Nutrient limitation of Philippine seagrasses (Cape Bolinao, NW Philippines): in situ experimental evidence

Abstract

Nutrient limitation of <i>Enhalus acoroides, Thalassia hemprichii</i> and <i>Cymodocea rotundata</i> in 2 mixed seagrass beds (Silaqui and Lucero) in Cape Bolinao, NW Philippines was investigated through a 4 mo in situ nutrient addition experiment. Leaf growth of <i>T. hemprichii</i> and <i>E. acoroides</i> significantly increased by 40 to 100% and 160%, respectively, following fertilization. Leaf biomass of the 3 species also increased significantly by 60 to 240% following nutrient additions. The increased growth and biomass with fertilization was supported by enhanced photosynthetic activity, consequently by higher chlorophyll and nutrient concentrations in the photosynthetic tissues. These results demonstrated nutrient limitation of seagrass growth and photosynthetic performance at the 2 sites in Cape Bolinao. The nature and extent of nutrient limitation, however, varied between sites and among species. <i>T. hemprichii</i> and <i>E. acoroides</i> appeared to be mainly P deficient and N deficient, respectively (from significant increases in tissue P and N concentration following fertilization, respectively). The deficiency was moderate (26% of requirement) for <i>T. hemprichii</i> but substantial for <i>E. acoroides</i> (54% of requirement). Moreover, N and P deficiency was greater in Lucero than in Silaqui, consistent with the higher ambient nutrient concentration in the porewater and sediment nutrient and organic matter content in Silaqui. These results emphasize the importance of local differences in the factors controlling nutrient losses and gains in seagrass meadows and, more importantly, the importance of identifying the species-specific traits that generate the interspecific plasticity of nutrient status.