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Overgrazing can deplete macroalgal communities and lead to reduction of habitat complexity and species diversity. To quantify the effects of sea urchin and fish herbivory on the rocky reefs of the North Aegean Sea, a six-month grazer exclusion experiment was conducted at the southeastern part of Lesvos Island at three rocky reef sites at depths between 1.0 and 4.7 m. The aim was to test whether (a) algal growth is hampered by the combined grazing activity of sea urchins and fish, and (b) algal growth is suppressed by the foraging of fish alone. At each site, three replicate cage, cage-control, and control treatments were applied and sampled every two weeks using photoquadrats: (1) fully-closed cages that excluded both sea urchins and large herbivorous fish; (2) open-top cages that excluded sea urchins only; and (3) control surfaces with no restrictions on herbivores. Algal biomass was estimated based on the percentage cover values of the analyzed images. Underwater surveys for the estimation of sea urchin population density (by quadrat sampling) and total fish biomass (by strip transects) were also conducted. The main grazers observed were the sea urchins Arbacia lixula and Paracentrotus lividus , and the herbivorous fish Sarpa salpa . Herbivorous fish dominated fish communities, but the overall fish biomass density was relatively low. The experimental monitoring of algal biomass showed that erect algal growth was significantly higher inside the fully-closed cages in contrast to the respective controls. Overall, algal growth was significantly higher inside both types of cages, when compared to the control surfaces. No significant differences among the two types of cages were found in terms of total algal growth, indicating that urchin grazing was the most important factor causing the hampering of algal growth and altering the structure of the macroalgal community towards a sparse vegetation of low complexity devoid of erect algae, while herbivore fish had a minor effect. [ABSTRACT FROM AUTHOR]

Abstract: Photographic quadrat sampling is commonly used for the study of sessile benthic communities. However, photoquadrat data handling is fragmented into several processing methods, and there is a scarcity of dedicated software tools that integrate all major analysis options. photoQuad is a new software system for advanced image processing of photographic samples, dedicated to ecological applications. The software integrates a series of methods for the extraction of species area, percentage coverage, or presence/absence information, including random point counts (RP), grid cell counts (CL), freehand regions (FH), and image segmentation-based regions (SG). These are simultaneously functional in a layer-based environment, further supported by a variety of tools for image enhancement, image calibration, automatic quadrat boundary detection, and management of user-specific species libraries. The paper documents the main features of photoQuad, and demonstrates its performance through the simultaneous application of the RP, CL, FH, and SG methods on identical datasets, and the comparison of errors in species area and coverage measurements. The simulated data used for reference are disk-shaped patches, whose area and density statistics are equivalent to three benthic species characteristic of Mediterranean coralligenous communities. The analysis indicated that measurement methods differed in area and coverage bias, as well as in their sensitivity to species size. Large patches were accurately measured by all methods in terms of mean scaled error, but CL and RP provided high error variance, and their performance deteriorated with decreasing patch size; the region-based SG and FH methods provided the lowest errors and were both robust to patch size. The image and quadrat calibration process showed no statistically significant effect on the outputs, although further analysis is needed to validate this result. Overall, photoQuad constitutes a powerful software for elaborate analysis of photoquadrat images, facilitating fast and comparable evaluation of the ecological information contained therein. The photoQuad software is freely available to download and use from: http://www.mar.aegean.gr/sonarlab/photoquad. [Copyright &y& Elsevier]