No abstract

ABSTRACT. Analyses of coral colony size-frequency distributions reveal Important characteristics of populations on the reef. Coral colony size-frequencies can be modelled by log normal d l s t r~b u t~o n s and coral populations can be described by statistics of frequency distlibutions such as coefficient of vanation, skewness and mode. Coefficient of vanatlon allows comparison of var~ation in coral populations of d~fferent mean colony size. Skewness reflects the proportion of sinall versus larger colonies, representing juvenile Input and longevity. The mode in the size-frequency dlstnbut~ons represents the most frequent colony size The mode appears ind~cative of a change in the relative impact of total and partial n~ortality in coral populat~ons. These size-frequency var~ables dlffer significantly between coral species and, w i t h~n specles, between reef localities. Coral colony size-frequency diagrams provide insight into past events and have some predictive power with respect to population development. Our data demonstrate that measurement of the above-ment~oned variables could be a tool to estlniate the response of coral populat~ons to the reef environment KEY WORDS Coral reef survey Colony slze Partial mortal~ty Modular demography

Colony size is an important characteristic for clonal modular organisms such as corals because life-history processes, e.g. reproduction and mortality, are strongly related to size. These processes are affected by the environment, and size-frequency distributions of coral populations provide information on the responses of populations to environmental conditions. We hypothesised that the size-frequency structure of coral populations in degraded environments would differ from those in a more pristine environment. We used log-transformed colony-size data of 13 coral species from the fringing reefs of Curaçao, Netherlands Antilles. Data were collected at 4 sites, representing 2 environments: a heavily urbanised coastal area with degraded reefs, and an upstream control area. Population structure was dependent on species and site. Size-frequency distributions of the same species from different sites were twice as similar as distributions of different species, indicating the strong effects of various life-history traits among species. Site effects were indicated by significant differences between distributions of 11 species at 4 sites. Mean colony size varied up to an order of magnitude between species, and much less between sites; however, in 10 species mean colony size was also significantly different among the 4 sites. The pattern of the differences was not consistent among species, indicating that mean colony size in degraded sites was higher for some species and lower for others. Parameters describing the shape of the population appeared to reflect a general response to reef condition. In the degraded area, frequency distributions tended to show increased negative skewness, occasionally extreme positive kurtosis, and smaller standard deviations: 11 populations were generally more negatively skewed, with the bulk of the population concentrated in the larger size classes; kurtosis was on average higher and extremely peaked; and standard deviations indicated that colony size varied less. This evidence suggests lower recruitment and higher partial mortality in larger colonies in this area. Lower recruitment decreased the influx into the smaller size classes of the populations, and higher partial colony-mortality decreased the proportion of colonies in the higher size classes while simultaneously increasing the proportion of colonies in the medium-sized classes. Species that attained large sizes (indicative of great age) appeared to have size-frequency distributions skewed to the left, while small species were more skewed to the right. The data indicate a general dichotomy in coral lifehistory strategies with respect to colony size, with small species generally having a shorter lifespan and reproduction being relatively frequent and successful. Thus, new input into smaller size classes occurs continuously. Species that attain large sizes live longer and are less dependent on frequent recruitment, and consequently populations tend to become 'impoverished' in small colonies, resulting in size-frequency distrib...

No abstract

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0–2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m−2 in summer and autumn, and 2.7 individuals m−2 in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0–2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0–2 m layer were higher than corresponding values from the 0–200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0–200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change.