Abstracts PA3

Coastal Development and Hinterland Dynamics



Abstract 144

Inventorizing dense mangrove forest canopies using instance and semantic segmentation networks on UAV imagery

by Daniel Schürholz | Arjun Chennu

Max Planck Institute for Marine Microbiology | ZMT


Mangrove forests provide invaluable ecosystem services to coastal communities across worldwide tropic and sub-tropic regions. Current anthropogenic stressors threaten these ecosystems and urge researchers to create improved monitoring methods. Recent studies have focused on automatically quantifying the above-ground biomass found in mangrove forests as an indicator of the ecosystem’s health. This is achieved by using remote sensing, computer vision and machine learning methods, showing promising results. Nonetheless, such studies focus on high resolution imagery of mangrove forests that have sparse vegetation. In this study we focus on mangrove-forest patches with dense vegetation, consisting of the endemic Pelliciera Rhizophorae and the invasive Rhizophora Mangle mangrove species, located in the Utria national park in the Colombian Pacific coast. Our developed workflow uses consumer-grade UAV imagery of the mangrove forests, from which we built large orthophotomosaics and digital elevation models. We apply convolutional neural networks for instance segmentation, to accurately delineate (36% AP) individual tree canopies for the Pelliciera Rhizophorae species. We also apply neural networks to accurately identify (82% recall) the area coverage of the Rhizophora Mangle mangrove tree species, as well as the area coverage of surrounding ground and water. Using the segmented ground areas we interpolate their height from the digital elevation model to generate a digital terrain model. Finally, we calculate a canopy height model from the digital elevation and terrain models, and combine it with the inventory of Pelliciera Rhizophorae trees to derive the height of each individual mangrove tree. Our workflow accurately predicts the number and height of trees, as shown by our validation with on- ground data, which was acquired simultaneously to the UAV footage. The resulting inventory of mangrove trees with height information enables the use of allometric equations to calculate important monitoring metrics, such as above ground biomass and carbon stocks.


Abstract 128

Anthropogenic nutrients and phytoplankton diversity metrics to assess coastal water quality in Kenya

by Nancy A. Oduor


Coastal habitats offer a wide range of services for human welfare and vital ecosystem functions used by various species of marine life during their life cycle. However, due to the growing human population and intense socio-economic activities, these habitats are becoming more vulnerable to anthropogenic nutrient pollution. Kenya’s seascape provides foraging habitats to five of the seven globally endangered sea turtles species. This study identified, mapped, and conducted a comprehensive assessment of ecological quality in sea turtle foraging sites using physicochemical indices and phytoplankton assemblages.

The study identified 154 turtle foraging grounds spread over 264 km of the Kenyan coast from Vanga in the south to Ng’omeni in the north. Ecological quality assessment was conducted in 68 sites selected in September 2021 and 30 sites in July 2022. The results established an average concentration of dissolved inorganic nutrients NOX (Nitrate + Nitrite), ammonium (NH4-N), phosphate, and silica of 9.98. 12.77, 0.67, and 6.40 µmol/L, respectively. It also revealed the presence of 154 phytoplankton species belonging to 119 taxa with the species composition dominated by diatoms (59 %,), dinoflagellates (23%), and cyanobacteria (9%), respectively. Based on chlorophyll a concentrations, about 60% of the assessed sites were under lower mesotrophic, within good and moderate ecological quality status during the study period. This study established for the first time an overview of the quality of coastal waters in Kenya. It however recommends longer, year-round studies to establish nutrient dynamics in the area.


Keywords: Eutrophication, nutrient pollution, phytoplankton assemblages, Kenya.

Abstract 143

Do seabird-derived nutrient subsidies influence the composition of reef-derived carbonate sediments in the Chagos Archipelago?

by Aitana Gea Neuhaus | Ines D Lange | Marleen Stuhr 

Leibniz Centre for Tropical Marine Research (ZMT), Bremen, Germany ; Institute for Chemistry and Biology of the Marine Environment (ICBM), University Oldenburg, Germany | Geography, University of Exeter, UK | Leibniz Centre for Tropical Marine Research (ZMT), Bremen, Germany

The production of carbonate sediments by coral reef organisms is essential for tropical beach and especially for low-lying island formation and maintenance. Even in areas that are characterised by minimal local human impacts, like the northern atolls of the Chagos Archipelago, global coral bleaching events had substantial impacts on reef communities and are hence expected to also influence sediment supply regimes. Therefore, with ongoing degradation of coral reef ecosystems and climate change-related sea level rise, a better understanding of sediment production has become increasingly important. Besides global stressors, local guano-derived nutrient runoff from islands with high seabird densities may play an important role by enhancing the productivity of reef organisms. Here, we examine the spatial distribution of main sediment components across different reef zones and in relationship to seabird derived-nutrient subsidies around six islands in the Chagos Archipelago, central Indian Ocean. We a) analyse spatial variations of organic carbon, nitrogen contents, and δ15N ratios, to quantify local nutrient distribution, b) quantify the overall contribution of different carbonate producers to sediments across islands with different seabird densities and between lagoon and forereef habitats, and c) use the benthic foraminifera community assemblages therein as bioindicators of environmental conditions. Initial results of the Salomon Atoll revealed coral fragments as dominant carbonate component followed by molluscs and foraminifera across reef zones, with increased abundance of green calcifying algae Halimeda on outer reef slopes. δ15N ratios in the sediment were considerably elevated on islands with high seabird densities, with peaks in the island and beach samples and declining with increasing distance to shore. Ongoing analyses will help to better understand the general sediment supply regimes of these reef islands and show if seabird-derived nutrient inputs affect sediment composition, allowing insights into the connectivity between local land-based activities and marine ecosystem functioning.


WordPress PopUp Plugin