Water conditions: The assessment of water conditions on La Azufrada reef was carried out using the data collected during an oceanographic sampling conducted near to the reef (<100m). In this station, a discrete sample of water from 1m and 10m depth was collected using a Niskin bottle. Temperature, salinity and dissolved oxygen were measured using an YSI 85 multi parameter probe in both water samples. Furthermore, three water aliquots from both depths were taken in order to measure in situ turbidity and chlorophyll using an AquaFluor® field fluorometer. A Spectroquant® Merck was used to determine nutrient concentration (nitrite, nitrate and phosphorus). Vertical records of temperature and salinity were carried out with a Seabird-19 CTD unit and water transparency was measured using a Secchi disk. A t-test was used to evaluate differences in chlorophyll-a and turbidity between 0m and 10m depths.

Sediment deposition: Sediment deposition rate was measured as a function of the trap collection rate, following methods used by Chou, Yu & Loh (2004), Dikou & Van Woesik (2006), Maldonado, Giraud & Carmona (2008) and Storlazzi, Field & Bothner (2011), with some modifications. We established nine stations along the coral reef (three stations per reef zone: backreef, flat and slope) and deployed sediment traps at each station (8cm diameter: 21cm heights; collector area: 54.7±0.8cm2) placed 20cm above the seabed. One trap consisting of a 2L plastic bottle was tied to a stainless steel rod and held in position by plastic bands. A replica was set 1m apart at each station (nine stations, 18 traps, six per reef zone), allowing us to evaluate the reproducibility of the collection rate measurement (Bothner, Reynolds, Casso, Storlazzi & Field, 2006).

Sediment traps were deployed on April 29th 2009, and were collected two days after, on May 1st 2009; the total exposure time was 44.3±0.6h). Every trap was filled with hypersaline borax-buffered 10% formaline solution before their deployment to minimize disturbance by aquatic organisms. The animals found in the filter contents were fixed and stored in a 4% formalin solution and frozen until identification was done. In the laboratory every individual was classified by taxonomic group according to Young, Sewell & Rice (2002). The sample design allowed us to compare the abundance (one way ANOVA) and diversity (Shannon index) between reef zones.

Trap contents, including all suspended particles, were filtered in a manifold with a vacuum bomb using pre-weighed cellulose filters (52mm) which were then dried at 65ºC for 24h and weighed again. The sediments were then removed from the cellulose filters to pre-weighed Whatman GFF filters (42mm) and combusted to ash at 450°C for 5h. The dry weight of the sediment samples was used to estimate the average trap collection rate (g dry sediment per m2 and day), while weight differences between dried and combusted samples allowed us to estimate mean organic content of the sediment entering the traps (% organic matter weight relative to dry sediment weight; Maldonado et al., 2008). A Kruskall Wallis test (KW) was used to evaluate differences in sediment deposition rate among backreef, flat and slope. A Dunn test (multiple t-test postanova) was carried out between each possible pair among the group (reef zones) when differences between the treatments were found.

RESULTS

A mean trap collection rate of 23.30±4.34g m-2d-1 (mean±confidence interval) was found for La Azufrada reef. Reproducibility of the collection-rate measurement by tube traps on each reef zone was 39%, 32% and 26%, respectively. In each reef zone the sediment deposition rate was highly variable. The particulate inorganic material deposition rate in the backreef ranged from 23.86 (20.42-29.38)g m-2 d-1[median (25%-75%)], in the flat zone from 18.29 (16.81-23.44)g m-2 d-1, and in the slope from 23.70 (19.16-29.32)g m-2 d-1. However, no significant differences were detected (K-W, H=0.38, p=0.85, Fig. 2). On the other hand, when the particulate organic material contribution to the collected sediment was evaluated, we detected differences between the backreef and the slope zones (KW, H=9.96, p=0.007, Table 1).

Physical and chemical measurements were similar to “normal conditions” previously reported for Gorgona Island (Giraldo 2008, Giraldo et al. 2008). Mean temperature and salinity in the coral reef depth zone (0-10m layer) were 26.98±0.19°C and 32.60±0.52, respectively. The water column around La Azufrada reef was stratified with a sharp thermocline (isotherm 21°C) at 17m depth (Fig. 3).

