Mapping Phytoplankton Community Physiology on a River Impacted Continental Shelf: Testing a Multifaceted Approach

Abstract

Seasonal phosphorus limitation occurs on the Louisiana continental shelf as a result of high nitrogen loads in the spring and early summer. Prior studies have assessed such nutrient limitation by laborious and timeconsuming nutrient analyses, enzyme assays, and nutrient addition bioassays. We undertook surface (0.5-1 m) mapping of fast repetition rate fluorescence (FRRF) parameters to assess nutrient limitation in real time on the Louisiana continental shelf and Mississippi River plume from 29 June to 08 July, 2002 in an effort to further understand phytoplankton productivity in this region, as well as to better inform effective nutrient management strategies. Surface nutrient concentrations, chlorophyll a biomass, alkaline phosphatase (AP) activity, and four FRRF parameters: the maximum quantum yield of photochemistry (F v /F m ), the functional absorption cross section for PSII, the time constant for Q A reoxidation, and the connectivity factor, were measured during continuous underway mapping. Results from traditional methods to assess phytoplankton nutrient stress indicated widespread phosphorus limitation from the Mississippi River plume to the Atchafalaya River, manifested as high inorganic N/P ratios and elevated AP activities associated with phytoplankton biomass. The FRRF data indicated complex patterns of phytoplankton physiology that were likely driven by the rapidly changing conditions in local surface waters and heterogeneous phytoplankton community structure. Correlations of nutrient data and enzyme assays with FRRF parameters were significant but low, potentially due to differences in the manner and time scale with which nutrient limitation affects the different techniques used, indicating that further work is needed to interpret FRRF parameters in large, heterogeneous environments such as estuaries and continental shelves.

Simulating the effects of phosphorus limitation in the Mississippi and Atchafalaya River plumes

“…A 7-yr simulation (2001)(2002)(2003)(2004)(2005)(2006)(2007) is analyzed to assess if and how P limitation affects patterns of primary production across the Louisiana shelf. The results compare well with spatial and temporal patterns of P and N limitation observed during the nutrient limitation studies of Sylvan et al (2006Sylvan et al ( , 2007Sylvan et al ( , 2011 and Quigg et al (2011) and corroborate their hypothesis of delayed primary production in the Mississippi delta resulting from P limitation. This effect is analyzed for several zones across the shelf, and nutrient dynamics in the Mississippi and Atchafalaya River plumes are compared.…”

“…However, while dissolved organic P (DOP) may be an important source of P on the Louisiana shelf (Dagg et al, 2007), DIP is the only form of P that is readily available to phytoplankton. Moreover, the DIN : DIP observations are consistent with physiological measurements and nutrient addition bioassays carried out near the Mississippi delta region (Dortch and Whitledge, 1992;Smith and Hitchcock, 1994) and across the Louisiana shelf (Sylvan et al, 2006(Sylvan et al, , 2007(Sylvan et al, , 2011Quigg et al, 2011) all indicating that P limitation occurs during spring and early summer. The period of P limitation coincides with the peak in primary production (Lohrenz et al, 1997) that is thought to contribute to the development of summer hypoxia (Rabalais et al, 2002).…”

Extracellular Enzyme Activity and Microbial Diversity Measured on Seafloor Exposed Basalts from Loihi Seamount Indicate the Importance of Basalts to Global Biogeochemical Cycling

“…7) species in response to C limitation, indicating a role for AP in obtaining organic C (9, 51-53). There is also strong support in the literature for the organic and inorganic P concentration being inversely proportional to AP activity (6,54). It is likely that the broad substrate specificity of AP indicated by the results just discussed implies that AP enzymes contribute to the processing of organic P moieties using different regulation systems in different environments.…”

Application of a one-dimensional model to explore the drivers and lability of carbon in the northern Gulf of Mexico

“…At station Z02 in June, it was phosphorus, as opposed to nitrogen, that was the limiting nutrient for the first few meters, the only site of the four studied that exhibited this behavior. This is not surprising and is consistent with recent literature showing that regions close to the plume may become phosphorus limited in the spring and early summer (Laurent et al, 2012;Quigg et al, 2011;Sylvan et al, 2006;Sylvan et al, 2011). These papers state that it is not necessarily a lack of phosphorus that limits the growth, but rather there is excess nitrate loaded to the shelf via the Mississippi and Atchafalaya rivers.…”