Effects of the nuisance algae Cladophora glomerata on the shoreline communities of Lake Michigan: Terrestrial plants and nearshore aquatic invertebrates

Background/Question/Methods

Productivity of Cladophora glomerata, a filamentous green alga native to the Laurentian Great Lakes, has increased in the past twenty years with the introduction of exotic zebra mussels, and has significantly impacted nearshore and shoreline communities. Much of our understanding of this nuisance algae comes from coarse scale models of its distribution in the pelagic zone; we understand little about its effects on ecological communities at the water’s edge.  On the shores of Beaver Island, Lake Michigan, we conducted a multi-scale study consisting of island-wide monitoring of Cladophora, medium scale transects quantifying factors driving terrestrial plant and nearshore invertebrate communities, and fine scale experiments to determine the mechanistic effect of Cladophora on key-indicator species.  The fine scale experiments tested the effects of shading, smothering, and/or nutrient release, of Cladophora on silverweed, a common shoreline plant species, and two common freshwater invertebrates, mites and isopods.

Results/Conclusions

We found that Cladophora presence along the shoreline varied spatially, with some beaches being consistently fouled, and others rarely.  Non-Metric Multidimensional Scaling analysis showed that sites subjected to Cladophora deposition had distinct community assemblages compared to Cladophora-free sites; however, co-variates such as substrate size, water depth, and shoreline slope will need to be disentangled with further data collection. Finally, in fine-scale microcosm experiments, we found that Cladophora presence lowered survivorship and leaf growth in invertebrates and plants, respectively. Our data suggests that these effects may be due to nutrient release of the algae as it decays, rather than shading effects. Taken together, these results suggest that mats of Cladophora have the potential to alter community composition in shoreline ecotones. Further research into the mechanistic effects of Cladophora on ecotone fauna and flora, along with longer term collection of medium and large scale data will be necessary to predict and manage effects of this nuisance alga in the Laurentian Great Lakes.