Physical and chemical conditions were in the optimal range for growth and development of coral reefs (Table 2; see Table 1 in Szmant, 2002). However, turbidity and chlorophyll-a concentration were above the threshold for optimal coral reef conditions. Although, we did not detect a significant difference in chlorophyll-a concentration between surface (0m) and 10m depth in La Azufrada reef (t-test. t=2.55, p=0.06), a significant turbidity difference was detected between surface and 10m depth (t-test, t=56.90, p<0.001; Fig. 4)

The fauna collected in the sediment traps was composed of fourteen taxonomic groups (Table 3). Bivalves were the most abundant group (39.3% of the whole sample) followed by copepods (23.8 %). The most frequent groups were copepods (100%), bivalves (71%) and shrimps (71%). There was no difference between abundance (KW, p=0.21) or diversity (KW, p=0.66) in the traps between the reef zones.

DISCUSSION

The mean sediment deposition rate estimated for La Azufrada reef is in accordance with the “normal” sedimentation rate expected on a coral reef (<100g m-2d-1, Rogers, 1990). Furthermore, it was equivalent to the 10% of sediment tolerance limit (300g m-2d-1) proposed for fringing reefs by Hawker & Connell (1992) mentioned by Dikou & Van Woesik (2006). Cantera et al. (2003) reported a complex spatial pattern of sedimentation for Playa Blanca reef (a coral reef adjacent to La Azufrada). These authors found a lower sediment deposition rate on the slope (10.28g m-2 d-1) than on the backreef (29.38g m-2 d-1); results that support the idea that terrestrial runoff from the island should be the main source of sediment in this reef (Zapata, 2001). Although our estimation of the sediment deposition rate on the backreef was similar to the one reported by Cantera, Orozco, Londoño-Cruz & Toro-Farmer (2003), on the slope zone the trap collection rate was threefold higher. This temporal difference may suggest that some years later not only sediments derived from Gorgona itself are arriving to La Azufrada reef, but also sediment from the mainland could be affecting the reef simultaneously. However, this should be confirmed with upcoming and more meticulous studies (e.g. monthly sampling, rainy and less rainy seasons) considering that results presented herein were obtained from a single sampling time point conducted during the rainy season and furthermore the methods used in this study differ from those used by Cantera et al. (2003).

In the last decade the coral cover on Gorgona Island reefs has decreased by 38.3% (Zapata et al., 2008) and it has been suggested that sedimentation stress may explain the coral-algal dynamics in this environment (Zapata et al. 2010). Apparently, this coral cover decrease could be connected to an increase in the net sediment deposition rate. Therefore, future research projects should ascertain the origin of this sediment (whether it’s from island runoff or if it has been transported from the mainland) because it has been reported that terrigenous sediment discharge reduces coral growth and coral reef biodiversity (Acevedo & Morelock, 1988; Rogers, 1990; Fabricius, 2005). In this sense, the nearest potential mainland source of sediment to Gorgona is the Patía Sanquianga river delta (c.a.<50km), with a sediment load equivalent to 14t yr -1 (Restrepo & Kjerfve, 2000).

The influence of mainland waters (Patia Sanquianga river plume) or runoff waters into La Azufrada reef could be modulated by local weather conditions. Two main climatic seasons have been described in Gorgona: a rainy period from May to November (c.a.>500mm month-1) and a less rainy period from December to April (c.a.<500mm month-1; Blanco, 2009). Therefore, a seasonal variation in sediment deposition, water transparency, nutrient availability, and sea surface temperature and salinity would be expected. The present study was carried out during the rainy season, where the influence of mainland waters is greater than in the less rainy season. Therefore, the concentration of nutrients and particulate matter in the water column will be higher and reduction in water column transparency and the growth of benthic macro-algae would be expected. These environmental conditions favour the growth of coral competitors, especially those that bore into coral skeletons, such as endolithic bioeroders, sponges, molluscs, worms and burrowing algae; and probably make corals more susceptible to diseases (Spencer & Viles, 2002; Wilkinson, 2004).

