Phytoplankton Productivity and Standing Crop in the Vicinity of the Lake Huron-Saginaw Bay Front

Abstract

Continuous underway sampling was used in four research cruises near the mouth of Saginaw Bay in order to locate the bay-lake front, where eutrophic bay water mixes with oligotrophic Lake Huron water. Although mixing between these water masses normally occurs in outer Saginaw Bay, the distribution was variable, depending on wind force and direction as well as water temperature. Large algal crops in inner Saginaw Bay had reduced soluble nutrients, particularly silicate and nitrate-nitrogen, in the outer bay to levels lower than the adjacent lake water. Nonetheless, outer bay water supported high algal biomass (up to 14 μg/L chlorophyll a) and 14C uptake (up to 73 μg /L/h). The phytoplankton community at the nutrient front between bay and lake waters was characterized by high chlorophyll a biomass ( x ¯ = 4.47 μg/L), low 14C uptake ( x ¯ = 7.12 μg/L/h), and high bacterial activity (V max x ¯ = 1.33) indicating the presence of a senescent assemblage. Decomposition and settling of phytoplankton in the vicinity of the front apparently served to remove much of the bay plankton from the water column before mixing into the open lake. But, during mid-summer, occasional large inputs of nutrients and/or phytoplankton from the bay into the open lake were observed.