Increasing nitrate and decreasing algae in Earth's biggest lake

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Five lakes contain 50% of Earth's liquid, surficial, freshwater.  However, assessing environmental changes occurring in Earth's largest lakes is often hampered by absent or highly spotty records of even the most basic limnological parameters.  Lake Superior, the headwaters of the Laurentian Great Lakes system, itself holds 10% of this freshwater.  One parameter with an unusually long and complete record is nitrate.  It demonstrates a seemingly linear increase over a time span of 100 years.  Evidence indicates that the lake's own biogeochemical processes are the immediate source of this excess nitrate.  Long-term records of NPP or even of algal abundance in Lake Superior are much less complete.  However, we have assembled at ~17 y record of chlorophyll and a shorter record of 14C in-situ measured production in the offshore.  In contrast to the increasing availability of nitrate, chlorophyll has diminished.  Especially the larger cells in the warm surface layer have dropped significantly.  Although Lake Superior is normally considered relatively pristine, there are significant environmental changes in this vast, oligotrophic freshwater ecosystem.