Decomposition dynamics of aquatic macrophytes in the lower Atchafalaya, a large floodplain river

Decomposition of aquatic macrophytes can considerably influence carbon cycl ing and energy flow in shallow freshwater aquatic ecosystems. The Atchafala ya River Basin (ARB) is a large floodplain river in southern Louisiana that experiences a seasonal floodpulse and is spatially composed of a mosaic of turbid riverine and stagnant backwater areas. During two seasons, winter a nd fall of 1995, we examined decomposition of four common aquatic macrophyt es in the ARB: water hyacinth ( Eichhornia crassipes), arrowhead (Sagittari a platyphylla), coontail (Ceratophyllum demersum) and hydrilla (Hydrilla ve rticillata). To determine decay rates, we used litter bags of two mesh size s (5 mm and 0.25 mm) and analyzed data with a single exponential decay mode l. Analysis of decay rates established several trends for aquatic macrophyt e decomposition in the ARB. First, macrophytes decayed faster in fall than winter due to the effect of increased temperature. Second, macroinvertebrat es were the primary decomposers of macrophytes in riverine sites and microb es were the primary decomposers in backwater areas. These trends may have b een related to decomposer-habitat interactions, with well-oxygenated riveri ne sites more hospitable to invertebrates and backwater areas more favorabl e to microbes because of high organic inputs and reduced flow. Decay rates for macrophytes, ranked from slowest to fastest, were E. crassipes < S. pla typhylla < C. demersum < H. verticillata. Slower decomposition of E. crassi pes was probably a result of microbial inhibition by the waxy-cutin outer l ayer and low nutritional value. The accelerated decomposition of C. demersu m and H. verticillata was most likely a function of the large surface area of the highly dissected leaves. Macroinvertebrate numbers were twice as hig h in riverine sites compared to backwater sites. In the winter, amphipods G ammarus spp. and Hyallela azteca composed a large percentage of the total d ensity on detritus. In the fall, Caenis sp. was prevalent in the backwater habitat and dipterans were abundant in the riverine site. We investigated t he microbial component involved in the decomposition of E. crassipes and S. platyphylla and found that the highest microbial respiration rates occurre d early in the winter at the backwater site. Bacterial density in the winte r on E. crassipes and S. platyphylla averaged 1.4x10(6) cm(-2) after two da ys and decreased to 2.0x10(5) cm(-2) after 28 d. Our results emphasized the importance of the microbial community in the decomposition of macrophytes in the ARB, especially in backwater habitats and in the early stages of dec ay.