Riegl B et al. (2019), Some environmental and biological determinants ...

ECB-ART-47314

Sci Rep 2019 Jul 16;91:10322. doi: 10.1038/s41598-019-46607-9.

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Some environmental and biological determinants of coral richness, resilience and reef building in Galápagos (Ecuador).

Riegl B , Johnston M , Glynn PW , Keith I , Rivera F , Vera-Zambrano M , Banks S , Feingold J , Glynn PJ .

Abstract
Throughout the Galápagos, differences in coral reef development and coral population dynamics were evaluated by monitoring populations from 2000-2019, and environmental parameters (sea temperatures, pH, NO3-, PO43-) from 2015-19. The chief goal was to explain apparent coral community differences between the northern (Darwin and Wolf) and southern (Sta. Cruz, Fernandina, San Cristóbal, Española, Isabela) islands. Site coral species richness was highest at Darwin and Wolf. In the three most common coral taxa, a declining North (N)-South (S) trend in colony sizes existed for Porites lobata and Pocillopora spp., but not for Pavona spp. Frequent coral recruitment was observed in all areas. Algal competition was highest at Darwin, but competition by bioeroding sea urchins and burrowing fauna (polychaete worms, bivalve mollusks) increased from N to S with declining coral skeletal density. A biophysical model suggested strong connectivity among southern islands with weaker connectivity to Wolf and even less to Darwin. Also, strong connectivity was observed between Darwin and Wolf, but from there only intermittently to the south. From prevailing ocean current trajectories, coral larvae from Darwin and Wolf drift primarily towards Malpelo and Cocos Islands, some reaching Costa Rica and Colombia. Mean temperature, pH, and PO43- declined from N to S. Strong thermocline shoaling, especially in the warm season, was observed at most sites. A single environmental factor could not explain the variability in observed coral community characteristics, with minimum temperature, pH and nutrient levels the strongest determinants. Thus, complex environmental determinants combined with larval connectivity patterns may explain why the northern Galápagos Islands (Darwin, Wolf) have higher coral richness and cover and also recover more rapidly than central/southern islands after region-wide disturbances. These northern islands are therefore potentially of critical conservation importance as important reservoirs of regional coral biodiversity and source of larvae.

PubMed ID: 31311961
PMC ID: PMC6635370
Article link: Sci Rep
Grant support: [+]

Species referenced: Echinodermata
Genes referenced: cse1l LOC100887844 LOC105441782 LOC115925415 ROCK

