Eutrophication‐induced phosphorus limitation in the Mississippi River plume: Evidence from fast repetition rate fluorometry

Abstract

We assessed nutrient limitation in the Mississippi River plume and Louisiana continental shelf during the summer of 2002 (04‐08 July). We measured nutrient concentrations, alkaline phosphatase (AP) activities, chlorophyll a (Chl a) concentrations, and four fast repetition rate fluorescence (FRRF) parameters: the maximum quantum yield of photochemistry in photosystem II (PSII), Fv:Fm; the functional absorption cross section for PSII, σPSII; the time for photosynthetic electron transport on the acceptor side of PSII, τQa; and the connectivity factor, p, in 24‐h‐long nutrient addition bioassays near the Mississippi River delta. Low phosphorus (P) concentrations, elevated inorganic nitrogen‐to‐phosphorus ratios, high AP activities, and Chl a increases in response to P additions in the bioassays all indicated phosphorus limitation that was confirmed by the response of FRRF parameters. This is the first study to use FRRF to confirm results from basic oceanographic methods to demonstrate phosphorus limitation in a marine setting. Fv:Fm and p responded positively to phosphorus addition, while σPSII and τQa decreased in the same treatments. When nitrate alone was added, none of the measured parameters differed significantly from the control. We therefore suggest that FRRF can be used to rapidly detect phosphorus limitation in marine ecosystems.