Observations of Isotopes and Biogeochemistry
at Twin Cays, Belize
Marilyn Fogel, Barbara Smallwood, and Matthew Wooller
With John Cheeseman, Myrna Jacobson, and Mike Kandianis
The Blue Moon Conference
October 2002

After two and a half years of field and analytical work in Twin Cays, Belize, we have made a number of observations that we can not explain: unusual nitrogen and carbon isotopic compositions, elevated nutrient levels in sediments and leaves, and distributions of trees in places that are outside of the “normal” tree gradient zones. At this meeting in Lafayette, Louisiana, a group of us met at the Blue Moon Hostel to discuss the research. In attendance were Barbara Smallwood, Marilyn Fogel, John Cheeseman, Myrna Jacobson, Mike Kandianis, Cathy Bermudez, and Matthew Wooller (Isabel Romero came later). A morning (22/10/02) of discussion on mangrove plant science and sedimentary geochemistry was followed by an afternoon discussion on Isotope Biogeochemistry of the mangroves and Landscape scale effects on Twin Cays.

This page summarizes a report written by Marilyn Fogel. The complete report, along with figures, is available in the All Hands 2002 folder on the Tangled Roots FTP server. Access to this site is restricted to members of the project team. Others should direct questions or correspondence directly to Dr. Fogel at fogel@gl.ciw.edu.

1. Question: What is the isotopic signature in the Rhizophora mangle leaves and trees on Twin Cays?

Observation: The nitrogen and carbon isotopic composition of R. mangle leaves ranges from –17 ‰ to +2 ‰ for N, and –30 to –22 ‰ for C in trees distributed around the island.

2. Question: What is the isotopic signature in the Avicennia germinans on Twin Cays?

Observation: The N and C isotopic signature of A. germinans is 0 ‰ ± 0.5 in an initial sampling of approx. 25 trees. After intensive searching, we found a distribution of N and C isotopes that is very similar to that determined in the R. mangle leaves.

3. Question: What is the isotopic composition of bacterial and algal mats distributed on Twin Cays?

Observation: All of the sediments and mats on the surface had &Mac182;¹⁵N of –2 to +2 ‰, with an average of 0 ‰ ± 1.0 ‰. These values indicate a source of nitrogen from nitrogen fixation.

4. Question: Is there any relationship of the isotopic composition in mangroves to the tree height zone or gradient?

Observation: In large measure the major variations are found in the dwarfed trees. All negative nitrogen isotopic compositions are ONLY measured in dwarf trees, but not all of them are negative. The fringe zone, and tall trees found in the interior have values that are closer to 0 ‰. Carbon isotopes are more enriched in the dwarf trees and more depleted in the fringe trees. The difference in a group of dwarf and fringe trees is about 2 ‰.

5. Question: Is there any relationship between the stable isotopic composition of mangrove trees to nutrient dynamics?

Observations: Fertilized trees with P have &Mac182;¹⁵N that are around 0 ‰ with no effect of tree height gradient. With the addition of N, the &Mac182;¹⁵N of dwarf trees reached values of –7 ‰, whereas the tall trees reached about –3 ‰. The range of values is not equivalent to those found on the landscape scale.

6. Question: Can the isotopic composition of a mangrove change if its nutrient status changes? How fast do those isotopic changes occur?

Observations: After 2-3 years, the isotopic signatures were set. In more recent experiments with low level P dosing, the change in the &Mac182;¹⁵N occurred after only 5 months.

7. Question: Are negative &Mac182;¹⁵N values only ever seen in leaf tissue?

Observations: Yes, but with some qualification. Roots and leaves from the same tree have differences in isotopic composition. Roots are about 4-6 ‰ heavier than the leaves.

8. Question: Are the unusual ¹⁵N that we find in mangroves unique to mangrove trees or are they found in other primary producing species?

Observations: We have found that lichens have &Mac182;¹⁵N that range from –4 ‰ in the fringe and transition zones to –21 ‰ in the floc zone. The &Mac182;¹⁵N of the lichens was NOT related to the &Mac182;¹⁵N of the tree that the lichens were found growing on.

9. Question: Has anyone else in the isotopic community found &Mac182;¹⁵N values that are this negative?

Observation: Based on literature searching and personal communication, values in &Mac182;¹⁵N down to –10 ‰ have been measured and related to mycrorhizzal fungi in roots in temperate terrestrial trees and arctic species. Some isotopically light &Mac182;¹⁵N have been measured in higher plants grown under nitrogen fertilized experiments.

