River reef impact studies I (RRIS I): reef responses to changing water temperatures

Abstract

Vietnam coastal waters host a wide range of marine ecosystems. Of these, coral reefs continue to be stressed by a variety of threats, particularly in areas of dense human populations, such as in Nha Trang Bay NTB. So far there are no studies available that try to establish direct causal links between natural or anthropogenic factors. This study attempts to do so by asking: what are average, max/min temperatures in NTB and how profound are seasonal changes?; can we identify major changes/ trends from year to year?; and, are there differences in coral cover and species composition and can these be related to temperature? For long term temperature measurements, 10 Hobo (USA) loggers were deployed at depths of six to 20m at selected locations directly in the coral reefs. Coral transects were performed according to the Lifeform standards and by using a video cam for permanent transects. Within few weeks two record lows and highs (in June 2012) and covering 10° temperature differences (in September 2011) in a depth of 12m, were recorded. In comparison to 2007 through 2009 not only were more extremes found, but a much higher intra-day variation, which can easily cover 5° difference, in exceptional cases even 8°C, was found. On coral species level only low to moderate coral cover was found in general. This is reflected in a clear gradient of coral cover and diversity from West to East. On a community level coral reefs are well developed in the southern area of Nha Trang Bay. The most immediate impact to reefs and the main driver of environmental degradation in NTB is thought to be from pollution and nutrient sources. As shown here, the temperature regime is providing a sub-optimal environment for coral growth. The overall situation is more complex than previously assumed and a number of stressors are important. For reefs in the south we predict an „increased resilience“.