G. Dogruer

Methods for Assessing Acropora spp. Populations in the Caribbean

Since the severe decline of the Acropora populations in Bonaire in the 1980s, no assessment has characterized the distribution of remnant colonies. Because of their patchy distribution, a large sampling effort is necessary to adequately describe their occurrence. However, the spatial scale at which this assessment needs to be carried out makes this prohibitive with approaches such as transects using SCUBA gear and photogrammetry. This internship project aimed to optimize and apply a simple methodology trialed by relevant stakeholders on the island to obtain coarse but spatially explicit data with relatively low time-investment. Snorkelers utilizing a waterproof GPS and a slate to record coarse categorical data outlined patches of Acropora cervicornis and Acropora palmata in-situ. These were processed with an ArcGIS workflow to create shapefiles of coral patches as polygons joined to their corresponding data. The resulting polygons were used to describe the distribution of Acropora spp. along the leeward coast of Bonaire. Furthermore, these were used as ground-truthing data to test whether remote sensing imagery can be used to detect A. cervicornis remotely. 466 polygons along 14.5km of the coast were created, showing a patchy distribution of both species, more frequent occurrence of A. palmata in the northern leeward coast compared to the southern, and vice-versa for A. cervicornis. A multinomial logistic regression, maximum likelihood classification, and forest-based classification all showed a high accuracy in labelling A. cervicornis correctly in remote sensing data, but all showed frequent misclassification of other reef structures as A. cervicornis. The mapping approach presented in this internship could be applied to investigate fragmentation effects in Acropora populations and to gather in-situ ground-truthing data for other benthic habitats.

Date
2023
Data type
Scientific article
Theme
Research and monitoring
Geographic location
Bonaire

Spatial and Temporal variation of Chlorophyll-a and nutrients in coastal waters of Saba

Anthropogenic stressors such as improper wastewater treatment, dump sites and construction sites can increase nutrient input into marine coastal waters, thereby declining water quality conditions for coral reefs and the surrounding marine environment. Increased nutrient flow in the form of dissolved inorganic nitrogen (DIN) and phosphorous (DIP) is taken up by phytoplankton leading to enhanced algal and cyanobacterial growth which can be measured by an increase of Chlorophyll – a (Chla). As part of the project, “Restoration of Nature and Resilience in the Dutch Caribbean”, water quality monitoring has been implemented on Dutch Caribbean islands to determine local stressors effecting costal water quality. Thus far, the role of land-based nutrient pollution has not been assessed on the Dutch Caribbean Island, Saba. This report analyses the water quality surrounding Saba’s coastal waters with a focus on Chla and nutrients and their relation to anthropogenic stressors on the island. We analysed the distribution of Chla and nutrients spatially and temporally. It is shown that Chla frequently exceeds the 0.2-0.3μM threshold around the harbour and in the two bays: Well’s Bay and Spring Bay/Cove Bay. Temporally, the highest elevated values can be attributed to the hurricane season form August-October. The first round of nutrient sampling from the week of 25-05-2022 shows that DIN exceeds its threshold for Cove Bay and Tent Reef, with ammonium contributing the largest to the DIN content. In future, attention needs to be paid to the DIN and DIP balance since high elevated ratios have been found at Tent Reef, possibly indicating DIP - limitation. With only one week of nutrient data available, no correlation between Chla and nutrients could be made. Overall, sites such as Tent Reef, Well’s Bay, and Spring Bay/Cove Bay demonstrate the most severely declining water quality based on frequently elevated Chla and nutrient levels. Further research on water quality data surrounding Saba is necessary to confirm these findings and to address stressors contributing to decreased water quality.

Date
2023
Data type
Scientific article
Theme
Research and monitoring
Journal
Geographic location
Saba