11. Question: Is there partitioning in organic pools of the &Mac182;¹⁵N in the sediments?

Observations: Isotopic compositions of leaf fragments in peat cores have isotopically light &Mac182;¹⁵N values ONLY in the surface layers (>50 cm). Leaf fragments in deeper layers range from –4 to +2 ‰. The &Mac182;¹⁵N of the peat/microbial biomass is routinely and monotonously near 0‰ ± 1.

12. Question: What is the distribution of the isotopic signatures on the landscape scale in Twin Cays?

Observations: The isotopically light &Mac182;¹⁵N values are only ever seen in trees growing in the interior of the islands. These trees always have very short internodal distances. Generally speaking, proportionally these trees are in locations with constant standing water. These &Mac182;¹⁵N depleted trees appear to be downwind from experimental and anthropogenic inputs. Larger, bushier trees have &Mac182;¹⁵N that are around –5 ‰, have large internodal distances. These trees seem to be associated with floc zones. In areas of floc zone, where bacterial algal mats are accumulating, the &Mac182;¹⁵N are more positive. All fringe trees, with and without encrusting sponges have &Mac182;¹⁵N around 0 ‰. The &Mac182;¹⁵N values of R. mangle are NOT ALWAYS associated with elevated concentrations of porewater ammonium concentrations. Almost exclusively, there appears to be more P in the sediments of trees with &Mac182;¹⁵N near 0 ‰, with one exception. The isotopic composition of transition zone trees is an enigma: they are relatively invariant, but it is unknown whether they hold a more pivotal role in the nutrient cycling on the island.

13. Question: Are the low &Mac182;¹⁵N restricted to Twin Cays, or have they been found in other mangrove forests worldwide?

Observations: In Panama, we have measured the &Mac182;¹⁵N down to –17 ‰ in dwarf trees from control sites taken adjacent to fertilization study sites. In Florida, there was no evidence of &Mac182;¹⁵N negative values in the experimental plots or other native trees. In this region, the R. mangle occur only in the fringe. The A. germinans in the interior of Florida have &Mac182;¹⁵N values down to about –7 ‰, but not less than that. These trees have anthropogenic input from human impacts in the Indian River lagoon. The Man of War Cay had values of &Mac182;¹⁵N that were +15 to +20 ‰, which are related to N source from nesting sea birds. At Turneffe Cay, the &Mac182;¹⁵N of dwarf trees living on the fringes were –2 to –5 ‰. Literature values range from –5 ‰ to +20 ‰ in unfertilized trees.

14. Question: What is the linkage between N:P in leaves to the isotopic compositions?

Observations: In McKee’s paper, she noted a relationship between N:P and &Mac182;¹³C, but the observation remains without an explanation. High N:P relate to elevated carbon isotopic compositions, that is taken to infer stressed trees. We all find that low N:P values are associated strongly with heavy &Mac182;¹⁵N. High N:P, which is basically determined by low P percentage, is strongly associated with &Mac182;¹⁵N that are most negative. The N:P is highest in leaves from unfertilized areas due to the low P, and the &Mac182;¹⁵N from P experiments in 2002 showed that it is not a simple relationship after 7 months.

15. Question: Is there isotopic variation seen on a small, tree to tree, or less than 5 m, scale?

Observation: Yes. Very small dwarf trees found in the shadows of larger trees are different in &Mac182;¹⁵N by up to 14 ‰. In spoke experiments, we determined this variation, and Cheeseman’s collections of trees right next to each other were different by the same range.

16. Question: Does the unusual isotopic &Mac182;¹⁵N transfer into animals? Are the C isotopes useful in discriminating animal diets?

Observation: Insects have isotopic C signals that show that mangroves are the dominant C source. Animals collected directly on dwarf R. mangle leaves having isotopically light &Mac182;¹⁵N values had world record N isotopic compositions. Animals collected in the fringe zone had N isotopic compositions indicating either a marine or a mangrove fringe N input. Animals collected from the water, e. g. fish or other swimming or floating invertebrates, used the bacterial/algal mat for C and N sources. Organisms that consume multiple sources of food have intermediate values, e. g. the tree snails.

17. Question: Are the biochemical pools in the modern leaf uniformly labeled in terms of N and C isotopic compositions?

Observations: For C, no. The isotopic composition of lipids is more depleted from the bulk, as expected. In a comparison of dwarf and tall leaves, the isotopic differences between the two were documented between the lipid, the amino acids, and the structural components of the leaves. The biochemical compositions themselves were identical between dwarf and tall trees.