Comment on the origin of the ‘‘fluid mud layer’’ in Lake Apopka, Florida

Bachmann et al. (2005) contend that a major fraction of flocculent sediments, a ‘‘fluid mud layer,’’ which has accumulated in Lake Apopka, Florida, since 1947, originated from sediment liquefaction of consolidated macrophyte-derived sediments. In addition, they contend that the underlying historic sediments are not only a major source of fluid mud, but also the source of organic matter for internal heterotrophy. They conclude that the ‘‘fluid mud layer is less a direct consequence of eutrophication than a consequence of enhanced wave action on the lake bed.’’ This conclusion is debatable (see Coveney et al. 2005; Schelske et al. 2005; Waters et al. 2005). All investigators agree that the primary producer community shifted abruptly from macrophyte to phytoplankton dominance in 1947 and that flocculent sediments that have accumulated since the 1947 shift differ from underlying macrophyte-derived sediments. Hypotheses about the origin of these flocculent sediments are addressed here using two conceptual models, a liquefaction model based on Bachmann et al. (2005) and a trophic dynamic model based on classic trophic dynamics (Lindemann 1942). The liquefaction and trophic dynamic models differ in several respects. Internal heterotrophy fueled by organic carbon from historic macrophyte-derived sediments and flocculent sediments derived from liquefaction of historic macrophyte-derived sediments are the main components of the liquefaction model. The trophic dynamic model based on classic Lindemann trophic dynamics (Lindemann 1942) assumes that autochthonous primary production is the major source of organic matter in post-1947 sediments. Phytoplankton synthesize organic matter in a shallow euphotic layer over the lake basin (Schelske et al. 2003). Phytoplankton then either settle to the bottom, become incorporated into the food chain, or both. In Lake Apopka, heterotrophic and diagenetic processes in the water column and sediments are hypothesized to consume some 90% of the organic matter (Schelske et al. 2003). The remaining organic matter is then the major source of organic matter in post-1947 sediments. Phytoplankton primary production is a minor source of organic matter in the liquefaction model because sediment liquefaction is purported to account for 59% of the organic carbon accumulation (Bachmann et al. 2005). Both models assume that allochthonous inputs of organic matter are not important. These conceptual models provide a framework for hypotheses. Hypotheses can be based on prediction or accommodation (Lipton 2005). In the case of prediction, though it may be partially based on an existing data set, the hypothesis ‘‘is formulated before the empirical claim in question is deduced and verified by observation.’’ Bachmann et al. (2005) predicted lutocline height from theory using wind energy, wave height, and water depth. This prediction, however, only demonstrated that predicted lutocline heights fall within the range of measured thickness of flocculent sediments and did not predict accumulation rate or extent of ‘‘fluid sediments.’’ In the case of accommodation, the hypothesis is ‘‘constructed to fit an observation that has already been made.’’ It is possible, however, to test predictions implicit in hypotheses based on accommodation with either new or existing data and observations. I use this approach here. The challenge here is to define tests that can be employed rigorously and without ambiguity to address the hypothesis that ‘‘fluid mud’’ originates primarily from sediment liquefaction of underlying macrophyte-derived sediments. Simply stated, sediment liquefaction is a physical change in sediments (decrease in wet bulk density) generated by wave action on the lake bottom. I utilize data on bulk composition of sediments, specifically sediment dry mass and its organic and inorganic components, including the total carbon (TC) : total nitrogen (TN) ratio of organic matter and total phosphorus (TP) concentration. I use mass balances on whole-lake scales to investigate the plausibility of the purported sediment liquefaction and address the environmental implications of such a process.