Effects of nitrogen enrichment on biomass allocation, growth, and leaf morphology of the seagrass Thalassia testudinum

Abstract

The effects of sediment ammonium (NH4+) enrichment on biomass allocation, growth, and leaf morphology of the seagrass Thalassia testudinum in Corpus Christi Bay (CCB) and lower Laguna Madre (LLM), Texas, were examined from May to October 1997. Prior studies had shown that shoot height and leaf biomass at CCB were significantly higher than those at LLM, and ambient sediment NH4+ concentrations in CCB (ca 100 μM) were significantly higher than those in LLM (ca 30 μM). It was hypothesized that the differences in plant morphology and biomass between the 2 areas could be related to differences in sediment nitrogen levels between 2 sites. To test this hypothesis, we conducted an in situ fertilization experiment at both sites over a 6 mo period. Results of this experiment revealed that seagrass growth, biomass and leaf size significantly increased as a result of sediment NH4+ enrichment at LLM, but had little effect on plant density, biomass and leaf morphology at CCB. In unfertilized plots, average leaf production rate (7.4 g dry wt m-2 d-1) and shoot height (43.3 cm) at CCB were significantly higher than those at LLM (2.5 g dry wt m-2 d-1 and 18.8 cm, respectively). After fertilization, leaf production rates and leaf size at LLM increased to reach equivalent levels of the CCB site. Leaf biomass at LLM increased significantly as a result of sediment NH4+ enrichment, but there was little change in below-ground biomass: The below-to above-ground biomass ratio at LLM (4.7) was about 3-fold higher than that at CCB (1.6) in unfertilized plots, but decreased significantly at LLM with sediment NH4+ enrichment, while the ratio at CCB remained unchanged. We conclude, based on seagrass growth responses to increases in sediment NH4+, that sediment nitrogen availability at LLM limits seagrass productivity. T. testudinum responded to limited nitrogen conditions by increasing below- to above-ground biomass ratios. An ambient sediment NH4+ level of about 100 μM was considered to be the threshold concentration for nitrogen limitation of seagrass growth.