Proceedings 9th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000
Coral population dynamics in a subtropical coral community, Solitary Islands Marine Park, Australia V. J. Harriott 1 and S.D.A. Smith2 ABSTRACT The population dynamics of coral communities have rarely been studied in subtropical coral communities, despite hypotheses that such communities may be subject to more variable mortality and recruitment rates than tropical coral reefs. In the Solitary Islands Marine Park (30ºS) in Eastern Australia, changes in cover of coral and other biota in mapped fixed quadrats were recorded at three island sites at approximately annual intervals between 1993 and 1998. At two of the sites, coral cover was relatively stable over time and ranged between 27% and 35% cover over the five years. At the third and most inshore site on the coastline at Coffs Harbour, coral cover approximately doubled during the study period, from 11% to 23% cover. For all times pooled, coral recruitment rates averaged 1.3 to 1.8 recruits/m2/year, while mortality rates were 0.7 – 2.0 corals/m2/year, with recruitment rate strongly correlated with mortality rate for different taxa. Pocillopora damicornis contributed approximately 50% of the population turnover, but only 23% of the coral cover at the three sites. Principle causes of coral mortality during the period were localised storm damage and overgrowth by worm tubes.
Keywords Subtropical reefs, Coral population dynamics Introduction Long-term studies of coral population dynamics in repeatedly-sampled quadrats have provided invaluable information on the dynamics and community characteristics of coral reefs (for example Connell 1978, Porter et al. 1981,1982, Hughes 1985, 1988, Connell et al. 1997, Hughes and Connell 1999). The ecological significance and timing of factors limiting coral recruitment and mortality are most readily derivable from repeated examination of the same communities at regular intervals. Such data can also be used to derive models of community population dynamics and trajectories over time (Preece and Johnson 1993, Tanner et al. 1994, 1996, Kudo and Yamano 1997, Mumby 1999). For high latitude coral communities, no such data sets have been published from which similar analyses and models can be derived. While coral reefs are largely limited to tropical regions where water temperature rarely falls below 18º C (reviews in Stoddart 1969, Rosen 1988, Veron 1995), many subtidal communities with significant coral cover occur at higher latitudes. Some of the better known of these communities include those present on both the east and west coasts of Australia, Japan, the Hawaiian chain of islands, Florida and the Gulf of Mexico, Bermuda, South America, and southern Africa. Examination of the factors regulating these communities can increase our understanding of the processes which control development of coral reefs (Grigg 1998) Hypotheses on limits of coral reef development include: a reduction in reproductive viability of corals at high latitudes (Yonge 1940, Wells 1957, Veron 1974), limited recruitment success of corals at high latitudes (reviewed in Harriott and Simpson 1997, Harriott 1999a), reduced growth rates at high latitudes (Grigg 1982, Harriott 1999b), reduced capacity of corals to compete with
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temperate algae or fouling organisms (Johannes et al. 1983, Miller and Hay 1996, Holmes et al. 1997, Miller 1998, McCook et al. 2001), high rates of carbonate solution at high latitudes associated with greater development of epilithic and other algal communities (Barnes and Lazar 1993), and high coral mortality rates (Edmondson 1929, Veron and Done 1979, Burns 1985, Tribble and Randall 1986). Edmondson (1929), Veron and Done (1979) further suggested that coral mortality caused by episodes of cold water exposure might limit the capacity for corals to survive at high latitudes. However, more recent evidence suggests that a significant number of species of hermatypic corals can survive at temperatures lower than 14ºC (Tribble and Randall 1986, Coles and Fadlalah 1991 reviewed in Veron 1995). Thus, we might hypothesise that an examination of coral population dynamics in a high latitude coral community would find high, and temporally variable, coral mortality rates relative to tropical sites. The absence of coral reproduction and successful recruitment has been argued to limit coral distribution and capacity of subtropical communities to recover from episodes of mortality (Yonge 1940, Wells 1957, Veron 1974, Harriott 1995). Studies at subtropical Australian sites have reported that total recruitment on settlement panels is somewhat less than for tropical Australian sites, and that most recruits are from a few brooding taxa; recruits of broadcast-spawning corals are uncommon (Harriott 1992, 1995, 1999a, Banks and Harriott 1996, Harriott and Simpson 1997). Recruitment of corals into the field has not previously been quantified from a high latitude site, but if recruitment reflected larval availability as recorded by settlement panel studies, it might be hypothesised that recruitment of corals at visible size might be lower in high latitude communities than at low latitude ones.
