ALBERT

Description: Globally, seagrass habitats have experienced sharp declines over the past century, with an annual loss of seagrass cover of 7%yr-1 since 1990. Despite the attention to seagrass this decline has brought, little research has been directed towards trends of seagrass habitats in Singapore. The research presented here developed and applied remote sensing methods to partially fill this gap, provide tools for more extensive monitoring in the future, and contribute to the global body of seagrass research. Satellite images from four different satellite sensors were used to estimate seagrass bed extent in Singapore’s second largest seagrass meadow, at Pulau Semakau, from 2001 to 2013. Statistical estimates of image signal-to-noise ratios were used to screen images for quality. Validation data collected in 2013 were used to estimate error for supervised classifications produced from each sensor. A novel method was explored to account for macroalgae blooms in the study area, but the resulting correction could not be validated and did not affect the overall trends in seagrass bed extent. In addition to the classification analysis, an empirical model linking remote sensing reflectance to above-ground biomass was constructed to examine the distribution of seagrass within the meadow. Applied to WV2 images from 2011 and 2013, this model produced estimates of above-ground biomass with root mean squared error (RMSE) of 54 gm-2 and 44.7 gm-2, respectively, within ranges of 0-288 gm-2 and 0-229 gm-2, respectively. A novel index to measure seagrass density non-destructively was developed to help conservation and monitoring efforts. This index, normalized canopy index (NCI), was estimated from satellite imagery more precisely than above-ground biomass, producing estimates from the 2013 WV2 image corresponding to field data with an R2 of 0.71 relative to the R2 of 0.39 produced by the above-ground biomass model. This index may be a promising, non-destructive alternative to above-ground biomass for remote sensing studies and should be pursued further in future research. Based on the time-series classification analysis, seagrass bed extent at Pulau Semakau declined from over 44.6 ha in April 2002 to 25.3 ha in June 2013. This decline occurred at an average of 5.1%yr-1 from 2001 to 2013, although this rate of decline slowed to 3.7%yr-1 in 2012. These declines are likely representative of other seagrass habitats in Singapore. Broader monitoring is required to determine to what extent Singapore’s seagrasses are disappearing. Although seagrass bed extent declined by 17% from April 2011 to June 2013, over the same time period total above-ground biomass in the seagrass meadow declined only 5%, from 41.6 Mg to 39.6 Mg. Two acute sedimentation events recorded over this time period corresponded to a large and permanent decrease in bed extent captured by WV2 imagery and a small and temporary decrease in bed extent captured by ALI imagery. I hypothesize that the discrepancy in decreases in extent and biomass, coupled with an increase in median biomass, is attributable to preferential survival and recolonization of dense-biomass seagrass species during these sedimentation events. Measurements of seagrass species abundance during this time period provide support for this hypothesis. This exercise demonstrates the advantages and limitations of monitoring seagrass bed extent and above-ground biomass. Bed extent provides a measure of overall viability of a seagrass meadow, but above-ground biomass provides a better index of spatially variable health and internal change. Coupled, these two measurements provide greater insight into complex seagrass bed processes and seagrass response to disturbance.

Description: Masters

Description: Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 125 (2020): e2019JF005446, doi: 10.1029/2019JF005446.

Description: Atoll reef islands primarily consist of unconsolidated sediment, and their ocean‐facing shorelines are maintained by sediment produced and transported across their reefs. Changes in incident waves can alter cross‐shore sediment exchange and, thus, affect the sediment budget and morphology of atoll reef islands. Here we investigate the influence of sea level rise and projected wave climate change on wave characteristics and cross‐shore sediment transport across an atoll reef at Kwajalein Island, Republic of the Marshall Islands. Using a phase‐resolving model, we quantify the influence on sediment transport of quantities not well captured by wave‐averaged models, namely, wave asymmetry and skewness and flow acceleration. Model results suggest that for current reef geometry, sea level, and wave climate, potential bedload transport is directed onshore, decreases from the fore reef to the beach, and is sensitive to the influence of flow acceleration. We find that a projected 12% decrease in annual wave energy by 2100 CE has negligible influence on reef flat hydrodynamics. However, 0.5–2.0 m of sea level rise increases wave heights, skewness, and shear stress on the reef flat and decreases wave skewness and shear stress on the fore reef. These hydrodynamic changes decrease potential sediment inputs onshore from the fore reef where coral production is greatest but increase potential cross‐reef sediment transport from the outer reef flat to the beach. Assuming sediment production on the fore reef remains constant or decreases due to increasing ocean temperatures and acidification, these processes have the potential to decrease net sediment delivery to atoll islands, causing erosion.

Description: This study was supported by the Strategic Environmental Research and Development Program through awards SERDP: RC‐2334, and RC‐2336. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Description: 2021-03-25

Description: These data files and MATLAB scripts reproduce the model data and figures as published in Bramante et al. (in prep) Modeling the impacts of a changing climate on cross-shore sediment transport: Kwajalein Atoll, Marshall Islands.

Description: Atoll reef islands primarily consist of unconsolidated sediment, and their ocean-facing shorelines are maintained by sediment produced and transported across their reefs. Changes in incident waves can alter cross-shore sediment exchange and thus affect the sediment budget and morphology of atoll reef islands. Here we investigate the influence of sea-level rise and projected wave climate change on wave characteristics and cross-shore sediment transport across an atoll reef at Kwajalein Island, Republic of the Marshall Islands. Using a phase-resolving model, we quantify the influence on sediment transport of quantities not well-captured by wave-averaged models, namely wave asymmetry and skewness and flow acceleration. Model results suggest that for current reef geometry, sea level, and wave climate, potential bedload transport is directed onshore, decreases from the fore reef to the beach, and is sensitive to the influence of flow acceleration. We find that a projected 12% decrease in annual wave energy by 2100 CE has negligible influence on reef flat hydrodynamics. However, 0.5-2.0 m of sea-level rise increases wave heights, skewness, and shear stress on the reef flat, and decreases wave skewness and shear stress on the fore reef. These hydrodynamic changes decrease potential sediment inputs onshore from the fore reef where coral production is greatest, but increase potential cross-reef sediment transport from the outer reef flat to the beach. Assuming sediment production on the fore reef remains constant or decreases due to increasing ocean temperatures and acidification, these processes have the potential to decrease net sediment delivery to atoll islands, causing erosion.

Description: This project was supported by the Strategic Environmental Research and Development Program through awards SERDP RC-2334 and RC-2336.