VARIABILITY OF DISSOLVED ORGANIC-CARBON IN SEDIMENTS OF A SEAGRASS BED AND AN UNVEGETATED AREA WITHIN AN ESTUARY IN SOUTHERN TEXAS

Pore-water dissolved organic carbon (PWDOC) concentrations were examin ed in vegetated and bare sediments of a Halodule wrightii seagrass bed , and in a mud bottom sediment of a southern Texas estuary. Temporal v ariability was examined at diel (dawn and noon) and bimonthly time sca les. Distribution patterns of PWDOC were compared with physical, chemi cal, and biological factors thought to exert control on PWDOC. Concent ration of PWDOC, bacterial production, and resultant PWDOC turnover ti mes displayed statistically significant spatial and temporal variabili ty. Concentration of PWDOC ranged from 14 mg C l(-1) to 107 mg C l(-1) of pore water, or 9-71 mu g C cm(-3) wet sediment. PWDOC was more var iable and was approximately 5 times higher than DOC concentrations in the water column. Low PWDOC concentrations (mean = 14.6 mu g C cm(-3)) and high bacterial production rates (mean = 1.92 mu g C cm(-3) h(-1)) were observed at the mud station, whereas PWDOC concentrations were h igh (mean = 24.6 mu g C cm(-3)) and bacterial production rates were lo w (mean = 0.43 mu g C cm(-3) h(-1)) at the bare station. PWDOC turnove r times (T-c), assuming 50% bacterial growth efficiency (1-840 h) were shortest at the mud station (mean = 13 h) and longest at the bare sta tion (mean = 180 h). In the overlying water column, T-t values were lo nger, ranging from 1,000-10,000 h. PWDOC concentrations were 25% highe r in vegetated sediments than in neighboring bare sediments. This diff erence was probably due to inputs of labile photosynthetic excretia, s ince bacterial production rates in vegetated sediments displayed signi ficant diel variability and were 4 times greater than that of bare sed iments. Based upon the entire data set, PWDOC was significantly relate d to macrofaunal biomass, sediment POC, sediment C:N ratios, and oxyge n metabolism, but was significantly correlated only to the latter two variables in stepwise multiple regression. Our findings suggest that o rganism activities and detrital quality are the major determinants con trolling variability in PWDOC.