The role of submerged macrophyte beds in phosphorus and sediment accumulation in Lake Memphremagog, Quebec, Canada

The relationships between long-term (;115-year) phosphorus and sediment accumulation and submerged macrophyte biomass were determined both within a submerged macrophyte bed and among beds in Lake Memphremagog, Quebec, Canada. Strong relationships between present submerged macrophyte biomass and littoral-zone sediment and phosphorus accumulation rates both within and among macrophyte beds yielded simple models for estimating long-term littoral-zone accumulation rates. Mean sediment particle size at the depth of maximum submerged macrophyte biomass, estimated from water content, was comparable with the mean particle size in profundal zone of Lake Memphremagog, suggesting that energy environments differ little between vegetated littoral zones and profundal zones. The mean total sediment-accumulation rate of 1.2 mm yr21 in the macrophyte beds was indistinguishable from average profundal rates of sediment accumulation in North American lakes. Although dense beds accumulated twice as much bulk sediment per unit area (g m22 yr21) compared with their profundal counterparts, phosphorus accumulation per unit bulk sediment in macrophyte beds with a biomass of 1,000 g m22 (wet weight) was one sixth that measured in profundal sediments, pointing to large postdepositional losses of sedimented phosphorus (.70%) from macrophyte beds. Finally, extrapolation of the present results to five nearby lakes suggests, based on still-limited data, that, in lakes in which half of the sediment surface is colonized by submerged macrophytes, the littoral zone accounts for roughly one third of whole-lake phosphorus and two thirds of whole-lake bulk sediment accumulated annually. The distribution, transport, and ultimate fate of sediments have large effects on phosphorus (P) cycling in lakes (Canfield et al. 1982; Kalff 2001). With respect to the composition and long-term accumulation of sediments, stratified lakes can be divided into three zones. The zone of sediment accumulation (ZSA) located in deep sites of low underwater slope and energy is dominated by fine, low-density inorganic and organic particles (Håkanson and Jansson 1983; Rowan et al. 1992a), and is characterized by disproportionately high rates of sediment accumulation. Located just above the ZSA is the zone of discontinuous sediment accumulation (ZDA), where the accumulation of fine sediment particles is interrupted by periodic resuspension and transport during rare storm events. Finally, the zone of sediment erosion (ZSE), located in high-energy, shallow environments, is characterized by coarse-grained, dense inorganic sediments and low sediment-accretion rates. This pattern, however, does not hold where the littoral zone is colonized by submerged vegetation (James and Barko 1990; Benoy and Kalff 1999; Bindler et al. 2001). Submerged macrophytes modify near-bed water flow and 1 Current address: Department of Zoology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada ([email protected]).