Variation in mangrove forest structure and sediment characteristics in Bocas del Toro, Panama

Mangrove forest structure and sediment characteristics were examined in the extensive mangroves of Bocas del Toro, Republic of Panama. Forest structure was characterized to determine if spatial vegetation patterns were repeated over the Bocas del Toro landscape. Using a series of permanent plots and transects we found that the forests of Bocas del Toro were dominated by Rhizophora maugle with very few individuals of Avicennia germinans and Laguncularia racemosa. Despite this low species diversity, there was large variation in forest structure and in edaphic conditions (salinity, concentration of available phosphorus, Eh and sulphide concentration). Aboveground biomass varied 20-fold, from 6.8 Mg ha(-1) in dwarf forests to 194.3 Mg ha(-1) in the forests fringing the land. But variation in forest structure was predictable across the intertidal zone. There was a strong tree height gradient from seaward fringe (mean tree height 3.9 m), decreasing in stature in the interior dwarf forests (mean tree height 0.7 m), and increasing in stature in forests adjacent to the terrestrial forest (mean tree height 4.1 m). The predictable variation in forest structure emerges due to the complex interactions among edaphic and plant factors. Identifying predictable patterns in forest structure will aid in scaling up the ecosystem services provided by mangrove forests in coastal landscapes.

Bacterial community structure associated with white band disease in the elkhorn coral Acropora palmata determined using culture-independent 16S rRNA techniques

Culture-independent molecular (16S ribosomal RNA) techniques showed distinct differences in bacterial communities associated with white band disease (WBD) Type I and healthy elkhorn coral Acropora palmata. Differences were apparent at all levels, with a greater diversity present in tissues of diseased colonies. The bacterial community associated with remote, non-diseased coral was distinct from the apparently healthy tissues of infected corals several cm from the disease lesion. This demonstrates a whole-organism effect from what appears to be a localised disease lesion, an effect that has also been recently demonstrated in white plague-like disease in star coral Montastraea annularis. The pattern of bacterial community structure changes was similar to that recently demonstrated for white plague-like disease and black band disease. Some of the changes are likely to be explained by the colonisation of dead and degrading tissues by a micro-heterotroph community adapted to the decomposition of coral tissues. However, specific ribosomal types that are absent from healthy tissues appear consistently in all samples of each of the diseases. These ribotypes are closely related members of a group of alpha-proteobacteria that cause disease, notably juvenile oyster disease, in other marine organisms. It is clearly important that members of this group are isolated for challenge experiments to determine their role in the diseases.

Genetic structure of a reef-building coral from thermally distinct environments on the Great Barrier Reef

Adaptation to localised thermal regimes is facilitated by restricted gene flow, ultimately leading to genetic divergence among populations and differences in their physiological tolerances. Allozyme analysis of six polymorphic loci was used to assess genetic differentiation between nine populations of the reef-building coral Acropora millepora over a latitudinal temperature gradient on the inshore regions of the Great Barrier Reef (GBR). Small but significant genetic differentiation indicative of moderate levels of gene flow (pairwise F-ST 0.023 to 0.077) was found between southern populations of A. millepora in cooler regions of the GBR and the warmer, central or northern GBR populations. Patterns of genetic differentiation at these putatively neutral allozyme loci broadly matched experimental variation in thermal tolerance and were consistent with local thermal regimes (warmest monthly-averages) for the A. millepora populations examined. It is therefore hypothesized that natural selection has influenced the thermal tolerance of the A. millepora populations examined and greater genetic divergence is likely to be revealed by examination of genetic markers under the direct effects of natural selection.

Linking physiological processes with mangrove forest structure: phosphorus deficiency limits canopy development, hydraulic conductivity and photosynthetic carbon gain in dwarf Rhizophora mangle

Spatial gradients in mangrove tree height in barrier islands of Belize are associated with nutrient deficiency and sustained flooding in the absence of a salinity gradient. While nutrient deficiency is likely to affect many parameters, here we show that addition of phosphorus (P) to dwarf mangroves stimulated increases in diameters of xylem vessels, area of conductive xylem tissue and leaf area index (LAI) of the canopy. These changes in structure were consistent with related changes in function, as addition of P also increased hydraulic conductivity (K-s), stomatal conductance and photosynthetic assimilation rates to the same levels measured in taller trees fringing the seaward margin of the mangrove. Increased xylem vessel size and corresponding enhancements in stern hydraulic conductivity in P fertilized dwarf trees came at the cost of enhanced midday loss of hydraulic conductivity and was associated with decreased assimilation rates in the afternoon. Analysis of trait plasticity identifies hydraulic properties of trees as more plastic than those of leaf structural and physiological characteristics, implying that hydraulic properties are key in controlling growth in mangroves. Alleviation of P deficiency, which released trees from hydraulic limitations, reduced the structural and functional distinctions between dwarf and taller fringing tree forms of Rhizophora mangle.

The genetic structure of Australasian green turtles (Chelonia mydas): exploring the geographical scale of genetic exchange

Ecological and genetic studies of marine turtles generally support the hypothesis of natal homing, but leave open the question of the geographical scale of genetic exchange and the capacity of turtles to shift breeding sites. Here we combine analyses of mitochondrial DNA (mtDNA) variation and recapture data to assess the geographical scale of individual breeding populations and the distribution of such populations through Australasia. We conducted multiscale assessments of mtDNA variation among 714 samples from 27 green turtle rookeries and of adult female dispersal among nesting sites in eastern Australia. Many of these rookeries are on shelves that were flooded by rising sea levels less than 10 000 years (c. 450 generations) ago. Analyses of sequence variation among the mtDNA control region revealed 25 haplotypes, and their frequency distributions indicated 17 genetically distinct breeding stocks (Management Units) consisting either of individual rookeries or groups of rookeries in general that are separated by more than 500 km. The population structure inferred from mtDNA was consistent with the scale of movements observed in long-term mark-recapture studies of east Australian rookeries. Phylogenetic analysis of the haplotypes revealed five clades with significant partitioning of sequence diversity (Phi = 68.4) between Pacific Ocean and Southeast Asian/Indian Ocean rookeries. Isolation by distance was indicated for rookeries separated by up to 2000 km but explained only 12% of the genetic structure. The emerging general picture is one of dynamic population structure influenced by the capacity of females to relocate among proximal breeding sites, although this may be conditional on large population sizes as existed historically across this region.