Does Body Type Really Matter? Relating Climate Change, Coral Morphology and Resiliency

Does body type really matter? Relating climate change, coral morphology and resiliency Miranda Camp 1, 2 Karsten Shein3 Kristi Foster1 Jim Hendee4 1 Little Cayman Research Centre, Central Caribbean Marine Institute, 2Stetson University, DeLand, FL 3NOAA National Centers for Environmental Information, 4NOAA Atlantic Oceanographic and Meteorological Laboratory Abstract Anthropogenic activities have reached nearly all coral reefs around the globe. Average sea temperatures in many tropical regions are rising approximately 1-2˚C per century as a result of human activities that release heat-trapping gases, namely, carbon dioxide into our atmosphere. This temperature change is thought to be a major driver of increased frequency of coral bleaching. Certain coral morphologies, however, appear to be more resilient to changes in the environment, particularly to sea temperature variations resulting from global climate change. The objective of this study was to compare coral resilience among morphology types in Little Cayman, a remote tropical island with <200 inhabitants, where it is possible to decouple environmental and anthropogenic stressors. Three morphological groups (branching, intermediary and massive) were surveyed at 17 sites to estimate the percent cover of each group. The relationship between coral morphological coverage and temperature variation at depth was assessed in the context of geographic variation around the island. Statistical analysis suggested that, although the total number of colonies did not change significantly with increased variability in temperature, total percent coverage of all coral morphologies (excluding an outlier in the massive category) tended to decrease at sites with higher temperature variability. Introduction Corals are the foundational component of reef ecosystems that are home to a significant proportion and diversity of marine life, and upon which many communities and economies depend. It is therefore critical to understand and address the negative effects of environmental change on corals (Hoegh-Guldberg, 1999). Hard corals especially, provide shelter and food for many reef fish and other accessory species. In fact, there is a direct correlation between hard coral cover and reef fish species diversity and richness (Noble et al., 2013). Coral cover is a determinant of the health of a reef, and coral mortality often precedes a collapse of the broader reef community. Corals have diverse morphologies, including branching corals, thicker columnar lobed corals, stacking plate-like corals, and large boulder-like massive corals. More massive coral morphologies are more resistant to changes and disturbances in the environment, meaning that they are more likely to survive in response to stressors like hurricanes, or a rise in sea temperature (Highsmith et al., 1980, Raymundo, 2001). Coral survivorship is a major concern presently because of the environmental repercussions of global climate change in the marine world (Hoegh-Guldberg et al., 2007). When a coral experiences environmental stress, it expels the endosymbiotic zooxanthellae that provide a majority of the coral’s nutritional needs. Zooxanthellae also provide color to the coral, and thus the loss of these endosmybionts causes a consequent loss of pigmentation, exposing the coral’s calcium carbonate skeleton through its now transparent tissue. It is this white coloration which has led to the process being termed “bleaching” (Dove and Hoegh-Guldberg, 2006). Loya et al. (2001) examined the fitness and survivability of branching corals compared to massive coral species during the 1998 global bleaching event that was caused by a rise in sea surface temperature as a result of a strong El Nino event. Branching corals showed much higher mortality rates than massive corals in every instance, even though branching corals had dominated the reef prior to the bleaching event (Dizon and Yap, 2006). In the Caribbean alone, scleractinian (e.g., hard or stony) coral coverage has been reduced by up to 80% in certain reef areas over the past few decades (Gardner et al., 2003). Branching coral species in the genus Acropora, in particular, have declined so dramatically that they are now listed on the International Union for Conservation of Nature’s (IUCN) Red List, indicating that they are facing a high risk of global extinction. Although branching corals (e.g., Acropora cervicornis, A. palmata) are highly susceptible to coral bleaching, this morphology is commonly the focus of coral restoration efforts, because of its fast growth rate (up to eight times faster than massive coral species) (Hoegh-Guldberg, 1999; Forrester et al. 2013). Massive corals show higher resistance and resilience to elevated temperature events than branching species, but are less common in coral nurseries. This study provides an initial assessment of fitness of varying coral morphologies around Little Cayman Island in the Cayman Islands (Fig. 1). Understanding the relationship between coral morphology and resilience to temperature variability will enhance current coral restoration practices by identifying which morphologies have the highest chance of long-term survivorship following relocation of colonies