CRC Reef Research Centre, PO Box 772, Townsville, 4810, Australia.
[email protected] Zoology Dept, University of New England, Armidale, 2351, Australia
Johannes and co-workers have argued that cold temperatures and high nutrient levels give macroalgal species a competitive advantage over corals, and that algae will progressively replace corals at high latitude sites (Johannes et al. 1983, Hatcher and Rimmer 1985, Crossland 1988). Miller and Hay (1996) also reported that macroalgae inhibited growth and recruitment of a temperate coral species in North Carolina, USA, thus suppressing the potential for reef development. Miller (1998) has reviewed the mechanisms by which temperate algal dominance might control community structure in coral/algal systems. The Solitary Islands are in a similar biogeographical zone in eastern Australia to the Houtman Abrolhos in western Australia, where Johannes and colleagues developed the hypothesis on macroalgal competition. Kelp and fleshy macroalgae form a significant component of the benthic community and, at nearshore sites, patches of coral are interspersed with kelp forests and stands of other macrophytes (Veron et al. 1974, Smith and Simpson 1991, Harriott et al. 1994). An examination of community dynamics might indicate whether competition between corals and macroalgae play a significant role in structuring these communities. The Solitary Islands Marine Park off the mid-north coast of NSW in eastern Australia, supports the most southerly significant coral communities in eastern Australia. Well developed coral communities exist in many sites, and over 90 coral species have been recorded in recent surveys (Harriott et al. 1994). The community structure, recruitment and growth of corals in the Solitary Islands have been reported in a series of recent papers (Harriott et al. 1994, Harriott 1999a, b). This paper reports on the community dynamics of coral populations at three sites within the marine park over a five year period. The study sought to investigate the hypotheses that: (a) coral recruitment success is limited at high latitudes; (b) coral mortality rates are higher and more variable than those reported from tropical sites; and (c) the outcomes of competitive interactions between corals and macroalgae structure high latitude coral communities. Methods A description and map of the Solitary Islands (153º E, 30º S) is provided in Harriott et al. (1994). The study sites were Muttonbird Island, which is close to shore at the township of Coffs Harbour and is connected to the shoreline by a breakwater; Split Solitary Islands, approx. 5 km north of Coffs Harbour and approx 2 km offshore, and North West Solitary Island, about 25 km north of Split Solitary Island and 5 km offshore. In April 1993, 20 fixed, 1 x 1 m quadrats were established on the leeward (north to north-west) side at each of island site. The location of each quadrat was marked using aluminium bars hammered into the rocky substrate, and later (when bars were becoming over grown with fouling biota) by pitons at fixed points. The locations of the quadrats at each site were marked on a map to assist relocation. Quadrats were placed on flat sections of the substratum to assist quadrat mapping. At Muttonbird
Island, flat areas of the benthic community are dominated by the kelp Ecklonia radiata, with corals restricted to patches several metres in breadth and generally close to more vertical areas. At Muttonbird Island, quadrats were located in the coral patches. Other than these limitations, quadrat locations were selected randomly within the available area at each location. A number of the markers were lost over the duration of the study and consequently data collection in subsequent years was from a subset of the original quadrats. In most cases quadrats were lost because markers had become detached from the substratum, presumably under the influence of wave-action which can be severe in this region. However, at two sites (North West Solitary and Split Solitary) at least some quadrat markers were temporarily lost due to their complete overgrowth by sandy tubes produced by dense aggregations of Chaetopterus worms (Smith and Harriott 1998). The timing of samples and number of quadrats which were relocated at each location for each sample time are presented in Table 1. Table 1. Sampling dates and number of quadrats relocated at each sample time. The number of quadrats per site for which 4 or 5 sample times were available is also listed.
Dates April 93 April 94 June 95 August 96 May 98 No. missing
