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 µM for the Columbia
River to > 2000 µM 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 µg
mg-1d-1 DOC in the Satilla to 1.7 µg mg-1d-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.