Cladophora, mass transport, and the nearshore phosphorus shunt

The nearshore phosphorus shunt hypothesis and the potential for mussels to excrete phosphorus sufficient to meet the growth requirements of Cladophora are now well accepted by scientists studying Great Lakes biogeochemistry. The response of algal growth to near bottom water column phosphorus concentrations and the interplay between excretion and mass transport in yielding those concentrations have, however, not been elucidated. Here we present soluble reactive phosphorus profiles from the near bottom environment of Lake Michigan at a site near Good Harbor Bay, Michigan, where both mussels and Cladophora were present. Soluble reactive phosphorus was observed to accumulate under quiescent conditions, establishing a concentration boundary layer (CBL), 5-15 cm thick, with near bottom concentrations on the order of 2-8 μg P/L. A one-dimensional model was applied to determine mass transport conditions mediating the transition from CBL formation to CBL destruction. Significant wave height (SWH) was used as an indicator of mass transport intensity, and it was determined that the formation/destruction transition occurred at a SWH of 0.2 m at the 8-m study site depth. The Great Lakes Cladophora Model was applied to determine the time intervals required to saturate (1 day with the CBL present) and deplete (14 days with the CBL absent) algal internal P stores. A review of SWH conditions at the study site indicated that a CBL would be expected to form at a frequency sufficient to support the phosphorus nutrition of Cladophora over the entire May to August interval.