Effect of species diversity on particle clearance and productivity in farmed bivalves

Most bivalve farms are designed as monocultures, yet diverse communities often outperform monocultures in various metrics of ecosystem functioning, including particle clearance and productivity. This study tested the feasibility of bivalve polyculture by examining particle clearance, growth, and survival of 4 species that are economically important to the northeastern USA. Three particle depletion experiments were conducted to determine if more diverse bivalve assemblages had greater clearance rates than those which were less diverse. Different assemblages of Crassostrea virginica, Spisula solidissima, Mercenaria mercenaria, and Mya arenaria were supplied with a single cultured algal species, a mix of 2 cultured algal species, or natural seston. To determine how species richness affects bivalve productivity, growth and survival were monitored in a flow-through mesocosm experiment, which simulated farm conditions. In the cultured algae experiments, more diverse assemblages did not exhibit significantly greater clearance rates than those that were less diverse. Instead, the clearance rates of each species were additive across assemblages. Surprisingly, most assemblages did not display a significant preference for the larger microalgae species, Pavlova lutheri (4.0-6.0 mu m), over Nannochloropsis oculata (1.90-3.75 mu m). Most notably, when supplied with natural seston, the 4-species polyculture demonstrated a significantly greater tank-level clearance rate for particles <25 mu m compared to most monocultures. However, nearly all productivity metrics were not significantly affected by species richness. This work suggests that some degree of complementarity exists among these bivalves, and that in non-food limited systems, these bivalves could be co-cultured without outcompeting one another.