Kevin Boto

Keystone species and mangrove forest dynamics: the influence of burrowing by crabs on soil nutrient status and forest productivity

Abstract The density of the burrowing crab fauna in a mangrove forest was reduced, using pitfall traps, to test the hypothesis that decreased burrowing would lead to: (1) increased soil sulphide concentrations; (2) altered nutrient concentrations; and (3) decreased forest productivity and growth. Experiments were conducted in Rhizophora -dominated forests in north Queensland, Australia, over a 12-month period. Crabs were trapped and removed from the experimental plots during 1 week each month for a year. Soil chemical and forest growth parameters were measured at monthly intervals in the experimental and appropriate control plots. Over the course of the experiment, soil sulphide and ammonium concentrations increased to levels which were significantly higher in plots from which crabs were being removed in comparison to controls. No differences were observed for either phosphate or nitrate plus nitrite. Cumulative forest growth, as measured by stipule fall, was significantly less in removal plots than in controls. Additionally, trees in the removal plots had significantly less reproductive output than did trees in control plots. These results support the hypothesis that burrowing by crabs is an important process in Australian Rhizophora forests. It appears that burrowing affects soil aeration which in turn affects the productivity and reproductive output of Rhizophora . Knowledge of the ecology of grapsid crabs from other continents, however, is very limited. It remains to be seen if the roles played by grapsid crabs are as important elsewhere as in Australia. This is particularly true for the Caribbean region from which many of our paradigms concerning mangrove forest ecology were developed.

The Relationship Between Nitrogen Fixation and Tidal Exports of Nitrogen in a Tropical Mangrove System

Various components (sediments, algal mats, decomposing logs and algal-covered prop roots) of a tropical mangrove forest showed low to moderate nitrogen fixation (acetylene reduction) rates. Measurements carried out in March, May and October 1989 revealed negligible or inconsistent seasonal variation in activity for all components. Prop roots with their associated algae showed much greater activity during night-time, typical of many substrates with associated cyanobacteria, whereas all other components had constant activity over 24 h periods. Extrapolation of the nitrogen fixation rates for each component to a ‘whole-forest’ basis, using field estimates of the densities of each component, indicated that sediments, prop roots and decomposing logs contributed approximately 3·5, 1·6 and 1·0%, respectively, of the nitrogen requirements for forest net primary production (FPPN). Blue-green algal mats on bare saltpan areas showed no significant nitrogen-fixing activity (in excess of that for the bare sediments in the same area). The total nitrogen contribution of the various components (6% of FPPN) closely matches previous esimates of the net nitrogen loss from the system through tidal fluxes of particulate and dissolved materials. The possible relative importance of other nitrogen input and loss mechanisms for this mangrove system is also discussed.

Composition and bacterial utilization of free amino acids in tropical mangrove sediments

Abstract The composition and bacterial utilization of dissolved free amino acids (DFAA) in tropical mangrove sediments was examined. Amino acid concentrations (300–900 ng total DFAA ml−1) and composition were similar to that of other organic-rich, anaerobic sediments with lowest and highest concentrations in the low and mid intertidal zones, respectively. The non-protein amino acid, β-glutamic acid, rarely reported in previous studies, was found as a major component of the interstitial DFAA pool. Intracellular amino acids from some cultured strains of sulphate-reducing bacteria (e.g. Desulfobacter app) showed the presence of β-glutamic acid as a major cellular constituent suggesting that these bacteria may be a source of this amino acid in mangrove pore waters. In high intertidal sediments, bacterial growth rates (μ) correlated significantly with total DFAA concentrations with depth. Amino acid concentrations and composition differed significantly between sediments and overlying tidal waters. Flux chamber experiments showed negligible amino acid flux out of the sediments in untreated chambers, but rates of amino acid flux ranged from 27 to 69 mgN m−2 day−1 (= 81–207 mg C.m−2 day−1) in chambers where poisons were applied to the sediment surface. Such fluxes could account for between 9–38% and 5–19% of the nitrogen and carbon required to support the levels of bacterial productivity measured in surface (0–1 cm) sediments. These experiments suggest that bacterial populations in surface sediments are capable of utilizing all of the amino acid flux to the sediment-water interface in tropical mangroves.

Fluorescent humic acid bands in coral skeletons originate from terrestrial runoff

Abstract Humic acid is a major contributor to the fluorescence of bands that are observed in the skeletons of inshore scleractinian corals from the Great Barrier Reef, Australia. Band sections contain a higher concentration of humic acid than non-band sections. During periods of low terrestrial runoff, temporal and spatial variations in the humic acid concentration of coastal waters are low. After heavy rainfall, the humic acid concentration in coastal waters, especially near river mouths, rises sharply. A source of the humic acid is plant matter. Humic acid is leached from plant matter into the soil, which acts as a massive reservoir. From the soil, humic acid is leached into rivers. On discharge into coastal waters, humic acid is conservatively diluted by seawater. Corals then appear to concentrate the humic acid during skeletogenesis. The fluorescent bands are considered to represent an accurate record of local river outflow and rainfall levels.