The zooplankton and fish actively access the inside of the sediment traps (Silver, Pilskaln & Steinberg, 1991, Steingber, Pilskaln & Silver, 1998, Buesseler et al., 2000; Kraft et al., 2012). Some of those organisms would be considered intruders (e.g. Ophiuridae, shrimp and fish) and some genuinely associated with detritus (e.g. bivalves, copepods or amphipods) that is immersed in the material collected inside the trap (Silver & Gowing, 1991; Kraft et al., 2012). Moreover, the sediment trap could have provided a new habitat or sheltered environment for some zooplankton and fishes where abundant food sources can be found (Kraft et al., 2012). Although the fauna associated with sediment in the eastern Tropical Pacific coral reef areas have not been extensively studied, recent research suggests that the factors responsible for the community structure in this environment is related to sediment characteristics, hydrodynamic conditions and specific region/habitat features (Semprucci et al., 2010; 2013). For instance, under a low flow regime of organic matter deposition, sediment-dwelling taxa (nematodes and annelids) are favored, while in a habitat dominated by rubble and dead coral fragments, there may be an increase in copepods and ostracods associated to soft bottom sediment areas (Semprucci et al., 2013). Therefore, inside a sediment trap deployment in a tropical coral reef the dominant fauna would be filter-feeder (e.g. bivalve) and dretitus-feeder (e.g. copepods), such as those groups that were dominant in this study.

Sediment traps have been used over the years to acquire a representative sample of the material settling vertically through the water column and to provide integrated particle collection rates and particle properties over the time of deployment (Rogers, Garrison, Grober, Hillis & Franke, 1994; Almada-Villela, Sale, Gold-Bouchot & Kjerfve, 2003; Bothner et al., 2006). In fact, now sediment traps in coral reef environments are being used as a standard method for monitoring the effects of sediment on coral reefs and as a method for determining the impact or effectiveness of land-use practices (Wilkinson, Green, Almany & Dionne, 2003; Hill & Wilkinson, 2004; Jordan Banks, Fisher, Walker & Gilliam, 2010). However, the information derived from sediment traps have been misinterpreted because of a lack of understanding of the trap design and the deployment method for the type and the amount of material being collected. It also has been misapplied due to a failure to understand the sediment and sediment trap dynamics in the shallow-water environment associated with the coral reef (Storlazzi et al., 2011). These two things must be taken into account when implementing this method in a coral reef health monitoring program.

In summary, the findings presented here constitute the first effort to describe the water conditions at La Azufrada coral reef and this is also the first report of potential sediment deposition rate in this important ecosystem at Gorgona Island. These results can be used as a baseline for future research, although further comprehensive studies (e.g. considering seasonal variation and sediment re-suspension) must be applied to reef monitoring and management practices. Research addressed to clarify the origin of the sediments (i.e. transported by river plume from the mainland or runoff waters from the island itself), and comparisons between rainy versus less rainy season (high precipitation may alter the sediment budget and generate a different spatial sediment deposition pattern) will be valuable for knowledge of the sediment dynamics at La Azufrada reef. This will provide useful information for the conservation of this valuable ecosystem.

ACKNOWLEDGMENTS

We thank Diego Ramírez, Jose Luis García, and Isabel Ocampo for their support during the field work and data collection. Special thanks to Emily Giles (King Abdullah University of Science and Technology) for correcting the English manuscript. This work was financed by Universidad del Valle and the Henry von Prahl Research Station (Parque Nacional Natural Gorgona). Research permit No DTSO-G-05 03/08 to A.G.

RESUMEN

Uno de los principales factores de estrés en los corales es el asentamiento de sedimento suspendido sobre su superficie. Esto conlleva al bloqueo de la luz, el cubrimiento superficial del mucus coralino e incremento del riesgo de enfermedades. Por esta razón, la sedimentación en considerada como un variable de alta importancia en el estudio de los arrecifes coralinos. Por medio de trampas de sedimentación (en nueve estaciones: seis por zona arrecifal), este proceso fue estudiado en un arrecife coralino (La Azufrada) del Pacífico Oriental Tropical, localizado en Isla Gorgona, Colombia. En las trampas se encontró una tasa promedio de colecta de sedimento de 23.30±4.34gm-2d-1 a y no se detectaron diferencias significativas entre las zonas arrecifales. La temperatura promedio y salinidad de la columna de agua donde habitan corales en este arrecife (entre 0 y 10m) fue 26.98±0.19°C y 32.60±0.52, respectivamente. Catorce grupos taxonómicos de invertebrados fueron encontrados en las trampas de sedimentación. Los bivalvos y copépodos fueron los más abundantes y frecuentes. Los resultados presentados constituyen el primer reporte de la tasa de deposición de sedimentos y de las condiciones del agua en La Azufrada. Estos resultados representan una línea base para estudios posteriores, aunque se necesita más investigación sobre estos temas que son esenciales para el manejo de los arrecifes coralinos.

Palabras clave: deposición de sedimentos, arrecifes de coral, condiciones oceanográficas, invertebrados, Pacífico Oriental Tropical, Isla Gorgona, Colombia.

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