Date of Award
Doctor of Philosophy in Environmental Engineering (PhD)
College, School or Department Name
Department of Civil and Environmental Engineering
Martin T. Auer
This dissertation supports the modeling of primary production in Lake Superior by offering site specific kinetics and algorithms developed from lab experiments performed on the natural phytoplankton assemblage of Lake Superior. Functions, developed for temperature, light and nutrient conditions and the maximum specific rate of primary production, were incorporated in a 1D specific primary production model and confirmed to published in-situ measured rates of primary production.
An extensive data set (supporting model calibration and confirmation), with a fine spatiotemporal resolution, was developed from field measurements taken bi-weekly during the sampling seasons of 2011, 2012 and 2014; considered to be meteorologically average, extremely warm and cold years, respectively. Samplings were taken at 11 stations along a 26 km transect extending lakeward from Michigan’s Keweenaw Peninsula covering the nearshore to offshore gradient. Measurements included: temperature, solar radiation, transparency, beam attenuation, chlorophyll-a fluorescence, colored dissolved organic matter, suspended solids and phosphorus and carbon constituents. Based on these measurements and application of the developed primary production model, patterns in primary production and driving forces (i.e. temperature, light and nutrients) are described in a seasonal, spatial, and interannual fashion.
The signal feature in 2011 was the development of a mid-summer “desert” in the offshore surface waters (a period of suboptimal temperatures coincident with a high degree of phosphorus limitation). The manifestation of the “summer desert”, however, was most extreme during the warm year and nonexistent during the cold year. Offshore primary production in all years manifested a subsurface maximum in the upper area of the metalimnion, distinctly above the deep chlorophyll maximum, with rates of production being highest In 2011 (~20 mg C m-3 d-1) followed by 2012 (~17 mg Cm-3 d-1) and lowest in 2014 (~12 mg Cm-3 d-1). Driven by variances in biomass and forcing conditions, offshore areal primary production manifested differences in seasonal patterns between years as well. In 2011 and 2014 a negatively skewed bell-shape pattern was observed, differing in magnitude and timing. The pattern in 2012 differed from these years in magnitude and timing, manifesting elevated production in April and decreased production in September. Greatest areal production in 2012 occurred in July and August (~320 mg Cm-2 d-1), in 2014 in August (~265 mg Cm-2 d-1) and in 2011 production was greatest in July (253 mg C m-2 d-1). Areal production in the summer of 1998, calculated for EPA’s 19 offshore stations in Lake Superior, manifested comparable rates and averaged 224 ± 90 mg C m-2·d-1.
Although in all years the development of the thermal bar (TB) occurred after the spring runoff event, an increase in chlorophyll-a concentration during the presence of the TB was observed in 2012. Rates of primary production during this period, however, decreased while the opposite occurred in 2014, signifying that changes in chlorophyll-a concentration should be interpreted carefully (especially if used to identify spring blooms).
The information presented in this work not only offers site specific kinetics, appropriate algorithms in support of primary production modeling and an extensive dataset supporting model calibration and confirmation, it also offers new insights into the dynamics of the Lake Superior ecosystem and the forces driving its function.
Dijkstra, Marcel L., "CLIMATE ANOMALIES AND PRIMARY PRODUCTION IN LAKE SUPERIOR", Dissertation, Michigan Technological University, 2015.