Article Images: [+] show captions

	Figure 1. (A) Galápagos Islands (Ecuador) study area in the Eastern Tropical Pacific. (B) Rendering of bathymetry around Darwin Island, based on 200 kHz single-beam surveys undertaken in this study. Wellington Reef, the only extensive coral framework in the Galápagos, is circled in red. It does not occupy the shallowest part of the Darwin platform. Image is tilted to provide best view of Wellington Reef. Elevations of the island are approximate and not based on a land-survey. (C) Contour chart of the same bathymetry as in (B). The Hidden Reef is not a framework structure, but an accumulation of relatively large, but isolated coral colonies. Map (A) plotted using R package “marmap” and NOAA bathymetry, maps (B,C) using Matlab and gridding data from present study.
	Figure 2. Differences in assemblages of scleractinian coral species based on presence/absence. (A) Multidimensional scaling based on city-Block (Manhattan) distance between sites (all individual sites were pooled for each island). (B) Statistical groupings of coral species assemblages shown in (A) superimposed as color-code on map of the Galápagos islands.
	Figure 3. Interpolation surfaces showing colony diameter size trends in the most common coral genera across the sampled area in the Galápagos. Note differences in respective scale sizes. In (A,C) a bimodal tendency of declining coral size from N and S towards the center of the archipelago is observed. The boxplots on which this figure is based are shown in the Supplemental Information Fig. SI-1. No size class information from San Cristobal was available, hence this region is not included in the prediction surface.
	Figure 4. Coral colony frequency of occurrence in 20 cm diameter size bins across all Galápagos survey sites.
	Figure 5. Regeneration versus degradation dynamics as observed at Darwin’s Wellington reef in response to bleaching mortality in response to the 1998 ENSO and an increasing algae invasion beginning shortly after 2007. (A) Upper row: Size distribution of linear skeletal growth axes (approximately the greatest vertical height of colony) of Porites lobata in four monitoring periods at three monitoring sites. Gray section of bars in (A) denote proportion of colonies with dead patches. Data from 2000, 2007 and 2012 from refs25,31. (B,C) Lower row: Logistic regression analyses show the decline in partial mortality that had occurred from bleaching with increasing time after the event, and partial mortality increase in response to increased algal overgrowth.
	Figure 6. Depth distribution of coral sizes (all species, all island locations) measured as (A) vertical growth axis (B) maximum horizontal diameter. Note that x-axis is not regularly continuous, but shows depths at which samples existed.
	Figure 7. Coral competitors and signs of predation. (A) Shows the incidence of interaction with corals by competitors, predators and bioeroders across the Galápagos archipelago. Y-axis represents the percentage of coral colonies affected. (B) Incidence of Caulerpa spp. in phototransects (N = 160 images, datapoints are means +/− S.D.) at Darwin. Y-axis represents percent cover on the reef framework. Some corals were overgrown, but the algae primarily covered coral rock.
	Figure 8. Biophysical model of larval connectivity in the Galápagos and the Equatorial Eastern Pacific, model region with source and settlement areas. The Galápagos Islands and other well-known coral areas in Costa Rica (Cocos Island), Panama (Gulf of Chiriquí), Colombia (Malpelo and Gorgona Islands) and Ecuador (Isla la Plata) are included in this analysis (A) Expected settlement density of larvae released from Darwin and Wolf. Red is highest density, blue is lowest (B) Expected settlement density of larvae released from the southern archipelago. Red is highest, blue is lowest density, white was not reached by any larvae and/or is outside of the model’s domain. (C) connectivity matrix. X-axis represents release sites, y-axis represents “recruitment to” sites. If dots fall within any of the vertical and horizontal boxes with the same name, self-recruitment occurred (i.e. dot falls into sGAL on x-axis but COC on y-axis indicates larva from south Galápagos settled at Cocos Island). Strong connectivity within the southern Galápagos, and sparser recruitment from northern islands into southern is evident. (DAR = Darwin, WOL = Wolf, sGAL = southern Galápagos, COC = Cocos Island, PAN = Panama, COL = Colombia, ISL = Isla la Plata, ECU = Ecuador).
	Figure 9. Time series decomposition of water temperatures (half-hourly) on Wellington Reef at Darwin Island (13 m depth) from 03/19/2016 to 4/4/2018. Upper graph: recorded temperatures. Middle graph: 7-point running mean as indicator of trend, Lower graph: Anomalies (deviation from overall mean). Strong downward spikes are the random component (time series – seasonal trend – trend cycle) and thus indicate events that occur irregularly (or at least not determined by the seasonal trend or cycle), such as thermocline shoaling (upwelling). The decomposition assumes a daily period (48 measurements per period; graph not shown since visually too dense). Blue bar = cool seasonal period, red bar = warm seasonal period.
	Figure 10. Phosphate (A), Nitrate (B), and pH (C) measurements across the Galápagos Archipelago in March 2016, 2018 and 2019 (“Months 1, 3, 4”), during the warm seasons, and November 2016 (“Month 2”) at the end of the cool season (compare with Fig. 9). Nutrient measurements in mg.l−1.
	Figure 11. Correspondence analysis triplot showing site separation along first two correspondence axes (CAs) between sites, biological and environmental determinants of clustering. The same colors as in Fig. 2 are used for showing groupings. Major groups of sites separate around the centroid (0, 0; origin of blue arrows)– clearly separating the northern, the western and the remaining central and southern islands. Text in red denotes the community characteristics that structure the ordination of the sites. The community biplot (community characters by sites) scales as community characteristics (red) explaining the site-pattern (scaling 143). Blue arrows are environmental variables used heuristically to explain the variation in community characteristics at sites (making this a triplot). (Abbreviations: CCA = canonical axis, in red: Pavona recruits = N Pavona recruits, Pocillopora recruits = N Pocillopora recruits, Porites recruits = N Porites recruits, skeletal density = skeletal density, med.size_Por = median size of Porites colonies, med.size_Poc = median size of Pocillopora colonies, ms.Pavona = median size of Pavona colonies, Number species = number of species; in blue: MinTemp = minimum HadISST temperature, NO3−= field-measured concentration in mg.l−1 at surface, PO43− = field-measured concentration in mg.l−1 at surface).

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