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"Armstrong, Roy"
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Localized inshore warming, acidification, and elevated particulate organic matter across a coupled mangrove, seagrass, and coral reef ecosystem in La Parguera, Puerto Rico
Global declines in mangroves, seagrasses, and corals threaten the provisioning of ecosystem services to coastal communities. However, potential feedbacks between these ecosystems are poorly understood owing to a lack of studies exploring functional links between these frequently coupled nearshore tropical ecosystems. To better understand these links, we sampled seawater temperature, salinity, and pH in addition to particulate organic carbon, particulate organic nitrogen, and their respective stable isotope composition (δ13C and δ15N) monthly from July 2018 to June 2019 at sites ranging from inshore to offshore in La Parguera Natural Reserve, Puerto Rico. We observed persistent, year-round warmer, more acidic, and higher particulate organic matter seawater at the most inshore Bioluminescent Bay station compared to sites further offshore. Particulate organic carbon values ranged from a maximum of 764 mg/m3 at the most inshore station to a minimum of 53.96 mg/m3 at the most offshore station while particulate organic nitrogen ranged from 166 mg/m3 at the most inshore station to 6.39 mg/m3 at the most offshore station. Biogeochemical variability across spatial scales largely followed gradients in biological and physical controls, with limited differences between the more offshore coral reef sites and the most intense biogeochemical modification occurring in the shallowest and most nearshore stations. Limited temporal variability was observed for most parameters except for seasonal variations in temperature, salinity, and pH. Particulate organic matter stable isotope composition were slightly enriched at inshore stations and suggested a mixture of primarily marine allochthonous and autochthonous organic matter sources. The importance of heterotrophy for coral resilience to warming and acidification suggests this localized nutrition-rich albeit warmer and acidified waters in nearshore mangrove and seagrass ecosystems is an important functional link to nearshore corals with the potential to modulate coral resilience to ocean warming and acidification.
Journal Article
Relative Pigment Composition and Remote Sensing Reflectance of Caribbean Shallow-Water Corals
Reef corals typically contain a number of pigments, mostly due to their symbiotic relationship with photosynthetic dinoflagellates. These pigments usually vary in presence and concentration and influence the spectral characteristics of corals. We studied the variations in pigment composition among seven Caribbean shallow-water Scleractinian corals by means of High Performance Liquid Chromatography (HPLC) analysis to further resolve the discrimination of corals. We found a total of 27 different pigments among the coral species, including some alteration products of the main pigments. Additionally, pigments typically found in endolithic algae were also identified. A Principal Components Analysis and a Hierarchical Cluster Analysis showed the separation of coral species based on pigment composition. All the corals were collected under the same physical environmental conditions. This suggests that pigment in the coral's symbionts might be more genetically-determined than influenced by prevailing physical conditions of the reef. We further investigated the use of remote sensing reflectance (Rrs) as a tool for estimating the total pigment concentration of reef corals. Depending on the coral species, the Rrs and the total symbiont pigment concentration per coral tissue area correlation showed 79.5-98.5% confidence levels demonstrating its use as a non-invasive robust technique to estimate pigment concentration in studies of coral reef biodiversity and health.
Journal Article
Hyperspectral Sensing of Disease Stress in the Caribbean Reef-Building Coral, Orbicella faveolata - Perspectives for the Field of Coral Disease Monitoring
The effectiveness of management plans developed for responding to coral disease outbreaks is limited due to the lack of rapid methods of disease diagnosis. In order to fulfill current management guidelines for responding to coral disease outbreaks, alternative methods that significantly reduce response time must be developed. Hyperspectral sensing has been used by various groups to characterize the spectral signatures unique to asymptomatic and bleached corals. The 2010 combined bleaching and Caribbean yellow band disease outbreak in Puerto Rico provided a unique opportunity to investigate the spectral signatures associated with bleached and Caribbean yellow band-diseased colonies of Orbicella faveolata for the first time. Using derivative and cluster analyses of hyperspectral reflectance data, the present study demonstrates the proof of concept that spectral signatures can be used to differentiate between coral disease states. This method enhanced predominant visual methods of diagnosis by distinguishing between different asymptomatic conditions that are identical in field observations and photographic records. The ability to identify disease-affected tissue before lesions become visible could greatly reduce response times to coral disease outbreaks in monitoring efforts. Finally, spectral signatures associated with the poorly understood Caribbean yellow band disease are presented to guide future research on the role of pigments in the etiology.
