Predicting Carbon and Nutrient Transformations in Tidal Freshwater Wetlands of the Hudson River

The exchange of water between the main channel of the tidal freshwater Hudson River and its tidal wetlands is a large proportion of the whole-river water volume and causes large changes in concentrations of some dissolved and suspended constituents. Ten representative wetlands were assessed for their ability to alter quantities of inorganic nutrients, suspended particles, dissolved organic carbon (DOC), and dissolved oxygen during tidal exchange. The majority of sites acted as sinks for oxygen and nitrate and as sources of DOC. For other variables such as phosphate and pigments, individual wetlands varied broadly in both the direction and magnitude of change. For some variables (oxygen, DOC) we found mechanistically plausible predictors for the magnitude of alteration. The proportional coverage of submerged vegetation or intertidal marsh graminoid vegetation was related to the degree of change in oxygen and DOC. For most cases, however, we did not find strong predictors and we attribute this to the spatial positioning of “hot spots” or redundancy in the processes actually responsible for the transformation. Our ability to predict ecosystem performance from whole-ecosystem attributes may be impeded by lack of consideration of within-system spatial contingencies or lack of knowledge of which process is actually responsible for the observed alteration in material flux.     

Ascorbic Acid Oxidase: An Enzyme in Search of a Role

Ascorbic acid oxidase (AAO) has been fully characterized at molecular level, yet its functional role is unclear. The properties of the enzyme and the main hypotheses on its function are discussed. Recent data and reappraisal of previous observations suggest that AAO could be part of a dynamic mechanism operating whenever plant cells have to control oxygen availability. This revised version was published online in July 2006 with corrections to the Cover Date.