turf algae

MSc internship report

The health and abundance of coral reefs are declining worldwide. Coastal development, terrestrial runoff, the temperature rise of the oceans and coral diseases have decreased the health of coral reef systems in the Caribbean. Terrestrial run off has been shown to affect water quality through nutrient influx. The water quality changes and community composition of the reefs of Bonaire have been recorded in the past but only on smaller scales and most of the time in combination with sites on Curaçao. The project “Resilience Restoration of Nature and Society in the Caribbean Netherlands” (the Resilience project) aims to improve the resilience of the coral reef ecosystem on Bonaire, by implementing an extensive monitoring plan for water quality and benthic community composition. This internship project continues surveys of benthic community composition of Bonaire (1), assess the risk of anthropogenic pollution through nutrient influx (2) and relate the water quality assessment of Bonaire to the benthic community data (3).

Benthic community composition was assessed at 8 sites on the leeward coast of Bonaire, between 5 and 60 meters depth. This was done by Remote Operated Vehicle assisted, picture gathering. Dissolved Inorganic Nitrogen (DIN) and Dissolved Inorganic Phospor (DIP) concentrations were also collected at 39 locations around the leeward coast of Bonaire. 11 of the locations were sampled biweekly between November 2021 and May 2022 and nutrient concentrations from these locations were used in a water quality Risk assessment for eutrophic pressure. This was done by calculating a Risk Quotient and comparing it to the percentage of exceedance of a nutrient concentration threshold for eutrophication on coral reefs. Additionally a "Generalized Additive Model" (GAM) was used to explore the relationship between mean coral coverage and mean DIN concentration.

Turf algae showed to be the most dominant functional group between 5 and 20 meters depth, followed by cyanobacteria as the most dominant group between 40 and 60 meters. Coral cover increased until 20 meters depth, up to 25%, and showed a sharp decline afterwards. The benthic index based on AGGRA benthic indicators suggests that the coral reef ecosystem is more dominated by algae and cyanobacteria than corals. DIN concentrations are close to the eutrophic threshold of 1 μm/L and display a relatively high exceedance percentage of that threshold. DIP concentrations stay safely below the threshold value of 0.1 μm/L. Risk assessment of the worst case scenario for DIN shows that almost all locations sampled have low risk or higher for pressure by nutrients. The GAM showed no significant relationship between DIN and coral coverage. Further nutrient concentrations need to be gathered on a temporal scale to get a better view of the water quality over the whole year. The benthic community composition of locations towards the north of the island also need to be mapped to assess the effect of the elevated nutrient concentrations on the coral reef. This study hopes to support the “Resilience Restoration of Nature and Society in the Caribbean Netherlands” project with their extensive monitoring plan.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

ABSTRACT  

With increasing maritime activities in the proximity of coral reefs, a growing number of manmade structures are becoming available for coral colonisation. Yet, little is known about the sessile community composition of such artificial reefs in comparison with that of natural coral reefs. Here, we compared the diversity of corals and their competitors for substrate space between a centuries-old manmade structure and the nearest natural reef at St. Eustatius, eastern Caribbean. The artificial reef had a significantly lower species richness and fewer competitive interactions than the natural reef. The artificial reef was dominated by a cover of crustose coralline algae and zoantharians, instead of turf algae and fire corals on the natural reef. Significant differences in species compo-sition were also found between exposed and sheltered sites on both reefs. Our study indicates that even a centuries-old manmade reef cannot serve as a surrogate for natural reefs

Algae-derived dissolved organic matter has been hypothesized to induce mortality of reef building corals. One proposed killing mechanism is a zone of hypoxia created by rapidly growing microbes. To investigate this hypothesis, biological oxygen demand (BOD) optodes were used to quantify the change in oxygen concentrations of microbial communities following exposure to exudates generated by turf algae and crustose coralline algae (CCA). BOD optodes were embedded with microbial communities cultured from Montastraea annularis and Mussismilia hispida, and respiration was measured during exposure to turf and CCA exudates. The oxygen concentrations along the optodes were visualized with a low-cost Submersible Oxygen Optode Recorder (SOOpR) system. With this system we observed that exposure to exudates derived from turf algae stimulated higher oxygen drawdown by the coral-associated bacteria than CCA exudates or seawater controls. Furthermore, in both turf and CCA exudate treatments, all microbial communities (coral-, algae-associated and pelagic) contributed significantly to the observed oxygen drawdown. This suggests that the driving factor for elevated oxygen consumption rates is the source of exudates rather than the initially introduced microbial community. Our results demonstrate that exudates from turf algae may contribute to hypoxia-induced coral stress in two different coral genera as a result of increased biological oxygen demand of the local microbial community. Additionally, the SOOpR system developed here can be applied to measure the BOD of any culturable microbe or microbial community.

Abstract

Despite the increasing dominance of turf algae in coral reefs, few studies have investigated their physiological and ecological responses to changes in abiotic factors. We tested the effects of depth and ultraviolet radiation on turf algae at different levels of successional stages using two experiments. Depth-related differences were found for all turf algal communities, characterized by a higher amount of the cyanobacteria taxonDichothrix and the red filamentous genera Poly-/Herposiphonia in the shallow and the appearance of oscillating cyanobacteria in deeper waters. In the first experiment, cross-depth transplantation of 153 days old communities influenced percentage cover, biomass and taxa composition. Downward transplantation lowered overall biomass and abundance of the foraminifera Sorites, whereas the crustose green alga Pringsheimiellaand filamentous cyanobacteria colonized the communities. A nearly reverse pattern was observed in upward transplanted communities. Overall we distinguished between sensitive taxa, like Oscillatoria, and taxa able to acclimate to alterations in their environment, like Pringsheimiella, Poly/Herposiphonia and Dichothrix. In the second experiment, algae grown for 285 days at 5 m were exposed together with a set of sterile settlement tiles to three UVR regimes at 2 m for 22 days. UVR had no effect on turf algal communities regardless of successional stage. This study highlights the presence of high light and UV tolerant species. The high UV tolerance of turf communities may confer a competitive advantage over other more sensitive coral reef biota, such as corals. This study demonstrates that turf algae are dynamic communities exhibiting species-specific resistance to environmental changes.