Herbivores effect on coral reefs : relationships between sea urchins, coral health and algae cover

Sammanfattning: Coral reefs are vital to marine ecology. The population dynamic on coral reefs are complex and sensitive to natural and anthropogenic changes. Various factors can affect the population dynamic and inflict change. Coral bleaching is threatening coral reefs worldwide and is a hot topic in today’s climate challenges. The main cause is increasing water temperatures. A bleached coral has higher mortality and is more likely to be covered in macro algae which reduces its chances to recover. The herbivores facilitate the competition between algae and corals through grazing. Thus, the herbivorous taxa may be crucial for the ability of reefs to recover after disturbance. This field study was done in north-west Madagascar, in the water channel between the two islands Nosy Be and Nosy Komba. The aim of this study is to better understand the role that herbivorous sea urchins and algae coverage has on the reef status. This will facilitate to understand the ecological processes in the reef system. Through diving surveys, data was collected to study the interactions between coral health, algae coverage and sea urchins. The main research questions are 1) how prevalent coral bleaching is in the surveyed areas, 2) if reef status and algae coverage is correlated in the area and 3) if the numbers of grazing sea urchins are related to reef status and algae cover. Along an 80 meter transect the benthic composition and number of sea urchins within 2.5 meter from the line was noted. For each survey, a new place was randomly chosen in the channel to get a broader understanding of the area between the islands Nosy Be and Nosy Komba. The results were used for statistical analysis in R. The results show no significant correlations between coral health, algae coverage and sea urchins. There are weak trends, which can be supported by earlier research in this field. These weak trends are that herbivores have a negative influence on algae cover and algae cover has a negative influence on healthy corals. On the area studied coral bleaching was 0.01% out of the total amount of coral, which was lower than expected. Degraded corals and algae cover were widespread in the surveyed area. One conclusion is that over-fishing of herbivorous fish can be one reason for the degradation rather than coral bleaching. Another conclusion is that the sampling number in this study is too small to get significant results. Improvements of the method used is needed to get a higher quality data suitable for statistical analysis.