포항 삼정리 해역 서식지 유형에 따른 해조군집과 성게 개체군 섭식 생태 연구

Abstract

This study compared two ecosystems, kelp beds and crustose coralline algae (CCA) beds in Samjeong-ri, Pohang, Korea. The overall goal of the study was to analyze feeding dynamics in sea urchins and kelp, the dominant species in these ecosystems. Specifically, our primary objective was to gain insights into the mechanisms driving the phase shift and to explore potential solutions. We determined the species composition and biomass (g/㎡) of the macroalgal community using a combination of quantitative surveys and DNA sequencing. We also investigated the density (ind./㎡) and distribution of sea urchin populations, and performed gut contents analysis. Using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios, we constructed a food web that implicitly describes feeding relationships between macroalgae and herbivores within communities of kelp beds and CCA beds. Following the installation of a protective fence to limit sea urchin access, we conducted experiments focused on the ecological restoration of the macroalgal community within the enclosed area. These experiments used both spore bags and transplantation techniques. The macroalgal communities in kelp beds and CCA beds showed notable differences in species composition and biomass. Although CCA beds showed few species diversity and biomass, DNA sequencing revealed the presence of eight species of turf-forming algae, including one potential new species, Gelidium sp. nov., highlighting the critical importance of continued research focused on understanding the species composition of CCA beds. We observed a two-fold difference in sea urchin density between kelp beds and CCA beds, with 2.89 ind./㎡ in kelp beds, versus 5.76 ind./㎡ in CCA beds. The sea urchin species Heliocidaris crassispina and Mesocentrotus nudus coexisted in the kelp beds, whereas only M. nudus were observed in CCA beds, suggesting a distinct partitioning of habitats in these species. Furthermore, populations of M. nudus from kelp beds and CCA beds exhibited significant differences in gonad index(%) corresponding to habitat (Welch's t-test, T₃₉.₁＝5.020, p＜0.001). Gut contents analysis revealed significant differences in food sources among species and habitats (Kruskal-Walis test, Rhodophyta, χ²₂＝9.653, p＝0.008; Phaeophyta, χ²₂＝26.452, p＜0.001; CCA, χ²₂＝21.905, p＜0.001; Balanus sp., χ²₂＝27.018, p＜0.001). The δ¹³C values of macroalgae in kelp beds ranged from –33.85 to –7.25‰, with δ¹⁵N values ranging from 2.65 to 7.80‰. The δ¹³C values of herbivores in kelp beds ranged from –32.88 to –9.21‰, with δ¹⁵N values ranged from 3.92 to 9.86‰. In CCA beds, the δ¹³C values of macroalgae ranged from –34.79 to –7.82‰, with δ¹⁵N values ranging from 4.14 to 7.24‰. Herbivores in CCA beds exhibited δ¹³C values between –19.69 and –14.70‰ with δ¹⁵N values ranged from 6.06 to 9.03‰. These data indicate that in both habitats, macroalgae serve as the primary food source for herbivores. The differences in δ¹³C values observed within M. nudus populations indicate distinct food source origins based on their respective habitats. These findings support the results of gut contents analysis and the correlation between gonad index and food sources. The results from spore bags and transplantation in the protective fence study confirmed that, in the absence of feeding pressure from sea urchins, both the growth rate of young thalli and the survival rate of transplanted individuals were notably higher. Our results revealed patterns in the feeding ecology of sea urchins and a correlation between food sources and gonad index in kelp beds and CCA beds. Continuous monitoring of macroalgal communities and sea urchin populations will be crucial in predicting changes in the marine ecosystem and understanding the factors behind phase shifts.