Open AccessDistribution and dynamics of nitrogen and microbial plankton in southern Lake Michigan during spring transition 1999–2000
Author(s) -
Gardner Wayne S.,
Lavrentyev Peter J.,
Cavaletto Joann F.,
McCarthy Mark J.,
Eadie Brian J.,
Johengen Thomas H.,
Cotner James B.
Publication year - 2004
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2002jc001588
Subject(s) - ammonium , phytoplankton , cycling , plankton , environmental chemistry , chlorophyll a , environmental science , nitrogen , chemistry , zoology , ecology , biology , nutrient , botany , history , organic chemistry , archaeology
Ammonium and amino acid fluxes were examined as indicators of N and microbial food web dynamics in southern Lake Michigan during spring. Either 15 NH 4 + or a mixture of 15 N‐labelled amino acids (both at 4 μM N final concentration) was added to Lake Michigan water. Net fluxes were measured over 24 h under natural light and dark conditions using deck‐top incubators and compared to microbial food web characteristics. Isotope dilution experiments showed similar light and dark NH 4 + regeneration rates at lake (6 versus 5 nM N h −1 ) and river‐influenced (20 versus 24 nM N h −1 ) sites. Ammonium uptake rates were similar to regeneration rates in dark bottles. Dark uptake (attributed mainly to bacteria) accounted for ∼70% of total uptake (bacteria plus phytoplankton) in the light at most lake sites but only ∼30% of total uptake at river‐influenced sites in or near the St. Joseph River mouth (SJRM). Cluster analysis grouped stations having zero, average, or higher than average N‐cycling rates. Discriminant analysis indicated that chlorophyll concentration, oligotrich ciliate biomass, and total P concentration could explain 66% of N‐cycling rate variation on average. Heterotrophic bacterial N demand was about one third of the NH 4 + regeneration rate. Results suggest that, with the exception of SJRM stations, bacterial uptake and protist grazing mediated much of the N dynamics during spring transition. Since NH 4 + is more available to bacteria than NO 3 − , regenerated NH 4 + may have a strong influence on spring, lake biochemical energetics by enhancing N‐poor organic matter degradation in this NO 3 − ‐replete ecosystem.