摘要:

IntroductionThe mangrove environment that dominates the world's tropical and subtropical coasts is currently threatened by clear cutting and nitrification.1,2Mangroves shield coasts from natural episodic events such as hurricanes; serve as nurseries for juvenile fish;3are an important source of nutrients for offshore biological production; and are essential for keeping coastal waters free of sediment, which subsequently helps preserve coral reefs.Rhizophora mangleL., the red mangrove, is one of the major species of mangrove in the Americas.2It has a broad range of stand structures (<1 m to >60 m) for trees of the same age.3,4These differences in height are due to variations in the balance and concentration of available nutrients (e.g., nitrogenvs.phosphorousvs.potassium), which can fluctuate over very small distances (i.e., meters).3,5–7The red mangrove's adaptability to nutrient alterations and dynamic water levels has ensured survival of this species through sea-level change and anthropogenic nutrient release.Twin Cays (16°50′N, 88°06′W), a peat-based, 92 ha archipelago of offshore (12 km) mangrove islands, is located inside the crest of the barrier reef of central Belize (BZ). The terrigenous input of organic matter is restricted to dust deposited by wind and rain. The shoreline gradient is intertidal and is interrupted by tidal creeks, open flats, and shallow interior ponds. The vegetation is dominated by the red mangrove with two other species,Avicennia germinans(L.) Stearn. (black mangrove) andLaguncularia racemosa(L.) Gaertn. f. (white mangrove), present in lower abundance. Mangrove forests at Twin Cays are characterized by a tree-height gradient that parallels other gradients, such as productivity and tidal flushing.3,6–8The tree-height gradient at the site can be subdivided into three zones: (1) A fringe of uniformly tall (5–6 m) red mangrove trees that dominate the seaward-most zone of the islands. (2) A transition zone (2–4 m in height), where all three mangrove species are present. (3) A zone of uniformly stunted (dwarf; ∼1–1.5 m) red mangrove trees, with stands in the interior of the islands.3Fertilization experiments at Twin Cays have shown that trees in the fringe zone are nitrogen limited; dwarf trees are phosphorous limited; and trees in the transition zone are limited by both nitrogen and phosphorous.3Previous studies at Twin Cays have described the bulk carbon and nitrogen isotopic composition of red mangrove leaves from experimental plots that traverse all three zones of nutrient limitation.7A wide range of carbon and nitrogen isotope values has been described for red mangrove, with dwarf trees having the highest carbon (meanδ13C = −25.3‰) and lowest nitrogen isotopic composition (meanδ15N = −10‰), compared with tall trees of the same species (meanδ13C = −28.3‰ and meanδ15N = 0.0‰1). This contrasts to relatively uniform carbon and nitrogen isotopic compositions of the peat matrix throughout the differing nutrient gradients at Twin Cays, previously reported.9,10These isotopic distinctions within the same species of red mangrove may permit inference about past stand structure if (1) it is established which chemical fractions hold the isotopic signal and (2) despite diagenesis, this isotopic signal is retained by the preserved leaves. This paper addresses both of these requirements by measuring the carbon and nitrogen isotopic composition of a variety of biochemical fractions (water soluble compounds, free lipids, acid hydrolysable compounds, individual amino acids and the insoluble residual compounds) in fresh and preserved red mangrove leaves sampled at Twin Cays, BZ. Although previous work has shown that amino acids are labile,11they are thought to degrade relatively slowly after burial.12–15Recent research has highlighted the immobilization of nitrogen by tannins in mangrove ecosystems;16nitrogen from amino acids is believed to be incorporated into complex, recalcitrant pools of organic matter and preserved in peat sediments. These factors provided a focus for the research presented in this paper.Fresh leaves were sampled from red mangrove trees growing in areas of apparent phosphorous limitation and flooding (dwarf trees) and in areas of nitrogen limitation (tall trees). Preserved leaves were analyzed from two peat cores (∼50 cm long) under two mangrove stands at Twin Cays, one from under a dwarf stand and one from under a tall stand. Woolleret al.,10provided information on the taphonomy of dwarf and tall trees in terms of their bulk isotopic characteristics, and this paper is essentially a continuation of that study.

ISSN:1467-4866    

DOI:10.1039/b308902a

出版商:RSC    

年代:2003

数据来源: RSC