Journal Article
Comparison of recent survey techniques for estimating benthic cover on Caribbean mesophotic reefs
Highly divergent estimates of benthic cover of sponges have been reported for Caribbean mesophotic reefs (90–100 m) based on quadrat point-intercept data collection using 2 methods: visual surveys conducted in situ by technical divers, and analyses of photographs taken by unmanned underwater vehicles (UUVs). The second method has been criticized for potential errors from image distortion caused by variable camera angle relative to the substratum, but without a broader comparison of both methods. We find that studies that have used the UUV-based method are advantageous for a number of reasons, most importantly: (1) access to the full mesophotic zone, (2) higher sample replication, and (3) reduced likelihood of sampling bias. For tech diving surveys conducted at 91 m, i.e. the deepest depth reported using this method but only midway through the mesophotic zone, studies have reported particularly high sponge cover (∼80 vs. <10% for UUV-based surveys), which may be a consequence of low replication and targeted sampling influenced by very short working times under hazardous conditions. When evaluating benthic abundance metrics from photographs, issues associated with variable substratum angle are common to any topographically complex surface, particularly within a quadrat. Nevertheless, point-intercept estimates are not dependent on quadrat area and are not subject to error due to image distortion or surface complexity. Unlike visual census data from tech dives, UUV photographs can be validated by taxonomic experts and archived for re-analysis. Past tech diving surveys should be repeated using the UUV-based method with greater replication over the full range of the mesophotic zone in order to reconcile divergent estimates of benthic cover.
Journal Article
Hypoxia driven by Caribbean Sargassum accumulation events
Sargassum accumulation events threaten coastal ecosystems across the Caribbean and have been associated with severe hypoxia. However, our understanding of Sargassum-induced hypoxia is limited by the lack of continuous monitoring of seawater dissolved oxygen during Sargassum accumulation events and the absence of decaying Sargassum oxygen uptake rates. Here, we combined time series of dissolved oxygen with remote sensing of Sargassum areal coverage in early summer and late winter at Isla Magueyes, Puerto Rico. While mild/moderate hypoxia was more frequent in summer than in winter, severe hypoxia was only observed following two distinct Sargassum accumulation events in early and mid-September 2023. We conducted incubation experiments with fresh decaying Sargassum to quantify mean ± s.e. oxygen demand rates of 0.0038 ± 0.0005 mg DO d−1 mg Sargassum−1 and incorporated these rates into a box model to show that modelled night-time was more severe with increasing Sargassum biomass and that less biomass was required to reach severe hypoxia under warmer and longer residence time scenarios. Our results demonstrate that Sargassum accumulation can drive local hypoxia and that the frequency and severity of Sargassum-induced hypoxia events in the Caribbean will likely increase under ongoing warming. These findings could be leveraged for an early warning system for future Sargassum-induced hypoxia events.
Journal Article
Water Quality Anomalies following the 2017 Hurricanes in Southwestern Puerto Rico: Absorption of Colored Detrital and Dissolved Material
Absorption of colored dissolved organic matter or detrital gelbstoff (aCDOM/ADG) and light attenuation coefficient (Kd490) parameters were studied at La Parguera Natural Reserve in southwestern Puerto Rico, before and following Hurricanes Irma (6–7 September) and María (20–21 September) in 2017. Water quality assessments involving Sentinel 3A ocean color products and field sample data was performed. The estimated mean of ADG in surface waters was calculated at >0.1 m−1 with a median of 0.05 m−1 and aCDOM443 ranged from 0.0023 to 0.1121 m−1 in field samples (n=21) in 2017. Mean ADG443 values increased from July to August at 0.167 to 0.353 m−1 in September–October over Turrumote reef (LP6) with a maximum value of 0.683 m−1. Values above 0.13 m−1 persisted at offshore waters off Guánica Bay and over coral reef areas at La Parguera for over four months. The ADG443 product presented values above the median and the second standard deviation of 0.0428 m−1 from September to October 2017 and from water sample measurement on 19 October 2017. Mean Kd490 values increased from 0.16 m−1 before hurricanes to 0.28 right after Hurricane Irma. The value remained high, at 0.34 m−1, until October 2017, a month after Hurricane María. Analysis of the Sentinel (S3) OLCI products showed a significant positive correlation (rs = 0.71, p = 0.0005) between Kd490_M07 and ADG_443, indicating the influence of ADG on light attenuation. These significant short-term changes could have ecological impacts on benthic habitats highly dependent on light penetration, such as coral reefs, in southwestern Puerto Rico.
