Biomass loss and nutrient redistribution in an Indonesian Thalassia hemprichii seagrass bed following seasonal low tide exposure during daylight

The intertidal reef flat of Barang Lompo Island, Indonesia, is exposed to air for several hours per day on the days around spring tides. The time of exposure shows a seasonal pattern. In the period January-June, the reef flat only runs dry at night, whereas in the period July-December, exposure only occurs during daylight. During the low tide daylight exposure period of July-December 1993, the leaf and rhizome biomasses (g m(-2)) declined significantly by 61 and 37%, respectively. Total rhizome length remained unchanged. C- and P-concentrations (% of dry weight) of leaves showed no change, while the leaf N- concentration increased by 25%. C-, N- and P-contents of leaves showed a decline that was strongly correlated to leaf biomass decline. In the rhizomes, C-concentration declined by 8%, but the N- and P-concentrations increased by 111 and 25%, respectively. Rhizome C-content (g m(-2)) declined (43%), N- content increased (46%) and P-content did not change. Total C- and P-contents of the summed biomass of leaves and rhizomes declined by 46 and 34%, respectively, but N-content showed no change. Ammonium and phosphate concentrations in the water column and pore water phosphate were not significantly different during daylight exposure compared to during nocturnal exposure. Ammonium concentration in the pore water, however, was 1.6 times higher during daylight exposure. Results show that during a period of frequent daylight exposure, the nutrient status in the intertidal Thalassia hemprichii seagrass bed changed considerably. Despite biomass reduction, the total nitrogen content in leaves and rhizomes together was constant, which was achieved by an enhanced nitrogen accumulation in the rhizomes. Two theories possibly responsible for the stable sum of the total leaf and rhizome nitrogen content are discussed. The first theory describes the seagrass bed as a relatively closed system with respect to nutrient cycles: detached leaf fragments remain trapped within the meadow. The second theory postulates that the loss of part of the photosynthesising canopy due to daylight exposure has a series of consequences for microbial N-transformation processes in the sediment, which indirectly affects the plants' nitrogen status. This study shows that Thalassia hemprichii, covering the reef flat of an intertidal tropical offshore coral island, which is often considered as a nutrient-poor environment, is rather resilient to a significant canopy die-off and concomitant nutrient losses. [KEYWORDS: daylight exposure; seagrass; biomass redistribution; nutrient contents; Thalassia hemprichii; Indonesia South sulawesi indonesia; banks ex konig; cymodocea-nodosa; syringodium filiforme; terrigenous sediments; zostera capricorni; halodule-wrightii; leaf growth; testudinum; carbonate