Wave velocity thresholds for fine sediment accumulation in lakes, and their effect on zoobenthic biomass and composition

Deposition of fine sediments occurs at depths where water velocity at the bottom is less than the threshold velocity required to keep the particles in suspension. In this paper we demonstrate that the depositional boundary depth (DBD) can be predicted from models based on wave height and lake morphometry, and that such depositional boundaries greatly affect the biomass and composition of the zoobenthos. Lake Memphremagog was chosen for this study since it exhibits a great deal of variation in exposure and bottom slope which make the DBD highly variable from place to place. In spite of this variation, our model predicted its location very accurately even along transects with highly complex topography. Sites that our models predicted would be depositional, always had fine sediments and had more than twice the zoobenthic biomass of non-depositional sites, which had sediments made up of silt or silty-sand. Epilimnetic sites had more than twice the average biomass of hypolimnetic sites of the same depositional type. PCA on the benthic community data ordinated our study sites along 2 distinct components: Component I reflected mainly the thermal environment and had positive loadings from epilimnetic taxa; Component II reflected mainly the depositional regime, and had positive loadings from most taxa. Most zoobenthos in lakes depend on sedimenting fine seston and its associated microflora for food. The close relationship between the depositional regime and the abundance and composition of the zoobenthos indicates that the sedimentation behavior of seston is very similar to that of fine clay-sized inorganic particles. This similarity allows us to use models developed for clay particle deposition to predict zones of high benthic productivity.