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“.
