Another look at the origin of the fluid mud layer in Lake Apopka, Florida

Another look at the origin of the fluid mud layer in Lake Apopka, Florida Schelske (2006) questions our conclusion (Bachmann et al. 2005) that a major portion of the fluid mud layer in Lake Apopka is derived from the liquefaction of underlying consolidated sediments. In particular, he questions our interpretation of mean depth measurements that did not decrease in the period 1968 to 1996 when the fluid mud layer increased in thickness by 37 cm. He points out that with a lower bulk density of liquefied consolidated sediments, the mean depth would also decrease. In response, we revisited the data and made a more detailed examination of what the expected net change in mean depth would be if sediment liquefaction were the major mechanism of fluid mud formation. We have also looked for other ways to test our hypothesis and have completed a new analysis of sediment microfossil data. To determine how much consolidated sediment might have been liquefied in the period 1968 to 1996, we started with the inventories of inorganic and organic materials and total phosphorus in the 47 cm of the fluid mud as calculated by Schelske (2006) for the 46 survey cores from Lake Apopka. Since there was 10 cm of fluid mud present in 1968, we needed to subtract the contents of that 10-cm layer to find the accumulation for the 28-yr period. We used the averages of the contents of the top 10 cm of fluid mud in the survey and historic cores from the tables in Schelske (1997). These values were subtracted from the total amounts to find the net accumulations from 1968 to 1996 in the top 37 cm of fluid mud (Table 1). The net accumulation of inorganic mass (0.604 g cm 22) was used as a basis to estimate the amount of consolidated sediment liquefied, since this would not change with the production or consumption of organic matter within the lake during the 28-yr period. We previously (Bachmann et al. 2005) determined that there was a net retention in the lake of externally supplied inorganic particles of 2.