Hong Kong's coastal waters afford a marginal environment for coral reef growth, with high seasonal and short-term variability in water temperatures (ranging from <14°C in winter to 31°C in summer), and low summer salinity (as low as 15 psu) due to runoff associated with the Asian wet monsoon season and the Pearl River Delta. Yet Hong Kong hosts 84 reef-building coral species in 28 genera of 12 families of the Scleractinia, distributed in 5 broad communities with strong geographic and environmental affinities and key indicator species. Coral communities farthest from the influence of the Pearl River Delta have relatively high sea bed coverage and species diversity (30 to 50% coverage, and > 30 spp. per site), and also host some large, old corals. X-radiographs of a core of one massive Porites colony confirms ~200 yr of growth, with 2 short-term periods of mortality, hidden in the skeleton by overgrowth. Very low linear extension rates (< 4 mm yr -1 ) in this coral and 2 others are anomalous for Porites corals and are indicative of the high-stress environment. Low growth rates suggest that other Hong Kong corals, despite the harsh environmental conditions, may also live for centuries, contributing to the development of 'incipient reefs'. KEY WORDS: Hong Kong · Long-lived massive corals · Porites · South China SeaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 426: 185-196, 2011 ate framework, slow or stunted growth rates, and decreased depth distribution (Harriott & Banks 2002). The lack of framework in these communities is primarily attributable to a lack of significant calcium carbonate accretion (Buddemeier & Smith 1999, Kleypas et al. 1999b. In many instances corals in these communities attach to hard substrata as isolated colonies growing on exposed bedrock (e.g. Macintyre 2003). Although coral communities can have species diversity similar to that of true coral reefs (>140 spp. in the Indo-Pacific region; Sheppard & Sheppard 1991, Perry 2003, most have much lower species diversity (< 90 spp.; Harriott & Banks 2002, Moyer et al. 2003, Nozawa et al. 2008.One such marginal environment hosting coral communities is the area around Hong Kong. Hong Kong (22' 20°N, 114' 11°E) lies on the southern Chinese coast, in the tropics, some 320 km south of the Tropic of Cancer (Fig. 1). Hong Kong waters experience a range of temperatures (from 14 to 31°C) and salinities (typically 24 to 25 practical salinity units, or psu, comparable to ppt) generated by the interplay of the monsoonal systems, changing ocean currents, and freshwater discharge (Tang & Ni 1996, Lee & Liu 1998. Hence the coral communities of Hong Kong are living at the edge of their physiological tolerances (e.g. Veron 1995, Harriott 1999. Episodic widespread partial-to-total colony mortality and localized community mortality of corals and other tropical marine species are observed at meteorological or oceanographic extremes (e.g. precipitation events or sea surface temperature [SST] minima;McCorry...

Southern China hosts coral communities in marginal environments that are characterized by low linear extension rates, low coral cover and/or no reef formation, thus providing natural laboratories to study coral communities with below average growth rates. Here we compare the annual linear extension rates over 10 years (range 1.2 to 11.4 mm yr−1) of six Porites sp. coral cores collected from Hong Kong with monthly hydrographic data from the Hong Kong Environmental Protection Department. At all sites, low-density, dry season extension were more variable than high-density, wet season extension and on average, was lower at two of the three sites. We applied multi-variate linear regressions that revealed high-density, wet season band extension to inversely correlate most significantly to temperature (r = −0.39, p<0.01). In contrast, low-density, dry season band extension was more variable and correlated most significantly with dry season chlorophyll-a (Chl-a) (r = 0.64, p<0.001). Additionally, we find that corals at the site with highest dry season Chl-a have the highest dry season extension lengths. Our findings indicate that relative mixing of fresh and salt water in the wet season and primary productivity in the dry season, and their influences on aragonite saturation, are likely to impact interannual coral extension variability in marginal environments.

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