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

Abstract

The effects of sediment ammonium (NH,') enrichment on biomass allocation, growth, and leaf morphology of the seagrass Thalassia testudrnum in Corpus Chnsti Bay (CCB) and lower Laguna Madre (LLM), Texas, were examined from May to October 1997. Prior stuhes 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 PM) were significantly higher than those in LLM (ca 30 PM). 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 sihr fertilization experiment at both sites over a 6 mo period. Results of t h~s experiment revealed that seagrass growth, biomass and leaf size significantly increased as a result of sedlment NHAt enrichment at LLM, but had Little effect on plant denslty, biomass and leaf morphology at CCB. In unfertilized plots, average leaf production rate (7.4 g dry wt m-' d-') and shoot height (43.3 cm) at CCB were significantly higher than those at LLM (2.5 g dry wt m-2 d-' and 18.8 cm, respectively). After fertilization, leaf production rates and leaf size at LLIM 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, hut decreased significantly at LLM with sediment NH,' enrichment, while the ratio at CCB remained unchanged. We conclude, based on seagrass growth responses to increases in sediment NH,,', that sediment nitrogen availabhty at LLM limits seagrass productivity. T. testudinum responded to lirmted nitrogen conditions by increasing below-to above-ground biomass ratios. An ambient sediment NH4+ level of about 100 PM was considered to be the threshold concentration for nitrogen limitation of seagrass growth.

Direct measurements of root growth and productivity in the seagrasses Posidonia australis and P. sinuosa

“…An increase in lateral roots, however, may be accompanied by decreased growth of main root axes (Linkohr et al 2002). In the few studies of seagrass root development in relation to nutrient availability (Kiswara et al 2009), species in lownutrient environments directed growth to developing more extensive root systems, increasing the potential for accessing nutrients from the sediments and also changing the ratio of above-and below-ground biomass (Powell et al 1989;Perez et al 1994;Lee and Dunton 2000).…”

Long-Term Effects of Adding Nutrients to an Oligotrophic Coastal Environment

“…Proximity to urbanized and cultivated terrestrial landscapes exposes seagrass beds to runoff replete with anthropogenic nutrients, which can subsequently alter vital ecological processes and economic functions of seagrass beds (Martinetto and others 2006). Within seagrass beds, nutrient input often increases productivity and plant biomass (Lee and Dunton 2000) and facilitates the overgrowth or replacement of seagrass by fast-growing, opportunistic primary producers (Fourqurean and others 1995;McGlathery 2001).…”

Quantification and classification of the main environmental impacts on a Halodule wrightii seagrass meadow on a tropical island in northeastern Brazil

“…The coastal environment is exposed to a large number of nutrients from human activities. These nutrients have harmful effects on the structure and function of the seagrass ecosystem (Lee and Dunton 2000). The input of organic matter causes an increase in the biomass of phytoplankton McComb 1992, McMahon et al 1997) and epiphytic algae, which reduces both light penetration and the concentration of dissolved oxygen (Huang et gatherers and containment structures.…”