Fine‐scale mapping of land‐derived nitrogen in coral reefs by δ¹⁵N in macroalgae

Abstract

We measured the C/N ratio and ␦ 15 N values of two brown macroalgae-Padina spp. and Dictyota sp., which are distributed over all the subtropical fringing reefs of the Ryukyu Islands, Japan-to evaluate the feasibility of these algae as indicators of the terrestrial nitrogen load to the reef. The correlations between the distance from the shoreline and algal C/N ratio and surrounding NO concentrations were not clear, although their average values among the Ϫ 3 reefs seemed to indicate differences in nitrogen loadings from the land. The ␦ 15 N values of these algae, on the other hand, linearly or curvilinearly decreased from ϩ8‰ to ϩ2‰ with increasing distance from the shoreline, indicating the difference in nitrogen sources available to macroalgae. The slope of the decline among eight study areas had different characters, which seemed to depend on the residence time of reef seawater and the fluxes of terrestrial nitrogen. Using ␦ 15 N values of brown algae as an indicator, we confirmed that primary producers, such as macroalgae on the reefs, assimilated land-derived nitrogen and successfully evaluated time-integrated effects of terrestrial nitrogen on coral reef algae, which had been missed by conventional monitoring of the water column nutrients.Direct anthropogenic influences on coral reefs are major concerns because the global population is increasing, especially in tropical and subtropical countries with coral reefs. Furthermore, coral reefs are threatened by global climate

Identifying nutrient sources to three lagoons at Ofu and Olosega, American Samoa using δ15N of benthic macroalgae

“…The low d 15 N values of the macroalgae, the low nutrient concentrations of lagoon waters, the lack of correlation between d 15 N values and distance from shore, and the low concentrations of nutrients in well-water samples indicate that the major sources of nutrients to the three Ofu-Olosega lagoons at the time of this study were most likely oceanic/atmospheric, and not animal/anthropogenic in origin. In contrast to this study, Umezawa et al (2002) found an inverse linear or curvilinear decrease in d 15 N values of macroalgae with distance from shore in the Ryuku Islands, Japan. This pattern was attributed to multiple sources of nitrogen, including terrestrial and human.…”

Applications of C and N stable isotopes to ecological and environmental studies in seagrass ecosystems

“…In these conditions, seagrasses should be poorer indicators of water column process than, for example, macroalgae which rely only on their surrounding water for their nutrient requirements (e.g. Gartner et al, 2002;Umezawa et al, 2002). On the other hand, the δ 15 N of species or populations which uptake a significant part of their inorganic N from sediment pore water, should be an informative indicator of environmental conditions in the sediment.…”

Selection of effective macroalgal species and tracing nitrogen sources on the different part of Yantai coast, China indicated by macroalgal δ15N values

“…For example, foliose and filamentous macroalgae such as Ulva intestinalis could more efficiently reflect the δ 15 N-DIN variations because their faster uptake and growth rates (Teichberg et al, 2008;Dailer et al, 2010;Ochoa-Izaguirre and Soto-Jiménez, 2015). Environmental factors such as seawater nutrient levels, temperature and light may also result in the difference of macroalgal δ 15 N values (Umezawa et al, 2002;Derse et al, 2007;Viana and Bode, 2015). Although the effect of environmental nitrogen status and concentration on macroalagal fractionation remains controversial (Cohen and Fong, 2004;Dailer et al, 2010;Kaldy, 2011), significant fractionation was showed with high nitrogen concentration in the water column and high nitrogen content in macroalgae (Thornber et al, 2008;Kaldy, 2011;Viana and Bode, 2015).…”