grown in a nursery to the reef. Methods Two categories of data were collected for this study: water column temperature profile data at six different sites around Little Cayman Island, and coral frequency and total coverage data from transect surveys at seventeen dive sites around the island (See Appendix A and Appendix C). After grouping the coral survey data in six groups based on the temperature profile near each survey site, survey and temperature data were compared and examined for variation in coral frequency and coverage with variation in daily temperature range at the reef level. Data collection: Environmental Figure 1. Sites were selected on the basis of a diverse geographic distribution and a depth to the sandy bottom of between 15 and 20m. Temperature was collected using Onset Corp. HOBO 64K Temperature and Light Pendant Loggers, which sampled conditions at fifteen minute intervals from the end of June to the beginning of November, 2015. Sensors were secured on a buoy line at 2m and 10 m below the surface, and 2 m from the ocean floor (See Appendix A for approximate depths in the water column at each site). The mooring lines were secured to the ocean floor using sand anchors (Figure 2). Temperature profiles were collected at six sites around the island to ensure systemic geographic representation. Due to limitations of the instrument system, we restricted site selection to locations with a depth to the sandy bottom between 15 and 20m. We ultimately chose to compare temperature data between sites using the deepest HOBO loggers because that sensor is at the same depth zone as the reefs we were surveying. Data collection: Coral Composition and Percent Coverage To evaluate their relative abundance, coral species were divided into three general morphology categories: branching, intermediary and massive (See Appendix B for species in each category) and counted along a transect. We used a 20m transect tape to make belt transects and surveyed seventeen sites around the island, including five of the six temperature profile buoy sites. The sixth site was not surveyed due to equipment availability and unfavorable diving conditions. Direction of the transect line was chosen somewhat haphazardly, ensuring that the entire length of the transect was over reef habitat, and that replicates of transects at each site were conducted in varying compass directions. At HOBO buoy sites, the mooring buoy was used as a focal point around which transects originated in subjective compass directions based on the same previous guidelines of reef covering the entire length of the transect. For sites where a HOBO logger buoy was not present, transects were made in varying directions from the boat mooring line under the aforementioned methodology. Each coral colony within 0.3 m of the transect line was categorized (Appendix B) and recorded. In addition to counting the number of coral colonies of each morphology along each transect, a video of the transect was simultaneously recorded so that still images could be extracted and processed using Coral Point Count (CPCe) software (Kohler & Gill, 2006). Videos were conducted at a height above the transect that produced approximately a 0.6 m field of view centered on the transect. Video frames were taken from each transect video at 5 second intervals. For each frame, CPCe was used to analyze the total surface area of each morphology group along a transect. A frame was not analyzed if there was no coral in the image or if the tape measure was not clearly legible as a reference. Data Analysis Temperature data were aggregated to six hour averages of four time bins for the purpose of comparing daily variation throughout the course of the day. Bins allowed for consolidation of data that were collected every 15 minutes. (Early Morning hours (0:00 – 5:59), Morning hours (6:00 – 11:59), Afternoon hours (12:00 – 17:59) and Night Time hours (18:00 – 23:59)). This study focused on the sea floor level sensor because that is the closest one to the reef where coral was surveyed. Variation in temperature at the bottom sensors between HOBO sites was assessed with ANOVA tests after detrending data to eliminate bias from diurnal or annual cycles. Detrending was done by plotting the binned averages over the course of the day for the entirety of the observation period per site. Each site was then assigned the most fitting second-degree polynomial trend line because the data was not linear. I recorded the equation of the trend line, which could be used to calculate a predicted temperature value at any given time. Using this equation, I subtracted the predicted value for temperature from the Figure 2. Temperature profiles were marker buoys anchored into the sand, with HOBO loggers attached 2m and 10m below the surface and 2m from the ocean floor. actual temperature value and obtained a residual temperature. I could then compare the amount of temperature variation between sites rather than just the daily averages with an ANOVA. Transect data were divided into six groups corresponding to the HOBO logger buoys. Non-HOBO logger sites were grouped with the closest buoy, under the assumption that these groups would be the most similar environments. At each site, percent of total coral cover by each coral morphology category based on the