A comparative study of nutrient processes along the salinity gradient of tropical and temperate estuaries.

The differences and similarities between near-pristine estuaries of different latitudinal regions were examined by selecting three tropical systems from North Queensland, Australia (Jardine, Annan, Daintree) and three temperate systems from Scotland, United Kingdom (Inverness, Cromarty, Dornoch Firths) for comparison. Although estuaries from the different regions have a number of unifying features, such as salinity gradients, tidal variations and terrestrial inputs they also have a number of important differences. The most distinct of these is the timing and variability of the major physical forcings on the estuary (e.g., river flow, insolation). The three tropical estuaries were much more episodic than their temperate counterparts, with a much more dynamic salinity structure and more variable riverwater concentrations, so that delivery of material to the estuary is dominated by short-lived flood events. In contrast, seawater concentrations were more stable in the tropical estuaries due to a more constant input of insolation, resulting in year round biological activity. There was biological removal of dissolved inorganic phosphorus in the low salinity region of the tropical Jardine and Daintree estuaries and a low salinity input of nitrate in the tropical Annan estuary most likely due to nitrification in the bottom sediments, and the biological reaction zone in the tropical Annan Estuary was flushed out of the estuarine basin to the edge of the offshore plume during a flood. Similar effects were not seen in the temperate Inverness, Cromarty, and Dornoch Firths. Similarities between estuaries include mid-estuary inputs of ammonium which were seen in both the temperate and tropical estuaries, although they occur under vastly contrasting conditions of low river discharge and periods of flood, respectively. Five of the estuaries show a general increase in dissolved inorganic phosphorus concentrations towards the sea during low flows, reflecting their pristine condition, and all six estuaries had low salinity silicate maxima probably sourced from the dissolution of freshwater biogenic silicate that has been carried seaward, except in the tropical estuaries during the dry season when a benthic source is proposed.