Spatial and temporal dynamics of phytoplankton nutrient status in the deep chlorophyll layer (DCL) of Lake Superior

Abstract

Total phosphorus and chlorophyll have been declining over the last few decades in the ultra-oligotrophic Lake Superior. A region in the water column where chlorophyll exceeds that of the mixed layer during the stratified period, referred to in this study as the deep chlorophyll layer (DCL), is presumably ideal in terms of nutrients and light for primary producers. Investigating the nutrient status of these primary producers could aid in understanding the nutrient dynamics in this large, low nutrient lake. Lake nutrient concentration and phytoplankton nutrient status measurements were determined to evaluate seasonal, temporal, and spatial patterns both across the lake and with depth. Additionally, active fluorometry was measured in order to compare its efficacy as a nutrient status indicator with more traditional methods. Water samples were collected above, within, and below the DCL across the lake during the summer of 2010. Chemical analyses including chlorophyll a, particulate nutrients (carbon, nitrogen, and phosphorus), soluble reactive phosphorus, total and total dissolved phosphorus, total and total dissolved nitrogen, ammonium, nitrate, and soluble reactive silica were performed in conjunction with bioassays (N and P debt, alkaline phosphatase activity) and active fluorometry (PhytoPAM) to fully assess phytoplankton nutrient status. Results indicated extreme P deficiency across the lake in the epilimnion and metalimnion with the most extreme deficiency seeming to occur in the epilimnion and occasional deficiency in the hypolimnion. Also, a P deficiency indicator exhibited a diurnal pattern with higher deficiency during the daytime. Deficiency also appeared to be highest in the spring and decline over the course of the summer; however, even in September, samples still indicated extreme deficiency. Additionally, N debt data suggests N deficiency in spring which had disappeared by August. Active fluorometry was consistent with nutrient bioassays in suggesting that the epilimnion was more stressed than the metalimnion or hypolimnion. Strong nutrient concentration gradients across the lake and with depth as well as with time were absent, although there appears to be a seasonal decrease in SRP, nitrate, and silica and an increase in TN and TDN from June to September. Nutrient concentration and phytoplankton nutrient status could have been influenced by many factors in Lake Superior including a seasonal shift in algal species composition, zooplankton nutrient translocation, contribution to deficiency by picobacterioplankton, borderline iron (Fe) limitation, and reallocation of nutrients for purposes other than photochemistry.