Seasonal distributions of organic nutrients on the Louisiana continental shelf and their implications for nutrient limitation and hypoxia formation

• Dissolved and particulate organic C, N & P were measured along the Louisiana shelf.
• Mid-salinity water on the shelf was a source of DOC, DON and DOP.
• The amount of DOP apparently utilized is not enough to relieve DIP-limitation.
• Total N to total P ratios were high, averaging 100 in March and May and 50 in July.
• TN:TP & DIN:DIP accurately assess nutrient limitation in this region; DIN:TP does not.

Nutrients delivered by the Mississippi and Atchafalaya Rivers to the Louisiana continental shelf cause local eutrophication and feed phytoplankton growth, resulting in a massive region of bottom water hypoxia each summer. Much of what is known about the drivers of this hypoxia is based on extensive datasets of inorganic nutrients. We measured dissolved organic and particulate organic carbon, nitrogen and phosphorus in surface water along the Louisiana continental shelf between the Mississippi and Atchafalaya Rivers during three cruises in March, May and July 2004 to develop a more comprehensive understanding of interactions between nutrients and eutrophication. We specifically sought to determine if sufficient organic P is present to relieve the inorganic P limitation that occurs during the spring and early summer. DOP is added to shelf waters in situ, but drawdown of DOP concentrations below conservative mixing lines indicate DOP is also being used as a source of P for growth. Calculations of the amount of DOP potentially utilized towards primary production reveal there is not enough to relieve P-limitation. Total nitrogen to total phosphorus ratios were high, averaging 86, 72 and 51 in March, May and July, respectively. Additionally, correlations between DIN and total nitrogen (TN) were identical to those between DIP and total phosphorus (TP), indicating that the use of either TN:TP or DIN:DIP ratios is equally relevant in assessing nutrient limitation in this region, but DIN:TP ratios are clearly underestimates. Organic matter on the shelf is best correlated to TN and TP loads measured upstream in the Mississippi River one month prior. The combination of low organic P concentrations and high total N:P ratios indicated P-limitation of phytoplankton biomass on the Louisiana shelf during the spring and early summer of 2004. This P-limitation occurs during the period of hypoxia formation as a result of excess N loads delivered by the rivers, supporting dual controls of both N and P to reduce the hypoxia.