Experimental test of N-limitation for Zostera nigricaulis seagrass at three sites reliant upon very different sources of N

The hypothesis that seagrasses may be nitrogen-limited has been examined in a number of studies, particularly in oligotrophic waters where nutrients are typically scarce. Here we examined nitrogen-limitation for the temperate seagrass, Zostera nigricaulis, at three sites where seagrasses use very different sources of nitrogen (N) based on stable isotopes of N: Swan Bay - fixed and recycled N, Kirk Point - N from treated sewage, and Blairgowrie - N from catchment. Dramatic changes to source inputs during a recent prolonged drought were linked to seagrass loss in parts of the bay dependent on catchment inputs. Here we test to what extent seagrasses dependent on different sources of N are N-limited, by enriching sediment pore water levels. Sediment pore water nutrient levels varied naturally between the sites by a factor of 14, from Swan Bay (highest) to Blairgowrie (lowest), and were much higher than water column levels. There was a detectable enrichment effect for pore water nutrient concentrations within the treatment plots after one month, although the effect was greatest for the Blairgowrie site, and was most pronounced for NH4 levels. There was a significant enrichment effect for seagrass growth after 5 months, but only at a single site, Blairgowrie. Seagrass cover, above-ground biomass and mean leaf length were substantially higher in the treatment than control plots at the Blairgowrie site, but not at the other two sites. Higher %N content and lower δ15N in the leaf tissues at Blairgowrie and Kirk Point indicated that N released from the fertilizer had been assimilated by the seagrass at both these sites, but in the case of Kirk Point this did not translate into greater seagrass growth. The results suggest that seagrasses at Blairgowrie, in the southern part of the bay, are highly N-limited. This is a region that is isolated from the major source of N (the catchment) utilised by seagrass in this part of the bay. The pattern of N-limitation in this and a previous study is consistent with regions that displayed major fluctuations in seagrass cover during the 'Millennium' drought from 1997 to 2009. Consequently, predicted decreases in long-term rainfall in south-eastern Australia could have negative consequences for seagrass extent in areas that are N-limited.