surface area (cm2) was calculated in each frame grab. Numbers of colonies and percent composition of morphologies was averaged within each of the six temperature profile regions, and then compared across profile regions using an ANOVA. Results Coral Counts and Composition All 17 survey sites were dominated by massive colonies, followed by intermediary, and had the least number of branching colonies. The total percent coverage of massive corals exceeded both mid-branching and branching corals at each site. Variation and the environment When comparing residual temperatures between sites, only two comparisons were not significantly different: BT vs. CC, and MF vs. SS. All other comparisons had significantly different variations in temperature over the study period, resulting in P values < 0.001 (Table 1). Table 1. P value matrix of ANOVA results comparing temperature variation throughout the day at the deepest HOBO sensor. Averages of HOBO logger readings were taken in four six hour periods: early morning, morning, afternoon and night. P values were calculated using residual figures after detrending the data. Two comparisons resulted in P values > 0.05, BT to CC, and MF to SS. All other comparisons between the two sites resulted in P values < 0.05. Figure 3. Map of Little Cayman Island identifying all coral survey sites corresponding to a HOBO temperature profile site. Sites CC GG SS MF BT MB CC 2.9E-09 3.63E-34 1.77E-29 0.094115 1.36291E-19 GG 2.14E-20 4.47E-15 2.45E-06 1.29681E-05 SS 0.13676 8.67E-35 2.03895E-07 MF 3.46E-28 0.0001842 BT 1.34921E-16 MB Linear regression analysis of the relationship between the range of residual temperatures and the number of coral colonies resulted in a near zero value of the coefficient of determination for all three morphology categories (Figure 5). For branching, intermediary and massive coral species, a larger variation in daily temperature range does not affect the number of colonies that are present. However, when comparing the range of residual temperature of each site group to the percent coral coverage (surface area), there is a near zero correlation for branching species, a negative correlation for intermediary species, and a positive correlation for massive species (Figure 5). A larger variation in daily temperature range had negligible effect on the surface area of colonies present, whereas intermediary colonies showed a decrease in total surface area and percent coverage at sites with a larger daily range. Contrastingly, massive coral species showed a higher percent surface area coverage in areas of higher daily variation. Discussion Coral frequency and percent cover analysis indicated there were both a larger number of massive colonies and a larger surface area of massive colonies than of either intermediary or Branching: R² = 0.0016 Intermediary: R² = 0.4979 Massive: R² = 0.0042 0 5 10 15 20 25 30 35 0 5 10 15 Number of Colonies Residual Temperature Range (˚C) Branching Intermediary Massive Figure 4. (a) Percent coverage per morphological category as a cumulative average of all coral survey sites within respective HOBO profile area. (b) Number of colonies (cumulative average per HOBO profile area). Number of transects for each group is as follows: BT = 2, CC = 4, GG = 4, MF = 7, MB = 10, SS = 5. No percent coverage data was collected for BT due to poor quality of video transect. branching colonies at each of the six survey sites (Figure 4). There were significant differences in the amount of temperature variation that occurred at each HOBO buoy site (Table 1). The differences in average temperatures between most of the sites could be a result of benthic topography. For example, Coral City (CC) is a spur and groove formation as opposed to the wall and patch reef environment of Mixing Bowl (MB) and Blacktip Tunnels (BT). Difference in impact from strong currents may be an alternative explanation. The Caribbean Current flows from the southeast to the northwest in the summer-autumn months of the year (Grant and Wyatt, 1980: Molinari et al., 1980). The south side, therefore, bears direct impact of the current hitting the island, resulting in a more energetic hydrodynamic regime, whereas the north side is sheltered by the island, resulting in generally calmer conditions. Additionally, a 10 km wide channel exists between Little Cayman and its neighboring island, Cayman Brac. Here, channelization accelerates turbulent flow, meaning that Blacktip Tunnels, which is within the channel would be expected to experience more variable and energetic conditions than other study sites. The presence of turbulent eddies in the channel may explain the thermal range at BT of 10.98˚C being the highest of all sites. The results of this study strongly suggest a connection between coral morphology and resilience. Though differences in temperature variation were not correlated with number of colonies of any of the three morphology categories, temperature variation is correlated with percent coral cover (percent surface area) in two of the three morphology categories. Since the reefs in Little Cayman are less likely to experience local anthropogenic stressors than reefs around more populous islands in the Caribbean, this island can be used as a baseline to extract and quantify global climate change drivers of reef