Journal Article
Deriving Bathymetry from Multispectral Remote Sensing Data
The use of passive satellite sensor data in shallow waters is complicated by the combined atmospheric, water, and bottom signals. Accurate determination of water depth is important for monitoring underwater topography and detection of moved sediments and in support of navigation. A Worldview 2 (WV2) image was used to develop high-resolution bathymetric maps (four meters) that were validated using bathymetry from an active sensor Light Detection and Ranging (LiDAR). The influence of atmospheric corrections in depth retrievals was evaluated using the Dark Substract, Fast Line-of-Sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) and the Cloud Shadow Approach (CSA) atmospheric corrections. The CSA combined with a simple band ratio (Band2/Band3) provided the best performance, where it explained 82% of model values. The WV2 depth model was validated at another site within the image, where it successfully retrieved depth values with a coefficient of determination (r2) of 0.90 for all the depth values sampled, and an r2 of 0.70, for a depth range to 20 m. The WV2 bands in the visible region were useful for testing different band combinations to derive bathymetry that, when combined with a robust atmospheric correction, provided depth retrievals even in areas with variable bottom composition and near the limits of detection.
Journal Article
Quantifying the Effects of Hurricanes Irma and Maria on Coastal Water Quality in Puerto Rico using Moderate Resolution Satellite Sensors
Coastal, benthic communities, such as coral reefs, are at particular risk due to poor water quality caused by hurricanes. In addition to the physical impacts from wave action and storm surge, hurricanes bring significant rainfall resulting in increased runoff from land. Hurricanes Irma and Maria caused record or near-record floods at many locations across Puerto Rico and resulted in major impacts on coastal and benthic ecosystems from heavy rainfall and river discharge. In this study, we use imagery from the moderate resolution Visible Infrared Imaging Radiometer Suite (VIIRS) satellite to quantify the impacts of hurricanes Irma and Maria, which struck Puerto Rico during September 2017, on the water quality of the coastal waters of Puerto Rico using the chlorophyll-a (Chl-a) and the diffuse attenuation coefficient at 490 nm (Kd490) products. The objectives include: (1) quantify the water quality and light attenuation after the hurricanes; (2) compare this event to the climatology of these parameters, and 3) evaluate long-term exposure and exceedances of various coastal areas to low levels of turbidity. The Chl-a inner shelf values increased in 2017 during the months of June (8% above baseline), July (17%), August (5%), September (8%), October (19%), and November (28%) when compared to 2012–2016 baseline data. The values for Chl-a concentration reached and exceeded 0.45 µg/L by August 2017 and persisted above that value until December 2017. The Kd490 inner shelf values for 2017 increased (in percent) for the months of June (4% above baseline), July (9%), August (10%), September (5%), October (12%), and November (7%) when compared to 2012–2016 baseline data. The values of Kd490 in August, September, and December 2017 were the highest seen during 2012–2017. Even with the limitations of spatial resolution and loss of data to cloud cover, the 6-year imagery time-series analysis can provide a useful evaluation of the effects of these two hurricanes on the coastal water quality in Puerto Rico, and quantify the exposure of benthic habitats to higher nutrient and turbidity levels.
Journal Article
Comparison of Satellite-Based Sea Surface Temperature to In Situ Observations Surrounding Coral Reefs in La Parguera, Puerto Rico
Coral reefs are among the most biologically diverse ecosystems on Earth. In the last few decades, a combination of stressors has produced significant declines in reef expanse, with declining reef health attributed largely to thermal stresses. We investigated the correspondence between time-series satellite remote sensing-based sea surface temperature (SST) datasets and ocean temperature monitored in situ at depth in coral reefs near La Parguera, Puerto Rico. In situ temperature data were collected for Cayo Enrique and Cayo Mario, San Cristobal, and Margarita Reef. The three satellite-based SST datasets evaluated were NOAA’s Coral Reef Watch (CoralTemp), the UK Meteorological Office’s Operational SST and Sea Ice Analysis (OSTIA), and NASA’s Jet Propulsion Laboratory (G1SST). All three satellite-based SST datasets assessed displayed a strong positive correlation (>0.91) with the in situ temperature measurements. However, all SST datasets underestimated the temperature, compared with the in situ measurements. A linear regression model using the SST datasets as the predictor for the in situ measurements produced an overall offset of ~1 °C for all three SST datasets. These results support the use of all three SST datasets, after offset correction, to represent the temperature regime at the depth of the corals in La Parguera, Puerto Rico.
Journal Article