SEASONAL AND INTERANNUAL PATTERNS OF SEDIMENT-WATER NUTRIENT AND OXYGEN FLUXES IN MOBILE BAY, ALABAMA (USA) - REGULATING FACTORS AND ECOLOGICAL SIGNIFICANCE

Sediment oxygen and nutrient fluxes were measured monthly for 2 yr in Mobile Bay, Alabama, USA. Rates of sediment oxygen consumption (0.1 to 1.25 gO(2) m(-2) d(-1)), ammonium flux (-22 to 181 mu mol m(-2) h(-1) ), nitrate flux (-14 to 67 mu mol m(-2) h(-1)), phosphate flux (-2 to 20.4 mu mol m(-2) h(-1)), and dissolved silicate flux (-1.5 to 342 mu mol m(-2) h(-1)) were moderate to high compared to values for other es tuaries. A step-wise regression analysis revealed that dissolved oxyge n concentration and temperature in bottom-waters explained much of the variance in fluxes. This is presumably because of their influence on rates of microbial and physico-chemical processes. Organic matter avai lability was not found to be an important factor in regulating tempora l (month to month) variability of fluxes, possibly because frequent re suspension of the sediments in this shallow system rendered indices of sediment organic matter nearly constant with time. However, warm seas on-averaged sediment nutrient releases were correlated with sediment c hlorophyll a. This relationship in Mobile Bay is in strong agreement w ith similar relationships found in other estuarine systems, and sugges ts that the availability of labile organic matter ultimately regulates the maximum rate of nutrient release by the sediments. Annually avera ged sediment fluxes supplied 36 % of the nitrogen (N) and 25 % of the phosphorus (P) required by phytoplankton in Mobile Bay. While this is not particularly high compared to other estuaries, monthly estimates s how that the sediments can supply from 0 to 94 % of the N, and 0 to 83 % of the P required by phytoplankton. In addition, flux ratios show t hat N and P are released from sediments at N:P ratios that rapidly swi tch from above (maximum 98) to below (minimum 1.2) that required for p hytoplankton growth. This pattern is different from cooler temperate s ystems, where such switching is seasonally based.