A Case for Mutualism: The Impact of Hard Clam Size and Density on Seagrass Restoration in North Carolina

Abstract

Seagrass ecosystems are vital to the health of coastal oceans and human well-being, yet they are declining at an alarming rate. Restoration managers are struggling to halt the tide of seagrass decline, with current restoration efforts being costly and majorly unsuccessful. Positive species interactions are a powerful way that species can thrive in environments they otherwise would not. In recent years, researchers have been looking to mutualistic relationships as a potential tool for habitat restoration. Here I examine the potential of the context dependent hard clam -seagrass mutualism to improve seagrass restoration outcomes. A prevalent threat to seagrass ecosystems is nutrient enrichment from anthropological activities that impedes light penetration in the water column and proliferates competitively dominate macroalgae. The filter-feeding bivalve mutualist Mercenaria mercenaria may pose a possible solution to this by clearing the water column and depositing nutrients into the sediment via psuedofeces, allowing seagrass to photosynthesize and absorb nutrients more easily. In this study, I utilize a fully factorial manipulative experiment to investigate how M. mercenaria size and density impacts the seagrass, Halodule wrightii, transplant success by implementing three levels of clam density (low, medium, and high) and two levels of clam size (small and large). At the end of the growing season, I observed that seagrasses in treatments with a low density of small clams had the least success with an average final area of 11.1cm2, and seagrasses in treatments with a medium density of small clams were the most successful with a final area of 43.44cm2. The inclusion of small clams at medium density resulted in statistically significant expansion of seagrass patch area (p=0.04), as well as non-statistically significant trends for improved above-ground biomass, growth, and grass height. Taken together, these results suggest that restorationists should utilize small bivalves at an intermediate density to maximize success of seagrass transplants. These results also emphasize that mutualistic interactions are not constant across mutualist size and density, which should be taken into consideration for restoration planning. Most importantly, by incorporating positive interactions into coastal restoration planning, managers may be able to improve success and cost efficiency.