Terrestrial inputs of organic matter to coastal ecosystems: An intercomparison of chemical characteristics and bioavailability

Dissolved and particulate organic matter (DOM and POM) collected from rivers or,groundwater feeding five estuaries along the east and west coasts of the USA were characterized with a variety of biogeochemical techniques and related to bioavailability to estuarine microbes. Surface water was sampled from the Columbia, Satilla, Susquehanna and Parker Rivers and groundwater was sampled from the Childs River. Several geochemical descriptors (percent organic matter of suspended particulate matter, C/N, lignin phenol content, ratio of vanillic acid to vanillin) suggested an ordering of the systems with respect to POM lability: Satilla < Parker < Columbia < Susquehanna. DOC concentrations in these systems ranged from <100 mu M for the Columbia River to >2000 mu M for the Satilla River. Elemental analysis of DOM concentrates (>1000 D) was used to predict organic matter composition and to calculate degree of substrate reduction using two different modeling approaches. Models predicted aliphatic carbon ranging between 43 and 60% and aromatic carbon between 26 and 36%, with aliphatic content lowest in the Satilla and highest in the Columbia River. The degree of substrate reduction of the organic matter concentrates followed a pattern similar to that for aliphatic C, being lowest in the Satilla (3.5) and highest in the Columbia (4.0). Extracellular enzyme activity varied broadly across the systems, but again ordered sites in the same way as did aliphatic content and degree of substrate reduction. Bacterial growth rates ranged from 1.3 mu g mg(-1) d(-1) DOC in the Satilla to 1.7 ug mg(-1) d(-1) DOC in the Parker River. Bioassays confirmed patterns of dissolved organic matter lability predicted by the chemical models. Between 67% to 75% of the variation in bacterial growth could be explained by differences in organic matter composition.