Ecological Interactions Among Bangia, Cladophora, and Ulothrix Along the Lake Erie Shoreline

Abstract

Bangia atropurpurea, a recent invader of Lake Erie, has altered the community structure of the littoral zone, which previously had been dominated by Cladophora or Ulothrix. Vertical and horizontal zonation patterns of these algae suggest that competition and differing physical tolerances may be involved. To determine the relative importance of these factors, seasonal changes in littoral algal distribution patterns and environmental conditions were monitored. Additionally, Bangia was examined in culture to relate growth and reproductive mechanisms to distribution patterns and competitive potential. Ulothrix established extensive cover immediately below mean water level (MWL) after thaw in mid-April along moderately exposed shores. By early May, Bangia colonized from +0.5–1.0 m to just below MWL coexisting with Ulothrix near MWL. As water temperature neared l5° C in late May, the distribution of Cladophora expanded from patches at −0.5–1.0 m upward into the Ulothrix band. In early June, Ulothrix sporulated, the remaining empty filaments disintegrated, and the alga was not observed for the remainder of the summer. Cladophora continued upward colonization, eventually dominating the substrate to the +0.5–1.0 m level. Bangia was dominant above the Cladophora zone, up to +2.0 m. During periods of moderate wave wash, Cladophora dominated up to the +1.0 m level. Heavy wave wash resulted in the detachment of Cladophora, allowing domination by Bangia from +0.5 m to 2.0 m. Bangia shares substrate with Ulothrix in the limited areas where their distributions overlap, and perhaps through greater desiccation tolerance can persist in areas where Cladophora does not grow. This partial niche segregation provides Bangia with refugia in time as well as space. Coexistence between Bangia and the competitively dominant Cladophora occurs as a result of partial niche segregation as well as the impact of wave action in initiating the detachment of Cladophora.