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

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, sPSII; the time for photosynthetic electron transport on the acceptor side of PSII, tQa; 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 sPSII and tQa 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. Nutrient limitation of net primary production can be an important control on phytoplankton growth in aquatic environments, and understanding it can help to limit eutrophication (Howarth and Marino 2006). Determining the extent of nutrient limitation has been a fundamentally important question of aquatic scientists for decades. Many methods, both direct and indirect, are available for addressing this problem, including nutrient concentrations and ratios, enzyme assays, fluorescence parameters, and nutrient addition bioassays (Beardall et al. 2001b). Fast repetition rate fluorescence (FRRF) allows quick, noninvasive assessment of phytoplankton in vivo fluorescence signatures that provides the user with photosynthetic parameters including Fv : Fm, sPSII, tQa, and p (Kolber et al. 1998). Fv : Fm is an indicator of the photosynthetic efficiency of a cell or community when measured in a darkacclimated state. Healthy algae can have an Fv : Fm as high as 0.65 (Kolber et al. 1998). The absorption cross section of PSII (sPSII) changes in response to cellular pigment concentrations and the efficiency of energy transfer from pigments to PSII reaction centers, thus making it subject to both nutrient and light availability (Kolber et al. 1988; Moore et al. 2006). sPSII is typically lower in nutrientreplete cells relative to unhealthy cells (Kolber et al. 1988). The time constant for photosynthetic electron transfer on the acceptor side of PSII (tQa) reflects the minimum turnover time for electron transport (Kolber et al. 1988). p is the probability of energy transfer between PSII reaction centers (Kolber et al. 1998). Higher p values indicate higher probabilities of electron transfer and have been implicated in recovery from iron limitation (Vassiliev et al. 1995). Collectively, these FRRF parameters can be used to assess the physiological response of phytoplankton cells and/or communities to nutrient stress, such as P limitation. The Mississippi River plume (MRP), herein defined as the area near the Mississippi Delta, especially Southwest Pass, and directly to the west of it (Fig. 1A), is a dynamic system for studying nutrient limitation. The widespread cultivation of maize and soybeans in the Mississippi River watershed has resulted in high nitrate loads in runoff to the Mississippi River that in turn have been implicated as a cause of the eutrophication in the river delta. The eutrophication here is thought to be the primary cause of the large hypoxic zone seen each summer off the Louisiana coast (Rabalais et al. 2002). Production in the MRP is affected by the annual river flow pattern and the water column light regime. While N is thought to control phytoplankton biomass on the Louisiana shelf because of its surplus in the system, a recent study convincingly documented spring and early summer P limitation in 2001 followed by a switch to N limitation in the fall (Sylvan et al. 2006). This is important because the P limitation occurred 1 Corresponding author ([email protected]). 2 Present address: Department of Marine Biology, Texas A&M University, 5007 Avenue U, Galveston, Texas 77551. 3 Present address: Virginia Institute of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, Virginia 23062-1346.