health as well as against which the local anthropogenic impact on reefs elsewhere may be evaluated. Annual environmental conditions, as recorded by HOBO loggers, can be used to analyze any correlations between morphology make up in a reef and the parameters of the environment, and ultimately, coral reef restoration practices can be evaluated to ensure the utmost ecological benefit. Acknowledgments I extend many thanks to all the staff (Lowell and Ms. Em especially) and mentors at the Central Caribbean Marine Institute (CCMI) for providing resources, time and advice. I also express my appreciation to all of my peers for their assistance in the field. Funding for this REU project was provided by the National Science Foundation’s Division of Ocean Sciences and private donations to the UK and US CCMI Research Fund. (b) (b) R² = 0.065 R² = 0.5458 R² = 0.4541 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 2 4 6 8 10 12 Percent Coral Coverage (decimal) Range in Residual Temperature (˚C) Figure 5. (a) Number of colonies compared to the range of residual temperatures of each site group. (b) Percent coral cover for each morphology category, expressed as a decimal, compared to the range in residual temperatures at each site. No surface area data was collected from BT or nearby dive sites to analyze surface area and percent coverage, so it is not included in the bottom figure. References Dizon RT, Yap HT. 2006. Effects of coral transplantation in sites of varying distances and environmental conditions. Marine Biology. 148: 933-943. Dove, G, Hoegh-Guldberg, IO .2006. The Cell Physiology of Coral Bleaching. In J T Phinney, Hoegh-Guldberg O, Kleypas J and Skirving W, Strong A (Ed.), Coral Reefs and Climate Change: Science and Management (pp. 55-71) Washington, DC: American Geophysical Union. Forrester GE, Taylor K, Schofield S, Maynard A. 2013. Colony growth of corals transplanted for restoration depends on their site of origin and environmental factors. Marine Ecology. 34:186-192 Grant CJ, Wyatt JR. Surface currents in the eastern Cayman and western Caribbean seas. Bulletin of Marine Science. 1980 Jul 1;30 (3):613-22. Highsmith RC, Riggs AC, D’Antonio CM. 1980. Survival of hurricane-generated coral fragments and a disturbance model of reef calcification/growth rates. Oecologia.46:322-329. Hoegh-Guldberg. 1999. Climate change, coral bleaching and the future of the world’s coral reefs. Marine and Freshwater Research. 50:839-866 Hoegh-Guldberg O, Mumby PJ, Hooten AJ, Steneck RS, Greenfield P, Gomez E, Harvell CD, Sale PF, Edwards AJ, Caldeira K, Knowlton N, Eakin CM, Iglesias-Prieto R, Muthiga N, Bradbury RH, Dubi A, Hatziolos ME. 2007. Coral Reefs Under Rapid Climate Change and Ocean Acidification. Science. 318: 1737-1742 Kohler, K.E. and S.M. Gill, 2006. Coral Point Count with Excel extensions (CPCe): A Visual Basic program for the determination of coral and substrate coverage using random point count methodology. Computers and Geosciences, Vol. 32, No. 9, pp. 1259-1269, Loya Y, Sakai K, Yamazato K, Nakano Y, Sambali H, van Woesik R. 2001. Coral bleaching: the winners and the losers. Ecology Letters. 4 (2): 122-131 Molinari RL, Atwood DK, Duckett C, Spillane M, Brooks I. 1980. Surface currents in the Caribbean Sea as deduced from satellite tracked drifting buoys. In Proceedings of the Gulf and Caribbean Fisheries Institute 31:106-115. 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Coral Reef. 20:263-272 Appendix A Location Latitude Longitude Seabed Depth (m) Sensor Depth (m) Snapshot (SS) 19°42'4.94"N 80° 3'26.89"W 14 12 7 2 2.5 Martha's Finyard (MF) 19°39'54.00"N 80° 6'38.43"W 16.5 14.5 9 2 2.5 Grundy's Gardens (GG) 19°39'24.72"N 80° 5'22.43"W 14.3 12 8 2 Blacktip Tunnels (BT) 19°42'50.97"N 79°57'33.30"W 13.4 11 7 2 Coral City (CC) 19°40'50.35"N 80° 1'24.18"W 13.4 11 8 2 Mixing Bowl (MB) 19°41'6.64"N 80° 4'38.52"W 14.3 12 7.6 2 Appendix B Branching Intermediary Massive Acropora cervicornis Orbicella annularis Montastrea cavernosa Acropora palmata Madracis decactis Orbicella franksi Millepora complanta Oculina diffusa Orbicella faveolata Porites porites Agaricia undaria Solenastrea bournoni Porites furcata Agaricia agaricia Siderastrea siderea Porites divaricata Agaricia helioseris Sierastrea radians Madracis auretenra Mussa angulosa Dichocoenia stokesi Eusmilia fastigiata Favia fragum Dendrogyra cylindricus Stephanocoenia intersepta Porites astreoides Diploria labrythiformis Colpophyllia natans Pseudodiploria strigosa Meandrina meandrites Pseudodiploria clivosa Mycetophyllia lamarckiana Isophyllia rigida Appendix C: Site Name * Site Code HOBO Group # transects per site Blacktip Tunnels BT BT 2 Coral City CC CC 2 Sand Shoot ST CC 2 Pirates Point PP GG 2 Gay's Reef GR GG 2 Mixing Bowl MB MB 2 Maryland's Cut MC MB 2 Great Wall (west) GW MB 2 Coconut Walk CW MB 2 Paul's Anchor PA MB 2 Marthas Finyard MF MF 5 Ron's Rocks RR MF 2 Tibit's Top TT MF 1 Dottie's DT MF 1 Snapshot SS SS 2 Rock Bottom RB SS 1 Crystal Palace CP SS 2 * Site names are the names used by the recreational scuba diving community to identify individual diving locations. Locations of each site are available from the public boat mooring list maintained by the Cayman Islands Department of Environment (http://www.doe.ky/wp-content/uploads/2009/09/public-moorings-list.pdf).