Selected Features of the Distribution of Chlorophyll along the Southern Shore of Lake Superior

Abstract

Monitoring was conducted over the April to October interval of 1999 and 2000, seeking signals in the spatiotemporal structure of algal biomass (chlorophyll) in Lake Superior. Sampling was conducted along three nearshore-offshore transects in the vicinity of Michigan's Keweenaw Peninsula. Levels of algal biomass in Lake Superior are the lowest in the Great Lakes, with chlorophyll concentrations ranging from ∼0.25 to 2.5 mg·m −3 and averaging less than 1 mg·m −3 in the surface waters. Several spatial signals were observed, including inter-transect differences, nearshore-offshore gradients, and vertical structure. These signals are thought to be mediated by several factors, including runoff from a major tributary, light availability and mixing depth, inter-transect variations in bathymetry and temperature gradients, e.g., the thermal bar in spring and vertical stratification in summer. Seasonally, surface water chlorophyll dynamics were characterized by an increase from late-winter concentrations in late April and early May, a continued increase in the nearshore and a decrease/stabilization at offshore sites from late May through July, a summer minimum in late July and August and an increase in September and October with the approach to turnover. These signals are striking given the modest levels of chlorophyll present in the lake and considering prevailing conditions of light, temperature and phosphorus availability. The spatial and temporal structure evidenced here likely resonates through the system, impacting the distribution of organisms both higher and lower in the food web. These results will find application in this and other similar systems for consideration of the vernal thermal bar as a factor mediating primary production and cross-margin transport of materials of biogeochemical significance, in developing a greater appreciation of the role of the microbial loop in fostering secondary production, in defining the relative roles of allochthonous and autochthonous contributions to the carbon budget and in supporting remote sensing studies of large scale transport phenomena in the lake.