Variability in patterns of growth and morphology of Posidonia oceanica exposed to urban and industrial wastes: contrasts with two reference locations

Urban and industrial wastes have been claimed to negatively affect Posidonia oceanica meadows, but few studies have addressed this issue by comparing disturbed locations with replicated reference locations. Here, we examined the general proposition that patterns of growth and morphology of P. oceanica exposed to urban and industrial effluents were different from those observed in reference meadows. Hypotheses were both on differences in mean values of response variables and on variation of these measures at a hierarchy of spatial scales (from centimetres to hundreds of metres). Results indicated a significant reduction in mean number of intermediate leaves at the outfall compared to reference locations, whereas the opposite pattern occurred for juvenile leaves. There were significant, though temporally variable differences in growth of rhizomes between disturbed and reference locations, with reduced growth at the outfall in 2 out of 3 years analysed. Measures of spatial variance in number of juvenile leaves and length of adult leaves at the scale of shoots were significantly larger at the outfall compared to reference locations. At the same scale, measures of spatial variance in length of juvenile and intermediate leaves were significantly lower at the putatively impacted location. Spatial variance in number of intermediate leaves was reduced at the outfall compared to reference locations at the scale of quadrats. Past values of spatial variance in number of leaves per shoot were lower at the outfall than at the two reference locations at the scale of shoots, whereas the opposite occurred at the scale of areas. None of the structural variables examined showed any difference between the putatively impacted location and the two reference locations, either in terms of mean response or as changes in spatial variance. These results indicated that integrating methods to examine present and past events of disturbance, including analyses to detect changes in spatial variance of response variables, may provide a powerful approach to the analysis of environmental impacts on P. oceanica.