavigating entire islands, these photos enable us quantify the spatial shifts in benthic communities as we traverse an island’s coastline.

A question then emerges: what do spatial shifts in coral reef benthic communities look like across scales and are they predictable?

Results

In our recent paper, we show that the benthic communities around Jarvis Island, a remote coral reef island in the Pacific Ocean, show predictable spatial patterns across different scales. Hard corals, crustose coralline algae, fleshy macroalgae and turf algae all fight for their place on the seafloor, helped or hindered in different places by gradients in key physical drivers. We found that these groups were spatially clustered across the seascape at scales of less than ~1 km.

A striking feature of this spatial arrangement was the aggregation of hard coral along ~44% of the island’s perimeter, including the majority of the western coastline of Jarvis, a feature that was stable during three survey years in 2006, 2008, and 2010. Moving around the island coastline, we then saw chunks of the coastline dominated by alternating benthic groups and very abrupt spatial transition points between them. We found that the winners and losers in the competition for space were best predicted by sub-surface gradients in temperature, driven primarily by localized (and relatively frequent) upwelling events along the western side of the island. The upwelling brings cool nutrient-rich water on to the shallow reefs. With a functionally intact community of herbivorous reef fish keeping the nutrient-fueled benthic algae at bay, the coral communities here are likely able to thrive on the additional source of nutrients and food, allowing them to grow and outcompete the other benthic groups and hold their ground (Fig. 2).

Interestingly, the spatial patterns in benthic communities around Jarvis were reminiscent of those around Palmyra Atoll, some ~700km NNW of Jarvis, at least in their patterns of spatial scaling (Gove et al. 2015). Tantalizingly, this suggests that benthic communities on coral reefs may display some universal scaling law, at least if they are undisturbed and protected from local human impacts that may act to distort and change these patterns.

Figure 2. A small group of Pacific snapper Lutjanus bohar and a grey reef shark Carcharinus amblyrhyncos swimming over the western side of Jarvis Island, which is dominated by hard plating Montipora corals (Photo credit: Brain Zgliczynski).

Comments