Broad-scale patterns of tissue-δ15N and tissue-N indices in frondose Ulva spp.; Developing a national baseline indicator of nitrogen-loading for coastal New Zealand

► Ulva growing in 'clean' coastal New Zealand shows a baseline range of tissue-δ15N. ► This baseline range is consistent in summer and winter, and is seasonally robust. ► Deviation in Ulva tissue-δ15N outside 6.6-8.8‰ indicates non-marine nitrogen. ► Ulva in sheltered estuaries with >3.1% tissue-N can indicate high N-availability. ► Shifts in tissue-δ15N and -N indicate change in source and amount of available-N.