Factors influencing the recovery of submersed macrophytes in four coastal marshes of Lake Ontario

Abstract

In this study, we investigated the impact of carp and turbidity on the growth of macrophytes from propagules in four Lake Ontario marshes with low submersed macrophyte abundance. A healthy propagule bank was transplanted into 4 m2 carp exclosures (5-cm-mesh cages), turbidity exclosures (enclosed in plastic), and open control sites, with four replicates per treatment used in each marsh. Carp exclosures were intended to protect the transplanted propagule banks from carp and other large aquatic organisms; turbidity exclosures were intended to also reduce wind exposure and inflowing suspended sediments, thus increasing the amount of light reaching bottom sediments. The mean density of shoots produced in the turbidity exclosures (256±46 shoots·m−2) was significantly higher than that produced in carp enclosures (20±7 shoots·m−2) and open controls (10±5 shoots·m−2); above-ground biomass (AGB) was also significantly greater in turbidity exclosures. The difference in protection afforded the developing submersed macrophyte shoots can be attributed to the lower concentration of total suspended solids and greater level of light penetration in the turbidity exclosures. There was a strong linear relationship between photosynthetically active radiation (PAR) reaching the substrate surface and shoot growth in terms of both shoot density and AGB. The growth response was more sensitive to PAR in the field than it was in a growth chamber, suggesting that light levels are more critical to shoot development when multiple stressors are involved. The two marshes exposed to high wave energy had very high levels of suspended solids, and the introduced propagule bank was eroded away in controls and carp exclosures. In such marshes, both turbidity and exposure would have to be addressed for macrophyte recovery.