Abstract
Rhizophora mangle L. (red mangrove) is the dominant species of mangrove in the Americas. At Twin Cays, Belize (BZ) red mangroves are present in a variety of stand structures (tall >5 m in height, transition ~2–4 m and dwarf ~1–1.5 m). These height differences are coupled with very different stable carbon and nitrogen isotopic values[1] (mean tall δ13C = -28.3‰, δ15N = 0‰; mean tall δ13C = -25.3‰, δ15N = -10‰). To determine the utility of using these distinct isotopic compositions as 'biomarkers' for paleoenvironmental reconstruction of mangrove ecosystems and nutrient availability, we investigated the distribution and isotopic (δ13C and δ15N) composition of different biochemical fractions (water soluble compounds, free lipids, acid hydrolysable compounds, individual amino acids, and the residual un-extractable compounds) in fresh and preserved red mangrove leaves from dwarf and tall trees. The distribution of biochemicals are similar in dwarf and tall red mangrove leaves, suggesting that, regardless of stand structure, red mangroves use nutrients for biosynthesis and metabolism in a similar manner. However, the δ13C and δ15N of the bulk leaf, the biochemical fractions, and seven amino acids can be used to distinguish dwarf and tall trees at Twin Cays, BZ. The data support the theory that the fractionation of carbon and nitrogen occurs prior to or during uptake in dwarf and tall red mangrove trees. Stable carbon and nitrogen isotopes could, therefore, be powerful tools for predicting levels of nutrient limitation at Twin Cays. The δ13C and δ15N of biochemical fractions within preserved leaves, reflect sedimentary cycling and nitrogen immobilization. The δ15N of the immobilized fraction reveals the overlying stand structure at the time of leaf deposition. The isotopic composition of preserved mangrove leaves could yield significant information about changes in ecosystem dynamics, nutrient limitation and past stand structure in mangrove paleoecosystems.
Highlights
The mangrove environment that dominates the world’s tropical and subtropical coasts is currently threatened by clear cutting and nitrification.[1,2] Mangroves shield coasts from natural episodic events such as hurricanes; serve as nurseries for juvenile fish;[3] are 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 mangle L., the red mangrove, is one of the major species of mangrove in the Americas.[2]
This contrasts to relatively uniform carbon and nitrogen isotopic compositions of the peat matrix throughout the differing nutrient gradients at Twin Cays, previously reported.[9,10]. These 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 in fresh and preserved red mangrove leaves sampled at Twin Cays, BZ
0.30 requirements for growth, and each are utilizing the available nutrients in a similar manner
Summary
The mangrove environment that dominates the world’s tropical and subtropical coasts is currently threatened by clear cutting and nitrification.[1,2] Mangroves shield coasts from natural episodic events such as hurricanes; serve as nurseries for juvenile fish;[3] are 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 mangle L., the red mangrove, is one of the major species of mangrove in the Americas.[2]. It has a broad range of stand structures (v1 m to w60 m) for trees of the same age.[3,4] These differences in height are due to variations in the balance and concentration of available nutrients (e.g., nitrogen vs. phosphorous vs. potassium), which can fluctuate over very small distances (i.e., meters).[3,5,6,7] The red mangrove’s adaptability to nutrient alterations and dynamic water levels has
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