The coral reefs of Eilat (Israeli Red Sea, 29oN) are biodiverse and rich in endemisms, but also fragile and susceptible to stress by natural and anthropogenic disturbances, including nutrient enrichment and recreational activities. The goal of my dissertation was to assess the ecological and economic impacts of anthropogenic stressors on these coral reefs.

Levels of coral cover and abundance on a coral reef flat at Eilat were estimated in 2001 by surveying 19, 10-m transects, and compared to the levels reported in the same area between 1966 and 1973. Lower values compared to 1966 levels are evident, and there has been only a modest recovery following a catastrophic low tide that killed a large proportion of the corals in 1970. Percent cover of soft and stony corals (16.1%) was less than half of that reported for 1969 (35%), when a sharp decrease in coral abundance had already been observed. In contrast to 1966, when half of the transects surveyed contained more than 30 coral colonies, no transects with this number of corals were observed. The cover of seven of the most common stony coral species was 841 cm, which is twice the coral cover of that in 1973, and only 22% of the 1969 level. Millepora dichotoma, an abundant species before 1970, has almost been depleted, and the soft coral Litophyton, abundant in 1972, was not observed. Anthropogenic nutrient enrichment is apparently among the causes for the lack of coral recovery in the studied reef flat. Reefs located further away from sources of pollution have recovered quickly after natural and anthropogenic disturbances, and have retained their coral abundance and diversity.

A photographic analysis was conducted at 5 shallow reef sites (5-6 m) at Eilat to study changes in live coral cover during a 2-year period. Logit regression analysis showed that levels of total oxidized nitrogen (TON; NO2 + NO3) and the presence of SCUBA divers were significant explicative variables of coral partial mortality, while sedimentation rate was not significant. Sites exposed to mean TON levels above 0.3 micro-M TON showed significantly lower live stony coral cover and abundance per m2, and higher partial mortality of coral colonies, than sites exposed to lower TON.

The relationship between levels of TON in the water column, and the worm-coral-zooxanthellae association at 5 shallow reef sites at Eilat were investigated. Of 656 stony corals examined, 218 (33.2%) were infested with spionid worms. There was a significant correlation between TON levels and percent infested coral colonies in the genera Leptastrea and Porites. High levels of sigma-PSII (the functional absorption cross section for Photosystem II) were measured in portions of colonies of Astreopora myriophthalma that were infested with spioinid polychaetes versus areas not infested, indicating relatively high concentrations of chlorophyll. It is suggested that nitrogen enrichment of the water column surrounding coral reefs at Eilat may cause corals to be more susceptible to infestation by spionid polychaetes, and affect the coral-zooxanthella symbiosis.

Effects on the welfare of coral reef users of decreases in reef quality were estimated by means of the Choice-Modelling methodology. The results showed that SCUBA divers are willing to pay an additional 11.86 NIS (New Israeli Shekels; 1 NIS is approximately US $0.22) for an improvement of 1 m in water clarity, and 5.46 NIS for an improvement of one unit in a coral and fish diversity index. From the standpoint of recreational diving welfare, the annual social costs of activities contributing to coral reef degradation are approximately 13 million NIS.