Phosphorus Limits Phytoplankton Growth on the Louisiana Shelf During the Period of Hypoxia Formation

Abstract

The Louisiana shelf is the largest zone of seasonally oxygen-depleted coastal bottom water in the U.S. This condition results from the high freshwater and nutrient input from the Mississippi River and the resulting high primary productivity in the river plume. The hypoxic zone has doubled in area since regular measurements began in 1985. Identification of the nutrient(s) limiting phytoplankton growth on the shelf and their sources is important for developing hypoxia-reduction strategies; nitrogen (N) has been considered the most important to date. In this study, we measured multiple parameters addressing nutrient limitation or stress (nutrient concentrations and ratios, alkaline phosphatase activity, phosphorus (P)turnover times, and changes in chlorophyll a concentrations in nutrient enrichment bioassays) in the Mississippi River plume in March, May, July, and September of 2001. All results indicate that phytoplankton growth on the Louisiana shelf was limited by P in May and July of 2001. P limitation was weakly evident in March, but N was limiting in September. The observed P limitation in spring and summer probably results from the historical increases in riverine N due to excessive N loading and has potential implications for developing hypoxia reduction strategies.

The Water Footprint of Biofuels: A Drink or Drive Issue?

“…The anticipated increase in corn cultivation would increase the annual average DIN load by 10-18%, which greatly exceeds the DIN export load targets. The role of P discharges in the formation of the hypoxic zone in the Gulf of Mexico has also been reassessed (37); resulting in a new goal for a 45% reduction in total phosphorus (TP) exports (Figure 3).…”

A coupled physical-biological model of the Northern Gulf of Mexico shelf: model description, validation and analysis of phytoplankton variability

“…However, a number of findings and ideas have been articulated recently that suggest the classic concept is too simplistic and that other factors are important as well (Rowe and Chapman, 2002;Hetland and DiMarco, 2008;Sylvan et al, 2006;Bianchi et al, 2010;Lehrter et al, 2009). For example, terrestrial organic matter load probably contributes significantly to oxygen consumption (Bianchi et al, 2010(Bianchi et al, , 2009, stratification is important for hypoxia formation in preventing supply of oxygen below the pycnocline (Wiseman et al, 1997), sediment oxygen demand is not directly related to river nutrient load (Morse and Rowe, 1999;Rowe and Chapman, 2002), and spatially varying rates of macrozooplankton grazing affect the rate of phytoplankton accumulation and the amount of organic matter reaching the bottom (Dagg, 1995).…”

“…We present a 15-yr simulation (from January 1990 to December 2004) comparing simulated distributions of nitrate and chlorophyll and modelpredicted rates with available observations. The simulation period overlaps with several observational programs, including the NOAA/NECOP program (1991)(1992)(1993), which provides estimates of primary production, phytoplankton growth rates, zooplankton grazing rates and sedimentation fluxes, a NOAA-funded program from 2001 to 2004 that provides surface nitrate distributions (Sylvan et al, 2006), and the SeaWiFS program which provides estimates of surface chlorophyll (starting from the end of 1998). After demonstrating that the model realistically reproduces many observed features of nitrate and phytoplankton dynamics, we analyze the environmental differences and phytoplankton source and sink terms along the ecological gradient from high-nutrient plume waters to low-nutrient waters far from the direct influence of the Mississippi River.…”

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“…The standard deviation of AP data from the MEA increases with increasing activity and deployment duration. The subset of higher AP activities from the MEA are >1000 nmol L -1 h -1 , a magnitude seen in phosphorus-limited, highly eutrophic coastal areas (Sylvan et al 2006). AP activity is more likely to be variable because of the shorter incubation times.…”

“…Leucine aminopeptidase (LAP) activity was assessed using L-leucine-7-amido-4-methylcoumarin hydrochloride (Leu-AMC) substrate and 7-amino-4-methylcoumarin (AMC) standard, and β-glucosidase (BGlc) activity with 4-methylumbelliferyl β-D-glucopyranoside (MUGlc) and 4-methylumbelliferone (MUF) from SigmaAldrich (Hoppe 1983). Alkaline phosphatase (AP) activity was assessed using a highly sensitive substrate from Invitrogen, 6,8-difluoro-4-methylumbelliferyl phosphate (DiFMUP), and the standard 6,8-difluoro-7-hydroxy-4-methylcoumarin (DiFMU) (Gee et al 1999;Sylvan et al 2006). Leu-AMC and BGlc were dissolved in pure 2-methoxyethanol, and DiFMUP and DiFMU were dissolved in dimethyl sulfoxide (DMSO), creating a 10-mM stock of each.…”