Seasonal variations in concentrations and fluxes of dissolved organic and inorganic materials in a tropical, tidally-dominated, mangrove waterway

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Abstract
Variations in concentrations, and extent of outwelling, or import, of dissolved inorganic and organic materials were examined over 16 tidial cycles during a 20 mo period, in a tropical mangrove system at Hinchinbrook Island, northern Australia. Previous studies of this system have shown that ca 35% of the forest net primary production (as carbon) is exported to nearby coastal waters through tidal export of particulate organic matter, primarily mangrove plant litter. Concentrations of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP), along with inorganic nitrogen and phosphorus species (ammonium, nitrate, nitrite and soluble reactive phosphate) were consistently much lower, by up to an order of magnitude, than those reported for many temperate salt marsh or estuarine mangrove waters. Significant (p < 0.05) variations in mean concentrations of all dissolved materials with time of year were found, although no consistent seasonal periodicity was apparent. Nitrate+nitrite was the only component which showed a strong relationship with simple variables, notably solar radiation and water temperature, as demonstrated by multiple regression analysis (R2 = 0.79). The dissolved organic materials did not appear to be linked with the photosynthetic activity of phytoplankton, benthic algae or the mangroves as there was no day-night variation in their concentrations. This contrasted with the inorganic nutrients which showed small but statistically significant differences between daytime and nighttime concentrations. A consistently high atomic C:N or C:P ratio for the dissolved organics (18 to 35:1 and 250 to 700:1 respectively) suggests that this material is mostly refractory and in an advanced diagenetic stage, for example, fulvic acids. Statistically significant net fluxes over individual tidal cycles were obtained in most cases for DOC, nitrate+nutrite, and soluble reactive phosphate. The other entities, particularly ammonium, showed very large and apparently random variations over space and time throughout a tidal cycle thus preventing accurate estimates of their fluxes. In all cases, however, there was no apparent seasonal variation in the direction or magnitude of net fluxes, and, integrating over a year, all components showed very small and negligible net transport to or from the system, except for total dissolved P which showed a net annual import amounting to 24% of net forest primary production requirements. The apparently finely balanced situation for dissolved material transport in this tidally dominated tropical system with virtually no terrestrial inputs is contrasted with the more usual ''outwelling'' of dissolved materials from temperate salt marsh systems.